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Sample records for positron annihilation lifetime

  1. Position-resolved Positron Annihilation Lifetime Spectroscopy

    NASA Astrophysics Data System (ADS)

    Wagner, A.; Butterling, M.; Fiedler, F.; Fritz, F.; Kempe, M.; Cowan, T. E.

    2013-06-01

    A new method which allows for position-resolved positron lifetime spectroscopy studies in extended volume samples is presented. In addition to the existing technique of in-situ production of positrons inside large (cm3) bulk samples using high-energy photons up to 16 MeV from bremsstrahlung production, granular position-sensitive photon detectors have been employed. A beam of intense bremsstrahlung is provided by the superconducting electron linear accelerator ELBE (Electron Linear Accelerator with high Brilliance and low Emittance) which delivers electron bunches of less than 10 ps temporal width and an adjustable bunch separation of multiples of 38 ns, average beam currents of 1 mA, and energies up to 40 MeV. Since the generation of bremsstrahlung and the transport to the sample preserves the sharp timing of the electron beam, positrons generated inside the entire sample volume by pair production feature a sharp start time stamp for positron annihilation lifetime studies with high timing resolutions and high signal to background ratios due to the coincident detection of two annihilation photons. Two commercially available detectors from a high-resolution medial positron-emission tomography system are being employed with 169 individual Lu2SiO5:Ce scintillation crystals, each. In first experiments, a positron-lifetime gated image of a planar Si/SiO2 (pieces of 12.5 mm × 25 mm size) sample and a 3-D structured metal in Teflon target could be obtained proving the feasibility of a three dimensional lifetime-gated tomographic system.

  2. Positron-Annihilation Lifetime Spectroscopy using Electron Bremsstrahlung

    NASA Astrophysics Data System (ADS)

    Wagner, A.; Anwand, W.; Butterling, M.; Cowan, T. E.; Fiedler, F.; Fritz, F.; Kempe, M.; Krause-Rehberg, R.

    2015-06-01

    A new type of an intense source of positrons for materials research has been set up at the superconducting electron linear. The source employs hard X-rays from electron- bremsstrahlung production generating energetic electron-positron pairs inside the sample under investigation. CW-operation allows performing experiments with significantly reduced pile-up artefacts in the detectors compared to pulsed mode operation in conventional accelerators. The high-resolution timing of the accelerator with bunch lengths below 10 ps full width at half maximum (FWHM) allows positron annihilation lifetime spectroscopy (PALS) measurements with high time resolution. A single-component annihilation lifetime of Kaptonhas been measured as (381.3 ± 0.3) ps. Employing segmented detectors for the detection of both annihilation photons allows for the first time to perform a 4D tomographic reconstruction of the annihilation sites including the annihilation lifetime.

  3. Positron annihilation lifetime spectroscopy study of Kapton thin foils

    NASA Astrophysics Data System (ADS)

    Kanda, G. S.; Ravelli, L.; Löwe, B.; Egger, W.; Keeble, D. J.

    2016-01-01

    Variable energy positron annihilation lifetime spectroscopy (VE-PALS) experiments on polyimide material Kapton are reported. Thin Kapton foils are widely used in a variety of mechanical, electronic applications. PALS provides a sensitive probe of vacancy-related defects in a wide range of materials, including open volume in polymers. Varying the positron implantation energy enables direct measurement of thin foils. Thin Kapton foils are also commonly used to enclose the positron source material in conventional PALS measurements performed with unmoderated radionuclide sources. The results of depth-profiled positron lifetime measurements on 7.6 μm and 25 μm Kapton foils are reported and determine a dominant 385(1) ps lifetime component. The absence of significant nanosecond lifetime component due to positronium formation is confirmed.

  4. Positron annihilation lifetime spectroscopy source correction determination: A simulation study

    NASA Astrophysics Data System (ADS)

    Kanda, Gurmeet S.; Keeble, David J.

    2016-02-01

    Positron annihilation lifetime spectroscopy (PALS) can provide sensitive detection and identification of vacancy-related point defects in materials. These measurements are normally performed using a positron source supported, and enclosed by, a thin foil. Annihilation events from this source arrangement must be quantified and are normally subtracted from the spectrum before analysis of the material lifetime components proceeds. Here simulated PALS spectra reproducing source correction evaluation experiments have been systematically fitted and analysed using the packages PALSfit and MELT. Simulations were performed assuming a single lifetime material, and for a material with two lifetime components. Source correction terms representing a directly deposited source and various foil supported sources were added. It is shown that in principle these source terms can be extracted from suitably designed experiments, but that fitting a number of independent, nominally identical, spectra is recommended.

  5. Study of Chemical Carcinogens by Positron Annihilation Lifetime Spectroscopy

    NASA Astrophysics Data System (ADS)

    Pivtsaev, A. A.; Razov, V. I.; Karasev, A. O.

    2013-11-01

    We have used positron annihilation lifetime spectroscopy to study the carcinogens C21H20BrN3, C4H7Cl2O4P, CCl4, CHCl3, AlF3, C8H12N4O, C6H4Cl2 and the non-carcinogens H2O, AlCl3, CH2Cl2, C2H6OS. We have established a correlation between the annihilation characteristics of the studied compounds and their degree of carcinogenicity.

  6. Positron and Positronium Annihilation Lifetime, and Free Volume in Polymers.

    NASA Astrophysics Data System (ADS)

    Yu, Zhibin

    1995-01-01

    Positron annihilation lifetime measurements were carried out for six polycarbonates of different structures and four polystyrenes of different molecular weight over a wide temperature range covering the glass transition region. The o-Ps mean lifetime is very sensitive to the changes of free volume in those polymers which occur due to change of molecular structure, chain length, and temperature. The influence of the unavoidable e^{+} irradiation and physical aging on the mean lifetime and the intensity of o-Ps annihilation were studied by conducting time dependent measurements on both very aged and rejuvenated samples. Both irradiation and physical aging reduce the formation of positronium, but have no effect on the mean lifetime of Ps atoms. The free volume fraction h obtained from the positron lifetime measurements was compared with the prediction of the statistical mechanical theory of Simha and Somcynsky; good agreement was found in the melt state though clear deviations were observed in the glassy state. A free volume quantity, computed from the bulk volume, which is in a good numerical agreement with the Simha-Somcynsky h-function in the melt, gives improved agreement with the h value calculated from the positron lifetime measurements. To investigate certain anomalies observed in the computer analysis of the positron annihilation lifetime spectra on polymers, we developed a computer simulation of the experimental data, which then was used to test the accuracy of the fitting results in the different circumstances. The influence caused by a possible distribution of the o-Ps mean lifetimes and the width of the spectrometer time resolution function were studied. The theoretical connection between the o-Ps mean lifetime and the free volume hole size was reviewed based on a finite spherical potential well model, and the status of the localized Ps atom in polymers was evaluated by calculation of the barrier transmission probability and the escaping probability of the

  7. Single-shot positron annihilation lifetime spectroscopy with LYSO scintillators

    NASA Astrophysics Data System (ADS)

    Alonso, A. M.; Cooper, B. S.; Deller, A.; Cassidy, D. B.

    2016-08-01

    We have evaluated the application of a lutetium yttrium oxyorthosilicate (LYSO) based detector to single-shot positron annihilation lifetime spectroscopy. We compare this detector directly with a similarly configured PbWO4 scintillator, which is the usual choice for such measurements. We find that the signal to noise ratio obtained using LYSO is around three times higher than that obtained using PbWO4 for measurements of Ps excited to longer-lived (Rydberg) levels, or when they are ionized soon after production. This is due to the much higher light output for LYSO (75% and 1% of NaI for LYSO and PbWO4 respectively). We conclude that LYSO is an ideal scintillator for single-shot measurements of positronium production and excitation performed using a low-intensity pulsed positron beam.

  8. Positron annihilation lifetime spectroscopy of ZnO bulk samples

    SciTech Connect

    Zubiaga, A.; Plazaola, F.; Garcia, J. A.; Tuomisto, F.; Munoz-Sanjose, V.; Tena-Zaera, R.

    2007-08-15

    In order to gain a further insight into the knowledge of point defects of ZnO, positron annihilation lifetime spectroscopy was performed on bulk samples annealed under different atmospheres. The samples were characterized at temperatures ranging from 10 to 500 K. Due to difficulties in the conventional fitting of the lifetime spectra caused by the low intensity of the defect signals, we have used an alternative method as a solution to overcome these difficulties and resolve all the lifetime components present in the spectra. Two different vacancy-type defects are identified in the samples: Zn vacancy complexes (V{sub Zn}-X) and vacancy clusters consisting of up to five missing Zn-O pairs. In addition to the vacancies, we observe negative-ion-type defects, which are tentatively attributed to intrinsic defects in the Zn sublattice. The effect of the annealing on the observed defects is discussed. The concentrations of the V{sub Zn}-X complexes and negative-ion-type defects are in the 0.2-2 ppm range, while the cluster concentrations are 1-2 orders of magnitude lower.

  9. Novel System for Potential Nondestructive Material Inspection Using Positron Annihilation Lifetime Spectroscopy

    NASA Astrophysics Data System (ADS)

    Yamawaki, Masato; Kobayashi, Yoshinori; Hattori, Kanehisa; Watanabe, Yoshihiro

    2011-08-01

    A new positron annihilation lifetime spectrometer consisting of a start γ-ray detector, a stop γ-ray detector, a digital oscilloscope, and a positron detector, which is a plastic scintillator coupled to a photomultiplier tube, is described. A 22Na source is placed between the positron detector and a sample to be studied. γ-ray signals related to positrons annihilating in the positron detector are rejected by anti-coincidence processing. Comparison of the positron lifetime spectrum of a steel strip collected with the new system with that collected with a conventional system using two specimens sandwiching the 22Na source shows that accurate positron lifetime measurements are possible with the new system. The new system does not require cutting of the samples and is potentially applicable to truly nondestructive onsite inspection of various materials such as those used in nuclear power plants, aircraft and cars, etc., by positron annihilation lifetime spectroscopy (PALS).

  10. Positron annihilation lifetime study of radiation-damaged natural zircons

    NASA Astrophysics Data System (ADS)

    Roberts, J.; Gaugliardo, P.; Farnan, I.; Zhang, M.; Vance, E. R.; Davis, J.; Karatchevtseva, I.; Knott, R. B.; Mudie, S.; Buckman, S. J.; Sullivan, J. P.

    2016-04-01

    Zircons are a well-known candidate waste form for actinides and their radiation damage behaviour has been widely studied by a range of techniques. In this study, well-characterised natural single crystal zircons have been studied using Positron Annihilation Lifetime Spectroscopy (PALS). In some, but not all, of the crystals that had incurred at least half of the alpha-event damage of ∼1019 α/g required to render them structurally amorphous, PALS spectra displayed long lifetimes corresponding to voids of ∼0.5 nm in diameter. The long lifetimes corresponded to expectations from published Small-Angle X-ray Scattering data on similar samples. However, the non-observation by PALS of such voids in some of the heavily damaged samples may reflect large size variations among the voids such that no singular size can be distinguished or. Characterisation of a range of samples was also performed using scanning electron microscopy, optical absorption spectroscopy, Raman scattering and X-ray scattering/diffraction, with the degree of alpha damage being inferred mainly from the Raman technique and X-ray diffraction. The observed void diameters and intensities of the long lifetime components were changed somewhat by annealing at 700 °C; annealing at 1200 °C removed the voids entirely. The voids themselves may derive from He gas bubbles or voids created by the inclusion of small quantities of organic and hydrous matter, notwithstanding the observation that no voidage was evidenced by PALS in two samples containing hydrous and organic matter.

  11. [Positron annihilation lifetime spectrometry (PALS) and its pharmaceutical applications].

    PubMed

    Sebe, István; Szabó, Barnabás; Zelkó, Romána

    2012-01-01

    PALS is one of the most widely used "nuclear probe" techniques for the tracking of the structural characteristics of materials. The method is based on the matter-energy equivalence principle recognized by Einstein: the electrons and positrons as particle-antiparticle pairs disappear in mutual destruction of particles, they annihilate with high-energy gamma-radiation, thus "particle-energy transition" occurs. The properties of the resulting radiation exactly correspond to the relevant properties of the electron and positron preceding the annihilation. Since electrons occur in all types of materials, the phenomenon of positron annihilation can play in any environment; consequently the method can be used for the analysis of each type of materials (crystalline and amorphous, organic and inorganic, biotic and abiotic). The present paper provides an overview of the theoretical physical background, the practical realization and evaluation of methods, their limitations, and summarizes the pharmaceutical applications published in recent years. PMID:22570984

  12. Positron annihilation lifetime study of polyvinylpyrrolidone for nanoparticle-stabilizing pharmaceuticals.

    PubMed

    Shpotyuk, O; Bujňáková, Z; Baláž, P; Ingram, A; Shpotyuk, Y

    2016-01-01

    Positron annihilation lifetime spectroscopy was applied to characterize free-volume structure of polyvinylpyrrolidone used as nonionic stabilizer in the production of many nanocomposite pharmaceuticals. The polymer samples with an average molecular weight of 40,000 g mol(-1) were pelletized in a single-punch tableting machine under an applied pressure of 0.7 GPa. Strong mixing in channels of positron and positronium trapping were revealed in the polyvinylpyrrolidone pellets. The positron lifetime spectra accumulated under normal measuring statistics were analysed in terms of unconstrained three- and four-term decomposition, the latter being also realized under fixed 0.125 ns lifetime proper to para-positronium self-annihilation in a vacuum. It was shown that average positron lifetime extracted from each decomposition was primary defined by long-lived ortho-positronium component. The positron lifetime spectra treated within unconstrained three-term fitting were in obvious preference, giving third positron lifetime dominated by ortho-positronium pick-off annihilation in a polymer matrix. This fitting procedure was most meaningful, when analysing expected positron trapping sites in polyvinylpyrrolidone-stabilized nanocomposite pharmaceuticals. PMID:26444751

  13. Positron annihilation lifetime study of interfaces in ternary polymer blends

    NASA Astrophysics Data System (ADS)

    Meghala, D.; Ramya, P.; Pasang, T.; Raj, J. M.; Ranganathaiah, C.; Williams, J. F.

    2013-06-01

    A new method based on positron lifetime spectroscopy is developed to characterize individual interfaces in ternary polymer blends and hence determine the composition dependent miscibility level. The method owes its origin to the Kirkwood-Risemann-Zimm (KRZ) model for the evaluation of the hydrodynamic interaction parameters (αij) which was used successfully for a binary blend with a single interface. The model was revised for the present work for ternary polymer blends to account for three interfaces. The efficacy of this method is shown for two ternary blends namely poly(styrene-co-acrylonitrile)/poly (ethylene-co-vinylacetate)/poly(vinyl chloride) (SAN/EVA/PVC) and polycaprolactone /poly(styrene-co-acrylonitrile)/poly(vinyl chloride) (PCL/SAN/PVC) at different compositions. An effective hydrodynamic interaction parameter, αeff, was introduced to predict the overall miscibility of ternary blends.

  14. Free volume structure of realgar α-As4S4 by positron annihilation lifetime spectroscopy

    NASA Astrophysics Data System (ADS)

    Shpotyuk, O.; Ingram, A.; Demchenko, P.

    2015-04-01

    Atomic-deficient free volume structure of realgar α-As4S4, the low-temperature modification, of tetraarsenic tetrasulfide polymorphs, is studied using positron annihilation lifetime spectroscopy. Eventual channels of positron annihilation in this molecular crystal are shown to be connected with low electron density entities around cage As4S4 molecules composing realgar-type structure of monoclinic P21/n space group. The overlapped spaces of bond-free solid angles around S atoms forming self-closed As4S4 molecules contribute preferentially to positron trapping modes, while a competitive influence of bound positron-electron states (positronium) stabilized in intermolecular spaces occurs also to be essential in the decomposed lifetime spectra too.

  15. The study of synthetic food dyes by positron annihilation lifetime spectroscopy.

    NASA Astrophysics Data System (ADS)

    Pivtsaev, A. A.; Razov, V. I.

    2015-06-01

    By method of positron annihilation lifetime spectroscopy (PALS), substances are food dyes were studied: E-102 (Tartrazine), E-124 (Ponso 4R), E 132 (Indigo carmine), E-133 (Brilliant Blue), E-151 (Black Shiny). They are examined for the presence of carcinogenic properties. The difference between dyes having explicit carcinogenic properties and mutagenic properties (non-explicit carcinogens) is established.

  16. Probing the amphiphile micellar to hexagonal phase transition using Positron Annihilation Lifetime Spectroscopy.

    PubMed

    Dong, Aurelia W; Fong, Celesta; Hill, Anita J; Boyd, Ben J; Drummond, Calum J

    2013-07-15

    Positron Annihilation Lifetime Spectroscopy (PALS) has been utilised only sparingly for structural characterisation in self assembled materials. Inconsistencies in approaches to experimental configuration and data analysis between studies has complicated comparisons between studies, meaning that the technique has not provided a cohesive data set across the study of different self assembled systems that advance the technique towards an important tool in soft matter research. In the current work a systematic study was conducted using ionic and non-ionic micellar systems with increasing surfactant concentration to probe positron behaviour on changes between micellar phase structures, and data analysed using contemporary approaches to fit four component spectra. A characteristic orthopositronium lifetime (in the organic regions) of 3.5±0.2 ns was obtained for the hexagonal phase for surfactants with C12 alkyl chains. Chemical quenching of the positron species was also observed for systems with ionic amphiphiles. The application of PALS has also highlighted an inconsistency in the published phase diagram for the octa(ethylene oxide) monododecyl ether (C12EO8) system. These results provide new insight into how the physical properties of micellar systems can be related to PALS parameters and means that the PALS technique can be applied to other more complex self-assembled amphiphile systems. PMID:23643250

  17. Photon-induced positron annihilation lifetime spectroscopy using ultrashort laser-Compton-scattered gamma-ray pulses

    SciTech Connect

    Taira, Y.; Toyokawa, H.; Kuroda, R.; Yamamoto, N.; Adachi, M.; Tanaka, S.; Katoh, M.

    2013-05-15

    High-energy ultrashort gamma-ray pulses can be generated via laser Compton scattering with 90 Degree-Sign collisions at the UVSOR-II electron storage ring. As an applied study of ultrashort gamma-ray pulses, a new photon-induced positron annihilation lifetime spectroscopy approach has been developed. Ultrashort gamma-ray pulses with a maximum energy of 6.6 MeV and pulse width of 2.2 ps created positrons throughout bulk lead via pair production. Annihilation gamma rays were detected by a BaF{sub 2} scintillator mounted on a photomultiplier tube. A positron lifetime spectrum was obtained by measuring the time difference between the RF frequency of the electron storage ring and the detection time of the annihilation gamma rays. We calculated the response of the BaF{sub 2} scintillator and the time jitter caused by the variation in the total path length of the ultrashort gamma-ray pulses, annihilation gamma rays, and scintillation light using a Monte Carlo simulation code. The positron lifetime for bulk lead was successfully measured.

  18. Photon-induced positron annihilation lifetime spectroscopy using ultrashort laser-Compton-scattered gamma-ray pulses.

    PubMed

    Taira, Y; Toyokawa, H; Kuroda, R; Yamamoto, N; Adachi, M; Tanaka, S; Katoh, M

    2013-05-01

    High-energy ultrashort gamma-ray pulses can be generated via laser Compton scattering with 90° collisions at the UVSOR-II electron storage ring. As an applied study of ultrashort gamma-ray pulses, a new photon-induced positron annihilation lifetime spectroscopy approach has been developed. Ultrashort gamma-ray pulses with a maximum energy of 6.6 MeV and pulse width of 2.2 ps created positrons throughout bulk lead via pair production. Annihilation gamma rays were detected by a BaF2 scintillator mounted on a photomultiplier tube. A positron lifetime spectrum was obtained by measuring the time difference between the RF frequency of the electron storage ring and the detection time of the annihilation gamma rays. We calculated the response of the BaF2 scintillator and the time jitter caused by the variation in the total path length of the ultrashort gamma-ray pulses, annihilation gamma rays, and scintillation light using a Monte Carlo simulation code. The positron lifetime for bulk lead was successfully measured. PMID:23742543

  19. Photon-induced positron annihilation lifetime spectroscopy using ultrashort laser-Compton-scattered gamma-ray pulses

    NASA Astrophysics Data System (ADS)

    Taira, Y.; Toyokawa, H.; Kuroda, R.; Yamamoto, N.; Adachi, M.; Tanaka, S.; Katoh, M.

    2013-05-01

    High-energy ultrashort gamma-ray pulses can be generated via laser Compton scattering with 90° collisions at the UVSOR-II electron storage ring. As an applied study of ultrashort gamma-ray pulses, a new photon-induced positron annihilation lifetime spectroscopy approach has been developed. Ultrashort gamma-ray pulses with a maximum energy of 6.6 MeV and pulse width of 2.2 ps created positrons throughout bulk lead via pair production. Annihilation gamma rays were detected by a BaF2 scintillator mounted on a photomultiplier tube. A positron lifetime spectrum was obtained by measuring the time difference between the RF frequency of the electron storage ring and the detection time of the annihilation gamma rays. We calculated the response of the BaF2 scintillator and the time jitter caused by the variation in the total path length of the ultrashort gamma-ray pulses, annihilation gamma rays, and scintillation light using a Monte Carlo simulation code. The positron lifetime for bulk lead was successfully measured.

  20. Positron annihilation lifetime characterization of oxygen ion irradiated rutile TiO2

    NASA Astrophysics Data System (ADS)

    Luitel, Homnath; Sarkar, A.; Chakrabarti, Mahuya; Chattopadhyay, S.; Asokan, K.; Sanyal, D.

    2016-07-01

    Ferromagnetic ordering at room temperature has been induced in rutile phase of TiO2 polycrystalline sample by O ion irradiation. 96 MeV O ion induced defects in rutile TiO2 sample has been characterized by positron annihilation spectroscopic techniques. Positron annihilation results indicate the formation of cation vacancy (VTi, Ti vacancy) in these irradiated TiO2 samples. Ab initio density functional theoretical calculations indicate that in TiO2 magnetic moment can be induced either by creating Ti or O vacancies.

  1. Positron annihilation lifetime spectroscopy (PALS): a probe for molecular organisation in self-assembled biomimetic systems.

    PubMed

    Fong, Celesta; Dong, Aurelia W; Hill, Anita J; Boyd, Ben J; Drummond, Calum J

    2015-07-21

    Positron annihilation lifetime spectroscopy (PALS) has been shown to be highly sensitive to conformational, structural and microenvironmental transformations arising from subtle geometric changes in molecular geometry in self-assembling biomimetic systems. The ortho-positronium (oPs) may be considered an active probe that can provide information on intrinsic packing and mobility within low molecular weight solids, viscous liquids, and soft matter systems. In this perspective we provide a critical overview of the literature in this field, including the evolution of analysis software and experimental protocols with commentary upon the practical utility of PALS. In particular, we discuss how PALS can provide unique insight into the macroscopic transport properties of several porous biomembrane-like nanostructures and suggest how this insight may provide information on the release of drugs from these matrices to aid in developing therapeutic interventions. We discuss the potentially exciting and fruitful application of this technique to membrane dynamics, diffusion and permeability. We propose that PALS can provide novel molecular level information that is complementary to conventional characterisation techniques. PMID:25948334

  2. Positron annihilation lifetime studies of changes in free volume on some biorelevant nitrogen heterocyclic compounds and their S-glycosylation.

    PubMed

    Mahmoud, K R; Khodair, A I; Shaban, S Y

    2015-11-01

    A series of N-heterocyclic compounds was investigated by positron annihilation lifetime spectroscopy as well as Doppler broadening of annihilation radiation (DBAR) at room temperature. The results showed that the formation probability and life time of ortho-positronium in this series are structure and electron-donation character dependent, and can give more information about the structure. The DBAR provides direct information about the change of core and valance electrons as well as the number of defect types present in these compounds. PMID:26272166

  3. Analysis of positron annihilation lifetime data by numerical Laplace inversion: Corrections for source terms and zero-time shift errors

    NASA Astrophysics Data System (ADS)

    Gregory, Roger B.

    1991-05-01

    We have recently described modifications to the program CONTIN [S.W. Provencher, Comput. Phys. Commun. 27 (1982) 229] for the solution of Fredholm integral equations with convoluted kernels of the type that occur in the analysis of positron annihilation lifetime data [R.B. Gregory and Yongkang Zhu, Nucl. Instr. and Meth. A290 (1990) 172]. In this article, modifications to the program to correct for source terms in the sample and reference decay curves and for shifts in the position of the zero-time channel of the sample and reference data are described. Unwanted source components, expressed as a discrete sum of exponentials, may be removed from both the sample and reference data by modification of the sample data alone, without the need for direct knowledge of the instrument resolution function. Shifts in the position of the zero-time channel of up to half the channel width of the multichannel analyzer can be corrected. Analyses of computer-simulated test data indicate that the quality of the reconstructed annihilation rate probability density functions is improved by employing a reference material with a short lifetime and indicate that reference materials which generate free positrons by quenching positronium formation (i.e. strong oxidizing agents) have lifetimes that are too long (400-450 ps) to provide reliable estimates of the lifetime parameters for the shortlived components with the methods described here. Well-annealed single crystals of metals with lifetimes less than 200 ps, such as molybdenum (123 ps) and aluminum (166 ps) do not introduce significant errors in estimates of the lifetime parameters and are to be preferred as reference materials. The performance of our modified version of CONTIN is illustrated by application to positron annihilation in polytetrafluoroethylene.

  4. A positron annihilation lifetime spectroscopic study of the corrosion protective properties of epoxy coatings

    SciTech Connect

    MacQueen, R.C.

    1992-01-01

    Positron Annihilation Lifetime Spectroscopy (PALS) was used to measure the free volume cavity sizes and free volume fractions of crosslinked epoxy coatings on steel before and after saturation with liquid water at 23[degrees]C. A direct linear relationship between the equilibrium volume fraction of water absorbed and the dry relative free volume fraction of bisphenol A epoxy coatings was found. The free volume cavity sizes and the number of free volume cavities per unit volume of these epoxies were found to decrease after water saturation. These decreases are ascribed to the occupation of 13-17% of the free volume cavities by 2-4 water molecules per cavity. The free volume cavity size of polyglycol diepoxides was found to increase after water saturation. This increase is ascribed to the expansion of the free volume cavities by water, which is substantiated by the macroscopic swelling observed in these coatings. An inverse, linear relationship between the equilibrium water uptake and the relative free volume fraction of these coatings were observed. This result coupled with the fact that less than one molecule of nitrobenzene was determined to fit into an epoxy free volume cavity, and that nitrobenzene is quite soluble in most of the epoxides, indicates that other factors besides the magnitude of the free volume fraction affect the amount of solvent absorbed by epoxy coatings. The small percentage of free volume occupied by water and the small number of water molecules capable of filling each void of the bisphenol A epoxies after water saturation correlate to the high impedance values and the good corrosion protection of these coatings, suggesting that water passes through these coatings by slow diffusion through the connected free volume cavities in the coating. Increases in the free volume cavity sizes of the polyglycol diepoxides after water saturation correlate to the low impedance and the poor corrosion protection of these coatings.

  5. Positron annihilation lifetime spectroscopy of poly(ethylene terephthalate): Contributions from rigid and mobile amorphous fractions

    NASA Astrophysics Data System (ADS)

    Olson, Brian; Lin, Jun; Nazarenko, Sergei; Jamieson, Alexander

    2004-03-01

    Systematic divergences in the orthopositronium (o-Ps) annihilation lifetimes, τ_3, and intensities, I_3, are observed, when comparing melt-crystallized and cold-crystallized poly(ethylene terephthalate) (PET) as a function of crystallinity. Following a previous analysis of corresponding deviations in oxygen permeability, the divergences in I3 and τ3 are traced to distinct characteristic values for the probability of o-Ps formation and o-Ps lifetime in the rigid amorphous phase (RAF) associated with the crystalline lamellae and the mobile amorphous regions (MAF) which are unperturbed by the presence of the crystal phase. Utilizing independent information on the volume fractions of RAF and MAF, a quantitative analysis of the o-Ps annihilation parameters is possible.

  6. Gas Permeations Studied by Positron Annihilation

    NASA Astrophysics Data System (ADS)

    Yuan, Jen-Pwu; Cao, Huimin; Jean, X.; Yang, Y. C.

    1997-03-01

    The hole volumes and fractions of PC and PET polymers are measured by positron annihilation lifetime spectroscopy. Direct correlations between the measured hole properties and gas permeabilities are observed. Applications of positron annihilation spectroscopy to study gas transport and separation of polymeric materials will be discussed.

  7. Positron annihilation lifetime measurement and X-ray analysis on 120 MeV Au+7 irradiated polycrystalline tungsten

    NASA Astrophysics Data System (ADS)

    Dube, Charu Lata; Kulriya, Pawan Kumar; Dutta, Dhanadeep; Pujari, Pradeep K.; Patil, Yashashri; Mehta, Mayur; Patel, Priyanka; Khirwadkar, Samir S.

    2015-12-01

    In order to simulate radiation damages in tungsten, potential plasma facing materials in future fusion reactors, surrogate approach of heavy ion irradiation on polycrystalline tungsten is employed. Tungsten specimen is irradiated with gold heavy ions of energy 120 MeV at different fluences. Positron annihilation lifetime measurements are carried out on pristine and ion beam irradiated tungsten specimens. The variation in positron annihilation lifetime in ion irradiated specimens confirms evolution of vacancy clusters under heavy ion irradiation. The pristine and irradiated tungsten specimens have also been characterized for their microstructural, structural, electrical, thermal, and mechanical properties. X-ray diffractograms of irradiated tungsten specimens show structural integrity of polycrystalline tungsten even after irradiation. Nevertheless, the increase in microstrain, electrical resistivity and microhardness on irradiation indicates creation of lattice damages inside polycrystalline tungsten specimen. On the other hand, the thermal diffusivity has not change much on heavy ion irradiation. The induction of damages in metallic tungsten is mainly attributed to high electronic energy loss, which is 40 keV/nm in present case as obtained from SRIM program. Although, concomitant effect of nuclear losses on damage creation cannot be ignored. It is believed that the energy received by the electronic system is being transferred to the atomic system by electron-phonon coupling. Eventually, elastic nuclear collisions and the transfer of energy from electronic to atomic system via inelastic collision is leading to significant defect generation in tungsten lattice.

  8. Utility of positron annihilation lifetime technique for the assessment of spectroscopic data of some charge-transfer complexes derived from N-(1-Naphthyl)ethylenediamine dihydrochloride.

    PubMed

    Refat, Moamen S; Adam, Abdel Majid A; Sharshar, T; Saad, Hosam A; Eldaroti, Hala H

    2014-03-25

    In this work, structural, thermal, morphological, pharmacological screening and positron annihilation lifetime measurements were performed on the interactions between a N-(1-Naphthyl)ethylenediamine dihydrochloride (NEDA·2HCl) donor and three types of acceptors to characterize these CT complexes. The three types of acceptors include π-acceptors (quinol and picric acid), σ-acceptors (iodine) and vacant orbital acceptors (tin(IV) tetrachloride and zinc chloride). The positron annihilation lifetime parameters were found to be dependent on the structure, electronic configuration, the power of acceptors and molecular weight of the CT complexes. The positron annihilation lifetime spectroscopy can be used as a probe for the formation of charge-transfer (CT) complexes. PMID:24291622

  9. Characteristics of vinyl-ester and carbon fiber composite dry and wet probe by Positron Annihilation Lifetime Spectroscopy

    NASA Astrophysics Data System (ADS)

    Madani, Mahmoud; Granata, Richard D.

    2015-03-01

    Carbon fiber composites of vinylester resins, Derakane 8084 and 510A, were studied dry and after water exposure. In this study, positron annihilation lifetime spectroscopy (PALS) was used to investigate the free volume fraction and the size of the free volume voids within the polymer matrix. The relative free volume (fractions replae by of positron lifetime intensities) in VE8084 polymer and in VE510A (Space) polymer were 35.2% and 13.8%, respectively. The free volume lifetime and intensities were determined as a function of the polymer thickness and significant differences were observed in both polymers with versus without post-curing. The effects of water uptake in these materials were also determined by PALS. Water uptake showed a 2% change in intensity of the longer lifetime (1.85 ns) in VE8084 polymer and in VE510A about 1.8%. The longer lifetime intensities in the wet composites were 17.1% in the 8084 polymer and its carbon fiber composite and 7.1% in the 510A polymer and its carbon fiber composite. For composite with 8084 polymer saturated (0.33% water gain) with seawater at 40 or 60 °C, no change in the longer lifetime intensity was observed which indicates no water entered the free volume voids (indicates replace by and) some differences between composite and neat polymer. For 510A resin the third lifetime intensity dropped from 7.1% to 3.9% indicating 48% of the free volume filled with water in the composite only after saturation with seawater with respect to dry one.

  10. Prediction of free-volume-type correlations in glassy chalcogenides from positron annihilation lifetime measurements

    NASA Astrophysics Data System (ADS)

    Shpotyuk, O.; Ingram, A.; Shpotyuk, M.; Filipecki, J.

    2014-11-01

    A newly modified correlation equation between defect-related positron lifetime determined within two-state trapping model and radius of corresponding free-volume-type defects was proposed to describe compositional variations in atomic-deficient structure of covalent-bonded chalcogenides like binary As-S/Se glasses. Specific chemical environment of free-volume voids around neighboring network-forming polyhedrons was shown to play a decisive role in this correlation, leading to systematically enhanced volumes in comparison with typical molecular substrates, such as polymers.

  11. The assessment of pore connectivity in hierarchical zeolites using positron annihilation lifetime spectroscopy: instrumental and morphological aspects.

    PubMed

    Zubiaga, Asier; Warringham, Robbie; Boltz, Marilyne; Cooke, David; Crivelli, Paolo; Gidley, David; Pérez-Ramírez, Javier; Mitchell, Sharon

    2016-04-01

    Recent studies demonstrated the power of positron annihilation lifetime spectroscopy (PALS) to characterise the connectivity and corresponding effectiveness of hierarchical pore networks in zeolites. This was based on the fractional escape of ortho-positronium (Ps), formed within the micropore framework, to vacuum. To further develop this technique, here we assess the impact of the positron implantation energy and of the zeolite crystal size and the particle morphology. Conventional measurements using fast positrons and beam measurements applying moderated positrons both readily distinguish purely microporous ZSM-5 zeolites comprised of single crystals or crystal aggregates. Unlike beam measurements, however, conventional measurements fail to discriminate model hierarchical zeolites with open or constricted mesopore architectures. Several steps are taken to rationalise these observations. The dominant contribution of Ps diffusion to the PALS response is confirmed by capping the external surface of the zeolite crystals with tetraethylorthosilicate, which greatly enhances the sensitivity to the micropore network. A one-dimensional model is constructed to predict the out-diffusion of Ps from a zeolite crystal, which is validated experimentally by comparing coffin-shaped single crystals of varying size. Calculation of the trends expected on the application of fast or moderated positrons indicates that the distinctions in the initial distribution of Ps at the crystal level cannot explain the limited sensitivity of the former to the mesopore architecture. Instead, we propose that the greater penetration of fast positrons within the sample increases the probability of Ps re-entry from intercrystalline voids into mesopores connected with the external surface of zeolite crystals, thereby reducing their fractional escape. PMID:26975204

  12. Confined water in controlled pore glass CPG-10-120 studied by positron annihilation lifetime spectroscopy and differential scanning calorimetry

    NASA Astrophysics Data System (ADS)

    Šauša, O.; Mat'ko, I.; Illeková, E.; Macová, E.; Berek, D.

    2015-06-01

    The solidification and melting of water confined in the controlled pore glass (CPG) with average pore size 12.6 nm has been studied by differential scanning calorimetry (DSC) and positron annihilation lifetime spectroscopy (PALS). The fully-filled sample of CPG by water as well as the samples of CPG with different content of water were used. The measurements show the presence of amorphous and crystalline phases of water in this type and size of pores, freezing point depression of a confined liquid and presence of certain transitions at lower temperatures, which could be detected only for cooling regime. The localization of confined water in the partially filled pores of CPG at room temperature was studied.

  13. Per-fluorinated sulfonic acid/PTFE copolymer studied by positron annihilation lifetime and gas permeation techniques

    NASA Astrophysics Data System (ADS)

    Mohamed, Hamdy F. M.; Abdel-Hady, E. E.; Ohira, A.

    2015-06-01

    The mechanism of gas permeation in per-fluorinated sulfonic acid/PTFE copolymer Fumapem® membranes for polymer electrolyte fuel cells has been investigated from the viewpoint of free volume. Three different samples, Fumapem® F-950, F-1050 and F-14100 membranes with ion exchange capacity (IEC) = 1.05, 0.95 and 0.71 meq/g, respectively were used after drying. Free volume was quantified using the positron annihilation lifetime (PAL) technique and gas permeabilities were measured for O2 and H2 as function of temperature. Good linear correlation between the logarithm of the permeabilities at different temperatures and reciprocal free volume indicate that gas permeation in dry Fumapem® is governed by the free volume. Nevertheless permeabilities are much smaller than the corresponding flexible chain polymer with a similar free volume size due to stiff chains of the perfluoroethylene backbone.

  14. Porous glasses as a matrix for incorporation of photonic materials. Pore determination by positron annihilation lifetime spectroscopy

    NASA Astrophysics Data System (ADS)

    Reisfeld, Pore determination by positron annihilation lifetime spectroscopy R.; Saraidarov, T.; Jasinska, B.

    2004-07-01

    Porous glasses prepared by the sol-gel technique have a variety of applications when incorporated by photonic materials: tunable lasers, sensors, luminescence solar concentrators, semiconductor quantum dots, biological markers. The known methods of pore size determinations, the nitrogen adsorption and mercury porosimetry allow to determine the sizes of open pores. Positron annihilation lifetime spectroscopy (PALS) allows to determine pore sizes also of closed pores. As an example we have performed measurements of non-doped zirconia-silica-polyurethane (ZSUR) ormocer glasses and the same glasses doped with lead sulfide quantum dots. The pore radii range between 0.25-0.38 nm, total surface area 15.5-23.8 m 2/g.

  15. Dependence of alpha particle track diameter on the free volume holes size using positron annihilation lifetime technique

    NASA Astrophysics Data System (ADS)

    El-Gamal, S.; Abdalla, Ayman M.; Abdel-Hady, E. E.

    2015-09-01

    The alpha particle track diameter dependence of the free volume holes size (Vf) in DAM-ADC and CR-39 nuclear track detectors was investigated using positron annihilation lifetime technique. The effect of temperature on the alpha particle track diameter and free volume were also investigated in the T-range (RT-130 °C). The obtained results revealed that the values of ortho-positronium lifetime τ3 and Vf increases while I3 slightly increases as T increases for the two detectors. The values of τ3, Vf and I3 are higher in CR-39 than DAM-ADC. The interpretation of obtained results is based on the fact that increasing T leads to significant enhancement of thermal expansion of the polymer matrix and consequently Vf increases. The track diameter increases as T increases. This can be explained by the fact that the increase in T increases the crystal size and Vf in the polymer. A relationship between Vf and the alpha particle track diameter was obtained. Moreover results of detector irradiation, along with free volume evaluation are addressed and thoroughly discussed.

  16. Free volume of mixed cation borosilicate glass sealants elucidated by positron annihilation lifetime spectroscopy and its correlation with glass properties

    NASA Astrophysics Data System (ADS)

    Ojha, Prasanta K.; Rath, Sangram K.; Sharma, Sandeep K.; Sudarshan, Kathi; Pujari, Pradeep K.; Chongdar, Tapas K.; Gokhale, Nitin M.

    2015-01-01

    The role of La+3/Sr+2 ratios, which is varied from 0.08 to 5.09, on density, molar volume, packing fraction, free volume, thermal and electrical properties in strontium lanthanum aluminoborosilicate based glass sealants intended for solid oxide fuel cell (SOFC) applications is evaluated. The studies reveal expansion of the glass network evident from increasing molar volume and decreasing packing fraction of glasses with progressive La+3 substitutions. The molecular origin of these macroscopic structural features can be accounted for by the free volume parameters measured from positron annihilation lifetime spectroscopy (PALS). The La+3 induced expanded glass networks show increased number of subnanoscopic voids with larger sizes, as revealed from the ortho-positronium (o-Ps) lifetime and its intensity. A remarkably direct correspondence between the molar volume and fractional free volume trend is established with progressive La2O3 substitution in the glasses. The effect of these structural changes on the glass transition temperature, softening temperature, coefficient of thermal expansion, thermal stability as well as electrical conductivity has been studied.

  17. Positron annihilation in transparent ceramics

    NASA Astrophysics Data System (ADS)

    Husband, P.; Bartošová, I.; Slugeň, V.; Selim, F. A.

    2016-01-01

    Transparent ceramics are emerging as excellent candidates for many photonic applications including laser, scintillation and illumination. However achieving perfect transparency is essential in these applications and requires high technology processing and complete understanding for the ceramic microstructure and its effect on the optical properties. Positron annihilation spectroscopy (PAS) is the perfect tool to study porosity and defects. It has been applied to investigate many ceramic structures; and transparent ceramics field may be greatly advanced by applying PAS. In this work positron lifetime (PLT) measurements were carried out in parallel with optical studies on yttrium aluminum garnet transparent ceramics in order to gain an understanding for their structure at the atomic level and its effect on the transparency and light scattering. The study confirmed that PAS can provide useful information on their microstructure and guide the technology of manufacturing and advancing transparent ceramics.

  18. Cholesterol-Ceramide Interactions in Phospholipid and Sphingolipid Bilayers As Observed by Positron Annihilation Lifetime Spectroscopy and Molecular Dynamics Simulations.

    PubMed

    García-Arribas, Aritz B; Axpe, Eneko; Mujika, Jon Iñaki; Mérida, David; Busto, Jon V; Sot, Jesús; Alonso, Alicia; Lopez, Xabier; García, Jose Ángel; Ugalde, Jesus M; Plazaola, Fernando; Goñi, Félix M

    2016-05-31

    Free volume voids in lipid bilayers can be measured by positron annihilation lifetime spectroscopy (PALS). This technique has been applied, together with differential scanning calorimetry and molecular dynamics (MD) simulations, to study the effects of cholesterol (Chol) and ceramide (Cer) on free volume voids in sphingomyelin (SM) or dipalmitoylphosphatidylcholine (DPPC) bilayers. Binary lipid samples with Chol were studied (DPPC:Chol 60:40, SM:Chol 60:40 mol ratio), and no phase transition was detected in the 20-60 °C range, in agreement with calorimetric data. Chol-driven liquid-ordered phase showed an intermediate free volume void size as compared to gel and fluid phases. For SM and SM:Cer (85:15 mol:mol) model membranes measured in the 20-60 °C range the gel-to-fluid phase transition could be observed with a related increase in free volume, which was more pronounced for the SM:Cer sample. MD simulations suggest a hitherto unsuspected lipid tilting in SM:Cer bilayers but not in pure SM. Ternary samples of DPPC:Cer:Chol (54:23:23) and SM:Cer:Chol (54:23:23) were measured, and a clear pattern of free volume increase was observed in the 20-60 °C because of the gel-to-fluid transition. Interestingly, MD simulations showed a tendency of Cer to change its distribution along the membrane to make room for Chol in ternary mixtures. The results suggest that the gel phase formed in these ternary mixtures is stabilized by Chol-Cer interactions. PMID:27158737

  19. Microscopic basis of free-volume concept as studied by quasielastic neutron scattering and positron annihilation lifetime spectroscopy

    NASA Astrophysics Data System (ADS)

    Kanaya, T.; Tsukushi, T.; Kaji, K.; Bartos, J.; Kristiak, J.

    1999-08-01

    We have reexamined the free-volume concept presented by Cohen and Turnbull on the basis of two microscopic quantities: the excess mean-square displacement f and the total free volume VPA,t, of poly- butadiene evaluated from the quasielastic neutron scattering and the positron annihilation lifetime spectroscopy (PALS) data, respectively. Comparing with the viscosity η we found two relations, η=η0 exp u 20/f and η=η0 exp V*PA,0/VPA,t=η0 exp V*PA,0/vPA,f, where u20, V*PA,0 and v*PA,0 are the critical values for the mean-square displacement, the total PALS free volume, and the PALS free volume per molecule, respectively, and further v*PA,0=V*PA,0/N, N being the total number of molecules or segments. On the basis of these relations, we discuss the microscopic basis of the free-volume theory. The experimentally evaluated critical values u20 and v*PA,0 are much larger than the average values of f and vPA,f calculated from the distributions. This has been explained from the low probability of escaping motions from a molecular cage. The free volume per monomer and the free-volume fraction were calculated from the excess mean-square displacement f. The former was compared with the free-volume hole obtained by PALS, suggesting that 22 monomers are required for one PALS free-volume hole. The free-volume fraction obtained from the excess mean-square displacement was found to be 6.4% at 250 K, which is in reasonable agreement with that evaluated from the rheological data (9.0%).

  20. Reinforcement Mechanism Of Polyurethane-Urea/Clay Nanocomposites Probed By Positron Annihilation Lifetime Spectroscopy And Dynamic Mechanical Analysis

    SciTech Connect

    Rath, S. K.; Patri, M.; Sudarshan, K.; Pujari, P. K.; Khakhar, D. V.

    2010-12-01

    A basis for quantitative analysis of the reinforcement mechanism of polyurethane-urea/clay nanocomposites using two characterization methods, positron annihilation life time spectroscopy (PALS) and dynamic mechanical analysis (DMA) is provided. DMA was used to measure the constrained volume fraction of amorphous soft segments induced by nanoclay and the storage modulus of the nanocomposites. The interfacial interactions in the nanocomposites were investigated by PALS. The modulus enhancement of the organoclay nanocomposites was found to have a good correlation with the volume fraction of the constrained region and the interfacial interactions.

  1. Drug release profiles and microstructural characterization of cast and freeze dried vitamin B12 buccal films by positron annihilation lifetime spectroscopy.

    PubMed

    Szabó, Barnabás; Kállai, Nikolett; Tóth, Gergő; Hetényi, Gergely; Zelkó, Romána

    2014-02-01

    Solvent cast and freeze dried films, containing the water-soluble vitamin B12 as model drug were prepared from two polymers, sodium alginate (SA), and Carbopol 71G (CP). The proportion of the CP was changed in the films. The microstructural characterization of various samples was carried out by positron annihilation lifetime spectroscopy (PALS). The drug release kinetics of untreated and stored samples was evaluated by the conventionally applied semi-empirical power law. Correlation was found between the changes of the characteristic parameters of the drug release and the ortho-positronium (o-Ps) lifetime values of polymer samples. The results indicated that the increase of CP concentration, the freeze-drying process and the storage at 75% R.H. decreased the rate of drug release. The PALS method enabled the distinction between the micro- and macrostructural factors influencing the drug release profile of polymer films. PMID:24269613

  2. Applications of slow positrons to cancer research: Search for selectivity of positron annihilation to skin cancer

    NASA Astrophysics Data System (ADS)

    Jean, Y. C.; Li, Ying; Liu, Gaung; Chen, Hongmin; Zhang, Junjie; Gadzia, Joseph E.

    2006-02-01

    Slow positrons and positron annihilation spectroscopy (PAS) have been applied to medical research in searching for positron annihilation selectivity to cancer cells. We report the results of positron lifetime and Doppler broadening energy spectroscopies in human skin samples with and without cancer as a function of positron incident energy (up to 8 μm depth) and found that the positronium annihilates at a significantly lower rate and forms at a lower probability in the samples having either basal cell carcinoma (BCC) or squamous cell carcinoma (SCC) than in the normal skin. The significant selectivity of positron annihilation to skin cancer may open a new research area of developing positron annihilation spectroscopy as a novel medical tool to detect cancer formation externally and non-invasively at the early stages.

  3. Investigation of the free volume and ionic conducting mechanism of poly(ethylene oxide)-LiClO4 polymeric electrolyte by positron annihilating lifetime spectroscopy

    NASA Astrophysics Data System (ADS)

    Gong, Jing; Gong, Zhen-Li; Yan, Xiao-Li; Gao, Shu; Zhang, Zhong-Liang; Wang, Bo

    2012-10-01

    The positron annihilation lifetime and ionic conductivity are each measured as a function of organophilic rectorite (OREC) content and temperature in a range from 160 K to 300 K. According to the variation of ortho-positronium (o-Ps) lifetime with temperature, the glassy transition temperature is determined. The continuous maximum entropy lifetime (MELT) analysis clearly shows that the OREC and temperature have important effects on o-Ps lifetime and free volume distribution. The experimental results show that the temperature dependence of ionic conductivity obeys the Vogel—Tammann—Fulcher (VTF) and Williams—Landel—Ferry (WLF) equations, implying a free-volume transport mechanism. A linear least-squares procedure is used to evaluate the apparent activation energy related to the ionic transport in the VTF equation and several important parameters in the WLF equation. It is worthwhile to notice that a direct linear relationship between the ionic conductivity and free volume fraction is established using the WLF equation based on the free volume theory for nanocomposite electrolyte, which indicates that the segmental chain migration and ionic migration and diffusion could be explained by the free volume theory.

  4. Determination of the activation enthalpy for migration of dislocations in plastically deformed 8006 Al-alloy by positron annihilation lifetime technique

    NASA Astrophysics Data System (ADS)

    Salah, Mohammed; Abdel-Rahman, M.; Badawi, Emad A.; Abdel-Rahman, M. A.

    2016-06-01

    The activation enthalpy for migration of dislocations of plastically deformed 8006 Al-alloy was investigated by positron annihilation lifetime technique. Plastic deformation using a hydraulic press produces mainly dislocations and may produce point defects. The type of defect was studied by isochronal annealing which determines the temperature range of recovery of each type. Only one type of defect (dislocations) was observed for the investigated sample and was found to be recovered within the range 455-700 K. Isothermal annealing by slow cooling was performed through this range and used in determination of the activation enthalpy of migration of dislocations which was found to be 0.26 ± 0.01 eV.

  5. Positron annihilation studies of organic superconductivity

    SciTech Connect

    Yen, H.L.; Lou, Y.; Ali, E.H.

    1994-09-01

    The positron lifetimes of two organic superconductors, {kappa}-(ET){sub 2}Cu(NCS){sub 2} and {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Br, are measured as a function of temperature across {Tc}. A drop of positron lifetime below {Tc} is observed. Positron-electron momentum densities are measured by using 2D-ACAR to search for the Fermi surface in {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Br. Positron density distributions and positron-electron overlaps are calculated by using the orthogonalized linear combination atomic orbital (OLCAO) method to interprete the temperature dependence due to the local charge transfer which is inferred to relate to the superconducting transition. 2D-ACAR results in {kappa}-(ET){sub 2}Cu[N(CN){sub 2}]Br are compared with theoretical band calculations based on a first-principles local density approximation. Importance of performing accurate band calculations for the interpretation of positron annihilation data is emphasized.

  6. Applications of positron annihilation spectroscopy in materials research

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.

    1988-01-01

    Positron Annihilation Spectroscopy (PAS) has emerged as a powerful technique for research in condensed matter. It has been used extensively in the study of metals, ionic crystals, glasses and polymers. The present review concentrates on applications of positron lifetime measurements for elucidation of the physicochemical structure of polymers.

  7. Positron annihilation in the interstellar medium

    NASA Technical Reports Server (NTRS)

    Guessoum, Nidhal; Ramaty, Reuven; Lingenfelter, Richard E.

    1991-01-01

    Positronium formation and annihilation are studied in a model for the interstellar medium consisting of cold cloud cores, warm partially ionized cloud envelopes, and hot intercloud gas. The gamma-ray spectra resulting from positron annihilation in these components of the interstellar medium are calculated. The spectra from the individual components are then combined, using two limiting assumptions for the propagation of the positrons, namely, that the positrons propagate freely throughout the interstellar medium, and that the positrons are excluded from the cold cloud cores. In the first case, the bulk of the positrons annihilate in the cloud cores and the annihilation line exhibits broad wings resulting from the annihilation of positronium formed by charge exchange in flight. In the second case, the positrons annihilate mainly in the warm envelopes, and the line wings are suppressed.

  8. Positron annihilation spectroscopy with magnetically analyzed beams

    NASA Technical Reports Server (NTRS)

    Singh, J. J.; Holt, W. H.; Mock, W., Jr.

    1982-01-01

    Lifetime measurements with magnetically analyzed positron beams were made in condensed media with uniform and non-uniform properties. As expected, the lifetime values with magnetically analyzed positron beams in uniform targets are similar to those obtained with conventional positron sources. The lifetime values with magnetically analyzed beams in targets which have non-uniform properties vary with positron energy and are different from the conventional positron source derived lifetime values in these targets.

  9. Measurements of defect structures by positron annihilation lifetime spectroscopy of the tellurite glass TeO2-P2O5-ZnO-LiNbO3 doped with ions of rare earth elements: Er3+, Nd3+ and Gd3+

    NASA Astrophysics Data System (ADS)

    Golis, E.; Yousef, El. S.; Reben, M.; Kotynia, K.; Filipecki, J.

    2015-12-01

    The objective of the study was the structural analysis of the TeO2-P2O5-ZnO-LiNbO3 tellurite glasses doped with ions of the rare-earth elements: Er3+, Nd3+ and Gd3+ based on the PALS (Positron Annihilation Lifetime Spectroscopy) method of measuring positron lifetimes. Values of positron lifetimes and the corresponding intensities may be connected with the sizes and number of structural defects, such as vacancies, mono-vacancies, dislocations or pores, the sizes of which range from a few angstroms to a few dozen nanometres. Experimental positron lifetime spectrum revealed existence of two positron lifetime components τ1 and τ2. Their interpretation was based on two-state positron trapping model where the physical parameters are the annihilation velocity and positron trapping rate.

  10. Positron annihilation induced Auger electron emission

    SciTech Connect

    Weiss, A.; Jibaly, M.; Lei, Chun; Mehl, D.; Mayer, R.; Lynn, K.G.

    1988-01-01

    We report on measurements of Auger electron emission from Cu and Fe due to core hole excitations produced by the removal of core electrons by matter-antimatter annihilation. Estimates are developed of the probability of positrons annihilating with a 3p electron in these materials. Several important advantages of Positron annihilation induced Auger Electron Spectroscopy (PAES) for surface analysis are suggested. 10 refs., 2 figs.

  11. Characterization of free volume during vulcanization of styrene butadiene rubber by means of positron annihilation lifetime spectroscopy and dynamic mechanical test.

    PubMed

    Marzocca, A J; Cerveny, S; Salgueiro, W; Somoza, A; Gonzalez, L

    2002-02-01

    An experimental investigation was performed to study the effect on the free volume of the advance of the cross-linking reaction in a copolymer of styrene butadiene rubber by sulfur vulcanization. The dynamic modulus and loss tangent were evaluated over samples cured for different times at 433 K by dynamic mechanical tests over a range of frequencies between 5 and 80 Hz at temperatures between 200 and 300 K. Using the William-Landel-Ferry relationship, master curves were obtained at a reference temperature of 298 K and the coefficients c(0)(1) and c(0)(2) were evaluated. From these parameters the dependence of the free volume on the cure time is obtained. Positron annihilation lifetime spectroscopy was also used to estimate the size and number density of free volume sites in the material. The spectra were analyzed in terms of continuous distributions of free volume size. The results suggest an increase of the lower free volume size when cross linking takes place. Both techniques give similar results for the dependence of free volume on the time of cure of the polymer. PMID:11863549

  12. Positron Annihilation Lifetime Spectroscopy Study of Neutron Irradiated High Temperature Superconductors YBa2Cu3O7-δ for Application in Fusion Facilities

    NASA Astrophysics Data System (ADS)

    Veterníková, J.; Chudý, M.; Slugeň, V.; Eisterer, M.; Weber, H. W.; Sojak, S.; Petriska, M.; Hinca, R.; Degmová, J.; Sabelová, V.

    2012-02-01

    This study focuses on the crystallographic defects introduced by neutron irradiation and the resulting changes of the superconducting properties in the high temperature superconductor YBa2Cu3O7-δ. This material is considered to be most promising for magnet systems in future fusion reactors. Two different bulk samples, pure non-doped YBa2Cu3O7-δ (YBCO) and multi-seed YBa2Cu3O7-δ doped by platinum (MS2F) were studied prior to and after irradiation in the TRIGA MARK II reactor in Vienna. Neutron irradiation is responsible for a significant enhancement of the critical current densities as well as for a reduction in critical temperature. The accumulation of small open volume defects (<0.5 nm) partially causes those changes. These defects were studied by positron annihilation lifetime spectroscopy at room temperature. A high concentration of Cu-O di-vacancies was found in both samples, which increased with neutron fluence. The defect concentration was significantly reduced after a heat treatment.

  13. Positron scattering and annihilation in hydrogenlike ions

    NASA Astrophysics Data System (ADS)

    Green, D. G.; Gribakin, G. F.

    2013-09-01

    Diagrammatic many-body theory is used to calculate the scattering phase shifts, normalized annihilation rates Zeff, and annihilation γ spectra for positron collisions with the hydrogenlike ions He+, Li2+, B4+, and F8+. Short-range electron-positron correlations and longer-range positron-ion correlations are accounted for by evaluating nonlocal corrections to the annihilation vertex and the exact positron self-energy. The numerical calculation of the many-body theory diagrams is performed using B-spline basis sets. To elucidate the role of the positron-ion repulsion, the annihilation rate is also estimated analytically in the Coulomb-Born approximation. It is found that the energy dependence and magnitude of Zeff are governed by the Gamow factor that characterizes the suppression of the positron wave function near the ion. For all of the H-like ions, the correlation enhancement of the annihilation rate is found to be predominantly due to corrections to the annihilation vertex, while the corrections to the positron wave function play only a minor role. Results of the calculations for s-, p-, and d-wave incident positrons of energies up to the positronium-formation threshold are presented. Where comparison is possible, our values are in excellent agreement with the results obtained using other, e.g., variational, methods. The annihilation-vertex enhancement factors obtained in the present calculations are found to scale approximately as 1+(1.6+0.46ℓ)/Zi, where Zi is the net charge of the ion and ℓ is the positron orbital angular momentum. Our results for positron annihilation in H-like ions provide insights into the problem of positron annihilation with core electrons in atoms and condensed matter systems, which have similar binding energies.

  14. Method for photon activation positron annihilation analysis

    DOEpatents

    Akers, Douglas W.

    2006-06-06

    A non-destructive testing method comprises providing a specimen having at least one positron emitter therein; determining a threshold energy for activating the positron emitter; and determining whether a half-life of the positron emitter is less than a selected half-life. If the half-life of the positron emitter is greater than or equal to the selected half-life, then activating the positron emitter by bombarding the specimen with photons having energies greater than the threshold energy and detecting gamma rays produced by annihilation of positrons in the specimen. If the half-life of the positron emitter is less then the selected half-life, then alternately activating the positron emitter by bombarding the specimen with photons having energies greater then the threshold energy and detecting gamma rays produced by positron annihilation within the specimen.

  15. Positron annihilation induced Auger electron spectroscopy

    NASA Technical Reports Server (NTRS)

    Weiss, Alex; Koymen, A. R.; Mehl, David; Jensen, K. O.; Lei, Chun; Lee, K. H.

    1990-01-01

    Recently, Weiss et al. have demonstrated that it is possible to excite Auger transitions by annihilating core electrons using a low energy (less than 30eV) beam of positrons. This mechanism makes possible a new electron spectroscopy, Positron annihilation induced Auger Electron Spectroscopy (PAES). The probability of exciting an Auger transition is proportional to the overlap of the positron wavefunction with atomic core levels. Since the Auger electron energy provides a signature of the atomic species making the transition, PAES makes it possible to determine the overlap of the positron wavefunction with a particular element. PAES may therefore provide a means of detecting positron-atom complexes. Measurements of PAES intensities from clean and adsorbate covered Cu surfaces are presented which indicate that approx. 5 percent of positrons injected into CU at 25eV produce core annihilations that result in Auger transitions.

  16. Positron Annihilation Spectroscopy Study of Barnett Shale Core

    NASA Astrophysics Data System (ADS)

    Ameena, Fnu; Alsleben, Helge; Quarles, Carroll A.

    Measurements are reported of positron annihilation lifetime and Doppler broadening parameters on 14 samples of Barnett shale core selected from 196 samples ranging from depths of 6107 to 6402 feet. The Barnett shale core was taken from EOG well Two-O-Five 2H located in Johnson county TX. The selected samples are dark clay-rich mudstone consisting of fine-grained clay minerals. The samples are varied in shape, typically a few inches long and about 1/2 inch in width and thickness, and are representative of the predominant facies in the core. X-ray fluorescence (XRF), X-ray diffraction (XRD), petrographic analysis and geochemical analysis of total organic carbon (TOC) were already available for each of the selected samples. The lifetime data are analyzed in terms of three lifetime components with the shortest lifetime fixed at 125 ps. The second lifetime is attributed to positron annihilation in the bulk and positron trapping; and the third lifetime is due to positronium. Correlations of the lifetimes, intensities, the average lifetime and S and W parameters with TOC, XRF and XRD parameters are discussed. The observed correlations suggest that positron spectroscopy may be a useful tool in characterizing shale.

  17. Positron Annihilation in Medical Substances of Insulin

    NASA Astrophysics Data System (ADS)

    Pietrzak, R.; Szatanik, R.

    2005-05-01

    Positrons lifetimes were measured in medical substances of insulin (human and animal), differing as far as the degree of purity and time of their activity in the organism are concerned. In all of the cases the spectrum of positron lifetime was distributed into three components, with the long-life component ranging from 1.8 to 2.08 ns and the intensity taking on values from 18 to 24%. Making use of Tao-Eldrup model, the average radius of the free volume, in which o-Ps annihilated, and the degree of filling in the volume were determined. It was found that the value of the long-life component for human insulin is higher than that of animal insulin. Moreover, the value of this component clearly depends on the manner of purification of the insulin. It was also noticed that there occurs a correlation between the value of this component and the time after which it begins to be active in the organism, as well as the total time of its activity.

  18. Positron Annihilation in the Bipositronium Ps2

    SciTech Connect

    Bailey, David H.; Frolov, Alexei M.

    2005-07-01

    The electron-positron-pair annihilation in the bipositronium PS2 is considered. In particular, the two-, three-, one- and zero-photon annihilation rates are determined to high accuracy. The corresponding analytical expressions are also presented. Also, a large number of bound state properties have been determined for this system.

  19. Nondestructive examination using neutron activated positron annihilation

    DOEpatents

    Akers, Douglas W.; Denison, Arthur B.

    2001-01-01

    A method is provided for performing nondestructive examination of a metal specimen using neutron activated positron annihilation wherein the positron emitter source is formed within the metal specimen. The method permits in situ nondestructive examination and has the advantage of being capable of performing bulk analysis to determine embrittlement, fatigue and dislocation within a metal specimen.

  20. Positron annihilation in gamma-ray bursts

    NASA Technical Reports Server (NTRS)

    Harding, Alice K.

    1990-01-01

    Emission features appear at energies of 350 to 450 keV in the spectra of a number of gamma ray burst sources. These features were interpreted as electron-positron annihilation lines, redshifted by the gravitational field near the surface of a neutron star. Evidence that gamma ray bursts originate at neutron stars with magnetic field strengths of approx. 10(exp 12) Gauss came from recent observations of cyclotron scattering harmonics in the spectra of two bursts. Positrons could be produced in gamma ray burst sources either by photon-photon pair production or by one-photon pair production in a strong magnetic field. The annihilation of positrons is affected by the presence of a strong neutron star magnetic field in several ways. The relaxation of transverse momentum conservation causes an intrinsic broadening of the two-photon annihilation line and there is a decrease in the annihilation cross section below the free-space value. An additional channel for one-photon annihilation also becomes possible in high magnetic fields. The physics of pair production and annihilation near strongly magnetized neutron stars will be reviewed. Results from a self-consistent model for non-thermal synchrotron radiation and pair annihilation are beginning to identify the conditions required to produce observable annihilation features from strongly magnetized plasmas.

  1. High sensitivity of positron annihilation to thermal oxidation of polyethylene

    NASA Astrophysics Data System (ADS)

    Ito, Kenji; Kobayashi, Yoshinori; Nanasawa, Atsushi

    2003-01-01

    We demonstrate the high sensitivity of positron annihilation to compositional changes related to the thermal degradation of polyethylene (PE). Positron annihilation γ-ray and lifetime measurements were conducted for PE films with and without antioxidant (1000-ppm Ciba® IRGANOX® 1076), subjected to heat treatment at 100 °C for different periods, to a maximum of 30 days. For the film without antioxidant, the positron Doppler parameter (S) and ortho-positronium formation probability (Io-Ps) appreciably decreased with increased heat treatment times, whereas they barely changed for the film with antioxidant. This, together with the Fourier transform infrared measurements, demonstrated that the variations of S and Io-Ps are caused by the thermal oxidation of PE. The S parameter was found to be sensitive to the early stage of degradation, where the carbonyl concentration is inferred to be lower than 100 ppm. The high sensitivity results from the large positron mobility in PE and from the high positron affinity of oxygen-containing polar groups. This work provides the basis for an application of positron annihilation to sensitive detection of the initial degradation of PE and other nonpolar polymers.

  2. Two-component density functional theory calculations of positron lifetimes for small vacancy clusters in silicon

    NASA Astrophysics Data System (ADS)

    Makhov, D. V.; Lewis, Laurent J.

    2005-05-01

    The positron lifetimes for various vacancy clusters in silicon are calculated within the framework of the two-component electron-positron density functional theory. The effect of the trapped positron on the electron density and on the relaxation of the structure is investigated. Our calculations show that, contrary to the usual assumption, the positron-induced forces do not compensate in general for electronic inward forces. Thus, geometry optimization is required in order to determine positron lifetime accurately. For the monovacancy and the divacancy, the results of our calculations are in good agreement with the experimental positron lifetimes, suggesting that this approach gives good estimates of positron lifetimes for larger vacancy clusters, required for their correct identification with positron annihilation spectroscopy. As an application, our calculations show that fourfold trivacancies and symmetric fourfold tetravacancies have positron lifetimes similar to monovacancies and divacancies, respectively, and can thus be confused in the interpretation of positron annihilation experiments.

  3. INSTRUMENTS AND METHODS OF INVESTIGATION: Positron annihilation spectroscopy in materials structure studies

    NASA Astrophysics Data System (ADS)

    Grafutin, Viktor I.; Prokop'ev, Evgenii P.

    2002-01-01

    A relatively new method of materials structure analysis — positron annihilation spectroscopy (PAS) — is reviewed. Measurements of positron lifetimes, the determination of positron 3γ- and 2γ-annihilation probabilities, and an investigation of the effects of different external factors on the fundamental characteristics of annihilation constitute the basis for this promising method. The ways in which the positron annihilation process operates in ionic crystals, semiconductors, metals and some condensed matter systems are analyzed. The scope of PAS is described and its prospects for the study of the electronic and defect structures are discussed. The applications of positron annihilation spectroscopy in radiation physics and chemistry of various substances as well as in physics and chemistry of solutions are exemplified.

  4. Positron annihilation gamma rays from novae

    NASA Technical Reports Server (NTRS)

    Leising, Mark D.; Clayton, Donald D.

    1987-01-01

    The potential for observing annihilation gamma rays from novae is investigated. These gamma rays, a unique signature of the thermonuclear runaway models of novae, would result from the annihilation of positrons emitted by beta(+)-unstable nuclei produced near the peak of the runaway and carried by rapid convection to the surface of the nova envelope. Simple models, which are extensions of detailed published models, of the expansion of the nova atmospheres are evolved. These models serve as input into investigations of the fate of nearby Galactic fast novae could yield detectable fluxes of electron-positron annihilation gamma rays produced by the decay of N-13 and F-18. Although nuclear gamma-ray lines are produced by other nuclei, it is unlikely that the fluxes at typical nova distances would be detectable to present and near-future instruments.

  5. Defects in metals. [Positron annihilation spectroscopy

    SciTech Connect

    Siegel, R.W.

    1982-06-01

    The application of positron annihilation spectroscopy (PAS) to the study of defects in metals has led to increased knowledge on lattice-defect properties during the past decade in two areas: the determination of atomic defect properties, particularly those of monovacancies, and the monitoring and characterization of vacancy-like microstructure development during post-irradiation and post-quench annealing. The study of defects in metals by PAS is reviewed within the context of the other available techniques for defect studies. The strengths and weaknesses of PAS as a method for the characterization of defect microstructures are considered. The additional possibilities for using the positron as a localized probe of the atomic and electronic structures of atomic defects are discussed, based upon theoretical calculations of the annihilation characteristics of defect-trapped positrons and experimental observations. Finally, the present status and future potential of PAS as a tool for the study of defects in metals is considered. 71 references, 9 figures.

  6. Positron annihilation in neutron irradiated iron-based materials

    NASA Astrophysics Data System (ADS)

    Lambrecht, M.; Almazouzi, A.

    2011-01-01

    The hardening and embrittlement of reactor pressure vessel steels is of great concern in the actual nuclear power plant life assessment. This embrittlement is caused by irradiation-induced damage, like vacancies, interstitials, solutes and their clusters. But the reason for the embrittlement of the material is not yet totally known. The real nature of the irradiation damage should thus be examined as well as its evolution in time. Positron annihilation spectroscopy has been shown to be a powerful method for analyzing some of these defects. In fact, both vacancy type clusters and precipitates can be visualized by positrons. Recently, at SCK·CEN, a new setup has been constructed, calibrated and optimized to measure the coincidence Doppler broadening and lifetime of neutron irradiated materials. To be able to compare the results obtained by the positron studies, with those of other techniques (such as transmission electron microscopy, atom probe tomography and small angle neutron scattering), quantitative estimations of the size and density of the annihilation sites are needed. Using the approach proposed by Vehanen et al., an attempt is made to calculate the needed quantities in Fe and Fe-Cu binary alloys that were neutron irradiated to different doses. The results obtained are discussed highlighting the difficulties in defining the annihilation centres even in these simple model alloys, in spite of using both lifetime and Doppler broadening measurements in the same samples.

  7. Positron annihilation studies in solid 2-aminopyridine, 3-aminopyridine, 4-aminopyridine and 2-aminopyrimidine

    NASA Astrophysics Data System (ADS)

    Netto, A. Marques; Bicalho, S. M. C. M.; Filgueiras, Ca. L.; Machado, J. C.

    1985-09-01

    Positron annihilation lifetimes and Doppler-broadened annihilation lines have been measured in solid 2-aminopyridine (2-APY), 3-aminopyridine (3-APY), 4-aminopyridine (4-APY) and 2-aminopyrimidine (2-APYM). The results point to the formation of positronium in the solid pyridines and the yields are discussed in terms of the structures and the electron donation character of the compounds.

  8. On the method of positron lifetime measurement

    NASA Technical Reports Server (NTRS)

    Nishiyama, F.; Shizuma, K.; Nasai, H.; Nishi, M.

    1983-01-01

    A fast-slow coincidence system was constructed for the measurement of positron lifetimes in material. The time resolution of this system was 270 ps for the (60)Co gamma rays. Positron lifetime spectra for 14 kinds of alkali halides were measured with this system. Two lifetime components and their intensities were derived from analyses of the lifetime spectra.

  9. Apparatus for photon activation positron annihilation analysis

    DOEpatents

    Akers, Douglas W.

    2007-06-12

    Non-destructive testing apparatus according to one embodiment of the invention comprises a photon source. The photon source produces photons having predetermined energies and directs the photons toward a specimen being tested. The photons from the photon source result in the creation of positrons within the specimen being tested. A detector positioned adjacent the specimen being tested detects gamma rays produced by annihilation of positrons with electrons. A data processing system operatively associated with the detector produces output data indicative of a lattice characteristic of the specimen being tested.

  10. Application of positron annihilation in materials science

    SciTech Connect

    Siegel, R.W.; Fluss, M.J.; Smedskjaer, L.C.

    1984-05-01

    Owing to the ability of the positron to annihilate from a variety of defect-trapped states, positron annihilation spectroscopy (PAS) has been applied increasingly to the characterization and study of defects in materials in recent years. In metals particularly, it has been demonstrated that PAS can yield defect-specific information which, by itself or in conjunction with more traditional experimental techniques, has already made a significant impact upon the determination of atomic-defect properties and the monitoring and characterization of vacancy-like microstructure development, as occurs during post-irradiation annealing. The applications of PAS are now actively expanding to the study of more complex defect-related phenomena in irradiated or deformed metals and alloys, phase transformations and structural disorder, surfaces and near-surface defect characterization. A number of these applications in materials science are reviewed and discussed with respect to profitable future directions.

  11. Positron annihilation radiation from solar flares

    NASA Technical Reports Server (NTRS)

    Share, G. H.; Chupp, E. L.; Forrest, D. J.; Rieger, E.

    1983-01-01

    Positron-annihilation radiation has been observed from the June 21, 1980 and June 3, 1982 flares by the gamma-ray spectrometer on the Solar Maximum Mission satellite. The observed 0.511-MeV line fluences from the flares were 14.6 + or - 3.3 gamma/sq cm and 103 + or - 8 gamma/sq cm, respectively. Measurement of the line width establishes an upper limit to the temperature in the annihilation region of 3 x 10 to the 6th K. The time dependence of the 0.511-MeV line during the 1980 flare is consistent with the calculations of Ramaty et al. (1983) for positrons created in the decay of radioactive nuclei. The time dependence of the 0.511-MeV line for the 1982 flare is more complex and requires more detailed study.

  12. Positron scattering and annihilation from hydrogenlike ions

    SciTech Connect

    Novikov, S.A.; Bromley, M.W.J.; Mitroy, J.

    2004-05-01

    The Kohn variational method is used with a configuration-interaction-type wave function to determine the J=0 and J=1 phase shifts and annihilation parameter Z{sub eff} for positron-hydrogenic ion scattering. The phase shifts are within 1-2% of the best previous calculations. The values of Z{sub eff} are small and do not exceed unity for any of the momenta considered. At thermal energies Z{sub eff} is minute with a value of order 10{sup -50} occurring for He{sup +} at k=0.05a{sub 0}{sup -1}. In addition to the variational calculations, analytic expressions for the phase shift and annihilation parameters within the Coulomb wave Born approximation are derived and used to help elucidate the dynamics of positron collisions with positive ions.

  13. Microemulsion systems studied by positron annihilation techniques

    SciTech Connect

    Boussaha, A.; Djermouni, B.; Fucugauchi, L.A.; Ache, H.J.

    1980-07-02

    The formation of thermalized positronium atoms is greatly reduced if increasing amounts of water become solubilized in reversed micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate in apolar solvents. Similar observations have been made if the surfactant is Triton X-100. The application of the positron annihilation technique to the study of microemulsions consisting of potassium oleate-alcohol-oil-water mixtures indicates, consistent with previous results, that microemulsion formation requires a certain water/oil ratio if the oil is a long-chain aliphatic hydrocarbon such as hexadecane. This ratio is 0.4 in the case of a 1-pentanol- and 0.2 for a 1-hexanol-containing mixture. This minimum water content is strongly reduced if the oil is an aromatic hydrocarbon. The positron annihilation data also sensitively reflect structural rearrangements in these solutions occurring upon further addition of water, such as the transition of spherical aggregates to a disk-like lamellae structure.

  14. Correlation of Gas Permeability in a Metal-Organic Framework MIL-101(Cr)–Polysulfone Mixed-Matrix Membrane with Free Volume Measurements by Positron Annihilation Lifetime Spectroscopy (PALS)

    PubMed Central

    Jeazet, Harold B. Tanh; Koschine, Tönjes; Staudt, Claudia; Raetzke, Klaus; Janiak, Christoph

    2013-01-01

    Hydrothermally stable particles of the metal-organic framework MIL-101(Cr) were incorporated into a polysulfone (PSF) matrix to produce mixed-matrix or composite membranes with excellent dispersion of MIL-101 particles and good adhesion within the polymer matrix. Pure gas (O2, N2, CO2 and CH4) permeation tests showed a significant increase of gas permeabilities of the mixed-matrix membranes without any loss in selectivity. Positron annihilation lifetime spectroscopy (PALS) indicated that the increased gas permeability is due to the free volume in the PSF polymer and the added large free volume inside the MIL-101 particles. The trend of the gas transport properties of the composite membranes could be reproduced by a Maxwell model. PMID:24957061

  15. Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy

    SciTech Connect

    Chai, L.; Schut, H.; Schaarenburg, L. C. van; Eijt, S. W. H.; Al-Sawai, W.; Barbiellini, B.; Bansil, A.; Gao, Y.; Houtepen, A. J.; Mijnarends, P. E.; Huis, M. A. van; Ravelli, L.; Egger, W.; Kaprzyk, S.

    2013-08-01

    Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy, show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.

  16. Positron annihilation investigation of BaSrFBr:Eu by X-ray irradiation

    NASA Astrophysics Data System (ADS)

    Lee, C. Y.

    2014-12-01

    The mechanical property of the BaSrFBr:Eu phosphor layer of X-ray image plates was investigated by using resolution (LP/mm) and coincidence Doppler broadening (CDB) positron annihilation as well as positron annihilation lifetime (PAL). The image plate samples of BaSrFBr:Eu phosphors in this experiment were irradiated by using hospital X-rays. The LP/mm values of the irradiated BaSrFBr:Eu image plates varied from 3.35 to 1.25 for up to 20,000 exposures. CDB positron annihilation and lifetime spectroscopy were used to analyze defect structures in the phosphor layer. Even when the LP/mm values were greatly decreased due to exposures, the S parameter and the lifetime ( τ 1, τ 2) values were almost constant with increasing number of exposures. A positive relationship existed between the SEM images and positron annihilation spectroscopy (PAS). According to the SEM images and the positron annihilation spectroscopy (PAS) results, measurements of the defects with PAS indicate that the image-plate phosphor can be safely used for hospital X-rays in the course of diagnostic radiography at an average rate of 20,000 times for one year.

  17. Energy-Resolved Positron Annihilation in Flight in Solid Targets

    NASA Astrophysics Data System (ADS)

    Weber, M. H.; Hunt, A. W.; Golovchenko, J. A.; Lynn, K. G.

    1999-11-01

    Energy-resolved two-quantum annihilation in flight of positrons with energies ranging from 10 to 71.6 keV was observed. An energy-dispersive two-detector coincidence system was used to observe the sum and difference energies of the γ rays from annihilating positron-electron pairs. For positrons penetrating carbon foils the c/v dependence of the annihilation cross section is confirmed. Spectra obtained from gold-coated carbon foils show evidence of in-flight annihilation with gold M-shell electrons.

  18. Positron Annihilation Spectroscopy of High Performance Polymer Films under CO2 Pressure

    SciTech Connect

    C.A. Quarles; John R. Klaehn; Eric S. Peterson; Jagoda M. Urban-Klaehn

    2010-08-01

    Positron annihilation Lifetime and Doppler broadening measurements are reported for six polymer films as a function of carbon dioxide absolute pressure ranging from 0 to 45 psi. Since the polymer films were thin and did not absorb all positrons, corrections were made in the lifetime analysis for the absorption of positrons in the positron source and sample holder using the Monte Carlo transport code MCNP. Different polymers are found to behave differently. Some polymers studied form positronium and some, such as the polyimide structures, do not. For those samples that form positronium an interpretation in terms of free volume is possible; for those that don’t form positronium, further work is needed to determine how best to describe the behavior in terms of the bulk positron annihilation parameters. Some polymers exhibit changes in positron lifetime and intensity under CO2 pressure which may be described by the Henry or Langmuir sorption models, while the positron response of other polymers is rather insensitive to the CO2 pressure. The results demonstrate the usefulness of positron annihilation spectroscopy in investigating the sorption of CO2 into various polymers at pressures up to about 3 atm.

  19. Characterization of the melting process of PTFE using positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Honda, Y.; Nishijima, S.

    2015-06-01

    Poly(tetrafluoroethylene) (PTFE) is a semi-crystalline polymer and the lifetime of ortho-positronium(o-Ps) is known to be able to be separated into two components due to annihilation in the crystal region and in the amorphous region. The melting process of PTFE was investigated using positron annihilation spectroscopy and X-ray diffraction. The results indicated that volume expansion with an increase of temperature is dominantly due to the expansion of the amorphous region and a Ps bubble is formed at melting in both regions. The o-Ps relating to the crystal region definitely remains on the surface of crystal at the time of annihilation. The production of lower energy electrons at melting was deduced by the analysis of the Doppler broadened annihilation photopeak, and the increase in the number of such electrons was found to have great influence on the formation of the o-Ps and annihilation processes of positron and o-Ps.

  20. Resolvability of defect ensembles with positron annihilation studies

    SciTech Connect

    Fluss, M.J.; Howell, R.H.; Rosenberg, I.J.; Meyer, P.

    1984-11-12

    Recent advances in the use of positron annihilation to study defect ensembles in and on the surfaces of metals, are pointing the way towards studies where particular positron-electron annihilation modes may be identified and studied in the presence of one another. Although a great deal is understood about the annihilation of positrons in ostensibly defect-free metals, much less is understood when the positron annihilates in complex defect systems such as liquid metals, amorphous solids, or at or near the vacuum-solid interface. In this paper the results of three experiments, all of which demonstrate means by which we can resolve various poistron annihilation channels from one another, are discussed.

  1. Positron annihilation in cardo-based polymer membranes.

    PubMed

    Kobayashi, Y; Kazama, Shingo; Inoue, K; Toyama, T; Nagai, Y; Haraya, K; Mohamed, Hamdy F M; O'Rouke, B E; Oshima, N; Kinomura, A; Suzuki, R

    2014-06-01

    Positron annihilation lifetime spectroscopy (PALS) is applied to a series of bis(aniline)fluorene and bis(xylidine)fluorene-based cardo polyimide and bis(phenol)fluorene-based polysulfone membranes. It was found that favorable amounts of positronium (Ps, the positron-electron bound state) form in cardo polyimides with the 2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) moiety and bis(phenol)fluorene-based cardo polysulfone, but no Ps forms in most of the polyimides with pyromellitic dianhydride (PMDA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BTDA) moieties. A bis(xylidine)fluorene-based polyimide membrane containing PMDA and BTDA moieties exhibits a little Ps formation but the ortho-positronium (o-Ps, the triplet state of Ps) lifetime of this membrane anomalously shortens with increasing temperature, which we attribute to chemical reaction of o-Ps. Correlation between the hole size (V(h)) deduced from the o-Ps lifetime and diffusion coefficients of O2 and N2 for polyimides with the 6FDA moiety and cardo polysulfone showing favorable Ps formation is discussed based on free volume theory of gas diffusion. It is suggested that o-Ps has a strong tendency to probe larger holes in rigid chain polymers with wide hole size distributions such as those containing cardo moieties, resulting in deviations from the previously reported correlations for common polymers such as polystyrene, polycarbonate, polysulfone, and so forth. PMID:24815092

  2. Defect identification in semiconductors with positron annihilation: experiment and theory

    NASA Astrophysics Data System (ADS)

    Tuomisto, Filip

    2015-03-01

    Positron annihilation spectroscopy is a very powerful technique for the detection, identification and quantification of vacancy-type defects in semiconductors. In the past decades, it has been used to reveal the relationship between opto-electronic properties and specific defects in a wide variety of materials - examples include parasitic yellow luminescence in GaN, dominant acceptor defects in ZnO and broad-band absorption causing brown coloration in natural diamond. In typical binary compound semiconductors, the selective sensitivity of the technique is rather strongly limited to cation vacancies that possess significant open volume and suitable charge (negative of neutral). On the other hand, oxygen vacancies in oxide semiconductors are a widely debated topic. The properties attributed to oxygen vacancies include the inherent n-type conduction, poor p-type dopability, coloration (absorption), deep level luminescence and non-radiative recombination, while the only direct experimental evidence of their existence has been obtained on the crystal surface. We will present recent advances in combining state-of-the-art positron annihilation experiments and ab initio computational approaches. The latter can be used to model both the positron lifetime and the electron-positron momentum distribution - quantities that can be directly compared with experimental results. We have applied these methods to study vacancy-type defects in III-nitride semiconductors (GaN, AlN, InN) and oxides such as ZnO, SnO2, In2O3andGa2O3. We will show that cation-vacancy-related defects are important compensating centers in all these materials when they are n-type. In addition, we will show that anion (N, O) vacancies can be detected when they appear as complexes with cation vacancies.

  3. Positron-annihilation spectroscopy of vacancy defects in aluminum

    SciTech Connect

    Chakraborty, B.; Berko, S.; Fluss, M.J.; Hoffmann, K.; Lippel, P.; Siegel, R.W.

    1982-06-01

    Positron-annihilation characteristics in a monovacancy and a divacancy in aluminium have been calculated self-consistently using a local density functional formalism, into which the many-body enhancement effects have been incorporated. Results for the theoretical two-dimensional angular correlation of annihilation radiation spectra are compared to experimental results obtained from an aluminum single crystal at 20/sup 0/C, where positrons annihilate from a Bloch-state, and at higher temperatures, 500/sup 0/C and 630/sup 0/C, where they annihilate primarily from vacancy-trapped states.

  4. Irradiation damage from low-dose high-energy protons on mechanical properties and positron annihilation lifetimes of Fe-9Cr alloy

    NASA Astrophysics Data System (ADS)

    Xu, Q.; Fukumoto, K.; Ishi, Y.; Kuriyama, Y.; Uesugi, T.; Sato, K.; Mori, Y.; Yoshiie, T.

    2016-01-01

    Nuclear reactions in accelerator-driven systems (ADS) result in the generation of helium within the ADS materials. The amount of helium produced in this way is approximately one order of magnitude higher than that generated by nuclear fusion. As helium is well-known to induce degradation in the mechanical properties of metals, its effect on ADS materials is an important factor to assess. The results obtained in this study show that low-dose proton irradiation (11 MeV at 573 K to 9.0 × 10-4 dpa and 150 MeV at room temperature to 2.6 × 10-6 dpa) leads to a decrease in yield stress and ultimate tensile strength in a Fe-9Cr alloy. Moreover, interstitial helium and hydrogen atoms, as well as the annihilation of dislocation jogs, were identified as key factors that determine the observed softening of the alloy.

  5. Positron annihilation studies of moisture in graphite-reinforced composites

    SciTech Connect

    Singh, J.J.; Holt, W.H.; Mock, W., Jr.

    1980-07-01

    The positron lifetime technique of monitoring absorbed moisture is applied to several composites, including graphite/polymides which are candidates for high-temperature (over 260 C) applications. The experimental setup is a conventional fast-slow coincidence system wherein the positron lifetime is measured with respect to a reference time determined by the detection of a nuclear gamma ray emitted simultaneously with the positron. From the experiments, a rate of change of positron mean lifetime per unit mass of water can be determined for each type of specimen. Positron lifetime spectra are presented for a graphite/polyimide composite and for a pure polyimide.

  6. Positron annihilation studies of moisture in graphite-reinforced composites

    NASA Technical Reports Server (NTRS)

    Singh, J. J.; Holt, W. H.; Mock, W., Jr.; Buckingham, R. D.

    1980-01-01

    The positron lifetime technique of monitoring absorbed moisture is applied to several composites, including graphite/polymides which are candidates for high-temperature (over 260 C) applications. The experimental setup is a conventional fast-slow coincidence system wherein the positron lifetime is measured with respect to a reference time determined by the detection of a nuclear gamma ray emitted simultaneously with the positron. From the experiments, a rate of change of positron mean lifetime per unit mass of water can be determined for each type of specimen. Positron lifetime spectra are presented for a graphite/polyimide composite and for a pure polyimide.

  7. Moisture dependence of positron annihilation spectra in nylon-6

    NASA Technical Reports Server (NTRS)

    Singh, J. J.; St. Clair, T. L.; Holt, W. H.; Mock, W., Jr.

    1984-01-01

    Positron annihilation time spectra have been measured in nylon-6 samples as a function of their moisture content. The measured average long life component lifetime values are: 1722 + or - 47 ps (dry), 1676 + or - 40 ps (14.6 percent saturation value), 1719 + or - 26 ps (29.3 percent saturation value), 1720 + or - 35 ps (50 percent of saturation value), 1857 + or - 35 ps (78.1 percent saturation value), and 1936 + or - 57 ps (saturated). It appears that nylon-6 has a special affinity for water at low concentration levels where H2O molecules enter between the (C = O - H-N) chemical bonds between nylon molecular chains. As the water concentration increases beyond a critical level, nylon-6 specimens start trapping H2O molecules in other bond sites or potential wells. The trapped water increases the free volume in the test specimens and reduces Ps atom formation as well as its subsequent decay rate.

  8. Positron annihilation characteristics in mesostructural silica films with various porosities

    SciTech Connect

    Xiong, Bangyun; Mao, Wenfeng; Tang, Xiuqin; He, Chunqing

    2014-03-07

    Porous silica films with various porosities were prepared via a sol-gel method using a nonionic amphiphilic triblock copolymer F127 as the structure-directing agent. Doppler broadening of positron annihilation radiation (DBAR) spectra were collected for the prepared films using a variable energy slow positron beam. Different linear relationships between positron annihilation line shape parameters S and W are found for the as-deposited films and calcined ones, indicative of the decomposition of the copolymer porogen in the as-deposited films upon calcination. This also reveals the variation of positron annihilation sites as a function of F127 loading or porosity. Strong correlations between positronium 3γ annihilation fraction, S parameter and porosity of the mesoporous silica films with isolated pores are obtained, which may provide a complementary method to determine closed porosities of mesoporous silica films by DBAR.

  9. Positron annihilation studies of Eu and Dy doped α'-Sr2SiO4

    NASA Astrophysics Data System (ADS)

    Gupta, S. K.; Sudarshan, K.; Sharma, S. K.; Pujari, P. K.; Natarajan, V.

    2015-06-01

    Sr2SiO4 is an important inorganic host for lanthanide doped white light emitting diodes (LEDs). Strontium silicate (Sr2SiO4) samples doped with 1.0 mol% of Eu3+ and Dy3+ content were prepared via sol-gel route and characterized by X-ray diffraction (XRD), Raman spectroscopy and positron annihilation spectroscopy (PAS). The concentration of the dopant ion and the temperature of annealing were optimized for maximum luminescence intensity. The positron annihilation lifetime and coincidence Doppler broadening (CDB) measurements indicated that the local environment around the positron annihilation site is different in Eu+3 doped and Dy+3 doped samples. The results could be explained based on the different local site occupancy of Eu+3 and Dy+3 in the matrix.

  10. Positron lifetime spectrometer using a DC positron beam

    DOEpatents

    Xu, Jun; Moxom, Jeremy

    2003-10-21

    An entrance grid is positioned in the incident beam path of a DC beam positron lifetime spectrometer. The electrical potential difference between the sample and the entrance grid provides simultaneous acceleration of both the primary positrons and the secondary electrons. The result is a reduction in the time spread induced by the energy distribution of the secondary electrons. In addition, the sample, sample holder, entrance grid, and entrance face of the multichannel plate electron detector assembly are made parallel to each other, and are arranged at a tilt angle to the axis of the positron beam to effectively separate the path of the secondary electrons from the path of the incident positrons.

  11. Positron annihilation and magnetic properties studies of copper substituted nickel ferrite nanoparticles

    NASA Astrophysics Data System (ADS)

    Kargar, Z.; Asgarian, S. M.; Mozaffari, M.

    2016-05-01

    Single phase copper substituted nickel ferrite Ni1-xCuxFe2O4 (x = 0.0, 0.1, 0.3 and 0.5) nanoparticles were synthesized by the sol-gel method. TEM images of the samples confirm formation of nano-sized particles. The Rietveld refinement of the X-ray diffraction patterns showed that lattice constant increase with increase in copper content from 8.331 for x = 0.0 to 8.355 Å in x = 0.5. Cation distribution of samples has been determined by the occupancy factor, using Rietveld refinement. The positron lifetime spectra of the samples were convoluted into three lifetime components. The shortest lifetime is due to the positrons that do not get trapped by the vacancy defects. The second lifetime is ascribed to annihilation of positrons in tetrahedral (A) and octahedral (B) sites in spinel structure. It is seen that for x = 0.1 and 0.3 samples, positron trapped within vacancies in A sites, but for x = 0.0 and 0.5, the positrons trapped and annihilated within occupied B sites. The longest lifetime component attributed to annihilation of positrons in the free volume between nanoparticles. The obtained results from coincidence Doppler broadening spectroscopy (CDBS) confirmed the results of positron annihilation lifetime spectroscopy (PALS) and also showed that the vacancy clusters concentration for x = 0.3 is more than those in other samples. Average defect density in the samples, determined from mean lifetime of annihilated positrons reflects that the vacancy concentration for x = 0.3 is maximum. The magnetic measurements showed that the saturation magnetization for x = 0.3 is maximum that can be explained by Néel's theory. The coercivity in nanoparticles increased with increase in copper content. This increase is ascribed to the change in anisotropy constant because of increase of the average defect density due to the substitution of Cu2+ cations and magnetocrystalline anisotropy of Cu2+ cations. Curie temperature of the samples reduces with increase in copper content which

  12. Primary cosmic ray positrons and galactic annihilation radiation

    NASA Technical Reports Server (NTRS)

    Lingenfelter, R. E.; Ramaty, R.

    1980-01-01

    The observation (Leventhal et al, 1978) of positron annihilation radiation at 0.511 MeV from the direction of the Galactic Center is reexamined, suggesting the possibility of a primary positron component of the cosmic rays. The observed 0.511 MeV emission requires a positron production rate nearly two orders of magnitude greater than the production rate of secondary cosmic ray positrons from pion decay produced in cosmic ray interactions. Possible sources of positrons are reviewed with both supernovae and pulsars appearing to be the more likely candidates. If only about 1% of these positrons were accelerated along with the cosmic ray nucleons and electrons to energies not less than 100 MeV, it is believed that these primary positrons would be comparable in intensity to those secondary positrons resulting from pion decay. Some observational evidence for the existence of primary positrons in the cosmic rays is also discussed.

  13. Coincidence Doppler Broadening of Positron Annihilation Radiation in Fe

    NASA Astrophysics Data System (ADS)

    do Nascimento, E.; Vanin, V. R.; Maidana, N. L.; Helene, O.

    2013-06-01

    We measured the Doppler broadening annihilation radiation spectrum in Fe, using 22NaCl as a positron source, and two Ge detectors in coincidence arrangement. The two-dimensional coincidence energy spectrum was fitted using a model function that included positron annihilation with the conduction band and 3d electrons, 3s and 3p electrons, and in-flight positron annihilation. Detectors response functions included backscattering and a combination of Compton and pulse pileup, ballistic deficit and shaping effects. The core electrons annihilation intensity was measured as 16.4(3) %, with almost all the remainder assigned to the less bound electrons. The obtained results are in agreement with published theoretical values.

  14. Effect of positron source irradiation on positronium annihilation in fine powdered alumina

    NASA Astrophysics Data System (ADS)

    Liu, Z. W.; Chen, Z. Q.

    2016-09-01

    Positron lifetime and Doppler broadening of positron annihilation radiation were measured as a function of time to study the irradiation effect by 22Na positron source in fine powdered alumina. The γ-Al2O3 samples were put in a vacuum chamber with a pressure of about 10-6 Torr and were cooled down to 10 K by a closed-cycle helium gas refrigerator. The irradiation of γ-Al2O3 samples by positron source was taken for a duration of about two days immediately after the sample was cooled down. After that, the sample was subjected to a warm up process from 10 K to 300 K with a step of 10 K. Positron lifetime and Doppler broadening spectra were measured simultaneously during these processes. Two long lifetime components corresponding to ortho-positronium annihilation were observed. A significant shortening of these long lifetime components and a large increase in S parameter is observed during irradiation. It is supposed that positron source irradiation creates a large number of paramagnetic centers on the surface of the γ-Al2O3 grains, which induce spin conversion quenching of positronium. The irradiation induced paramagnetic centers are unstable above 70 K and are nearly annealed out when the temperature rises to 190 K. After warming up of the sample to room temperature, the positron lifetime spectrum is identical to that before irradiation. It was also found that after irradiation, a medium long lifetime component of about 5 ns appears, of which the intensity increases with increasing irradiation time. This may be originated from the formation of the surface o-Ps state. This surface o-Ps state is also inhibited at elevated temperatures. Our results indicate that positronium is a very sensitive probe for the surface defects in porous materials.

  15. Energy-resolved positron annihilation rates for molecules

    SciTech Connect

    Barnes, L. D.; Young, J. A.; Surko, C. M.

    2006-07-15

    The development of high resolution positron beams has enabled measurements of annihilation rates for molecules as a function of incident positron energy. Vibrational Feshbach resonances in these spectra provide evidence for the existence of positron-molecule bound states. In this paper we present further studies of this phenomenon. Evidence is presented for positronically excited bound states (i.e., in addition to the ground state) in C{sub 12}H{sub 26} and C{sub 14}H{sub 30}. Measurements of the annihilation spectra of the halomethanes, CH{sub 3}F, CH{sub 3}Cl, and CH{sub 3}Br, exhibit strong resonances that vary significantly with the substituted halogen. Annihilation spectra for linear alkanes and ring molecules are compared. Annihilation spectra and infrared absorption spectra are compared for a number of molecules. Finally, annihilation rate measurements are presented for a variety of molecules at energies {>=}0.5 eV (i.e., above the vibrational resonances). These provide a measure of the annihilation rates in the absence of vibrational resonances.

  16. Identification of vacancy-type defects in ZnTe using positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Hamid, A. S.; Shaban, H.; Mansour, B. A.; Uedono, A.

    2005-08-01

    The species of defects in Mg0.5Zn0.5Te, Li-doped and P-doped ZnTe samples were investigated by using positron-based experiments. The positron lifetime experiments along with the atomic superposition (AT-SUP) method were used to predict the positron trapping centers in the samples. The results of lifetime of positron were combined with the 2D angular correlation of annihilation radiation (ACAR) experiments to get information on the electronic structure of these defects. The momentum density distributions permitted the vacancy-type defect to be distinguished in each sample. In addition, the results of the 2D-ACAR experiments disclosed a relaxation of the atoms around the vacancy in the P-doped ZnTe sample.

  17. Positron annihilation study of the micro-defects induced by cavitation in mild steel

    NASA Astrophysics Data System (ADS)

    Zhao, Ming; Wang, Jiadao; Chen, Darong; Hao, Xiaopeng; Wang, Baoyi

    2008-08-01

    Cavitation-induced micro-defects in mild steel after cavitation experiment in the fluid field have been studied by positron Doppler broadening measurement and positron annihilation lifetime spectra (PALS). Depth-resolved positron Doppler S-parameter (DPDS) results showed that S-parameter increased and micro-defects between the surface and the bulk has obvious variation with depth during the cavitation process. From the positron lifetime results, it was found that the size and number of micro-defects increase with the development of cavitation. These results suggest that more micro-defects are generated in mild steel bulk during the cavitation process than those in the mild steel surface layer region, although more mico-defects seen in the mild steel surface layer. Moreover, the size of micro-defects in mild steel bulk increases remarkably owing to their transfer and aggregation.

  18. Time-resolved positron annihilation spectroscopy study of relaxation dynamics of ion damage in fused quartz

    NASA Astrophysics Data System (ADS)

    Tsuchida, Hidetsugu; Mizuno, Shohei; Tsutsumi, Hironori; Kinomura, Atsushi; Suzuki, Ryoichi; Itoh, Akio

    2016-05-01

    Relaxation dynamics of ion damage in fused quartz is investigated by our newly developed pump–probe technique combining energetic ions (pump) with slow positrons (probe). This method enables determination of time-resolved positron lifetime. We study the time-dependent relaxation of ion damage, by analyzing the intensity variation in the ortho-positronium lifetime component associated with irradiation damage. For irradiation with 160 keV He ions in the temperature range of 300–573 K, the positron annihilation lifetime spectra are obtained as a function of time after ion irradiation. We observe that the relaxation time of ion damage is strongly influenced by specimen temperatures; the relaxation time constant is approximately 400 ms at room temperature (300 K) and becomes smaller with an increasing temperature. Analysis for the effect of temperature on damage accumulation reveals that the activation energy for thermal annealing of the observed damage is approximately 0.1 eV.

  19. Phase behavior of microemulsion systems studied by positron annihilation techniques

    SciTech Connect

    Serrano, J.; Reynoso, R.; Lopez, R.; Olea, O.; Fucugauchi, L.A.

    1983-02-17

    The positron annihilation technique was applied to the study of phase behavior of sodium stearate (or oleate)-alcohol-oil-water microemulsion systems. The positron annihilation parameters revealed a dependence of the water/oil ratio at which microemulsion formation occurs on the hydrocarbon chain length of both alcohol cosurfactant and solvent as well as surfactant concentration. Dynamic laser light scattering has been utilized for substantiating the phase transitions determined in the different microemulsion systems by positron annihilation. The difference in the behavior between saturated and unsaturated surfactants is the most remarkable result of the investigation. Thus, replacing sodium stearate by sodium oleate in the surfactant-1-hexanol-isooctane systems obviated microemulsion formation. This behavior has been rationalized by considering packing and kink presence in microemulsion formation. 26 references.

  20. Temperature Dependence of Positron Annihilation in beta-Cyclodextrin and beta-Cyclodextrin Complexes

    NASA Astrophysics Data System (ADS)

    Hu, Y.; Hsu Hadley, F. H., Jr.; Trinh, T.

    1996-11-01

    The effects of temperature on positron annihilation in beta-cyclodextrin and beta-cyclodextrin complexed with benzyl salicylate, benzyl acetate, ethyl salicylate, geraniol, linalool and nerol were studied. Samples were prepared by slurry, air-dried and freeze-dried methods. Lifetime spectra were measured as a function of temperature for each sample. Comparison of the annihilation rate and intensity of the longer-lived component showed that positronium formation was affected by guest molecules, preparation methods and temperature variations. Results can be used to explain beta-cyclodextrin complex formation with different guest molecules.

  1. Thermal Stability of MgyTi1-y Thin Films Investigated by Positron Annihilation Spectroscopy

    NASA Astrophysics Data System (ADS)

    Anastasopol, A.; Eijt, S. W. H.; Schut, H.; Mulder, F. M.; Plazaola, F.; Dam, B.

    Mg-Ti compounds are attractive candidates as hydrogen storage materials for their fast sorption kinetics and high storage capacity. In this context, an investigation of their thermal stability is of great importance. The thermal stability of MgyTi1-y thin films was investigated using positron annihilation spectroscopy. Despite the positive enthalpy of mixing of Mg and Ti, positron Doppler Broadening of Annihilation Radiation (DBAR) depth profiling showed that Mg0.9Ti0.1 films are stable up to 300°C. However, for Mg0.7Ti0.3 films, segregation of Mg and Ti was observed at 300oC by the appearance of a clear Ti signature in the S-W diagrams and in the Doppler broadening depth profiles analyzed using VEPFIT. The thickness of the 250-300 nm thin films remained unchanged during the heating treatments. We further present ab-initio calculations of positron lifetimes of the corresponding metal and metal hydride phases for comparison to our previous positron annihilation lifetime spectroscopy (PALS) study.

  2. Study of the structure of porous silicon via positron annihilation experiments

    NASA Astrophysics Data System (ADS)

    Biasini, M.; Ferro, G.; Monge, M. A.; Di Francia, G.; La Ferrara, V.

    2000-07-01

    We performed two-dimensional angular correlation of the electron-positron annihilation radiation (2D-ACAR) and positron lifetime measurements on a porous Si sample. From the width of the narrow 2D-ACAR component, attributed to the positronium atom, we estimated the average size of the pores to be ~2.4 nm and did not find evidence of a preferential propagation of the pores. Moreover, by comparing the 2D-ACAR spectrum with that observed for a pure Si crystal, we isolated a further isotropic component attributable to crystal defects of unknown origin.

  3. Coincidence Efficiency of Sodium Iodide Detectors for Positron Annihilation

    NASA Astrophysics Data System (ADS)

    Eckert, Thomas; Vincett, Laurel; Yuly, Mark; Padalino, Stephen; Russ, Megan; Bienstock, Mollie; Simone, Angela; Ellison, Drew; Desmitt, Holly; Sangster, Craig; Regan, Sean

    2014-10-01

    One possible diagnostic technique for characterizing inertial confinement fusion reactions uses tertiary neutron activation of 12C via the 12C(n, 2n)11C reaction. A recent experiment to measure this cross section involved counting the positron annihilation gamma rays from the 11C decay by using sodium iodide detectors in coincidence. To determine the number of 11C decays requires an accurate value for the full-peak coincidence efficiency for the detector system. A new technique has been developed to measure this coincidence efficiency by detecting the positron prior to its annihilation, and vetoing events in which decay gamma rays other than the 511 keV annihilation gamma rays could enter the detectors. Measurements and simulation results for the absolute coincidence total and full-peak efficiencies are presented. Funded in part by a grant from the DOE through the Laboratory for Laser Energetics.

  4. Study of radiation damage in ODS steels by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Bartošová, I.; Bouhaddane, A.; Dománková, M.; Slugeň, V.; Wall, D.; Selim, F. A.

    2016-01-01

    Microstructure of various oxide-dispersion-strengthened (ODS) steels with 15% chromium content was studied in term of vacancy defects presence and their accumulation after defined irradiation treatment, respectively. Studied materials originated from Kyoto University and studied via IAEA collaborative project. Samples were characterized “as received” by positron annihilation lifetime spectroscopy and their microstructure was examined by transmission electron microscopy as well. Samples were afterwards irradiated in Washington State University Nuclear Radiation Center via a strong gamma source (6TBq). Damage induced by gamma irradiation was evaluated by positron lifetime measurements in emphasis on defect accumulation in the materials. We have demonstrated strong defect production induced by gamma irradiation which results from positron measurement data.

  5. Positron annihilation in the hydrogenated granular superconductor of YBa 2Cu 3O 7-δ

    NASA Astrophysics Data System (ADS)

    Lam, C. C.; Shao, L. W.; Jin, X.; Chen, W. M.; Xiong, H. Q.; Feng, J.; Qi, S. R.; Hung, K. C.

    1996-02-01

    In this paper, we have used the sputtering neutral-particles mass spectroscopy (SNMS) and positron-annihilation technique to investigate the effect of hydrogenation on the physical properties of different oxygenated YBa 2Cu 3O 7-δ superconductors. Under the same of hydrogenation treatment, the hydrogenation effects on the superconductors are compared to the non-superconductors. It was shown that the hydrogen concentration in the superconductors is about eight times of the non-superconductor's. It was proven that the long lifetime of positrons in the annihilation process is determined by the variation of the concentrations of monovacancies and microvoids, which takes place in both of intragrain and intergrain samples. The hydrogenation effect can be classified into four stages. At the first stage, the hydrogen atoms fill both monovacancy and microvoid. At the end of the first stage, the long lifetime τ2 reaches the maximum value which is determined by the lifetime of the positron in the monovacancy-free and nearly microvoid-exhausted YBCO sample. In the second stage, the hydrogen charging will lead to creation of new monovacancies; this will make the long lifetime τ2 drop monotonically to its minimum value. In the third stage, further hydrogen charging promotes the formation of microvoids, and leads to an increasing τ2 up to a saturation value, which indicates the equilibrium concentrations of monovacancy and microvoid at that temperature.

  6. Effect of metal ions on positron annihilation characteristics in metal ion containing epoxies

    NASA Technical Reports Server (NTRS)

    Singh, J. J.; St. Clair, A. K.; Stoakley, D. M.; Holt, W. H.; Mock, W., Jr.

    1984-01-01

    In the course of developing improved moisture-resistant epoxy resins, two different types of epoxy resins containing variable mole ratios of chromium ions per polymer repeat unit were developed. Positron annihilation characteristics have been investigated in these resins as a function of their metal ion content. In both cases, the presence of metal ions reduces the lifetime as well as the intensity of the long life component. The long life component intensity reduction is considerably more pronounced than the lifetime reduction. These results have been discussed in terms of increased unpaired electron density at Ps formation sites due to the presence of chromium ions in the matrix.

  7. Theory of Positron Annihilation in Helium-Filled Bubbles in Plutonium

    SciTech Connect

    Sterne, P A; Pask, J E

    2003-02-13

    Positron annihilation lifetime spectroscopy is a sensitive probe of vacancies and voids in materials. This non-destructive measurement technique can identify the presence of specific defects in materials at the part-per-million level. Recent experiments by Asoka-Kumar et al. have identified two lifetime components in aged plutonium samples--a dominant lifetime component of around 182 ps and a longer lifetime component of around 350-400ps. This second component appears to increase with the age of the sample, and accounts for only about 5 percent of the total intensity in 35 year-old plutonium samples. First-principles calculations of positron lifetimes are now used extensively to guide the interpretation of positron lifetime data. At Livermore, we have developed a first-principles finite-element-based method for calculating positron lifetimes for defects in metals. This method is capable of treating system cell sizes of several thousand atoms, allowing us to model defects in plutonium ranging in size from a mono-vacancy to helium-filled bubbles of over 1 nm in diameter. In order to identify the defects that account for the observed lifetime values, we have performed positron lifetime calculations for a set of vacancies, vacancy clusters, and helium-filled vacancy clusters in delta-plutonium. The calculations produced values of 143ps for defect-free delta-Pu and 255ps for a mono-vacancy in Pu, both of which are inconsistent with the dominant experimental lifetime component of 182ps. Larger vacancy clusters have even longer lifetimes. The observed positron lifetime is significantly shorter than the calculated lifetimes for mono-vacancies and larger vacancy clusters, indicating that open vacancy clusters are not the dominant defect in the aged plutonium samples. When helium atoms are introduced into the vacancy cluster, the positron lifetime is reduced due to the increased density of electrons available for annihilation. For a mono-vacancy in Pu containing one helium

  8. When some elementary free volumes in polymers are not seen by positron annihilation experiments

    NASA Astrophysics Data System (ADS)

    Shantarovich, V. P.; Bekeshev, V. G.; Pastukhov, A. V.; Davankov, V. A.; Krasil'nikova, O. K.; Belousova, E. V.; Kevdina, I. B.; Filimonov, M. K.; Gustov, V. W.

    2015-06-01

    Size distributions of elementary free volumes have been studied in mesoporous micro-heterogeneous polymer sorbents. Positron annihilation lifetime spectroscopy (PALS), low temperature gas adsorption (BET) and thermo-stimulated luminescence (TSL) measurements are employed as complementary instruments for the study. It is shown that small admixtures of rubbers are very effective for variations of the pore size distribution. While BET technique was very informative for measurements of mesopores(2-50 nm), positron annihilation was sensitive to micropores(<2 nm), but not for mesopores. The last specificity is explained by the limited positronium diffusion length in a polymer and also by inhomogeneous distribution of mesoporesin heterogeneous systems. TSL measurements gave information on sizes of rubber inclusions in compositions.

  9. Analysis of positron lifetime spectra in polymers

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Mall, Gerald H.; Sprinkle, Danny R.

    1988-01-01

    A new procedure for analyzing multicomponent positron lifetime spectra in polymers was developed. It requires initial estimates of the lifetimes and the intensities of various components, which are readily obtainable by a standard spectrum stripping process. These initial estimates, after convolution with the timing system resolution function, are then used as the inputs for a nonlinear least squares analysis to compute the estimates that conform to a global error minimization criterion. The convolution integral uses the full experimental resolution function, in contrast to the previous studies where analytical approximations of it were utilized. These concepts were incorporated into a generalized Computer Program for Analyzing Positron Lifetime Spectra (PAPLS) in polymers. Its validity was tested using several artificially generated data sets. These data sets were also analyzed using the widely used POSITRONFIT program. In almost all cases, the PAPLS program gives closer fit to the input values. The new procedure was applied to the analysis of several lifetime spectra measured in metal ion containing Epon-828 samples. The results are described.

  10. Positron annihilation study of vacancy-type defects in Al single crystal foils with the tweed structures across the surface

    SciTech Connect

    Kuznetsov, Pavel; Cizek, Jacub Hruska, Petr; Anwad, Wolfgang; Bordulev, Yuri; Lider, Andrei; Laptev, Roman; Mironov, Yuri

    2015-10-27

    The vacancy-type defects in the aluminum single crystal foils after a series of the cyclic tensions were studied using positron annihilation. Two components were identified in the positron lifetime spectra associated with the annihilation of free positrons and positrons trapped by dislocations. With increasing number of cycles the dislocation density firstly increases and reaches a maximum value at N = 10 000 cycles but then it gradually decreases and at N = 70 000 cycles falls down to the level typical for the virgin samples. The direct evidence on the formation of a two-phase system “defective near-surface layer/base Al crystal” in aluminum foils at cyclic tension was obtained using a positron beam with the variable energy.

  11. Gamma-induced positron annihilation spectroscopy and application to radiation-damaged alloys

    NASA Astrophysics Data System (ADS)

    Wells, D. P.; Hunt, A. W.; Tchelidze, L.; Kumar, J.; Smith, K.; Thompson, S.; Selim, F.; Williams, J.; Harmon, J. F.; Maloy, S.; Roy, A.

    2006-06-01

    Radiation damage and other defect studies of materials are limited to thin samples because of inherent limitations of well-established techniques such as diffraction methods and traditional positron annihilation spectroscopy (PAS) [P. Hautojarvi, et al., Positrons in Solids, Springer, Berlin, 1979, K.G. Lynn, et al., Appl. Phys. Lett. 47 (1985) 239]. This limitation has greatly hampered industrial and in-situ applications. ISU has developed new methods that use pair-production to produce positrons throughout the volume of thick samples [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427, F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D.P. Wells, et al., Nucl. Instr. and Meth. B. 241 (2005) 262]. Unlike prior work at other laboratories that use bremsstrahlung beams to create positron beams (via pair-production) that are then directed at a sample of interest, we produce electron-positron pairs directly in samples of interest, and eliminate the intermediate step of a positron beam and its attendant penetrability limitations. Our methods include accelerator-based bremsstrahlung-induced pair-production in the sample for positron annihilation energy spectroscopy measurements (PAES), coincident proton-capture gamma-rays (where one of the gammas is used for pair-production in the sample) for positron annihilation lifetime spectroscopy (PALS), or photo-nuclear activation of samples for either type of measurement. The positrons subsequently annihilate with sample electrons, emitting coincident 511 keV gamma-rays [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427, F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D

  12. A modified positron lifetime spectrometer as method of non-destructive testing in materials

    NASA Astrophysics Data System (ADS)

    Chen, Z. Q.; Shi, J. J.; Jiang, J.; Liu, X. B.; Wang, R. S.; Wu, Y. C.

    2015-02-01

    This paper aims to develop a new non-destructive testing (NDT) method using positron annihilation spectroscopy, a powerful tool to detect vacancy-type defects and defect's chemical environment. A positron NDT system was designed and constructed by modifying the "sandwich" structure of sample-source-sample in the conventional positron lifetime spectrometer. The positron lifetime spectra of one single sample can be measured and analyzed by subtracting the contribution of a reference sample. The feasibility and reliability of the positron NDT system have been tested by analyzing nondestructively deformation damage caused by mechanical treatment in metals and steels. This system can be used for detecting defects and damage in thick or large-size samples without cutting off the sample materials, as well as for detecting two-dimensional distribution of defects.

  13. A study of vacancy defects related to gray tracks in KTiOPO{sub 4} (KTP) using positron annihilation

    SciTech Connect

    Zhang, Yang; Li, Jing; Wang, Jiyang Jiang, Huaidong; Cao, Xingzhong; Yang, Jing

    2014-12-15

    For the first time to our knowledge, positron annihilation spectroscopy (PAS) was used to study vacancy defects in KTiOPO{sub 4} (KTP) single crystals. Positron annihilation lifetime spectroscopy combined with dielectric measurements identified the existence of oxygen vacancies and reflected the concentration of vacancy defects in three samples. The vacancy defects in KTP do not consist of monovacancies, but rather vacancy complexes. Doppler broadening indicates that the vacancy defects are distributed uniformly. A relationship is established where a crystal with a low oxygen vacancy concentration and a highly balanced stoichiometry has a higher resistance to gray track formation.

  14. A study of vacancy defects related to gray tracks in KTiOPO4 (KTP) using positron annihilation

    NASA Astrophysics Data System (ADS)

    Zhang, Yang; Li, Jing; Cao, Xingzhong; Yang, Jing; Wang, Jiyang; Jiang, Huaidong

    2014-12-01

    For the first time to our knowledge, positron annihilation spectroscopy (PAS) was used to study vacancy defects in KTiOPO4 (KTP) single crystals. Positron annihilation lifetime spectroscopy combined with dielectric measurements identified the existence of oxygen vacancies and reflected the concentration of vacancy defects in three samples. The vacancy defects in KTP do not consist of monovacancies, but rather vacancy complexes. Doppler broadening indicates that the vacancy defects are distributed uniformly. A relationship is established where a crystal with a low oxygen vacancy concentration and a highly balanced stoichiometry has a higher resistance to gray track formation.

  15. Positron annihilation study of aluminum, titanium, and iron alloys surface after shot peening

    NASA Astrophysics Data System (ADS)

    Zaleski, R.; Zaleski, K.; Gorgol, M.; Wiertel, M.

    2015-08-01

    Shot peening influence on alloys based on iron, aluminum, and titanium was studied using positron annihilation lifetime spectroscopy (PALS) and residual stress measurements. The PALS spectra were analyzed assuming two lifetime components. While the residual stresses change in a similar way in all the samples, the PALS results show an opposite tendency of a component relative intensities change with the time of shot peening for the Ti alloy as compared to steel or the Al alloy. A comparison between the depth profiles of positron implantation and the residual stress distribution reveals that the positron range covers a whole depth where residual stress is observed only in the Ti alloy. Based on this observation, the evolution of the defect concentration is presumed, consisting in migration of large defects away from the surface, while only smaller ones remain close to the surface. Furthermore, the positron lifetime distribution in the Al alloy was determined using the MELT program. The results showed that the initial single, wide distribution of lifetime splits into two narrower ones with increasing shot peening time.

  16. Positron annihilation spectroscopy of biological tissue in 11C irradiation

    NASA Astrophysics Data System (ADS)

    Sakurai, Hiroshi; Itoh, Fumitake; Hirano, Yoshiyuki; Nitta, Munetaka; Suzuki, Kosuke; Kato, Daisuke; Yoshida, Eiji; Nishikido, Fumihiko; Wakizaka, Hidekatsu; Kanai, Tatsuaki; Yamaya, Taiga

    2014-11-01

    Positron annihilation spectroscopy (PAS) spectra of biological tissue in 11C irradiation are reported and spatial resolution coefficient of positron emission tomography (PET) obtained from the PAS spectrum is discussed for 11C irradiation. A PAS spectrum of the biological tissue with water is the same as that of the water pool phantom in 11C irradiation. However, a PAS spectrum of the biological tissue with less water differs from that of the water pool phantom. The PET spatial resolution coefficient depends on the kind of biological tissue. However, the PET spatial resolution coefficient, 0.00243  ±  0.00014, can be used as a common value of maximum limit.

  17. Vacancy-type defects and electronic structure of perovskite-oxide SrTiO3 from positron annihilation

    NASA Astrophysics Data System (ADS)

    Hamid, A. S.; Uedono, A.; Chikyow, T.; Uwe, K.; Mochizuki, K.; Kawaminami, S.

    2006-02-01

    The vacancy-type defects in Nb-doped SrTiO3 and in undoped SrTiO3, annealed in H2 flow, were investigated by means of positron lifetime and 2D angular correlation of annihilation radiation (ACAR) experiments. The calculations of the lifetime of positron were performed by using atomic superposition (AT-SUP) method. The results showed that positrons annihilate from a free state in the Nb-doped SrTiO3. The trapping centers in the annealed sample were found to be oxygen vacancies VO associated with relaxation of the surrounding ions. Moreover, the momentum distributions of the samples studied were correlated to the variation of their electronic structure. It was proposed from the drastic change in the momentum distribution upon introduction of VO, that 2D-ACAR technique is a sensitive tool for acquiring information on the electronic and bond structure of the perovskite-oxides.

  18. Positron annihilation study of neutron irradiated model alloys and of a reactor pressure vessel steel

    NASA Astrophysics Data System (ADS)

    Lambrecht, M.; Almazouzi, A.

    2009-03-01

    The hardening and embrittlement of reactor pressure vessel steels are of great concern in the actual nuclear power plant life assessment. This embrittlement is caused by irradiation-induced damage, and positron annihilation spectroscopy has been shown to be a suitable method for analysing most of these defects. In this paper, this technique (both positron annihilation lifetime spectroscopy and coincidence Doppler broadening) has been used to investigate neutron irradiated model alloys, with increasing chemical complexity and a reactor pressure vessel steel. It is found that the clustering of copper takes place at the very early stages of irradiation using coincidence Doppler broadening, when this element is present in the alloy. On the other hand, considerations based on positron annihilation spectroscopy analyses suggest that the main objects causing hardening are most probably self-interstitial clusters decorated with manganese in Cu-free alloys. In low-Cu reactor pressure vessel steels and in (Fe, Mn, Ni, Cu) alloys, the main effect is still due to Cu-rich precipitates at low doses, but the role of manganese-related features becomes pre-dominant at high doses.

  19. Calculation of positron annihilation characteristics of six main defects in 6 H -SiC and the possibility to distinguish them experimentally

    NASA Astrophysics Data System (ADS)

    Linez, F.; Makkonen, I.; Tuomisto, F.

    2016-07-01

    We have determined positron annihilation characteristics (lifetime and Doppler broadening) in six basic vacancy-type defects of 6 H -SiC and two nitrogen-vacancy complexes using ab initio calculations. The positron characteristics obtained allow us to point out which positron technique in the most adapted to identify a particular defect. They show that the coincidence Doppler broadening technique is the most relevant for observing the silicon vacancy-nitrogen complexes, VSiNC , and carbon vacancy-carbon antisite ones, VCCSi . For the other studied defects, the calculated positron characteristics are found to be too close for the defects to be easily distinguished using a single positron annihilation technique. Then it is required to use complementary techniques, positron annihilation based or other.

  20. Defect structures of F82H irradiated at SINQ using positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Sato, K.; Ikemura, K.; Krsjak, V.; Vieh, C.; Brun, R.; Xu, Q.; Yoshiie, T.; Dai, Y.

    2016-01-01

    The growth process of He-filled vacancy clusters during annealing was investigated with positron annihilation lifetime spectroscopy and coincidence Doppler broadening (CDB) measurements. The reduced activation ferritic/martensitic steel F82H was irradiated with high-energy protons and spallation neutrons. The He-filled vacancy clusters absorbed more He atoms when annealed below 673 K, and the long and mean positron lifetimes decreased. When annealed above 873 K, the vacancies (V)-Hem or Vn-Hem complexes dissociated (n and m are the number of vacancies and He atoms, respectively). The He-filled vacancy clusters then absorb these dissociated vacancies and He atoms. Therefore, the size of the He-filled vacancy clusters increased, and the He-to-vacancy ratio decreased. These annealing-induced phenomena increased the long positron lifetime in addition to the higher positron trapping rates of the He-filled vacancy clusters. By comparing electron-irradiated samples that did not contain He atoms to the proton- and neutron-irradiated samples containing He atoms, the effects of He atoms on the CDB ratio curves were studied. The results agreed with the previous study of He-ion-implanted Fe-Cr alloys.

  1. Moisture determination in composite materials using positron lifetime techniques

    NASA Technical Reports Server (NTRS)

    Singh, J. J.; Holt, W. R.; Mock, W., Jr.

    1980-01-01

    A technique was developed which has the potential of providing information on the moisture content as well as its depth in the specimen. This technique was based on the dependence of positron lifetime on the moisture content of the composite specimen. The positron lifetime technique of moisture determination and the results of the initial studies are described.

  2. Positron-annihilation spectroscopy of defects in metals: an assessment

    SciTech Connect

    Siegel, R.W.

    1982-06-01

    Positron annihilation spectroscopy (PAS) has made significant contributions to our knowledge regarding lattice defects in metals in two areas: (i) the determination of atomic defect properties, particularly those of monovacancies, and (ii) the monitoring and characterization of vacancy-like microstructure development during post-irradiation of post-quench annealing. The application of PAS to the study of defects in metals is selectively reviewed and critically assessed within the context of other available techniques for such investigations. Possibilities for using the positron as a localized probe of the structure of atomic defects are discussed. Finally, the present status and future potential of PAS as a tool for the study of defects in metals are considered relative to other available techniques. 92 references, 20 figures.

  3. Electron momentum distribution in amorphous metals investigated by positron annihilation

    NASA Astrophysics Data System (ADS)

    Kristiakova, K.; Kristiak, J.; Galan, P.

    Electron momentum distribution in amorphous metals and the crystalline form of Ni xFe 80- xB 20 ( x = 10, 20, 30, 40) were investigated by positron annihilation. The samples were 30 mn thick ribbons, produced by rapid quenching of liquid metal on a rotating Cu wheel. The positron source was 22Na on mylar or blotting paper between two samples in sandwich arrangement. The γ-radiation was detected by a Ge(Li) detector which has a resolution at 511.9 keV ( 106Ru) of 1.6 keV. An unfolding method based on Bayes principle was applied to calculate the Doppler-broadening; S-parameter values were also determined. The calculated momentum distribution revealed a difference for the Ni 30Fe 50B 20 sample.

  4. Development of positron annihilation spectroscopy for characterizing neutron irradiated tungsten

    SciTech Connect

    C.N. Taylor; M. Shimada; D.W. Akers; M.W. Drigert; B.J. Merrill; Y. Hatano

    2013-05-01

    Tungsten samples (6 mm diameter, 0.2 mm thick) were irradiated to 0.025 and 0.3 dpa with neutrons in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Samples were then exposed to deuterium plasma in the tritium plasma experiment (TPE) at 100, 200 and 500ºC to a total fluence of 1 x 1026 m-2. Nuclear reaction analysis (NRA) and Doppler broadening positron annihilation spectroscopy (DB-PAS) were performed at various stages to characterize damage and retention. We present the first known results of neutron damaged tungsten characterized by DB-PAS in order to study defect concentration. Two positron sources, 22Na and 68Ge, probe ~58 µm and through the entire 200 µm thick samples, respectively. DB-PAS results reveal clear differences between the various irradiated samples. These results, and the calibration of DB-PAS to NRA data are presented.

  5. Positron annihilation study of Fe-ion irradiated reactor pressure vessel model alloys

    NASA Astrophysics Data System (ADS)

    Chen, L.; Li, Z. C.; Schut, H.; Sekimura, N.

    2016-01-01

    The degradation of reactor pressure vessel steels under irradiation, which results from the hardening and embrittlement caused by a high number density of nanometer scale damage, is of increasingly crucial concern for safe nuclear power plant operation and possible reactor lifetime prolongation. In this paper, the radiation damage in model alloys with increasing chemical complexity (Fe, Fe-Cu, Fe-Cu-Si, Fe-Cu-Ni and Fe-Cu-Ni-Mn) has been studied by Positron Annihilation Doppler Broadening spectroscopy after 1.5 MeV Fe-ion implantation at room temperature or high temperature (290 oC). It is found that the room temperature irradiation generally leads to the formation of vacancy-type defects in the Fe matrix. The high temperature irradiation exhibits an additional annealing effect for the radiation damage. Besides the Cu-rich clusters observed by the positron probe, the results show formation of vacancy-Mn complexes for implantation at low temperatures.

  6. Investigation Of Helium Implanted Fe-Cr Alloys By Means Of X-Ray Diffraction And Positron Annihilation Spectroscopy

    NASA Astrophysics Data System (ADS)

    Novák, Patrik; Gokhman, Aleksandr; Dobročka, Edmund; Bokor, Jozef; Pecko, Stanislav

    2015-11-01

    X-ray diffraction (XRD) and positron annihilation spectroscopy (PAS) have been used for the characterization of the two binary alloys Fe-Cr with Cr content 2.36 and 8.39 wt%. The influence of ion implantation on these alloys was studied. Different implantation doses of helium, up to 0.5 C/cm2, were used to simulate neutron-induced damage in a sub-surface region. To characterize the damage, a lattice parameter, coherent domain size, residual stress and a crystallographic texture have been studied by grazing incidence X-ray diffraction (GIXRD). It was found out that these parameters showed a similar dependence on the implantation dose as the positron lifetime determined by positron annihilation spectroscopy.

  7. Positron states and annihilation in nanometric semiconducting superlattices

    NASA Astrophysics Data System (ADS)

    Sekkal, Nadir; Arutyunov, N. Yu.

    2009-12-01

    The electron and positron states in the band structure of GaAs-AlAs, (GaAs) m((AlAs) 1-x(vacancy/pore) x) n and (GaAs) m((AlAs) 1-xGe x) n superlattices have been analyzed on the basis of empirical pseudopotential method of calculations. When possible, the validity of results has been checked by comparing them with the data obtained for relevant constituents of the superlattices with the help of the angular correlation of annihilation radiation (ACAR) spectroscopy. The most important finding is that positron is capable of being selectively confined in the superlattices even when they are defect-free. In the presence of defects of a vacancy-type, the regime of confinement may be changed to the one of trapping of positron. Being “fingerprints” of certain types of defects, the ACAR spectra may be used for non-destructive characterization of superlattices. The question of sensitivity of the positron particle microprobe for studying electron structure of superlattices is discussed.

  8. Atomic Ionization at the Positron-Electron Annihilation in Process of β+-DECAY

    NASA Astrophysics Data System (ADS)

    Fedotkin, S. N.; Zheltonozhskii, V. A.

    2013-03-01

    The process of atomic shell ionization during the annihilation of a positron, emitted at β+- decay with K-electron of daughter's atom, is considered. The ratio of probabilities of these processes to the probability of ordinary β+-decay is found. It excitation of atomic shell at the positron annihilation at β+-decay of was studied the 4522Ti.

  9. Positron and gamma-ray signatures of dark matter annihilation and big-bang nucleosynthesis

    SciTech Connect

    Hisano, Junji; Kawasaki, Masahiro; Kohri, Kazunori; Nakayama, Kazunori

    2009-03-15

    The positron excess observed by the PAMELA experiment may come from dark matter annihilation, if the annihilation cross section is large enough. We show that the dark matter annihilation scenarios to explain the positron excess may also be compatible with the discrepancy of the cosmic lithium abundances between theory and observations. The winolike neutralino in the supersymmetric standard model is a good example for it. This scenario may be confirmed by Fermi satellite experiments.

  10. Positron-annihilation study of the equilibrium vacancy ensemble in aluminum

    SciTech Connect

    Fluss, M.J.; Berko, S.; Chakraborty, B.; Hoffmann, K.; Lippel, P.; Siegel, R.W.

    1982-06-01

    A preliminary report is presented of a positron-annihilation study of the equilibrium vacancy ensemble in aluminum using one- and two-dimensional angular correlation of annihilation radiation (ACAR) measurements versus temperature. The annihilation characteristics of a positron from the Bloch state, and the monovancy- and divacancy-trapped states have been calculated self-consistently within a supercell, including many-body enhancement effects, and are compared with experiment. 4 figures.

  11. Determination and applications of enhancement factors for positron and ortho-positronium annihilations

    NASA Astrophysics Data System (ADS)

    Mitroy, J.

    2005-12-01

    Electron-positron annihilation rates calculated directly from the electron and positron densities are known to underestimate the true annihilation rate. A correction factor, known as the enhancement factor, allows for the local increase of the electron density around the positron caused by the attractive electron-positron interaction. Enhancement factors are given for positrons annihilating with the 1s electron in H, He+ , He, Li2+ , and Li+ . The enhancement factor for a free positron annihilating with He+ and He is found to be close to that of ortho-positronium (i.e., Ps in its triplet state) annihilating with these atoms. The enhancement factor for Ps-He scattering is used in conjunction with the known annihilation rate for pickoff annihilation to derive a scattering length of 1.47a0 for Ps-He scattering. Further, enhancement factors for e+ -Ne and e+ -Ar annihilation are used in conjunction with the pickoff annihilation rate to estimate scattering lengths of 1.46a0 for Ps-Ne scattering and 1.75a0 for Ps-Ar scattering.

  12. Determination and applications of enhancement factors for positron and ortho-positronium annihilations

    SciTech Connect

    Mitroy, J.

    2005-12-15

    Electron-positron annihilation rates calculated directly from the electron and positron densities are known to underestimate the true annihilation rate. A correction factor, known as the enhancement factor, allows for the local increase of the electron density around the positron caused by the attractive electron-positron interaction. Enhancement factors are given for positrons annihilating with the 1s electron in H, He{sup +}, He, Li{sup 2+}, and Li{sup +}. The enhancement factor for a free positron annihilating with He{sup +} and He is found to be close to that of ortho-positronium (i.e., Ps in its triplet state) annihilating with these atoms. The enhancement factor for Ps-He scattering is used in conjunction with the known annihilation rate for pickoff annihilation to derive a scattering length of 1.47a{sub 0} for Ps-He scattering. Further, enhancement factors for e{sup +}-Ne and e{sup +}-Ar annihilation are used in conjunction with the pickoff annihilation rate to estimate scattering lengths of 1.46a{sub 0} for Ps-Ne scattering and 1.75a{sub 0} for Ps-Ar scattering.

  13. Positron Annihilation Spectroscopy as a Probe of Microscopic Structure and Physical Aging in Polymer.

    NASA Astrophysics Data System (ADS)

    Yu, Minzi

    Positron annihilation is studied as a characterization method for the properties of polymers. Previous studies indicate that the ortho-positronium lifetime tau _3 and intensity I_3 is correlated to the free volume "hole" size and number density of holes in a polymer. Positron annihilation lifetime (PAL) studies in polymers measure the change in free volume, and they are sensitive to different physical environments. PAL studies of the temperature dependence of a bisphenol-A polycarbonate shows that the free volume increases with increasing temperature, and it also obtains the transition temperatures T_{rm g} and T_beta^', from the tau_3 curve and the I_3 curve, respectively. The isothermal aging in polycarbonate shows that: I_3 decreases while tau_3 remains constant during a long-time annealing at a temperature far below T_{rm g}; and I_3 remains constant while tau_3 goes through a "over shooting" in the first few hours after quenching and annealing at a temperature just below T_{rm g}. The free volume in polycarbonate increases (as a result of an increase in tau_3 ) with applied tensile strain up to 4%, then levels off. Similarly, the free volume in polymethyl methacrylate (PMMA) decreases (as the result of tau_3 ) with applied compressional strain also up to -4% then levels off. A negative change in both tau_3 and I _3 has been observed when polycarbonate is under 3% tensile strain and after release of strain. A more advance technique of positron annihilation, PAL-momentum correlation which can give more detailed information about free volume structure in polymers, has also been studied and improved. Two 5-cm-diameter, 5-cm-long CsF scintillation detectors for lifetime measurement, and a 30-cm-diameter Anger camera whose y-analog pulse gives one-dimensional ACAR information, comprise a new experimental arrangement of PAL-momentum correlation system. Its triple -coincidence counting rate is about 2.5 per minute per microcurie of positron source and system time

  14. Positron annihilation studies of 4-n-butyl-4'-isothiocyanato-1,1'-biphenyl.

    PubMed

    Dryzek, E; Juszyńska, E; Zaleski, R; Jasińska, B; Gorgol, M; Massalska-Arodź, M

    2013-08-01

    Positron annihilation lifetime spectroscopy (PALS) measurements were performed between 93 and 293 K in order to study the supercooled smectic-E (Sm-E) phase of 4-n-butyl-4'-isothiocyanato-1,1'-biphenyl (4TCB), the ordered molecular crystal of 4TCB, and the phase transition between the Sm-E phase and the ordered molecular crystal of 4TCB. The phase transition was well reflected in the abrupt increase of the ortho-positronium (o-Ps) lifetime and intensity. The value of the o-Ps lifetime in the Sm-E liquid crystalline phase of 4TCB, i.e., 2.21 ns at room temperature, was explained by the formation of bubbles induced by Ps atoms, which are created due to a liquidlike state of the butyl chains of 4TCB molecules in the Sm-E phase. The temperature dependence of the o-Ps intensity for the supercooled Sm-E phase can be explained by thermal generation of sites where bubbles are formed; an activation energy equal to 0.30±0.02 eV was estimated. This value was compared with the activation energies of molecular motions. The o-Ps lifetime in the ordered molecular crystal was interpreted as originating from the annihilation of o-Ps confined in molecular vacancy-type imperfections in the crystal lattice. The value of the o-Ps pickoff annihilation between 1.8 and 1.9 ns is in accordance with the size of the molecular vacancy for the 4TCB crystal lattice. Its intensity is lower than 5%. The isothermal crystallization of the 4TCB Sm-E phase was observed by PALS. The low-dimensional crystal growth was concluded from the Avrami equation fitted to the time dependence of the o-Ps intensity, which resulted in an Avrami exponent equal to 1.73. PMID:24032853

  15. On the potential of positron lifetime spectroscopy for the study of early stages of zeolites formation from their amorphous precursors

    NASA Astrophysics Data System (ADS)

    Bosnar, S.; Kosanović, C.; Subotić, B.; Bosnar, D.; Kajcsos, Zs.; Liszkay, L.; Lohonyai, L.; Molnár, B.; Lázár, K.

    2007-02-01

    The applicability of positron lifetime (LT) spectroscopy to the study of progress of formation of Secondary Building Units (SBU) in gels yielding in FAU and LTA type zeolites was investigated. Samples were prepared from aluminosilicate gels with various degrees of local structural order. LT measurements were performed at room temperature in air and in vacuum. Coexistence of annihilation modi with long lifetime components was shown; a correlation with precursors of nucleation and type of exchanged ions was also indicated.

  16. Fragmentation production of charmed hadrons in electron-positron annihilation

    SciTech Connect

    Novoselov, A. A.

    2010-10-15

    Processes involving the production of D* mesons and {Lambda}{sub c} baryons in electron-positron annihilation at the energies of 10.58 and 91.18 GeV are considered. At the energy of 10.58 GeV, the production of pairs of B mesons that is followed by their decay to charmed particles is analyzed along with direct charm production. The violation of scaling in the respective fragmentation functions is taken into account in the next-to-leading-logarithmic approximation of perturbative QCD. The required nonperturbative fragmentation functions are extracted numerically from experimental data obtained at B factories and are approximated by simple analytic expressions. It is shown that the difference in the nonperturbative fragmentation functions for transitions to mesons and baryons can readily be explained on the basis of the quark-counting rules.

  17. Positron annihilation studies of vacancy related defects in ceramic and thin film Pb(Zr,Ti)O{sub 3} materials

    SciTech Connect

    Keeble, D.J.; Krishnan, A.; Umlor, M.T.; Lynn, K.G.; Warren, W.L.; Dimos, D.; Tuttle, B.A.; Ramesh, R.; Poindexter, E.H.

    1994-07-01

    Preliminary positron annihilation studies of ceramic and thin film Pb(Zr,Ti)O{sub 3} (PZT) materials have been completed. This paper examines effects of processing conditions on vacancy related defects. Positron lifetime measurements on bulk PLZT plates showed an increase in positron trapping to a defect state with increasing grain size consistent with trapping to lead vacancy related defects formed through lead oxide loss during processing. Variable energy positron beam measurements were completed on bulk PLZT plates, sol-gel PZT thin films and laser ablated PLZT thin films. Films processed in a reduced oxygen atmosphere were found to give a higher S-parameter, due to an increase in concentration of neutral or negatively charged vacancy type defects, compared with material processed in an oxidizing ambient.

  18. Positron annihilation in AlN and GaN

    NASA Astrophysics Data System (ADS)

    Arutyunov, N. Yu.; Emtsev, V. V.; Mikhailin, A. V.; Davidov, V. Yu.

    2001-12-01

    The measurements of one-dimensional angular correlation of the annihilation radiation (1D-ACAR) have been carried out for AlN and GaN as well as for some related materials (Al, Ga, GaP, GaAs, GaSb) which have been used as samples of references the analysis of results. The numeral values of characteristic length of radius of spherical volume to be occupied by annihilating electron ( rs‧) have differed significantly from the corresponding values ( rs) calculated by the conventional independent-particle-model (IPM) for ideal Fermi-gas: rs‧ (AlN)≃1.28 rs, where rs (AlN)≃1.61 a.u., and rs‧ (GaN)≃1.66 rs, where rs (GaN)≃1.64 a.u. The electron-positron “ion radii” reconstructed by the high-momentum components (HMC) of 1D-ACAR for Al 3+, Ga 3+ cores as well as numeral rs‧ values provide some reasons to believe that Ga- and Al-vacancies and their impurity complexes are effective centers of the positron localization in AlN and GaN; it is assumed that these complexes include V Ga, V Al, and N atom (V Ga-N Ga in GaN and V Al-N Al in AlN) where the nitrogen atom is likely to be in the configuration of substitution (anti-site), N +Ga and N +Al, respectively.

  19. Development of positron annihilation spectroscopy for characterizing neutron irradiated tungsten

    NASA Astrophysics Data System (ADS)

    Taylor, C. N.; Shimada, M.; Merrill, B. J.; Drigert, M. W.; Akers, D. W.; Hatano, Y.

    2014-04-01

    Tungsten samples (6 mm diameter and 0.2 mm thick) were irradiated to 0.025 and 0.3 dpa with neutrons in the High Flux Isotope Reactor at Oak Ridge National Laboratory as part of the US/Japan Tritium, Irradiation and Thermofluids for America and Nippon (TITAN) collaboration. Samples were then exposed to deuterium plasma in Idaho National Laboratory's Tritium Plasma Experiment at 100, 200 and 500 °C to a total fluence of 1 × 1026 m-2. Nuclear reaction analysis (NRA) and Doppler broadening positron annihilation spectroscopy (DB-PAS) were performed at various stages to characterize radiation damage and retention. We present the first results of neutron irradiated tungsten characterized by DB-PAS in order to study defect concentration. Two positron sources, 22Na and 68Ge, probe ˜58 μm and through the entire 200 μm thick samples, respectively. DB-PAS results reveal clear differences between the various irradiated samples. These results, and a correlation between DB-PAS and NRA data, are presented.

  20. Positron lifetime studies in thermoplastic polyimide test specimens

    NASA Technical Reports Server (NTRS)

    Singh, J. J.; Stclair, T. L.; Holt, W. H.; Mock, W., Jr.

    1982-01-01

    Positron lifetime measurements were made in two thermoplastic polyimide materials recently developed at Langley. The long component lifetime values in polyimidesulfone samples are 847 + or - 81 Ps (dry) and 764 + or - 91 Ps (saturated). The corresponding values in LARC thermoplastic imides are 1080 + or - 139 Ps (dry) and 711 + or - 96 Ps (saturated). Clearly, the presence of moisture has greater effect on positron lifetime in LARC thermoplastic imides than in the case of polyimidesulfones. This result is consistent with the photomicrographic observations made on frozen water saturated specimens of these materials.

  1. Positron annihilation studies in binary solid solutions and mechanical mixtures of lanthanide dipivaloylmethanate complexes

    NASA Astrophysics Data System (ADS)

    Fulgêncio, F.; Oliveira, F. C.; Windmöller, D.; Araujo, M. H.; Marques-Netto, A.; Machado, J. C.; Magalhães, W. F.

    2015-11-01

    Measurements using positron annihilation lifetime (PALS) and Doppler broadening annihilation radiation lineshape (DBARLS) spectroscopies were performed in several lanthanide dipivaloylmethanate complexes, Ln(dpm)3 where Ln = Sm3+, Gd3+, Tb3+, Ho3+, Er3+, Yb3+ and dpm = 2,2,6,6-tetramethyl-3,5-pentanedionate, and also on their binary solid solutions and mechanical mixtures, biphasic systems, of the general formula Ln1-xEux(dpm)3. Expressive positronium formation was observed in all Ln(dpm)3 complexes, except in Eu(dpm)3 complex. The results indicate formation of solid solutions in the Sm3+, Gd3+and Tb3+ systems, where total inhibition of positronium formation was observed. A Stern-Volmer type equation, I30/I3 = 1 + kx, was used to fit the data, enabling the calculation of the inhibition constants, k. A mechanical mixture behavior, with linear variation of I3 between the I3 values of the pure complexes, was observed in systems containing Ho3+, Er3+ and Yb3+ complexes, where no effective solid solution formation occurred due to differences between the crystalline structures of these complexes and Eu(dpm)3. No positronium quenching reactions were observed in the solid solutions. DBARLS results confirmed those of PALS, evidencing that the positron annihilation spectroscopies are useful techniques to characterize the formation of solid solutions. PALS measurements at 80 K were performed in the Sm1-xEux(dpm)3 and Gd1-xEux(dpm)3 solid solutions. The results indicate that, despite a contraction in the crystalline structures, the solid solution structure remains intact at low temperatures. The temperature dependence of the inhibition constant do not seem to be understood from the positronium formation spur model and might be related to intra and intermolecular energy and charge transfer processes in the solid solutions, which is currently being studied.

  2. Positron Annihilation 3-D Momentum Spectrometry by Synchronous 2D-ACAR and DBAR

    NASA Astrophysics Data System (ADS)

    Burggraf, Larry W.; Bonavita, Angelo M.; Williams, Christopher S.; Fagan-Kelly, Stefan B.; Jimenez, Stephen M.

    2015-05-01

    A positron annihilation spectroscopy system capable of determining 3D electron-positron (e--e+) momentum densities has been constructed and tested. In this technique two opposed HPGe strip detectors measure angular coincidence of annihilation radiation (ACAR) and Doppler broadening of annihilation radiation (DBAR) in coincidence to produce 3D momentum datasets in which the parallel momentum component obtained from the DBAR measurement can be selected for annihilation events that possess a particular perpendicular momentum component observed in the 2D ACAR spectrum. A true 3D momentum distribution can also be produced. Measurement of 3-D momentum spectra in oxide materials has been demonstrated including O-atom defects in 6H SiC and silver atom substitution in lithium tetraborate crystals. Integration of the 3-D momentum spectrometer with a slow positron beam for future surface resonant annihilation spectrometry measurements will be described. Sponsorship from Air Force Office of Scientific Research

  3. Positron line radiation from halo WIMP annihilations as a dark matter signature

    NASA Technical Reports Server (NTRS)

    Turner, Michael S.; Wilczek, Frank

    1989-01-01

    We suggest a new signature for dark matter annihilation in the halo: high energy positron line radiation. Because the cosmic ray positron spectrum falls rapidly with energy, e+'s from halo WIMP annihilations can be a significant, clean signal for very massive WIMP's (approx. greater than 30 GeV). In the case that the e+e- annihilation channel has an appreciable branch, the e+ signal should be above background in a future detector, such as have been proposed for ASTROMAG, and of potential importance as a dark matter signature. A significant e+e- branching ratio can occur for neutralinos or Dirac neutrinos. High-energy, continuum positron radiation may also be an important signature for massive neutralino annihilations, especially near or above the threshold of the W+W- and ZoZo annihilation channels.

  4. Relative Defect Density Measurements of Laser Shock Peened 316L Stainless Steel Using Positron Annihilation Spectroscopy

    SciTech Connect

    Marcus A. Gagliardi; Bulent H. Sencer; A. W. Hunt; Stuart A. Maloy; George T. Gray III

    2011-12-01

    The surface of an annealed 316L stainless steel coupon was laser shock peened and Vickers hardness measurements were subsequently taken of its surface. This Vickers hardness data was compared with measurements taken using the technique of positron annihilation Doppler broadening spectroscopy. When compared, a correlation was found between the Vickers hardness data measurements and those made using Doppler broadening spectroscopy. Although materials with a high defect density can cause the S-parameter measurements to saturate, variations in the Sparameter measurements suggest that through further research the Doppler broadening technique could be used as a viable alternative to measuring a material's hardness. In turn, this technique, could be useful in industrial settings where surface hardness and surface defects are used to predict lifetime of components.

  5. Neutrino emissivity from electron-positron annihilation in hot matter in a strong magnetic field

    SciTech Connect

    Amsterdamski, P.; Haensel, P. )

    1990-10-15

    The neutrino emissivity due to electron-positron annihilation in a strong magnetic field is computed. A strong magnetic field can significantly increase the neutrino emissivity at {ital T}{similar to}10{sup 9} K.

  6. Synthesis and characterization of highly conductive charge-transfer complexes using positron annihilation spectroscopy.

    PubMed

    Adam, Abdel Majid A; Refat, Moamen S; Sharshar, T; Heiba, Z K

    2012-09-01

    Molecular charge-transfer complexes of the tetramethylethylenediamine (TMEDA) with picric acid (Pi-OH), benzene-1,4-diol (QL), tin(IV) tetrachloride (SnCl(4)), iodine, bromine, and zinc chloride (ZnCl(2)) have been synthesized and investigated by elemental and thermal analysis, electronic, infrared, Raman and proton-NMR, energy-dispersive X-ray spectroscopy, X-ray powder diffraction and positron annihilation lifetime spectroscopy, and scanning electron microscopy. In this work, three types of acceptors π-acceptors (Pi-OH and QL), σ-acceptors (iodine and bromine), and vacant orbital acceptors (SnCl(4) and ZnCl(2)) were covered. The results of elemental analysis indicated that the CT complexes were formed with ratios 1:1 and 1:2 for QL, SnCl(4), and ZnCl(2) acceptors and iodine, Pi-OH, and Br(2) acceptors, respectively. The type of chelating between the TMEDA donor and the mentioned acceptors depends upon the behavior of both items. The positron annihilation lifetime parameters were found to be dependent on the structure, electronic configuration, and the power of acceptors. The correlation between these parameters and the molecular weight and biological activities of studied complexes was also observed. Regarding the electrical properties, the AC conductivity and the dielectric coefficients were measured as a function of frequency at room temperature. The TMEDA charge-transfer complexes were screened against antibacterial (Escherichia coli, Staphylococcus aureus, Bacillus subtilis, and Pseudomonas aeruginosa) and antifungal (Aspergillus flavus and Candida albicans) activities. PMID:22595252

  7. Synthesis and characterization of magnesium oxide nanocrystallites and probing the vacancy-type defects through positron annihilation studies

    NASA Astrophysics Data System (ADS)

    Das, Anjan; Mandal, Atis Chandra; Roy, Soma; Prashanth, Pendem; Ahamed, Sk Izaz; Kar, Subhrasmita; Prasad, Mithun S.; Nambissan, P. M. G.

    2016-09-01

    Magnesium oxide nanocrystallites exhibit certain abnormal characteristics when compared to those of other wide band gap oxide semiconductors in the sense they are most prone to water absorption and formation of a hydroxide layer on the surface. The problem can be rectified by heating and pure nanocrystallites can be synthesized with controllable sizes. Inevitably the defect properties are distinctly divided between two stages, the one with the hydroxide layer (region I) and the other after the removal of the layer by annealing (region II). The lattice parameters, the optical band gap and even the positron annihilation characteristics are conspicuous by their distinct behavior in the two stages of the surface configurations of nanoparticles. While region I was specific with the formation of positronium-hydrogen complexes that drastically altered the defect-specific positron lifetimes, pick-off annihilation of orthopositronium atoms marked region II. The vacancy clusters within the nanocrystallites also trapped positrons. They agglomerated due to the effect of the higher temperatures and resulted in the growth of the nanocrystallites. The coincidence Doppler broadening spectroscopic measurements supported these findings and all the more indicated the trapping of positrons additionally into the neutral divacancies and negatively charged trivacancies. This is apart from the Mg2+ monovacancies which acted as the dominant trapping centers for positrons.

  8. Surface states and annihilation characteristics of positrons trapped at the (100) and (111) surfaces of silicon

    NASA Astrophysics Data System (ADS)

    Fazleev, N. G.; Fry, J. L.; Weiss, A. H.

    2004-10-01

    Recent studies of Si(100) and Si(111) using positron annihilation induced Auger-electron spectroscopy (PAES) reveal that experimental annihilation probabilities of surface trapped positrons with relevant Si core-level electrons differ significantly for two faces of clean Si, an elemental semiconductor. These experimental results are investigated theoretically by performing calculations of the “image-potential” positron surface states and annihilation characteristics of the surface trapped positrons with relevant Si core-level electrons for the ideally terminated, nonreconstructed and reconstructed Si(100)-(2×1) and Si(111)-(7×7) surfaces. Computed positron surface binding energies demonstrate their sensitivity to the specific atomic structure of the topmost layers of surfaces, and, when compared to positron work functions, the stability of positron surface states on all studied Si(100) and Si(111) surfaces. The positron surface state wave function was found to be localized in a potential well on the vacuum side at both nonreconstructed semiconductor surfaces. The (2×1) reconstruction of the Si(100) surface causes the positron surface state wave function to extend into the lattice in the regions where atoms are displaced away from their ideal terminated positions. A comparison of theoretical and experimental positron surface binding energies for Si(100) shows that the best agreement is achieved when the reconstructed Si(100)-(2×1) surface is described within the asymmetric dimer model. Calculations indicate that the positron surface state wave function is localized in all three dimensions in the corner hole regions of the reconstructed Si(111)-(7×7) surface. This localization provides an explanation for previous experiments that failed to show the anisotropy in the electron-positron pair momentum density distribution expected for a positron surface state delocalized in the plane of the surface. Positron annihilation characteristics are calculated for each

  9. Study of the ionic transport in polymer electrolyte using positron lifetime distribution method

    NASA Astrophysics Data System (ADS)

    Peng, Z. L.; Itoh, Y.; Li, S. Q.; Wang, S. J.

    1996-09-01

    Positron annihilation spectroscopy has been applied to measure the free-volume hole distribution in poly(ether urethane) as a function of temperature. The hole radius distribution determined from orthopositronium lifetime distribution is found to shift to a larger values with increasing temperature. This result, combined with the variation of ionic conductivity, suggests that carrier ions do not migrate naked but are bound to polymer segments through ion-dipole interaction forces, and the ion migration is controlled primarily by segmental motion of the polymer.

  10. Moisture dependence of positron lifetime in Kevlar-49

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Holt, William H.; Mock, Willis, Jr.

    1984-01-01

    Because of filamentary character of Kevlar-49 aramid fibers, there is some concern about the moisture uptake and its effect on plastic composites reinforced with Kevlar-49 fibers. As part of continuing studies of positron lifetime in polymers, we have measured positron lifetime spectra in Kevlar-49 fibers as a function of their moisture content. The long lifetime component intensities are rather low, being only of the order of 2-3 percent. The measured values of long component lifetimes at various moisture levels in the specimens are as follows: 2072 +/- 173 ps (dry); 2013 +/- 193 ps (20.7 percent saturation); 1665 +/- 85 ps (25.7 percent saturation); 1745 +/- 257 ps (32.1 percent saturation); and 1772 +/- 217 ps (100 percent saturation). It is apparent that the long component lifetime at first decreases and then increases as the specimen moisture content increases. These results have been compared with those inferred from Epon-815 and Epon-815/K-49 composite data.

  11. Study of local structure in hyper-eutectic Zr-Cu-Al bulk glassy alloys by positron annihilation techniques

    NASA Astrophysics Data System (ADS)

    Ishiyama, T.; Ishii, K.; Yokoyama, Y.; Konno, T. J.; Iwase, A.; Hori, F.

    2016-01-01

    The Zr-Cu-Al bulk glassy (BG) alloy, which has amorphous structure, possesses various properties such as high strength and toughness with compositional dependence. In the present study, density, positron annihilation lifetime and coincidence Doppler Broadening measurement have been performed for various compositional hyper-eutectic Zr-Cu-Al BG alloys. The density of hyper-eutectic Zr-Cu-Al BG alloys increases with decreasing of Zr fraction. In contrast, positron lifetime for all compositional alloys is almost constant about 165 psec. In addition, the CDB ratio profile is almost the same for hyper-eutectic alloys. This unchanging trend of CDB ratio profile is quite different from that of hypo-eutectic BG alloys. These results reveal that different internal structure exists in hyper and hypo-eutectic BG alloys.

  12. Low energy positron flux generator for lifetime studies in thin films

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; St. Clair, Terry L.; Eftekhari, Abe

    1991-01-01

    A slow positron flux generator for positron annihilation spectroscopic measurements in thin polymer films is described. The advantages of this generator include operability at room temperature and atmospheric pressure without special test film preparaton requirements.

  13. Positron annihilation study for cadmium (electronic structure and enhancement effect)

    NASA Astrophysics Data System (ADS)

    Hamid, A.

    2003-12-01

    The three dimensional electron density in momentum space ρ(p) and in wave vector space n(k) was reconstructed for cadmium (Cd). The measurements were performed using the two dimensional angular correlation of annihilation radiation (2D-ACAR) technique. Enhanced contributions in the spectra were observed around 5.5 mrad, discussed in terms of a Kahana-like enhancement effect. From another viewpoint, Fermi radii were analyzed in the (λM K), (ALM) and (AHK) planes, and they showed a maximum deviation of about 4% from the free electron Fermi radius. Moreover, comparisons to a radio-frequency size effect (RFSE) experiment and theoretical band structure calculations (using augmented plane wave (APW), linear combination of atomic orbital (LCAO) and linear muffin tin orbital (LMTO) methods) were examined. The results showed a qualitative agreement with both APW and LCAO calculations. However, a favorable agreement with the APW method was determined via Fermi surface dimensions. The differences of bands' occupation of n(k) between the current work and the APW method were argued in view of positron wave function in Cd.

  14. Timelike Virtual Compton Scattering from Electron-Positron Radiative Annihilation

    SciTech Connect

    Afanasev, Andrei; Brodsky, Stanley J.; Carlson, Carl E.; Mukherjee, Asmita; /Indian Inst. Tech., Mumbai

    2009-03-31

    We propose measurements of the deeply virtual Compton amplitude (DVCS) {gamma}* {yields} H{bar H}{gamma} in the timelike t = (p{sub H} + p{sub {bar H}}){sup 2} > 0 kinematic domain which is accessible at electron-positron colliders via the radiative annihilation process e{sup +}e{sup -} {yields} H{bar H}{gamma}. These processes allow the measurement of timelike deeply virtual Compton scattering for a variety of H{bar H} hadron pairs such as {pi}{sup +}{pi}{sup -}, K{sup +}K{sup -}, and D{bar D} as well as p{bar p}. As in the conventional spacelike DVCS, there are interfering coherent amplitudes contributing to the timelike processes involving C = - form factors. The interference between the amplitudes measures the phase of the C = + timelike DVCS amplitude relative to the phase of the timelike form factors and can be isolated by considering the forward-backward e{sup +} {leftrightarrow} e{sup -} asymmetry. The J = 0 fixed pole contribution which arises from the local coupling of the two photons to the quark current plays a special role. As an example we present a simple model.

  15. Timelike Virtual Compton Scattering from Electron-Positron Radiative Annihilation

    SciTech Connect

    Andrei Afanaciev,Andrei Afanasev, Stanley J. Brodsky, Carl E. Carlson, Asmita Mukherjee

    2010-02-01

    We propose measurements of the deeply virtual Compton amplitude (DVCS), gamma* to H H-bar gamma, in the timelike t = (p_{H} + p_{H-bar})^2 > 0 kinematic domain which is accessible at electron-positron colliders via the radiative annihilation process e+ e- to H H-bar gamma. These processes allow the measurement of timelike deeply virtual Compton scattering for a variety of H H-bar hadron pairs such as pi+ pi-, K+ K-, and D D-bar as well as p p-bar. As in the conventional spacelike DVCS, there are interfering coherent amplitudes contributing to the timelike processes involving C= - form factors. The interference between the amplitudes measures the phase of the C=+ timelike DVCS amplitude relative to the phase of the timelike form factors and can be isolated by considering the forward-backward e+ \\leftrightarrow e- asymmetry. The J=0 fixed pole contribution which arises from the local coupling of the two photons to the quark current plays a special role. As an example we present a simple model.

  16. Positron lifetime measurements in chiral nematic liquid crystals

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Eftekhari, Abe; Parmar, Devendra S.

    1991-01-01

    Positron lifetimes in the isotropic phases of chiral nematic liquid crystal formulations and their mixtures up to the racemic level were measured. The lifetime spectra for all liquid crystal systems were analyzed into three components. Although the individual spectra in the left- and right-handed components are identical, their racemic mixtures exhibit much larger orthopositronium lifetimes; these larger lifetimes indicate the presence of larger microvoids. This result is consistent with the reportedly higher thermodynamic stability and color play range in the racemic mixtures of chiral nematic liquid crystals.

  17. Investigation and calculation of positron lifetimes of monovacancies in crystals

    NASA Astrophysics Data System (ADS)

    Huang, Shijuan; Liu, Jiandang; Ye, Bangjiao

    2016-01-01

    The first-principles calculations of positron lifetimes of mono-vacancies in crystals were investigated. We use the two-component density functional theory to respectively compute positron lifetimes of neutral charge state of VAl defect in aluminium, VSi defect in silicon, VC, VSi and VC+CSi defects in 3C silicon carbide, VGa and VAs defects in gallium arsenide, taking into account atomic relaxation due to vacancy and electronic structural relaxation due to the presence of the positron. Three different calculation schemes are used. We find that the electron density inside the vacancy more or less increases due to the presence of the positron if the ionic positions are kept fixed, and the positron becomes more localized after the electronic structural relaxation for the case of VAl defect in aluminium and VSi defect in 3C silicon carbide, but it is opposite for the case of VGa defect in gallium arsenide and VC defect in 3C silicon carbide. The results with no consideration of the relaxation are even much closer to the experimental ones, therefore the atomic relaxation due to the position play an important role in calculating the positron lifetime of mono-vacancies in crystals.

  18. Microstructural Characterization of Polymers by Positron Lifetime Spectroscopy

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.

    1996-01-01

    Positrons provide a versatile probe for monitoring microstructural features of molecular solids. In this paper, we report on positron lifetime measurements in two different types of polymers. The first group comprises polyacrylates processed on earth and in space. The second group includes fully-compatible and totally-incompatible Semi-Interpenetrating polymer networks of thermosetting and thermoplastic polyimides. On the basis of lifetime measurements, it is concluded that free volumes are a direct reflection of physical/electromagnetic properties of the host polymers.

  19. Monte Carlo modelling of the propagation and annihilation of nucleosynthesis positrons in the Galaxy

    NASA Astrophysics Data System (ADS)

    Alexis, A.; Jean, P.; Martin, P.; Ferrière, K.

    2014-04-01

    Aims: We want to estimate whether the positrons produced by the β+-decay of 26Al, 44Ti, and 56Ni synthesised in massive stars and supernovae are sufficient to explain the 511 keV annihilation emission observed in our Galaxy. Such a possibility has often been put forward in the past. In a previous study, we showed that nucleosynthesis positrons cannot explain the full annihilation emission. Here, we extend this work using an improved propagation model. Methods: We developed a Monte Carlo Galactic propagation code for ~MeV positrons in which the Galactic interstellar medium, the Galactic magnetic field, and the propagation are finely described. This code allows us to simulate the spatial distribution of the 511 keV annihilation emission. We tested several Galactic magnetic fields models and several positron escape fractions from type-Ia supernova for 56Ni positrons to account for the large uncertainties in these two parameters. We considered the collisional/ballistic transport mode and then compared the simulated 511 keV intensity spatial distributions to the INTEGRAL/SPI data. Results: Regardless of the Galactic magnetic field configuration and the escape fraction chosen for 56Ni positrons, the 511 keV intensity distributions are very similar. The main reason is that ~MeV positrons do not propagate very far away from their birth sites in our model. The direct comparison to the data does not allow us to constrain the Galactic magnetic field configuration and the escape fraction for 56Ni positrons. In any case, nucleosynthesis positrons produced in steady state cannot explain the full annihilation emission. The comparison to the data shows that (a) the annihilation emission from the Galactic disk can be accounted for; (b) the strongly peaked annihilation emission from the inner Galactic bulge can be explained by positrons annihilating in the central molecular zone, but this seems to require more positron sources than the population of massive stars and type Ia

  20. Characterization of point defects in CdTe by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Elsharkawy, M. R. M.; Kanda, G. S.; Abdel-Hady, E. E.; Keeble, D. J.

    2016-06-01

    Positron lifetime measurements on CdTe 0.15% Zn-doped by weight are presented, trapping to monovacancy defects is observed. At low temperatures, localization at shallow binding energy positron traps dominates. To aid defect identification density functional theory, calculated positron lifetimes and momentum distributions are obtained using relaxed geometry configurations of the monovacancy defects and the Te antisite. These calculations provide evidence that combined positron lifetime and coincidence Doppler spectroscopy measurements have the capability to identify neutral or negative charge states of the monovacancies, the Te antisite, A-centers, and divacancy defects in CdTe.

  1. The annihilation of positrons in the cold phase of the interstellar medium revisited

    NASA Astrophysics Data System (ADS)

    Wallyn, P.; Durouchoux, Ph.; Chapuis, C.; Leventhal, M.

    1994-02-01

    The positron cross sections in H and H2 media are reevaluated, taking into account new experimental results. Using a Monte Carlo simulation, we find a positronium fraction before thermalization of 0.90 for H2, in good agreement with the previous experimental result given by Brown et al. (1986). For H we obtain an upper limit of 0.98. We study the behavior of the charge exchange annihilation in a cold phase (molecular cloud). We calculate a formula for the slowing-down time t, before annihilation lasting Delta t, via charge exchange, of a positron beam with a given energy for different medium densities and initial energies. An upper limit of 0.7 MeV for the initial energy of the positrons, annihilating in the molecular cloud G0.86 - 0.08 near the gamma ray source positronium and gives new time constraints on their possible observation.

  2. The annihilation of positrons in the cold phase of the interstellar medium revisited

    NASA Technical Reports Server (NTRS)

    Wallyn, P.; Durouchoux, PH.; Chapuis, C.; Leventhal, M.

    1994-01-01

    The positron cross sections in H and H2 media are reevaluated, taking into account new experimental results. Using a Monte Carlo simulation, we find a positronium fraction before thermalization of 0.90 for H2, in good agreement with the previous experimental result given by Brown et al. (1986). For H we obtain an upper limit of 0.98. We study the behavior of the charge exchange annihilation in a cold phase (molecular cloud). We calculate a formula for the slowing-down time t, before annihilation lasting Delta t, via charge exchange, of a positron beam with a given energy for different medium densities and initial energies. An upper limit of 0.7 MeV for the initial energy of the positrons, annihilating in the molecular cloud G0.86 - 0.08 near the gamma ray source positronium and gives new time constraints on their possible observation.

  3. Positron-Electron Annihilation Process in (2,2)-Difluoropropane Molecule

    NASA Astrophysics Data System (ADS)

    Yang, Liu; Xiao-Guang, Ma; Ying-Hao, Zhu

    2016-04-01

    The positron-electron annihilation process in (2,2)-difluoropropane molecule and the corresponding gamma-ray spectra are studied by quantum chemistry method. The positrophilic electrons in (2,2)-difluoropropane molecule are found for the first time. The theoretical predictions show that the outermost 2s electrons of fluoride atoms play an important role in positron-electron annihilation process of (2,2)-difiuoropropane. In the present scheme, the correlation coefficient between the theoretical gamma-ray spectra and the experiments can be 99%. The present study gives an alternative annihilation model for positron-electron pair in larger molecules. Supported by the National Natural Science Foundation of China under Grant No. 11347011 and the Natural Science Foundation Project of Shandong Province under Grant No. ZR2011AM010 and 2014 Technology Innovation Fund of Ludong University under Grant Nos. 1d151007 and ld15l016

  4. Observation of a charge delocalization from Se vacancies in Bi2Se3 : A positron annihilation study of native defects

    NASA Astrophysics Data System (ADS)

    Unzueta, I.; Zabala, N.; Marín-Borrás, V.; Muñoz-Sanjosé, V.; García, J. A.; Plazaola, F.

    2016-07-01

    By means of positron annihilation lifetime spectroscopy, we have investigated the native defects present in Bi2Se3 , which belongs to the family of topological insulators. We experimentally demonstrate that selenium vacancy defects (VSe1) are present in Bi2Se3 as-grown samples, and that their charge is delocalized as temperature increases. At least from 100 K up to room temperature both VSe10 and VSe1+ charge states coexist. The observed charge delocalization determines the contribution of VSe1 defects to the n -type conductivity of Bi2Se3 . These findings are supported by theoretical calculations, which show that vacancies of nonequivalent Se1 and Se2 selenium atoms are clearly differentiated by positron annihilation lifetime spectroscopy, enabling us to directly detect and quantify the most favorable type of selenium vacancy. In addition to open-volume defects, experimental data indicate the presence of defects that act as shallow traps, suggesting that more than one type of native defects coexist in Bi2Se3 . As will be discussed, the presence of a dislocation density around 1010cm-2 could be the source of the detected shallow traps. Understanding the one-dimensional defects and the origin of the charge delocalization that leads Bi2Se3 to be an n -type semiconductor will help in the development of high-quality topological insulators based on this material.

  5. Electron beam induced microstructural changes and electrical conductivity in Bakelite polymer RPC detector material: A positron lifetime study

    NASA Astrophysics Data System (ADS)

    Aneesh Kumar, K. V.; Ningaraju, S.; Munirathnamma, L. M.; Ravikumar, H. B.; Ranganathaiah, C.

    2015-06-01

    In order to explore the structural modification induced electrical conductivity, samples of Bakelite RPC polymer detector materials were exposed to 8 MeV of electron beam with the irradiation dose from 20 kGy to 100 kGy in steps of 20 kGy. The microstructural changes upon electron beam irradiation have been studied using Positron Annihilation Lifetime Spectroscopy (PALS) and Fourier Transform Infrared (FTIR) Spectroscopy. Positron lifetime parameters viz., o-Ps lifetime and its intensity show chain scission at lower doses (20 kGy, 40 kGy) followed by cross-linking beyond 40 kGydue to the radical reactions. The reduction in electrical conductivity of Bakelite material beyond 60 kGy is correlated to the conducting pathways and cross-links in the polymer matrix. The appropriate doses of electron beam irradiation of Bakelite material may reduce the leakage current and hence improves the performance of the detector.

  6. Positron-annihilation 2D-ACAR studies of disordered and defected alloys

    SciTech Connect

    Bansil, A.; Prasad, R.; Smedskjaer, L.C.; Benedek, R.; Mijnarends, P.E.

    1987-09-01

    Theoretical and experimental progess in connection with 2D-ACAR positron annihilation studies of ordered, disordered, and defected alloys is discussed. We present, in particular, some of the recent developments concerning the electronic structure of disordered alloys, and the work in the area of annihilation from positrons trapped at vacancy-type defects in metals and alloys. The electronic structure and properties of a number of compounds are also discussed briefly; we comment specifically on high T/sub c/ ceramic superconductors, Heusler alloys, and transition-metal aluminides. 58 refs., 116 figs.

  7. Jahn-Teller distortion of neutral divacancy in Si studied by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Nagai, Y.; Inoue, K.; Tang, Z.; Yonenaga, I.; Chiba, T.; Saito, M.; Hasegawa, M.

    2003-12-01

    Jahn-Teller (JT) distortion of the diamagnetic neutral divacancy (V20) in Si is studied by positron annihilation spectroscopy. Based on the anisotropy of electron momentum distribution around the divacancies obtained by two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) technique, the preferred alignment of the divacancy in a specimen by compressing uniaxial stress along the [0 1 1] axis is detected. The direction of the alignment evidences the pairing JT distortion rather than the resonant bonding distortion, which is similar to the distortion in the singly negative divacancy (V2-).

  8. Jet algorithms in electron-positron annihilation: perturbative higher order predictions

    NASA Astrophysics Data System (ADS)

    Weinzierl, Stefan

    2011-02-01

    This article gives results on several jet algorithms in electron-positron annihilation: Considered are the exclusive sequential recombination algorithms Durham, Geneva, Jade-E0 and Cambridge, which are typically used in electron-positron annihilation. In addition also inclusive jet algorithms are studied. Results are provided for the inclusive sequential recombination algorithms Durham, Aachen and anti- k t , as well as the infrared-safe cone algorithm SISCone. The results are obtained in perturbative QCD and are N3LO for the two-jet rates, NNLO for the three-jet rates, NLO for the four-jet rates and LO for the five-jet rates.

  9. Asymmetric 511 keV Positron Annihilation Line Emission from the Inner Galactic Disk

    NASA Technical Reports Server (NTRS)

    Skinner, Gerry; Weidenspointner, Georg; Jean, Pierre; Knodlseder, Jurgen; Ballmoos, Perer von; Bignami, Giovanni; Diehl, Roland; Strong, Andrew; Cordier, Bertrand; Schanne, Stephane; Winkler, Christoph

    2008-01-01

    A recently reported asymmetry in the 511 keV gamma-ray line emission from the inner galactic disk is unexpected and mimics an equally unexpected one in the distribution of LMXBs seen at hard X-ray energies. A possible conclusion is that LMXBs are an important source of the positrons whose annihilation gives rise to the line. We will discuss these results, their statistical significance and that of any link between the two. The implication of any association between LMXBs and positrons for the strong annihilation radiation from the galactic bulge will be reviewed.

  10. Is there a dark matter signal in the galactic positron annihilation radiation?

    PubMed

    Lingenfelter, R E; Higdon, J C; Rothschild, R E

    2009-07-17

    Assuming Galactic positrons do not go far before annihilating, a difference between the observed 511 keV annihilation flux distribution and that of positron production, expected from beta+ decay in Galactic iron nucleosynthesis, was evoked as evidence of a new source and signal of dark matter. We show, however, that the dark matter sources cannot account for the observed positronium fraction without extensive propagation. Yet with such propagation, standard nucleosynthetic sources can fully account for the spatial differences and positronium fraction, leaving no new signal for dark matter to explain. PMID:19659265

  11. Spectra of Positrons Lifetimes in Choose Gel Drugs

    NASA Astrophysics Data System (ADS)

    Pietrzak, R.; Muszyńska, J.; Kajdas, A.

    2006-11-01

    Spectra of positrons lifetimes in selected gel non-steride analgesic and antiphlogistic drugs were investigated. The basic components in them were sodium salts of diclophenac and they differed from one another with the chemical composition of other components. It was found that in all of the investigated spectra there occurred a component which testified to the formation of positronium. The differences in the values of ortho-Ps component lifetimes and their intensity can be attributed to the presence of ingredients modifying the effects of the drug.

  12. Positron annihilation response and viscosity of a glass-forming system within the two-order parameter model of liquids

    NASA Astrophysics Data System (ADS)

    Bartoš, J.

    2008-07-01

    This paper presents a combined description of the ortho-positronium (o-Ps) response from positron annihilation lifetime spectroscopy (PALS) and viscosity data on bis[m-(m-phenoxy) phenoxyphenyl] ether within the free volume version of the two-order parameter (TOP) model of disordered phase. The quasi-sigmoidal form of the temperature dependence of the o-Ps lifetime, τ3, over a wide temperature interval, can be described by an expression with the distribution function for liquid-like domains 1-F(T) which follows from a phenomenological analysis of the viscosity using the modified Vogel-Fulcher-Tamman-Hesse (MVFTH) equation. This simultaneous description indicates a close connection between the PALS response and the viscosity behavior for the matrix and gives support for the liquid-like and solid-like domain physical picture of all the physical states of the disordered phase.

  13. Characterization of Al-ALLOYS (50xx) by Using Positron Annihilation, X-Ray Diffraction and Vibrating Reed Techniques

    NASA Astrophysics Data System (ADS)

    Kumar, Uday; Badawi, Emad; Mukhopadhyay, P. K.

    A series of Al-Mgx alloys, with x = 0.82, 2.09, 2.28, 2.49 and 4.47 wt.%, respectively were characterized by using positron annihilation lifetime studies (PAL), X-ray diffraction (XRD), and sound velocity and internal friction using a vibrating reed technique (VRT). PAL lifetime values increase linearly as the composition is varied, but texturing or preferential orientation is maximum at an intermediate value of composition (x = 2.49%). The internal friction shows a minimum at the same composition, and the sound velocity changes show the maximum value here too. This means that at this composition the sample is the most ordered and defect free.

  14. Miscibility and crystallization behavior of poly (3-hydroxybutyrate) and poly (ethylene glycol) blends studied by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Abdel-Hady, E. E.; Abdel-Hamed, M. O.; Hammam, A. M.

    2011-01-01

    Positron annihilation Lifetime (PAL) spectroscopy has been used to study the effect of PEG concentrations on the free volume properties of PHB. The data revealed that the ortho-positronium (o-Ps) lifetime τPs increases with 20% increase in concentration, decrease as the concentration increases to 40%, then rapid increase at 50% concentration of PEG. The o-Ps intensity, I3, shows a linear dependence as the concentration increases with a discontinuity at 20% concentration of PEG. Furthermore, the results presented and discussed in this work show that the PHB and PEG are miscible up to 40% of PEG but greater than 40%, the blend is immiscible. In addition, the mechanical properties of PHB are well improved by the addition of PEG with a low concentration up to 20%, while at higher concentration the blend becomes waxy.

  15. Whole-Pattern Fitting and Positron Annihilation Studies of Magnetic PVA/α-Fe2O3 Nanocomposites

    NASA Astrophysics Data System (ADS)

    Prashanth, K. S.; Mahesh, S. S.; Prakash, M. B. Nanda; Ningaraju, S.; Ravikumar, H. B.; Somashekar, R.; Nagabhushana, B. M.

    2016-03-01

    A low-temperature solution combustion method was used to synthesize α-Fe2O3 nanoparticles. Magnetic polyvinyl alcohol (PVA)/α-Fe2O3/NaCl nanocomposites were prepared by solvent cast method. The Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) results are in confirmation with X-ray diffraction (XRD) results indicating the formation of nanocomposites. The microcrystalline parameters, crystallite size (), lattice strain (g in %), stacking faults (α d ), and twin faults (β) of prepared polymer nanocomposites were evaluated by whole-pattern fitting technique. The refinement was carried out using the computed microstructural parameters in which the twin faults and stacking faults did not vary much and statistical deviation was less than 5 %. Positron annihilation lifetime spectroscopy (PALS) was used for microstructural characterization. PALS results show that the ortho-positronium (o-Ps) lifetime (τ3) increases gradually as a function of nanoparticle concentration and about 219 ps increase observed from1.50 to1.71 ns at 3 wt%. This indicates the increase of free volume hole size (V f ) from 54.47 to 72.18 Å3. The o-Ps intensities (I 3) decrease indicating the inhibition of o-Ps formation upon incorporation of nanoparticles into PVA. The increase in I 2 values suggests the increased annihilation at the interface region. Positron lifetime parameters, viz., o-Ps lifetime, and its intensities indicate the effect of quenching and inhibition upon incorporation of metal oxide nanoparticles and inorganic salt into PVA.

  16. Whole-Pattern Fitting and Positron Annihilation Studies of Magnetic PVA/α-Fe2O3 Nanocomposites

    NASA Astrophysics Data System (ADS)

    Prashanth, K. S.; Mahesh, S. S.; Prakash, M. B. Nanda; Ningaraju, S.; Ravikumar, H. B.; Somashekar, R.; Nagabhushana, B. M.

    2016-06-01

    A low-temperature solution combustion method was used to synthesize α-Fe2O3 nanoparticles. Magnetic polyvinyl alcohol (PVA)/α-Fe2O3/NaCl nanocomposites were prepared by solvent cast method. The Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) results are in confirmation with X-ray diffraction (XRD) results indicating the formation of nanocomposites. The microcrystalline parameters, crystallite size ( ), lattice strain ( g in %), stacking faults ( α d ), and twin faults ( β) of prepared polymer nanocomposites were evaluated by whole-pattern fitting technique. The refinement was carried out using the computed microstructural parameters in which the twin faults and stacking faults did not vary much and statistical deviation was less than 5 %. Positron annihilation lifetime spectroscopy (PALS) was used for microstructural characterization. PALS results show that the ortho-positronium (o-Ps) lifetime (τ3) increases gradually as a function of nanoparticle concentration and about 219 ps increase observed from1.50 to1.71 ns at 3 wt%. This indicates the increase of free volume hole size ( V f ) from 54.47 to 72.18 Å3. The o-Ps intensities ( I 3) decrease indicating the inhibition of o-Ps formation upon incorporation of nanoparticles into PVA. The increase in I 2 values suggests the increased annihilation at the interface region. Positron lifetime parameters, viz., o-Ps lifetime, and its intensities indicate the effect of quenching and inhibition upon incorporation of metal oxide nanoparticles and inorganic salt into PVA.

  17. Point defects in YBa2Cu3O7-x studied using positron annihilation

    NASA Astrophysics Data System (ADS)

    Chudy, Michal; Eisterer, M.; Weber, H. W.; Veterníková, J.; Sojak, S.; Slugeň, V.

    2012-07-01

    Fast neutron irradiation is a powerful technique for introducing additional pinning centers into high temperature superconductors. The spherical defects with sizes of a few nanometers are considered to be effective pinning centers, enhancing Jc. Their morphology is well-known and has already been investigated by several authors in great detail. However, only very little is known about the nature and density of smaller and point defects, which are invisible in transmission electron microscopy. Positron annihilation lifetime spectroscopy was applied to investigate the nature and the concentration of small point-like defects. In this work, the influence of small point defects, such as vacancies and vacancy clusters, on the superconducting properties of YBa2Cu3O7-x bulks was studied; these were introduced by irradiation in the TRIGA Mark II reactor in Vienna. Jc and Tc measurements were performed prior to and after each irradiation step. The samples were irradiated up to a fast neutron ( > 0.1 MeV) fluence of 6 × 1021 m-2. The two kinds of defects—the large collision cascades and the small point-like defects—contribute to the decrease of Tc as well as to the Jc enhancement in astonishingly similar ways.

  18. Study of Benzyl Salicylate/beta-Cyclodextrin Inclusion Complex Formation by Positron Annihilation

    NASA Astrophysics Data System (ADS)

    Bellitto, V. J.; Hsu Hadley, F. H., Jr.; Trinh, T.

    1996-11-01

    Results of positron annihilation lifetime spectra of beta-cyclodextrin and beta-cyclodextrin complexed with benzyl salicylate,benzyl acetate, or ethyl salicylate in air and vacuum were used to determine the fraction of beta-cyclodextrin which remains uncomplexed in the benzyl salicylate/beta-cyclodextrin 1:2 molar ratio inclusion complex. The intensity of the longest-lived component in vacuum was shown to decrease when the beta-cyclodextrin cavity was filled with benzyl salicylate, benzyl acetate, or ethyl salicylate guest molecules. Comparison of the intensity for beta-cyclodextrin, benzyl salicylate/beta-cyclodextrin 1:2 molar ratio, and 1:1 molar ratio indicated that the benzyl and salicylate moieties each formed an inclusion complex with a molecule of beta-cyclodextrin in the benzyl salicylate/beta-cyclodextrin 1:2 complex. It was determined that the benzyl moiety of the benzyl salicylate molecule is preferred by the beta-cyclodextrin "host" and that only 34of the salicylate moieties are complexed in the benzyl salicylate/beta-cyclodextrin 1:2 sample.

  19. Theoretical studies of positron states and annihilation characteristics at the oxidized Cu(100) surface

    SciTech Connect

    Fazleev, N. G.; Weiss, A. H.

    2013-04-19

    In this work we present the results of theoretical studies of positron surface and bulk states and annihilation probabilities of surface-trapped positrons with relevant core electrons at the oxidized Cu(100) surface under conditions of high oxygen coverage. An ab-initio study of the electronic properties of the Cu(100) missing row reconstructed surface at various on surface and sub-surface oxygen coverages has been performed on the basis of the density functional theory (DFT) using the Dmol3 code and the generalized gradient approximation (GGA). Surface structures in calculations have been constructed by adding oxygen atoms to various surface hollow and sub-surface octahedral sites of the 0.5 monolayer (ML) missing row reconstructed phase of the Cu(100) surface with oxygen coverages ranging from 0.5 to 1.5 ML. The charge redistribution at the surface and variations in atomic structure and chemical composition of the topmost layers associated with oxidation and surface reconstruction have been found to affect the spatial extent and localization of the positron surface state wave function and annihilation probabilities of surface trapped positrons with relevant core electrons. Theoretical results are compared with experimental data obtained from studies of oxidation of the Cu(100) surface using positron annihilation induced Auger electron spectroscopy (PAES). It has been shown that positron annihilation probabilities with Cu 3s and 3p core electrons decrease when total (on-surface and sub-surface) oxygen coverage of the Cu(100) surface increases up to 1 ML. The calculations show that for high oxygen coverage when total oxygen coverage is 1. 5 ML the positron is not bound to the surface.

  20. Positron annihilation process in Ni/sub c/Cu/sub 1-c/ alloys

    SciTech Connect

    Szotek, Z.; Gyorffy, B.L.; Stocks, G.M.; Temmerman, W.M.

    1982-01-01

    New, accurate, calculations of the electron momentum distribution function for the Cu/sub 60/Ni/sub 40/ random solid solution are presented and the role played by the positron wavefunction in determining the Angular Correlation of the Annihilation Radiation (ACAR) is discussed in quantitative terms.

  1. Positron annihilation studies in Li-implanted alumina

    NASA Astrophysics Data System (ADS)

    Gaikwad, Prashant V.; Sharma, S. K.; Mukherjee, S.; Sudarshan, K.; Maheshwari, P.; Pujari, P. K.; Kshirsagar, A.

    2015-06-01

    Depth dependent Doppler broadening of annihilation radiation (DBAR) measurements are carried out for a sample of Li ion implanted in alumina. The effect of Li ion implantation and the subsequent isochronal annealing at the temperatures up to 1100 °C on the Doppler broadening annihilation parameters (S-parameter) are studied. The S-parameter around the Li implantation depth (∼191 nm) increased with annealing temperature up to 700 °C and reduced beyond. The results suggest possible Li cluster formation in annealed sample.

  2. Measuring electron-positron annihilation radiation from laser plasma interactions

    SciTech Connect

    Chen, Hui; Tommasini, R.; Seely, J.; Szabo, C. I.; Feldman, U.; Pereira, N.; Gregori, G.; Falk, K.; Mithen, J.; Murphy, C. D.

    2012-10-15

    We investigated various diagnostic techniques to measure the 511 keV annihilation radiations. These include step-wedge filters, transmission crystal spectroscopy, single-hit CCD detectors, and streaked scintillating detection. While none of the diagnostics recorded conclusive results, the step-wedge filter that is sensitive to the energy range between 100 keV and 700 keV shows a signal around 500 keV that is clearly departing from a pure Bremsstrahlung spectrum and that we ascribe to annihilation radiation.

  3. Positron annihilation spectroscopy of vacancy-type defects hierarchy in submicrocrystalline nickel during annealing

    SciTech Connect

    Kuznetsov, Pavel V.; Mironov, Yuri P. E-mail: tolmach@ispms.tsc.ru Tolmachev, Aleksey I. E-mail: tolmach@ispms.tsc.ru Rakhmatulina, Tanzilya V. E-mail: tolmach@ispms.tsc.ru; Bordulev, Yuri S. E-mail: laptev.roman@gmail.com Laptev, Roman S. E-mail: laptev.roman@gmail.com Lider, Andrey M. E-mail: laptev.roman@gmail.com Mikhailov, Andrey A. E-mail: laptev.roman@gmail.com; Korznikov, Alexander V.

    2014-11-14

    Positron annihilation and X-ray diffraction analysis have been used to study submicrocrystalline nickel samples prepared by equal channel angular pressing. In the as-prepared samples the positrons are trapped at dislocation-type defects and in vacancy clusters that can include up to 5 vacancies. The study has revealed that the main positron trap centers at the annealing temperature of ΔT= 20°C-180°C are low-angle boundaries enriched by impurities. At ΔT = 180°C-360°C, the trap centers are low-angle boundaries providing the grain growth due to recrystallization in-situ.

  4. Study of gamma irradiation effect on positron annihilation mechanism in PFA

    NASA Astrophysics Data System (ADS)

    Yang, J.; Li, Z. X.; Zhao, B. Z.; Zhang, P.; Lu, E. Y.; Zhang, J.; Yuan, D. Q.; Cao, X. Z.; Yu, R. S.; Wang, B. Y.

    2014-03-01

    Gamma irradiation effect on annihilation characteristics of positronium and free positron in tetrafluoroethylene-perluoro (alkoxy vinyl ether) copolymer (PFA) were studied independently by age momentum correlation (AMOC) and the correlation between Doppler broadening S parameter and o-Ps fraction (S-Io-Ps correlation). AMOC results revealed decreases in S parameter of o-Ps, owing to accumulation of polar atoms around free volume. S-Io-Ps correlation indicated a reduced intrinsic S parameter of free positron in irradiated PFA, which was caused by enhanced positron trapping on polar atoms due to densification of local segments and variation in the elemental environment around free volumes.

  5. Spin dependent momentum density and Fermi surface of ferromagnetic Ni obtained by positron annihilation experiments

    NASA Astrophysics Data System (ADS)

    Hamid, A. S.; Uedono, A.

    2004-11-01

    The spin-dependent momentum density and Fermi surface of ferromagnetic Ni have been obtained through positron annihilation experiments. The measurements were carried out through 2D angular correlation of annihilation radiation (ACAR) using longitudinally polarized positrons. The magnetic field direction was reversed in order to study the effect of the spin-dependent positron-electron momentum space density on the Fermi surface of Ni. The results showed that ferromagnetic Ni had different Fermi surfaces for the majority-spin and minority-spin states. The differences due to the spin-states were studied in the momentum space and in the wave vector space. In general, the experimental results showed good agreement with previous theoretical calculations.

  6. Positron annihilation characterization of free volume in micro- and macro-modified Cu0.4Co0.4Ni0.4Mn1.8O4 ceramics

    NASA Astrophysics Data System (ADS)

    Klym, H.; Ingram, A.; Shpotyuk, O.; Hadzaman, I.; Solntsev, V.; Hotra, O.; Popov, A. I.

    2016-07-01

    Free volume and pore size distribution size in functional micro and macro-micro-modified Cu0.4Co0.4Ni0.4Mn1.8O4 ceramics are characterized by positron annihilation lifetime spectroscopy in comparison with Hg-porosimetry and scanning electron microscopy technique. Positron annihilation results are interpreted in terms of model implication positron trapping and ortho-positronium decaying. It is shown that free volume of positron traps are the same type for macro and micro modified Cu0.4Co0.4Ni0.4Mn1.8O4 ceramics. Classic Tao-Eldrup model in spherical approximation is used to calculation of the size of nanopores smaller than 2 nm using the ortho-positronium lifetime.

  7. High elemental selectivity to Sn submonolayers embedded in Al using positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Hugenschmidt, C.; Pikart, P.; Stadlbauer, M.; Schreckenbach, K.

    2008-03-01

    In the present work, we demonstrate that metal layers in the submonolayer range embedded in a matrix are revealed with unprecedented sensitivity by coincident Doppler-broadening spectroscopy of the positron annihilation using a monoenergetic positron beam. The measured electron momentum distribution specific for Sn is clearly observable in Al/Sn/Al -layered samples even at a Sn area density of as low as 7.3×10-2μg/cm2 below 200nm Al. An explanation for the high elemental selectivity for the thin Sn layers is set forward in terms of efficient positron trapping due to the changing positron affinity at the Al/Sn -interface and quantum-dot-like positron states in Sn nanoparticles.

  8. Electron momentum distribution and singlet-singlet annihilation in the organic anthracene molecular crystals using positron 2D-ACAR and fluorescence spectroscopy.

    PubMed

    Selvakumar, Sellaiyan; Sivaji, Krishnan; Arulchakkaravarthi, Arjunan; Sankar, Sambasivam

    2014-08-14

    We present the mapping of electron momentum distribution (EMD) in a single crystal of anthracene by two-dimensional angular correlation of positron annihilation radiation (2D-ACAR). The projected EMD is explained on the basis of the crystallographic features of the material. The EMD spectra provide information about the positron states and their behavior and also about the hindrance of the positronium (Ps) formation in this material. The EMD has exhibited evidence for the absence of free volume defects. The characteristic EMD features regarding the delocalized electronic states are explained. Further, scintillation characteristics such as fluorescence and time-correlated single photon counting have also been studied. The emission peaks are attributed to vibrational bands of fluorescence emission from the singlet excitons and lifetime components are observed to be due to singlet fission and the singlet-singlet excitons annihilation. PMID:24963608

  9. Studies of iron exposed to heavy ion implantation using positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Horodek, P.; Dryzek, J.; Skuratov, V. A.

    2016-05-01

    Variable energy positron beam and positron lifetime spectroscopy were used to study pure iron exposed to irradiation with 167 MeV Xe26+ heavy ions with different doses of 1012, 1013, 5×1013, 1014 ions/cm2. The positron lifetime spectroscopy revealed the presence of large cluster of about 15-27 vacancies and dislocations. The dislocations are distributed at the depth of about 18 μm i.e. almost twice deeper than the ion implantation range from the surface exposed to the heavy ions implantation. Possible explanation is the long-range effect attributed to the ion implantation into materials.

  10. Study of positrons from cosmic rays interactions and cold dark matter annihilations in the galactic environment

    NASA Astrophysics Data System (ADS)

    Lineros, Roberto A.

    2008-12-01

    Positron and electron cosmic rays provide a complementary way to study the galactic environment. The actual cosmic rays experiments, for instance PAMELA and HEAT, have presented very exciting results in this field. The observed positron fraction appears larger than the actual theoretical predictions for energies larger than 10 GeV. The indirect evidences of Dark Matter in connection with Beyond the Standard Model theories would suggest the existence of an extra contribution present in the cosmic ray signal. We study and calculate the positron signal produced by the annihilation of a generic Dark Matter candidate. Especially, We analyze typical annihilation signatures and the impact of CR propagation physics on the positron signal. In addition, we study the positron signal related to spallation processes between nuclei cosmic--rays and the interstellar gas. We analyze the effects of uncertainties present in nuclear cross section, nuclei cosmic--ray and CR propagation physics. The propagation of positrons is modeled according to the Two--Zone Propagation Model which has been successfully tested in the study of nuclei cosmic--ray and present an analytical approach to study the cosmic--ray physics.

  11. Annihilation momentum density of positrons trapped at vacancy-type defects in metals and alloys

    SciTech Connect

    Bansil, A.; Prasad, R.; Benedek, R.

    1988-01-01

    Positron annihilation, especially the angular correlation of annihilation radiation, is a powerful tool for investigating the electronic spectra of ordered as well as defected materials. The tendency of positrons to trap at vacancy-type defects should enable this technique to study the local environment of such defects. However, we need to develop a theoretical basis for calculating the two-photon annihilation momentum density rho/sub 2gamma/(p-vector). We have recently formulated and implemented a theory of rho/sub 2gamma/(p-vector) from vacancy-type defects in metals and alloys. This article gives an outline of our approach together with a few of our results. Section 2 summarizes the basic equations for evaluating rho/sub 2gamma/(p-vector). Our Green's function-based approach is nonperturbative and employs a realistic (one-particle) muffin-tin Hamiltonian for treating electrons and positrons. Section 3 presents and discusses rho/sub 2gamma/(p-vector) results for a mono-vacancy in Cu. We have neglected the effects of electron-positron correlations and of lattice distortion around the vacancy. Section 4 comments briefly on the question of treating defects such as divacancies and metal-impurity complexes in metals and alloys. Finally, in Section 5, we remark on the form of rho/sub 2gamma/(p-vector) for a mono-vacancy in jellium. 2 figs.

  12. Study of PRIMAVERA steel samples by a positron annihilation spectroscopy technique

    NASA Astrophysics Data System (ADS)

    Grafutin, V.; Ilyukhina, O.; Krsjak, V.; Burcl, R.; Hähner, P.; Erak, D.; Zeman, A.

    2010-11-01

    In the present article, a positron annihilation spectroscopy investigation of VVER-440/230 weld materials is discussed. Important characteristics of metals such as Fermi energy, concentration of electrons in the conduction band, size and concentration of defects were experimentally determined for three model materials with higher level of copper (0.16 wt.%) and phosphorus (0.027-0.038 wt.%). The impact of neutron irradiation and subsequent annealing on crystal lattice parameters was investigated. The experiments with the angular correlation of positron annihilation radiation (ACAR) complement the published positron annihilation spectroscopy (PAS) studies of the radiation treated VVER materials as well as previous experiments on PRIMAVERA materials. The availability of the experimental reactor to prepare strong 64Cu positron sources provided for unique experimental conditions, such as good resolution of spectra (0.4 mrad) and reasonable short time of measurement (36 h). The present paper aims to contribute to further understanding of RPV (reactor pressure vessel) steels behaviour under irradiation conditions as well as annealing recovery procedures, which have already been applied at several VVER NPP units in Europe.

  13. On positron annihilation in concentrated random alloys and superconducting cuprates

    SciTech Connect

    Szotek, Z.; Temmerman, W.M.; Gyorffy, B.L.; Stocks, G.M.

    1988-01-01

    We discuss an application of a generalisation of the Lock-Crisp-West theorem to concentrated random alloys. Using a theory developed for binary random alloys we explore a possibility of positron localisation in the new high temperature superconductors. 7 refs., 1 fig.

  14. Free volume study on the miscibility of PEEK/PEI blend using positron annihilation and dynamic mechanical thermal analysis

    NASA Astrophysics Data System (ADS)

    Ramani, R.; Alam, S.

    2015-06-01

    High performance polymer blend of poly(ether ether ketone) (PEEK) and poly(ether imide) (PEI) was examined for their free volume behaviour using positron annihilation lifetime spectroscopy and dynamic mechanical thermal analysis methods. The fractional free volume obtained from PALS shows a negative deviation from linear additivity rule implying good miscibility between PEEK and PEI. The dynamic modulus and loss tangent were obtained for the blends at three different frequencies 1, 10 and 100 Hz at temperatures close to and above their glass transition temperature. Applying Time-Temperature-Superposition (TTS) principle to the DMTA results, master curves were obtained at a reference temperature To and the WLF coefficients c01 and c02 were evaluated. Both the methods give similar results for the dependence of fractional free volume on PEI content in this blend. The results reveal that free volume plays an important role in determining the visco-elastic properties in miscible polymer blends.

  15. Two magnetic states of iron atoms in Invar Fe-Ni alloys and positron annihilation

    NASA Astrophysics Data System (ADS)

    Sedov, V. L.; Tsigel'nik, O. A.

    1999-11-01

    The temperature dependence of angular correlation annihilation radiation (ACAR) in Invar Fe-Ni alloys is investigated. It is found that the ACAR distribution in the Curie temperature region TC depends on temperature. This effect is created only by those positrons that are trapped by vacancies. The effect is enhanced if the positrons trapped by vacancy-hydrogen complexes. The ACAR distribution is changed due to enhanced interaction of these positrons with 3d electrons. A simple interpretation of this phenomenon can be given on the basis of the model of two magnetic states of Fe atoms in Invar alloys. According to this model the enhancement of the electron-positron correlation interaction in the TC region occurs as a result of the convergence of the energy levels εHS and εLS corresponding to the high-spin (HS) and low-spin (LS) states of Fe atoms.

  16. Time of flight spectrometer for background-free positron annihilation induced Auger electron spectroscopy.

    PubMed

    Mukherjee, S; Shastry, K; Anto, C V; Joglekar, P V; Nadesalingam, M P; Xie, S; Jiang, N; Weiss, A H

    2016-03-01

    We describe a novel spectrometer designed for positron annihilation induced Auger electron spectroscopy employing a time-of-flight spectrometer. The spectrometer's new configuration enables us to implant monoenergetic positrons with kinetic energies as low as 1.5 eV on the sample while simultaneously allowing for the detection of electrons emitted from the sample surface at kinetic energies ranging from ∼500 eV to 0 eV. The spectrometer's unique characteristics made it possible to perform (a) first experiments demonstrating the direct transition of a positron from an unbound scattering state to a bound surface state and (b) the first experiments demonstrating that Auger electron spectra can be obtained down to 0 eV without the beam induced secondary electron background obscuring the low energy part of the spectra. Data are presented which show alternative means of estimating positron surface state binding energy and background-free Auger spectra. PMID:27036826

  17. Time of flight spectrometer for background-free positron annihilation induced Auger electron spectroscopy

    NASA Astrophysics Data System (ADS)

    Mukherjee, S.; Shastry, K.; Anto, C. V.; Joglekar, P. V.; Nadesalingam, M. P.; Xie, S.; Jiang, N.; Weiss, A. H.

    2016-03-01

    We describe a novel spectrometer designed for positron annihilation induced Auger electron spectroscopy employing a time-of-flight spectrometer. The spectrometer's new configuration enables us to implant monoenergetic positrons with kinetic energies as low as 1.5 eV on the sample while simultaneously allowing for the detection of electrons emitted from the sample surface at kinetic energies ranging from ˜500 eV to 0 eV. The spectrometer's unique characteristics made it possible to perform (a) first experiments demonstrating the direct transition of a positron from an unbound scattering state to a bound surface state and (b) the first experiments demonstrating that Auger electron spectra can be obtained down to 0 eV without the beam induced secondary electron background obscuring the low energy part of the spectra. Data are presented which show alternative means of estimating positron surface state binding energy and background-free Auger spectra.

  18. Development and Testing of the Positron Identification By Coincident Annihilation Photons (PICAP) System

    NASA Astrophysics Data System (ADS)

    Tran, D.; Connell, J. J.; Lopate, C.; Bickford, B.

    2014-12-01

    Moderate energy positrons (~few to 10 MeV) have seldom been observed in the Heliosphere, due primarily to there not having been dedicated instruments for such measurements. Their detection would have implications in the study of Solar energetic particle events and the transport and modulation of the Solar wind and Galactic cosmic rays. The Positron Identification by Coincident Annihilation Photons (PICAP) system is designed specifically to measure these moderate energy positrons by simultaneously detecting the two 511-keV γ-ray photons that result from a positron stopping in the instrument and the subsequent electron-positron annihilation. This method is also expected to effectively discriminate positrons from protons by measuring the amount of energy deposited in the detectors (dE/dx versus residual energy). PICAP offers a low-mass, low-power option for measuring positrons, electrons, and ions in space. Following Monte Carlo modeling, a PICAP laboratory prototype, adaptable to a space-flight design, was designed, built, and tested. This instrument is comprised of (Si) solid-state detectors, plastic scintillation detectors, and high-Z BGO crystal scintillator suitable for detecting the 511-keV γ rays. The prototype underwent preliminary laboratory testing and calibration using radioactive sources for the purpose of establishing functionality. It has since been exposed to beams of energetic protons (up to ~200 MeV) at Massachusetts General Hospital's Francis H. Burr Proton Beam Therapy Center and positrons and electrons (up to ~10 MeV) at Idaho State University's Idaho Accelerator Center. The goal is to validate modeling and determine the performance of the instrument concept. We will present a summary of modeling calculations and analysis of data taken at the accelerator tests. This work is 95% supported by NASA Grant NNX10AC10G.

  19. Positron annihilation signatures associated with the outburst of the microquasar V404 Cygni

    NASA Astrophysics Data System (ADS)

    Siegert, Thomas; Diehl, Roland; Greiner, Jochen; Krause, Martin G. H.; Beloborodov, Andrei M.; Bel, Marion Cadolle; Guglielmetti, Fabrizia; Rodriguez, Jerome; Strong, Andrew W.; Zhang, Xiaoling

    2016-03-01

    Microquasars are stellar-mass black holes accreting matter from a companion star and ejecting plasma jets at almost the speed of light. They are analogues of quasars that contain supermassive black holes of 106 to 1010 solar masses. Accretion in microquasars varies on much shorter timescales than in quasars and occasionally produces exceptionally bright X-ray flares. How the flares are produced is unclear, as is the mechanism for launching the relativistic jets and their composition. An emission line near 511 kiloelectronvolts has long been sought in the emission spectrum of microquasars as evidence for the expected electron-positron plasma. Transient high-energy spectral features have been reported in two objects, but their positron interpretation remains contentious. Here we report observations of γ-ray emission from the microquasar V404 Cygni during a recent period of strong flaring activity. The emission spectrum around 511 kiloelectronvolts shows clear signatures of variable positron annihilation, which implies a high rate of positron production. This supports the earlier conjecture that microquasars may be the main sources of the electron-positron plasma responsible for the bright diffuse emission of annihilation γ-rays in the bulge region of our Galaxy. Additionally, microquasars could be the origin of the observed megaelectronvolt continuum excess in the inner Galaxy.

  20. Positron annihilation signatures associated with the outburst of the microquasar V404 Cygni.

    PubMed

    Siegert, Thomas; Diehl, Roland; Greiner, Jochen; Krause, Martin G H; Beloborodov, Andrei M; Bel, Marion Cadolle; Guglielmetti, Fabrizia; Rodriguez, Jerome; Strong, Andrew W; Zhang, Xiaoling

    2016-03-17

    Microquasars are stellar-mass black holes accreting matter from a companion star and ejecting plasma jets at almost the speed of light. They are analogues of quasars that contain supermassive black holes of 10(6) to 10(10) solar masses. Accretion in microquasars varies on much shorter timescales than in quasars and occasionally produces exceptionally bright X-ray flares. How the flares are produced is unclear, as is the mechanism for launching the relativistic jets and their composition. An emission line near 511 kiloelectronvolts has long been sought in the emission spectrum of microquasars as evidence for the expected electron-positron plasma. Transient high-energy spectral features have been reported in two objects, but their positron interpretation remains contentious. Here we report observations of γ-ray emission from the microquasar V404 Cygni during a recent period of strong flaring activity. The emission spectrum around 511 kiloelectronvolts shows clear signatures of variable positron annihilation, which implies a high rate of positron production. This supports the earlier conjecture that microquasars may be the main sources of the electron-positron plasma responsible for the bright diffuse emission of annihilation γ-rays in the bulge region of our Galaxy. Additionally, microquasars could be the origin of the observed megaelectronvolt continuum excess in the inner Galaxy. PMID:26934231

  1. Positron annihilation studies of the electronic structure and fermiology of the high-{Tc} superconductors

    SciTech Connect

    Smedskjaer, L.C.; Bansil, A.

    1992-09-01

    We discuss the application of the positron annihilation angular correlation (ACAR) spectroscopy for investigating the electronic structure and Fermiology of the high-T{sub c} superconductors, with focus on the YBa{sub 2}Cu{sub 3}O{sub 7} system where most of the experimental and theoretical work has to date been concentrated. Comparisons between measured 2D-ACAR positron spectra and band theory predictions show a remarkable agreement (for the normal state), indicating that the electronic structure and Fermi surface of this material is described reasonably by the conventional picture.

  2. Positron annihilation studies of the electronic structure and fermiology of the high-[Tc] superconductors

    SciTech Connect

    Smedskjaer, L.C. ); Bansil, A. . Dept. of Physics)

    1992-09-01

    We discuss the application of the positron annihilation angular correlation (ACAR) spectroscopy for investigating the electronic structure and Fermiology of the high-T[sub c] superconductors, with focus on the YBa[sub 2]Cu[sub 3]O[sub 7] system where most of the experimental and theoretical work has to date been concentrated. Comparisons between measured 2D-ACAR positron spectra and band theory predictions show a remarkable agreement (for the normal state), indicating that the electronic structure and Fermi surface of this material is described reasonably by the conventional picture.

  3. Macromolecular conformation in solution. Study of carbonic anhydrase by the positron annihilation technique.

    PubMed Central

    Handel, E D; Graf, G; Glass, J C

    1980-01-01

    The structural features of carbonic anhydrase (carbonate hydro-lyase; EC 4.2.1.1) in aqueous solution were probed by the positron annihilation technique. The data obtained under varying conditions of temperature, pH, and enzyme concentration were interpreted in terms of the free volume model. The change of enzymic activity with temperature is accompanied by a change in free volume of the protein. Upon thermal denaturation an irreversible change in free volume of the molecule occurred. At low temperatures the protein-water interactions were investigated. These results are discussed in terms of current concepts of structure-function relationships in proteins. This study shows the sensitivity of the positron annihilation method toward the structure of proteins related to their overall conformation and to the nature of bound water. PMID:6789901

  4. Order-disorder transition in clathrate Ba6Ge25 studied by positron annihilation

    NASA Astrophysics Data System (ADS)

    Li, X. F.; Zhao, B.; Zhang, T.; He, H. F.; Zhang, Q.; Yang, D. W.; Chen, Z. Q.; Tang, X. F.

    2015-07-01

    Clathrate Ba6Ge25 is prepared by melt method and spark plasma sintering. Structural transition below room temperature is studied by positron annihilation and X-ray diffraction measurements. There is a pronounced transition in the temperature range of 200-250 K which might be involved with the movement of Ba atoms in Ge cages and result in disordered structure. This transition is further confirmed by the theoretical calculation of positron annihilation states. Thus our results confirm the structural models proposed by Carrillo-Cabrera et al. (2005). The measured specific heat capacity, electric resistivity and magnetic susceptibility all show anomalous transition in the same temperature range, indicating that the movement of Ba atoms in the cage has influence on the thermal, electric as well as magnetic properties of Ba6Ge25.

  5. INTEGRAL/SPI Limits on Electron-Positron Annihilation Radiation from the Galactic Plane

    NASA Technical Reports Server (NTRS)

    Teegarden, B. J.; Watanabe, K.; Jean, P.; Knoedlseder, J.; Lonjou, V.; Roques, J. P.; Skinner, G. K.; vonBallmoos, P.; Weidenspointner, G.; Bazzano, A.

    2005-01-01

    The center of our Galaxy is a known strong source of electron-positron 511- keV annihilation radiation. Thus far, however, there have been no reliable detections of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (INTErnational Gamma-RAy Astrophysics Laboratory) mission, launched in Oct. 2002, is the detailed study of this radiation. The Spectrometer on INTEGRAL (SPI) is a high resolution coded-aperture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution and energy resolution. We report results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic Plane. No positive annihilation flux was detected outside of the central region (|l| greater than 40 degrees) of our Galaxy. In this paper we describe the observations and data analysis methods and give limits on the 511-keV flux.

  6. INTEGRAL/SPI Observations of Electron-Positron Annihilation Radiation from our Galaxy

    NASA Technical Reports Server (NTRS)

    Teegarden, B. J.; Watanabe, K.; Knoedlseder, J.; Jean, P.; Lonjou, V.; Weidenspointer, G.; Skinner, G.; Vedrenne, G.; Roques, J.-P.; Schanne, S.; Schoenfelder, V.

    2005-01-01

    The spectrometer on INTEGRAL (SPI) is a coded-aperture gamma-ray telescope with moderate angular resolution (3 deg) and superior energy resolution (2 keV at 511 kev). One of it's principal science goals is the detailed study of 511 keV electron-positron annihilation from our Galaxy. The origin of this radiation remains a mystery, however current morphological studies suggest an older stellar population. There has also been recent speculation on the possibility of the existence of light (< 100 MeV) dark matter particles whose annihilation or decay could produce the observed 511 keV emission. In this paper we summarize the current results from SPI, compare them with previous results and discuss their implication on possible models for the production of the annihilation radiation.

  7. Different ways of dealing with Compton scattering and positron annihilation experimental data

    NASA Astrophysics Data System (ADS)

    Kontrym-Sznajd, G.; Samsel-Czekała, M.

    2005-05-01

    Different ways of dealing with one-dimensional (1D) spectra, measured e.g., in the Compton scattering or angular correlation of positron annihilation radiation (ACAR) experiments, are presented. Using the example of divalent hexagonal close packed metals, we show what kind of information on the electronic structure one can get from 1D profiles interpreted in terms of either 2D or 3D momentum densities.

  8. Dynamics of defects in x-ray irradiated alkali chloride crystals studied by positron annihilation

    SciTech Connect

    Stern, S.H.

    1982-01-01

    Data on the time dependence of positron-electron annihilation characteristics in single crystals of the homologous series NaCl, KCl, RbCl, and CsCl after large doses of x irradiation are reported. A new instrument, the ..pi..-radian coincidence apparatus (PICA), recorded the coincidence count rate P of the two 0.5-MeV annihilation ..gamma.. rays emerging 180/sup 0/ apart from the crystal during isothermal and isochronal heating conditions. In most crystals an initial rapid increase of P to a maximum followed by a slow decline toward the coincidence count rate corresponding to the pre-irradiation state of the crystal was observed. Positron-annihilation data were completed by independent measurements of the optical absorption in KCl and NaCl crystals after various durations of isothermal heating. Absorption spectrophotometry revealed enhancement of the M band in KCl, of the R and N bands in NaCl, at the expense of the F band during the interpretation that positrons are trapped by radiation-induced color centers in which they annihilate with a higher P than in the bulk of the crystal. The dynamics associated with the incipient rise of P during the initial heating period is attributable to the agglomeration of F centers into aggregate centers. The rise times of P give access to the diffusion rates for agglomeration. At equal temperatures, a strong dependence of the rate of defect diffusion on the size of the cation was observed. The data must be corrected for the effects of decoloration of the crystals by the positrons during the measurements. Activation energies for defect diffusion annealing are extracted.

  9. Extragalactic Inverse Compton Light from Dark Matter annihilation and the Pamela positron excess

    SciTech Connect

    Profumo, Stefano; Jeltema, Tesla E. E-mail: tesla@ucolick.org

    2009-07-01

    We calculate the extragalactic diffuse emission originating from the up-scattering of cosmic microwave photons by energetic electrons and positrons produced in particle dark matter annihilation events at all redshifts and in all halos. We outline the observational constraints on this emission and we study its dependence on both the particle dark matter model (including the particle mass and its dominant annihilation final state) and on assumptions on structure formation and on the density profile of halos. We find that for low-mass dark matter models, data in the X-ray band provide the most stringent constraints, while the gamma-ray energy range probes models featuring large masses and pair-annihilation rates, and a hard spectrum for the injected electrons and positrons. Specifically, we point out that the all-redshift, all-halo inverse Compton emission from many dark matter models that might provide an explanation to the anomalous positron fraction measured by the Pamela payload severely overproduces the observed extragalactic gamma-ray background.

  10. Temperature dependence of the positron lifetime spectra of rubber-carbon black composites

    NASA Astrophysics Data System (ADS)

    Wang, Jingyi; Quarles, C. A.

    2003-10-01

    We have constructed a temperature controlled sample chamber, which uses a 30 liter liquid nitrogen Dewar and dipstick originally used for a Germanium detector, to study the temperature dependence of the positron lifetime and Doppler broadening spectra of polymer composite samples. In order to understand how carbon black (CB) affects positron lifetime (LT) in rubber materials, we also investigated cross-link density by studying positron lifetime in rubber with different sulfur concentration. We provide experimental data that will characterize the temperature dependence of the positron lifetime. Data is provided for two polymers, natural rubber and SnSSBR, and for these polymers mixed with different types of CB (N-115 and N-762). The temperature range studied was from room temperature to below the glass transition temperature. We will also provide experimental data that will show the relationship between lifetime and different sulfur concentration. We will conclude with a discussion of how CB affects the positron lifetime in polymers.

  11. Probing polymeric thin films using beam-based positronium annihilation lifetime spectroscopy

    NASA Astrophysics Data System (ADS)

    Peng, Huagen

    Positrounium annihilation lifetime spectroscopy (PALS) utilizing a focused low energy beam of positrons to control implantation depth enables the analysis of very thin films. Beam-PALS was used to study confinement and interfacial effects on polymer mobility in ultra-thin polycarbonate films and to characterize nanoporous structures of polymeric low dielectric constant (low-k ) thin films. Three complementary techniques were used to address the apparent discrepancies in recent polymer film results. Beam-PALS (probing positronium nanovoid lifetime, tau), specular X-ray reflectivity (SXR, monitoring film thickness, h) and incoherent neutron scattering (INS, characterizing mean-square atomic displacements, ) were combined to study the thermophysical properties of Bisphenol-A polycarbonate (PC) ultra-thin films (60 A to ˜1000 A) supported on an oxidized silicon wafer surface. As h decreased the concomitant reduction in thermal expansion coefficients of h, tau and < u2>, as well as the decreased amplitudes of < u2>, indicated that thin film confinement produces suppressed molecular mobility in PC. These films were modeled with an immobile interface layer ranging from 38 A to 130 A, depending on the measurement technique and the temperature range. No clear trends in the apparent glass transition temperature (Tg) emerged from these techniques, thus rendering Tg shifts inconclusive and of less fundamental importance. Beam-PALS was also applied to characterize several generations of porous SiLK (Trade Mark of Dow Chemical) low-k films to reveal the size, size distribution, interconnectivity and possible morphology of the engineered nanopores. The dependence of these properties on porogen loading/porosity was carefully analyzed and compared, when possible, with results from other techniques such as small-angle X-ray scattering and AFM. Unique to the PALS technique is the ability to quantify the pore interconnection length. Porous SiLK (V9), U.2 and Y were found to have

  12. Hunting for glueballs in electron-positron annihilation

    SciTech Connect

    Stanley Brodsky; Alfred Scharff Goldhaber; Jungil Lee

    2003-05-01

    We calculate the cross section for the exclusive production of J{sup PC} = 0{sup ++} glueballs G{sub 0} in association with the J/{psi} in e{sup +}e{sup -} annihilation using the pQCD factorization formalism. The required long-distance matrix element for the glueball is bounded by CUSB data from a search for resonances in radiative {Upsilon} decay. The cross section for e{sup +}e{sup -} {yields} J/{psi} + G{sub 0} at {radical}s = 10.6 GeV is similar to exclusive charmonium-pair production e{sup +}e{sup -} {yields} J/{psi} + h for h = {eta}{sub c} and {chi}{sub c0}, and is larger by a factor 2 than that for h = {eta}{sub c}(2S). As the subprocesses {gamma}* {yields} (c {bar c}) (c {bar c}) and {gamma}* {yields} (c {bar c}) (g g) are of the same nominal order in perturbative QCD, it is possible that some portion of the anomalously large signal observed by Belle in e{sup +}e{sup -} {yields} J/{psi} X may actually be due to the production of charmonium-glueball J/{psi} G{sub J} pairs.

  13. Hunting for Glueballs in Electron-Positron Annihilation

    NASA Astrophysics Data System (ADS)

    Brodsky, Stanley J.; Goldhaber, Alfred Scharff; Lee, Jungil

    2003-09-01

    We calculate the cross section for the exclusive production of JPC=0++ glueballs G0 in association with the J/ψ in e+e- annihilation using the perturbative QCD factorization formalism. The required long-distance matrix element for the glueball is bounded by CUSB data from a search for resonances in radiative ϒ decay. The cross section for e+e-→J/ψ+G0 at (s)=10.6 GeV is similar to exclusive charmonium-pair production e+e-→J/ψ+h for h=ηc and χc0, and is larger by a factor of 2 than that for h=ηc(2S). As the subprocesses γ*→(cc¯)(cc¯) and γ*→(cc¯)(gg) are of the same nominal order in perturbative QCD, it is possible that some portion of the anomalously large signal observed by Belle in e+e-→J/ψX may actually be due to the production of charmonium-glueball J/ψGJ pairs.

  14. The 511 keV emission from positron annihilation in the Galaxy

    SciTech Connect

    Prantzos, N.; Boehm, C.; Bykov, A. M.; Diehl, R.; Ferriere, K.; Guessoum, N.; Jean, P.; Knoedlseder, J.; Marcowith, A.; Moskalenko, I. V.; Strong, A.; Weidenspointner, G.

    2011-07-01

    The first {gamma}-ray line originating from outside the Solar System that was ever detected is the 511 keV emission from positron annihilation in the Galaxy. Despite 30 years of intense theoretical and observational investigation, the main sources of positrons have not been identified up to now. Observations in the 1990s with OSSE/CGRO (Oriented Scintillation Spectrometer Experiment on GRO satellite/Compton Gamma Ray Observatory) showed that the emission is strongly concentrated toward the Galactic bulge. In the 2000s, the spectrometer SPI aboard the European Space Agency's (ESA) International Gamma Ray Astrophysics Laboratory (INTEGRAL) allowed scientists to measure that emission across the entire Galaxy, revealing that the bulge-to-disk luminosity ratio is larger than observed at any other wavelength. This mapping prompted a number of novel explanations, including rather ''exotic'' ones (e.g., dark matter annihilation). However, conventional astrophysical sources, such as type Ia supernovae, microquasars, or x-ray binaries, are still plausible candidates for a large fraction of the observed total 511 keV emission of the bulge. A closer study of the subject reveals new layers of complexity, since positrons may propagate far away from their production sites, making it difficult to infer the underlying source distribution from the observed map of 511 keV emission. However, in contrast to the rather well-understood propagation of high-energy (>GeV) particles of Galactic cosmic rays, understanding the propagation of low-energy ({approx}MeV) positrons in the turbulent, magnetized interstellar medium still remains a formidable challenge. The spectral and imaging properties of the observed 511 keV emission are reviewed and candidate positron sources and models of positron propagation in the Galaxy are critically discussed.

  15. Ab initio calculation of positron distribution, ACAR and lifetime in TTF-TCNQ

    NASA Astrophysics Data System (ADS)

    Ishibashi, Shoji; Kohyama, Masanori

    2000-06-01

    We have performed ab initio calculations of positron distribution, ACAR and lifetime in the quasi-one-dimensional organic conductor TTF-TCNQ. The electronic structure is obtained within the LDA, while the positron state is calculated either with the LDA or with the GGA. Except the positron lifetime, differences between the LDA and GGA results are rather small. The obtained results are compared with our previous experiments and calculations.

  16. Positron annihilation Doppler broadening measurement for bulk amorphous alloy by using high energy positron generated from LCS gamma-ray at NEW SUBARU

    NASA Astrophysics Data System (ADS)

    Hori, F.; Ueno, Y.; Ishii, K.; Ishiyama, T.; Iwase, A.; Miyamoto, S.; Terasawa, T.

    2016-01-01

    A simple positron annihilation measurement apparatus via pair creation has been developed using high energetic gamma beam generated by laser Compton scattering (LCS) of 1 GeV electrons circulated in a storage ring and laser light with the power more than 1 W at the New SUBARU synchrotron radiation facility, University of Hyogo. This MeV ordered energy changeable positron apparatus is useful to study defects in bulk materials. In this study, the average energy of 8MeV positron was selected by the wavelength of laser light and circulated electron energy in photon factory. As a demonstrate of non-destruction positron measurement by this apparatus, positron annihilation Doppler broadening measurement has performed for bulk size of amorphous and crystal structured Zr based alloys. The larger Doppler broadening S parameter for amorphous alloy than that for crystallized one has been successfully measured.

  17. Quark Flavor Identification in Electron-Positron Annihilation.

    NASA Astrophysics Data System (ADS)

    Kaye, Harold Stephen

    The MAC (Magnetic Calorimeter) Detector at the PEP electron-positron storage ring at SLAC is used to obtain multihadron events at a center of mass energy of 29 GeV. Particles that penetrate the one-meter thickness of steel contained in the calorimetric detector are tracked by drift chambers and identified as muon candidates. The momentum of the muons is obtained by measurement of the curvature of the track through the magnetized steel. Events with a muon candidate with momentum greater than 2 GeV/c are studied in this analysis. The momentum of the muon transverse to the event thrust axis is used to obtain samples enriched in events with either b or c parent quarks. Background from light quark events is concentrated at low values of the transverse momentum, so that the high transverse momentum sample contains mostly b quark events. The total momentum spectrum of the muons is used to infer the fragmentation function of the b quark. It is found that the B meson carries away most of the momentum of the b quark in the fragmentation process. The semimuonic branching fraction of the B mesons, averaged over the mixture of charged and neutral mesons present, is. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). The invariant mass is computed for the jets in these events and is used to confirm the presence of heavy quark events in the sample. By the same technique, an additional one-third charged quark with mass less than 14 GeV is ruled out. Also, charged Higgs particles and technipions with masses between 9 and 13 GeV are ruled out, with more than 95% confidence, if their predominant decay mode is to the heaviest available quarks. The charged multiplicity of the events is indicative of the presence of weak decays. The forward/backward asymmetry of the b quark events is consistent with the predicted value. Pairs of oppositely charged muons in the same jet are studied, and an upper limit of 0.8% is established for the dimuon branching fraction of the b. This result rules

  18. Microstructure Evaluation of Fe-BASED Amorphous Alloys Investigated by Doppler Broadening Positron Annihilation Technique

    NASA Astrophysics Data System (ADS)

    Lu, Wei; Huang, Ping; Wang, Yuxin; Yan, Biao

    2013-07-01

    Microstructure of Fe-based amorphous and nanocrystalline soft magnetic alloy has been investigated by X-ray diffraction (XRD), transmission electronic microscopy (TEM) and Doppler broadening positron annihilation technique (PAT). Doppler broadening measurement reveals that amorphous alloys (Finemet, Type I) which can form a nanocrystalline phase have more defects (free volume) than alloys (Metglas, Type II) which cannot form this microstructure. XRD and TEM characterization indicates that the nanocrystallization of amorphous Finemet alloy occurs at 460°C, where nanocrystallites of α-Fe with an average grain size of a few nanometers are formed in an amorphous matrix. With increasing annealing temperature up to 500°C, the average grain size increases up to around 12 nm. During the annealing of Finemet alloy, it has been demonstrated that positron annihilates in quenched-in defect, crystalline nanophase and amorphous-nanocrystalline interfaces. The change of line shape parameter S with annealing temperature in Finemet alloy is mainly due to the structural relaxation, the pre-nucleation of Cu nucleus and the nanocrystallization of α-Fe(Si) phase during annealing. This study throws new insights into positron behavior in the nanocrystallization of metallic glasses, especially in the presence of single or multiple nanophases embedded in the amorphous matrix.

  19. Measurement of the hadronic cross section in electron-positron annihilation

    SciTech Connect

    Clearwater, S.

    1983-11-01

    This thesis describes the most precise measurement to date of the ratio R, the hadronic cross section in lowest order electron-positron annihilation to the cross section for muon pair production in lowest order electron-positron annihilation. This experiment is of interest because R is a fundamental parameter that tests in a model independent way the basic assumptions of strong interaction theories. According to the assumptions of one of these theories the value of R is determined simply from the electric charges, spin, and color assignments of the produced quark-pairs. The experiment was carried out with the MAgnetic Calorimeter using collisions of 14.5 GeV electrons and positrons at the 2200m circumference PEP storage ring at SLAC. The MAC detector is one of the best-suited collider detectors for measuring R due to its nearly complete coverage of the full angular range. The data for this experiment were accumulated between February 1982 and April 1983 corresponding to a total event sample of about 40,000 hadronic events. About 5% of the data were taken with 14 GeV beams and the rest of the data were taken with 14.5 GeV beams. A description of particle interactions and experimental considerations is given.

  20. Novel time-of-flight spectrometer for the analysis of positron annihilation induced Auger electrons

    SciTech Connect

    Hugenschmidt, Christoph; Legl, Stefan

    2006-10-15

    Positron annihilation induced Auger-electron spectroscopy (PAES) has several advantages over conventional Auger-electron spectroscopy such as extremely high surface sensitivity and outstanding signal-to-noise ratio at the Auger-transition energy. In order to benefit from these prominent features a low-energy positron beam of high intensity is required for surface sensitive PAES studies. In addition, an electron energy analyzer is required, which efficiently detects the Auger electrons with acceptable energy resolution. For this reason a novel time-of-flight (TOF) spectrometer has been developed at the intense positron source NEPOMUC that allows PAES studies within short measurement time. This TOF-PAES setup combines a trochoidal filter and a flight tube in a Faraday cage in order to achieve an improved energy resolution of about 1 eV at high electron energies up to E{approx_equal}1000 eV. The electron flight time is the time between the annihilation radiation at the sample and when the electron hits a microchannel plate detector at the end of the flight tube.

  1. Local electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation

    NASA Astrophysics Data System (ADS)

    Ceeh, Hubert; Weber, Josef Andreass; Böni, Peter; Leitner, Michael; Benea, Diana; Chioncel, Liviu; Ebert, Hubert; Minár, Jan; Vollhardt, Dieter; Hugenschmidt, Christoph

    2016-02-01

    We employ a positron annihilation technique, the spin-polarized two-dimensional angular correlation of annihilation radiation (2D-ACAR), to measure the spin-difference spectra of ferromagnetic nickel. The experimental data are compared with the theoretical results obtained within a combination of the local spin density approximation (LSDA) and the many-body dynamical mean-field theory (DMFT). We find that the self-energy defining the electronic correlations in Ni leads to anisotropic contributions to the momentum distribution. By direct comparison of the theoretical and experimental results we determine the strength of the local electronic interaction U in ferromagnetic Ni as 2.0 ± 0.1 eV.

  2. Local electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation

    PubMed Central

    Ceeh, Hubert; Weber, Josef Andreass; Böni, Peter; Leitner, Michael; Benea, Diana; Chioncel, Liviu; Ebert, Hubert; Minár, Jan; Vollhardt, Dieter; Hugenschmidt, Christoph

    2016-01-01

    We employ a positron annihilation technique, the spin-polarized two-dimensional angular correlation of annihilation radiation (2D-ACAR), to measure the spin-difference spectra of ferromagnetic nickel. The experimental data are compared with the theoretical results obtained within a combination of the local spin density approximation (LSDA) and the many-body dynamical mean-field theory (DMFT). We find that the self-energy defining the electronic correlations in Ni leads to anisotropic contributions to the momentum distribution. By direct comparison of the theoretical and experimental results we determine the strength of the local electronic interaction U in ferromagnetic Ni as 2.0 ± 0.1 eV. PMID:26879249

  3. Local electron-electron interaction strength in ferromagnetic nickel determined by spin-polarized positron annihilation.

    PubMed

    Ceeh, Hubert; Weber, Josef Andreass; Böni, Peter; Leitner, Michael; Benea, Diana; Chioncel, Liviu; Ebert, Hubert; Minár, Jan; Vollhardt, Dieter; Hugenschmidt, Christoph

    2016-01-01

    We employ a positron annihilation technique, the spin-polarized two-dimensional angular correlation of annihilation radiation (2D-ACAR), to measure the spin-difference spectra of ferromagnetic nickel. The experimental data are compared with the theoretical results obtained within a combination of the local spin density approximation (LSDA) and the many-body dynamical mean-field theory (DMFT). We find that the self-energy defining the electronic correlations in Ni leads to anisotropic contributions to the momentum distribution. By direct comparison of the theoretical and experimental results we determine the strength of the local electronic interaction U in ferromagnetic Ni as 2.0 ± 0.1 eV. PMID:26879249

  4. Cosmic-ray antiprotons, positrons, and gamma rays from halo dark matter annihilation

    NASA Technical Reports Server (NTRS)

    Rudaz, S.; Stecker, F. W.

    1988-01-01

    The subject of cosmic ray antiproton production is reexamined by considering other choices for the nature of the Majorana fermion chi other than the photino considered in a previous article. The calculations are extended to include cosmic-ray positrons and cosmic gamma rays as annihilation products. Taking chi to be a generic higgsino or simply a heavy Majorana neutrino with standard couplings to the Z-zero boson allows the previous interpretation of the cosmic antiproton data to be maintained. In this case also, the annihilation cross section can be calculated independently of unknown particle physics parameters. Whereas the relic density of photinos with the choice of parameters in the previous paper turned out to be only a few percent of the closure density, the corresponding value for Omega in the generic higgsino or Majorana case is about 0.2, in excellent agreement with the value associated with galaxies and one which is sufficient to give the halo mass.

  5. AC Dielectric Properties and Positron Annihilation Study on Co and Ti Substitution Effect on Ca-Sr M-Hexaferrites

    NASA Astrophysics Data System (ADS)

    Mahmoud, K. R.; Eraky, M. R.

    2016-06-01

    The dependence of AC conductivity σ AC, dielectric constant έ, and dielectric loss tangent tan δ on frequency and composition have been investigated at room temperature for polycrystalline Ca0.5Sr0.5Co x Ti x Fe12 - 2 x O19 (where 0.0 ≤ x ≤ 0.8) hexaferrites. It was found that the parameters σ AC, ɛ ', and tan δ have maximum values at x = 0.4 of the Co and Ti substitution. The behavior of σ AC, ɛ ', and tan δ with frequency and composition was explained on the basis of the hopping conduction mechanism and the Koops model. Positron annihilation lifetime spectroscopy (PALS) was used to investigate the defects and changes in electron density for hexaferrite samples. The PAL parameters ( τ 1, I 1, τ 2, I 2, and mean lifetime) show that altering the doping percentage of the Co and Ti ions affects the size and concentration of defects. The results reveal that there are some large voids in the studied samples. The obtained results indicate the high sensitivity of the PALS technique to the enhanced structure changes with changing composition of the investigated samples and correlate the results with the measured electrical parameters.

  6. Possible presence of hydrophilic SO3H nanoclusters on the surface of dry ultrathin Nafion® films: a positron annihilation study.

    PubMed

    Mohamed, Hamdy F M; Kuroda, S; Kobayashi, Y; Oshima, N; Suzuki, R; Ohira, A

    2013-02-01

    Solutions of Nafion® with an ion exchange capacity (IEC) of 0.91 meq g(-1), which are on the verge of the formation of SO(3)H nanoclusters, were spin coated on silicon (Si), glassy carbon (GC) and platinum/silicon (Pt/Si) substrates to form films of up to 256 nm thickness. Nanostructure of the films was studied using Doppler broadening of annihilation radiation (DBAR), positron annihilation lifetime (PAL), X-ray photoelectron spectroscopy (XPS), an atomic force microscope (AFM) and contact angle measurements. Contact angles as low as 10 degrees indicate that the surface of dry ultrathin Nafion® films on Si is highly hydrophilic. XPS data of 10 nm thick, ultrathin film on Si show that oxygen concentration is enhanced and the SO(3)H group concentration, in other words, IEC on the surface is much higher than other films. The S parameter measured by DBAR of an ultrathin Nafion® film on Si is much higher than that of the films on the other substrates. We consider that a large number of hydrophilic, reversed micelle like SO(3)H groups are on the surface of the ultrathin Nafion® film on Si but not on the surface of other films. Positrons implanted into the film are trapped by the SO(3)H clusters, annihilating with the electrons of oxygen and exhibit the high S parameter. The SO(3)H concentration on the surface of thin Nafion® films on GC and Pt/Si substrates may not be so high as the threshold for the formation of a large number of SO(3)H clusters. Positrons implanted into the films annihilate mostly with fluorine atoms, resulting in a low S parameter. The film-substrate interaction plays an essential role in nanostructuring of Nafion® thin films, which may also be the case for Nafion® on the catalysts of polymer electrolyte fuel cells. PMID:23238425

  7. Gamma-ray lines from novae. [relationship to radioactive decay and positron annihilation

    NASA Technical Reports Server (NTRS)

    Clayton, D. D.; Hoyle, F.

    1974-01-01

    An appropriate gamma-ray telescope could detect the gamma-rays associated with radioactive decays. The observable lines would be the annihilation radiation following the positron emission of N-13, O-14, O-15, and Na-22 and the 2.312-MeV line emitted following the O-14 decay and the 1.274-MeV line emitted following the Na-22 decay. The experimental possibility should be borne in mind for the occurrence of novae within a few kiloparsecs.

  8. Production of {omega}{pi}{sup 0} pairs in electron-positron annihilation

    SciTech Connect

    Arbuzov, A. B.; Kuraev, E. A.; Volkov, M. K.

    2011-04-15

    The process of electron-positron annihilation into a pair of {pi}{sup 0} and {omega} mesons is considered in the framework of the SU(2)xSU(2) Nambu-Jona-Lasinio model. Contributions of intermediate photons, {rho}(770) and {rho}{sup '}(1450) vector mesons are taken into account. It is shown that the bulk of the cross section at energies below 2 GeV is provided by the process with intermediate {rho}{sup '}(1450) state. The contribution due to single photon and {rho}(770) exchange is in agreement with the vector meson dominance model. Numerical results are compared with experimental data.

  9. Positron annihilation spectroscopy: a new frontier for understanding nanoparticle-loaded polymer brushes

    NASA Astrophysics Data System (ADS)

    Panzarasa, Guido; Aghion, Stefano; Soliveri, Guido; Consolati, Giovanni; Ferragut, Rafael

    2016-01-01

    Nanoparticle-loaded polymer brushes are powerful tools for the development of innovative devices. However, their characterization is challenging and arrays of different techniques are typically required to gain sufficient insight. Here we demonstrate for the first time the suitability of positron annihilation spectroscopy (PAS) to investigate, with unprecedented detail and without making the least damage to samples, the physico-chemical changes experienced by pH-responsive polymer brushes after protonation and after loading of silver nanoparticles. One of the most important findings is the depth profiling of silver nanoparticles inside the brushes. These results open up a completely new way to understand the structure and behavior of such complex systems.

  10. Dynamics of Defects in X-Ray Irradiated Alkali Chloride Crystals Studied by Positron Annihilation.

    NASA Astrophysics Data System (ADS)

    Stern, Stanley Hy.

    This thesis reports first data on the time dependence of positron-electron annihilation characteristics in single crystals of the homologous series NaCl, KCl, RbCl, and CsCl after large doses of x irradiation. A new instrument, the (pi)-radian coincidence apparatus (PICA), recorded the coincidence count rate P of the two 0.5-MeV annihilation (gamma) rays emerging 180(DEGREES) apart from the crystal during isothermal and isochronal heating conditions. In most crystals one observes an initial rapid increase of P to a maximum followed by a slow decline toward the coincidence count rate corresponding to the pre-irradiation state of the crystal. Positron-annihilation data were completed by independent measurements of the optical absorption in KCl and NaCl crystals after various durations of isothermal heating. Absorption spectrophotometry revealed enhancement of the M band in KCl, of the R and N bands in NaCl, at the expense of the F band during the interval of increasing P. The PICA results are consistent with the interpretation that positrons are trapped by radiation-induced color centers in which they annihilate with a higher P than in the bulk of the crystal. The dynamics associated with the incipient rise of P during the initial heating period is attributable to the agglomeration of F centers into aggregate centers. The rise times of P give access to the diffusion rates for agglomeration. At equal temperatures, we observe a strong dependence of the rate of defect diffusion on the size of the cation. For example, it is 100 times faster in CsCl than in NaCl at 120(DEGREES)C. The data must be corrected for the effects of decoloration of the crystals by the positrons during the measurements. Activation energies for defect diffusion annealing are extracted. They test the hypotheses underlying the theories of macroscopic transport properties in these crystals in that they are indicative of the dominant microscopic lattice processes and their dependence on the crystal

  11. Study on the Enhanced Contribution in Noble Metals from Positron Annihilation

    NASA Astrophysics Data System (ADS)

    Hamid, A. S.; Ahmed, M. M.; Abu-Elmagd, M. S. M.; Rizk, R. A. M.

    2007-02-01

    Our motivations in the current work were to inspect various enhancement formulae along with to classify the electronic structure of noble metals Cu, Ag and Au. The measurements were performed via the two dimensional angular correlation of annihilation radiation 2D-ACAR apparatus. The electron density in momentum space ρ(p) was reconstructed and it displayed the following features. Initially, the reciprocal lattice points underscored the calculations, and they revealed Fermi surface features. Additionally, enhanced anisotropy exposed nearby Fermi momentum. They attributed to enhancement of the electronic wave function at the position of the positron. Finally, the high momentum contributions, due to interaction of positron with core-like-state, conducted the electronic structure of the metals under investigations. From another viewpoint, the features of Fermi surface of Cu, Ag and Au showed an expected analogous behavior as multiply connected sphere inside the first Brillouin zone.

  12. Study of microvoids in high-rate a-Si:H using positron annihilation

    SciTech Connect

    Zou, X.; Webb, D.P.; Lin, S.H.; Lam, Y.W.; Chan, Y.C.; Hu, Y.F.; Beling, C.D.; Fung, S.

    1997-07-01

    In this paper, the authors have carried out the positron annihilation measurement on high-rate and low-rate a-Si:H thin films deposited by PECVD. By means of the slow positron beam Doppler-broadening technique, the depth profiles of microvoids in a-Si:H have been determined. They have also studied the vacancy-type defect in the surface region in high-rate grown a-Si:H, making comparison between high-rate and low-rate a-Si:H. By plotting S and W parameters in the (S, W) plane, they have shown that the vacancies in all of the high-rate and low-rate deposited intrinsic samples, and in differently doped low-rate samples are of the same nature.

  13. Material characterization for advanced Si LSI process technology by means of positron annihilation

    NASA Astrophysics Data System (ADS)

    Uedono, A.; Oshima, N.; Ohdaira, T.; Suzuki, R.; Ishibashi, S.

    2013-06-01

    Vacancy-type defects in gas cluster ion implanted Si and electroless deposited Cu were studied by monoenergetic positron beams. For Ar gas cluster ion implanted Si, we found that the vacancy-rich region was localized at a depth of 0-13 nm. Two different defect species were found to coexist in the damaged region, and they were identified as divacancy-type defects and vacancy clusters filled with Ar. For electroless deposited Cu films, the major defect species were identified as vacancy complexes (V3-V4) and larger vacancy clusters (~V10). Annealing behaviours of the defects and the relation between the defects and impurities were also discussed. We have demonstrated the efficacy of positron annihilation to aid in the optimization of process parameters for advanced Si LSI processes.

  14. Assay of weathering effects on protective polymer coatings using positron annihilation spectroscopy

    SciTech Connect

    Hulett, L.D. Jr.; Wallace, S.; Xu, Jun; Nielsen, B.; Szeles, Cs.; Lynn, K.G.; Pfau, J.; Schaub, A.

    1995-02-01

    Polymer coatings, both with and without pigments, have been subjected to solar radiation and water spray weathering. The degrees of penetration of the weathering effects have been measured by injecting positrons of varying energy, i.e. to variable depths, into the films and observing the Doppler broadening of the annihilation radiation. The method is capable of detecting changes due to weathering effects at very early stages, long before visual examination reveals degradation. As little as one week of exposure caused measurable changes in the polymer structure, which were reflected in the Doppler broadening. Given further development, positron spectroscopy could possibly become a useful complement to the other methods of determining weatherabilities of protective polymer coatings.

  15. Astrophysical uncertainties in the cosmic ray electron and positron spectrum from annihilating dark matter

    SciTech Connect

    Simet, Melanie; Hooper, Dan E-mail: dhooper@fnal.gov

    2009-08-01

    In recent years, a number of experiments have been conducted with the goal of studying cosmic rays at GeV to TeV energies. This is a particularly interesting regime from the perspective of indirect dark matter detection. To draw reliable conclusions regarding dark matter from cosmic ray measurements, however, it is important to first understand the propagation of cosmic rays through the magnetic and radiation fields of the Milky Way. In this paper, we constrain the characteristics of the cosmic ray electron/positron propagation model through comparison with observational inputs, including recent data from the CREAM experiment, and use these constraints to estimate the corresponding uncertainties in the spectrum of cosmic ray electrons and positrons from dark matter particles annihilating in the halo of the Milky Way.

  16. Modification of steel surfaces induced by turning: non-destructive characterization using Barkhausen noise and positron annihilation

    NASA Astrophysics Data System (ADS)

    Čížek, J.; Neslušan, M.; Čilliková, M.; Mičietová, A.; Melikhova, O.

    2014-11-01

    This paper deals with the characterization of sub-surface damage caused by the machining of 100Cr6 roll bearing steel. The samples turned using tools with variable flank wears were characterized by two non-destructive techniques sensitive to defects introduced by plastic deformation: magnetic Barkhausen noise and positron annihilation. These techniques were combined with light and electron microscopy, x-ray diffraction and microhardness testing. The results of the experiment showed that damage in the sub-surface region increases with increasing flank wear, but from a certain critical value dynamic recovery takes place. The intensity of Barkhausen noise strongly decreases with increasing flank wear due to the increasing density of the dislocations pinning the Bloch walls and suppressing their motion. This was confirmed by positron annihilation spectroscopy, which enables the determination of the dislocation density directly. Hence, a good correlation between Barkhausen noise emission and positron annihilation spectroscopy was found.

  17. SYNCHROTRON RADIATION, FREE ELECTRON LASER, APPLICATION OF NUCLEAR TECHNOLOGY, ETC.: Identification of pore size in porous SiO2 thin film by positron annihilation

    NASA Astrophysics Data System (ADS)

    Zhang, Zhe; Qin, Xiu-Bo; Wang, Dan-Ni; Yu, Run-Sheng; Wang, Qiao-Zhan; Ma, Yan-Yun; Wang, Bao-Yi

    2009-02-01

    Positron annihilation lifetime and Doppler broadening of annihilation line techniques have been used to obtain information about the small pore structure and size of porous SiO2 thin film produced by sputtered Al-Si thin film and etched Al-Si thin film. The film is prepared by an Al/Si 75:25 at.-% (Al75Si25) target with the radiofrequency (RF) power of 66 W at room temperature. A 5 wt.-% phosphoric acid solution is used to etch the Al cylinders. All the Al cylinders dissolved in the solution after 15 h at room temperature, and the sample is subsequently rinsed in pure water. In this way, the porous SiO2 on the Si substrate is produced. From our results, the values of all lifetime components in the spectra of Al-Si thin film are less than 1 ns, but the value of one of the lifetime components in the spectra of porous SiO2 thin film is τ = 7.80 ns. With these values of lifetime, RTE (Rectangular Pore Extension) model has been used to analyze the pore size.

  18. Role of Se vacancies on Shubnikov-de Haas oscillations in Bi2Se3: A combined magneto-resistance and positron annihilation study

    NASA Astrophysics Data System (ADS)

    Devidas, T. R.; Amaladass, E. P.; Sharma, Shilpam; Rajaraman, R.; Sornadurai, D.; Subramanian, N.; Mani, Awadhesh; Sundar, C. S.; Bharathi, A.

    2014-12-01

    Magneto-resistance measurements coupled with positron lifetime measurements, to characterize the vacancy-type defects, have been carried out on the topological insulator (TI) system Bi2Se3 of varying Se/Bi ratio. Pronounced Shubnikov-de Haas (SdH) oscillations are seen in nominal Bi2Se3.1 crystals for measurements performed in magnetic fields up to 15 T in the 4 K-10 K temperature range, with field applied perpendicularly to the (001) plane of the crystal. The quantum oscillations, characteristic of 2D electronic structure, are seen only in the crystals that have a lower concentration of Se vacancies, as inferred from positron annihilation spectroscopy.

  19. Positron annihilation studies of defects in molecular beam epitaxy grown III-V layers

    SciTech Connect

    Umlor, M.T.; Keeble, D.J.; Asoka-Kumar, P.; Lynn, K.G.; Cooke, P.W.

    1994-08-01

    A summary of recent positron annihilation experiments on molecular beam epitaxy (MBE) grown III-V layers is Presented. Variable energy positron beam measurements on Al{sub 0.32}Ga{sub 0.68}As undoped and Si doped have been completed. Positron trapping at a open volume defect in Al{sub 0.32}Ga{sub 0.68}:Si for temperatures from 300 to 25 K in the dark was observed. The positron trap was lost after 1.3 eV illumination at 25K. These results indicate an open volume defect is associated with the local structure of the deep donor state of the DX center. Stability of MBE GaAs to thermal annealing war, investigated over the temperature range of 230 to 700{degrees}C, Proximity wafer furnace anneals in flowing argon were used, Samples grown above 450{degrees}C were shown to be stable but for sample below this temperature an anneal induced vacancy related defect was produced for anneals between 400 and 500{degrees}C. The nature of the defect was shown to be different for material grown at 350 and 230{degrees}C. Activation energies of 2.5 eV to 2.3 eV were obtained from isochronal anneal experiments for samples grown at 350 and 230{degrees}C, respectively.

  20. Free volume and ionic conductivity of poly(ether urethane)-LiClO4 polymeric electrolyte studied by positron annihilation

    NASA Astrophysics Data System (ADS)

    Peng, Z. L.; Wang, B.; Li, S. Q.; Wang, S. J.

    1995-01-01

    The positron lifetime spectra and ionic conductivity have been measured for poly(ether urethane)-LiClO4 polymeric electrolyte as a function of temperature. The glass transition temperature T(sub g), free-volume V(sub f), and fractional free-volume f were derived from the positron annihilation parameters. A correlation between fractional free-volume f(T) and conductivity sigma above T(sub g), log(sigma/sigma(T(sub g))) = C(sub 1)(f(T) - f(T(sub g)))/f(T), was first experimentally confirmed using measured positron annihilation results. The comparison of the value of the obtained constant C(sub 1) with the universal values for the segmental diffusion of amorphous polymers indicated that the critical free volume required for the ion transport is much smaller than that required for polymer chain segment mobility. Carrier transport and the segmental motion are discussed in terms of the free-volume theory.

  1. Quantification of stress history in type 304L stainless steel using positron annihilation spectroscopy

    SciTech Connect

    Walters, Thomas W.; Walters, Leon C.; Schoen, Marco P.; Naidu, D. Subbaram; Dickerson, Charles; Perrenoud, Ben C.

    2011-04-15

    Five Type 304L stainless steel specimens were subjected to incrementally increasing values of plastic strain. At each value of strain, the associated static stress was recorded and the specimen was subjected to positron annihilation spectroscopy (PAS) using the Doppler Broadening method. A calibration curve for the 'S' parameter as a function of stress was developed based on the five specimens. Seven different specimens (blind specimens labeled B1-B7) of 304L stainless steel were subjected to values of stress inducing plastic deformation. The values of stress ranged from 310 to 517 MPa. The seven specimens were subjected to PAS post-loading using the Doppler Broadening method, and the results were compared against the developed curve from the previous five specimens. It was found that a strong correlation exists between the 'S' parameter, stress, and strain up to a strain value of 15%, corresponding to a stress value of 500 MPa, beyond which saturation of the 'S' parameter occurs. Research Highlights: {yields} Specimens were initially in an annealed/recrystallized condition. {yields} Calibration results indicate positron annihilation measurements yield correlation. {yields} Deformation produced by cold work was likely larger than the maximum strain.

  2. The gamma-ray spectra of 5-carbon alkane isomers in the positron annihilation process

    NASA Astrophysics Data System (ADS)

    Ma, Xiaoguang; Zhu, Yinghao; Liu, Yang

    2016-05-01

    The gamma-ray spectra of pentane (C5H12) and its two isomers, i.e., 2-Methylbutane (CH3C(CH3)HC2H5) and 2,2-Dimethylpropane (C(CH3)4) have been studied theoretically in the present work. The recent experimental gamma-ray spectra of these three molecules show that they have the same Doppler shifts, although their molecular structures are dramatically different. In order to reveal why the gamma-ray spectra of these molecules are less sensitive to the molecular structures, the one-dimensional gamma-ray spectra and spherically averaged momentum (SAM) distributions, the two-dimensional angular correlation of annihilation radiation (ACAR), and the three-dimensional momentum distributions of the positron-electron pair are studied. The one-centered momentum distributions of the electrons are found to play more important role than the multi-centered coordinate distributions. The present theoretical predictions have confirmed the experimental findings for the first time. The dominance of the inner valence electrons in the positron-electron annihilation process has also been suggested in the present work.

  3. The influence of microstructure on the sintering process in crystalline metal powders investigated by positron lifetime spectroscopy: II. Tungsten powders with different powder-particle sizes

    NASA Astrophysics Data System (ADS)

    Staab, T. E. M.; Krause-Rehberg, R.; Vetter, B.; Kieback, B.; Lange, G.; Klimanek, P.

    1999-02-01

    Compacts of tungsten powder with five different powder-particle sizes (from 0953-8984/11/7/010/img7 to 0953-8984/11/7/010/img8) are subjected to pressureless sintering. We investigate the change in microstructure during the sintering process by positron lifetime spectroscopy. So as to be able to distinguish between defects having the same positron lifetime, we investigate their kinetics when the sample is annealed. In particular, we consider the annealing out of vacancy clusters after low-temperature electron irradiation, as well as recovery and recrystallization of a tungsten sheet, in as-manufactured form. Making measurements on uncompacted powder, we find an increasing fraction of positrons annihilating in surface states with decreasing powder-particle size. The powder-particle and grain sizes (influencing the x-ray domain size) are monitored additionally by means of metallography and x-ray diffraction. We find that all of the methods give results in agreement with each other. The small grain sizes at lower temperature, about one fifth of the powder-particle size, cause positrons to annihilate at grain boundaries, leading to vacancy-cluster-like signals. At the intensive-shrinkage stage, there are certainly contributions from different shrinkage mechanisms. The observed shrinkage rates can be explained by Coble creep. It is possible that dislocations also play a role as vacancy sources and sinks, since the intensive-shrinkage stage occurs in a temperature region wherein recrystallization takes place.

  4. An investigation of molecular structure of copolymers using positron annihilation spectroscopy

    NASA Technical Reports Server (NTRS)

    Singh, J. J.; St.clair, T. L.; Holt, W. H.; Mock, W., Jr.

    1985-01-01

    Positron lifetime measurements were made in copolyimides synthesized from linear 4,4 prime-bis(3,4-dicarboxyphenoxy) diphenylsulfide dianhydride (BDSDA)/4,4 prime-diaminodiphenyl (ODA) and BDSDA/1,3-diaminobenzene (m-phenylene diamine) homopolymers. The probability of positronium formation as well as its subsequent lifetime are lower in the BDSDA/ODA/MPD (50-50) copolyimide, indicating the presence of a transition molecular architecture characterized by higher electron density and stronger bonds which permit both chemical as well as physical entry of water molecules into it. The presence of this transition region imparts unique physical and mechanical properties to the copolyimide.

  5. On-ground detection of an electron-positron annihilation line from thunderclouds.

    PubMed

    Umemoto, D; Tsuchiya, H; Enoto, T; Yamada, S; Yuasa, T; Kawaharada, M; Kitaguchi, T; Nakazawa, K; Kokubun, M; Kato, H; Okano, M; Tamagawa, T; Makishima, K

    2016-02-01

    Thunderclouds can produce bremsstrahlung gamma-ray emission, and sometimes even positrons. At 00:27:00 (UT) on 13 January 2012, an intense burst of gamma rays from a thundercloud was detected by the GROWTH experiment, located in Japan, facing the Sea of Japan. The event started with a sharp gamma-ray flash with a duration of <300 ms coincident with an intracloud discharge, followed by a decaying longer gamma-ray emission lasting for ∼60 s. The spectrum of this prolonged emission reached ∼10 MeV, and contained a distinct line emission at 508±3(stat.)±5(sys.) keV, to be identified with an electron-positron annihilation line. The line was narrow within the instrumental energy resolution (∼80keV), and contained 520±50 photons which amounted to ∼10% of the total signal photons of 5340±190 detected over 0.1-10 MeV. As a result, the line equivalent width reached 280±40 keV, which implies a nontrivial result. The result suggests that a downward positron beam produced both the continuum and the line photons. PMID:26986281

  6. Study of the Fermi surface of molybdenum and chromium via positron annihilation experiments

    NASA Astrophysics Data System (ADS)

    Biasini, Maurizio

    2000-02-01

    A quantitative mapping of the Fermi surface (FS) of molybdenum and chromium was sought by modelling the three-dimensional k-space occupancy with a small number of parameters which were determined by a least-squares fit to the two-dimensional angular correlation of the electron-positron annihilation radiation (2D-ACAR) data subjected to a Lock-Crisp-West (LCW) transformation. The resulting FS topology of molybdenum, unlike what was assumed in previous 2D-ACAR studies, does not support the nesting of its two main FS sheets. In the case of chromium, although the overall discrepancy with the FS expected from the theory is larger, the difference in shape between the same two FS sheets is of lesser extent. According to this analysis the ratio of the electron Fermi volume to the hole Fermi volume is found to deviate from unity, the value expected for compensated metals, for both materials. We suggest that these discrepancies might be due to positron wave function and/or electron-positron many-body distortions not predicted by the theory.

  7. On-ground detection of an electron-positron annihilation line from thunderclouds

    NASA Astrophysics Data System (ADS)

    Umemoto, D.; Tsuchiya, H.; Enoto, T.; Yamada, S.; Yuasa, T.; Kawaharada, M.; Kitaguchi, T.; Nakazawa, K.; Kokubun, M.; Kato, H.; Okano, M.; Tamagawa, T.; Makishima, K.

    2016-02-01

    Thunderclouds can produce bremsstrahlung gamma-ray emission, and sometimes even positrons. At 00:27:00 (UT) on 13 January 2012, an intense burst of gamma rays from a thundercloud was detected by the GROWTH experiment, located in Japan, facing the Sea of Japan. The event started with a sharp gamma-ray flash with a duration of <300 ms coincident with an intracloud discharge, followed by a decaying longer gamma-ray emission lasting for ˜60 s. The spectrum of this prolonged emission reached ˜10 MeV, and contained a distinct line emission at 508 ±3 (stat .)±5 (sys .) keV, to be identified with an electron-positron annihilation line. The line was narrow within the instrumental energy resolution (˜80 keV) , and contained 520 ±50 photons which amounted to ˜10 % of the total signal photons of 5340 ±190 detected over 0.1-10 MeV. As a result, the line equivalent width reached 280 ±40 keV, which implies a nontrivial result. The result suggests that a downward positron beam produced both the continuum and the line photons.

  8. A positron annihilation study on the microstructure of the interpenetration polymer networks of cyanate ester resin/epoxy resin

    NASA Astrophysics Data System (ADS)

    Chenze, Qi; Chunqing, Li; Minfeng, Zeng; Baoyi, Wang; Jian, Zhang

    2010-04-01

    Cyanate ester (CE) resin was blended with epoxy resin (EP) at different mass ratios (CE/EP: 100/0, 90/10, 70/30, 50/50, 30/70, 10/90, 0/100). The free volume size of CE/EP IPNs has been determined by positron annihilation lifetime spectroscopy (PALS). The size decreased as the epoxy resin content increased. The PALS results are consistent with the chemical structure changes for the copolymerizing between CE and EP. The crosslinking units of curing products (oxazoline, oxazolidinone, and polyether network) of the blends are all smaller in size than those of triazine ring structure from neat CE. Therefore, the free volume size of the blends decreases with increase of EP content. Examination of the mechanical properties, thermal stability, and morphology of the blend systems showed that addition of epoxy resin resulted in improved toughness but a little sacrifice in thermal stability when compared with pure CE. The correlations between the free volume properties and physical properties (thermal stability and mechanical properties) have been discussed.

  9. Structural analysis of IPC zeolites and related materials using positron annihilation spectroscopy and high-resolution argon adsorption.

    PubMed

    Jagiello, J; Sterling, M; Eliášová, P; Opanasenko, M; Zukal, A; Morris, R E; Navaro, M; Mayoral, A; Crivelli, P; Warringham, R; Mitchell, S; Pérez-Ramírez, J; Čejka, J

    2016-06-01

    The advanced investigation of pore networks in isoreticular zeolites and mesoporous materials related to the IPC family was performed using high-resolution argon adsorption experiments coupled with the development of a state-of-the-art non-local density functional theory approach. The optimization of a kernel for model sorption isotherms for materials possessing the same layer structure, differing only in the interlayer connectivity (e.g. oxygen bridges, single- or double-four-ring building units, mesoscale pillars etc.) revealed remarkable differences in their porous systems. Using high-resolution adsorption data, the bimodal pore size distribution consistent with crystallographic data for IPC-6, IPC-7 and UTL samples is shown for the first time. A dynamic assessment by positron annihilation lifetime spectroscopy (PALS) provided complementary insights, simply distinguishing the enhanced accessibility of the pore network in samples incorporating mesoscale pillars and revealing the presence of a certain fraction of micropores undetected by gas sorption. Nonetheless, subtle differences in the pore size could not be discriminated based on the widely-applied Tao-Eldrup model. The combination of both methods can be useful for the advanced characterization of microporous, mesoporous and hierarchical materials. PMID:27210107

  10. Accounting for the lack of nano-effect in a thermoset/clay nanocomposite: A positron annihilation study

    NASA Astrophysics Data System (ADS)

    Rath, S. K.; Sudarshan, K.; Patri, M.; Pujari, P. K.

    2015-06-01

    The effect of nanoclay dispersion on the thermo-mechanical properties of an unsaturated polyester thermoset resin was studied by flexural and dynamic mechanical property measurements. Transmission electron microscopy studies revealed intercalated clay dispersion morphology in the nanocomposites. The thermomechanical measurements showed a steady decrease in the flexural strength and a relaxation temperature, with only moderate increase in the storage modulus at 1% clay loading, followed by a drop at higher clay loadings. In order to understand the absence of nano-effect in this case, free volume measurements were carried out by using positron annihilation lifetime spectroscopy. A bimodal distribution of o-Ps life times was observed. Nanoclay loading resulted in the increase of the o-Ps intensity corresponding to the longest life time as well as free volume fraction suggesting diminished chain packing efficiency in the nanocomposites. We posit that nanoclay induced decreased chain packing efficiency and the presence of higher free volume size elements might cause deterioration in mechanical properties of the nanocomposites.

  11. Positron annihilation spectroscopy of vacancy-related defects in CdTe:Cl and CdZnTe:Ge at different stoichiometry deviations

    PubMed Central

    Šedivý, L.; Čížek, J.; Belas, E.; Grill, R.; Melikhova, O.

    2016-01-01

    Positron annihilation spectroscopy (PAS) was used to examine the effect of defined Cd-rich and Te-rich annealing on point defects in Cl-doped CdTe and Ge-doped CdZnTe semi-insulating single crystals. The as-grown crystals contain open-volume defects connected with Cd vacancies . It was found that the Cd vacancies agglomerate into clusters coupled with Cl in CdTe:Cl, and in CdZnTe:Ge they are coupled with Ge donors. While annealing in Cd pressure reduces of the density, subsequent annealing in Te pressure restores . The CdTe:Cl contains negatively-charged shallow traps interpreted as Rydberg states of A-centres and representing the major positron trapping sites at low temperature. Positrons confined in the shallow traps exhibit lifetime, which is shorter than the CdTe bulk lifetime. Interpretation of the PAS data was successfully combined with electrical resistivity, Hall effect measurements and chemical analysis, and allowed us to determine the principal point defect densities. PMID:26860684

  12. Positron annihilation spectroscopy of vacancy-related defects in CdTe:Cl and CdZnTe:Ge at different stoichiometry deviations.

    PubMed

    Šedivý, L; Čížek, J; Belas, E; Grill, R; Melikhova, O

    2016-01-01

    Positron annihilation spectroscopy (PAS) was used to examine the effect of defined Cd-rich and Te-rich annealing on point defects in Cl-doped CdTe and Ge-doped CdZnTe semi-insulating single crystals. The as-grown crystals contain open-volume defects connected with Cd vacancies . It was found that the Cd vacancies agglomerate into clusters coupled with Cl in CdTe:Cl, and in CdZnTe:Ge they are coupled with Ge donors. While annealing in Cd pressure reduces of the density, subsequent annealing in Te pressure restores . The CdTe:Cl contains negatively-charged shallow traps interpreted as Rydberg states of A-centres and representing the major positron trapping sites at low temperature. Positrons confined in the shallow traps exhibit lifetime, which is shorter than the CdTe bulk lifetime. Interpretation of the PAS data was successfully combined with electrical resistivity, Hall effect measurements and chemical analysis, and allowed us to determine the principal point defect densities. PMID:26860684

  13. Positron annihilation spectroscopy of vacancy-related defects in CdTe:Cl and CdZnTe:Ge at different stoichiometry deviations

    NASA Astrophysics Data System (ADS)

    Šedivý, L.; Čížek, J.; Belas, E.; Grill, R.; Melikhova, O.

    2016-02-01

    Positron annihilation spectroscopy (PAS) was used to examine the effect of defined Cd-rich and Te-rich annealing on point defects in Cl-doped CdTe and Ge-doped CdZnTe semi-insulating single crystals. The as-grown crystals contain open-volume defects connected with Cd vacancies . It was found that the Cd vacancies agglomerate into clusters coupled with Cl in CdTe:Cl, and in CdZnTe:Ge they are coupled with Ge donors. While annealing in Cd pressure reduces of the density, subsequent annealing in Te pressure restores . The CdTe:Cl contains negatively-charged shallow traps interpreted as Rydberg states of A-centres and representing the major positron trapping sites at low temperature. Positrons confined in the shallow traps exhibit lifetime, which is shorter than the CdTe bulk lifetime. Interpretation of the PAS data was successfully combined with electrical resistivity, Hall effect measurements and chemical analysis, and allowed us to determine the principal point defect densities.

  14. Positron annihilation studies of fluorine-vacancy complexes in Si and SiGe

    SciTech Connect

    Edwardson, C. J.; Coleman, P. G.; El Mubarek, H. A. W.; Gandy, A. S.

    2012-04-01

    The formation of fluorine-vacancy (FV) complexes in strained Si-SiGe-Si multilayer structures and relaxed SiGe layers of varying Ge content has been investigated using variable-energy positron annihilation spectroscopy, including Doppler-broadened spectra ratio curves. It has been found that in all sample types there are two distinct regions defined only by the damage created by the implanted F ions. The first, shallower region (from the surface to a depth of {approx}200 nm) was found to contain a mixture of undecorated vacancies and FV complexes; there is no correlation between the vacancy or F concentration in this region and the Ge content. The multi-layer samples may also have O contamination that is not present in the relaxed samples. The second region (at depths {approx}200-440 nm) contains primarily FV complexes in all samples. In the multi-layer samples secondary ion mass spectrometry (SIMS) results show peaks of F accumulating in, or at the interfaces of, each SiGe multi-layer; the FV complexes, however, are distributed over depths similar to those in the relaxed samples, with some localization at the SiGe layer located within the second region. The positron response is primarily to FV complexes formed by the F implant in all samples. The F: FV ratios are approximately 3-7: 1 in the relaxed samples. Positrons appear to be relatively insensitive to the largest of the F SIMS peaks which lies beyond the second region. This is probably because the F has filled all the open volume at the SiGe layer, leaving no positron trapping sites.

  15. Positron annihilation studies of the AlOx/SiO2/Si interface in solar cell structures

    NASA Astrophysics Data System (ADS)

    Edwardson, C. J.; Coleman, P. G.; Li, T.-T. A.; Cuevas, A.; Ruffell, S.

    2012-03-01

    Film and film/substrate interface characteristics of 30 and 60 nm-thick AlOx films grown on Si substrates by thermal atomic layer deposition (ALD), and 30 nm-thick AlOx films by sputtering, have been probed using variable-energy positron annihilation spectroscopy (VEPAS) and Doppler-broadened spectra ratio curves. All samples were found to have an interface which traps positrons, with annealing increasing this trapping response, regardless of growth method. Thermal ALD creates an AlOx/SiOx/Si interface with positron trapping and annihilation occurring in the Si side of the SiOx/Si boundary. An induced positive charge in the Si next to the interface reduces diffusion into the oxides and increases annihilation in the Si. In this region there is a divacancy-type response (20 ± 2%) before annealing which is increased to 47 ± 2% after annealing. Sputtering seems to not produce samples with this same electrostatic shielding; instead, positron trapping occurs directly in the SiOx interface in the as-deposited sample, and the positron response to it increases after annealing as an SiO2 layer is formed. Annealing the film has the effect of lowering the film oxygen response in all film types. Compared to other structural characterization techniques, VEPAS shows larger sensitivity to differences in film preparation method and between as-deposited and annealed samples.

  16. A study on symmetrization of 2D ACAR positron annihilation data

    NASA Astrophysics Data System (ADS)

    Smedskjaer, L. C.; Legnini, D. G.

    1990-07-01

    The important problem of symmetrization of two-dimensional angular correlation positron annihilation data is discussed in detail. Interest in this problem is motivated by the potential for a substantial improvement of the data quality. The artefacts present in our Anger cameras have been studied experimentally, and form the basis for a quantitative discussion of the symmetrization operation. The main conclusion is that symmetrization of the two-dimensional angular correlation spectra is allowed, if the symmetry center can be defined. It is argued that the center can be defined if the instrumental artefacts are small. Finally, it is shown that it is unlikely that the instrumental artefacts interfere constructively during the symmetrization operation.

  17. Study on Momentum Density in Semiconductor Alloys GeC and SnC by Positron Annihilation

    NASA Astrophysics Data System (ADS)

    Amrane, N.

    The independent particle model (IPM) coupled with empirical pseudopotential method (EPM) was used to compute the thermalized positron charge densities in specific family of binary tetrahedrally coordinated crystals of formula ANB8-N. Initial results show a clear asymmetrical positron charge distribution relative to the bond center. It is observed that the positron density is maximum in the open interstices and is excluded not only from the ion cores but also to a considerable degree from the valence bonds. Electron-positron momentum densities are calculated for the (001, 110) planes. The results are used to analyze the positron effects in GeC and SnC. Our computational technique provides the theoretical means of interpreting the k-space densities obtained experimentally using the two-dimensional angular correlation of annihilation radiation (2D-ACAR).

  18. Hydrodynamic and kinetic models for spin-1/2 electron-positron quantum plasmas: Annihilation interaction, helicity conservation, and wave dispersion in magnetized plasmas

    NASA Astrophysics Data System (ADS)

    Andreev, Pavel A.

    2015-06-01

    We discuss the complete theory of spin-1/2 electron-positron quantum plasmas, when electrons and positrons move with velocities mach smaller than the speed of light. We derive a set of two fluid quantum hydrodynamic equations consisting of the continuity, Euler, spin (magnetic moment) evolution equations for each species. We explicitly include the Coulomb, spin-spin, Darwin and annihilation interactions. The annihilation interaction is the main topic of the paper. We consider the contribution of the annihilation interaction in the quantum hydrodynamic equations and in the spectrum of waves in magnetized electron-positron plasmas. We consider the propagation of waves parallel and perpendicular to an external magnetic field. We also consider the oblique propagation of longitudinal waves. We derive the set of quantum kinetic equations for electron-positron plasmas with the Darwin and annihilation interactions. We apply the kinetic theory to the linear wave behavior in absence of external fields. We calculate the contribution of the Darwin and annihilation interactions in the Landau damping of the Langmuir waves. We should mention that the annihilation interaction does not change number of particles in the system. It does not related to annihilation itself, but it exists as a result of interaction of an electron-positron pair via conversion of the pair into virtual photon. A pair of the non-linear Schrodinger equations for the electron-positron plasmas including the Darwin and annihilation interactions is derived. Existence of the conserving helicity in electron-positron quantum plasmas of spinning particles with the Darwin and annihilation interactions is demonstrated. We show that the annihilation interaction plays an important role in the quantum electron-positron plasmas giving the contribution of the same magnitude as the spin-spin interaction.

  19. Hydrodynamic and kinetic models for spin-1/2 electron-positron quantum plasmas: Annihilation interaction, helicity conservation, and wave dispersion in magnetized plasmas

    SciTech Connect

    Andreev, Pavel A.

    2015-06-15

    We discuss the complete theory of spin-1/2 electron-positron quantum plasmas, when electrons and positrons move with velocities mach smaller than the speed of light. We derive a set of two fluid quantum hydrodynamic equations consisting of the continuity, Euler, spin (magnetic moment) evolution equations for each species. We explicitly include the Coulomb, spin-spin, Darwin and annihilation interactions. The annihilation interaction is the main topic of the paper. We consider the contribution of the annihilation interaction in the quantum hydrodynamic equations and in the spectrum of waves in magnetized electron-positron plasmas. We consider the propagation of waves parallel and perpendicular to an external magnetic field. We also consider the oblique propagation of longitudinal waves. We derive the set of quantum kinetic equations for electron-positron plasmas with the Darwin and annihilation interactions. We apply the kinetic theory to the linear wave behavior in absence of external fields. We calculate the contribution of the Darwin and annihilation interactions in the Landau damping of the Langmuir waves. We should mention that the annihilation interaction does not change number of particles in the system. It does not related to annihilation itself, but it exists as a result of interaction of an electron-positron pair via conversion of the pair into virtual photon. A pair of the non-linear Schrodinger equations for the electron-positron plasmas including the Darwin and annihilation interactions is derived. Existence of the conserving helicity in electron-positron quantum plasmas of spinning particles with the Darwin and annihilation interactions is demonstrated. We show that the annihilation interaction plays an important role in the quantum electron-positron plasmas giving the contribution of the same magnitude as the spin-spin interaction.

  20. PREFACE: The 16th International Conference on Positron Annihilation (ICPA-16)

    NASA Astrophysics Data System (ADS)

    Alam, Ashraf; Coleman, Paul; Dugdale, Stephen; Roussenova, Mina

    2013-06-01

    The 16th International Conference on Positron Annihilation (ICPA-16) was held at the University of Bristol, United Kingdom during 19-24 August, 2012. This triennial conference is the foremost gathering of the Positron Annihilation Physics community and it was hosted in the UK for the first time since the series of meetings first started back in 1965. The University of Bristol, the Alma Mater of Paul Dirac, is situated at the heart of the city, and it has established a worldwide reputation in research and teaching. Many of the topics which were discussed during ICPA-16 form an integral part of the research themes in the schools of Physics, Chemistry and Engineering of this University. ICPA-16 attracted a diverse audience, both from academic and industrial institutions, with over 200 participants from 29 countries. It continued the long held tradition of showcasing novel research in the field of positron annihilation and a total of 170 papers were presented as talks and posters. The papers reported studies of metallic and semi-conducting solids, polymers and soft matter, porous materials, surfaces and interfaces, as well as advances in experimental, analytical and biomedical applications. The high quality of the presented work, coupled with the enthusiastic exchange of ideas, provided an invaluable forum, especially for younger researchers and postgraduate students. The excellence of student presentations was acknowledged by the award of prizes for the best student posters, which were received by David Billington (University of Bristol, UK), Moussa Sidibe (CEMHTI, France) and Hongxia Xu (Tohoku University, Japan). All papers published in the Conference Proceedings were reviewed by ICPA-16 participants. We are indebted to all reviewers who contributed their time and intellectual resources, allowing the refereeing and editing process to move smoothly toward the compilation of the Proceedings. Our sincere thanks and gratitude go to everyone who contributed to the

  1. Positron lifetimes in TTF-TCNQ and κ-(BEDT-TTF) 2Cu(NCS) 2 single crystals

    NASA Astrophysics Data System (ADS)

    Ishibashi, Shoji; Tokumoto, Madoka; Kinoshita, Nobumori; Terada, Norio; Ihara, Hideo; Suzuki, Ryoichi; Ohdaira, Toshiyuki; Mikado, Tomohisa; Anzai, Hiroyuki

    1997-05-01

    Positron lifetimes in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane) and κ-(BEDT-TTF) 2Cu(SCN) 2 (BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene) single crystals have been measured utilizing a pulsed variable-energy positron beam. The bulk positron lifetimes are 338 and 357 ps, respectively. The incident positron energy dependence of the lifetime is rather weak for both materials. Theoretical simulations have been also performed with several descriptions of the electronic wave functions and the electron-positron correlation. Results are compared with each other and the experiments.

  2. Positron annihilation spectroscopic studies of Mn substitution-induced cubic to tetragonal transformation in ZnFe2-xMnxO4 (x = 0.0-2.0) spinel nanocrystallites

    NASA Astrophysics Data System (ADS)

    Cyriac, Jincemon; Mundiyaniyil Thankachan, Rahul; Raneesh, B.; Nambissan, P. M. G.; Sanyal, D.; Kalarikkal, Nandakumar

    2015-12-01

    The replacement of cations at the B-sites in the spinel ferrite ZnFe2O4 by Mn3+ ions brings in several interesting changes, the most striking among them being a transformation from the spinel cubic structure to a body-centered tetragonal one. Concomitantly, there are variations in the nanocrystallite sizes and also in the lattice parameters. These are examined through high-precision X-ray diffraction measurements and transmission electron microscopic analysis. A more interesting aspect is the success of positron annihilation spectroscopy comprising of the measurements of positron lifetime and coincidence Doppler broadening measurements in understanding the effects of cation replacement and the resultant generation of vacancy-type defects. There are definite changes in the positron lifetimes and intensities which show positron trapping in trivacancy-type defect clusters and the nanocrystallite surfaces. The presence of ortho-positronium atoms within the extended intercrystallite region is also identified, although in small concentrations. The cubic to tetragonal transformation is indicated through definite decrease in the values of the positron lifetimes. We also performed a model analysis to predict the expected effect of substitution on the positron lifetime in the bulk of the sample and the experimentally obtained positron lifetimes significantly differed, indirectly hinting at the possibility of a structural transformation. Finally, Mössbauer spectroscopic studies have indicated a ferromagnetic nature for one of the samples, i.e. the one with Mn3+ doping concentration x = 0.4, which incidentally had the lowest crystallite size ~10 nm.

  3. Optimization of three-dimensional positron annihilation spectroscopy system (3DPASS) for three-dimensional momentum measurements

    NASA Astrophysics Data System (ADS)

    Williams, Christopher S.; Burggraf, Larry W.; Adamson, Paul E.; Petrosky, James C.

    2011-02-01

    A three-dimensional positron annihilation spectroscopy system (3DPASS) was characterized. 3DPASS permits determination of three-dimensional electron-positron ( e-- e+) momentum distributions by simultaneously measuring angles and energies for coincident two-gamma annihilation photons. 3DPASS collects a single dataset of correlated energies and positions for two coincident annihilation photons from a pair of solid-state double-sided strip detectors (DSSDs). Subpixel-interpolated positions are determined by transient charge analysis. 3DPASS performs simultaneous two-dimensional angular correlation of annihilation radiation (2D ACAR) and two-detector coincidence Doppler-broadening of annihilation radiation (CDBAR) measurements, which are typically collected independently. The 2D ACAR response of 3DPASS was measured for single-crystal Cu and 6H-SiC, with and without compensation for subpixel detection efficiency. Variation of efficiency across the width of DSSD charge collection electrodes was dominated by the event selection criteria required by the subpixel interpolation method. The DBAR resolution was optimized by adjusting the energy range of CDBAR events included in the Doppler-broadening (DB) lineshape. 2D ACAR and DBAR spectra from 3DPASS were compared to previously published results for single-crystal Cu and 6H-SiC. Detailed analysis of the ACAR spectra and the DB lineshapes highlighted momentum features not previously reported.

  4. Gamma Spectra Resulting From the Annihilation of Positrons with Electrons in Single, Selected Core Levels of Cu, Ag and Au

    SciTech Connect

    Kim, S; Eshed, A; Goktepeli, S; Sterne, P A; Koymen, A R; Chen, W C; Weiss, A H

    2005-07-25

    The {gamma}-ray energy spectra due to positron annihilation with the 3p core-level of Cu, the 4p core-level of Ag, and 5p core level of Au were obtained separately from the total annihilation spectrum by measuring the energies of {gamma}-rays time coincident with Auger electrons emitted as a result of filling the core-hole left by annihilation. The results of these measurements are compared to the total annihilation spectra and with LDA based theoretical calculations. A comparison of area normalized momentum distributions with the individual cores extracted from the Doppler measurements shows good qualitative agreement, however, in all three spectra, the calculated values of the momentum density appears to fall below the measured values as the momentum increases. The discrepancies between theory and experiment are well outside the statistical uncertainties of the experiment and become more pronounced with increasing Z going down the column from Cu to Ag to Au. The comparison with the experimental results clearly indicates that the calculations are not predicting the correct ratio of high momentum to low momentum spectral weight and suggest the need to improve the treatment of many body electron-positron correlation effects in annihilation as they pertain to core levels.

  5. Positron annihilation spectroscopic study of high performance semi-interpenetrating network polyimids

    NASA Technical Reports Server (NTRS)

    Ray, Asit K.

    1995-01-01

    Semi-interpenetrating (S-IPN) network polyimids were made from different proportions of LaRC RP46 (a thermosetpolyimid) and LaRC BDTA-ODA (a thermoplastic polyimid). The ultimate goal of this networking is to improve the mechanical properties of the thermoset polyimid. Positron lifetime study was made to calculate lifetime based on second component of the life time spectra and the free volume & microvoid size. All these properties tend to decrease steadily with increasing thermoset content except at the 50 percent thermoset level where these properties show sudden drop. This result contradicts with the initial expectation that the blend properties should change gradually if it were a solid solution of thermoset (TSP) and thermoplastic (TPP) components. Thermal analyses (TMA, DSC, DMA & TGA) were run to complement the positron life time studies. The TMA and DSC studies confirm the contradiction mentioned above. Further experimentation with S-IPN polymers made at TSP/TTP content around 50/50 level are being conducted to explain this anomaly. Scanning electron microscope study of the S-IPN polyimid samples is under way in order to detect morphological differences which might help explain the phenomenon mentioned above.

  6. Graphene networks and their influence on free-volume properties of graphene-epoxidized natural rubber composites with a segregated structure: rheological and positron annihilation studies.

    PubMed

    He, Canzhong; She, Xiaodong; Peng, Zheng; Zhong, Jieping; Liao, Shuangquan; Gong, Wei; Liao, Jianhe; Kong, Lingxue

    2015-05-14

    Epoxidized natural rubber-graphene (ENR-GE) composites with segregated GE networks were successfully fabricated using the latex mixing combined in situ reduced technology. The rheological behavior and electrical conductivity of ENR-GE composites were investigated. At low frequencies, the storage modulus (G') became frequency-independent suggesting a solid-like rheological behavior and the formation of GE networks. According to the percolation theory, the rheological threshold of ENR-GE composites was calculated to be 0.17 vol%, which was lower than the electrical threshold of 0.23 vol%. Both percolation thresholds depended on the evolution of the GE networks in the composites. At low GE concentrations (<0.17 vol%), GE existed as individual units, while a "polymer-bridged GE network" was constructed in the composites when GE concentrations exceeded 0.17 vol%. Finally, a "three-dimensional GE network" with percolation conductive paths was formed with a GE concentration of 0.23 vol%, where a remarkable increase in the conductivity of ENR-GE composites was observed. The effect of GE on the atom scale free-volume properties of composites was further studied by positron annihilation lifetime spectroscopy and positron age momentum correlation measurements. The motion of ENR chains was retarded by the geometric confinement of "GE networks", producing a high-density interfacial region in the vicinity of GE nanoplatelets, which led to a lower ortho-positronium lifetime intensity and smaller free-volume hole size. PMID:25881784

  7. Interstitial oxygen related defects and nanovoids in Au implanted a-SiO2 glass depth profiled by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Ravelli, L.; Macchi, C.; Mariazzi, S.; Mazzoldi, P.; Egger, W.; Hugenschmidt, C.; Somoza, A.; Brusa, R. S.

    2015-12-01

    Samples of amorphous silica were implanted with Au ions at an energy of 190 keV and fluences of 1× {{10}14} ions cm-2and 5× {{10}14} ions cm-2 at room temperature. The damage produced by ion implantation and its evolution with the thermal treatment at 800 °C for one hour in nitrogen atmosphere was depth profiled using three positron annihilation techniques: Doppler broadening spectroscopy, positron annihilation lifetime spectroscopy and coincidence Doppler broadening spectroscopy. Around the ion projected range of {{R}\\text{p}}=67 nm, a size reduction of the silica matrix intrinsic nanovoids points out a local densification of the material. Oxygen related defects were found to be present at depths four times the ion projected range, showing a high mobility of oxygen molecules from the densified and stressed region towards the bulk. The 800 °C thermal treatment leads to a recovery of the silica intrinsic nanovoids only in the deeper damaged region and the defect distribution, probed by positrons, shrinks around the ion projected range where the Au atoms aggregate. Open volume defects at the interface between Au and the amorphous matrix were evidenced in both the as implanted and in the thermal treated samples. A practically complete disappearance of the intrinsic nanovoids was observed around {{R}\\text{p}} when the implantation fluence was increased by two orders of magnitude (3× {{10}16} ions cm-2). In this case, the oxygen defects move to a depth five times larger than {{R}\\text{p}} .

  8. Detection of 511 keV positron annihilation radiation from the galactic center direction. [gamma ray astronomy

    NASA Technical Reports Server (NTRS)

    Leventhal, M.; Maccallum, C. J.; Stang, P. D.

    1978-01-01

    A balloon-borne gamma ray telescope with an approximately 130 cu cm high purity germanium detector was flown over Australia to detect sharp spectral features from the galactic center direction. A 511 keV positron annihilation line was observed at a flux level of (1.21 plus or minus 0.22) x (10/cu cm) photons/sec/sp cm. Suggestive evidence for the detection of the three-photon positronium continuum is presented. The possible origin of the positrons is discussed.

  9. Positrons from supernovae

    NASA Technical Reports Server (NTRS)

    Chan, Kai-Wing; Lingenfelter, Richard E.

    1993-01-01

    Positrons are produced in the ejecta of supernovae by the decay of nucleosynthetic Co-56, Ti-44, and Al-26. We calculate the probability that these positrons can survive without annihilating in the supernova ejecta, and we show that enough of these positrons should escape into the interstellar medium to account for the observed diffuse Galactic annihilation radiation. The surviving positrons are carried by the expanding ejecta into the interstellar medium where their annihilation lifetime of 10 exp 5 - 10 exp 6 yr is much longer than the average supernovae occurrence time of about 100 yr. Thus, annihilating positrons from thousands of supernovae throughout the Galaxy produce a steady diffuse flux of annihilation radiation. We further show that combining the calculated positron survival fractions and nucleosynthetic yields for current supernova models with the estimated supernova rates and the observed flux of diffuse Galactic annihilation radiation suggests that the present Galactic rate of Fe-56 nucleosynthesis is about 0.8 +/- 0.6 solar mass per 100 yr.

  10. Effect of electron- and neutron-irradiation on Fe-Cu model alloys studied by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Nagai, Y.; Takadate, K.; Tang, Z.; Ohkubo, H.; Sunaga, H.; Takizawa, H.; Hasegawa, M.

    2011-01-01

    Electron- and neutron-irradiation effects on dilute Fe-Cu model alloys of nuclear reactor pressure vessel steels are studied by positron annihilation spectroscopy. We have found that, not only by high-dose neutron-irradiation but also by low-dose electron-irradiation, the aggregation of Cu atoms and vacancies takes place and the ultrafine Cu precipitates are formed after post-irradiation annealing at 400°C. In spite of large difference in the irradiation doses between the electron- and the neutron-irradiated samples, no significant difference is observed in the isochronal annealing behaviour above 400°C of positron annihilation and micro-hardness, indicating that small amount of extra vacancies enhance the aggregation of Cu atoms in Fe during the annealing-out process of the vacancies.

  11. The free volume in dried and H2O-loaded biopolymers studied by positron lifetime measurements.

    PubMed

    Hugenschmidt, Christoph; Ceeh, Hubert

    2014-08-01

    We present experiments on glucose-gelatin compounds using positron annihilation lifetime spectroscopy (PALS) in order to study the behavior of the free volume dependent on H2O loading, drying, and uniaxial pressure. A semiempirical quantum mechanical model was applied in order to correlate the lifetime of orthopositronium in nanoscaled voids to the void size. This allowed us to determine the absolute value of the mean void radius in the biopolymer samples. In addition, the variation of the total free volume of the differently treated samples is quantified and illustrated by a log-normal distribution function. Most interesting results have been obtained after saturation loading with H2O that leads to the formation of voids with a mean size of 84.3(1.9) Å(3) and to an increase of the total free volume by a factor of 2.5. This observation in the swelled sample is explained by the entropy elastic regime well above the glass transition temperature that greatly facilitates the formation of free volume. Differential scanning calorimetry (DSC) measurements were performed in order to determine the glass transition temperature and to support the interpretation of the results obtained by PALS. PMID:25046083

  12. A study of defects in iron-based binary alloys by the Mössbauer and positron annihilation spectroscopies

    SciTech Connect

    Idczak, R. Konieczny, R.; Chojcan, J.

    2014-03-14

    The room temperature positron annihilation lifetime spectra and {sup 57}Fe Mössbauer spectra were measured for pure Fe as well as for iron-based Fe{sub 1−x}Re{sub x}, Fe{sub 1−x}Os{sub x}, Fe{sub 1−x}Mo{sub x}, and Fe{sub 1−x}Cr{sub x} solid solutions, where x is in the range between 0.01 and 0.05. The measurements were performed in order to check if the known from the literature, theoretical calculations on the interactions between vacancies and solute atoms in iron can be supported by the experimental data. The vacancies were created during formation and further mechanical processing of the iron systems under consideration so the spectra mentioned above were collected at least twice for each studied sample synthesized in an arc furnace— after cold rolling to the thickness of about 40 μm as well as after subsequent annealing at 1270 K for 2 h. It was found that only in Fe and the Fe-Cr system the isolated vacancies thermally generated at high temperatures are not observed at the room temperature and cold rolling of the materials leads to creation of another type of vacancies which were associated with edge dislocations. In the case of other cold-rolled systems, positrons detect vacancies of two types mentioned above and Mössbauer nuclei “see” the vacancies mainly in the vicinity of non-iron atoms. This speaks in favour of the suggestion that in iron matrix the solute atoms of Os, Re, and Mo interact attractively with vacancies as it is predicted by theoretical computations and the energy of the interaction is large enough for existing the pairs vacancy-solute atom at the room temperature. On the other hand, the corresponding interaction for Cr atoms is either repulsive or attractive but smaller than that for Os, Re, and Mo atoms. The latter is in agreement with the theoretical calculations.

  13. Enhanced positron annihilation in small gaseous hydrocarbons: Threshold effects from symmetric C-H bond deformations

    SciTech Connect

    Nishimura, Tamio; Gianturco, Franco A.

    2005-08-15

    The present results report a computational analysis of the effects of symmetric bond stretching during positron scattering from polyatomic hydrocarbon molecules in the gas phase. The collisions are considered at very low energies where the behavior of the s-wave scattering length can be analyzed and where signatures of virtual state formation appear for all the three systems considered (C{sub 2}H{sub 2}, C{sub 2}H{sub 4}, and C{sub 2}H{sub 6}). Furthermore, the present calculations show that the stretching of the CH bonds in all molecules causes the moving of the existing virtual state closer to threshold and further makes it become a bound state whenever highly distorted molecules are involved. The effects of these changes are further seen to cause a marked enhancing of the corresponding annihilation parameters Z{sub eff} at low collision energies, in line with what is experimentally observed for such gases. The significance of such model calculations is discussed in some detail.

  14. Assessment of the fatigue transformation zone in bulk metallic glasses using positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Liu, M.; Vallery, R. S.; Gidley, D. W.; Launey, M. E.; Kruzic, J. J.

    2009-05-01

    Depth-profiled Doppler broadening spectroscopy of positron annihilation on fatigue fracture surfaces of two amorphous Zr44Ti11Ni10Cu10Be25 metallic glass specimens reveals the presence of a layer of increased free volume induced by cyclic deformation, as compared to surfaces that have been etched to remove any surface damage. The damage layer, or fatigue transformation zone (FTZ), is generated by the propagating fatigue crack tip and the deduced size of that zone is similar to the predicted cyclic plastic zone size at a number of locations where the crack grew at different stress intensities. The presence of the FTZ is independent of the initial amount of bulk free volume, which was varied between the two specimens by structural relaxation via annealing, and the free volume sites generated in the zone are distinct from those typical of the bulk, as evidenced by the higher S parameter. Such observations support the concept that the mechanically induced free volume within the FTZ zone controls the fatigue crack growth rates rather than the initial free volume of the bulk material.

  15. Structure dependence of gamma-ray irradiation effects on polyethylenes studied by positron annihilation

    NASA Astrophysics Data System (ADS)

    Suzuki, Takenori; Oki, Yuichi; Numajiri, Masaharu; Miura, Taichi; Kondo, Kenjiro; Ito, Yasuo

    1995-05-01

    The irradiation effects on five kinds of polyethylenes (PEs) were studied from the viewpoints of mechanical properties, free radicals and positron annihilation. The degradation of the mechanical strength of samples irradiated by 60Co γ-rays was evaluated based on the elongation at break and the tensile strength. Fully stretched PE with a crystallinity larger than 99% was greatly affected by γ-irradiation. In low-density PEs free radicals were detectable only above 70 kGy, and sharply increased above 100 kGy. A detectable number of free radicals was obtained even at 0.1 kGy in PEs haaving high crystallinity. At around 100 kGy, the increase in the intensity ( I3) of ortho-positronium ( o-Ps), seems to be associated with an increase in the mechanical strength. Contrary to the expectation that the o-Ps seeks intermolecular space holes only in amorphous regions, the I3 obtained in PEs with nearly 100% crystallinity was almost half that low-density PEs, and τ 3 had similar values. This suggests that even in crystalline regions the o-Ps seeks intermolecular space holes comparable to those in the amorphous regions.

  16. Microstructural evolution of RPV steels under proton and ion irradiation studied by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Jiang, J.; Wu, Y. C.; Liu, X. B.; Wang, R. S.; Nagai, Y.; Inoue, K.; Shimizu, Y.; Toyama, T.

    2015-03-01

    The microstructural evolution of reactor pressure vessel (RPV) steels induced by proton and heavy ion irradiation at low temperature (∼373 K) has been investigated using positron annihilation spectroscopy (PAS), atom probe tomography (APT), transmission electron microscopy (TEM) and nanoindentation. The PAS results indicated that both proton and heavy ion irradiation produce a large number of matrix defects, which contain small-size defects such as vacancies, vacancy-solute complexes, dislocation loops, and large-size vacancy clusters. In proton irradiated RPV steels, the size and number density of vacancy cluster defects increased rapidly with increasing dose due to the migration and agglomeration of vacancies. In contrast, for Fe ion irradiated steels, high density, larger size vacancy clusters can be easily induced at low dose, showing saturation in PAS response with increasing dose. No clear precipitates, solute-enriched clusters or other forms of solute segregation were observed by APT. Furthermore, dislocation loops were observed by TEM after 1.0 dpa, 240 keV proton irradiation, and an increase of the average nanoindentation hardness was found. It is suggested that ion irradiation produces many point defects and vacancy cluster defects, which induce the formation of dislocation loops and the increase of nanoindentation hardness.

  17. Development of positron annihilation spectroscopy for investigating deuterium decorated voids in neutron-irradiated tungsten

    NASA Astrophysics Data System (ADS)

    Taylor, C. N.; Shimada, M.; Merrill, B. J.; Akers, D. W.; Hatano, Y.

    2015-08-01

    The present work is a continuation of a recent research to develop and optimize positron annihilation spectroscopy (PAS) for characterizing neutron-irradiated tungsten. Tungsten samples were exposed to neutrons in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory and damaged to 0.025 and 0.3 dpa. Subsequently, they were exposed to deuterium plasmas in the Tritium Plasma Experiment (TPE) at Idaho National Laboratory. The implanted deuterium was desorbed through sample heating to 900 °C, and Doppler broadening (DB)-PAS was performed both before and after heating. Results show that deuterium impregnated tungsten is identified as having a smaller S-parameter. The S-parameter increases after deuterium desorption. Microstructural changes also occur during sample heating. These effects can be isolated from deuterium desorption by comparing the S-parameters from the deuterium-free back face with the deuterium-implanted front face. The application of using DB-PAS to examine deuterium retention in tungsten is examined.

  18. Theoretical and positron annihilation study of point defects in intermetallic compound Ni{sub 3}Al

    SciTech Connect

    Jian Sun; Dongliang Lin

    1994-01-01

    The equilibrium equation of point defects in Ll{sub 2} types of intermetallic compounds was established in a new simple method, which is independent of the chemical potentials. The formation energies of the relevant point defects in Ni{sub 3}Al were calculated by EAM potentials and statical relaxations. The concentration of point defects at 1,000 K as a function of bulk composition and the effect of temperature on them were studied for Ni{sub 3}Al alloy. The results show that the Al-antisites are the constitutional defects in hypostoichiometric Ni{sub 3}Al, and the Ni-antisite defects in hyperstoichiometric Ni{sub 3}Al. The two types of vacancies belong to thermal defects. The positron annihilation technique was also conducted to measure the concentration of vacancies in Ni{sub 3}Al alloys with and without boron. Although vacancies interact with the boron dopant, the changes of vacancy concentration Ni{sub 3}Al alloys can not be considered as the main reason in explaining the effect of stoichiometry on the segregation of boron. The effect of stoichiometry on diffusion in Ni{sub 3}Al alloys was discussed additionally.

  19. Reverse Micelle Formation of Triton X-100 in Butanol and n-Heptane Mixed Solvents Studied by the Positron Annihilation Technique

    PubMed

    Das; Ganguly

    1997-08-01

    The positron annihilation technique (PAT) has been applied to study the molecular association phenomenon of Triton X-100 (TX-100) and formation of reversed micelles in the mixed solvent of butanol and n-heptane. The results indicate the sensitivity of positronium (Ps) parameters to the phase transition region due to the self-aggregation phenomenon of TX-100 within the system. The intensity of the long-lived ortho-Ps component, I3, and its lifetime, tau3, show a remarkable change at a critical concentration of the surfactant at approximately 1.5 mM coined as the operational CMC of TX-100 in both (1:1) and (1:2) butanol-n-heptane (BuHp) systems. The narrow component/para-Ps intensity as computed from Doppler broadening of annihilation radiation indicates discernable changes at the same concentration region ( approximately 1.5 mM) of TX-100 in the system. Further, microphase changes due to the association of water molecules within the nonaqueous phase has been studied by Ps parameters, which reveals a clear demarcation of the polar and nonpolar zones. PMID:9268557

  20. Three-dimensional positron annihilation momentum measurement technique applied to measure oxygen-atom defects in 6H silicon carbide

    NASA Astrophysics Data System (ADS)

    Williams, Christopher S.

    A three-dimensional Positron Annihilation Spectroscopy System (3DPASS) capable to simultaneously measure three-dimensional electron-positron (e--e+) momentum densities measuring photons derived from e--e+ annihilation events was designed and characterized. 3DPASS simultaneously collects a single data set of correlated energies and positions for two coincident annihilation photons using solid-state double-sided strip detectors (DSSD). Positions of photons were determined using an interpolation method which measures a figure-of-merit proportional to the areas of transient charges induced on both charge collection strips directly adjacent to the charge collection strips interacting with the annihilation photons. The subpixel resolution was measured for both double-sided strip detectors (DSSD) and quantified using a new method modeled after a Gaussian point-spread function with a circular aperture. Error associated with location interpolation within an intrinsic pixel in each of the DSSDs, the subpixel resolution, was on the order of +/- 0.20 mm (this represents one-standard deviation). The subpixel resolution achieved was less than one twenty-fifth of the 25-mm2 square area of an intrinsic pixel created by the intersection of the DSSDs' orthogonal charge collection strips. The 2D ACAR and CDBAR response for single-crystal copper and 6H silicon carbide (6H SiC) was compared with results in the literature. Two additional samples of 6H SiC were irradiated with 24 MeV O+ ions, one annealed and one un-annealed, and measured using 3DPASS. Three-dimensional momentum distributions with correlated energies and coincident annihilation photons' positions were presented for all three 6H SiC samples. 3DPASS was used for the first experimental measurement of the structure of oxygen defects in bulk 6H SiC.

  1. Nano-pore size and porosity study by means of Nuclear Magnetic Resonance and Positronium Annihilation Lifetime

    NASA Astrophysics Data System (ADS)

    Chesta, M. A.; Ramia, M. E.; Jeandrevin, S.; Martín, C. A.

    2009-11-01

    The present work involves a comprehensive experimental determination of porosity and pore size distribution in rocks from oil fields formations by deuterium (2H) Nuclear Magnetic Resonance (NMR) and Positronium Annihilation Lifetime Spectroscopy (PALS). Both techniques yield complementary results; PALS measures the average pore size providing bulk information from which the most abundant pore size can be obtained, and NMR allows for the determination of the relative pore size distribution accurately. Both techniques give complementary information to obtain an absolute pore size distribution.

  2. ON THE MORPHOLOGY OF THE ELECTRON-POSITRON ANNIHILATION EMISSION AS SEEN BY SPI/INTEGRAL

    SciTech Connect

    Bouchet, L.; Roques, J. P.; Jourdain, E.

    2010-09-10

    The 511 keV positron annihilation emission remains a mysterious component of the high energy emission of our Galaxy. Its study was one of the key scientific objectives of the SPI spectrometer on board the International Gamma-Ray Astrophysics Laboratory satellite. In fact, a lot of observing time has been dedicated to the Galactic disk with a particular emphasis on the central region. A crucial issue in such an analysis concerns the reduction technique used to treat this huge quantity of data, and more particularly the background modeling. Our method, after validation through a variety of tests, is based on detector pattern determination per {approx}6 month period, together with a normalization variable on a few hour timescale. The Galactic bulge is detected at a level of {approx}70{sigma}, allowing more detailed investigations. The main result is that the bulge morphology can be modeled with two axisymmetric Gaussians of 3.{sup 0}2 and 11.{sup 0}8 FWHM and respective fluxes of 2.5 and 5.4 x10{sup -4} photons cm{sup -2} s{sup -1}. We found a possible shift of the bulge center toward negative longitude at l = -0.{sup 0}6 {+-} 0.{sup 0}2. In addition to the bulge, a more extended structure is detected significantly with flux ranging from 1.7 to 2.9 x10{sup -3} photons cm{sup -2} s{sup -1} depending on its assumed geometry (pure disk or disk plus halo). The disk emission is also found to be symmetric within the limits of the statistical errors.

  3. Mn2+-induced substitutional structural changes in ZnS nanoparticles as observed from positron annihilation studies

    NASA Astrophysics Data System (ADS)

    Biswas, Subhajit; Kar, Soumitra; Chaudhuri, Subhadra; Nambissan, P. M. G.

    2008-06-01

    Zinc sulfide nanoparticles doped with different concentrations of manganese ions (Mn2+) were synthesized at various temperatures to investigate the effects of substitution and the associated defect evolution. Positron lifetime and Doppler broadening measurements were used as probes. The initial stage of defect recovery was dominated by the occupation of Zn2+ vacancies by Mn2+ ions, bringing in characteristic changes in the positron lifetimes, intensities and Doppler broadened lineshape parameters. Detailed analyses considering the presence of one and two types of defects were carried out to identify the type of defects which trap positrons at the different dopant concentrations. Electron paramagnetic resonance studies indicated increased Mn-Mn interaction and the formation of Mn clusters with further doping. The results are in striking contrast to those for nanorods, where vacancy recombination transformed their interior into regions free of defects.

  4. Mn(2+)-induced substitutional structural changes in ZnS nanoparticles as observed from positron annihilation studies.

    PubMed

    Biswas, Subhajit; Kar, Soumitra; Chaudhuri, Subhadra; Nambissan, P M G

    2008-06-11

    Zinc sulfide nanoparticles doped with different concentrations of manganese ions (Mn(2+)) were synthesized at various temperatures to investigate the effects of substitution and the associated defect evolution. Positron lifetime and Doppler broadening measurements were used as probes. The initial stage of defect recovery was dominated by the occupation of Zn(2+) vacancies by Mn(2+) ions, bringing in characteristic changes in the positron lifetimes, intensities and Doppler broadened lineshape parameters. Detailed analyses considering the presence of one and two types of defects were carried out to identify the type of defects which trap positrons at the different dopant concentrations. Electron paramagnetic resonance studies indicated increased Mn-Mn interaction and the formation of Mn clusters with further doping. The results are in striking contrast to those for nanorods, where vacancy recombination transformed their interior into regions free of defects. PMID:21694317

  5. Free volume from positron lifetime and pressure-volume-temperature experiments in relation to structural relaxation of van der Waals molecular glass-forming liquids.

    PubMed

    Dlubek, G; Shaikh, M Q; Rätzke, K; Paluch, M; Faupel, F

    2010-06-16

    Positron annihilation lifetime spectroscopy (PALS) is employed to characterize the temperature dependence of the free volume in two van der Waals liquids: 1, 1'-bis(p-methoxyphenyl)cyclohexane (BMPC) and 1, 1'-di(4-methoxy-5-methylphenyl)cyclohexane (BMMPC). From the PALS spectra analysed with the routine LifeTime9.0, the size (volume) distribution of local free volumes (subnanometer size holes), its mean, [v(h)], and mean dispersion, σ(h), were calculated. A comparison with the macroscopic volume from pressure-volume-temperature (PV T) experiments delivered the hole density and the specific hole free volume and a complete characterization of the free volume microstructure in that sense. These data are used in correlation with structural (α) relaxation data from broad-band dielectric spectroscopy (BDS) in terms of the Cohen-Grest and Cohen-Turnbull free volume models. An extension of the latter model allows us to quantify deviations between experiments and theory and an attempt to systematize these in terms of T(g) or of the fragility. The experimental data for several fragile and less fragile glass formers are involved in the final discussion. It was concluded that, for large differences in the fragility of different glass formers, the positron lifetime mirrors clearly the different character of these materials. For small differences in the fragility, additional properties like the character of bonds and chemical structure of the material may affect size, distribution and thermal behaviour of the free volume. PMID:21393763

  6. Influence of O-Co-O layer thickness on the thermal conductivity of Na{sub x}Co{sub 2}O{sub 4} studied by positron annihilation

    SciTech Connect

    Li, H. Q.; Zhao, B.; Zhang, T.; Li, X. F.; He, H. F.; Chen, Z. Q.; Su, X. L.; Tang, X. F.

    2015-07-21

    Nominal stoichiometric Na{sub x}Co{sub 2}O{sub 4} (x = 1.0, 1.2, 1.4, 1.6, 1.8, and 2.0) polycrystals were synthesized by a solid-state reaction method. They were further pressed into pellets by the spark plasma sintering. The crystal structure and morphology of Na{sub x}Co{sub 2}O{sub 4} samples were characterized by X-ray diffraction and scanning electron microscopy measurements. Good crystallinity and layered structures were observed for all the samples. Positron annihilation measurements were performed for Na{sub x}Co{sub 2}O{sub 4} as a function of Na content. Two lifetime components are resolved. τ{sub 1} is attributed mainly to positron annihilation in the O-Co-O layers and shifts to Na layers only in the H3 phase. The second lifetime τ{sub 2} is due to positron annihilation in vacancy clusters which may exist in the Na layers or grain boundary region. The size of vacancy clusters grow larger but their concentration decreases with increasing Na content in the range of 1.0 < x < 1.8. The thickness of O-Co-O layer also shows continuous increase with increasing Na content, which is reflected by the increase of τ{sub 1}. The thermal conductivity κ, on the other hand, shows systematic decrease with increasing Na content. This suggests that the increasing spacing of O-Co-O layer could effectively reduce the thermal conductivity of Na{sub x}Co{sub 2}O{sub 4}.

  7. Positron annihilation study for enhanced nitrogen-vacancy center formation in diamond by electron irradiation at 77 K

    SciTech Connect

    Tang, Z.; Chiba, T.; Nagai, Y.; Inoue, K.; Toyama, T.; Hasegawa, M.

    2014-04-28

    A compact ensemble of high density nitrogen-vacancy (NV) centers in diamond is essential to sense various external fields with a high precision at the nanoscale. Here, defects in type IIa and type Ib diamonds induced by 28 MeV electron irradiation at 77 K were studied by combining the positron annihilation spectroscopy and first-principles calculations. It is shown that the electron irradiation at 77 K can significantly enhance the NV center formation by directly converting 24% vacancies into the NV centers, indicating that it is an efficient way to produce the high density NV centers in the type Ib diamond.

  8. Mechanism of phosphorescence quenching in photomagnetic molecules determined by positron annihilation spectroscopy

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Eftekhari, A.; Naidu, S. V. N.

    1994-01-01

    Platinum Octaethyl Porphyrin (Pt.OEP) is an efficient phosphor under ultraviolet excitation. The phosphorescent triplet state (T(sub 1)) is readily quenched by the oxygen (O2) molecules. This phenomenon is being utilized as the basis for global air pressure measurements in aerodynamic facilities at various laboratories. The exact mechanism by which O2 molecules quench the (T(sub 1) - S(sub 0)) transitions is still unknown. The diamagnetic S(sub n) singlet states, which feed T(sub 1) states via intersystem crossings, would presumably not be affected by O2. It must be the magnetic T(sub 1) states, which can interact with the paramagnetic O2 molecules, that are affected. However, our positron lifetime and Doppler broadening studies suggest the formation of (S(sub n) central dot O2) complexes which can also eventually reduce the population of the T(sub 1) states (i.e. quench phosphorescence). This is possible since higher triplet states in (Pt.OEP) are admixed with the S(sub n) states via spin orbit interactions. The experimental procedures and the results of various measurements are discussed in this paper.

  9. Mechanism of phosphorescence quenching in photomagnetic molecules determined by positron annihilation spectroscopy

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Eftekhari, A.; Naidu, S. V. N.

    1994-01-01

    Platinum Octaethyl Porphyrin (Pt.OEP) is an efficient phosphor under ultraviolet excitation. The phosphorescent triplet state (T(sub 1)) is readily quenched by the oxygen (O2) molecules. This phenomenon is being utilized as the basis for global air pressure measurements in aerodynamic facilities at various laboratories. The exact mechanism by which O2 molecules quench the (T(sub 1)-S(sub 0)) transitions is still unknown. The diamagnetic S(sub n) singlet states, which feed T(sub 1) states via intersystem crossings, would presumably not be affected by O2. It must be the magnetic T(sub 1) states, which can interact with the paramagnetic O2 molecules, that are affected. However, our positron lifetime and Doppler broadening studies suggest the formation of (S(sub n).02) complexes which can also eventually reduce the population of the T(sub 1) states (i.e. quench phosphorescence). This is possible since higher triplet states in (Pt-OEP) are admixed with the S(sub n) states via spin orbit interactions. The experimental procedures and the results of various measurements are discussed in this paper.

  10. Investigation of Oxygen-Induced Quenching of Phosphorescence in Photoexcited Aromatic Molecules by Positron Annihilation Spectroscopy

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Eftekhari, Abe

    1996-01-01

    Platinum OctaEthyl Porphyrin (Pt.OEP) is an efficient phosphor under ultraviolet excitation. The phosphorescent triplet state P(T(Sup 1)) is readily quenched by the oxygen O2 molecules. This phenomenon is being utilized as the basis for global air pressure measurements in aerodynamic facilities at various laboratories. The exact mechanism by which O2 molecules quench the P(T(Sup 1) approaches P(S(Sub O)) transitions is still unknown. The diamagnetic singlet states P(S(Sub n)), which feed P(T(Sub 1)) states via intersystem crossings, would presumably not be affected by O2. It must be only the magnetic P(T(Sub 1)) states, which can interact with the paramagnetic O2 molecules, that are affected. However, our positron lifetime and Doppler broadening studies suggest the formation of O2P(S(Sub n)), complexes which can also eventually reduce the population of the P(T(Sub 1)) states (i.e., quench phosphorescence). This reduction is possible because higher triplet states in (Pt.OEP) are admixed with the P(S(Sub 1)), states via spin orbit interactions. The experimental procedures and the results of various measurements are presented in this paper.

  11. Physical Selectivity of Molecularly Imprinted polymers evaluated through free volume size distributions derived from Positron Lifetime Spectroscopy

    NASA Astrophysics Data System (ADS)

    Pasang, T.; Ranganathaiah, C.

    2015-06-01

    The technique of imprinting molecules of various sizes in a stable structure of polymer matrix has derived multitudes of applications. Once the template molecule is extracted from the polymer matrix, it leaves behind a cavity which is physically (size and shape) and chemically (functional binding site) compatible to the particular template molecule. Positron Annihilation Lifetime Spectroscopy (PALS) is a well known technique to measure cavity sizes precisely in the nanoscale and is not being used in the field of MIPs effectively. This method is capable of measuring nanopores and hence suitable to understand the physical selectivity of the MIPs better. With this idea in mind, we have prepared molecular imprinted polymers (MIPs) with methacrylicacid (MAA) as monomer and EGDMA as cross linker in different molar ratio for three different size template molecules, viz. 4-Chlorophenol (4CP)(2.29 Å), 2-Nephthol (2NP) (3.36 Å) and Phenolphthalein (PP) (4.47Å). FTIR and the dye chemical reactions are used to confirm the complete extraction of the template molecules from the polymer matrix. The free volume size and its distribution have been derived from the measured o-Ps lifetime spectra. Based on the free volume distribution analysis, the percentage of functional cavities for the three template molecules are determined. Percentage of functional binding cavities for 4-CP molecules has been found out to be 70.2% and the rest are native cavities. Similarly for 2NP it is 81.5% and nearly 100% for PP. Therefore, PALS method proves to be very precise and accurate for determining the physical selectivity of MIPs.

  12. Positron annihilation study of the electronic structure of LaB{sub 6} and CeB{sub 6}

    SciTech Connect

    Biasini, M.; Fretwell, H.M.; Dugdale, S.B.; Alam, M.A.; Kubo, Y.; Harima, H.; Sato, N.

    1997-10-01

    We measured the two-dimensional angular correlation of the positron annihilation radiation (2D-ACAR) on a single crystal of LaB{sub 6} for two projections. The anisotropies of the 2D electron-positron momentum density were very similar to those observed for the isostructural heavy-fermion (HF) system CeB{sub 6} in the paramagnetic phase and consistent with those of the calculated electron-positron momentum density of LaB{sub 6}. The standard Lock-Crisp-West (LCW) analysis was in reasonable agreement with the LCW folding of the calculated 2D-ACAR spectrum and the de Haas{endash}van Alphen findings. From the projected {bold {ital k}}-space density we could evaluate the Fermi volume, corresponding to 1.10{plus_minus}0.04 electrons per formula unit, and deduce that the effect of the nonuniform positron density does not play a significant role. The apparent discrepancy with the LCW analysis of CeB{sub 6}, where filtering procedures were required to recover a k-space density similar to that obtained for LaB{sub 6}, is discussed. {copyright} {ital 1997} {ital The American Physical Society}

  13. Optical and positron annihilation spectroscopic studies on PMMA polymer doped by rhodamine B/chloranilic acid charge transfer complex: Special relevance to the effect of γ-ray irradiation.

    PubMed

    Hassan, H E; Refat, Moamen S; Sharshar, T

    2016-04-15

    Polymeric sheets of poly (methylmethaclyerate) (PMMA) containing charge transfer (CT) complex of rhodamine B/chloranilic acid (Rho B/CHA) were synthesized in methanol solvent at room temperature. The systematic analysis done on the Rho B and its CT complex in the form of powder or polymeric sheets confirmed their structure and thermal stability. The IR spectra interpreted the charge transfer mode of interaction between the CHA central positions and the terminal carboxylic group. The polymer sheets were irradiated with 70kGy of γ radiation using (60)Co source to study the enhanced changes in the structure and optical parameters. The microstructure changes of the PMMA sheets caused by γ-ray irradiation were analyzed using positron annihilation lifetime (PAL) and positron annihilation Doppler broadening (PADB) techniques. The positron life time components (τi) and their corresponding intensities (Ii) as well as PADB line-shape parameters (S and W) were found to be highly sensitive to the enhanced disorder occurred in the organic chains of the polymeric sheets due to γ-irradiation. PMID:26867205

  14. Optical and positron annihilation spectroscopic studies on PMMA polymer doped by rhodamine B/chloranilic acid charge transfer complex: Special relevance to the effect of γ-ray irradiation

    NASA Astrophysics Data System (ADS)

    Hassan, H. E.; Refat, Moamen S.; Sharshar, T.

    2016-04-01

    Polymeric sheets of poly (methylmethaclyerate) (PMMA) containing charge transfer (CT) complex of rhodamine B/chloranilic acid (Rho B/CHA) were synthesized in methanol solvent at room temperature. The systematic analysis done on the Rho B and its CT complex in the form of powder or polymeric sheets confirmed their structure and thermal stability. The IR spectra interpreted the charge transfer mode of interaction between the CHA central positions and the terminal carboxylic group. The polymer sheets were irradiated with 70 kGy of γ radiation using 60Co source to study the enhanced changes in the structure and optical parameters. The microstructure changes of the PMMA sheets caused by γ-ray irradiation were analyzed using positron annihilation lifetime (PAL) and positron annihilation Doppler broadening (PADB) techniques. The positron life time components (τi) and their corresponding intensities (Ii) as well as PADB line-shape parameters (S and W) were found to be highly sensitive to the enhanced disorder occurred in the organic chains of the polymeric sheets due to γ-irradiation.

  15. Positron annihilation 2D-ACAR study of semi-coherent Li nanoclusters in MgO( 1 0 0 ) and MgO( 1 1 0 )

    NASA Astrophysics Data System (ADS)

    Falub, C. V.; Mijnarends, P. E.; Eijt, S. W. H.; van Huis, M. A.; van Veen, A.; Schut, H.

    2002-05-01

    Depth selective positron annihilation two-dimensional angular correlation of annihilation radiation (2D-ACAR) is used to determine the electronic structure of Li nanoclusters formed by implantation of 10 16 cm -26Li ions (with an energy of 30 keV) in MgO(1 0 0) and (1 1 0) crystals, and subsequently annealed at 950 K. The 2D-ACAR spectra of Li-implanted MgO obtained with 4 keV positrons reveal the semi-coherent ordering state of the embedded metallic Li nanoclusters. The results agree with ab initio Korringa-Kohn-Rostoker calculations.

  16. Fully self-consistent calculations of momentum distributions of annihilating electron-positron pairs in SiC

    NASA Astrophysics Data System (ADS)

    Wiktor, Julia; Jomard, Gérald; Torrent, Marc; Barthe, Marie-France; Bertolus, Marjorie

    2016-05-01

    We performed calculations of momentum distributions of annihilating electron-positron pairs in various fully relaxed vacancy defects in SiC. We used self-consistent two-component density functional theory schemes to find the electronic and positronic densities and wave functions in the considered systems. Using the one-dimensional momentum distributions (Doppler-broadened annihilation radiation line shapes) we calculated the line-shape parameters S and W . We emphasize the effect of the experimental resolution and the choice of the integration ranges for the S and W parameters on the distributions of the points corresponding to different defects in the S (W ) plot. We performed calculation for two polytypes of SiC, 3 C , and 6 H and showed that for silicon vacancies and clusters containing this defect there were no significant differences between the Doppler spectra. The results of the Doppler spectra calculations were compared with experimental data obtained for n -type 6 H -SiC samples irradiated with 4-MeV Au ions. We observed a good general agreement between the measured and calculated points.

  17. Identification of open-volume defects in disordered and amorphized Si: A depth-resolved positron annihilation study

    SciTech Connect

    Amarendra, G.; Rajaraman, R.; Venugopal Rao, G.; Nair, K. G. M.; Viswanathan, B.; Suzuki, R.; Ohdaira, T.; Mikado, T.

    2001-06-01

    Depth-resolved positron beam studies have been carried out on Ar-irradiated Si using Doppler S parameter and lifetime measurements. Si samples have been irradiated with 140-keV Ar ions to a dose of 2{times}10{sup 13} and 5{times}10{sup 16} Ar/cm{sup 2}, respectively, so as to produce disordered and amorphous states in near-surface regions. The observed features of the defect sensitive line shape S parameter indicate the presence of small vacancylike defects in the disordered sample and higher-order vacancy clusters in an amorphous sample. Pulsed positron beam lifetime results indicate that the disordered Si sample exhibits lifetime distribution ascribable to mostly divacancies. In the case of an amorphous sample, the lifetime distribution is broad with larger lifetime values indicating the presence of a distribution of large vacancy clusters or nanovoids. By using theoretical lifetime values for Si reported in the literature, an empirical fit to the lifetime variation as a function of vacancy cluster size is obtained. By comparing the experimental lifetime distribution with this data, the vacancy cluster size distribution in disordered and amorphous Si is deduced. In disordered Si, divacancies are found to be the dominant defects species followed by small concentration of V{sub 3}. In amorphous Si, nanovoids in the size range of four to seven vacancy clusters are present with V{sub 5} and V{sub 6} clusters being the dominant defect species. The implication of these results is discussed in light of recent computer-simulation studies.

  18. Free volume in imidazolium triflimide ([C3MIM][NTf2]) ionic liquid from positron lifetime: amorphous, crystalline, and liquid states.

    PubMed

    Dlubek, G; Yu, Yang; Krause-Rehberg, R; Beichel, W; Bulut, S; Pogodina, N; Krossing, I; Friedrich, Ch

    2010-09-28

    Positron annihilation lifetime spectroscopy (PALS) is used to study the ionic liquid 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide [C(3)MIM][NTf(2)] in the temperature range between 150 and 320 K. The positron decay spectra are analyzed using the routine LifeTime-9.0 and the size distribution of local free volumes (subnanometer-size holes) is calculated. This distribution is in good agreement with Fürth's classical hole theory of liquids when taking into account Fürth's hole coalescence hypothesis. During cooling, the liquid sample remains in a supercooled, amorphous state and shows the glass transition in the ortho-positronium (o-Ps) lifetime at 187 K. The mean hole volume varies between 70 Å(3) at 150 K and 250 Å(3) at 265-300 K. From a comparison with the macroscopic volume, the hole density is estimated to be constant at 0.20×10(21) g(-1) corresponding to 0.30 nm(-3) at 265 K. The hole free volume fraction varies from 0.023 at 185 K to 0.073 at T(m)+12 K=265 K and can be estimated to be 0.17 at 430 K. It is shown that the viscosity follows perfectly the Cohen-Turnbull free volume theory when using the free volume determined here. The heating run clearly shows crystallization at 200 K by an abrupt decrease in the mean <τ(3)> and standard deviation σ(3) of the o-Ps lifetime distribution and an increase in the o-Ps intensity I(3). The parameters of the second lifetime component <τ(2)> and σ(2) behave parallel to the o-Ps parameters, which also shows the positron's (e(+)) response to structural changes. During melting at 253 K, all lifetime parameters recover to the initial values of the liquid. An abrupt decrease in I(3) is attributed to the solvation of e(-) and e(+) particles. Different possible interpretations of the o-Ps lifetime in the crystalline state are briefly discussed. PMID:20886945

  19. Free volume in imidazolium triflimide ([C{sub 3}MIM][NTf{sub 2}]) ionic liquid from positron lifetime: Amorphous, crystalline, and liquid states

    SciTech Connect

    Dlubek, G.; Beichel, W.; Bulut, S.; Pogodina, N.; Krossing, I.; Friedrich, Ch.

    2010-09-28

    Positron annihilation lifetime spectroscopy (PALS) is used to study the ionic liquid 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide [C{sub 3}MIM][NTf{sub 2}] in the temperature range between 150 and 320 K. The positron decay spectra are analyzed using the routine LifeTime-9.0 and the size distribution of local free volumes (subnanometer-size holes) is calculated. This distribution is in good agreement with Fuerth's classical hole theory of liquids when taking into account Fuerth's hole coalescence hypothesis. During cooling, the liquid sample remains in a supercooled, amorphous state and shows the glass transition in the ortho-positronium (o-Ps) lifetime at 187 K. The mean hole volume varies between 70 A{sup 3} at 150 K and 250 A{sup 3} at 265-300 K. From a comparison with the macroscopic volume, the hole density is estimated to be constant at 0.20x10{sup 21} g{sup -1} corresponding to 0.30 nm{sup -3} at 265 K. The hole free volume fraction varies from 0.023 at 185 K to 0.073 at T{sub m}+12 K=265 K and can be estimated to be 0.17 at 430 K. It is shown that the viscosity follows perfectly the Cohen-Turnbull free volume theory when using the free volume determined here. The heating run clearly shows crystallization at 200 K by an abrupt decrease in the mean <{tau}{sub 3}> and standard deviation {sigma}{sub 3} of the o-Ps lifetime distribution and an increase in the o-Ps intensity I{sub 3}. The parameters of the second lifetime component <{tau}{sub 2}> and {sigma}{sub 2} behave parallel to the o-Ps parameters, which also shows the positron's (e{sup +}) response to structural changes. During melting at 253 K, all lifetime parameters recover to the initial values of the liquid. An abrupt decrease in I{sub 3} is attributed to the solvation of e{sup -} and e{sup +} particles. Different possible interpretations of the o-Ps lifetime in the crystalline state are briefly discussed.

  20. Positron lifetime studies of decomposition in 2024 (Al-Cu-Mg) and 7010 (Al-Zn-Cu-Mg) alloys

    SciTech Connect

    Dlubek, G. |; Lademann, P.; Krause, H.; Krause, S.; Unger, R.

    1998-09-04

    In the current paper, the decomposition behavior of the engineering alloys 2024 (Al-Cu-Mg) and 7010 (Al-Zn-Cu-Mg) is studied using positron lifetime measurements. Positrons probe open volume defects such as vacancies and dislocations. However, they may also be used to investigate coherent zones and incoherent precipitates. In order to understand the rather complicated precipitation sequences and the response of positrons to different type of precipitates occurring in 2024 and 7010 alloys, binary and ternary laboratory alloys were also investigated under the same experimental conditions as the engineering alloys. The interpretations of the results are based on experiences of the group from extensive positron studies of laboratory alloys such as Al-Zn, Al-Zn-Mg, Al-Cu, and further Al alloys (see also the review (4)). Their collected results are shown as lifetimes and curve-shape parameters S of the electron-positron momentum distribution curves characteristic for different precipitates in Al alloys.

  1. Spectra of electrons emitted as a result of the sticking and annihilation of low energy positrons to the surfaces of graphene and highly oriented pyrolytic graphite (HOPG)

    NASA Astrophysics Data System (ADS)

    Chrysler, M.; Chirayath, V.; McDonald, A.; Lim, Z.; Shastry, K.; Gladen, R.; Fairchild, A.; Koymen, A.; Weiss, A.

    Positron annihilation induced Auger electron spectroscopy (PAES) was used to study the positron induced low energy electron spectra from HOPG and a sample composed of 6-8 layers of graphene grown on polycrystalline copper. A low energy (~2eV) beam of positrons was used to implant positrons into a surface localized state on the graphene and HOPG samples. Measurements of the energy spectra of the positron induced electrons obtained using a TOF spectrometer indicate the presence of an annihilation induced KLL C Auger peak (at ~263 eV) along with a narrow low energy secondary peak due to an Auger mediated positron sticking (AMPS) process. A broad spectral feature was also observed below ~15 eV which we believe may be due to a VVV C Auger transition not previously observed. The energy dependence of the integrated intensity of the AMPS peak was measured for a series of incident positron kinetic energies ranging from ~1.5 eV up to 11 eV from which the binding energy of the surface localized positron state on graphene and HOPG was estimated. The implication of our results regarding the applicability of AMPS and PAES to the study of graphene surfaces and interfaces will be discussed. This work was supported by NSF Grant No. DMR 1508719 and DMR 1338130.

  2. Photo-degradation of Lexan polycarbonate studied using positron lifetime spectroscopy

    SciTech Connect

    Hareesh, K.; Sanjeev, Ganesh; Pandey, A. K.; Meghala, D.; Ranganathaiah, C.

    2013-02-05

    The free volume properties of pristine and UV irradiated Lexan polycarbonate have been investigated using Positron Lifetime Spectroscopy (PLS). The decrease in o-Ps life time and free volume size of irradiated sample is attributed to free volume modification and formation of more stable free radicals. These free radicals are formed due to the breakage of C-O bonds in Lexan polycarbonate after irradiation. This is also supported by the decrease in the intensity of C-O bond after exposure to UV-radiation as studied from Fourier Transform Infrared (FTIR) spectroscopy and it also shows that benzene ring does not undergo any changes after irradiation.

  3. Elementally specific electron-positron annihilation radiation emitted from ion cores of group-V impurity-vacancy complexes in germanium

    NASA Astrophysics Data System (ADS)

    Arutyunov, N. Yu.; Emtsev, V. V.

    2007-12-01

    High-momentum component (HMC) of the electron-positron annihilation has been detected by the angular correlation of annihilation radiation (ACAR) technique in order to obtain elementally specific information about the ion cores of the donor-vacancy complexes (DV) formed by irradiation with 60Co γ-rays at Tirr.≈280 K in oxygen-lean n-Ge doped with group-V donors (D=As, Sb, and Bi). The probability of annihilation of positrons with the core electrons of DV complexes reconstructed from ACAR spectra increases in passing from AsV to SbV and BiV complexes. This increase correlates with the shift of the D atom from its regular position towards the vacancy site predicted by the results of spin-density functional modeling study. The data obtained suggest inward relaxation of the ion cores of DV complexes (including the one directed inward towards the vacancy).

  4. Microstructural Characterization of Semi-Interpenetrating Polymer Networks by Positron Lifetime Spectroscopy

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Pater, Ruth H.; Eftekhari, Abe

    1996-01-01

    Thermoset and thermoplastic polyimides have complementary physical and mechanical properties. Whereas thermoset polyimides are brittle and generally easier to process, thermoplastic polyimides are tough but harder to process. A combination of these two types of polyimides may help produce polymers more suitable for aerospace applications. Semi-Interpenetrating Polymer Networks (S-IPN) of thermoset LaRC(TM)-RP46 and thermoplastic LaRC(TM)-IA polyimides were prepared in weight percent ratios ranging from 100:0 to 0:100. Positron lifetime measurements were made in these samples to correlate their free volume features with physical and mechanical properties. As expected, positronium atoms are not formed in these samples. The second lifetime component has been used to infer the positron trap dimensions. The 'free volume' goes through a minimum at a ratio of about 50:50, and this suggests that S-IPN samples are not merely solid solutions of the two polymers. These data and related structural properties of the S-IPN samples are discussed.

  5. Identification of the Native Vacancy Defects in Both Sublattices of ZnSxSe1-x by Positron Annihilation

    NASA Astrophysics Data System (ADS)

    Saarinen, K.; Laine, T.; Skog, K.; Mäkinen, J.; Hautojärvi, P.; Rakennus, K.; Uusimaa, P.; Salokatve, A.; Pessa, M.

    1996-10-01

    We show how positron annihilation can distinguish vacancies in the different sublattices of a binary compound by performing experiments in ZnSxSe1-x layers. We identify the Se vacancies \\(VSe\\) in N-doped and the Zn vacancies \\(VZn\\) in Cl-doped material by the shape of the core electron momentum distribution. The charge of the defect involving VSe is neutral or negative in p-type ZnSxSe1-x, suggesting that VSe is complexed with an acceptor. The concentration of the VSe complexes is high \\(>=1018 cm-3\\), indicating that their role is important in the electrical compensation of p-type ZnSxSe1-x.

  6. GRO: Red-shifted electron-positron annihilation gamma-rays from radiopulsars

    NASA Technical Reports Server (NTRS)

    Ruderman, Malvin

    1993-01-01

    Reported red-shifted e(+) + e(-) yields gamma + gamma 511 keV gamma-rays from the Crab pulsar would, if ultimately confirmed, provide crucial clues about the structure of the powerful magnetospheric accelerator in that rapidly spinning gamma-ray pulsar. In an attempt to understand the origin of this component of the Crab pulsar's emission, we try to account for the following: (1) a flow of approximately 10 exp 40 e(+/-)/s into near the surface of the neutron star; (2) a relatively narrow annihilation line implying that the annihilating e(+/-) pairs probably had a velocity (along vector B) less than or approximately = 10(exp -1)c; and (3) a tentative light curve suggesting a doubly peaked structure different from that of the rest of the Crab pulsar's nonthermal radiation.

  7. CHARACTERIZATION OF PLASTICALLY-INDUCED STRUCTURAL CHANGES IN A Zr-BASED BULK METALLIC GLASS USING POSITRON ANNIHILATION SPECTROCOPY

    SciTech Connect

    Flores, K M; Kanungo, B P; Glade, S C; Asoka-Kumar, P

    2005-09-16

    Flow in metallic glasses is associated with stress-induced cooperative rearrangements of small groups of atoms involving the surrounding free volume. Understanding the details of these rearrangements therefore requires knowledge of the amount and distribution of the free volume and how that distribution evolves with deformation. The present study employs positron annihilation spectroscopy to investigate the free volume change in Zr{sub 58.5}Cu{sub 15.6}Ni{sub 12.8}Al{sub 10.3}Nb{sub 2.8} bulk metallic glass after inhomogeneous plastic deformation by cold rolling and structural relaxation by annealing. Results indicate that the size distribution of open volume sites is at least bimodal. The size and concentration of the larger group, identified as flow defects, changes with processing. Following initial plastic deformation the size of the flow defects increases, consistent with the free volume theory for flow. Following more extensive deformation, however, the size distribution of the positron traps shifts, with much larger open volume sites forming at the expense of the flow defects. This suggests that a critical strain is required for flow defects to coalesce and form more stable nanovoids, which have been observed elsewhere by high resolution TEM. Although these results suggest the presence of three distinct open volume size groups, further analysis indicates that all groups have the same line shape parameter. This is in contrast to the distinctly different interactions observed in crystalline materials with multiple defect types. This similarity may be due to the disordered structure of the glass and positron affinity to particular atoms surrounding open-volume regions.

  8. Characterization of radiation-induced lattice vacancies in intermetallic compounds by means of positron-lifetime studies

    NASA Astrophysics Data System (ADS)

    Würschum, R.; Badura-Gergen, K.; Kümmerle, E. A.; Grupp, C.; Schaefer, H.-E.

    1996-07-01

    In the present paper a characterization of atomic vacancies in intermetallic compounds is given by means of positron-lifetime measurements after electron irradiation and comparison with the states after preparation, after long-time annealing, or in high-temperature equilibrium. In TiAl, Ti3Al, and Ni3Al no structural vacancies (detection limit CV=10-6) are observed at ambient temperature. This confirms that in these compounds slight deviations from stoichiometry are compensated by antisite atoms. In the Al-poor B2 alloys FeAl and NiAl, on the other hand, remnant vacancies exist due to the high thermal equilibrium vacancy concentrations and their slow diffusivities. The kinetics of vacancy elimination in FeAl and NiAl is discussed. A substantial temperature dependence of the positron lifetime in vacancies is detected in close-packed intermetallics which is attributed to an increased atomic relaxation or partial positron detrapping at high temperatures. In contrast to that, the temperature dependence of the positron lifetime in vacancies is small in the open-structured B2 aluminides. The lifetimes τf of free delocalized positrons in transition-metal aluminides and in NiZr and NiTi can be correlated to those of the pure components, taking into account the densities of valence electrons. For the positron lifetimes τ1 of vacancies in intermetallic compounds, values of τ1/τf=1.5-1.7 are observed similar as in the pure metals. Annealing studies of B2-FeAl after electron irradiation yield time constants for the disappearance of vacancies identical to those deduced recently for the equilibration of thermal vacancies. In electron-irradiated Ti aluminides annealing processes at 250 K and 450 K are observed where the latter process is tentatively attributed to long-range migration of vacancies.

  9. Spin dependent momentum density and Fermi surface of ferromagnetic Ni obtained by positron annihilation experiments

    NASA Astrophysics Data System (ADS)

    Hamid, A. S.; Uedono, A.

    2004-11-01

    The cover picture of this issue, taken from [1], shows a cross section of the Fermi surface in the basal plane of nickel. The measurements were carried out using 2D angular correlation of annihilation radiation (ACAR) experiments. The intersecting plane is normal to the c-axis, passing through the and X points. The light regions correspond to a high electron momentum density. The Fermi surface is presented as two hole surfaces around the point and two electron surfaces around the X point.

  10. Lambda production in electron-positron annihilation at 29 GeV

    SciTech Connect

    Baden, A.R.

    1986-08-01

    The inclusive cross-secton for the production of the singly-strange baryons lambda and anti lambda, along with the differential cross-sections in momentum and energy, are measured by e/sup +/e/sup -/ annihilation at a center-of-mass energy of 29GeV. The charged decay mode lambda ..-->.. p..pi.. is used in a search for polarization. Such a polarization may be used as a check of CP invariance in lambda production. The sample of events with two detected decays is analyzed for correlations in production angle. 43 refs., 44 figs.

  11. Collins effect in semiinclusive deeply inelastic scattering and in electron-positron-annihilation

    SciTech Connect

    Efremov, A.V.; Goeke, K.; Schweitzer, P.

    2006-05-01

    The Collins fragmentation function is extracted from HERMES data on azimuthal single spin asymmetries in semi-inclusive deeply inelastic scattering, and BELLE data on azimuthal asymmetries in e{sup +}e{sup -}-annihilations. A Gaussian model is assumed for the distribution of transverse parton momenta and predictions are used from the chiral quark-soliton model for the transversity distribution function. We find that the HERMES and BELLE data yield a consistent picture of the Collins fragmentation function which is compatible with COMPASS data and the information previously obtained from an analysis of DELPHI data. Estimates for future experiments are made.

  12. Spectroscopic, Elemental and Thermal Analysis, and Positron Annihilation Studies on Ca(II), Sr(II), Ba(II), Pb(II), and Fe(III) Penicillin G Potassium Complexes

    NASA Astrophysics Data System (ADS)

    Refat, M. S.; Sharshara, T.

    2015-11-01

    The [Pb(Pin)2] · 3H2O, [M(Pin)(H2O)2(Cl)] · nH2O (M = SrII, CaII or BaII; n = 0-1), and [Fe(Pin)2(Cl)(H2O)] · H2O penicillin G potassium (Pin) complexes were synthesized and characterized using elemental analyses, molar conductivity, thermal analysis and electronic spectroscopy techniques. The positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been employed to probe the defects and structural changes of Pin ligand and its complexes. The PAL and DB line-shape parameters were discussed in terms of the structure, molecular weight, ligand-metal molar ratio, and other properties of the Pin complexes.

  13. Electronic structure and orientation relationship of Li nanoclusters embedded in MgO studied by depth-selective positron annihilation two-dimensional angular correlation

    NASA Astrophysics Data System (ADS)

    Falub, C. V.; Mijnarends, P. E.; Eijt, S. W.; van Huis, M. A.; van Veen, A.; Schut, H.

    2002-08-01

    Quantum-confined positrons are sensitive probes for determining the electronic structure of nanoclusters embedded in materials. In this work, a depth-selective positron annihilation 2D-ACAR (two-dimensional angular correlation of annihilation radiation) method is used to determine the electronic structure of Li nanoclusters formed by implantation of 1016-cm-2 30-keV 6Li ions in MgO (100) and (110) crystals and by subsequent annealing at 950 K. Owing to the difference between the positron affinities of lithium and MgO, the Li nanoclusters act as quantum dots for positrons. 2D-ACAR distributions for different projections reveal a semicoherent fitting of the embedded metallic Li nanoclusters to the host MgO lattice. Ab initio Korringa-Kohn-Rostoker calculations of the momentum density show that the anisotropies of the experimental distributions are consistent with an fcc crystal structure of the Li nanoclusters. The observed reduction of the width of the experimental 2D-ACAR distribution is attributed to positron trapping in vacancies associated with Li clusters. This work proposes a method for studying the electronic structure of metallic quantum dots embedded in an insulating material.

  14. Nondestructive Induced Residual Stress Assessment in Superalloy Turbine Engine Components Using Induced Positron Annihilation (IPA)

    SciTech Connect

    Rideout, C. A.; Ritchie, S. J.; Denison, A.

    2007-03-21

    Induced Positron Analysis (IPA) has demonstrated the ability to nondestructively quantify shot peening/surface treatments and relaxation effects in single crystal superalloys, steels, titanium and aluminum with a single measurement as part of a National Science Foundation SBIR program and in projects with commercial companies. IPA measurement of surface treatment effects provides a demonstrated ability to quantitatively measure initial treatment effectiveness along with the effect of operationally induced changes over the life of the treated component. Use of IPA to nondestructively quantify surface and subsurface residual stresses in turbine engine materials and components will lead to improvements in current engineering designs and maintenance procedures.

  15. Study on Defects in H+ion Implanted B2 type Fe-Al Alloy using Slow Positron Beam

    NASA Astrophysics Data System (ADS)

    Komagata, S.; Kawasuso, A.; Yabuuchi, A.; Maekawa, M.; Batchulun, C.; Yasuda, K.; Ishigami, R.; Kume, K.; Iwase, A.; Hori, F.

    Fe48-at.%Al alloy were implanted with 50 keV H+ ions to the fluence of 3×1016 and 1×1018/cm2 at room temperature. Positron annihilation Doppler broadening and lifetime measurements for these alloys have been carried out using slow positron beam apparatus with an energy range of 0.2 to 30.2 keV. The positron annihilation S-parameter decreased by H+ ion irradiation. Also the positron lifetimes for hydrogen deposited region in the alloy decreased by the irradiation. These results show that implanted H atoms were trapped by vacancy type defects.

  16. Effect of phosphorus on vacancy-type defect behaviour in electron-irradiated Ni studied by positron annihilation

    NASA Astrophysics Data System (ADS)

    Druzhkov, A. P.; Danilov, S. E.; Perminov, D. A.; Arbuzov, V. L.

    2015-02-01

    Very dilute Ni-P system (containing 50-240 appm phosphorus) irradiated by 5 MeV electrons at various temperatures (270-543 K) was studied by positron annihilation spectroscopy (PAS) and the electrical resistivity measurements. Under irradiation at 270 K (below stage III in Ni), the accumulation of the monovacancies in the Ni-P system is 1.5-2.0 times greater than that in pure Ni irradiated in the same conditions. This fact attests to the strong interaction between P atoms and self-interstitial atoms (SIAs). As a result of the non-mobile SIA-P complexes formation, the mutual recombination of point defects is suppressed and the vacancy accumulation is, respectively, enhanced. During post-irradiation annealing, the vacancy migration induces the transport process of the phosphorus atoms and leads to the formation of the vacancy clusters decorated with P atoms. The annealing behaviour of the defect structures in Ni-P systems after irradiation at enhanced temperatures was also studied. The influence of phosphorus on the formation and further evolution of the vacancy aggregates decrease with increasing of the irradiation temperature.

  17. A comparative study on ferromagnetic C/O-implanted GaN films by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Xu, Juping; Li, Qiang; Liu, Jiandang; Ye, Bangjiao

    2016-05-01

    Room temperature ferromagnetism was observed in both C- and O-implanted GaN films, which were irradiated by 80 keV C/O-ions with respective dose of 5 × 1016 and 2 × 1017 ions/cm2. Positron annihilation spectroscopy was used to explore the magnetic origin and the correlation between the magnetism and structural features. The results reveal that carbon-ions play an important role in the stable ferromagnetism in C-implanted GaN films, while oxygen has no effect on the magnetic properties, even than a weak hysteresis loop was observed in O-implanted sample. This weak ferromagnetism is demonstrated as originated from Ga-related vacancies which induced by implantation. With first-principle calculations, we confirmed that substitutional C-ion at N-site can introduce magnetic moment for 0.8 μB and stabilize ferromagnetic coupling with adjacent Ga-vacancy at room temperature. Moreover, the effect of O-ions was clearly ruled out. Our discussion gives an experimental and theoretical insight of the different origin of ferromagnetism between acceptor and donor non-metal-doped GaN materials.

  18. Positron Annihilation Study of Zr-2.5 wt.% Nb alloy Irradiated by Ar9+ heavy ions

    NASA Astrophysics Data System (ADS)

    Devi, Aruna; Menon, Ranjini; Maheshwari, Priya; Neogy, S.; Mukherjee, P.; Nabhiraj, P. Y.; Pujari, P. K.; Srivastava, D.; Dey, G. K.

    2015-06-01

    Zr-2.5 Nballoy is used as a pressure tube material in pressurized heavy water reactor (PHWR). It is one of the most critical component which decides the lifespan of the reactor. The in-reactor degrading phenomenon of prime concern is dimensional changes caused by irradiation induced creep and growth processes. The present study aims to understand the mechanism of irradiation damage by irradiating the alloy with heavy ion. Such type of irradiation study would facilitate larger damage of material in a shorter time. Zr-2.5Nb alloy samples were irradiated using 315 keV Ar9+ ion for different durations. The irradiation doses were varied in the range of 3.1X1015 to 4.17X1016 Ar9+/cm2. SRIM calculation was carried out to evaluate damage profile in the irradiated samples. Beam based Positron Annihilation Spectroscopy (PAS) technique was used for depth profiling to characterize defect distribution in the alloys. The no. of defects generated is seen to increase with the increase in the fluence.

  19. Positronics of radiation-induced effects in chalcogenide glassy semiconductors

    SciTech Connect

    Shpotyuk, O.; Kozyukhin, S. A.; Shpotyuk, M.; Ingram, A.; Szatanik, R.

    2015-03-15

    Using As{sub 2}S{sub 3} and AsS{sub 2} glasses as an example, the principal possibility of using positron annihilation spectroscopy methods for studying the evolution of the free volume of hollow nanoobjects in chalcogenide glassy semiconductors exposed to radiation is shown. The results obtained by measurements of the positron annihilation lifetime and Doppler broadening of the annihilation line in reverse chronological order are in full agreement with the optical spectroscopy data in the region of the fundamental absorption edge, being adequately described within coordination defect-formation and physical-aging models.

  20. Analysis of exclusive kT jet algorithms in electron-positron annihilation

    NASA Astrophysics Data System (ADS)

    Chay, Junegone; Kim, Chul; Kim, Inchol

    2015-10-01

    We study the factorization of the dijet cross section in e+e- annihilation using the generalized exclusive jet algorithm which includes the cone-type, the JADE, the kT, the anti-kT and the Cambridge/Aachen jet algorithms as special cases. In order to probe the characteristics of the jet algorithms in a unified way, we consider the generalized kT jet algorithm with an arbitrary weight of the energies, in which various types of the kT-type algorithms are included for specific values of the parameter. We show that the jet algorithm respects the factorization property for the parameter α <2 . The factorized jet function and the soft function are well defined and infrared safe for all the jet algorithms except the kT algorithm. The kT algorithm (α =2 ) breaks the factorization since the jet and the soft functions are infrared divergent and are not defined for α =2 , though the dijet cross section is infrared finite. In the jet algorithms which enable factorization, we give a phenomenological analysis using the resummed and the fixed-order results.

  1. Hard photon processes in electron-positron annihilation at 29 GeV

    SciTech Connect

    Gold, M.S.

    1986-11-01

    The hard photon processes ..mu mu gamma.. and hadrons + ..gamma.. in e/sup +/e/sup -/ annihilation at 29 GeV have been studied. The study is based on an integrated luminosity of 226 pb/sup -1/ taken at PEP with the Mark II detector. For the ..mu mu gamma.. process, a small fraction of non-planar events are observed with missing momentum along the beam direction. The resulting missing energy spectrum is consistent with that expected from higher order effects. The observed cross section is consistent with the predicted cross section for this process, sigma/sup exp/sigma/sup th/ = .90 +- .05 +- .06. The observed hard photon energy spectrum and mass distributions are found to be in agreement with O(..cap alpha../sup 3/) QED. The measured charge asymmetry is in good agreement with the predicted value, A/sub exp/A/sub th/ = .83 +- .25 +- .12. The ..mu gamma.. invariant mass distribution is used to place a limit on a possible excited muon coupling G..gamma../M* for excited muon masses in the range 1 < M* < 21 GeV of (G..gamma../M*)/sup 2/ < 10/sup -5/ GeV/sup -2/ at a 95% confidence level. In the hadrons + ..gamma.. process, evidence for final state radiation is found in an excess of events over that predicted from initial state radiation alone of 253 +- 54 +- 60 events. Further evidence for final state radiation is found in a large hadronic charge asymmetry A/sub Had+..gamma../= (-24.6 +- 5.5)%.

  2. Defects in electron-irradiated GaAs studied by positron lifetime spectroscopy

    SciTech Connect

    Polity, A.; Rudolf, F.; Nagel, C.; Eichler, S.; Krause-Rehberg, R.

    1997-04-01

    A systematic study of electron-irradiation-induced defects in GaAs was carried out. The irradiation was performed at low temperature (4 K) with an incident energy of 2 MeV. Both, the defect formation and annealing behavior were studied in dependence on the fluence (10{sup 15}--10{sup 19} cm{sup {minus}2}) in undoped, n-, and p-doped GaAs. Temperature-dependent positron lifetime measurements were performed between 20 and 600 K. The thermal stability of defects was studied by annealing experiments in the temperature range of 90--600 K. A defect complex, which anneals in a main stage at 300 K, was found in all GaAs samples after electron irradiation. A possible candidate for this defect is a complex of a vacancy connected with an intrinsic defect. A second vancancylike defect was observed in n-type material after annealing at 550 K. This defect was assumed to be in the As sublattice. {copyright} {ital 1997} {ital The American Physical Society}

  3. Free volumes in bulk nanocrystalline metals studied by the complementary techniques of positron annihilation and dilatometry

    PubMed Central

    Würschum, Roland; Oberdorfer, Bernd; Steyskal, Eva-Maria; Sprengel, Wolfgang; Puff, Werner; Pikart, Philip; Hugenschmidt, Christoph; Pippan, Reinhard

    2012-01-01

    Free-volume type defects, such as vacancies, vacancy-agglomerates, dislocations, and grain boundaries represent a key parameter in the properties of ultrafine-grained and nanocrystalline materials. Such free-volume type defects are introduced in high excess concentration during the processes of structural refinement by severe plastic deformation. The direct method of time-differential dilatometry is applied in the present work to determine the total amount and the kinetics of free volume by measuring the irreversible length change upon annealing of bulk nanocrystalline metals (Fe, Cu, Ni) prepared by high-pressure torsion (HPT). In the case of HPT-deformed Ni and Cu, distinct substages of the length change upon linear heating occur due to the loss of grain boundaries in the wake of crystallite growth. The data on dilatometric length change can be directly related to the fast annealing of free-volume type defects studied by in situ Doppler broadening measurements performed at the high-intensity positron beam of the FRM II (Garching, Munich, Germany). PMID:23471443

  4. Free volumes in bulk nanocrystalline metals studied by the complementary techniques of positron annihilation and dilatometry.

    PubMed

    Würschum, Roland; Oberdorfer, Bernd; Steyskal, Eva-Maria; Sprengel, Wolfgang; Puff, Werner; Pikart, Philip; Hugenschmidt, Christoph; Pippan, Reinhard

    2012-07-15

    Free-volume type defects, such as vacancies, vacancy-agglomerates, dislocations, and grain boundaries represent a key parameter in the properties of ultrafine-grained and nanocrystalline materials. Such free-volume type defects are introduced in high excess concentration during the processes of structural refinement by severe plastic deformation. The direct method of time-differential dilatometry is applied in the present work to determine the total amount and the kinetics of free volume by measuring the irreversible length change upon annealing of bulk nanocrystalline metals (Fe, Cu, Ni) prepared by high-pressure torsion (HPT). In the case of HPT-deformed Ni and Cu, distinct substages of the length change upon linear heating occur due to the loss of grain boundaries in the wake of crystallite growth. The data on dilatometric length change can be directly related to the fast annealing of free-volume type defects studied by in situ Doppler broadening measurements performed at the high-intensity positron beam of the FRM II (Garching, Munich, Germany). PMID:23471443

  5. Positron annihilation Doppler broadening spectroscopy study on Fe-ion irradiated NHS steel

    NASA Astrophysics Data System (ADS)

    Zhu, Huiping; Wang, Zhiguang; Gao, Xing; Cui, Minghuan; Li, Bingsheng; Sun, Jianrong; Yao, Cunfeng; Wei, Kongfang; Shen, Tielong; Pang, Lilong; Zhu, Yabin; Li, Yuanfei; Wang, Ji; Song, Peng; Zhang, Peng; Cao, Xingzhong

    2015-02-01

    In order to study the evolution of irradiation-induced vacancy-type defects at different irradiation fluences and temperatures, a new type of ferritic/martensitic (F/M) steel named NHS (Novel High Silicon) was irradiated by 3.25 MeV Fe-ion at room temperature and 723 K to fluences of 4.3 × 1015 and 1.7 × 1016 ions/cm2. After irradiation, vacancy-type defects were investigated with variable-energy positron beam Doppler broadening spectra. Energetic Fe-ions produced a large number of vacancy-type defects in the NHS steel, but one single main type of vacancy-type defect was observed in both unirradiated and irradiated samples. The concentration of vacancy-type defects decreased with increasing temperature. With the increase of irradiation fluence, the concentration of vacancy-type defects increased in the sample irradiated at RT, whereas for the sample irradiated at 723 K, it decreased. The enhanced recombination between vacancies and excess interstitial Fe atoms from deeper layers, and high diffusion rate of self-interstitial atoms further improved by diffusion via grain boundary and dislocations at high temperature, are thought to be the main reasons for the reversed trend of vacancy-type defects between the samples irradiated at RT and 723 K.

  6. Positron beam investigations of natural cubic and coated diamonds

    NASA Astrophysics Data System (ADS)

    Shiryaev, A. A.; van Veen, A.; Schut, H.; Kruseman, A. C.; Zakharchenko, O. D.

    2000-06-01

    Positron beam and 2D-ACAR investigation of cubic and coated diamonds are reported. In type IIA diamonds, positrons are mostly trapped in vacancies in the carbon lattice; in type Ia diamonds, two main defect-related annihilation sites are nitrogen-vacancy complexes (H2, H3) and the vicinity of split interstitial atoms. No correlation between principal nitrogen defects and annihilation rate was found. PAS data indicate the presence of a significant amount of vacancies in all studied diamonds, which increases the rate of nitrogen aggregation. It is shown that pressurised fluid inclusions may serve as a positron trap, giving rise to the long component in the lifetime spectra.

  7. Positronics of subnanometer atomistic imperfections in solids as a high-informative structure characterization tool.

    PubMed

    Shpotyuk, Oleh; Filipecki, Jacek; Ingram, Adam; Golovchak, Roman; Vakiv, Mykola; Klym, Halyna; Balitska, Valentyna; Shpotyuk, Mykhaylo; Kozdras, Andrzej

    2015-01-01

    Methodological possibilities of positron annihilation lifetime (PAL) spectroscopy applied to characterize different types of nanomaterials treated within three-term fitting procedure are critically reconsidered. In contrast to conventional three-term analysis based on admixed positron- and positronium-trapping modes, the process of nanostructurization is considered as substitutional positron-positronium trapping within the same host matrix. Developed formalism allows estimate interfacial void volumes responsible for positron trapping and characteristic bulk positron lifetimes in nanoparticle-affected inhomogeneous media. This algorithm was well justified at the example of thermally induced nanostructurization occurring in 80GeSe2-20Ga2Se3 glass. PMID:25852373

  8. Positronics of subnanometer atomistic imperfections in solids as a high-informative structure characterization tool

    NASA Astrophysics Data System (ADS)

    Shpotyuk, Oleh; Filipecki, Jacek; Ingram, Adam; Golovchak, Roman; Vakiv, Mykola; Klym, Halyna; Balitska, Valentyna; Shpotyuk, Mykhaylo; Kozdras, Andrzej

    2015-02-01

    Methodological possibilities of positron annihilation lifetime (PAL) spectroscopy applied to characterize different types of nanomaterials treated within three-term fitting procedure are critically reconsidered. In contrast to conventional three-term analysis based on admixed positron- and positronium-trapping modes, the process of nanostructurization is considered as substitutional positron-positronium trapping within the same host matrix. Developed formalism allows estimate interfacial void volumes responsible for positron trapping and characteristic bulk positron lifetimes in nanoparticle-affected inhomogeneous media. This algorithm was well justified at the example of thermally induced nanostructurization occurring in 80GeSe2-20Ga2Se3 glass.

  9. General relativistic ray-tracing algorithm for the determination of the electron-positron energy deposition rate from neutrino pair annihilation around rotating neutron and quark stars

    NASA Astrophysics Data System (ADS)

    Kovács, Z.; Harko, T.

    2011-11-01

    We present a full general relativistic numerical code for estimating the energy-momentum deposition rate (EMDR) from neutrino pair annihilation (?). The source of the neutrinos is assumed to be a neutrino-cooled accretion disc around neutron and quark stars. We calculate the neutrino trajectories by using a ray-tracing algorithm with the general relativistic Hamilton's equations for neutrinos and derive the spatial distribution of the EMDR due to the annihilations of neutrinos and antineutrinos around rotating neutron and quark stars. We obtain the EMDR for several classes of rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the Massachusetts Institute of Technology (MIT) bag model equation of state and in the colour-flavour-locked (CFL) phase. The distribution of the total annihilation rate of the neutrino-antineutrino pairs around rotating neutron and quark stars is studied for isothermal discs and accretion discs in thermodynamical equilibrium. We demonstrate both the differences in the equations of state for neutron and quark matter and rotation with the general relativistic effects significantly modify the EMDR of the electrons and positrons generated by the neutrino-antineutrino pair annihilation around compact stellar objects, as measured at infinity.

  10. SYNCHROTRON RADIATION, FREE ELECTRON LASER, APPLICATION OF NUCLEAR TECHNOLOGY, ETC.: Positronium annihilation in silica aerogel studied by a positron age-momentum correlation technique

    NASA Astrophysics Data System (ADS)

    Wang, Dan-Ni; Zhang, Lan-Zhi; Wang, Bao-Yi; Yu, Run-Sheng; Zhang, Zhi-Ming; Li, Dao-Wu; Wei, Long

    2009-01-01

    A high-performance positron age-momentum correlation (AMOC) spectrometer was newly developed. The counting rate is increased up to 200 cps much larger than the value 20 cps reported by other international groups. And at the same time, the time resolution still keeps at the international level of 220 ps. Furthermore, positronium (Ps) annihilation in silica aerogel was investigated by AMOC, which indicates: (1) Ps annihilation between the grains dominantly undergoes pick-off process and spin conversion from o-Ps to p-Ps; (2) Annealing below 400 °C changes the grain surface conditions, i. e. the desorption of hydrogen and the decrease of the defect centers concentration.

  11. Positron spectroscopy for materials characterization

    SciTech Connect

    Schultz, P.J.; Snead, C.L. Jr.

    1988-01-01

    One of the more active areas of research on materials involves the observation and characterization of defects. The discovery of positron localization in vacancy-type defects in solids in the 1960's initiated a vast number of experimental and theoretical investigations which continue to this day. Traditional positron annihilation spectroscopic techniques, including lifetime studies, angular correlation, and Doppler broadening of annihilation radiation, are still being applied to new problems in the bulk properties of simple metals and their alloys. In addition new techniques based on tunable sources of monoenergetic positron beams have, in the last 5 years, expanded the horizons to studies of surfaces, thin films, and interfaces. In the present paper we briefly review these experimental techniques, illustrating with some of the important accomplishments of the field. 40 refs., 19 figs.

  12. A Study of Double-Charm and Charm-Strange Baryons inElectron-Positron Annihilations

    SciTech Connect

    Edwards, Adam J.; /SLAC

    2007-10-15

    In this dissertation I describe a study of double-charm and charm-strange baryons based on data collected with the BABAR Detector at the Stanford Linear Accelerator Center. In this study I search for new baryons and make precise measurements of their properties and decay modes. I seek to verify and expand upon double-charm and charm-strange baryon observations made by other experiments. The BABAR Detector is used to measure subatomic particles that are produced at the PEP-II storage rings. I analyze approximately 300 million e+e- {yields} c{bar c} events in a search for the production of double-charm baryons. I search for the double-charm baryons {Xi}{sup +}{sub cc} (containing the quarks ccd) and {Xi}{sup ++}{sub cc} (ccu) in decays to {Lambda}{sup +}{sub c}K{sup -}{pi}{sup +} and {Lambda}{sup +}{sub c}K{sup -}{pi}{sup +}{pi}{sup +}, respectively. No statistically significant signals for their production are found, and upper limits on their production are determined. Statistically significant signals for excited charm-strange baryons are observed with my analysis of approximately 500 million e+e- {yields} c{bar c} events. The charged charm-strange baryons {Xi}{sub c}(2970){sup +}, {Xi}{sub c}(3055){sup +}, {Xi}{sub c}(3123){sup +} are found in decays to {Lambda}{sup +}{sub c}K{sup -}{pi}{sup +}, the same decay mode used in the {Xi}{sup +}{sub cc} search. The neutral charm-strange baryon {Xi}{sub c}(3077){sup 0} is observed in decays to {Lambda}{sup +}{sub c}K{sub 8}{pi}{sup -}. I also search for excited charm-strange baryon decays to {Lambda}{sup +}{sub c}K{sub 8}, {Lambda}{sup +}{sub c}K{sup -}, {Lambda}{sup +}{sub c}K{sub 8}{pi}{sup -}{pi}{sup +}, and {Lambda}{sup +}{sub c}K{sup -}{pi}{sup -}{pi}{sup +}. No significant charm-strange baryon signals a f h these decay modes. For each excited charm-strange baryon state that I observe, I measure its mass, natural width (lifetime), and production rate. The properties of these excited charm-strange baryons and their

  13. Diffuse galactic annihilation radiation from supernova nucleosynthesis

    NASA Technical Reports Server (NTRS)

    Higdon, J. C.

    1985-01-01

    The propagation of MeV positrons in the outer ejecta of type I supernovae was investigated. It was found that the positrons created at times of approx 100 days propagated along magnetic field lines in the outer ejecta without any appreciable pitch-angle scattering or excitation of hydromagnetic waves. The lack of significant pitch-angle scattering is well consistent with models of wave excitation and scattering by resonant interactions. This occurs because time periods to scatter the particles or to excite waves are significantly longer than escape times. Thus it is expected that, when positrons are not coupled to the ejecta by Coulomb collisions, they escape from the relatively cold, dense ejecta and reside predominantly in the tenuous, hotter, shock-heated interstellar gas. In the tenuous shock-heated gas the positron lifetime against annihilation is much greater than lifetimes in the dense ejectra. Thus the production of steady-state diffuse annihilation radiation by some fraction of these escaped positrons seems probable.

  14. Observation of the electron ridge Fermi surface in YBa{sub 2}Cu{sub 3}O{sub 7-x} by positron annihilation

    SciTech Connect

    Smedskjaer, L.C.; Fang, Y.; Bailey, K.G.; Welp, U.; Bansil, A.

    1991-04-01

    Positron annihilation (two-dimensional-angular-correlation) experiments on an untwinned single crystal of metallic YBa{sub 2}Cu{sub 3}O{sub 7{minus}x} sample are reported in the c-projection. The measurements were carried out at room temperature and involved 94 Mcounts. An analysis of the spectra reveals clearly for the first time the presence of the electron ridge Fermi surface associated with the one-dimensional chain bands, and orthorhombic anisotropies in momentum density in good agreement with the band theory predictions.

  15. Ab initio study of the positronation of the CaO and SrO molecules including calculation of annihilation rates.

    PubMed

    Buenker, Robert J; Liebermann, Heinz-Peter

    2012-07-15

    Ab initio multireference single- and double-excitation configuration interaction calculations have been performed to compute potential curves for ground and excited states of the CaO and SrO molecules and their positronic complexes, e(+)CaO, and e(+)SrO. The adiabatic dissociation limit for the (2)Σ(+) lowest states of the latter systems consists of the positive metal ion ground state (M(+)) and the OPs complex (e(+)O(-)), although the lowest energy limit is thought to be e(+)M + O. Good agreement is found between the calculated and experimental spectroscopic constants for the neutral diatomics wherever available. The positron affinity of the closed-shell X (1)Σ(+) ground states of both systems is found to lie in the 0.16-0.19 eV range, less than half the corresponding values for the lighter members of the alkaline earth monoxide series, BeO and MgO. Annihilation rates (ARs) have been calculated for all four positronated systems for the first time. The variation with bond distance is generally similar to what has been found earlier for the alkali monoxide series of positronic complexes, falling off gradually from the OPs AR value at their respective dissociation limits. The e(+)SrO system shows some exceptional behavior, however, with its AR value reaching a minimum at a relatively large bond distance and then rising to more than twice the OPs value close to its equilibrium distance. PMID:22522712

  16. Bounds on Cross-sections and Lifetimes for Dark Matter Annihilation and Decay into Charged Leptons from Gamma-ray Observations of Dwarf Galaxies

    SciTech Connect

    Essig, Rouven; Sehgal, Neelima; Strigari, Louis E.; /KIPAC, Menlo Park

    2009-06-19

    We provide conservative bounds on the dark matter cross-section and lifetime from final state radiation produced by annihilation or decay into charged leptons, either directly or via an intermediate particle {phi}. Our analysis utilizes the experimental gamma-ray flux upper limits from four Milky Way dwarf satellites: HESS observations of Sagittarius and VERITAS observations of Draco, Ursa Minor, and Willman 1. Using 90% confidence level lower limits on the integrals over the dark matter distributions, we find that these constraints are largely unable to rule out dark matter annihilations or decays as an explanation of the PAMELA and ATIC/PPB-BETS excesses. However, if there is an additional Sommerfeld enhancement in dwarfs, which have a velocity dispersion {approx} 10 to 20 times lower than that of the local Galactic halo, then the cross-sections for dark matter annihilating through {phi}'s required to explain the excesses are very close to the cross-section upper bounds from Willman 1. Dark matter annihilation directly into {tau}'s is also marginally ruled out by Willman 1 as an explanation of the excesses, and the required cross-section is only a factor of a few below the upper bound from Draco. Finally, we make predictions for the gamma-ray flux expected from the dwarf galaxy Segue 1 for the Fermi Gamma-ray Space Telescope. We find that for a sizeable fraction of the parameter space in which dark matter annihilation into charged leptons explains the PAMELA excess, Fermi has good prospects for detecting a gamma-ray signal from Segue 1 after one year of observation.

  17. A position-sensitive γ-ray detector for positron annihilation 2D-ACAR based on metal package photomultiplier tubes

    NASA Astrophysics Data System (ADS)

    Inoue, Koji; Saito, Haruo; Nagashima, Yasuyuki; Hyodo, Toshio; Nagai, Yasuyoshi; Muramatsu, Shinichi; Nagai, Shota; Masuda, Keisuke

    2002-07-01

    A new position-sensitive γ-ray detector to be used in a two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) apparatus has been developed. It consists of 36 compact position-sensitive photomultiplier tubes (PS-PMT: HAMAMATSU R5900-00-C8), a light guide, and 2676 Bi 4Ge 3O 12 (BGO) scintillator pieces of size 2.6 mm×2.6 mm×18 mm. A high detection efficiency for 511 keV γ-ray is achieved with the length of BGO scintillators used. The detection area is about 160 mm×160 mm. The 288 anode outputs of the PS-PMTs are wired and connected to resistor chains from which 16 outputs (8 outputs each along the X and Y directions) are taken to identify the incident position of the γ-ray. The spatial resolution is about 3 mm (FWHM). The timing signal taken from the last dynodes of the PS-PMTs gives a timing resolution of 7.7 ns (FWHM) for 511 keV positron annihilation γ-rays.

  18. Ligand-surface interactions and surface oxidation of colloidal PbSe quantum dots revealed by thin-film positron annihilation methods

    NASA Astrophysics Data System (ADS)

    Shi, Wenqin; Eijt, Stephan W. H.; Suchand Sandeep, C. S.; Siebbeles, Laurens D. A.; Houtepen, Arjan J.; Kinge, Sachin; Brück, Ekkes; Barbiellini, Bernardo; Bansil, Arun

    2016-02-01

    Positron Two Dimensional Angular Correlation of Annihilation Radiation (2D-ACAR) measurements reveal modifications of the electronic structure and composition at the surfaces of PbSe quantum dots (QDs), deposited as thin films, produced by various ligands containing either oxygen or nitrogen atoms. In particular, the 2D-ACAR measurements on thin films of colloidal PbSe QDs capped with oleic acid ligands yield an increased intensity in the electron momentum density (EMD) at high momenta compared to PbSe quantum dots capped with oleylamine. Moreover, the EMD of PbSe QDs is strongly affected by the small ethylenediamine ligands, since these molecules lead to small distances between QDs and favor neck formation between near neighbor QDs, inducing electronic coupling between neighboring QDs. The high sensitivity to the presence of oxygen atoms at the surface can be also exploited to monitor the surface oxidation of PbSe QDs upon exposure to air. Our study clearly demonstrates that positron annihilation spectroscopy applied to thin films can probe surface transformations of colloidal semiconductor QDs embedded in functional layers.

  19. Mössbauer and positron annihilation studies in plastically deformed Fe 72- xAl 28Ti x ( x=0, 2, 9) alloys

    NASA Astrophysics Data System (ADS)

    Pandey, Brajesh; Nambissan, P. M. G.; Suwas, Satyam; Verma, H. C.

    2003-07-01

    Positron lifetime measurements in deformed Fe-Al-Ti alloys, obtained by filing the homogenized ingots show that monovacancies are created during the filing process. This is in contrast with known results on deformation due to rolling where dislocations have been reported as the major defect type. These results together with those from Mössbauer spectroscopy suggest that the vacancies are dominantly created due to aluminum atoms being displaced to the nearby sites where they replace iron atoms. Annealing at 900°C for extended periods caused defect concentration to greatly reduce. Positron lifetime spectroscopy has been successfully used to detect agglomeration of vacancies to form di- or tri-vacancies for the first time. Addition of titanium is found to facilitate fast removal of defects during controlled heat treatments.

  20. Positron bunching and electrostatic transport system for the production and emission of dense positronium clouds into vacuum

    NASA Astrophysics Data System (ADS)

    Aghion, S.; Amsler, C.; Ariga, A.; Ariga, T.; Belov, A. S.; Bonomi, G.; Bräunig, P.; Bremer, J.; Brusa, R. S.; Cabaret, L.; Caccia, M.; Caravita, R.; Castelli, F.; Cerchiari, G.; Chlouba, K.; Cialdi, S.; Comparat, D.; Consolati, G.; Demetrio, A.; Di Noto, L.; Doser, M.; Dudarev, A.; Ereditato, A.; Evans, C.; Fesel, J.; Fontana, A.; Forslund, O. K.; Gerber, S.; Giammarchi, M.; Gligorova, A.; Gninenko, S.; Guatieri, F.; Haider, S.; Holmestad, H.; Huse, T.; Jernelv, I. L.; Jordan, E.; Kaltenbacher, T.; Kellerbauer, A.; Kimura, M.; Koetting, T.; Krasnicky, D.; Lagomarsino, V.; Lebrun, P.; Lansonneur, P.; Lehner, S.; Liberadzka, J.; Malbrunot, C.; Mariazzi, S.; Marx, L.; Matveev, V.; Mazzotta, Z.; Nebbia, G.; Nedelec, P.; Oberthaler, M.; Pacifico, N.; Pagano, D.; Penasa, L.; Petracek, V.; Pistillo, C.; Prelz, F.; Prevedelli, M.; Ravelli, L.; Rienäcker, B.; Røhne, O. M.; Rosenberger, S.; Rotondi, A.; Sacerdoti, M.; Sandaker, H.; Santoro, R.; Scampoli, P.; Sorrentino, F.; Spacek, M.; Storey, J.; Strojek, I. M.; Testera, G.; Tietje, I.; Vamosi, S.; Widmann, E.; Yzombard, P.; Zavatarelli, S.; Zmeskal, J.

    2015-11-01

    We describe a system designed to re-bunch positron pulses delivered by an accumulator supplied by a positron source and a Surko-trap. Positron pulses from the accumulator are magnetically guided in a 0.085 T field and are injected into a region free of magnetic fields through a μ -metal field terminator. Here positrons are temporally compressed, electrostatically guided and accelerated towards a porous silicon target for the production and emission of positronium into vacuum. Positrons are focused in a spot of less than 4 mm FWTM in bunches of ∼8 ns FWHM. Emission of positronium into the vacuum is shown by single shot positron annihilation lifetime spectroscopy.

  1. An upper bound on the energy of a gravitationally redshifted electron-positron annihilation line from the Crab pulsar

    NASA Astrophysics Data System (ADS)

    Negi, P. S.

    2005-02-01

    results requires evidence of the observation of the gravitationally redshifted electron-positron annihilation line in the energy range of about 0.414-0.418 MeV from the Crab pulsar, which is in agreement with the energy of the gamma-ray line at about 0.40 MeV, observed in the mid 1970s.

  2. Bulk Fermi surface and momentum density in heavily doped La2-xSrxCuO4 using high-resolution Compton scattering and positron annihilation spectroscopies

    NASA Astrophysics Data System (ADS)

    Al-Sawai, W.; Barbiellini, B.; Sakurai, Y.; Itou, M.; Mijnarends, P. E.; Markiewicz, R. S.; Kaprzyk, S.; Wakimoto, S.; Fujita, M.; Basak, S.; Lin, H.; Wang, Yung Jui; Eijt, S. W. H.; Schut, H.; Yamada, K.; Bansil, A.

    2012-03-01

    We have observed the bulk Fermi surface (FS) in an overdoped (x=0.3) single crystal of La2-xSrxCuO4 by using Compton scattering. A two-dimensional (2D) momentum density reconstruction from measured Compton profiles yields a clear FS signature in the third Brillouin zone along [100]. The quantitative agreement between density functional theory (DFT) calculations and momentum density experiment suggests that Fermi-liquid physics is restored in the overdoped regime. In particular the predicted FS topology is found to be in good accord with the corresponding experimental data. We find similar quantitative agreement between the measured 2D angular correlation of positron annihilation radiation (2D-ACAR) spectra and the DFT-based computations. However, 2D-ACAR does not give such a clear signature of the FS in the extended momentum space in either the theory or the experiment.

  3. Direct Observation of the Surface Segregation of Cu in Pd by Time-Resolved Positron-Annihilation-Induced Auger Electron Spectroscopy

    SciTech Connect

    Mayer, J.; Hugenschmidt, C.; Schreckenbach, K.

    2010-11-12

    Density functional theory calculations predict the surface segregation of Cu in the second atomic layer of Pd which has not been unambiguously confirmed by experiment so far. We report measurements on Pd surfaces covered with three and six monolayers of Cu using element selective positron-annihilation-induced Auger electron spectroscopy (PAES) which is sensitive to the topmost atomic layer. Moreover, time-resolved PAES, which was applied for the first time, enables the investigation of the dynamics of surface atoms and hence the observation of the segregation process. The time constant for segregation was experimentally determined to {tau}=1.38(0.21) h, and the final segregated configuration was found to be consistent with calculations. Time-dependent PAES is demonstrated to be a novel element selective technique applicable for the investigation of, e.g., heterogeneous catalysis, corrosion, or surface alloying.

  4. Positron annihilation study of the influence of doping on the 3 d electron states in the Ni3Al intermetallic compound

    NASA Astrophysics Data System (ADS)

    Druzhkov, A. P.; Perminov, D. A.; Stepanova, N. N.

    2010-10-01

    The 3 d electron states in Ni3Al single crystals doped with Fe, Co, and Nb have been investigated using angular correlation of annihilation radiation (ACAR). The ACAR spectra contain information on the momentum distribution of valence electrons and strongly bound 3 d electrons of the intermetallic compound. It has been established that the positrons in the Ni3Al crystals predominantly annihilate in the nickel sublattice from delocalized states. The doping of the compound by the third element leads to a variation in the momentum distribution of Ni 3 d electrons due to the change in the character of interatomic bonds. An analysis of the momentum distribution has demonstrated that the niobium atoms increase the covalent component of the chemical bond as compared to the binary compound due to the d Nb- d Ni hybridization. The doping with cobalt atoms also enhances the tendency toward the formation of the covalent bond. At the same time, iron atoms have a weak effect on the electronic structure of the intermetallic compound.

  5. The Effect of Alloying with Magnesium on the Annealing Behavior of Aluminum Alloys Studied by Positron Lifetime Technique

    NASA Astrophysics Data System (ADS)

    Abdel-Hamed, M. O.

    The migration enthalpy Hivm for point defects and dislocations is estimated by using positron lifetime technique; point defects and dislocations are produced as a result of plastic deformation at room temperature (RT) for the decomposition sequence, namely 5005, 5052 and 5083, of commercial Al-Mg systems. The results show that Hivm for the three systems increases as the Mg content is increased to u1=0.34±0.09 eV, u2=0.39±0.12 eV, and u3=0.42±0.08 eV for the point defect state, and u1=1.12±0.08 eV and u2=1.37±0.13 eV for the dislocation state to 5005 and 5052, respectively. All the data are analyzed in terms of the two state trapping model.

  6. Nanocluster-associated vacancies in nanocluster-strengthened ferritic steel as seen via positron-lifetime spectroscopy

    NASA Astrophysics Data System (ADS)

    Xu, Jun; Liu, C. T.; Miller, M. K.; Chen, Hongmin

    2009-01-01

    Nanocluster-strengthened ferritic alloys are promising as structural materials because of their excellent high-temperature strength and radiation-damage resistance. Recently, Fu [Phys. Rev. Lett. 99, 225502 (2007)] predicted that vacancies play an essential role in the formation and stabilization of nanoclusters in these materials. Positron-lifetime spectroscopy has been used to test this theoretical prediction in a nanocluster-strengthened Fe-based alloy. Nanoclusters (2-4 nm in diameter) containing Ti, Y, and O have been observed in a mechanically alloyed ferritic steel by atom-probe tomography. Vacancy clusters containing four to six vacancies have also been found in this material. In contrast, no vacancy clusters were detected in similar alloys containing no nanoclusters. These results indicate that vacancies are a vital component of the nanoclusters in these alloys.

  7. Study of the effect of annealing on defects in Fe Mn Si Cr Ni C alloy by slow positron beam

    NASA Astrophysics Data System (ADS)

    Mostafa, Khaled. M.; De Baerdemaeker, J.; Van Caenegem, N.; Segers, D.; Houbaert, Y.

    2008-10-01

    FeMnSi shape memory alloys (SMAs) have received much attention as one-way SMAs due to their cost-effectiveness. Variable-energy (0-30 keV) positron beam studies have been carried out on a Fe-Mn-Si-Cr-Ni-C alloy with different degrees of deformation. Doppler broadening profiles of the positron annihilation as a function of incident positron energy were shown to be quite sensitive to defects introduced by deformation. The variation of the nature and the concentration of defects are studied as a function of isochronal annealing temperature. These results are correlated with the data measured with the positron annihilation lifetime spectroscopy (PALS). The positron annihilation results are compared to XRD and optical microscopy (OM).

  8. Gamma-ray spectroscopy of the galactic center region: Confirmation of the time-variability of the positron annihilation line

    NASA Technical Reports Server (NTRS)

    Paciesas, W. S.; Cline, T. L.; Teegarden, B. J.; Tueller, J.; Durouchoux, P.; Hameury, J. M.

    1982-01-01

    The GSFC Low-Energy Gamma-Ray Spectrometer observed the region of the galactic center during a balloon flight from Alice Springs, Australia, on 1981 November 20. No significant excess over background was evident in the 511 keV annihilation line. A 98 percent confidence upper limit is derived for this line of 1.2 x .001 photons/sq. cm-s. Continuum emission was detected above 100 keV with a best-fitting power law spectrum.

  9. Formation and evolution of intermetallic nanoparticles and vacancy defects under irradiation in Fesbnd Nisbnd Al ageing alloy characterized by resistivity measurements and positron annihilation

    NASA Astrophysics Data System (ADS)

    Druzhkov, A. P.; Danilov, S. E.; Perminov, D. A.; Arbuzov, V. L.

    2016-08-01

    In this paper, we study the effects of intermetallic nanoparticles like Ni3Al on the evolution of vacancy defects in the fcc Fesbnd Nisbnd Al alloy under electron irradiation using positron annihilation spectroscopy. Electrical resistivity measurements have been used as a testing method for characterizing the evolution in the underlying precipitate microstructure due to heat treatment and irradiation. It was shown that the nanosized (∼4.5 nm) intermetallic precipitates homogeneously distributed in the alloy matrix caused a several-fold decrease in the accumulation of vacancies as compared to their accumulation in the pre-quenched alloy. This effect was enhanced with the irradiation temperature. The irradiation-induced growth of intermetallic nanoparticles was also observed in the pre-quenched Fesbnd Nisbnd Al alloy under irradiation at 573 K. Thus, resistivity measurement and positron confinement in ultrafine intermetallic particles, which we revealed earlier, provided the control over the evolution of coherent precipitates, along with vacancy defects, during irradiation and annealing.

  10. Positron sensing of distribution of defects in depth materials

    NASA Astrophysics Data System (ADS)

    Kupchishin, A. I.; Kupchishin, A. A.; Voronova, N. A.; Kirdyashkin, V. I.

    2016-02-01

    It was developed a non-destructive method of positron sensing, which allows to determine the distribution of defects in the depth of the material. From the analysis we can conclude that the angular distribution curves of annihilation photons (well as and on the characteristics in experiments on the lifetime, 3γ - angular correlation, Doppler effect) is influenced by three main factors: a) The distribution of defects in the depth of the material, their dimensions as well as parameters of the interaction of positrons with defects. With increasing the concentration of defects the intensity Jγ(a, ξ) varies more; b) Modification of the energy spectrum of slow positrons due to the influence of defects, wherein the spectrum of positrons becomes softer, and the average energy of the positron annihilation is reduced; c) Deformation of the momentum distribution of the electrons in the region of defect. The energy spectrum of electrons is also becomes softer, and the average energy of the electrons (on which positrons annihilate) is less. The experimentally were measured spectra of photons in the zone of annihilation and were calculated the distribution of defects in depth for a number of metals.

  11. Positron measurements in 2H-TaSe/sub 2/ crystals

    SciTech Connect

    Jean, Y.C.; Fluss, M.J.

    1985-01-01

    Temperature-dependent positron annihilation lifetime and Doppler broadening experiments are reported on single crystals of 2H-TaSe/sub 2/ to search for effects from known charge-density-wave (CDW) phase transitions. The positron lifetime in the perfect lattice and in positron trapping sites were found to be 0.173 and 0.378 ns, respectively. The apparent activation energy for the thermally generated trapping sites was found to be 0.12 eV. Doppler broadening spectra exhibited no response to the known CDW phase transitions, nor any significant overall anisotropy in their temperature dependence.

  12. Cold positrons from decaying dark matter

    NASA Astrophysics Data System (ADS)

    Boubekeur, Lotfi; Dodelson, Scott; Vives, Oscar

    2012-11-01

    Many models of dark matter contain more than one new particle beyond those in the Standard Model. Often, heavier particles decay into the lightest dark matter particle as the Universe evolves. Here, we explore the possibilities which arise if one of the products in a (heavyparticle)→(darkmatter) decay is a positron, and the lifetime is shorter than the age of the Universe. The positrons cool down by scattering off the cosmic microwave background and eventually annihilate when they fall into Galactic potential wells. The resulting 511 keV flux not only places constraints on this class of models, but might even be consistent with that observed by the INTEGRAL satellite.

  13. Effect of free volume and temperature on the structural relaxation in polymethylphenylsiloxane: a positron lifetime and pressure-volume-temperature study.

    PubMed

    Dlubek, G; Shaikh, M Q; Krause-Rehberg, R; Paluch, M

    2007-01-14

    The microstructure of the free volume and its temperature dependence in polymethylphenylsiloxane (PMPS) have been examined using positron annihilation lifetime spectroscopy (PALS) and pressure-volume-temperature experiments. The hole-free volume fraction h and the specific hole-free and occupied volumes, Vf=hV and Vocc=(1-h)V, were estimated employing the Simha-Somcynsky (SS) lattice-hole theory. From the PALS spectra analyzed with the new routine LT9.0 the hole size distribution, its mean, , and mean dispersion, sigmah, were calculated. A comparison of with V and Vf delivered a constant specific hole number Nh'. Using a fluctuation approach the temperature dependency of the volume of the smallest representative freely fluctuating subsystem, , is estimated to vary from approximately 8.5 nm3 at Tg to approximately 3 nm3 at T/Tg>or=1.15. Unlike other polymers, the segmental relaxation from dielectric spectroscopy of PMPS follows the Cohen-Turnbull free volume theory almost perfectly in the temperature and pressure ranges between 243 and 279 K and 0 and approximately 100 MPa. This behavior correlates with the small mass of the SS lattice mer which indicates the high flexibility of the PMPS chain. Above 293 K and approximately 150 MPa, the free volume prediction gives relaxation times that are too small, which indicates that effects of thermal energy must be included in the analysis. To quantify the degree to which volume and thermal energy govern the structural dynamics the ratio of the activation enthalpies, Ei=R[(d ln taudT1)]i (tau-relaxation time of alpha relaxation), at constant volume V and constant pressure P, EV/EP, is frequently determined. The authors present arguments for necessity to substitute EV with EVf, the activation enthalpy at constant (hole) free volume, and show that EVf/EP changes as expected: increasing with increasing free volume, i.e., with increasing temperature and decreasing pressure. EVf/EP (=0.04-0.1) exhibits remarkably

  14. Modeling of the energy resolution of a 1 meter and a 3 meter time of flight positron annihilation induced Auger electron spectrometers

    NASA Astrophysics Data System (ADS)

    Fairchild, A.; Chirayath, V.; Gladen, R.; McDonald, A.; Lim, Z.; Chrysler, M.; Koymen, A.; Weiss, A.

    Simion 8.1®simulations were used to determine the energy resolution of a 1 meter long Time of Flight Positron annihilation induced Auger Electron Spectrometer (TOF-PAES). The spectrometer consists of: 1. a magnetic gradient section used to parallelize the electrons leaving the sample along the beam axis, 2. an electric field free time of flight tube and 3. a detection section with a set of ExB plates that deflect electrons exiting the TOF tube into a Micro-Channel Plate (MCP). Simulations of the time of flight distribution of electrons emitted according to a known secondary electron emission distribution, for various sample biases, were compared to experimental energy calibration peaks and found to be in excellent agreement. The TOF spectra at the highest sample bias was used to determine the timing resolution function describing the timing spread due to the electronics. Simulations were then performed to calculate the energy resolution at various electron energies in order to deconvolute the combined influence of the magnetic field parallelizer, the timing resolution, and the voltage gradient at the ExB plates. The energy resolution of the 1m TOF-PAES was compared to a newly constructed 3 meter long system. The results were used to optimize the geometry and the potentials of the ExB plates for obtaining the best energy resolution. This work was supported by NSF Grant NSF Grant No. DMR 1508719 and DMR 1338130.

  15. Correlation of polycrystalline Cu(In,Ga)Se{sub 2} device efficiency with homojunction depth and interfacial structure: X-ray photoemission and positron annihilation spectroscopic characterization

    SciTech Connect

    Nelson, A.J.; Sobol, P.E.; Gabor, A.M.; Contreras, M.A.; Asoka-Kumar, P.; Lynn, K.G.

    1994-06-01

    Angled-resolved high resolution photoemission measurements on valence band electronic structure and Cu 2p, In 3d, Ga 2p, and Se 3d core lines were used to evaluate surface and near-surface chemistry of CuInSe{sub 2} and Cu(In,Ga)Se{sub 2} device grade thin films. XPS compositional depth profiles were also acquired from the near-surface region, and bonding of the Cu, In, Ga, and Se was determined as a function of depth. A Cu-poor region was found, indicating CuIn{sub 5}Se{sub 8} or a CuIn{sub 3}Se{sub 5}-In{sub 2}Se{sub 3} mixture. Correlation between the depth of the Cu-poor region/bulk interface and device efficiency showed that the depth was 115 {angstrom} for a 16.4% CIGS device, 240 {angstrom} for a 15.0% CIGS, and 300 {angstrom} for 14.0% CIGS, with similar trends for CIS films. The surface region is n-type, the bulk is p-type, with a 0.5 eV valence band offset. Depth of homojunction may be the determining factor in device performance. Positron annihilation spectroscopy gave similarly illuminating results.

  16. Positron annihilation and nuclear magnetic resonance study of the phase behavior of water confined in mesopores at different levels of hydration.

    PubMed

    Maheshwari, Priya; Dutta, Dhanadeep; Mukherjee, Saurabh; Madhu, Perunthiruthy K; Mote, Kaustubh R; Pujari, Pradeep K

    2016-05-14

    We investigated the molecular origin of the phase behavior of water confined in MCM 41 mesopores at different levels of hydration using positron annihilation spectroscopic and nuclear magnetic resonance techniques. The level of hydration influenced the phase behavior of the nanoconfined water. Two transitions above and below the bulk freezing temperature were observed depending on the level of hydration. At the highest level of hydration, nucleation seemed to predominate over the effect of confinement, leading to the complete freezing of water, whereas disrupted H-bonding dominated at the lowest level of hydration, leading to the disappearance of the transitions. A transition at c. T = 188 K (close to the reported glass transition temperature of interface-affected water) was observed at intermediate hydration level. This study suggests that the H-bonding network within nanoconfined water, which can be tampered by the degree of hydration, is the key factor responsible for the phase behavior of supercooled water. This study on the phase behavior and associated transitions of nanoconfined water has implications for nanofluidics and drug-delivery systems, in addition to understanding the fundamentals of water in confinement. PMID:27105178

  17. High resolution positron annihilation induced Auger electron spectroscopy of the CuM 2,3VV-transition and of Cu sub-monolayers on Pd and Fe

    NASA Astrophysics Data System (ADS)

    Mayer, J.; Hugenschmidt, C.; Schreckenbach, K.

    2010-09-01

    We present a high resolution positron annihilation induced Auger Electron Spectroscopy (PAES) of the CuM 2,3VV-transition with the unprecedented energy resolution of Δ/EE <1%. This energy resolution and the highly intense positron source NEPOMUC enabled us to resolve the double peak structure with PAES for the first time within a measurement time of only 5.5 h. In addition, sub-monolayers of Cu were deposited on Fe- and Pd-samples in order to investigate the surface selectivity of PAES in comparison with EAES. The extremely high surface selectivity of PAES due to the different positron affinity of Cu and Fe lead to the result that with only 0.96 monolayer of Cu on Fe more than 55% of the emitted Auger electrons stem from Cu, whereas with EAES the Cu Auger fraction amounted to less than 6%.

  18. Oxygen-Atom Defects In 6H Silicon Carbide Implanted Using 24- MeV O3+ Ions Measured Using Three-Dimensional Positron Annihilation Spectroscopy System (3DPASS)

    NASA Astrophysics Data System (ADS)

    Williams, Christopher S.; Duan, Xiaofeng F.; Petrosky, James C.; Burggraf, Larry W.

    2011-06-01

    Three dimensional electron-positron (e--e+) momentum distributions were measured for single crystal 6H silicon carbide (SiC); both virgin and having implanted oxygen-atom defects. 6H SiC samples were irradiated by 24- MeV O3+ ions at 20 particle-nanoamps at the Sandia National Laboratory's Ion Beam Facility. O3+ ions were implanted 10.8 μm deep normal to the (0001) face of one side of the SiC samples. During positron annihilation measurements, the opposite face of the 254.0-μm thick SiC samples was exposed to positrons from a 22Na source. This technique reduced the influence on the momentum measurements of vacancy-type defects resulting from knock-on damage by the O3+ ions. A three-dimensional positron annihilation spectroscopy system (3DPASS) was used to measure e--e+ momentum distributions for virgin and irradiated 6H SiC crystal both before and following annealing. 3DPASS simultaneously measures coincident Doppler-broadening (DBAR) and angular correlation of annihilation radiation (ACAR) spectra. DBAR ratio plots and 2D ACAR spectra are presented. Changes in the momentum anisotropies relative to crystal orientation observed in 2D ACAR spectra for annealed O-implanted SiC agree with the local structure of defect distortion predicted using Surface Integrated Molecular Orbital/Molecular Mechanics (SIMOMM). Oxygen atoms insert between Si and C atoms increasing their separation by 0.9 Å forming a Si-O-C bond angle of ˜150°.

  19. Oxygen-Atom Defects In 6H Silicon Carbide Implanted Using 24- MeV O{sup 3+} Ions Measured Using Three-Dimensional Positron Annihilation Spectroscopy System (3DPASS)

    SciTech Connect

    Williams, Christopher S.; Petrosky, James C.; Burggraf, Larry W.

    2011-06-01

    Three dimensional electron-positron (e{sup -}-e{sup +}) momentum distributions were measured for single crystal 6H silicon carbide (SiC); both virgin and having implanted oxygen-atom defects. 6H SiC samples were irradiated by 24- MeV O{sup 3+} ions at 20 particle-nanoamps at the Sandia National Laboratory's Ion Beam Facility. O{sup 3+} ions were implanted 10.8 {mu}m deep normal to the (0001) face of one side of the SiC samples. During positron annihilation measurements, the opposite face of the 254.0-{mu}m thick SiC samples was exposed to positrons from a {sup 22}Na source. This technique reduced the influence on the momentum measurements of vacancy-type defects resulting from knock-on damage by the O{sup 3+} ions. A three-dimensional positron annihilation spectroscopy system (3DPASS) was used to measure e{sup -}-e{sup +} momentum distributions for virgin and irradiated 6H SiC crystal both before and following annealing. 3DPASS simultaneously measures coincident Doppler-broadening (DBAR) and angular correlation of annihilation radiation (ACAR) spectra. DBAR ratio plots and 2D ACAR spectra are presented. Changes in the momentum anisotropies relative to crystal orientation observed in 2D ACAR spectra for annealed O-implanted SiC agree with the local structure of defect distortion predicted using Surface Integrated Molecular Orbital/Molecular Mechanics (SIMOMM). Oxygen atoms insert between Si and C atoms increasing their separation by 0.9 A forming a Si-O-C bond angle of {approx}150 deg.

  20. Positron accumulation effect in particles embedded in a low-density matrix

    SciTech Connect

    Dryzek, Jerzy; Siemek, Krzysztof

    2015-02-07

    Systematic studies of the so-called positron accumulation effect for samples with particles embedded in a matrix are reported. This effect is related to energetic positrons which penetrate inhomogeneous medium. Due to differences in the linear absorption coefficient, different amounts of positrons are accumulated and annihilate in the identical volume of both materials. Positron lifetime spectroscopy and Doppler broadening of the annihilation line using Na-22 positrons were applied to the studies of the epoxy resin samples with embedded micro-sized particles of transition metals, i.e., Ni, Sn, Mo, W, and nonmetal particles, i.e., Si and NaF. The significant difference between the determined fraction of positrons annihilating in the particles and the particle volume fraction indicates the positron accumulation effect. The simple phenomenological model and Monte Carlo simulations are able to describe the main features of the obtained dependencies. The aluminum alloy with embedded Sn nanoparticles is also considered for demonstration differences between the accumulation and another related effect, i.e., the positron affinity.

  1. Positron trapping and possible presence of SO3H clusters in dry fluorinated polymer electrolyte membranes

    NASA Astrophysics Data System (ADS)

    Mohamed, Hamdy F. M.; Kobayashi, Y.; Kuroda, S.; Ohira, A.

    2012-08-01

    The behavior of positrons that do not form positronium in dry fluorinated polymer electrolyte membranes (Nafion®, Fumapem® and Aquivion®) with various ion exchange capacities (IECs) was studied by the combined use of Doppler broadening of annihilation radiation (DBAR) and the positron lifetime technique. The drastic increase of the S parameter, measured by DBAR, with increasing IEC above 0.91 meq/g indicates that increasing numbers of positrons are trapped by oxygen atoms and annihilate with the electrons bound in them. Reversed micelle like SO3H nanoclusters to trap positrons possibly appear at IEC = 0.91 meq/g and their concentration increases with increasing IEC.

  2. Positron trapping in vacancies in indium doped CdTe crystals

    NASA Astrophysics Data System (ADS)

    Gély-Sykes, C.; Corbel, C.; Triboulet, R.

    1991-10-01

    In weakly n-type CdTe(In) crystals grown by the travelling heater method, positrons annihilate in vacancy-type defects with a lifetime of 320 ± 4 ps. The concentration of these native defects varies with the concentration of indium and electron in agreement with the model of self-compensation where the indium donors are compensated by indium-vacancy complexes. These defects are assumed to be (V CdIn) - complexes. The positron trapping in these complexes disappears at low temperature. This phenomenon is attributed to competing trapping of positrons by negative ions which are either residual impurities or intrinsic defects.

  3. Annihilation in Gases and Galaxies

    NASA Technical Reports Server (NTRS)

    Drachman, Richard J. (Editor)

    1990-01-01

    This publication contains most of the papers, both invited and contributed, that were presented at the Workshop of Annihilation in Gases and Galaxies. This was the fifth in a biennial series associated with the International Conference on the Physics of Electronic and Atomic Collisions. Subjects covered included the scattering and annihilation of positrons and positronium atoms in various media, including those of astrophysical interest. In addition, the topics of antimatter and dark matter were covered.

  4. Interaction between HfC precipitates and vacancies in quenched Cu:Hf as studied by TDPAC and positron lifetime measurements

    NASA Astrophysics Data System (ADS)

    Govindaraj, R.; Rajaraman, R.

    2004-09-01

    A Cu:Hf sample with 1 wt% Hf as prepared by arc melting is characterized by TEM and microdiffraction analysis to contain HfC precipitates. HfC precipitates in a Cu matrix bind vacancies and divacancies strongly in the quenched Cu:Hf sample as deduced by time differential perturbed angular correlation (TDPAC) studies. Isochronal annealing studies using TDPAC and positron lifetime measurements indicate the stability of these vacancy complexes in the quenched sample for annealing treatments up to 1200 K, beyond which the de-trapping of the vacancies from HfC precipitates is observed to occur. This shows that HfC precipitates present in Cu inhibit the formation of voids by strongly binding quenched vacancies.

  5. Two-component density functional theory within the projector augmented-wave approach: Accurate and self-consistent computations of positron lifetimes and momentum distributions

    NASA Astrophysics Data System (ADS)

    Wiktor, Julia; Jomard, Gérald; Torrent, Marc

    2015-09-01

    Many techniques have been developed in the past in order to compute positron lifetimes in materials from first principles. However, there is still a lack of a fast and accurate self-consistent scheme that could handle accurately the forces acting on the ions induced by the presence of the positron. We will show in this paper that we have reached this goal by developing the two-component density functional theory within the projector augmented-wave (PAW) method in the open-source code abinit. This tool offers the accuracy of the all-electron methods with the computational efficiency of the plane-wave ones. We can thus deal with supercells that contain few hundreds to thousands of atoms to study point defects as well as more extended defects clusters. Moreover, using the PAW basis set allows us to use techniques able to, for instance, treat strongly correlated systems or spin-orbit coupling, which are necessary to study heavy elements, such as the actinides or their compounds.

  6. Nuclear Instruments and Methods in Physics Research. Section B; Microstructural Characterization of Semi-Interpenetrating Polymer Networks by Positron Lifetime Spectroscopy

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Pater, Ruth H.; Eftekhari, Abe

    1998-01-01

    Thermoset and thermoplastic polyimides have complementary physical/mechanical properties. Whereas thermoset polyimides are brittle and generally easier to process, thermoplastic polyimides are tough but harder to process. It is expected that a combination of these two types of polyimides may help produce polymers more suitable for aerospace applications. Semi-Interpenetrating Polymer Networks (S-IPNs) of thermoset LaRC(Trademark)-RP46 and thermoplastic LARC(Trademark)-IA polyimides were prepared in weight percent ratios ranging from 100:0 to 0: 100. Positron lifetime measurements were made in these samples to correlate their free volume features with physical/mechanical properties. As expected, positronium atoms are not formed in these samples. The second life time component has been used to infer the positron trap dimensions. The "free volume" goes through a minimum at about 50:50 ratio, suggesting that S-IPN samples are not merely solid solutions of the two polymers. These data and related structural properties of the S-IPN samples have been discussed in this paper.

  7. Cold Positrons from Decaying Dark Matter

    SciTech Connect

    Boubekeur, Lotfi; Dodelson, Scott; Vives, Oscar

    2012-11-01

    Many models of dark matter contain more than one new particle beyond those in the Standard Model. Often heavier particles decay into the lightest dark matter particle as the Universe evolves. Here we explore the possibilities that arise if one of the products in a (Heavy Particle) $\\rightarrow$ (Dark Matter) decay is a positron, and the lifetime is shorter than the age of the Universe. The positrons cool down by scattering off the cosmic microwave background and eventually annihilate when they fall into Galactic potential wells. The resulting 511 keV flux not only places constraints on this class of models but might even be consistent with that observed by the INTEGRAL satellite.

  8. Implications of the first AMS-02 measurement for dark matter annihilation and decay

    SciTech Connect

    Jin, Hong-Bo; Wu, Yue-Liang; Zhou, Yu-Feng E-mail: ylwu@itp.ac.cn

    2013-11-01

    In light of the first measurement of the positron fraction by the AMS-02 experiment, we perform a detailed global analysis on the interpretation of the latest data of PAMELA, Fermi-LAT, and AMS-02 in terms of dark matter (DM) annihilation and decay in various propagation models. The allowed regions for the DM particle mass and annihilation cross section or decay life-time are obtained for channels with leptonic final states: 2e, 2μ, 2τ, 4e, 4μ and 4τ. We show that for the conventional astrophysical background the AMS-02 positron fraction data alone favour a DM particle mass ∼ 500(800) GeV if DM particles annihilate dominantly into 2μ(4μ) final states, which is significantly lower than that favoured by the Fermi-LAT data of the total flux of electrons and positrons. The allowed regions by the two experiments do not overlap at a high confidence level (99.99999%C.L.). We consider a number of propagation models with different halo height Z{sub h}, diffusion parameters D{sub 0} and δ{sub 1/2}, and power indices of primary nucleon sources γ{sub p1/p2}. The normalization and the slope of the electron background are also allowed to vary. We find that the tension between the two experiments can be only slightly reduced in the propagation model with large Z{sub h} and D{sub 0}. The consistency of fit is improved for annihilation channels with 2τ and 4τ final states which favour TeV scale DM particle with large cross sections above ∼ 10{sup −23} cm{sup 3}s{sup −1}. In all the considered leptonic channels, the current data favour the scenario of DM annihilation over DM decay. In the decay scenario, the charge asymmetric DM decay is slightly favoured.

  9. Positron-electron autocorrelation function study of E-center in silicon

    NASA Astrophysics Data System (ADS)

    Ho, K. F.; Ching, H. M.; Beling, C. D.; Fung, S.; Ng, K. P.; Biasini, M.; Ferro, G.; Gong, M.

    2003-11-01

    Two-dimensional angular correlation of annihilation radiation (2D-ACAR) spectra have been taken for 1019cm-3 phosphorus-doped Si in the as-grown state after having been subjected to 1.8 MeV electron fluences of 1×1018 and 2×1018 cm-2. Positron annihilation lifetime spectroscopy confirms, in accordance with previous works, that positrons are saturation trapping into (VSi:P) pair defect (E-center) monovacancy sites in the electron irradiated samples. In the as-grown case, the positron-electron autocorrelation functions along the [111] and [1-10] directions, obtained through Fourier transformation of the 2D-ACAR data, reveal zero-crossings that deviate only slightly from the lattice points, in a manner consistent with positron-electron correlation effects. Conversely, in the spectra of the irradiated samples, the zero-crossing points are observed to move outward further by between 0.15 and 0.50 Å. This displacement is associated with positron annihilation with electrons in localized orbitals at the defect site. An attempt is made to extract just the component of the defect's positron-electron autocorrelation function that relates to the localized defect orbitals. In doing this features are found that correspond to the expected atomic positions at the vacancy defect site suggesting that this real-space function may provide a convenient means for obtaining a mapping of localized orbitals. The observed approximate separability of positron and electron wave-function autocorrelates leads to an estimate of 0.22 eV for the positron binding energy to the E-center.

  10. Positron annihilation study of Sr Doping in La{sub 2-x}Sr{sub x}CuO{sub 4}

    SciTech Connect

    Sterne, P.A. |; Howell, R.H.; Fluss, M.J.; Kaiser, J.H.; Kitazawa, K.; Kojima, H.

    1993-04-22

    We present a combined experimental and threshold study of effects of Sr doping on electronic structure of La{sub 2-x}Sr{sub x}CuO{sub 4}. Electron-positron momentum distributions have been measured to high statistical precision (> 4 {times} 10{sup 8} counts) at room temperature for samples with Sr concentrations of x = 0.0, 0.1, 0.13 and 0.2. Analysis of all four spectra reveal strong features due to electron-positron wavefunction overlap, in quantitative agreement with theoretical calculations. The Sr doped samples show discontinuities consistent with presence of a Fermi surface. The form and position of these features are in general agreement with the predictions of band theory. Correspondence between theory and experiment, as well as some differences, are revealed by a detailed study of the changes in electron-position momentum density with increasing Sr doping.

  11. The formation and evolution of vacancy-type defects in Ar-implanted silicon studied by slow-positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Li, B. S.; Zhang, C. H.; Zhong, Y. R.; Wang, D. N.; Zhou, L. H.; Yang, Y. T.; Zhang, H. H.; Zhang, L. Q.

    2009-07-01

    The Doppler broadening spectrum of a silicon wafer was measured using a variable-energy positron beam to investigate the effects of vacancy-type defects induced by 180 keV Ar ion implantation. The S-parameter in the damaged layer decreases with annealing temperature up to 673 K, and then increases with annealing temperature from 673 to 1373 K. At low annealing temperatures ranging from room temperature to 673 K, argon-decorated vacancies are formed by argon atoms combining with open-volume defects at inactive positron sites. With further increase of annealing temperature, argon-decorated vacancies dissociate and subsequently migrate and coalesce, leading to an increase of S-parameter. Furthermore, the buried vacancy-layer becomes narrow with increasing annealing temperature. At 1373 K, the buried vacancy-layer moved towards the sample surface.

  12. High-field penning-malmberg trap: confinement properties and use in positron accumulation

    SciTech Connect

    Hartley, J.H.

    1997-09-01

    This dissertation reports on the development of the 60 kG cryogenic positron trap at Lawrence Livermore National Laboratory, and compares the trap`s confinement properties with other nonneutral plasma devices. The device is designed for the accumulation of up to 2{times}10{sup 9} positrons from a linear-accelerator source. This positron plasma could then be used in Bhabha scattering experiments. Initial efforts at time-of-flight accumulation of positrons from the accelerator show rapid ({approximately}100 ms) deconfinement, inconsistent with the long electron lifetimes. Several possible deconfinement mechanisms have been explored, including annihilation on residual gas, injection heating, rf noise from the accelerator, magnet field curvature, and stray fields. Detailed studies of electron confinement demonstrate that the empirical scaling law used to design the trap cannot be extrapolated into the parameter regime of this device. Several possible methods for overcoming these limitations are presented.

  13. Precise Determination of the Strong Coupling Constant at NNLO in QCD from the Three-Jet Rate in Electron-Positron Annihilation at LEP

    SciTech Connect

    Dissertori, G.; Gehrmann-DeRidder, A.; Gehrmann, T.; Glover, E. W. N.; Heinrich, G.; Stenzel, H.

    2010-02-19

    We present the first determination of the strong coupling constant from the three-jet rate in e{sup +}e{sup -} annihilation at LEP, based on a next-to-next-to-leading-order (NNLO) perturbative QCD prediction. More precisely, we extract {alpha}{sub s}(M{sub Z}) by fitting perturbative QCD predictions at O({alpha}{sub s}{sup 3}) to data from the ALEPH experiment at LEP. Over a large range of the jet-resolution parameter y{sub cut}, this observable is characterized by small nonperturbative corrections and an excellent stability under renormalization scale variation. We find {alpha}{sub s}(M{sub Z})=0.1175+-0.0020(expt)+-0.0015(theor), which is more accurate than the values of {alpha}{sub s}(M{sub Z}) from e{sup +}e{sup -} event-shape data currently used in the world average.

  14. Positron emission tomography.

    PubMed

    Hoffman, E J; Phelps, M E

    1979-01-01

    Conventional nuclear imaging techniques utilizing lead collimation rely on radioactive tracers with little role in human physiology. The principles of imaging based on coincidence detection of the annihilation radiation produced in positron decay indicate that this mode of detection is uniquely suited for use in emission computed tomography. The only gamma-ray-emitting isotopes of carbon, nitrogen, and oxygen are positron emitters, which yield energies too high for conventional imaging techniques. Thus development of positron emitters in nuclear medicine imaging would make possible the use of a new class of physiologically active, positron-emitting radiopharmaceuticals. The application of these principles is described in the use of a physiologically active compound labeled with a positron emitter and positron-emission computed tomography to measure the local cerebral metabolic rate in humans. PMID:440173

  15. Portable Positron Measurement System (PPMS)

    SciTech Connect

    2011-01-01

    Portable Positron Measurement System (PPMS) is an automated, non-destructive inspection system based on positron annihilation, which characterizes a material's in situatomic-level properties during the manufacturing processes of formation, solidification, and heat treatment. Simultaneous manufacturing and quality monitoring now are possible. Learn more about the lab's project on our facebook site http://www.facebook.com/idahonationallaboratory.

  16. Portable Positron Measurement System (PPMS)

    ScienceCinema

    None

    2013-05-28

    Portable Positron Measurement System (PPMS) is an automated, non-destructive inspection system based on positron annihilation, which characterizes a material's in situatomic-level properties during the manufacturing processes of formation, solidification, and heat treatment. Simultaneous manufacturing and quality monitoring now are possible. Learn more about the lab's project on our facebook site http://www.facebook.com/idahonationallaboratory.

  17. Thermal vacancy formation and positron-vacancy interaction in Ti3Al at high temperatures

    NASA Astrophysics Data System (ADS)

    Würschum, R.; Kümmerle, E. A.; Badura-Gergen, K.; Seeger, A.; Herzig, Ch.; Schaefer, H.-E.

    1996-07-01

    In order to study the formation of thermal vacancies in the Ti-Al alloy system, high-temperature positron lifetime measurements together with a modeling of defect formation in the framework of nearest-neighbor pair bonds were performed for α2Ti3Al and compared to recent results on γTiAl [U. Brossmann, R. Würschum, K. Badura, and H.-E. Schaefer, Phys. Rev. B 49, 6457 (1994)]. Substantial increases of the positron lifetime τ were observed for Ti65.6Al34.4 and Ti77.1Al22.9 in the temperature range T≳1200 K where thermal vacancy concentrations above the detection limit of positron annihilation are expected from the model calculations for the α2 phase. Within the high-temperature increase of the positron lifetime in the α2 and the β phase single-component positron lifetime spectra were observed. This behavior is in contrast to the two-component spectra observed conventionally at intermediate positron trapping rates and is attributed to a fast detrapping and retrapping of positrons at vacancies due to a low positron-vacancy binding energy. For this case, a vacancy formation enthalpy of HFV=(1.55±0.2) eV in α2Ti65.6Al34.4 and HFV=(1.8±0.2) eV in βTi77.1Al22.9 can be derived. These results are discussed in the context of recent 44Ti tracer diffusion studies.

  18. Fermi-surface reconstruction from two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) data using maximum-likelihood fitting of wavelet-like functions

    NASA Astrophysics Data System (ADS)

    G, A., Major; Fretwell, H. M.; Dugdale, S. B.; Alam, M. A.

    1998-11-01

    A novel method for reconstructing the Fermi surface from experimental two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) projections is proposed. In this algorithm, the 3D electron momentum-density distribution is expanded in terms of a basis of wavelet-like functions. The parameters of the model, the wavelet coefficients, are determined by maximizing the likelihood function corresponding to the experimental data and the projections calculated from the model. In contrast to other expansions, in the case of that in terms of wavelets a relatively small number of model parameters are sufficient for representing the relevant parts of the 3D distribution, thus keeping computation times reasonably short. Unlike other reconstruction methods, this algorithm takes full account of the statistical information content of the data and therefore may help to reduce the amount of time needed for data acquisition. An additional advantage of wavelet expansion may be the possibility of retrieving the Fermi surface directly from the wavelet coefficients rather than indirectly using the reconstructed 3D distribution.

  19. Electron-positron annihilation from 3 to 11 GeV. First progress report, 18 August 1980-15 Mar 1981

    SciTech Connect

    Darden, C.W.

    1981-01-01

    The ARGUS collaboration started forming in mid 1978. It was a natural outgrowth of the CASP II collaboration which had grown up to run the old DASP detector in late 1977. That was the year that the Upsilon meson was discovered at Fermilab. Both the PLUTO and DASP II collaborations were able to see the Upsilon in e/sup +/e/sup -/ annihilations the following year and to confirm that it was indeed a narrow resonance. Late in 1978 the ARGUS collaboration submitted a proposal to the DESY Directorium to build a new universal magnetic solenoid detector that would allow the investigation of reactions of the type e/sup +/e/sup -/ > anything at energies that could be reached by the storage ring DORIS. This energy range included the Charm region and the first two Upsilon states, 1S and 2S. At the same time, the Directorium was urged to rebuild the storage ring DORIS to increase its luminosity and raise its maximum energy so that Upsilon spectroscopy could be done. Following the approval of the ARGUS detector, the finishing touches were put on its design and various parts either put out for bid or spoken for by different groups in the collaboration. Progress toward completion has been satisfactory and the status of each is discussed.

  20. A calorimetric measurement of the strong coupling constant in electron-positron annihilation at a center-of-mass energy of 91.6 GeV

    SciTech Connect

    Martirena, S.G.

    1994-04-01

    In this work, a measurement of the strong coupling constant {alpha}{sub s} in e{sup +}e{sup {minus}} annihilation at a center-of-mass energy of 91.6 GeV is presented. The measurement was performed with the SLD at the Stanford Linear Collider facility located at the Stanford Linear Accelerator Center in California. The procedure used consisted of measuring the rate of hard gluon radiation from the primary quarks in a sample of 9,878 hadronic events. After defining the asymptotic manifestation of partons as `jets`, various phenomenological models were used to correct for the hadronization process. A value for the QCD scale parameter {Lambda}{sub bar MS}, defined in the {sub bar MS} renormalization convention with 5 active quark flavors, was then obtained by a direct fit to O({alpha}{sub s}{sup 2}) calculations. The value of {alpha}{sub s} obtained was {alpha}{sub s}(M{sub z0}) = 0.122 {plus_minus} 0.004 {sub {minus}0.007} {sup +0.008} where the uncertainties are experimental (combined statistical and systematic) and theoretical (systematic) respectively. Equivalently, {Lambda}{sub bar MS} = 0.28 {sub {minus}0.10}{sup +0.16} GeV where the experimental and theoretical uncertainties have been combined.

  1. Position annihilation radiation from neutron stars

    NASA Technical Reports Server (NTRS)

    Ramaty, R.; Borner, G. A.; Cohen, J. M.

    1972-01-01

    Matter accreted on the surfaces of neutron stars consists of energetic particles of a few tens to a couple hundred MeV/nucleon, depending on the neutron star mass. In addition to heat, such particles produce nuclear reactions with the surface material. It is proposed that the recently observed 473 + or - 30 keV spectral feature from the galactic center is gravitationally red-shifted positron annihilation radiation produced at the surfaces of old neutron stars. The principal observational tests of the model would be the detection of nuclear gamma ray lines from the galactic center and red-shifted positron annihilation radiation from the galactic disk.

  2. Slow-Positron Generator For Studying Polymer Films

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; St. Clair, Terry L.; Eftekhari, Abe

    1992-01-01

    Aspects of molecular structures probed by positron-annihilation spectroscopy (PAS). Slow-positron-beam generator suitable for PAS measurements in thin polymer films. Includes Na22 source of positrons and two moderators made of well-annealed tungsten foil. With proper choice of voltage, positrons emitted by inward-facing surfaces of moderators made to stop in polymer films tested.

  3. Measurement of the (27)P lifetime

    NASA Astrophysics Data System (ADS)

    Freeman, Charles George

    The lifetime of 27P has been measured using the Recoil Mass Spectrometer (RMS) at the Nuclear Structure Research Laboratory (NSRL) at the University of Rochester. 27P was produced by bombarding a BeO target with a 24Mg beam at a lab energy of 118 MeV. A focal-plane detector system, consisting of a parallel-grid avalanche counter (PGAC) backed by an ionization counter (IC) and a silicon detector, was used to provide particle identification. A sodium iodide detector array was used to detect the 511 keV positron annihilation radiation produced by the decay of 27P. The result obtained for the half-life of 27P is 0.32 -0.15+0.22 s.

  4. The Lone Loop Radiative Corrections to W Pair Production in Electron Positron Annihilation in the Supersymmetric Extension of the Salam-Weinberg Model of the Electroweak Interactions.

    NASA Astrophysics Data System (ADS)

    Alam, S.

    1992-01-01

    The one loop radiative corrections to W pair production in e^+e^- annihilation in the supersymmetric extension of the Salam-Weinberg (SW) model of the electroweak interactions are calculated. Since our model contains the SW theory, and several calculations have been reported on the latter we compare these results with ours. In general agreement is found, a detailed comparison is not possible since the explicit details have not been published. However we have cross checked many of SW model results with Sundaresan and Kalyniak who have performed the calculation using the same renormalization scheme as ours. The virtual corrections are determined in the on-mass-shell renormalization scheme (OMRS) of Sakakibara. The OMRS scheme has several advantages, one being that it is a transparent (i.e. in terms of the physics) renormalization procedure. Moreover the fundamental set of input parameters of OMRS is well determined. By this we mean the accurate determination of the Z-boson mass at LEP I and the expected precise measurement of the W mass at LEP II, and the already well determined alpha value constitute a good set of the fundamental input parameters. Of course the Higgs boson mass and the fermion mass have also to be put in. So far the top quark and the Higgs boson have eluded detection and consequently their masses have to be put in as free parameters. One very important feature in determining how good the standard model (SM) is involves the measurement of the tri-boson coupling. Such a coupling occurs in e^+e^- to W^+W ^- in the SM, at the tree level. For a precision check, one must calculate one loop radiative corrections in SM, using the tri-boson coupling. We have also evaluated W pair production with non standard coupling to get an intuitive feel for deviations away from the SM. Supersymmetry has the effect of reducing the virtual corrections. The effect of supersymmetry is examined on the differential cross section of e^+e^- to W^+W^-, the 'A' term, the magnetic

  5. Positronium formation and annihilation in liquid crystalline smectic-E phase revisited

    NASA Astrophysics Data System (ADS)

    Dryzek, E.; Juszyńska-Gałazka, E.

    2016-02-01

    The results of the positron lifetime measurements of the quenched smectic-E (Sm-E ) phase of 4-butyl-4'-isothiocyano-1,1'-biphenyl (4TCB) are revisited. The sites of positronium formation and annihilation, according to the model with nanosegregated layered structure of the Sm-E phase and molten state of alkyl chains of molecules, are identified in the sublayer containing alkyl chains of molecules. The possibility of vitrification of the Sm-E phase for 4TCB consisting in freezing of the alkyl chain motions is considered as a cause of the thermally activated creation of sites where o -Ps is formed and annihilates in the quenched Sm-E phase. The description of the temperature dependence of ortho-positronium intensity is performed using the glass transition model which assumes that the molecules occupy two thermodynamic states: solidlike or liquidlike regarding mobility of their alkyl chains. The equilibrium temperature between solidlike and liquidlike domains of the model obtained from positron lifetime measurements coincides with the exothermic effect in the temperature dependence of the heat capacity.

  6. Microstructural probing of ferritic/martensitic steels using internal transmutation-based positron source

    NASA Astrophysics Data System (ADS)

    Krsjak, Vladimir; Dai, Yong

    2015-10-01

    This paper presents the use of an internal 44Ti/44Sc radioisotope source for a direct microstructural characterization of ferritic/martensitic (f/m) steels after irradiation in targets of spallation neutron sources. Gamma spectroscopy measurements show a production of ∼1MBq of 44Ti per 1 g of f/m steels irradiated at 1 dpa (displaced per atom) in the mixed proton-neutron spectrum at the Swiss spallation neutron source (SINQ). In the decay chain 44Ti → 44Sc → 44Ca, positrons are produced together with prompt gamma rays which enable the application of different positron annihilation spectroscopy (PAS) analyses, including lifetime and Doppler broadening spectroscopy. Due to the high production yield, long half-life and relatively high energy of positrons of 44Ti, this methodology opens up new potential for simple, effective and inexpensive characterization of radiation induced defects in f/m steels irradiated in a spallation target.

  7. Positron microprobe at LLNL

    SciTech Connect

    Asoka, P; Howell, R; Stoeffl, W

    1998-11-01

    The electron linac based positron source at Lawrence Livermore National Laboratory (LLNL) provides the world's highest current beam of keV positrons. We are building a positron microprobe that will produce a pulsed, focused positron beam for 3-dimensional scans of defect size and concentration with sub-micron resolution. The widely spaced and intense positron packets from the tungsten moderator at the end of the 100 MeV LLNL linac are captured and trapped in a magnetic bottle. The positrons are then released in 1 ns bunches at a 20 MHz repetition rate. With a three-stage re-moderation we will compress the cm-sized original beam to a 1 micro-meter diameter final spot on the target. The buncher will compress the arrival time of positrons on the target to less than 100 ps. A detector array with up to 60 BaF2 crystals in paired coincidence will measure the annihilation radiation with high efficiency and low background. The energy of the positrons can be varied from less than 1 keV up to 50 keV.

  8. Limits on dark matter from AMS-02 antiproton and positron fraction data

    NASA Astrophysics Data System (ADS)

    Lu, Bo-Qiang; Zong, Hong-Shi

    2016-05-01

    Herein we derive limits on dark matter annihilation cross section and lifetime using measurements of the AMS-02 antiproton ratio and positron fraction data. In deriving the limits, we consider the scenario of secondary particles accelerated in supernova remnants (SNRs), which has been argued to be able to reasonably account for the AMS-02 high-energy positron/antiproton fraction/ratio data. We parametrize the contribution of secondary particles accelerated in SNRs and then fit the observational data within the conventional cosmic ray propagation model by adopting the galprop code. We use the likelihood ratio test to determine the 95% confidence level upper limits of possible dark matter (DM) contribution to the antiproton/positron fractions measured by AMS-02. Under the assumption taken in this work, we find that our limits are stronger than that set by the Fermi-LAT gamma ray Pass 8 data observation on the dwarf spheroidal satellite galaxies. We show that the solar modulation (cosmic ray propagation) parameters can play a non-negligible role in modifying the constraints on dark matter annihilation cross section and lifetime for mχ<100 GeV (mχ>100 GeV ), where mχ is the rest mass of dark matter particles. We also find that constrains on DM parameters from AMS-02 data would become more stringent when the solar modulation is weak. Using these results, we also put limits on the effective field theory of dark matter.

  9. Positron emission tomography wrist detector

    DOEpatents

    Schlyer, David J.; O'Connor, Paul; Woody, Craig; Junnarkar, Sachin Shrirang; Radeka, Veljko; Vaska, Paul; Pratte, Jean-Francois

    2006-08-15

    A method of serially transferring annihilation information in a compact positron emission tomography (PET) scanner includes generating a time signal representing a time-of-occurrence of an annihilation event, generating an address signal representing a channel detecting the annihilation event, and generating a channel signal including the time and address signals. The method also includes generating a composite signal including the channel signal and another similarly generated channel signal concerning another annihilation event. An apparatus that serially transfers annihilation information includes a time signal generator, address signal generator, channel signal generator, and composite signal generator. The time signal is asynchronous and the address signal is synchronous to a clock signal. A PET scanner includes a scintillation array, detection array, front-end array, and a serial encoder. The serial encoders include the time signal generator, address signal generator, channel signal generator, and composite signal generator.

  10. The intense slow positron beam facility at the PULSTAR reactor and applications in nano-materials study

    SciTech Connect

    Liu, Ming; Moxom, Jeremy; Hawari, Ayman I.; Gidley, David W.

    2013-04-19

    An intense slow positron beam has been established at the PULSTAR nuclear research reactor of North Carolina State University. The slow positrons are generated by pair production in a tungsten moderator from gammarays produced in the reactor core and by neutron capture reactions in cadmium. The moderated positrons are electrostatically extracted and magnetically guided out of the region near the core. Subsequently, the positrons are used in two spectrometers that are capable of performing positron annihilation lifetime spectroscopy (PALS) and positron Doppler broadening spectroscopy (DBS) to probe the defect and free volume properties of materials. One of the spectrometers (e{sup +}-PALS) utilizes an rf buncher to produce a pulsed beam and has a timing resolution of 277 ps. The second spectrometer (Ps-PALS) uses a secondary electron timing technique and is dedicated to positronium lifetime measurements with an approximately 1 ns timing resolution. PALS measurements have been conducted in the e{sup +}-PALS spectrometer on a series of nano-materials including organic photovoltaic thin films, membranes for filtration, and polymeric fibers. These studies have resulted in understanding some critical issues related to the development of the examined nano-materials.

  11. The intense slow positron beam facility at the PULSTAR reactor and applications in nano-materials study

    NASA Astrophysics Data System (ADS)

    Liu, Ming; Moxom, Jeremy; Hawari, Ayman I.; Gidley, David W.

    2013-04-01

    An intense slow positron beam has been established at the PULSTAR nuclear research reactor of North Carolina State University. The slow positrons are generated by pair production in a tungsten moderator from gammarays produced in the reactor core and by neutron capture reactions in cadmium. The moderated positrons are electrostatically extracted and magnetically guided out of the region near the core. Subsequently, the positrons are used in two spectrometers that are capable of performing positron annihilation lifetime spectroscopy (PALS) and positron Doppler broadening spectroscopy (DBS) to probe the defect and free volume properties of materials. One of the spectrometers (e+-PALS) utilizes an rf buncher to produce a pulsed beam and has a timing resolution of 277 ps. The second spectrometer (Ps-PALS) uses a secondary electron timing technique and is dedicated to positronium lifetime measurements with an approximately 1 ns timing resolution. PALS measurements have been conducted in the e+-PALS spectrometer on a series of nano-materials including organic photovoltaic thin films, membranes for filtration, and polymeric fibers. These studies have resulted in understanding some critical issues related to the development of the examined nano-materials.

  12. Intense positron beam at KEK

    NASA Astrophysics Data System (ADS)

    Kurihara, Toshikazu; Yagishita, Akira; Enomoto, Atsushi; Kobayashi, Hitoshi; Shidara, Tetsuo; Shirakawa, Akihiro; Nakahara, Kazuo; Saitou, Haruo; Inoue, Kouji; Nagashima, Yasuyuki; Hyodo, Toshio; Nagai, Yasuyoshi; Hasegawa, Masayuki; Inoue, Yoshi; Kogure, Yoshiaki; Doyama, Masao

    2000-08-01

    A positron beam is a useful probe for investigating the electronic states in solids, especially concerning the surface states. The advantage of utilizing positron beams is in their simpler interactions with matter, owing to the absence of any exchange forces, in contrast to the case of low-energy electrons. However, such studies as low-energy positron diffraction, positron microscopy and positronium (Ps) spectroscopy, which require high intensity slow-positron beams, are very limited due to the poor intensity obtained from a conventional radioactive-isotope-based positron source. In conventional laboratories, the slow-positron intensity is restricted to 10 6 e +/s due to the strength of the available radioactive source. An accelerator based slow-positron source is a good candidate for increasing the slow-positron intensity. One of the results using a high intensity pulsed positron beam is presented as a study of the origins of a Ps emitted from SiO 2. We also describe the two-dimensional angular correlation of annihilation radiation (2D-ACAR) measurement system with slow-positron beams and a positron microscope.

  13. Positrons as imaging agents and probes in nanotechnology

    NASA Astrophysics Data System (ADS)

    Smith, Suzanne V.

    2009-09-01

    Positron emission tomography (PET) tracks a positron emitting radiopharmaceutical injected into the body and generates a 3-dimensional image of its location. Introduced in the early 70s, it has now developed into a powerful medical diagnostic tool for routine clinical use as well as in drug development. Unrivalled as a highly sensitive, specific and non-invasive imaging tool, PET unfortunately lacks the resolution of Computer Tomography (CT) and Magnetic Resonance Imaging (MRI). As the resolution of PET depends significantly on the energy of the positron incorporated in the radiopharmaceutical and its interaction with its surrounding tissue, there is growing interest in expanding our understanding of how positrons interact at the atomic and molecular level. A better understanding of these interactions will contribute to improving the resolution of PET and assist in the design of better imaging agents. Positrons are also used in Positron Annihilation Lifetime Spectroscopy (PALS) to determine electron density and or presence and incidence of micro- and mesopores (0.1 to 10 nm) in materials. The control of porosity in engineered materials is crucial for applications such as controlled release or air and water resistant films. Equally important to the design of nano and microtechnologies, is our understanding of the microenvironments within these pores and on surfaces. Hence as radiopharmaceuticals are designed to track disease, nuclear probes (radioactive molecules) are synthesized to investigate the chemical properties within these pores. This article will give a brief overview of the present role of positrons in imaging as well as explore its potential to contribute in the engineering of new materials to the marketplace.

  14. Transient ions in electron and positron scattering

    NASA Astrophysics Data System (ADS)

    d'A Sanchez, Sergio; de Oliveira, Eliane M.; dos Santos, Josué S.; da Costa, Romarly F.; Bettega, Márcio H. F.; Lima, Marco A. P.; Varella, Márcio T. do N.

    2009-11-01

    We report on recent advances in studies of transient ions formed in electron and positron scattering by molecules. We briefly discuss elastic electron collisions against pyrrole and glycine, as well as electron affinities of glycine-water clusters. Positron scattering and annihilation on small molecules is also discussed.

  15. [Fundamentals of positron emission tomography].

    PubMed

    Ostertag, H

    1989-07-01

    Positron emission tomography is a modern radionuclide method of measuring physiological quantities or metabolic parameters in vivo. The method is based on: (1) radioactive labelling with positron emitters; (2) the coincidence technique for the measurement of the annihilation radiation following positron decay; (3) analysis of the data measured using biological models. The basic aspects and problems of the method are discussed. The main fields of future research are the synthesis of new labelled compounds and the development of mathematical models of the biological processes to be investigated. PMID:2667029

  16. Time-dependent behavior of positrons in noble gases

    SciTech Connect

    Wadehra, J.M. . Dept. of Physics and Astronomy); Drallos, P.J. )

    1990-01-01

    Both equilibrium and nonequilibrium behaviors of positrons in several noble gases are reviewed. Our novel procedure for obtaining the time-dependent behavior of various swarm parameters -- such as the positron drift velocity, average positron energy, positron annihilation rate (or equivalently Z{sub eff}) etc. -- for positrons in pure ambient gases subjected to external electrostatic fields is described. Summaries of time-dependent as well as electric field-dependent results for positron swarms in various noble gases are presented. New time-dependent results for positron swarms in neon are also described in detail. 36 refs., 4 figs., 3 tabs.

  17. ZnO Luminescence and scintillation studied via photoexcitation, x-ray excitation, and gamma-induced positron spectroscopy"

    SciTech Connect

    Ji, C; Colosimo, A; Anwand, W; Boatner, Lynn A; Wagner, A; Stepanov, P S; Trinh, t t; Liedke, m o; Krause-Rehberg, R; Cowan, T E; Selim, F. A.

    2016-01-01

    Luminescence and scintillation in ZnO single crystals were measured by photoluminescence and X-ray-induced luminescence (XRIL). XRIL allowed a direct comparison to be made between the near-band emission (NBE) and trap emissions providing insight into the carrier recombination efficiency in the ZnO crystals. The origin of green emission, the dominant trap emission in ZnO, was investigated by gamma-induced positron spectroscopy (GIPS) - a unique defect spectroscopy method that enables positron lifetime measurements to be made for a sample without contributions from positron annihilation in the source materials or the surroundings. The measurements showed the absence of positron traps in the crystals and yielded a bulk positron lifetime value that is in complete agreement with the predicted theoretical value = thereby confirming the advantage of the GIPS method. By combining scintillation measurements with XRIL, the fast scintillation in ZnO crystals was found to be strongly correlated with the ratio between the defect luminescence and NBE.

  18. ZnO Luminescence and scintillation studied via photoexcitation, x-ray excitation, and gamma-induced positron spectroscopy"

    DOE PAGESBeta

    Ji, C; Colosimo, A; Anwand, W; Boatner, Lynn A; Wagner, A; Stepanov, P S; Trinh, t t; Liedke, m o; Krause-Rehberg, R; Cowan, T E; et al

    2016-01-01

    Luminescence and scintillation in ZnO single crystals were measured by photoluminescence and X-ray-induced luminescence (XRIL). XRIL allowed a direct comparison to be made between the near-band emission (NBE) and trap emissions providing insight into the carrier recombination efficiency in the ZnO crystals. The origin of green emission, the dominant trap emission in ZnO, was investigated by gamma-induced positron spectroscopy (GIPS) - a unique defect spectroscopy method that enables positron lifetime measurements to be made for a sample without contributions from positron annihilation in the source materials or the surroundings. The measurements showed the absence of positron traps in the crystalsmore » and yielded a bulk positron lifetime value that is in complete agreement with the predicted theoretical value = thereby confirming the advantage of the GIPS method. By combining scintillation measurements with XRIL, the fast scintillation in ZnO crystals was found to be strongly correlated with the ratio between the defect luminescence and NBE.« less

  19. Positron deposition in plasmas by positronium beam ionization and transport of positrons in tokamak plasmas

    SciTech Connect

    Murphy, T.J.

    1986-11-01

    In a recently proposed positron transport experiment, positrons would be deposited in a fusion plasma by forming a positronium (Ps) beam and passing it through the plasma. Positrons would be deposited as the beam is ionized by plasma ions and electrons. Radial transport of the positrons to the limiter could then be measured by detecting the gamma radiation produced by annihilation of positrons with electrons in the limiter. This would allow measurements of the transport of electron-mass particles and might shed some light on the mechanisms of electron transport in fusion plasmas. In this paper, the deposition and transport of positrons in a tokamak are simulated and the annihilation signal determined for several transport models. Calculations of the expected signals are necessary for the optimal design of a positron transport experiment. There are several mechanisms for the loss of positrons besides transport to the limiter. Annihilation with plasma electrons and reformation of positronium in positron-hydrogen collisions are two such processes. These processes can alter the signal and place restrictions ons on the plasma conditions in which positron transport experiments can be effectively performed.

  20. Exciton annihilation in dye-sensitized nanocrystalline semiconductor films

    NASA Astrophysics Data System (ADS)

    Namekawa, Akihiro; Katoh, Ryuzi

    2016-08-01

    Exciton annihilation in dye-sensitized nanocrystalline semiconductor (Al2O3) films has been studied through laser-induced fluorescence spectroscopy. The relative quantum yield of the fluorescence decreases with increasing excitation light intensity, the indication being that exciton annihilation occurred. The rate constants of the annihilation were estimated for three dyes, N719, D149, and MK2, that are known to be sensitizing dyes for efficient dye-sensitized solar cells. The hopping time between dye molecules and the diffusion length of excitons within their lifetime were also estimated to facilitate discussion of the relevance of exciton annihilation to primary processes in dye-sensitized solar cells.

  1. What is the fate of runaway positrons in tokamaks?

    DOE PAGESBeta

    Liu, Jian; Qin, Hong; Fisch, Nathaniel J.; Teng, Qian; Wang, Xiaogang

    2014-06-19

    In this study, massive runaway positrons are generated by runaway electrons in tokamaks. The fate of these positrons encodes valuable information about the runaway dynamics. The phase space dynamics of a runaway position is investigated using a Lagrangian that incorporates the tokamak geometry, loop voltage, radiation and collisional effects. It is found numerically that runaway positrons will drift out of the plasma to annihilate on the first wall, with an in-plasma annihilation possibility less than 0.1%. The dynamics of runaway positrons provides signatures that can be observed as diagnostic tools.

  2. What is the fate of runaway positrons in tokamaks?

    SciTech Connect

    Liu, Jian; Qin, Hong; Fisch, Nathaniel J.; Teng, Qian; Wang, Xiaogang

    2014-06-15

    Massive runaway positrons are generated by runaway electrons in tokamaks. The fate of these positrons encodes valuable information about the runaway dynamics. The phase space dynamics of a runaway position is investigated using a Lagrangian that incorporates the tokamak geometry, loop voltage, radiation and collisional effects. It is found numerically that runaway positrons will drift out of the plasma to annihilate on the first wall, with an in-plasma annihilation possibility less than 0.1%. The dynamics of runaway positrons provides signatures that can be observed as diagnostic tools.

  3. Annihilation physics of exotic galactic dark matter particles

    NASA Technical Reports Server (NTRS)

    Stecker, F. W.

    1990-01-01

    Various theoretical arguments make exotic heavy neutral weakly interacting fermions, particularly those predicted by supersymmetry theory, attractive candidates for making up the large amount of unseen gravitating mass in galactic halos. Such particles can annihilate with each other, producing secondary particles of cosmic-ray energies, among which are antiprotons, positrons, neutrinos, and gamma-rays. Spectra and fluxes of these annihilation products can be calculated, partly by making use of positron electron collider data and quantum chromodynamic models of particle production derived therefrom. These spectra may provide detectable signatures of exotic particle remnants of the big bang.

  4. Low energy positron interactions with biological molecules

    NASA Astrophysics Data System (ADS)

    Wanniarachchi, Indika L.

    Calculations of the positron density distribution which can be used for positrons bound to midsize and larger molecules have been tested for smaller molecules and subsequently applied to investigate the most likely e +e-- annihilation sites for positrons interacting with biological molecules containing C, H, O, and N. In order to allow consideration of positrons bound to extended molecules with regions of different character and no particular symmetry, atom-centered positron basis sets of Gaussian-type functions were developed for positrons bound to molecules containing O, N, C, H, Li, Na, and Be. Testing shows that there is no need to scale the positron basis functions to take into account different effective charges on the atoms in different molecules. Even at the HF level of theory the calculated positron and the contact density of e+LiH system is in qualitative agreement with the most accurate calculation was done in ECG method. Also it has been found that for larger biological molecules such as derivation of formaldehyde can leave out positron basis sets centered on H atoms and still get qualitatively acceptable contact density distribution. According to our results, the electronic and positronic wavefunctions have the most overlap in the regions of most negative electrostatic potential in the parent molecule, and we can expect that a positron bound to the molecule will be more likely to annihilate with one of the electrons in these regions. Also we find that the highest energy occupied electronic orbital often does not make the largest contribution to e+e -- annihilation, and that the energy liberated by subsequent electronic relaxation is sufficient to break the backbone in several places in di-peptides and other organic molecules.

  5. Resolvability of positron decay channels

    SciTech Connect

    Fluss, M.J.; Howell, R.H.; Rosenberg, I.J.; Meyer, P.

    1985-03-07

    Many data analysis treatments of positron experiments attempt to resolve two or more positron decay or exist channels which may be open simultaneously. Examples of the need to employ such treatments of the experimental results can be found in the resolution of the constituents of a defect ensemble, or in the analysis of the complex spectra which arise from the interaction of slow positrons at or near the surfaces of solids. Experimental one- and two-dimensional angular correlation of annihilation radiation experiments in Al single crystals have shown that two defect species (mono- and divacancies) can be resolved under suitable conditions. Recent experiments at LLNL indicate that there are a variety of complex exit channels open to positrons interacting at surfaces, and ultimely these decay channels must also be suitably resolved from one another. 6 refs., 4 figs.

  6. Positrons from accelerated particle interactions

    NASA Technical Reports Server (NTRS)

    Kozlovsky, B.; Lingenfelter, R. E.; Ramaty, R.

    1987-01-01

    Positron production from the decay of radioactive nuclei produced in nuclear interactions of accelerated particles is treated in detail. Laboratory data as well as theoretical considerations are used to construct energy-dependent cross sections for the production of a large number of radioactive positron emitters resulting from proton and alpha-particle interactions with ambient cosmic matter. Using these cross sections, positron production rates are calculated for a variety of energetic particle spectra, assuming solar abundances for both the energetic particles and the ambient medium. These results can be used for the study of astrophysical sites which emit annihilation radiation. In particular, the results have been applied to solar flares, where the observed 0.511 MeV line is shown to be due to positrons resulting from accelerated particle reactions.

  7. ZnO Luminescence and scintillation studied via photoexcitation, X-ray excitation, and gamma-induced positron spectroscopy.

    PubMed

    Ji, J; Colosimo, A M; Anwand, W; Boatner, L A; Wagner, A; Stepanov, P S; Trinh, T T; Liedke, M O; Krause-Rehberg, R; Cowan, T E; Selim, F A

    2016-01-01

    The luminescence and scintillation properties of ZnO single crystals were studied by photoluminescence and X-ray-induced luminescence (XRIL) techniques. XRIL allowed a direct comparison to be made between the near-band emission (NBE) and trap emissions providing insight into the carrier recombination efficiency in the ZnO crystals. It also provided bulk luminescence measurements that were not affected by surface states. The origin of a green emission, the dominant trap emission in ZnO, was then investigated by gamma-induced positron spectroscopy (GIPS) - a unique defect spectroscopy method that enables positron lifetime measurements to be made for a sample without contributions from positron annihilation in the source materials. The measurements showed a single positron decay curve with a 175 ps lifetime component that was attributed to Zn vacancies passivated by hydrogen. Both oxygen vacancies and hydrogen-decorated Zn vacancies were suggested to contribute to the green emission. By combining scintillation measurements with XRIL, the fast scintillation in ZnO crystals was found to be strongly correlated with the ratio between the defect luminescence and NBE. This study reports the first application of GIPS to semiconductors, and it reveals the great benefits of the XRIL technique for the study of emission and scintillation properties of materials. PMID:27550235

  8. ZnO Luminescence and scintillation studied via photoexcitation, X-ray excitation, and gamma-induced positron spectroscopy

    PubMed Central

    Ji, J.; Colosimo, A. M.; Anwand, W.; Boatner, L. A.; Wagner, A.; Stepanov, P. S.; Trinh, T. T.; Liedke, M. O.; Krause-Rehberg, R.; Cowan, T. E.; Selim, F. A.

    2016-01-01

    The luminescence and scintillation properties of ZnO single crystals were studied by photoluminescence and X-ray-induced luminescence (XRIL) techniques. XRIL allowed a direct comparison to be made between the near-band emission (NBE) and trap emissions providing insight into the carrier recombination efficiency in the ZnO crystals. It also provided bulk luminescence measurements that were not affected by surface states. The origin of a green emission, the dominant trap emission in ZnO, was then investigated by gamma-induced positron spectroscopy (GIPS) - a unique defect spectroscopy method that enables positron lifetime measurements to be made for a sample without contributions from positron annihilation in the source materials. The measurements showed a single positron decay curve with a 175 ps lifetime component that was attributed to Zn vacancies passivated by hydrogen. Both oxygen vacancies and hydrogen-decorated Zn vacancies were suggested to contribute to the green emission. By combining scintillation measurements with XRIL, the fast scintillation in ZnO crystals was found to be strongly correlated with the ratio between the defect luminescence and NBE. This study reports the first application of GIPS to semiconductors, and it reveals the great benefits of the XRIL technique for the study of emission and scintillation properties of materials. PMID:27550235

  9. Positron clouds within thunderstorms

    NASA Astrophysics Data System (ADS)

    Dwyer, Joseph R.; Smith, David M.; Hazelton, Bryna J.; Grefenstette, Brian W.; Kelley, Nicole A.; Lowell, Alexander W.; Schaal, Meagan M.; Rassoul, Hamid K.

    2015-08-01

    We report the observation of two isolated clouds of positrons inside an active thunderstorm. These observations were made by the Airborne Detector for Energetic Lightning Emissions (ADELE), an array of six gamma-ray detectors, which flew on a Gulfstream V jet aircraft through the top of an active thunderstorm in August 2009. ADELE recorded two 511 keV gamma-ray count rate enhancements, 35 s apart, each lasting approximately 0.2 s. The enhancements, which were approximately a factor of 12 above background, were both accompanied by electrical activity as measured by a flat-plate antenna on the underside of the aircraft. The energy spectra were consistent with a source mostly composed of positron annihilation gamma rays, with a prominent 511 keV line clearly visible in the data. Model fits to the data suggest that the aircraft was briefly immersed in clouds of positrons, more than a kilometre across. It is not clear how the positron clouds were created within the thunderstorm, but it is possible they were caused by the presence of the aircraft in the electrified environment.

  10. Positron implantation in solids

    SciTech Connect

    Ghosh, V.J.; Lynn, K.G.; Welch, D.O.

    1993-12-31

    The Monte Carlo technique for modeling positron prior to annihilation and electron implantation in semi-infinite metals is described. Particle implantation is modelled as a multistep process, a series of collisions with the atoms of the host material. In elastic collisions with neutral atoms there is no transfer of energy. The particle loses energy by several different channels, excitation of the electron gas, ionization of the ion cores, or, at low energies, by phonon excitation. These competing scattering mechanisms have been incorporated into the Monte Carlo framework and several different models are being used. Brief descriptions of these Monte Carlo schemes, as well as an analytic model for positron implantation are included. Results of the Monte Carlo simulations are presented and compared with expermental data. Problems associated with modeling positron implantation are discuss and the need for more expermental data on energy-loss in different materials is stressed. Positron implantation in multilayers of different metals is briefly described and extensions of this work to include a study of multilayers and heterostructures is suggested.

  11. Recent Developments in Positron Emission Tomography (PET) Instrumentation

    DOE R&D Accomplishments Database

    Derenzo, S. E.; Budinger, T. F.

    1986-04-01

    This paper presents recent detector developments and perspectives for positron emission tomography (PET) instrumentation used for medical research, as well as the physical processes in positron annihilation, photon scattering and detection, tomograph design considerations, and the potentials for new advances in detectors.

  12. CALET's sensitivity to Dark Matter annihilation in the galactic halo

    NASA Astrophysics Data System (ADS)

    Motz, H.; Asaoka, Y.; Torii, S.; Bhattacharyya, S.

    2015-12-01

    CALET (Calorimetric Electron Telescope), installed on the ISS in August 2015, directly measures the electron+positron cosmic rays flux up to 20 TeV. With its proton rejection capability of 1 : 105 and an aperture of 1200 cm2· sr, it will provide good statistics even well above one TeV, while also featuring an energy resolution of 2%, which allows it to detect fine structures in the spectrum. Such structures may originate from Dark Matter annihilation or decay, making indirect Dark Matter search one of CALET's main science objectives among others such as identification of signatures from nearby supernova remnants, study of the heavy nuclei spectra and gamma astronomy. The latest results from AMS-02 on positron fraction and total electron+positron flux can be fitted with a parametrization including a single pulsar as an extra power law source with exponential cut-off, which emits an equal amount of electrons and positrons. This single pulsar scenario for the positron excess is extrapolated into the TeV region and the expected CALET data for this case are simulated. Based on this prediction for CALET data, the sensitivity of CALET to Dark Matter annihilation in the galactic halo has been calculated. It is shown that CALET could significantly improve the limits compared to current data, especially for those Dark Matter candidates that feature a large fraction of annihilation directly into e+ + e-, such as the LKP (Lightest Kaluza-Klein particle).

  13. Positron microscopy

    SciTech Connect

    Hulett, L.D. Jr.; Xu, J.

    1995-02-01

    The negative work function property that some materials have for positrons make possible the development of positron reemission microscopy (PRM). Because of the low energies with which the positrons are emitted, some unique applications, such as the imaging of defects, can be made. The history of the concept of PRM, and its present state of development will be reviewed. The potential of positron microprobe techniques will be discussed also.

  14. Positron astrophysics and areas of relation to low-energy positron physics

    NASA Astrophysics Data System (ADS)

    Guessoum, Nidhal

    2014-05-01

    I briefly review our general knowledge of positron astrophysics, focusing mostly on the theoretical and modelling aspects. The experimental/observational aspects of the topic have recently been reviewed elsewhere [E. Churazov et al., Mon. Nat. R. Astron. Soc. 411, 1727 (2011); N. Prantazos et al., Rev. Mod. Phys. 83, 1001 (2011)]. In particular, I highlight the interactions and cross sections of the reactions that the positrons undergo in various cosmic media. Indeed, these must be of high interest to both the positron astrophysics community and the low-energy positron physics community in trying to find common areas of potential collaboration for the future or areas of research that will help the astrophysics community make further progress on the problem. The processes undergone by positrons from the moments of their birth to their annihilation (in the interstellar medium or other locations) are thus examined. The physics of the positron interactions with gases and solids (dust grains) and the physical conditions and characteristics of the environments where the processes of energy loss, positronium formation, and annihilation take place, are briefly reviewed. An explanation is given about how all the relevant physical information is taken into account in order to calculate annihilation rates and spectra of the 511 keV emission in the ISM; special attention is paid to positron interactions with dust and with polycyclic aromatic hydrocarbons. In particular, an attempt is made to show to what extent the interactions between positrons and interstellar dust grains are similar to laboratory experiments in which beams of low-energy positrons impinge upon solids and surfaces. Sample results are shown for the effect of dust grains on positron annihilation spectra in some phases of the ISM which, together with high resolution spectra measured by satellites, can be used to infer useful knowledge about the environment where the annihilation is predominantly taking place

  15. Positrons in the Galaxy: Their Births, Marriages and Deaths

    NASA Technical Reports Server (NTRS)

    Skinner, Gerald K.

    2010-01-01

    High energy (approximately GeV) positrons are seen within cosmic rays and observation of a narrow line at 511 keV shows that positrons are annihilating in the galaxy after slowing down to approximately keV energies or less. Our state of knowledge of the origin of these positrons, of the formation of positronium 'atoms', and of the circumstances of their annihilation or escape from the galaxy are reviewed and the question of whether the two phenomena are linked is discussed.

  16. Positron and positronium studies of irradiation-induced defects and microvoids in vitreous metamict silica

    NASA Astrophysics Data System (ADS)

    Hasegawa, M.; Saneyasu, M.; Tabata, M.; Tang, Z.; Nagai, Y.; Chiba, T.; Ito, Y.

    2000-05-01

    To study irradiation-induced defects and structural microvoids in vitreous silica (v-SiO2), positron lifetime, angular correlation of positron annihilation radiation (ACAR), and electron spin resonance (ESR) were measured on v-SiO2 and quartz (c-SiO2) samples irradiated with fast neutrons up to a dose of 4.1×1020 n/cm2. Two kinds of positron-trapping defects have been found to form in v-SiO2 by fast neutron irradiation: type-I and type-II defects. Similar defects also appear in the irradiated c-SiO2, indicating that both the defects are common in v-SiO2 and c-SiO2. The detailed annealing and photo-illumination studies of positron annihilation and ESR for these two defects suggest that the type-I defects are non-bridging oxygen hole centers (NBOHC), while the type-II defects are oxygen molecules which cannot be detected by ESR. Higher dose irradiation than 1.0×1020 n/cm2 causes c-SiO2 to change to metamict (amorphous) phase (m-SiO2). Positronium (Ps) atoms are found to form in microvoids with an average radius of about 0.3 nm in the v-SiO2 and m-SiO2. This suggests that microvoids proved by Ps are structurally intrinsic open spaces and reflect the topologically disordered structure of these phases in the subnanometer scale.

  17. Positron Spectroscopy Investigation of Normal Brain Section and Brain Section with Glioma Derived from a Rat Glioma Model

    NASA Astrophysics Data System (ADS)

    Yang, SH.; Ballmann, C.; Quarles, C. A.

    2009-03-01

    The application of positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS) to the study of animal or human tissue has only recently been reported [G. Liu, et al. phys. stat. sol. (C) 4, Nos. 10, 3912-3915 (2007)]. We have initiated a study of normal brain section and brain section with glioma derived from a rat glioma model. For the rat glioma model, 200,000 C6 cells were implanted in the basal ganglion of adult Sprague Dawley rats. The rats were sacrificed at 21 days after implantation. The brains were harvested, sliced into 2 mm thick coronal sections, and fixed in 4% formalin. PALS lifetime runs were made with the samples soaked in formalin, and there was not significant evaporation of formalin during the runs. The lifetime spectra were analyzed into two lifetime components. While early results suggested a small decrease in ortho-Positronium (o-Ps) pickoff lifetime between the normal brain section and brain section with glioma, further runs with additional samples have showed no statistically significant difference between the normal and tumor tissue for this type of tumor. The o-Ps lifetime in formalin alone was lower than either the normal tissue or glioma sample. So annihilation in the formalin absorbed in the samples would lower the o-Ps lifetime and this may have masked any difference due to the glioma itself. DBS was also used to investigate the difference in positronium formation between tumor and normal tissue. Tissue samples are heterogeneous and this needs to be carefully considered if PALS and DBS are to become useful tools in distinguishing tissue samples.

  18. Positron Spectroscopy Investigation of Normal Brain Section and Brain Section with Glioma Derived from a Rat Glioma Model

    SciTech Connect

    Yang, SH.; Ballmann, C.; Quarles, C. A.

    2009-03-10

    The application of positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS) to the study of animal or human tissue has only recently been reported [G. Liu, et al. phys. stat. sol. (C) 4, Nos. 10, 3912-3915 (2007)]. We have initiated a study of normal brain section and brain section with glioma derived from a rat glioma model. For the rat glioma model, 200,000 C6 cells were implanted in the basal ganglion of adult Sprague Dawley rats. The rats were sacrificed at 21 days after implantation. The brains were harvested, sliced into 2 mm thick coronal sections, and fixed in 4% formalin. PALS lifetime runs were made with the samples soaked in formalin, and there was not significant evaporation of formalin during the runs. The lifetime spectra were analyzed into two lifetime components. While early results suggested a small decrease in ortho-Positronium (o-Ps) pickoff lifetime between the normal brain section and brain section with glioma, further runs with additional samples have showed no statistically significant difference between the normal and tumor tissue for this type of tumor. The o-Ps lifetime in formalin alone was lower than either the normal tissue or glioma sample. So annihilation in the formalin absorbed in the samples would lower the o-Ps lifetime and this may have masked any difference due to the glioma itself. DBS was also used to investigate the difference in positronium formation between tumor and normal tissue. Tissue samples are heterogeneous and this needs to be carefully considered if PALS and DBS are to become useful tools in distinguishing tissue samples.

  19. Nuclear annihilation by antinucleons

    DOE PAGESBeta

    Lee, Teck-Ghee; Wong, Cheuk-Yin

    2016-01-25

    We examine the momentum dependence ofmore » $$\\bar p$$$p$ and $$\\bar n$$$p$ annihilation cross sections by considering the transmission through a nuclear potential and the $$\\bar p p$$ Coulomb interaction. Compared to the $$\\bar n p$$ annihilation cross section, the $$\\bar p p$$ annihilation cross section is significantly enhanced by the Coulomb interaction for projectile momenta below $$p_{\\rm lab} <$$ 500 MeV/$c$$, and the two annihilation cross sections approach the Pomeranchuk's equality limit [JETP {\\bb 30}, 423 (1956)] at $$p_{\\rm lab}\\sim 500$ MeV/$c$. Using these elementary cross sections as the basic input data, the extended Glauber model is employed to evaluate the annihilation cross sections for $$\\bar n$$ and $$\\bar p$$ interaction with nuclei and the results compare well with experimental data.« less

  20. 2D ACAR momentum density study of the nature of the positron surface state on Al(100)

    SciTech Connect

    Berko, S.; Canter, K.F.; Lynn, K.G.; Mills, A.P.; Roellig, L.O.; West, R.N.

    1985-01-01

    The two-dimensional angular correlation of the 2..gamma.. annihilation radiation (2D ACAR) has been measured from an Al(100) surface bombarded by 200-eV positrons. After removing the contribution of fast para-positronium annihilation, the spectrum from positrons annihilating at the surface exhibits a nearly isotropic conical shape with a (7.1 +- 0.5) mrad FWHM. 5 refs., 6 figs.

  1. Instrumentation in positron emission tomography

    SciTech Connect

    Not Available

    1988-03-11

    Positron emission tomography (PET) is a three-dimensional medical imaging technique that noninvasively measures the concentration of radiopharmaceuticals in the body that are labeled with positron emitters. With the proper compounds, PET can be used to measure metabolism, blood flow, or other physiological values in vivo. The technique is based on the physics of positron annihilation and detection and the mathematical formulations developed for x-ray computed tomography. Modern PET systems can provide three-dimensional images of the brain, the heart, and other internal organs with resolutions on the order of 4 to 6 mm. With the selectivity provided by a choice of injected compounds, PET has the power to provide unique diagnostic information that is not available with any other imaging modality. This is the first five reports on the nature and uses of PET that have been prepared for the American Medical Association's Council on Scientific Affairs by an authoritative panel.

  2. Constraint on the velocity dependent dark matter annihilation cross section from Fermi-LAT observations of dwarf galaxies

    NASA Astrophysics Data System (ADS)

    Zhao, Yi; Bi, Xiao-Jun; Jia, Huan-Yu; Yin, Peng-Fei; Zhu, Feng-Rong

    2016-04-01

    The γ -ray observation of dwarf spheroidal satellites (dSph's) is an ideal approach for probing the dark matter (DM) annihilation signature. The latest Fermi-LAT dSph searches have set stringent constraints on the velocity independent annihilation cross section in the small DM mass range, which gives very strong constraints on the scenario to explain the AMS-02 positron excess by DM annihilation. However, the dSph constraints would change in the velocity dependent annihilation scenarios, because the velocity dispersion in the dSph's varies from that in the Milky Way. In this work, we use a likelihood map method to set constraints on the velocity dependent annihilation cross section from the Fermi-LAT observation of six dSph's. We consider three typical forms of the annihilation cross section, i.e. p-wave annihilation, Sommerfeld enhancement, and Breit-Wigner resonance. For the p-wave annihilation and Sommerfeld enhancement, the dSph limits would become much weaker and stronger compared with those for the velocity independent annihilation, respectively. For the Breit-Wigner annihilation, the dSph limits would vary depending on the model parameters. We show that the scenario to explain the AMS-02 positron excess by DM annihilation is still viable in the velocity dependent cases.

  3. Characterization of dynamics in complex lyophilized formulations: II. Analysis of density variations in terms of glass dynamics and comparisons with global mobility, fast dynamics, and Positron Annihilation Lifetime Spectroscopy (PALS)

    PubMed Central

    Chieng, Norman; Cicerone, Marcus T.; Zhong, Qin; Liu, Ming; Pikal, Michael J.

    2013-01-01

    Amorphous HES/disaccharide (trehalose or sucrose) formulations, with and without added polyols (glycerol and sorbitol) and disaccharide formulations of human growth hormone (hGH), were prepared by freeze drying and characterized with particular interest in methodology for using high precision density measurements to evaluate free volume changes and a focus on comparisons between “free volume” changes obtained from analysis of density data, fast dynamics (local mobility), and PALS characterization of “free volume” hole size. Density measurements were performed using a helium gas pycnometer, and fast dynamics was characterized using incoherent neutron scattering spectrometer. Addition of sucrose and trehalose to hGH decreases free volume in the system with sucrose marginally more effective than trehalose, consistent with superior pharmaceutical stability of sucrose hGH formulations well below Tg relative to trehalose. We find that density data may be analyzed in terms of free volume changes by evaluation of volume changes on mixing and calculation of apparent specific volumes from the densities. Addition of sucrose to HES decreases free volume, but the effect of trehalose is not detectable above experimental error. Addition of sorbitol or glycerol to HES/trehalose base formulations appears to significantly decrease free volume, consistent with the positive impact of such additions on pharmaceutical stability (i.e., degradation) in the glassy state. Free volume changes, evaluated from density data, fast dynamics amplitude of local motion, and PALS hole size data generally are in qualitative agreement for the HES/disaccharide systems studied. All predict decreasing molecular mobility as disaccharides are added to HES. Global mobility as measured by enthalpy relaxation times, increases as disaccharides, particularly sucrose, are added to HES. PMID:23623797

  4. The Buffer-Gas Positron Accumulator and Resonances in Positron-Molecule Interactions

    NASA Technical Reports Server (NTRS)

    Surko, C.M.

    2007-01-01

    This is a personal account of the development of our buffer-gas positron trap and the new generation of cold beams that these traps enabled. Dick Drachman provided much appreciated advice to us from the time we started the project. The physics underlying trap operation is related to resonances (or apparent resonances) in positron-molecule interactions. Amusingly, experiments enabled by the trap allowed us to understand these processes. The positron-resonance "box score" to date is one resounding "yes," namely vibrational Feshbach resonances in positron annihilation on hydrocarbons; a "probably" for positron-impact electronic excitation of CO and NZ;an d a "maybe" for vibrational excitation of selected molecules. Two of these processes enabled the efficient operation of the trap, and one almost killed it in infancy. We conclude with a brief overview of further applications of the trapping technology discussed here, such as "massive" positron storage and beams with meV energy resolution.

  5. Gamma-Ray background spectrum and annihilation rate in the baryon-symmetric big-bang cosmology

    NASA Technical Reports Server (NTRS)

    Puget, J. L.

    1973-01-01

    An attempt was made to extract experimental data on baryon symmetry by observing annihilation products. Specifically, gamma rays and neutrons with long mean free paths were analyzed. Data cover absorption cross sections and radiation background of the 0.511 MeV gamma rays from positron annihilations and the 70 MeV gamma rays from neutral pion decay.

  6. A positron study of the defect structures in the D0{sub 3} and B2 phases in the Fe-Al system

    SciTech Connect

    Diego, N. de . E-mail: nievesd@fis.ucm.es; Plazaola, F.; Jimenez, J.A.; Rio, J. del

    2005-01-03

    The positron annihilation technique was used to identify the nature of the vacancy-type defects in the D0{sub 3} and B2 phases of the Fe-Al system. Seven alloys with Al concentrations in the range 22.7-48 at.% Al and with different thermal treatments were examined. Positron lifetime calculations for the expected defects in the two phases were also performed in order to facilitate the defect identification. In the B2 phase, two types of defects were identified: a thermal complex formed by a Fe-divacancy and an Al antisite, and a Fe-vacancy. No constitutional vacancies were found in the D0{sub 3} phase.

  7. Dark matter annihilation and the PAMELA, FERMI, and ATIC anomalies

    SciTech Connect

    El Zant, A. A.; Okada, H.; Khalil, S.

    2010-06-15

    If dark matter annihilation accounts for the tantalizing excess of cosmic ray electron/positrons, as reported by the PAMELA, ATIC, HESS, and FERMI observatories, then the implied annihilation cross section must be relatively large. This results, in the context of standard cosmological models, in very small relic dark matter abundances that are incompatible with astrophysical observations. We explore possible resolutions to this apparent conflict in terms of nonstandard cosmological scenarios; plausibly allowing for large cross sections, while maintaining relic abundances in accord with current observations.

  8. Pair momentum distribution in Bi sub 2 Sr sub 2 CaCu sub 2 O sub 8+. delta. measured by positron annihilation: Existence and nature of the Fermi surface

    SciTech Connect

    Chan, L.P. ); Harshman, D.R. ); Lynn, K.G. ); Massidda, S. , PHB Ecublens, CH-105 Lausanne ); Mitzi, D.B. )

    1991-09-02

    We report the first measurement of the positron-electron momentum density in superconducting single-crystal Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} ({ital T}{sub {ital c}}{approx}90 K). The observed anisotropy exhibits a twofold (rather than fourfold) symmetry, which is attributed to the superlattice modulation along the {ital b} axis of the BiO{sub 2} layers. Subtraction of the superlattice contribution also reveals a pair momentum distribution consistent with the CuO{sub 2} and BiO{sub 2} Fermi surfaces, and in reasonable agreement with the theoretical pair momentum density derived from band theory.

  9. Low energy positrons as probes of reconstructed semiconductor surfaces.

    NASA Astrophysics Data System (ADS)

    Fazleev, Nail G.; Weiss, Alex H.

    2007-03-01

    Positron probes of semiconductor surfaces that play a fundamental role in modern science and technology are capable to non-destructively provide information that is both unique to the probe and complimentary to that extracted using other more standard techniques. We discuss recent progress in studies of the reconstructed Si(100), Si(111), Ge(100), and Ge(111) surfaces, clean and exposed to hydrogen and oxygen, using a surface characterization technique, Positron-Annihilation-Induced Auger-Electron Spectroscopy (PAES). Experimental PAES results are analyzed by performing first-principles calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons for the reconstructed surfaces, taking into account discrete lattice effects, the electronic reorganization due to bonding, and charge redistribution effects at the surface. Effects of the hydrogen and oxygen adsorption on semiconductor surfaces on localization of positron surface state wave functions and annihilation characteristics are also analyzed. Theoretical calculations confirm that PAES intensities, which are proportional to annihilation probabilities of the surface trapped positrons that results in a core hole, are sensitive to the crystal face, surface structure and elemental content of the semiconductors.

  10. Electroweak fragmentation functions for dark matter annihilation

    SciTech Connect

    Cavasonza, Leila Ali; Krämer, Michael; Pellen, Mathieu

    2015-02-18

    Electroweak corrections can play a crucial role in dark matter annihilation. The emission of gauge bosons, in particular, leads to a secondary flux consisting of all Standard Model particles, and may be described by electroweak fragmentation functions. To assess the quality of the fragmentation function approximation to electroweak radiation in dark matter annihilation, we have calculated the flux of secondary particles from gauge-boson emission in models with Majorana fermion and vector dark matter, respectively. For both models, we have compared cross sections and energy spectra of positrons and antiprotons after propagation through the galactic halo in the fragmentation function approximation and in the full calculation. Fragmentation functions fail to describe the particle fluxes in the case of Majorana fermion annihilation into light fermions: the helicity suppression of the lowest-order cross section in such models cannot be lifted by the leading logarithmic contributions included in the fragmentation function approach. However, for other classes of models like vector dark matter, where the lowest-order cross section is not suppressed, electroweak fragmentation functions provide a simple, model-independent and accurate description of secondary particle fluxes.

  11. Reduction of Positron Range Effects by the Application of a Magnetic Field: for Use with Positron Emission Tomography

    NASA Astrophysics Data System (ADS)

    Raylman, Raymond Robert

    The process of positron emission tomography has become a valuable medical research tool. This procedure involves the administration of a radiopharmaceutical labelled with a positron-emitting isotope to a living organism. Upon the emission and subsequent annihilation of a positron, the gamma rays produced are detected to create an image of metabolic activity within the subject. Many factors such as Compton scattering and photoelectric absorption of the gamma rays tend to limit the quality of these images. Another important limitation is the non-negligible distance the positron travels prior to annihilation. This phenomenon leads to the misplacement of data in the final image. A method for reducing this effect utilizing a magnetic field has been tested and evaluated. The application of a magnetic field constrains the positrons to travel in helical paths instead of their relatively straight courses. Thus, the effective distance the positrons travel from their point of emission is reduced. Results indicate that this technique is successful in reducing the blurring caused in PET images by positron range. The results also indicate that the amount of resolution improvement depends upon the choice of positron emitter and scanner resolution. Reduction of this blurring helps to produce clearer PET images which can allow for more precise localization of tumors, in addition to better measurement of metabolic rate constants. The use of a magnetic field to reduce the range of positrons will lead to more useful images produced by positron emission tomography.

  12. Positron-molecule bound states and positive ion production

    NASA Technical Reports Server (NTRS)

    Leventhal, M.; Passner, A.; Surko, C. M.

    1990-01-01

    The interaction was studied of low energy positrons with large molecules such as alkanes. These data provide evidencce for the existence of long lived resonances and bound states of positrons with neutral molecules. The formation process and the nature of these resonances are discussed. The positive ions produced when a positron annihilates with an electron in one of these resonances were observed and this positive ion formation process is discussed. A review is presented of the current state of the understanding of these positron-molecule resonances and the resulting positive ion formation. A number of outstanding issues in this area is also discussed.

  13. Characterization of defects in Si and SiO{sub 2}-Si using positrons

    SciTech Connect

    Asoka-Kumar, P.; Lynn, K.G.

    1993-12-31

    Positron annihilation spectroscopy of overlayers, interfaces, and buried regions of semiconductors has seen a rapid growth in recent years. The characteristics of the annihilation gamma rays depend strongly on the local environment of the annihilation sites, and can be used to probe defect concentrations in a range inaccessible to conventional defect probes. Some of the recent success of the technique in examining low concentrations of point defects in technologically important Si-based structures is discussed.

  14. Positron production during relativistic runaway processes associated with thunderstorms

    NASA Astrophysics Data System (ADS)

    Dwyer, J. R.; Smith, D. M.; Rassoul, H.; Cramer, E. S.; Schaal, M.; Saleh, Z. H.; Grefenstette, B.; Hazelton, B. J.; Splitt, M. E.; Lazarus, S. M.; Fishman, G. J.; Briggs, M. S.; Connaughton, V.

    2009-12-01

    Recent spacecraft observations of terrestrial gamma-ray flashes (TGFs) by Fermi/GBM and aircraft observations of the gamma-ray emissions from thunderclouds by ADELE have shown prominent 511 keV positron annihilation lines, demonstrating large enhancements in the positron populations. These observations show that significant pair-production must be taking place, most likely in association with the production of relativistic runaway electron avalanches. Using detailed Monte Carlo simulations, we investigate the production and subsequent transport of positrons by strong electric fields associated with thunderstorms. It will be shown that intense high-energy beams of positrons can be produced with energies reaching 100 MeV, well exceeding the average energy of the runaway electron population. These positrons, which may travel many kilometers before annihilating, generate a substantial amount of bremsstrahlung x-rays and annihilation gamma-rays. In this presentation, we shall discuss the theory of positron production by runaway electron avalanches and the feedback effects produced by these positrons. In addition, we shall use the Monte Carlo simulations to model the recent Fermi/GBM TGF and ADELE gamma-ray data.

  15. Positron research at the University of Texas at Austin

    NASA Astrophysics Data System (ADS)

    Goktepeli, Sinan

    The objective of the research presented in this dissertation is to advance the applications of positron annihilation research. An intense positron beam facility was designed and constructed, and a method was developed to better analyze the defect structure of solids. The Texas Reactor-based Intense Positron beam facility (TRIP) was designed to provide a monoenergetic/monodirectional beam of at least 108 e +/sec on a sample. This increase in beam intensity will enhance many positron research techniques both in atomic physics and materials science. The TRIP facility, the result of a collaboration between UT Austin and UT Arlington, is being developed around the concept of multiple scattering of positrons from solid krypton. A large area copper source will be irradiated in a beam port of the 1 MW TRIGA Mark-II research reactor at UT Austin. The source will form the bottom face of a cube with the remaining faces made of tungsten. All surfaces will be cooled down to 22 K and coated with krypton. The top face of the cube has a 1 cm diameter hole to allow for the passage of positrons. The fast beta particles emitted from the β+ decay of 64Cu will be moderated while passing through the krypton. The non- moderated positrons will lose their energy while interacting with the remaining walls. The positrons will be removed from the box by an electric field and electrostatically delivered to the sample. The work on the TRIP facility is ongoing. The results of early measurements at UT Arlington have shown that the facility will be able to achieve its goals. The method developed to better analyze the positron depth profiling (PDP) experiments uses the difference spectra of the measured Doppler broadened annihilation peaks. The difference spectra, which are obtained by subtracting the bulk peak shape from the peaks recorded for each incident positron energy, enhance the differences of the observed peaks, while removing the bulk annihilation term from the multi-state annihilation

  16. Positron kinetics in an idealized PET environment

    PubMed Central

    Robson, R. E.; Brunger, M. J.; Buckman, S. J.; Garcia, G.; Petrović, Z. Lj.; White, R. D.

    2015-01-01

    The kinetic theory of non-relativistic positrons in an idealized positron emission tomography PET environment is developed by solving the Boltzmann equation, allowing for coherent and incoherent elastic, inelastic, ionizing and annihilating collisions through positronium formation. An analytic expression is obtained for the positronium formation rate, as a function of distance from a spherical source, in terms of the solutions of the general kinetic eigenvalue problem. Numerical estimates of the positron range - a fundamental limitation on the accuracy of PET, are given for positrons in a model of liquid water, a surrogate for human tissue. Comparisons are made with the ‘gas-phase’ assumption used in current models in which coherent scattering is suppressed. Our results show that this assumption leads to an error of the order of a factor of approximately 2, emphasizing the need to accurately account for the structure of the medium in PET simulations. PMID:26246002

  17. Positron kinetics in an idealized PET environment.

    PubMed

    Robson, R E; Brunger, M J; Buckman, S J; Garcia, G; Petrović, Z Lj; White, R D

    2015-01-01

    The kinetic theory of non-relativistic positrons in an idealized positron emission tomography PET environment is developed by solving the Boltzmann equation, allowing for coherent and incoherent elastic, inelastic, ionizing and annihilating collisions through positronium formation. An analytic expression is obtained for the positronium formation rate, as a function of distance from a spherical source, in terms of the solutions of the general kinetic eigenvalue problem. Numerical estimates of the positron range - a fundamental limitation on the accuracy of PET, are given for positrons in a model of liquid water, a surrogate for human tissue. Comparisons are made with the 'gas-phase' assumption used in current models in which coherent scattering is suppressed. Our results show that this assumption leads to an error of the order of a factor of approximately 2, emphasizing the need to accurately account for the structure of the medium in PET simulations. PMID:26246002

  18. Positron kinetics in an idealized PET environment

    NASA Astrophysics Data System (ADS)

    Robson, R. E.; Brunger, M. J.; Buckman, S. J.; Garcia, G.; Petrović, Z. Lj.; White, R. D.

    2015-08-01

    The kinetic theory of non-relativistic positrons in an idealized positron emission tomography PET environment is developed by solving the Boltzmann equation, allowing for coherent and incoherent elastic, inelastic, ionizing and annihilating collisions through positronium formation. An analytic expression is obtained for the positronium formation rate, as a function of distance from a spherical source, in terms of the solutions of the general kinetic eigenvalue problem. Numerical estimates of the positron range - a fundamental limitation on the accuracy of PET, are given for positrons in a model of liquid water, a surrogate for human tissue. Comparisons are made with the ‘gas-phase’ assumption used in current models in which coherent scattering is suppressed. Our results show that this assumption leads to an error of the order of a factor of approximately 2, emphasizing the need to accurately account for the structure of the medium in PET simulations.

  19. Asymmetries involving dihadron fragmentation functions: from DIS to e+ e- annihilation

    SciTech Connect

    Bacchetta, Alessandro; Radici, M.; Mukherjee, Asmita; Ceccopieri, Federico

    2009-01-01

    Using a model calculation of dihadron fragmentation functions, we fit the spin asymmetry recently extracted by HERMES for the semi-inclusive pion pair production in deep-inelastic scattering on a transversely polarized proton target. By evolving the obtained dihadron fragmentation functions, we make predictions for the correlation of the angular distributions of two pion pairs produced in electron-positron annihilations at BELLE kinematics. Our study shows that the combination of two-hadron inclusive deep-inelastic scattering and electron-positron annihilation measurements can provide a valid alternative to Collins effect for the extraction of the quark transversity distribution in the nucleon.

  20. Three-dimensional electron-positron momentum distribution of O3+-irradiated 6H SiC using two positron spectroscopy techniques simultaneously

    NASA Astrophysics Data System (ADS)

    Williams, Christopher; Burggraf, Larry; Adamson, Paul; Petrosky, James

    2011-01-01

    A three-dimensional (3D) positron annihilation spectroscopy system (3DPASS) capable of determining 3D electron-positron (e--e+) momentum densities from measurements of deviations from co-linearity and energies of photons from e--e+ annihilation events was employed to examine the effects of O-atom defects in 6H SiC. Three-dimensional momentum datasets were determined for 6H SiC irradiated with 24 MeV O3+ ions. Angular correlation of annihilation radiation (ACAR) and coincidence Doppler-broadening of annihilation radiation (CDBAR) analyses are presented. In addition, a novel technique is illustrated for analyzing 3D momentum datasets in which the parallel momentum component, p|| (obtained from the CDBAR measurement) is selected for annihilation events that possess a particular perpendicular momentum component, p- observed in the 2D ACAR spectrum.

  1. Electron capture from solids by positrons

    SciTech Connect

    Howell, R.

    1987-08-01

    The capture of electrons in solids is modified from that in gasses by several factors. The most important is the collective interaction of the electrons which results in a density of electron states in the solid in wide bands. Also the high density of electrons in many solids gives a high frequency of interaction as compared to gasses, and quickly destroys any electron-positron states in the metal matrix. Consequently, most positrons implanted in a metal will rapidly thermalize, and unless they reach the surface will annihilate with an electron in an uncorrelated state. Positronium formation from positrons scattered at a metal surface is analogous to ion neutralization however, most of the positronium comes from positrons passing through the surface from the bulk. The dominant motivation for studying positronium formation has been the hope that the distribution of the electrons at the surface would be obtained through the annihilation properties of positrons trapped at the surface or through analysis of the energy and angular distributions of the positronium emitted into the vacuum. These distributions have been measured and are included in this paper. 17 refs.

  2. Measurement of the positron diffusion constants in polycrystalline molybdenum by the observation of positronium negative ions

    NASA Astrophysics Data System (ADS)

    Suzuki, Takuji; Iida, Simpei; Yamashita, Takashi; Nagashima, Yasuyuki

    2015-06-01

    We have measured the positron diffusion constants in polycrystalline molybdenum by the observation of positronium negative ions (Ps-). The Ps- ions emitted from the sample surface coated with Na were accelerated. The γ-rays from the accelerated Ps- ions were Doppler- shifted and thus the signals of self-annihilation of the Ps- ions were isolated from those of self-annihilation of para-positronium (p-Ps) or pair-annihilation of positrons in the bulk. Clear and reliable values of the diffusion constants have been obtained.

  3. A method to detect positron anisotropies with Pamela data

    NASA Astrophysics Data System (ADS)

    Panico, B.; Adriani, O.; Barbarino, G. C.; Bazilevskaya, G. A.; Bellotti, R.; Boezio, M.; Bogomolov, E. A.; Bongi, M.; Bonvicini, V.; Bottai, S.; Bruno, A.; Cafagna, F.; Campana, D.; Carbone, R.; Carlson, P.; Casolino, M.; Castellini, G.; De Donato, C.; De Santis, C.; De Simone, N.; Di Felice, V.; Formato, V.; Galper, A. M.; Giaccari, U.; Karelin, A. V.; Koldashov, S. V.; Koldobskiy, S.; Krutkov, S. Y.; Kvashnin, A. N.; Leonov, A.; Malakhov, V.; Marcelli, L.; Martucci, M.; Mayorov, A. G.; Menn, W.; Merge, M.; Mikhailov, V. V.; Mocchiutti, E.; Monaco, A.; Mori, N.; Munini, R.; Osteria, G.; Palma, F.; Papini, P.; Pearce, M.; Picozza, P.; Pizzolotto, C.; Ricci, M.; Ricciarini, S. B.; Sarkar, R.; Scotti, V.; Simon, M.; Sparvoli, R.; Spillantini, P.; Stozhkov, Y. I.; Vacchi, A.; Vannuccini, E.; Vasilyev, G. I.; Voronov, S. A.; Yurkin, Y. T.; Zampa, G.; Zampa, N.; Zverev, V. G.

    2014-11-01

    The PAMELA experiment is collecting data since 2006; its results indicate the presence of a large flux of positron with respect to electrons in the CR spectrum above 10 GeV. This excess might also be originated in objects such as pulsars and microquasars or through dark matter annihilation. Here the electrons and positrons events collected by PAMELA have been analized searching for anisotropies. The analysis is performed at different angular scales and results will be presented at the conference.

  4. The Isotropic Radio Background and Annihilating Dark Matter

    SciTech Connect

    Hooper, Dan; Belikov, Alexander V.; Jeltema, Tesla E.; Linden, Tim; Profumo, Stefano; Slatyer, Tracy R.

    2012-11-01

    Observations by ARCADE-2 and other telescopes sensitive to low frequency radiation have revealed the presence of an isotropic radio background with a hard spectral index. The intensity of this observed background is found to exceed the flux predicted from astrophysical sources by a factor of approximately 5-6. In this article, we consider the possibility that annihilating dark matter particles provide the primary contribution to the observed isotropic radio background through the emission of synchrotron radiation from electron and positron annihilation products. For reasonable estimates of the magnetic fields present in clusters and galaxies, we find that dark matter could potentially account for the observed radio excess, but only if it annihilates mostly to electrons and/or muons, and only if it possesses a mass in the range of approximately 5-50 GeV. For such models, the annihilation cross section required to normalize the synchrotron signal to the observed excess is sigma v ~ (0.4-30) x 10^-26 cm^3/s, similar to the value predicted for a simple thermal relic (sigma v ~ 3 x 10^-26 cm^3/s). We find that in any scenario in which dark matter annihilations are responsible for the observed excess radio emission, a significant fraction of the isotropic gamma ray background observed by Fermi must result from dark matter as well.

  5. Fluxes and spectra of quasimonochromatic annihilation photons for studying E1 giant resonances in nuclei

    SciTech Connect

    Dzhilavyan, L. Z.

    2014-12-15

    The fluxes and spectra of quasimonochromatic photons originating from the in-flight annihilation of positrons interacting with electrons of targets are analyzed in the energy region characteristic of the excitation of E1 giant resonances in nuclei. Targets of small thickness and low atomic number are used. The dependences of the spectra on the energy and angle (and their scatter) for positrons incident to the target, on the collimation angle for photons, and on the target thickness are studied.

  6. Positron density enhancements recorded within a thunderstorm by ADELE

    NASA Astrophysics Data System (ADS)

    Dwyer, J. R.; Smith, D. M.; Hazelton, B. J.; Grefenstette, B.; Kelley, N. A.; Lowell, A. W.; Schaal, M.; Rassoul, H.

    2015-12-01

    We report the observation of two unusual positron density enhancements made inside an active thunderstorm by the Airborne Detector for Energetic Lightning Emissions (ADELE) onboard a Gulfstream V aircraft in August 2009. ADELE recorded two count rate enhancements of 511 keV annihilation gamma rays, 35 seconds apart, that lasted approximately 0.2 seconds each. The enhancements were about a factor of 12 above background and had energy spectra consistent with clouds of positrons, approximately 1 km across, briefly surrounding the aircraft. A flat-plate antenna on the underside of the aircraft also recorded electrical activity during the positron enhancements. It is not clear how the positron clouds were created within the thunderstorm or whether the presence of the aircraft played a role in their production. In this presentation, we will show the ADELE data along with model fits of the positron spectra. We shall also discuss possible sources of the positron excesses.

  7. [Basic principles of 18F-fluorodeoxyglucose positron emission tomography].

    PubMed

    Standke, R

    2002-01-01

    Positron emission tomography uses photons to receive regional information about dynamic, physiologic, and biochemical processes in the living body. A positron decay is measured indirectly by the simultaneous registration of both gamma rays created by the annihilation. The event is counted, if two directly opposite located detectors register gamma rays in coincidence. Unfortunately the detectors of a positron emission tomography system do not register only true coincident events. There are also scattered and random coincidences. Different types of positron tomographs are presented and scintillation crystals, which are in use for positron emission tomography are discussed. The 2D- and 3D-acquisition methods are described as well as preprocessing methods, such as correction for attenuation, scatter and dead time. For quantification the relative parameter standard uptake value (SUV) is explained. Finally hybrid systems, such as combined positron emission tomography/computed tomography scanners and the use of computed tomography data for attenuation correction are introduced. PMID:12506765

  8. PhytoBeta imager: a positron imager for plant biology

    SciTech Connect

    Weisenberger, Andrew G; Lee, Seungjoon; McKisson, John; McKisson, J E; Xi, Wenze; Zorn, Carl; Reid, Chantal D; Howell, Calvin R; Crowell, Alexander S; Cumberbatch, Laurie; Fallin, Brent; Stolin, Alexander; Smith, Mark F

    2012-06-01

    Several positron emitting radioisotopes such as 11C and 13N can be used in plant biology research. The 11CO2 tracer is used to facilitate plant biology research toward optimization of plant productivity, biofuel development and carbon sequestration in biomass. Positron emission tomography (PET) imaging has been used to study carbon transport in live plants using 11CO2. Because plants typically have very thin leaves, little medium is present for the emitted positrons to undergo an annihilation event. The emitted positrons from 11C (maximum energy 960 keV) could require up to approximately 4 mm of water equivalent material for positron annihilation. Thus many of the positrons do not annihilate inside the leaf, resulting in limited sensitivity for PET imaging. To address this problem we have developed a compact beta-positive, beta-minus particle imager (PhytoBeta imager) for 11CO2 leaf imaging. The detector is based on a Hamamatsu H8500 position sensitive photomultiplier tube optically coupled via optical grease to a 0.5 mm thick Eljen EJ-212 plastic scintillator. The detector is equipped with a flexible arm to allow its placement and orientation over or under the leaf to be studied while maintaining the leaf's original orientation. To test the utility of the system the detector was used to measure carbon translocation in a leaf of the spicebush (Lindera benzoin) under two transient light conditions.

  9. PhytoBeta imager: a positron imager for plant biology

    NASA Astrophysics Data System (ADS)

    Weisenberger, Andrew G.; Kross, Brian; Lee, Seungjoon; McKisson, John; McKisson, J. E.; Xi, Wenze; Zorn, Carl; Reid, Chantal D.; Howell, Calvin R.; Crowell, Alexander S.; Cumberbatch, Laurie; Fallin, Brent; Stolin, Alexander; Smith, Mark F.

    2012-07-01

    Several positron emitting radioisotopes such as 11C and 13N can be used in plant biology research. The 11CO2 tracer is used to facilitate plant biology research toward optimization of plant productivity, biofuel development and carbon sequestration in biomass. Positron emission tomography (PET) imaging has been used to study carbon transport in live plants using 11CO2. Because plants typically have very thin leaves, little medium is present for the emitted positrons to undergo an annihilation event. The emitted positrons from 11C (maximum energy 960 keV) could require up to approximately 4 mm of water equivalent material for positron annihilation. Thus many of the positrons do not annihilate inside the leaf, resulting in limited sensitivity for PET imaging. To address this problem we have developed a compact beta-positive, beta-minus particle imager (PhytoBeta imager) for 11CO2 leaf imaging. The detector is based on a Hamamatsu H8500 position sensitive photomultiplier tube optically coupled via optical grease to a 0.5 mm thick Eljen EJ-212 plastic scintillator. The detector is equipped with a flexible arm to allow its placement and orientation over or under the leaf to be studied while maintaining the leaf's original orientation. To test the utility of the system the detector was used to measure carbon translocation in a leaf of the spicebush (Lindera benzoin) under two transient light conditions.

  10. Models of pair annihilation in 1E 1740.7-2942 and the HEAO 1 A-4 annihilation source

    NASA Technical Reports Server (NTRS)

    Maciolek-Niedzwiecki, Andrzej; Zdziarski, Andrzej

    1994-01-01

    We study possible models of two Galactic sources of transient pair annihilation radiation, 1E 1740.7-2942 and a source observed by High Energy Astronomy Observatory (HEAO) 1 A-4. We fit the observed spectral features by thermal annihilation spectra and find that the redshifts obtained by us are much larger than those obtained from fitting Caussian lines centered on 511 keV. This effect, which is due to the net blueshift (with respect to 511 keV) of the annihilation spectrum due to the thermal energies of pairs, puts strong constraints on models of sources. We consider those constraints first without considering the mechanism of positron production. From the shape of the observed spectra, we are able to rule out both spherical clouds and layers above cold matter as possible source geometries. The observed spectra are compatible with two source geometries: (1) a nearly face-on disk in the Kerr metric and (2) a jet close to a black hole. We consider, then, the origin of the pairs. Theories of both thermal and nonthermal pair equilibria predict that photon-pair production is unable to produce annihilation features that contain as much as half of the bolometric luminosity, which is observed. A possible solution to this problem is obscuration of a nonthermal source (in which pairs are produced by photon-photon collisions) and an outflow of pairs to an unobscured region. This makes annihilation in a jet the most likely model of the considered sources.

  11. Development of a method to study positron diffusion in metals by the observation of positronium negative ions

    NASA Astrophysics Data System (ADS)

    Suzuki, Takuji; Terabe, Hiroki; Iida, Shimpei; Yamashita, Takashi; Nagashima, Yasuyuki

    2014-09-01

    We have developed a new method to study positron diffusion in metals. In this method, we observe positronium negative ions emitted from the sample surfaces after coating with alkali-metals to evaluate the yields of the positrons which return to the surfaces. γ-rays from the ions accelerated using an electric field are clearly distinguished from those emitted from pair-annihilation of positrons in the bulk or on the surface and self-annihilation of emitted positronium atoms. Reliable studies on positron diffusion in metals have been enabled by this method.

  12. Positron Physics

    NASA Technical Reports Server (NTRS)

    Drachman, Richard J.

    2003-01-01

    I will give a review of the history of low-energy positron physics, experimental and theoretical, concentrating on the type of work pioneered by John Humberston and the positronics group at University College. This subject became a legitimate subfield of atomic physics under the enthusiastic direction of the late Sir Harrie Massey, and it attracted a diverse following throughout the world. At first purely theoretical, the subject has now expanded to include high brightness beams of low-energy positrons, positronium beams, and, lately, experiments involving anti-hydrogen atoms. The theory requires a certain type of persistence in its practitioners, as well as an eagerness to try new mathematical and numerical techniques. I will conclude with a short summary of some of the most interesting recent advances.

  13. Annihilation of strained vortices

    NASA Astrophysics Data System (ADS)

    Kimura, Yoshifumi

    2014-11-01

    As an initial stage of vortex reconnection, approach of nearly anti-parallel vortices has often been observed experimentally and studied numerically. Inspired by the recent experiment by Kleckner and Irvine on the dynamics of knotted vortices, we have studied the motion of two anti-parellel Burgers vortices driven by an axisymmetric linear straining field. We first extend the Burgers vortex solution which is a steady exact solution of the Navier-Stokes equation to a time-dependent exact solution. Then by superposing two such solutions, we investigate the annihilation process analytically. We can demonstrate that during the annihilation process the total vorticity decays exponentially on a time-scale proportional to the inverse of the rate of strain, even as the kinematic viscosity tends to 0. The analytic results are compared with the numerical simulations of two strained vortices with the vortex-vortex nonlinear interaction by Buntine and Pullin.

  14. Positron range estimations with PeneloPET

    NASA Astrophysics Data System (ADS)

    Cal-González, J.; Herraiz, J. L.; España, S.; Corzo, P. M. G.; Vaquero, J. J.; Desco, M.; Udias, J. M.

    2013-08-01

    Technical advances towards high resolution PET imaging try to overcome the inherent physical limitations to spatial resolution. Positrons travel in tissue until they annihilate into the two gamma photons detected. This range is the main detector-independent contribution to PET imaging blurring. To a large extent, it can be remedied during image reconstruction if accurate estimates of positron range are available. However, the existing estimates differ, and the comparison with the scarce experimental data available is not conclusive. In this work we present positron annihilation distributions obtained from Monte Carlo simulations with the PeneloPET simulation toolkit, for several common PET isotopes (18F, 11C, 13N, 15O, 68Ga and 82Rb) in different biological media (cortical bone, soft bone, skin, muscle striated, brain, water, adipose tissue and lung). We compare PeneloPET simulations against experimental data and other simulation results available in the literature. To this end the different positron range representations employed in the literature are related to each other by means of a new parameterization for positron range profiles. Our results are generally consistent with experiments and with most simulations previously reported with differences of less than 20% in the mean and maximum range values. From these results, we conclude that better experimental measurements are needed, especially to disentangle the effect of positronium formation in positron range. Finally, with the aid of PeneloPET, we confirm that scaling approaches can be used to obtain universal, material and isotope independent, positron range profiles, which would considerably simplify range correction.

  15. Monte Carlo analysis of germanium detector performance in slow positron beam experiments

    NASA Astrophysics Data System (ADS)

    Heikinheimo, J.; Tuominen, R.; Tuomisto, F.

    2016-01-01

    Positron annihilation Doppler broadening spectroscopy is one of the most popular positron annihilation vacancy characterization techniques in experimental materials research. The measurements are often carried out with a slow positron beam setup, which enables depth profiling of the samples. The key measurement devices of Doppler broadening spectroscopy setups are high-purity germanium detectors. Since Doppler broadening spectroscopy is one of the standard techniques in defect characterization, there is a demand to evaluate different kinds of factors that might have an effect on the results. Here we report the results of Monte Carlo simulations of detector response in different geometries and compare the data to experiments.

  16. Correlation between the segmental motion and ionic conductivity of poly(ether urethane)-LiClO4 complex studied by positron spectroscopy

    NASA Astrophysics Data System (ADS)

    Wang, B.; Li, S. Q.; Wang, S. J.

    1997-11-01

    Positron annihilation lifetime and conductivity measurements have been performed for the poly(ether urethane) PEU-LiClO4 complex as a function of temperature in the temperature range from 120 to 360 K and from 280 to 360 K, respectively. According to the variations of free volume and fractional free volume, the structural transition of PEU-LiClO4 has been determined. Based on a polymer lattice model, the formation energy of a free-volume hole has been calculated in terms of fractional free volume derived from positron annihilation parameters. The temperature dependence of ionic conductivity obeys the Vogel-Tammann-Fulcher and Williams-Landel-Ferry equations, implying a free-volume transport mechanism. A direct relationship between the ionic conductivity and the fractional free volume has been established based on the experimental measurements. A linear least-squares procedure was used to evaluate the apparent activation energy in the Vogel-Tammann-Fulcher equation and several important parameters in the Williams-Landel-Ferry and Vogel-Tammann-Fulcher equations. The correlation between the segmental motion and the conductivity could be explained by means of the free-volume theory.

  17. Advances in defect characterizations of semiconductors using positrons

    SciTech Connect

    Lynn, K.G.; Asoka-Kumar, P.

    1996-12-31

    Positron Annihilation Spectroscopy (PAS) is a sensitive probe for studying the electronic structure of defects in solids. The authors summarize recent developments in defect characterization of semiconductors using depth-resolved PAS. The progress achieved in extending the capabilities of the PAS method is also described.

  18. Positron states on the Cs/Cu(100) surface

    SciTech Connect

    Koeymen, A.R.; Lee, K.H.; Mehl, D.; Weiss, A. ); Jensen, K.O. )

    1991-02-01

    The attenuation of the CuM{sub 23}VV Auger peak with Cs coverage on Cu(100) is measured using both positron-annihilation-induced Auger electron emission (PAES) and conventional (electron induced) Auger electron spectroscopy (EAES). The Cs coverage varies from 0 to 1 physical monolayer (ML). The data indicates that below 0.5 ML in agreement with first order theoretical calculations the positrons are trapped at the Cu/Cs interface. At higher Cs coverages the thermal desorption of the positrons as positronium drops the PAES intensity to zero whereas the EAES signal changes linearly as expected.

  19. Positron production using a 1.7 MV pelletron accelerator

    SciTech Connect

    Alcantara, K. F.; Santos, A. C. F.; Crivelli, P.

    2013-04-19

    We report the foremost phase of a fourth generation positron source, being constructed at the Federal University of Rio de Janeiro. Positron yields are reported by making use of the {sup 19}F(p,{alpha}e{sup +}e{sup -}){sup 16}O reaction, where the fluorine target is in the form of a CaF{sub 2} pellet. Positron production has been observed by detecting 511 keV annihilation gamma rays emerging from the irradiated CaF{sub 2} target.

  20. Microstructural Characterization of Polymers with Positrons

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.

    1997-01-01

    Positrons provide a versatile probe for monitoring microstructural features of molecular solids. In this paper, we report on positron lifetime measurements in two different types of polymers. The first group comprises polyacrylates processed on earth and in space. The second group includes fully-compatible and totally-incompatible Semi-Interpenetrating polymer networks of thermosetting and thermoplastic polyimides. On the basis of lifetime measurements, it is concluded that free volumes are a direct reflection of physical/electromagnetic properties of the host polymers.

  1. Vacancy-type defects induced by grinding of Si wafers studied by monoenergetic positron beams

    SciTech Connect

    Uedono, Akira; Yoshihara, Nakaaki; Mizushima, Yoriko; Kim, Youngsuk; Nakamura, Tomoji; Ohba, Takayuki; Oshima, Nagayasu; Suzuki, Ryoichi

    2014-10-07

    Vacancy-type defects introduced by the grinding of Czochralski-grown Si wafers were studied using monoenergetic positron beams. Measurements of Doppler broadening spectra of the annihilation radiation and the lifetime spectra of positrons showed that vacancy-type defects were introduced in the surface region (<98 nm), and the major defect species were identified as (i) relatively small vacancies incorporated in dislocations and (ii) large vacancy clusters. Annealing experiments showed that the defect concentration decreased with increasing annealing temperature in the range between 100 and 500°C. After 600–700°C annealing, the defect-rich region expanded up to about 170 nm, which was attributed to rearrangements of dislocation networks, and a resultant emission of point defects toward the inside of the sample. Above 800°C, the stability limit of those vacancies was reached and they started to disappear. After the vacancies were annealed out (900°C), oxygen-related defects were the major point defects and they were located at <25 nm.

  2. Compact Beta Particle/Positron Imager for Plant Biology

    SciTech Connect

    Weisenberger, Andrew; Lee, Seung Joon; McKisson, John; Xi, Wenze; Zorn, Carl; Stolin, Alexander; Majewski, Stan; Majewski, Stanislaw; Howell, Calvin; Crowell, Alec; Smith, Mark

    2011-06-01

    The 11CO2 tracer is used to facilitate plant biology research towards optimization of plant productivity, biofuel development and carbon sequestration in biomass. Positron emission tomography (PET) imaging has been used to study carbon transport in live plants using 11CO2. Plants typically have very thin leaves resulting in little medium for the emitted positrons to undergo an annihilation event. For the emitted positron from 11C decay approximately 1mm of water equivalent material is needed for positron annihilation. Thus most of the positrons do not annihilate inside the leaf, resulting in limited sensitivity for PET imaging. To address this problem we have developed a compact beta-positive beta-minus particle (BPBM) imager for 11CO2 leaf imaging. The detector is based on a Hamamatsu H8500 position sensitive photomultiplier tube optically coupled via optical grease and a 3mm thick glass plate to a 0.5mm thick Eljin EJ-212 plastic scintillator. The detector is equipped with a flexible arm to allow its placement and orientation on the leaf of the plant of interest while maintaining the leaf's original orientation. We are planning to utilize the imaging device at the Duke University Phytotron to investigate dynamic carbon transport differences between invasive and native species.

  3. Experiments Enabled by a New High-Resolution Positron Beam

    NASA Astrophysics Data System (ADS)

    Natisin, Mike; Danielson, James; Surko, Cliff

    2016-05-01

    The ability to make state-resolved measurements of positron interactions with atoms and molecules is limited by difficulties encountered in creating beams with narrow energy spreads. Recent experiments and simulations of buffer gas positron cooling and trap-based beam formation have enabled the design and construction of a cryogenic buffer-gas trap with total beam energy spreads as low as 7 meV FWHM and temporal spreads of sub-microsecond duration. The potential effect of this narrow energy spread on the ability to probe new physics in positron scattering and annihilation experiments will be discussed. For example, beams with such energy spreads are expected to enable the first measurements of state-resolved excitation of molecular rotations by positron impact (i.e., H2). Further, these narrow spreads and resulting enhanced resolving power are expected to permit the study of new features in annihilation energy spectra, such as possible overtone, combination, and IR-inactive vibrational modes in Feshbach-resonant positron annihilation. Work supported by NSF Grant PHY-1401794.

  4. Eternal annihilations: New constraints on long-lived particles from big-bang nucleosynthesis

    NASA Astrophysics Data System (ADS)

    Frieman, Joshua A.; Kolb, Edward W.; Turner, Michael S.

    1990-05-01

    In the early Universe, the relative abundance of a massive weakly interacting particle species ``freezes out'' when the annihilation rate becomes less than the expansion rate. Although ineffective in reducing the total number of the species, occasional annihilations still occur after freeze-out. The residual annihilations of massive particles (10 MeV<~mX<~1 GeV) after primordial nucleosynthesis can strongly alter the light-element abundances through photodissociation. For particles with typical weak-interaction cross sections and lifetimes τX>~5×106 sec, we find that the mass range mX<~1 GeV is ruled out, independent of how they subsequently decay.

  5. Quark flavor identification in electron-positron annihilation

    SciTech Connect

    Kaye, H.S.

    1983-09-01

    The theoretical issues relevant to inclusive muon analysis, the MAC detector and its data flow structure, the identification of muons in hadronic events and the measurement of their momenta, and the selection of events so as to minimize background are described. Experimental results are presented describing the fragmentation of heavy quarks into hadrons, the semimuonic branching fractions of the heavy quarks, the asymmetry in the angular distribution of the heavy quarks, and the invariant mass and charged multiplicity of heavy quark jets. In addition, lower limits are set on the masses of certain proposed particles that are expected to decay semileptonically. Finally, events containing two muons are analyzed in order to investigate the possibility of mixing in the B-B system and whether the b might form its own SU(2) singlet.

  6. Some Annihilating Particle Systems.

    NASA Astrophysics Data System (ADS)

    Balding, David

    Available from UMI in association with The British Library. Requires signed TDF. Systems of annihilating and coalescing particles on both infinite and periodic one-dimensional state spaces are studied. These systems have various applications in the physical sciences, in particular they are useful as simple models of diffusion-limited reactions. A unified approach to computing properties of the systems using duality methods is presented and it is shown that many results in the scientific literature, derived using diverse techniques, are readily obtained in this general framework. The transition distributions of the processes with arbitrary initial configurations are characterized in terms of two-particle annihilation processes. Further, a concise expression for the distribution of the cardinality of the processes with finite initial configurations is given and particular cases of interest from the applications perspective are described in detail. Asymptotic site occupancies, previously known for certain classes of initial configurations, are derived for all spatially stationary configurations. The asymptotic spatial structure is described for many cases by showing convergence to point processes whose properties are given.

  7. PET iterative reconstruction incorporating an efficient positron range correction method.

    PubMed

    Bertolli, Ottavia; Eleftheriou, Afroditi; Cecchetti, Matteo; Camarlinghi, Niccolò; Belcari, Nicola; Tsoumpas, Charalampos

    2016-02-01

    Positron range is one of the main physical effects limiting the spatial resolution of positron emission tomography (PET) images. If positrons travel inside a magnetic field, for instance inside a nuclear magnetic resonance (MR) tomograph, the mean range will be smaller but still significant. In this investigation we examined a method to correct for the positron range effect in iterative image reconstruction by including tissue-specific kernels in the forward projection operation. The correction method was implemented within STIR library (Software for Tomographic Image Reconstruction). In order to obtain the positron annihilation distribution of various radioactive isotopes in water and lung tissue, simulations were performed with the Monte Carlo package GATE [Jan et al. 2004 [1

  8. Brane annihilations during inflation

    SciTech Connect

    Battefeld, Diana; Battefeld, Thorsten; Firouzjahi, Hassan; Khosravi, Nima E-mail: tbattefe@princeton.edu E-mail: nima@ipm.ir

    2010-07-01

    We investigate brane inflation driven by two stacks of mobile branes in a throat. The stack closest to the bottom of the throat annihilates first with antibranes, resulting in particle production and a change of the equation of state parameter w. We calculate analytically some observable signatures of the collision; related decays are common in multi-field inflation, providing the motivation for this case study. The discontinuity in w enters the matching conditions relating perturbations in the remaining degree of freedom before and after the collision, affecting the power-spectrum of curvature perturbations. We find an oscillatory modulation of the power-spectrum for scales within the horizon at the time of the collision, and a slightly redder spectrum on super-horizon scales. We comment on implications for staggered inflation.

  9. PREFACE: The International Workshop on Positron Studies of Defects 2014

    NASA Astrophysics Data System (ADS)

    Sugita, Kazuki; Shirai, Yasuharu

    2016-01-01

    The International Workshop on Positron Studies of Defects 2014 (PSD-14) was held in Kyoto, Japan from 14-19 September, 2014. The PSD Workshop brought together positron scientists interested in studying defects to an international platform for presenting and discussing recent results and achievements, including new experimental and theoretical methods in the field. The workshop topics can be characterized as follows: • Positron studies of defects in semiconductors and oxides • Positron studies of defects in metals • New experimental methods and equipment • Theoretical calculations and simulations of momentum distributions, positron lifetimes and other characteristics for defects • Positron studies of defects in combination with complementary methods • Positron beam studies of defects at surfaces, interfaces, in sub-surface regions and thin films • Nanostructures and amorphous materials

  10. Photoinduced carrier annihilation in silicon pn junction

    NASA Astrophysics Data System (ADS)

    Sameshima, Toshiyuki; Motoki, Takayuki; Yasuda, Keisuke; Nakamura, Tomohiko; Hasumi, Masahiko; Mizuno, Toshihisa

    2015-08-01

    We report analysis of the photo-induced minority carrier effective lifetime (τeff) in a p+n junction formed on the top surfaces of a n-type silicon substrate by ion implantation of boron and phosphorus atoms at the top and bottom surfaces followed by activation by microwave heating. Bias voltages were applied to the p+ boron-doped surface with n+ phosphorus-doped surface kept at 0 V. The values of τeff were lower than 1 × 10-5 s under the reverse-bias condition. On the other hand, τeff markedly increased to 1.4 × 10-4 s as the forward-bias voltage increased to 0.7 V and then it leveled off when continuous-wave 635 nm light was illuminated at 0.74 mW/cm2 on the p+ surface. The carrier annihilation velocity S\\text{p + } at the p+ surface region was numerically estimated from the experimental τeff. S\\text{p + } ranged from 4000 to 7200 cm/s under the reverse-bias condition when the carrier annihilation velocity S\\text{n + } at the n+ surface region was assumed to be a constant value of 100 cm/s. S\\text{p + } markedly decreased to 265 cm/s as the forward-bias voltage increased to 0.7 V.

  11. Pair annihilation into neutrinos in strong magnetic fields.

    NASA Technical Reports Server (NTRS)

    Canuto, V.; Fassio-Canuto, L.

    1973-01-01

    Among the processes that are of primary importance for the thermal history of a neutron star is electron-positron annihilation into neutrinos and photoneutrinos. These processes are computed in the presence of a strong magnetic field typical of neutron stars, and the results are compared with the zero-field case. It is shown that the neutrino luminosity Q(H) is greater than Q(O) for temperatures up to T about equal to 3 x 10 to the 8th power K and densities up to 1,000,000 g/cu cm.

  12. Inclusive Production Λ c in the e + e - - Annihilation

    NASA Astrophysics Data System (ADS)

    Osati, T.; Movlanaei, M.

    2016-09-01

    In the framework of the quark-diquark model of baryons, Λ c can be considered as constituent c quark and ud diquark. In hadronizasion process baryon Λ c can be produced from ud scalar diquark and c quark fragmentation. So in this work, in the lowest order of perturbative QCD, fragmentation functions c → Λ c and ud → Λ c are calculated and finally in the electron-positron annihilation in LEP, the inclusive cross section production of Λ c is calculated about pole of z 0.

  13. Singlet-triplet annihilation in single LHCII complexes.

    PubMed

    Gruber, J Michael; Chmeliov, Jevgenij; Krüger, Tjaart P J; Valkunas, Leonas; van Grondelle, Rienk

    2015-08-14

    In light harvesting complex II (LHCII) of higher plants and green algae, carotenoids (Cars) have an important function to quench chlorophyll (Chl) triplet states and therefore avoid the production of harmful singlet oxygen. The resulting Car triplet states lead to a non-linear self-quenching mechanism called singlet-triplet (S-T) annihilation that strongly depends on the excitation density. In this work we investigated the fluorescence decay kinetics of single immobilized LHCIIs at room temperature and found a two-exponential decay with a slow (3.5 ns) and a fast (35 ps) component. The relative amplitude fraction of the fast component increases with increasing excitation intensity, and the resulting decrease in the fluorescence quantum yield suggests annihilation effects. Modulation of the excitation pattern by means of an acousto-optic modulator (AOM) furthermore allowed us to resolve the time-dependent accumulation and decay rate (∼7 μs) of the quenching species. Inspired by singlet-singlet (S-S) annihilation studies, we developed a stochastic model and then successfully applied it to describe and explain all the experimentally observed steady-state and time-dependent kinetics. That allowed us to distinctively identify the quenching mechanism as S-T annihilation. Quantitative fitting resulted in a conclusive set of parameters validating our interpretation of the experimental results. The obtained stochastic model can be generalized to describe S-T annihilation in small molecular aggregates where the equilibration time of excitations is much faster than the annihilation-free singlet excited state lifetime. PMID:26156159

  14. Compact conscious animal positron emission tomography scanner

    DOEpatents

    Schyler, David J.; O'Connor, Paul; Woody, Craig; Junnarkar, Sachin Shrirang; Radeka, Veljko; Vaska, Paul; Pratte, Jean-Francois; Volkow, Nora

    2006-10-24

    A method of serially transferring annihilation information in a compact positron emission tomography (PET) scanner includes generating a time signal for an event, generating an address signal representing a detecting channel, generating a detector channel signal including the time and address signals, and generating a composite signal including the channel signal and similarly generated signals. The composite signal includes events from detectors in a block and is serially output. An apparatus that serially transfers annihilation information from a block includes time signal generators for detectors in a block and an address and channel signal generator. The PET scanner includes a ring tomograph that mounts onto a portion of an animal, which includes opposing block pairs. Each of the blocks in a block pair includes a scintillator layer, detection array, front-end array, and a serial encoder. The serial encoder includes time signal generators and an address signal and channel signal generator.

  15. The HAWC Sensitivity to Dark Matter Annihilation and Decay

    NASA Astrophysics Data System (ADS)

    Yapici, Tolga; HAWC Collaboration

    2016-03-01

    The High Altitude Water Cherenkov (HAWC) Observatory is an extensive air shower array in the state of Puebla, Mexico at an altitude of 4100m. The HAWC observatory will perform an indirect search for dark matter via GeV-TeV photons resulting from dark matter annihilation and decay, including annihilation from extended dark matter sources. We consider the HAWC sensitivity to a subset of the sources, including the M31 galaxy, the Virgo cluster, and the Galactic center. We simulate the HAWC response to gamma rays from the sources in well-motivated dark matter annihilation channels. We show the limits HAWC can place on the dark matter cross-section or lifetime from these sources if gamma-ray excess is not observed. In particular, for dark matter annihilating into gauge bosons, HAWC will be able to measure a narrow range of dark matter masses to cross-sections below that expected for a thermal relic. HAWC should also be sensitive to cross-sections higher than thermal for masses up to nearly 1000 TeV. HAWC will be sensitive to decaying dark matter for these masses as well. HAWC can explore higher dark matter masses than are currently constrained.

  16. Luminosity Lifetime

    SciTech Connect

    Zisman, M.S.

    1997-04-01

    In a symmetric or 'energy transparent' relativistic collider, the luminosity is given by L = N{sup 2}f{sub c}/4{pi}{sigma}*{sub x}{sigma}*{sub y} where N is the number of electrons or positrons per bunch, {sigma}*{sub x} ({sigma}*{sub y}) is the horizontal (vertical) rms beam size at the interaction point (IP), and f{sub c} is the collision frequency. If the beam sizes remain constant as the luminosity decreases, then the time dependence of luminosity is contained entirely in the time dependence of the beam currents, i.e., N O N(t), and we can rewrite the equation as L(t) = N{sup 2}(t)f{sub c}/4{pi}{sigma}*{sub x}{sigma}*{sub y}. There are two distinct categories for luminosity loss. In the first category are loss processes due to collisions between the two beams, that is, processes associated directly with the luminosity. In the second category (see below) are single-beam loss processes. The processes in the first category relevant to a high-energy collider are Bhabha scattering (e{sup +}e{sup -} O e{sup +}e{sup -}) and 'radiative' Bhabha scattering (e{sup +}e{sup -} O e{sup +}e{sup -}{gamma}). In the first process, a beam particle is lost if its angular deflection is beyond the ring's transverse acceptance; in the second process, loss occurs if the beam particle's momentum change is outside the longitudinal acceptance of the ring (typically determined by the RF bucket height).

  17. An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays.

    PubMed

    Weidenspointner, Georg; Skinner, Gerry; Jean, Pierre; Knödlseder, Jürgen; von Ballmoos, Peter; Bignami, Giovanni; Diehl, Roland; Strong, Andrew W; Cordier, Bertrand; Schanne, Stéphane; Winkler, Christoph

    2008-01-10

    Gamma-ray line radiation at 511 keV is the signature of electron-positron annihilation. Such radiation has been known for 30 years to come from the general direction of the Galactic Centre, but the origin of the positrons has remained a mystery. Stellar nucleosynthesis, accreting compact objects, and even the annihilation of exotic dark-matter particles have all been suggested. Here we report a distinct asymmetry in the 511-keV line emission coming from the inner Galactic disk ( approximately 10-50 degrees from the Galactic Centre). This asymmetry resembles an asymmetry in the distribution of low mass X-ray binaries with strong emission at photon energies >20 keV ('hard' LMXBs), indicating that they may be the dominant origin of the positrons. Although it had long been suspected that electron-positron pair plasmas may exist in X-ray binaries, it was not evident that many of the positrons could escape to lose energy and ultimately annihilate with electrons in the interstellar medium and thus lead to the emission of a narrow 511-keV line. For these models, our result implies that up to a few times 10(41) positrons escape per second from a typical hard LMXB. Positron production at this level from hard LMXBs in the Galactic bulge would reduce (and possibly eliminate) the need for more exotic explanations, such as those involving dark matter. PMID:18185581

  18. Positronic complexes with unnatural parity

    SciTech Connect

    Bromley, M. W. J.; Mitroy, J.; Varga, K.

    2007-06-15

    The structure of the unnatural parity states of PsH, LiPs, NaPs, and KPs are investigated with the configuration interaction and stochastic variational methods. The binding energies (in hartree) are found to be 8.17x10{sup -4}, 4.42x10{sup -4}, 15.14x10{sup -4}, and 21.80x10{sup -4}, respectively. These states are constructed by first coupling the two electrons into a configuration which is predominantly {sup 3}P{sup e}, and then adding a p-wave positron. All the active particles are in states in which the relative angular momentum between any pair of particles is at least L=1. The LiPs state is Borromean since there are no three-body bound subsystems (of the correct symmetry) of the (Li{sup +}, e{sup -}, e{sup -}, e{sup +}) particles that make up the system. The dominant decay mode of these states will be radiative decay into a configuration that autoionizes or undergoes positron annihilation.

  19. The multi-scattering model for calculations of positron spatial distribution in the multilayer stacks, useful for conventional positron measurements

    SciTech Connect

    Dryzek, Jerzy; Institute of Physics, Opole University, ul. Oleska 48, 45-052 Opole ; Siemek, Krzysztof

    2013-08-21

    The spatial distribution of positrons emitted from radioactive isotopes into stacks or layered samples is a subject of the presented report. It was found that Monte Carlo (MC) simulations using GEANT4 code are not able to describe correctly the experimental data of the positron fractions in stacks. The mathematical model was proposed for calculations of the implantation profile or positron fractions in separated layers or foils being components of a stack. The model takes into account only two processes, i.e., the positron absorption and backscattering at interfaces. The mathematical formulas were applied in the computer program called LYS-1 (layers profile analysis). The theoretical predictions of the model were in the good agreement with the results of the MC simulations for the semi infinite sample. The experimental verifications of the model were performed on the symmetrical and non-symmetrical stacks of different foils. The good agreement between the experimental and calculated fractions of positrons in components of a stack was achieved. Also the experimental implantation profile obtained using the depth scanning of positron implantation technique is very well described by the theoretical profile obtained within the proposed model. The LYS-1 program allows us also to calculate the fraction of positrons which annihilate in the source, which can be useful in the positron spectroscopy.

  20. Compton-backscattered annihilation radiation from the Galactic Center region

    NASA Technical Reports Server (NTRS)

    Smith, D. M.; Lin, R. P.; Feffer, P.; Slassi, S.; Hurley, K.; Matteson, J.; Bowman, H. B.; Pelling, R. M.; Briggs, M.; Gruber, D.

    1993-01-01

    On 1989 May 22, the High Energy X-ray and Gamma-ray Observatory for Nuclear Emissions, a balloon-borne high-resolution germanium spectrometer with an 18-deg FOV, observed the Galactic Center (GC) from 25 to 2500 keV. The GC photon spectrum is obtained from the count spectrum by a model-independent method which accounts for the effects of passive material in the instrument and scattering in the atmosphere. Besides a positron annihilation line with a flux of (10.0 +/- 2.4) x 10 exp -4 photons/sq cm s and a full width at half-maximum (FWHM) of (2.9 + 1.0, -1.1) keV, the spectrum shows a peak centered at (163.7 +/- 3.4) keV with a flux of (1.55 +/- 0.47) x 10 exp -3 photons/sq cm s and a FWHM of (24.4 +/- 9.2) keV. The energy range 450-507 keV shows no positronium continuum associated with the annihilation line, with a 2-sigma upper limit of 0.90 on the positronium fraction. The 164 keV feature is interpreted as Compton backscatter of broadened and redshifted annihilation radiation, possibly from the source 1E 1740.7-2942.

  1. A new look at the cosmic ray positron fraction

    NASA Astrophysics Data System (ADS)

    Boudaud, M.; Aupetit, S.; Caroff, S.; Putze, A.; Belanger, G.; Genolini, Y.; Goy, C.; Poireau, V.; Poulin, V.; Rosier, S.; Salati, P.; Tao, L.; Vecchi, M.

    2015-03-01

    Context. The positron fraction in cosmic rays has recently been measured with improved accuracy up to 500 GeV, and it was found to be a steadily increasing function of energy above ~10 GeV. This behaviour contrasts with standard astrophysical mechanisms, in which positrons are secondary particles, produced in the interactions of primary cosmic rays during their propagation in the interstellar medium. The observed anomaly in the positron fraction triggered a lot of excitement, as it could be interpreted as an indirect signature of the presence of dark matter species in the Galaxy, the so-called weakly interacting massive particles (WIMPs). Alternatively, it could be produced by nearby sources, such as pulsars. Aims: These hypotheses are probed in light of the latest AMS-02 positron fraction measurements. As regards dark matter candidates, regions in the annihilation cross section to mass plane, which best fit the most recent data, are delineated and compared to previous measurements. The explanation of the anomaly in terms of a single nearby pulsar is also explored. Methods: The cosmic ray positron transport in the Galaxy is described using a semi-analytic two-zone model. Propagation is described with Green functions as well as with Bessel expansions. For consistency, the secondary and primary components of the positron flux are calculated together with the same propagation model. The above mentioned explanations of the positron anomaly are tested using χ2 fits. The numerical package MicrOMEGAs is used to model the positron flux generated by dark matter species. The description of the positron fraction from conventional astrophysical sources is based on the pulsar observations included in the Australia Telescope National Facility (ATNF) catalogue. Results: The masses of the favoured dark matter candidates are always larger than 500 GeV, even though the results are very sensitive to the lepton flux. The Fermi measurements point systematically to much heavier

  2. Excited S-symmetry states of positronic lithium and beryllium

    NASA Astrophysics Data System (ADS)

    Strasburger, Krzysztof

    2016-04-01

    The possibility of the existence of excited S-symmetry states of positronic lithium and beryllium, resulting from the positron attachment to high-spin P parent atomic states, is examined and confirmed with variational calculations in the basis of explicitly correlated Gaussian functions. The unexpectedly different order of the energies of the S and P states is explained by the formation of the positronium cluster structure and associated disappearance of the destabilizing centrifugal force. The annihilation properties of newly discovered states are discussed in the context of prospective experimental detection.

  3. Excited S-symmetry states of positronic lithium and beryllium.

    PubMed

    Strasburger, Krzysztof

    2016-04-14

    The possibility of the existence of excited S-symmetry states of positronic lithium and beryllium, resulting from the positron attachment to high-spin P parent atomic states, is examined and confirmed with variational calculations in the basis of explicitly correlated Gaussian functions. The unexpectedly different order of the energies of the S and P states is explained by the formation of the positronium cluster structure and associated disappearance of the destabilizing centrifugal force. The annihilation properties of newly discovered states are discussed in the context of prospective experimental detection. PMID:27083730

  4. Models for the positive latitude e{-}e{+} annihilation feature

    NASA Astrophysics Data System (ADS)

    von Ballmoos, P.; Guessoum, N.; Jean, P.; Knödlseder, J.

    2003-01-01

    Galactic maps of e-e+ annihilation radiation based on CGRO-OSSE, SMM and TGRS data have indicated the existence of an extended component at positive Galactic latitudes (l~ -2 deg, b~ 7 deg), in addition to the emission from the galactic bulge and disk (Purcell et al. \\cite{Purcell97}; Cheng et al. \\cite{Cheng97}; Milne et al. \\cite{Milne00}; Milne et al. \\cite{Milne01}). This Positive Latitude Enhancement (PLE) was first attributed to an ``annihilation fountain" in the Galactic center (Dermer & Skibo \\cite{Dermer97}) but has since been the object of several models. After discussing the observational evidence for the PLE, we investigate various models for the PLE: besides the scenarios proposed in the literature, we have introduced a number of models requiring relatively modest positron rates due to a local origin of the e-e+ emission (local galactic-, solar system-, earth- and spacecraft-environment origins). The various scenarios for the PLE are constrained in the light of the latest OSSE-SMM-TGRS data analysis results: we have looked at the possible positron production mechanisms as well as the annihilation conditions in the different physical environments (temperature and dust grain content) proposed for the positive-latitude region. By constraining those parameters, based on the recent limits for the line width and the positronium fraction, we found that some of the models can essentially be discarded. A number of other scenarios will have to await further measurements and maps, such as will be possible with INTEGRAL's SPI and IBIS instruments. We present a table/checklist of model-falsification criteria.

  5. Atomic Composition of the Positron-Sensitive Vacancy Complexes in GaN

    NASA Astrophysics Data System (ADS)

    Arutyunov, Nikolai; Emtsev, Vadim

    2004-03-01

    The positron probing of the as-grown point defects in GaN and related materials has been conducted by the one-dimensional angular correlation of the annihilation radiation (1D-ACAR) as well as by some other positron annihilation techniques. It has been established that the electron density around the positron is lowered suggesting that positrons are trapped by the defects of a vacancy type in the crystal lattice of GaN. The electron-positron radii fitted to the conventional ion radii for the relevant anion and cation sites have formed a basis for a comparative analysis of the obtained results. The electron-positron cation core radius whose length is determined by the atoms in the nearest environment of the annihilating positrons has been estimated by the parameters of the high-momentum component of 1D-ACAR curves. The electron-positron momentum distribution outside the cation cores has been used for the estimations of the electron-positron anion core radius. A comparison of the electron-positron ion radii with a certain standards obtained for B, Al, Ga, related III-V compounds GaP, GaAs, GaSb, BN, and AlN has been done systematically at the processing of results. A plausible explanation of the experimental data obtained for GaN may be given assuming that the positron annihilation occurs in the complex of a vacancy-type where the nitrogen atom is shifted from its regular position and, probably, is trapped by the gallium vacancy forming the antisite configuration (N.Yu. Arutyunov, A.V.Mikhailin, V.Yu. Davidov, V.V.Emtsev, G.A. Oganesyan, and E.E.Haller, Semiconductors, 36 (2002) 1106.) (N.Yu. Arutyunov, V.V.Emtsev, A.V.Mikhailin, and C.J. Humphreys, Physica B, (2003), in press.) Similar complexes in GaN have been discussed theoretically. (D.J. Chadi, Appl. Phys. Lett. 71 (1997) 2970.) In this connection the results of the positron annihilation studies in GaN:Mg are also considered.

  6. Elastic positron-cadmium scattering at low energies

    SciTech Connect

    Bromley, M. W. J.; Mitroy, J.

    2010-05-15

    The elastic and annihilation cross sections for positron-cadmium scattering are reported up to the positronium-formation threshold (at 2.2 eV). The low-energy phase shifts for the elastic scattering of positrons from cadmium were derived from the bound and pseudostate energies of a very large basis configuration-interaction calculation of the e{sup +}-Cd system. The s-wave binding energy is estimated to be 126{+-}42 meV, with a scattering length of A{sub scat}=(14.2{+-}2.1)a{sub 0}, while the threshold annihilation parameter, Z{sub eff}, was 93.9{+-}26.5. The p-wave phase shift exhibits a weak shape resonance that results in a peak Z{sub eff} of 91{+-}17 at a collision energy of about 490{+-}50 meV.

  7. Photon spectra from WIMP annihilation

    SciTech Connect

    Cembranos, J. A. R.; Cruz-Dombriz, A. de la; Dobado, A.; Maroto, A. L.; Lineros, R. A.

    2011-04-15

    If the present dark matter in the Universe annihilates into standard model particles, it must contribute to the fluxes of cosmic rays that are detected on the Earth and, in particular, to the observed gamma-ray fluxes. The magnitude of such a contribution depends on the particular dark matter candidate, but certain features of the produced photon spectra may be analyzed in a rather model-independent fashion. In this work we provide the complete photon spectra coming from WIMP annihilation into standard model particle-antiparticle pairs obtained by extensive Monte Carlo simulations. We present results for each individual annihilation channel and provide analytical fitting formulas for the different spectra for a wide range of WIMP masses.

  8. Effects of vertical aperture on beam lifetime at the Advanced Photon Source (APS) storage ring

    SciTech Connect

    Bizek, H.M.

    1995-06-01

    When a positron`s energy deviation {delta}E/E exceeds the rf acceptance, or when it receives an angular kick for the betatron motion that exceeds some limiting admittance, the positron will be lost. The main contributions to the total beam lifetime come from single Coulomb and Touschek scattering. In this report we investigate the dependence of the residual gas pressure and the vertical aperture of the Advanced Photon Source storage ring on the total beam. lifetime. We present results of calculating the total beam lifetime as a function of vertical aperture for varying average ring pressure, beam current, and coupling coefficient.

  9. Positron spectroscopy of 2D materials using an advanced high intensity positron beam

    NASA Astrophysics Data System (ADS)

    McDonald, A.; Chirayath, V.; Lim, Z.; Gladen, R.; Chrysler, M.; Fairchild, A.; Koymen, A.; Weiss, A.

    An advanced high intensity variable energy positron beam(~1eV to 20keV) has been designed, tested and utilized for the first coincidence Doppler broadening (CDB) measurements on 6-8 layers graphene on polycrystalline Cu sample. The system is capable of simultaneous Positron annihilation induced Auger electron Spectroscopy (PAES) and CDB measurements giving it unparalleled sensitivity to chemical structure at external surfaces, interfaces and internal pore surfaces. The system has a 3m flight path up to a micro channel plate (MCP) for the Auger electrons emitted from the sample. This gives a superior energy resolution for PAES. A solid rare gas(Neon) moderator was used for the generation of the monoenergetic positron beam. The positrons were successfully transported to the sample chamber using axial magnetic field generated with a series of Helmholtz coils. We will discuss the PAES and coincidence Doppler broadening measurements on graphene -Cu sample and present an analysis of the gamma spectra which indicates that a fraction of the positrons implanted at energies 7-60eV can become trapped at the graphene/metal interface. This work was supported by NSF Grant No. DMR 1508719 and DMR 1338130.

  10. Indirect and direct signatures of Higgs portal decaying vector dark matter for positron excess in cosmic rays

    SciTech Connect

    Baek, Seungwon; Ko, P.; Park, Wan-Il; Tang, Yong E-mail: pko@kias.re.kr E-mail: ytang@kias.re.kr

    2014-06-01

    We investigate the indirect signatures of the Higgs portal U(1){sub X} vector dark matter (VDM) X{sub μ} from both its pair annihilation and decay. The VDM is stable at renormalizable level by Z{sub 2} symmetry, and thermalized by Higgs-portal interactions. It can also decay by some nonrenormalizable operators with very long lifetime at cosmological time scale. If dim-6 operators for VDM decays are suppressed by 10{sup 16} GeV scale, the lifetime of VDM with mass ∼ 2 TeV is just right for explaining the positron excess in cosmic ray observed by PAMELA and AMS02 Collaborations. The VDM decaying into μ{sup +}μ{sup −} can fit the data, evading various constraints on cosmic rays. We give one UV-complete model as an example. This scenario for Higgs portal decaying VDM with mass around ∼ 2 TeV can be tested by DM direct search at XENON1T, and also at the future colliders by measuring the Higgs self-couplings.

  11. Precise tests of QCD in e{sup +}e{sup {minus}} annihilation

    SciTech Connect

    Burrows, P.N.

    1997-03-01

    A pedagogical review is given of precise tests of QCD in electron-positron annihilation. Emphasis is placed on measurements that have served to establish QCD as the correct theory of strong interactions, as well as measurements of the coupling parameter {alpha}{sub s}. An outlook is given for future important tests at a high-energy e{sup +}e{sup {minus}} collider.

  12. Measurements of heavy quark and lepton lifetimes

    SciTech Connect

    Jaros, J.A.

    1985-02-01

    The PEP/PETRA energy range has proved to be well-suited for the study of the lifetimes of hadrons containing the b and c quarks and the tau lepton for several reasons. First, these states comprise a large fraction of the total interaction rate in e/sup +/e/sup -/ annihilation and can be cleanly identified. Second, the storage rings have operated at high luminosity and so produced these exotic states copiously. And finally, thanks to the interplay of the Fermi coupling strength, the quark and lepton masses, and the beam energy, the expected decay lengths are in the 1/2 mm range and so are comparatively easy to measure. This pleasant coincidence of cleanly identified and abundant signal with potentially large effects has made possible the first measurements of two fundamental weak couplings, tau ..-->.. nu/sub tau/W and b ..-->.. cW. These measurements have provided a sharp test of the standard model and allowed, for the first time, the full determination of the magnitudes of the quark mixing matrix. This paper reviews the lifetime studies made at PEP during the past year. It begins with a brief review of the three detectors, DELCO, MAC and MARK II, which have reported lifetime measurements. Next it discusses two new measurements of the tau lifetime, and briefly reviews a measurement of the D/sup 0/ lifetime. Finally, it turns to measurements of the B lifetime, which are discussed in some detail. 18 references, 14 figures, 1 table.

  13. Annihilation of defects in liquid crystals

    NASA Astrophysics Data System (ADS)

    Svetec, M.; Ambrožič, M.; Kralj, S.

    The annihilation of defect is studied theoretically in liquid crystals (LCs). We consider the annihilation of point disclinations in nematic and line edge dislocations in smectic A LC phase, respectively. We stress qualitative similarities in these processes. The whole annihilation regime is taken into account, consisting of the pre-collision, collision, and post-collision stage.

  14. Extracting the Size of the Cosmic Electron-Positron Anomaly

    NASA Astrophysics Data System (ADS)

    Auchettl, Katie; Balazs, C.

    2011-09-01

    Over the last few decades, numerous observations have hinted at an excess of high energy positrons in our locality. The most recent of these experiments has been the positron fraction measured by the PAMELA satellite and the electron plus positron spectrum as measured by the Fermi-LAT satellite. Since the release of these measurements, there have been a plethora of papers where authors invoke new physics ranging from, modification of the cosmic ray propagation, supernova remnants and dark matter annihilation. Using a Bayesian likelihood analysis, we isolate the anomalous contribution of the cosmic electron-positron flux. A significant tension was found between the electron positron related data and non-electron-positron cosmic ray fluxes. Using 219 recent cosmic ray datum, we extracted the preferred values of the selected cosmic ray propagation parameters from the non-electron-positron related measurements. Based on these parameter values we calculated background predictions with uncertainties for PAMELA and Fermi-LAT. We found a deviation between the PAMELA and Fermi-LAT data and the predicted background even when uncertainties, including systematics, were taken into account. Interpreting this as a hint of new physics, we subtracted the background from the data extracting the size, shape and uncertainty of the anomalous contribution in a model independent fashion. We briefly compared the extracted signal to some theoretical results predicting such an anomaly.

  15. Induced Positron Annihiliation Investigation of Cadmium Zinc Telluride Crystal Microstructures

    SciTech Connect

    D. W. Akers

    2005-06-01

    Cadmium-Zinc-Telluride (CZT) crystals are used in semiconductor radiation detectors for the detection of x-ray and gamma radiation. However, production of detector grade crystals is difficult as small variations in compositional uniformity and primarily the zinc content can significantly affect the ability of the CZT crystal to function as a radiation detector. Currently there are no known nondestructive methods that can be used to identify detector grade crystals. The current test method is to fabricate and test the detector to determine if the crystal is sufficiently uniform and of the correct composition to be considered a detector grade crystal. Consequently, nondestructive detection methods are needed to identify detector grade crystals prior to the fabrication process. The purpose of this feasibility study was to perform a preliminary assessment of the ability of several new, nondestructive technologies based on Induced Positron Annihilation (IPA) to determine if detector grade CZT crystals can be identified. Results of measurements performed on specimens from Fisk University and EV Products, Inc. indicate that both the near surface Distributed Source Positron Annihilation (up to 3 mm penetration) and the volumetric Photon Induced Positron Annihilation methods may be suitable for determining CZT crystal quality. Further work on CZT crystals with a broader range of compositions and detector characteristics is needed to provide a well defined, calibrated, method for assessing CZT crystal quality.

  16. PIMC Simulation of Ps Annihilation: From Micro to Mesopores

    SciTech Connect

    Bug, A R; Sterne, P A

    2005-08-23

    Path Integral Monte Carlo (PIMC) can reproduce the results of simple analytical calculations in which a single quantum particle is used to represent positronium within an idealized, spherical pore. Our calculations improve on this approach by explicitly treating the positronium as a two-particle e{sup -}, e{sup +} system interacting via the Coulomb interaction. We study the lifetime and the internal contact density, {kappa}, which controls the self-annihilation behavior, for positronium in model spherical pores, as a function of temperature and pore size. We compare the results with both PIMC and analytical calculations for a single-particle model.

  17. A fiber-optically coupled positron-sensitive surgical probe

    SciTech Connect

    Raylman, R.R.; Wahl, R.L.

    1994-05-01

    Positron-emitting radiopharmaceuticals such as {sup 18}F-labeled 2-deoxy-D-glucose (FDG) have considerable utility in the noninvasive imaging of cancers due to their rapid and excellent tumor-localizing properties. In addition, the relatively short range of positrons in tissue facilitates the precise delineation of FDG-avid tumors. Therefore, FDG used in conjunction with a positron-sensitive probe may be capable of guiding surgical procedures. Many of the current probe systems, however, are sensitive to the intense flux of background photons produced by positron annihilation. The authors describe the design, manufacture and initial in vitro and in vivo testing of a probe well-suited to the detection of positron-emitting isotopes in a high-photon background. The device consists of a small piece of plastic scintillator coupled by fiber-optic cable to a photomultiplier tube. Measurements of resolution and detector sensitivity were obtained. In addition, the reduction in resolution caused by the effects of various levels of background photon flux was determined. These measurements indicate that resolution is degraded minimally ({approximately}5% with a background-to-source ratio of 2:1) due to annihilation photon background. Sensitivity for positrons is good, detecting amounts of radioactivity as low as 10.2 nCi of FDG in vitro. In rats given FDG subcutaneously, lymph nodes containing as little as 11 nCi of FDG could be detected above the background activity levels present in normal surrounding tissues. A plastic scintillator probe system has been devised which may be highly suitable for intraoperative FDG-guided (or other positron or beta emitting-tracer) surgery. 29 refs., 7 figs.

  18. p -wave annihilating dark matter from a decaying predecessor and the Galactic Center excess

    NASA Astrophysics Data System (ADS)

    Choquette, Jeremie; Cline, James M.; Cornell, Jonathan M.

    2016-07-01

    Dark matter (DM) annihilations have been widely studied as a possible explanation of excess gamma rays from the Galactic Center seen by Fermi/LAT. However most such models are in conflict with constraints from dwarf spheroidals. Motivated by this tension, we show that p -wave annihilating dark matter can easily accommodate both sets of observations due to the lower DM velocity dispersion in dwarf galaxies. Explaining the DM relic abundance is then challenging. We outline a scenario in which the usual thermal abundance is obtained through s -wave annihilations of a metastable particle, that eventually decays into the p -wave annihilating DM of the present epoch. The couplings and lifetime of the decaying particle are constrained by big bang nucleosynthesis, the cosmic microwave background and direct detection, but significant regions of parameter space are viable. A sufficiently large p -wave cross section can be found by annihilation into light mediators, that also give rise to Sommerfeld enhancement. A prediction of the scenario is enhanced annihilations in galaxy clusters.

  19. AC quantum efficiency harmonic analysis of exciton annihilation in organic light emitting diodes (Presentation Recording)

    NASA Astrophysics Data System (ADS)

    Giebink, Noel C.

    2015-10-01

    Exciton annihilation processes impact both the lifetime and efficiency roll-off of organic light emitting diodes (OLEDs), however it is notoriously difficult to identify the dominant mode of annihilation in operating devices (exciton-exciton vs. exciton-charge carrier) and subsequently to disentangle its magnitude from competing roll-off processes such as charge imbalance. Here, we introduce a simple analytical method to directly identify and extract OLED annihilation rates from standard light-current-voltage (LIV) measurement data. The foundation of this approach lies in a frequency domain EQE analysis and is most easily understood in analogy to impedance spectroscopy, where in this case both the current (J) and electroluminescence intensity (L) are measured using a lock-in amplifier at different harmonics of the sinusoidal dither superimposed on the DC device bias. In the presence of annihilation, the relationship between recombination current and light output (proportional to exciton density) becomes nonlinear, thereby mixing the different EQE harmonics in a manner that depends uniquely on the type and magnitude of annihilation. We derive simple expressions to extract different annihilation rate coefficients and apply this technique to a variety of OLEDs. For example, in devices dominated by triplet-triplet annihilation, the annihilation rate coefficient, K_TT, is obtained directly from the linear slope that results from plotting EQE_DC-EQE_1ω versus L_DC (2EQE_1ω-EQE_DC). We go on to show that, in certain cases it is sufficient to calculate EQE_1ω directly from the slope of the DC light versus current curve [i.e. via (dL_DC)/(dJ_DC )], thus enabling this analysis to be conducted solely from common LIV measurement data.

  20. Vacancy-type defects in In{sub x}Ga{sub 1−x}N grown on GaN templates probed using monoenergetic positron beams

    SciTech Connect

    Uedono, Akira; Watanabe, Tomohito; Kimura, Shogo; Zhang, Yang; Lozac'h, Mickael; Sang, Liwen; Sumiya, Masatomo; Ishibashi, Shoji; Oshima, Nagayasu; Suzuki, Ryoichi

    2013-11-14

    Native defects in In{sub x}Ga{sub 1−x}N layers grown by metalorganic chemical vapor deposition were studied using monoenergetic positron beams. Measurements of Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons for a 200-nm-thick In{sub 0.13}Ga{sub 0.87}N layer showed that vacancy-type defects were introduced by InN alloying, and the major species of such defects was identified as complexes between a cation vacancy and nitrogen vacancies. The presence of the defects correlated with lattice relaxation of the In{sub 0.13}Ga{sub 0.87}N layer and the increase in photon emissions from donor-acceptor-pair recombination. The species of native defects in In{sub 0.06}Ga{sub 0.94}N layers was the same but its concentration was decreased by decreasing the InN composition. With the layer thickness increased from 120 nm to 360 nm, a defect-rich region was introduced in the subsurface region (<160 nm), which can be associated with layer growth with the relaxation of compressive stress.