Sample records for positron annihilation lifetime

  1. Tomographic Positron Annihilation Lifetime Spectroscopy

    NASA Astrophysics Data System (ADS)

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

    2014-04-01

    Positron annihilation lifetime spectroscopy serves as a perfect tool for studies of open-volume defects in solid materials such as vacancies, vacancy agglomerates, and dislocations. Moreover, structures in porous media can be investigated ranging from 0.3 nm to 30 nm employing the variation of the Positronium lifetime with the pore size. While lifetime measurements close to the material's surface can be performed at positron-beam installations bulk materials, fluids, bio-materials or composite structures cannot or only destructively accessed by positron beams. Targeting those problems, a new method of non-destructive positron annihilation lifetime spectroscopy has been developed which features even a 3-dimensional tomographic reconstruction of the spatial lifetime distribution. A beam of intense bremsstrahlung is provided by the superconducting electron linear accelerator ELBE (Electron Linear Accelerator with high Brilliance and low Emittance) at Helmholtz-Zentrum Dresden-Rossendorf. 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 lifetime studies. 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. The detector system will be described and results for experiments using samples with increasing complexity will be presented. The Lu2SiO5:Ce scintillation crystals allow resolving the total energy to 5.1 % (root-mean-square, RMS) and the annihilation lifetime to 225 ps (RMS). 3-dimensional annihilation lifetime maps have been created in an offline-analysis employing well-known techniques from PET.

  2. 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.

  3. 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.

  4. 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.

  5. 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 trapped Ps atom. We conclude that there is no justification for a one-to-one correlation of lifetime components with specific hole sizes.

  6. Positron annihilation lifetime measurement of irradiated stainless steels

    NASA Astrophysics Data System (ADS)

    Kuramoto, E.; Tsukuda, N.; Aono, Y.; Takenaka, M.; Takano, Y.; Yoshida, H.; Shiraishi, K.

    1985-08-01

    Two types of austenitic stainless steels (316SS and JPCA) and two types of ferritic/martensitic stainless steels (HT-9 and JFMS) were irradiated by high energy electrons at 77 K and positron annihilation lifetime measurements were carried out to obtain the isochronal annealing behaviour above room temperature. The main decrease of the intensity of the second component, namely, the migration of vacancies to sinks was observed at 250°C for austenitic stainless steels and at 150°C for ferritic/martensitic stainless steels. By assuming the number of jumps to sinks as 10 3, the vacancy migration energy was obtained as 1.36 and 1.10 eV for austenitic and ferritic/martensitic stainless steels, respectively. This result was used to discuss the low swelling behaviour of the ferritic/martensitic stainless steels.

  7. Single shot positron annihilation lifetime spectroscopy D. B. Cassidy,a

    E-print Network

    Mills, Allen P.

    Single shot positron annihilation lifetime spectroscopy D. B. Cassidy,a S. H. M. Deng, H. K. M in positron trapping technology have made possible experimentation with dense interacting positronium gases developed a method to measure positronium lifetimes from a single intense burst of positrons. Our method

  8. Materials characterization of free volume and void properties by two-dimensional positron annihilation lifetime spectroscopy

    NASA Astrophysics Data System (ADS)

    Chen, Hongmin; Van Horn, J. David; Jean, Y. C.; Hung, Wei-Song; Lee, Kueir-Rarn

    2013-04-01

    Positron annihilation lifetime spectroscopy (PALS) has been widely used to determine the free volume and void properties in polymeric materials. Recently, a two dimensional positron annihilation lifetime spectroscopy (2DPALS) system has been developed for membrane applications. The system measures the coincident signals between the lifetime and the energy which could separate the 2? and 3? annihilations and improve the accuracy in the determination of the free volume and void properties. When 2D-PALS is used in coupling with a variable mono-energy slow positron beam, it could be applied to a variety of material characterization. Results of free volumes and voids properties in a multi-layer polymer membrane characterized using 2D-PALS are presented.

  9. Digitized detection of gamma-ray signals concentrated in narrow time windows for transient positron annihilation lifetime spectroscopy

    NASA Astrophysics Data System (ADS)

    Kinomura, A.; Suzuki, R.; Oshima, N.; O'Rourke, B. E.; Nishijima, T.; Ogawa, H.

    2014-12-01

    A pulsed slow-positron beam generated by an electron linear accelerator was directly used for positron annihilation lifetime spectroscopy without any positron storage devices. A waveform digitizer was introduced to simultaneously capture multiple gamma-ray signals originating from positron annihilation events during a single accelerator pulse. The positron pulse was chopped and bunched with the chopper signals also sent to the waveform digitizer. Time differences between the annihilation gamma-ray and chopper peaks were calculated and accumulated as lifetime spectra in a computer. The developed technique indicated that positron annihilation lifetime spectroscopy can be performed in a 20 ?s time window at a pulse repetition rate synchronous with the linear accelerator. Lifetime spectra of a Kapton sheet and a thermally grown SiO2 layer on Si were successfully measured. Synchronization of positron lifetime measurements with pulsed ion irradiation was demonstrated by this technique.

  10. Positron Annihilation Lifetime Study of Steel Surface Modification by Shot Peening

    NASA Astrophysics Data System (ADS)

    Zaleski, Rados?aw; Gorgol, Marek; Zaleski, Kazimierz

    Steel made machine components are often exposed to variable loads during the operation. Thus, their important characteristic is fatigue strength, which largely depends on the residual stress in the surface layer of these elements. The fatigue strength of components can be increased e.g. by exposing them to shot peening. Influence of shot peening on the defect structure in the samples observed by positron annihilation lifetime spectroscopy was studied for various steels. The differences between unprocessed and shot peened samples reflect mostly in the intensity ratio decrease of the components attributed to monovacancies and vacancy clusters. Shot peening result in uniformization of the vacancy clusters size. The carburized steel does not exhibit any changes in positron annihilation lifetime spectra caused by shot peening. On the other hand, the changes in the chromium plated steel are considerable. Their origin lies is compression of the microcracks present in the chromium layer caused by shot peening.

  11. Positron annihilation lifetime measurements of austenitic stainless and ferritic/martensitic steels irradiated in the SINQ target irradiation program

    NASA Astrophysics Data System (ADS)

    Sato, K.; Xu, Q.; Yoshiie, T.; Dai, Y.; Kikuchi, K.

    2012-12-01

    Titanium-doped austenitic stainless steel (JPCA) and reduced activated ferritic/martensitic steel (F82H) irradiated with high-energy protons and spallation neutrons were investigated by positron annihilation lifetime measurements. Subnanometer-sized (<˜0.8 nm) helium bubbles, which cannot be observed by transmission electron microscopy, were detected by positron annihilation lifetime measurements for the first time. For the F82H steel, the positron annihilation lifetime of the bubbles decreased with increasing irradiation dose and annealing temperature because the bubbles absorb additional He atoms. In the case of JPCA steel, the positron annihilation lifetime increased with increasing annealing temperature above 773 K, in which case the dissociation of complexes of vacancy clusters with He atoms and the growth of He bubbles was detected. He bubble size and density were also discussed.

  12. Alkali-doped heavy metal fluoride glasses studied by positron annihilation lifetime spectroscopy

    Microsoft Academic Search

    A. J. Hill; P. J. Newman; J. Javorniczky; D. R. MacFarlane

    1995-01-01

    Heavy metal fluoride glasses of varying alkali metal fluoride have been studied by positron annihilation lifetime spectroscopy. The alkali series 53ZrF4 · (40 - x)BaF2 · 4LaF3 · 3AlF3 · xLiF and 53ZrF4 · (40 - x)BaF2 · 4LaF3 · 3AlF3 · xCsF are compared with the mixed alkali series 53ZrF4 · 20BaF2 · 4LaF3 · 4LaF3 · 3AlF3 ·

  13. 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

    2015-07-01

    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

  14. Positron Lifetimes in Metals

    Microsoft Academic Search

    Howard Weisberg; Stephan Berko

    1967-01-01

    Positron decay curves in various metals have been measured, using careful sample preparation techniques and a delayed-coincidence system with 0.30-nsec (full width at half-maximum) prompt time resolution and 104:1 peak-to-background ratio. Evidence was found that the annihilation in pure metals under ideal conditions is characterized by a single exponential decay, the second lifetime usually observed being an artifact of sample

  15. Detection of interstitial clusters in neutron irradiated Ni?Hf alloy by perturbed angular correlation and positron annihilation lifetime measurements

    NASA Astrophysics Data System (ADS)

    Okazawa, H.; Yoshiie, T.; Ishizai, T.; Sato, K.; Xu, Q.; Satoh, Y.; Ohkubo, Y.; Kawase, Y.

    2004-08-01

    In order to understand the effect of alloying elements on the damage structure evolution in metals, defect clusters near Hf in Ni were studied using the perturbed angular correlation (PAC) technique and positron annihilation spectroscopy (PAS). The volume size factor of Hf in Ni was determined to be 61% by measuring the lattice parameter. The positron annihilation mean lifetime of Ni-0.5 at.%Hf after neutron irradiation at 473 K up to a dose of 0.0053 dpa was 132 ps. The lifetime decreased with increasing annealing temperature and by annealing at 723 K the recovery was finished. The PAC spectrum of 181Ta (? 181Hf) taken after the irradiation indicated that there were two components. The first component consisted of a Larmor frequency of 534 mega-radian/s (Mrad/s). The second consisted of a very broad range of frequencies which brought destructive interference among them and made the contribution to the spectrum almost zero. The component disappeared with annealing at 873 K. It was concluded that the first component and the second component represent Hf in the regular substitutional site and Hf with defect clusters, respectively. From the difference of annealing out temperature of defect clusters, clusters annihilated by 723 K were assigned as stacking fault tetrahedra (SFTs) and those by 873 K as interstitial clusters.

  16. Positron annihilation in germanium

    Microsoft Academic Search

    M. A. Shulman; G. M. Beardsley; S. Berko

    1975-01-01

    High precision long-slit and cross-slit geometry angular distributions of annihilation radiation from oriented germanium are\\u000a presented. The momentum distributions from the long-slit measurements are compared with recent Compton profile data to test\\u000a the importance of the positron wavefunction and positron-electron correlations. Evidence for Umklapp annihilation is discussed.\\u000a No observable differences are found between distributions from heavily doped and intrinsic germanium

  17. Positron annihilation in flight

    NASA Astrophysics Data System (ADS)

    Tudor Jones, Goronwy

    1999-09-01

    In this resource article, an exceptional bubble chamber picture - showing the annihilation of a positron (antielectron e+ ) in flight - is discussed in detail. Several other esoteric phenomena (some not easy to show on their own!) also manifest themselves in this picture - pair creation or the materialization of a high energy photon into an electron-positron pair; the `head-on' collision of a positron with an electron, from which the mass of the positron can be estimated; the Compton Effect ; an example of the emission of electromagnetic radiation (photons) by accelerating charges (bremsstrahlung ).

  18. Variation of free volume size and content of shape memory polymer — Polyurethane — Upon temperature studied by positron annihilation lifetime techniques and infrared spectroscopy

    Microsoft Academic Search

    K. Ito; K. Abe; H. L. Li; Y. Ujihira; N. Ishikawa; S. Hayashi

    1996-01-01

    Positron annihilation lifetime measurement and Fourier transform infrared spectrometry were applied to the study of temperature dependencies of free volume parameters and hydrogen bonds in segmented polyurethane, specially fabricated as a shape memory polymer. The variation of free volumes in amorphous region were correlated to that of hydrogen bonding and the shape memory mechanism of polyurethane is elucidated from a

  19. 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.

  20. 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.

  1. Positron annihilation processes update

    NASA Technical Reports Server (NTRS)

    Guessoum, Nidhal; Skibo, Jeffrey G.; Ramaty, Reuven

    1997-01-01

    The present knowledge concerning the positron annihilation processes is reviewed, with emphasis on the data of the cross sections of the various processes of interest in astrophysical applications. Recent results are presented including results on reaction rates and line widths, the validity of which is verified.

  2. 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.

  3. Evolution of the positron annihilation lifetime for ageing in {beta} phase Cu-Al-Ni-(Ti)-(Mn) shape memory alloys

    SciTech Connect

    Hurtado, I.; Van Humbeeck, J. [Katholieke Universiteit Leuven (Belgium). Dept. Metaalkunde en Toegepaste Materiaalkunde] [Katholieke Universiteit Leuven (Belgium). Dept. Metaalkunde en Toegepaste Materiaalkunde; Segers, D.; Dorikens-Vanpraet, L.; Dauwe, C. [Rijksuniversiteit Gent (Belgium). Vakgroep Subatomaire en Stralingsfysica] [Rijksuniversiteit Gent (Belgium). Vakgroep Subatomaire en Stralingsfysica

    1995-09-01

    During the last years, the positron annihilation technique has proven to be a sensitive tool for the characterization of phase transitions, and in particular of the martensitic transformation in Cu-based alloys. The differences in structure between the high temperature phase (cubic) and the martensite (monoclinic) produce distinctive positron annihilation characteristics which allow the determination of the transformation temperature range. In the present article the influence of these parameters on the positron annihilation characteristics of three Cu-Al-Ni shape memory alloys is discussed. Two of the chosen compositions contain Ti and Mn, which have been added to the ternary Cu-Al-Ni alloy in order to improve its mechanical properties. The effect of those elements has also been evaluated.

  4. Comparative studies of positron annihilation lifetime and coincident Doppler broadening spectra for a binary Cd-based quasicrystal and 1/1-approximant crystal

    SciTech Connect

    Takagiwa, Y.; Kanazawa, I.; Sato, K.; Murakami, H.; Kobayashi, Y.; Tamura, R.; Takeuchi, S. [Department of Physics, Tokyo Gakugei University, 4-1-1 Koganei, Tokyo 184-0051 (Japan); Department of Environmental Science, Tokyo Gakugei University, 4-1-1 Koganei, Tokyo 184-0051 (Japan); National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan); Department of Materials Science and Technology, Tokyo University of Science, Noda, Chiba 278-8501 (Japan)

    2006-03-01

    We performed the positron annihilation lifetime and coincident Doppler broadening measurements for binary icosahedral quasicrystal Cd{sub 5.7}Ca and its 1/1-cubic Cd{sub 6}Ca and Cd{sub 6}Yb approximants. Since the obtained positron lifetimes are quite similar to one another, it is likely that the same type of structural vacancies exists in quasicrystal Cd{sub 5.7}Ca and 1/1-cubic Cd{sub 6}Ca and Cd{sub 6}Yb approximants. The vacancy-type defects are concluded to be surrounded mostly by Cd atoms in both quasicrystal Cd{sub 5.7}Ca and its 1/1-cubic approximant Cd{sub 6}Ca from the high-momentum Doppler broadening spectra. In addition, we studied the temperature dependence of the positron annihilation lifetime in the low temperature region from 10 to 300 K for 1/1-cubic approximant Cd{sub 6}Ca and Cd{sub 6}Yb crystals. As a whole, in both 1/1-cubic Cd{sub 6}Ca and Cd{sub 6}Yb approximants the positron lifetime {tau}{sub 1} gradually increases with increasing temperature due to isotropic thermal expansion. However, the positron lifetime {tau}{sub 1} does not change at the order-disorder transition temperature, namely, 100 and 110 K for 1/1-cubic Cd{sub 6}Ca and Cd{sub 6}Yb approximants, respectively. These results suggest that the size of the structural vacancies and local electron density do not change with the ordering.

  5. Gamma radiation production using channeled positron annihilation in crystals

    NASA Astrophysics Data System (ADS)

    Gevorkyan, A. S.; Oganesyan, Koryun B.; Rostovtsev, Yuri V.; Kurizki, Gershon

    2015-07-01

    The possibility of channeling low-energy relativistic positrons in some ionic crystals with axial symmetry is shown. The annihilation processes of positrons with medium electrons are investigated in detail. The lifetime of a positron in the regime of channeling is estimated; the existence of a long relaxation lifetime has been shown.

  6. Free-Radical Quenching of Positron Lifetimes

    Microsoft Academic Search

    Stephan Berko; A. Joseph Zuchelli

    1956-01-01

    The annihilation lifetime of positrons stopping in benzene has been measured as a function of the percentage of added free radical, diphenylpicrylhydrazyl. The long component of the complex annihilation curve quenches from 2.67×10-9 sec in pure benzene to 5.3×10-10 sec at a 3% free radical concentration. An annihilation model following Bell and Graham's assumptions is discussed, postulating the partial formation

  7. Behaviour of Pluronic P84 block copolymer micelles above the gelification temperature as probed by positron annihilation lifetime spectroscopy.

    PubMed

    Messaoud, Taoufik; Duplâtre, Gilles; Waton, Gilles; Michels, Bernard

    2005-11-21

    The new method based on positron annihilation lifetime spectroscopy (PALS) to determine both the mean core radius, R(core), and aggregation number, N(ag), of micelles is applied to the study of aqueous solutions of the triblock Pluronic P84 copolymer as a function of temperature (T), beyond the gelification point (334 K). Two long-lived components appear in the PALS spectra, ascribed to triplet positronium in the water bulk (o-Ps(aq)) and in the organic core of the micelles (o-Ps(org)). Of the various fitting parameters, only the lifetime of the latter species, tau4, and the micellar parameters, R(core) and N(ag), disclose the occurrence of gelification by first increasing up to 334 K, then decreasing. By contrast to what is known in case of phase transition, none of the parameters shows any abrupt change at 334 K, whereas the macroscopic viscosity of the solutions suffers a drastic increase. This is attributed to the fact that positronium is sensitive to the microviscosity of the solutions. At the transition point, the properties of the polyoxipropylene aggregates forming the organic core of the P84 micelles are not greatly affected. Furthermore, the fact that the experimental N(ag) values coincide with those calculated for spheres, from the R(core) values, indicates that the shape of the P84 cores does not change significantly after gelification. The onset of gelification results from a decrease in the hydrogen bonding interactions in the solution with an ensuing relative increase in the interactions between the polyoxipropylene (PPO) groups, initially forming the corona of the P84 micelles, in an intermicellar mode. This increased solicitation of the PPO groups outside their initial location would result in depletion in the number of surfactant molecules forming the micelles, viz. a decrease in both R(core) and N(ag) above 334 K. From the data, additional information can be gained regarding the local viscosity and surface tension in the micellar cores. PMID:16358034

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

    SciTech Connect

    Rath, S. K.; Patri, M. [Naval Materials Research Laboratory, Shil-Badlapur Road, Ambernath 421506, Maharastra (India); Sudarshan, K.; Pujari, P. K. [Radiochemistry Division, Bhabha Atomic Research Centre, Mumbai-400085 (India); Khakhar, D. V. [Department of Chemical Engineering, Indian Institute of Technology, Powai-76 (India)

    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.

  9. Positron lifetime setup based on DRS4 evaluation board

    NASA Astrophysics Data System (ADS)

    Petriska, M.; Sojak, S.; Sluge?, V.

    2014-04-01

    A digital positron lifetime setup based on DRS4 evaluation board designed at the Paul Scherrer Institute has been constructed and tested in the Positron annihilation laboratory Slovak University of Technology Bratislava. The high bandwidth, low power consumption and short readout time make DRS4 chip attractive for positron annihilation lifetime (PALS) setup, replacing traditional ADCs and TDCs. A software for PALS setup online and offline pulse analysis was developed with Qt,Qwt and ALGLIB libraries.

  10. Application of positron annihilation lifetime spectroscopy (PALS) to study the nanostructure in amphiphile self-assembly materials: phytantriol cubosomes and hexosomes.

    PubMed

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

    2015-01-21

    Self-assembled amphiphile nanostructures of colloidal dimensions such as cubosomes and hexosomes are of interest as delivery vectors in pharmaceutical and nanomedicine applications. Translation would be assisted through a better of understanding of the effects of drug loading on the internal nanostructure, and the relationship between this nanostructure and drug release profile. Positron annihilation lifetime spectroscopy (PALS) is sensitive to local microviscosity and is used as an in situ molecular probe to examine the Q2 (cubosome) ? H2 (hexosome) ? L2 phase transitions of the pharmaceutically relevant phytantriol-water system in the presence of a model hydrophobic drug, vitamin E acetate (VitEA). It is shown that the ortho-positronium lifetime (?) is sensitive to molecular packing and mobility and this has been correlated with the rheological properties of individual lyotropic liquid crystalline mesophases. Characteristic PALS lifetimes for L2 (?4? 4 ns) ? H2 (?4? 4 ns) > Q(2?Pn3m) (?4? 2.2 ns) are observed for the phytantriol-water system, with the addition of VitEA yielding a gradual increase in ? from ?? 2.2 ns for cubosomes to ?? 3.5 ns for hexosomes. The dynamic chain packing at higher temperatures and in the L2 and H2 phases is qualitatively less "viscous", consistent with rheological measurements. This information offers increased understanding of the relationship between internal nanostructure and species permeability. PMID:25459998

  11. THEORY OF POSITRON ANNIHILATION ON MOLECULES

    E-print Network

    Gribakin, Gleb

    THEORY OF POSITRON ANNIHILATION ON MOLECULES Gleb Gribakin Department of Applied Mathematics mechanisms of positron annihilation in binary collisions with molecules are considered ­ direct and resonant. The contribution of the former is enhanced, together with the elastic scattering cross section, if the positron has

  12. 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.

  13. Positron Annihilation in Superconducting 123 Compounds

    NASA Astrophysics Data System (ADS)

    Peter, M.; Manuel, A. A.; Erb, A.

    After a brief review of the theory of angular correlation of positron annihilation radiation (ACAR), we illustrate experimental principles and give examples of successful determination of electron momentum density (EMD) and of positron lifetime in solids. The central question which we try to answer concerns the contribution of positron spectroscopy to the knowledge and understanding of the new high temperature superconducting oxides. We find that in these oxides also, partially filled bands exist and we can observe parts of their Fermi surface and measure lifetimes in accordance with band theoretical calculations. There are characteristic differences, however. The intensity of the anisotropy of the ACAR signal is below theoretical expectation and signals depend on sample preparation. Recent studies by the Geneva group have concerned dependence of the signals on impurities, on oxygen content and on the thermal history of preparation. Of particular interest are correlations between the variations of these signals and between the variations of structural and transport properties in these substances. Besides deliberate additions of impurities, the Geneva group also reports progress in the preparations of samples of highest purity (barium zirconate crucibles). The alloy series PrxY1-xBa2Cu3O7-? is of special interest because of exceptional transport properties. The recent positron results on these materials will be presented and commented in the light of theoretical models and in the light of the reported superconductivity of the Pr-compound.

  14. Polarized positron annihilation in ferromagnetic gadolinium

    Microsoft Academic Search

    C. Hohenemser; J. M. Weingart; S. Berko

    1968-01-01

    Measurements of the angular correlation of positron annihilation radiation in ferromagnetic Gd are presented, and observed polarization effects are attributed to anisotropic spin alignment of the conduction band.

  15. The temperature dependence of free volume in phenyl salicylate and its relation to structural dynamics: A positron annihilation lifetime and pressure-volume-temperature study

    NASA Astrophysics Data System (ADS)

    Dlubek, G.; Shaikh, M. Q.; Raetzke, K.; Faupel, F.; Pionteck, J.; Paluch, M.

    2009-04-01

    Positron annihilation lifetime spectroscopy (PALS) and pressure-volume-temperature (PVT) experiments were performed to characterize the temperature dependent microstructure of the hole free volume in the low molecular weight glass-former phenyl salicylate (salol). The PALS spectra were analyzed with the new routine LT9.0 and the volume distribution of subnanometer size holes characterized by its mean ?vh? and standard deviation ?h was calculated. Crystallization of the amorphous sample was observed in the temperature range above 250 K, which leads to a vanishing of the positronium formation. The positronium signal recovered after melting at 303 K. A combination of PALS with PVT data enabled us to calculate the specific density Nh', the specific volume Vf, and the fraction of holes fh in the amorphous state. From comparison with dielectric measurements in the temperature range above TB=265 K, it was found that the primary structural relaxation slows down with temperature, faster than the shrinkage of the hole free volume Vf would predict, on the basis of the Cohen-Turnbull (CT) free volume theory. CT plots can be linearized by replacing Vf of the CT theory by (Vf-?V), where ?V is a volume correction term. This was interpreted as indication that the lower wing of the hole size distribution contains holes too small to show a liquidlike behavior in their surroundings. Peculiarities of the relaxation behavior below TB=265 K and the possible validity of the Cohen-Grest free volume model are discussed.

  16. Positron annihilation study on hafnium metals given various treatments

    SciTech Connect

    Min, Duck Ki; Kang, Myung Soo (Korea Atomic Energy Research Inst., Taejon (Korea, Republic of)); Yoon, Young Ku (Korea Advanced Inst. of Science and Tech., Taejon (Korea, Republic of))

    1993-08-01

    The positron annihilation technique that enables measurements of positron lifetime, two-photon angular correlation and Doppler broadening due to annihilation radiation has been established for studies of the electronic configuration and defect properties in solids. In metals, positrons can be trapped at vacancies and their agglomerates as well as at dislocations, but not at interstitials. Because of these interactions, the positron annihilation method can be applied to studies of the behavior of dislocations during annealing of plastically deformed metals. Furthermore, it is possible by measurements of annihilation characteristics to identify defects such as vacancies, dislocations and vacancy-clusters, and to determine spatial dimensions of defects. In this work, positron annihilation measurements for annealed, cold worked, annealed and then quenched, and cold worked and then cathodically hydrogen charged hafnium specimens were made to obtain information on (a) positron annihilation characteristics of hafnium metal, (b) role of vacancy-type defects on hydrogen charging, (c) defects produced during hydrogen charging and (d) recovery of lattice defects in hafnium and effects of hydrogen on defects recovery upon annealing.

  17. Mechanisms of positron annihilation on molecules

    Microsoft Academic Search

    G. F. Gribakin

    2000-01-01

    The aim of this work is to identify the mechanisms responsible for very large rates and other peculiarities observed in low-energy positron annihilation on molecules. The two mechanisms considered are the following: (i) Direct annihilation of the incoming positron with one of the molecular electrons. This mechanism dominates for atoms and small molecules. I show that its contribution to the

  18. Positron annihilation in superconducting 123 compounds

    SciTech Connect

    Peter, M.; Manuel, A.A.; Erb, A. (Univ. of Geneva (Switzerland). Dept. of Physics of Condensed Matter)

    1998-12-20

    After a brief review of the theory of angular correlation of positron annihilation radiation (ACAR), the authors illustrate experimental principles and give examples of successful determination of electron momentum density (EMD) and of positron lifetime in solids. The central question which the authors try to answer concerns the contribution of positron spectroscopy to the knowledge and understanding of the new high temperature superconducting oxides. They find that in these oxides also, partially filled bands exist and they can observe parts of their Fermi surface and measure lifetimes in accordance with band theoretical calculations. There are characteristic differences, however. The intensity of the anisotropy of the ACAR signal is below theoretical expectation and signals depend on sample preparation. Recent studies by the Geneva group have concerned dependence of the signals on impurities, on oxygen content and on the thermal history of preparation. Of particular interest are correlations between the variations of these signals and between the variations of structural and transport properties in these substances. Besides deliberate additions of impurities, the Geneva group also reports progress in the preparations of samples of highest purity (barium zirconate crucibles). The alloy series Pr[sub x]Y[sub 1[minus]x]Ba[sub 2]Cu[sub 3]O[sub 7[minus][delta

  19. Distribution of Positron Annihilation Radiation

    E-print Network

    Peter A. Milne

    2006-02-28

    The SPI instrument on-board the ESA/INTEGRAL satellite is engaged in a mission-long study of positron annihilation radiation from the Galaxy. Early results suggest that the disk component is only weakly detected at 511 keV by SPI. We review CGRO/OSSE, TGRS and SMM studies of 511 keV line and positronium continuum emission from the Galaxy in light of the early INTEGRAL/SPI findings. We find that when similar spatial distributions are compared, combined fits to the OSSE/SMM/TGRS data-sets produce bulge and disk fluxes similar in total flux and in B/D ratio to the fits reported for SPI observations. We further find that the 511 keV line width reported by SPI is similar to the values reported by TGRS, particularly when spectral fits include both narrow-line and broad-line components. Collectively, the consistency between these four instruments suggests that all may be providing an accurate view of positron annihilation in the Galaxy.

  20. Positronium formation and inhibition in binary ionic solid solutions of general formula Al (1 ? x) Cr ( x) K(SO 4) 2 · 12 H 2O: A positron annihilation lifetime and Doppler broadening spectroscopy study

    Microsoft Academic Search

    J. C. Machado; G. M. de Lima; F. C. Oliveira; I. M. Marzano

    2006-01-01

    Positron annihilation lifetime and Doppler broadening spectroscopies were applied in solid mixtures and solutions of AlK(SO4)2·12H2O and CrK(SO4)2·12H2O. For the solid solutions of the general formula Al(1?x)Cr(x)K(SO4)2·12H2O (x=Cr(III)), a strong inhibition of positronium formation is observed with increasing chromium complex concentration. In the solid mixtures, the positronium formation decreases linearly with CrK(SO4)2·12H2O concentration, while the positronium lifetime remains constant for

  1. 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.

  2. 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.

  3. Positron beam lifetime spectroscopy at Lawrence Livermore National Laboratory

    SciTech Connect

    Howell, R.H.; Cowan, T.E.; Hartley, J.H. [Lawrence Livermore National Lab., CA (United States); Stern, P.A. [Lawrence Livermore National Lab., CA (United States)]|[California Univ., Davis, CA (United States). Dept. of Physics

    1996-10-01

    Defect analysis is needed for samples ranging in thickness from thin films to large engineering parts. We are meeting that need with two positron beam lifetime spectrometers: on on a 3 MeV electrostatic accelerator and the second on our high current linac beam. The high energy beam spectrometer performs positron lifetime analysis on thick samples which can be encapsulated for containment or for in situ measurements in controlled environments. At our high current beam, we are developing a low energy, microscopically focused, pulsed positron beam to enable positron annihilation lifetime spectroscopy for defect specific, 3-D maps with sub-micron location resolution. The data from these instruments with the aid of first principles calculations of defect specific positron lifetimes.

  4. Positron annihilation in flight: experiment with slow and fast positrons

    NASA Astrophysics Data System (ADS)

    ?ížek, J.; Vl?ek, M.; Luká?, F.; Melikhova, O.; Procházka, I.; Anwand, W.; Wagner, A.; Butterling, M.; Krause-Rehberg, R.

    2014-04-01

    A novel digital coincidence Doppler broadening (D-CDB) spectrometer was employed for energy resolved investigations of two-quantum annihilation-in-flight (TQAF). The TQAF phenomenon was studied using monoenergetic positrons produced in a slow positron beam and also using fast positrons. Because of a low background the measurements on the slow positron beam could be performed in a close geometry and the TQAF contribution in the two-dimensional gamma ray energy spectra fills a 'bowl-like' area delimited by a hyperbolic curve and a kinematical cut-off determined by the kinetic energy of positrons. With decreasing positron energy the area of TQAF contribution becomes smaller and disappears completely for slow positrons with energies below ~ 100 eV. The measurements with fast positrons were restricted to a limited range of angles between the annihilation gamma rays and the TQAF events contribute to a hyperbolic band in gamma ray energy spectrum.

  5. Application of positron annihilation spectroscopy to the characterization of rocks

    NASA Astrophysics Data System (ADS)

    Urban-Klaehn, Jagoda Maria

    1998-11-01

    This work is a first comprehensive study on rocks by use of the positron annihilation methods: Doppler Broadening Spectroscopy (DBS) and the lifetime measurements (LT). The complementary methods like Scanning Electron Microscopy (SEM), the residual gas analysis method (RGAS) and gas absorption (BET) methods were used also. Several rock samples with the known composition and porosity, mainly natural sandstones, carbonates but also anhydrite, opals and synthetic sandstones have been studied. Rocks have been investigated in different states, dry versus soaked (in water, brine or hydrocarbons) and heated (20-220°C) versus non-heated For the rocks with similar structure and porosity, the positron annihilation (PA) signal has been found to depend primarily on the rock composition (S-parameter and the mean lifetime values were higher for sandstones than for carbonates) and on the rock state, but to a lesser extent. We found also how the structural differences affect the PA signal by studying powderized versus non-powderized samples and opals versus sandstones. Our measurements have proved that rocks can be segregated according to their PA- parameter values. The positron annihilation parameters, like S,SW,W-parameters and the mean lifetime values have been found to be characteristic for each rock. Physical models explaining how positron annihilation response is sensitive to the rock composition, structure and state have been developed. We also propose a new analysis method to get more insight into the rock microstructure. In the long perspective, the understanding of the physics of positron interactions in rocks can lead to the development of a new nuclear well-logging tool using the positron annihilation technique, which would characterize the rock and its state uniquely.

  6. 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.

  7. 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.

  8. Defect characterization with positron annihilation. [Review

    SciTech Connect

    Granatelli, L; Lynn, K G

    1980-01-01

    Positron annihilation in metal crystals is reviewed. A brief introduction to the positron annihilation technique is presented first. Then the ability of the positron technique to perform microstructural characterization of four types of lattice defects (vacancies, voids, dislocations, grain boundaries) is discussed. It is frequently not possible to obtain samples that contain only one type of defect in nonnegligible concentrations. Such situations exist for some alloys and for fatigued metal samples. Finally, the current limitations and some future prospects of the technique are presented. 79 references, 14 figures, 1 table. (RWR)

  9. Positron annihilation on large molecules Koji Iwata,1,

    E-print Network

    Gribakin, Gleb

    Positron annihilation on large molecules Koji Iwata,1, * G. F. Gribakin,2, R. G. Greaves,1, C. Kurz May 1999; published 18 January 2000 Positron annihilation on molecules is known to depend sensitively of the annihilation are considered theoretically: direct annihilation of the positron with one of the molecular

  10. Characterization of bulk AlN crystals with positron annihilation spectroscopy

    Microsoft Academic Search

    F. Tuomisto; J.-M. Mäki; T. Yu. Chemekova; Yu. N. Makarov; O. V. Avdeev; E. N. Mokhov; A. S. Segal; M. G. Ramm; S. Davis; G. Huminic; H. Helava; M. Bickermann; B. M. Epelbaum

    2008-01-01

    We have applied positron annihilation spectroscopy to study in-grown vacancy defects in bulk aluminium nitride (AlN) crystals grown by physical vapor transport. We interpret the lowest lifetime value of about 155ps, measured at low temperatures, to represent the annihilations from the free state of the positron in the crystal lattice. The increased lifetime at high temperatures is an indication of

  11. 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 ?-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.

  12. Microstructural Characterization of Thin Polyimide Films by Positron Lifetime Spectroscopy

    NASA Technical Reports Server (NTRS)

    Eftekhari, A.; St.Clair, A. K.; Stoakley, D. M.; Sprinkle, Danny R.; Singh, J. J.

    1996-01-01

    Positron lifetimes have been measured in a series of thin aromatic polyimide films. No evidence of positronium formation was observed in any of the films investigated. All test films exhibited only two positron lifetime components, the longer component corresponding to the positrons annihilating at shallow traps. Based on these trapped positron lifetimes, free volume fractions have been calculated for all the films tested. A free volume model has been developed to calculate the dielectric constants of thin polyimide films. The experimental and the calculated values for the dielectric constants of the films tested are in reasonably good agreement. It has been further noted that the presence of bulky CF(sub 3) groups and meta linkages in the polyimide structure results in higher free volume fraction and, consequently, lower dielectric constant values for the films studied.

  13. Increased Elemental Specificity of Positron Annihilation Spectra

    Microsoft Academic Search

    P. Asoka-Kumar; M. Alatalo; V. J. Ghosh; A. C. Kruseman; B. Nielsen; K. G. Lynn

    1996-01-01

    Positron annihilation spectroscopy (PAS) is a sensitive probe for studying the electronic structure of defects in solids. We show that the high-momentum part of the Doppler-broadened annihilation spectra can be used to distinguish different elements. This is achieved by using a new two-detector coincidence system to examine the line shape variations originating from high-momentum core electrons. Because the core electrons

  14. Nondestructive detection of fatigue damage in austenitic stainless steel by positron annihilation

    Microsoft Academic Search

    Petra Schaaff; Uwe Holzwarth

    2005-01-01

    Austenitic stainless steel specimens have been examined by positron-lifetime measurements at various stages until failure\\u000a during fatigue tests at constant stress or plastic strain amplitudes. A positron-source-detector assembly has been mounted\\u000a on the servohydraulic testing machines that allowed truly non-destructive positron annihilation studies without removing the\\u000a specimens from the load train. Positrons were generated by a 72Se\\/72 As source with

  15. Mechanisms of positron annihilation on molecules G. F. Gribakin*

    E-print Network

    Gribakin, Gleb

    Mechanisms of positron annihilation on molecules G. F. Gribakin* School of Physics, University in low-energy positron annihilation on molecules. The two mechanisms considered are the following: i Direct annihilation of the incoming positron with one of the molecular electrons. This mechanism

  16. Resonant positron annihilation in ammonia G. F. Gribakin

    E-print Network

    Gribakin, Gleb

    Resonant positron annihilation in ammonia G. F. Gribakin Department of Applied Mathematics.gribakin@qub.ac.uk Abstract. Positron annihilation in ammonia is analyzed using the framework of resonant annihilation [G. F to broadening of vibrational Feshbach resonances. Rotations also allow a distinct contribution at low positron

  17. Positron-molecule interactions: Resonant attachment, annihilation, and bound states

    E-print Network

    Gribakin, Gleb

    Positron-molecule interactions: Resonant attachment, annihilation, and bound states G. F. Gribakin This article presents an overview of current understanding of the interaction of low-energy positrons with molecules with emphasis on resonances, positron attachment, and annihilation. Measurements of annihilation

  18. Slow positron beam generator for lifetime studies

    NASA Technical Reports Server (NTRS)

    Singh, Jag J. (inventor); Eftekhari, Abe (inventor); St.clair, Terry L. (inventor)

    1991-01-01

    A slow positron beam generator uses a conductive source residing between two test films. Moderator pieces are placed next to the test film on the opposite side of the conductive source. A voltage potential is applied between the moderator pieces and the conductive source. Incident energetic positrons: (1) are emitted from the conductive source; (2) are passed through test film; and (3) isotropically strike moderator pieces before diffusing out of the moderator pieces as slow positrons, respectively. The slow positrons diffusing out of moderator pieces are attracted to the conductive source which is held at an appropriate potential below the moderator pieces. The slow positrons have to pass through the test films before reaching the conductive source. A voltage is adjusted so that the potential difference between the moderator pieces and the conductive source forces the positrons to stop in the test films. Measurable annihilation radiation is emitted from the test film when positrons annihilate (combine) with electrons in the test film.

  19. 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.

  20. High energy positrons from annihilating dark matter

    SciTech Connect

    Cholis, Ilias; Goodenough, Lisa; Weiner, Neal [Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, New York 10003 (United States); Hooper, Dan [Theoretical Astrophysics Group, Fermi National Accelerator Laboratory, Batavia, Illinois 60510 (United States); Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637 (United States); Simet, Melanie [Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637 (United States); Theoretical Astrophysics Group, Fermi National Accelerator Laboratory, Batavia, Illinois 60510 (United States)

    2009-12-15

    Results from the PAMELA experiment indicate the presence of an excess of cosmic ray positrons above 10 GeV. In this paper, we consider the possibility that this signal is the result of dark matter annihilations taking place in the halo of the Milky Way. Rather than focusing on a specific particle physics model, we take a phenomenological approach and consider a variety of masses and two-body annihilation modes, including W{sup +}W{sup -}, Z{sup 0}Z{sup 0}, bb, {tau}{sup +}{tau}{sup -}, {mu}{sup +}{mu}{sup -}, and e{sup +}e{sup -}. We also consider a range of diffusion parameters consistent with current cosmic ray data. We find that the significant upturn in the positron fraction above 10 GeV can be explained by dark matter annihilation to leptons, although very large annihilation cross sections and/or boost factors arising from inhomogeneities in the local dark matter distribution are required to produce the observed intensity of the signal. We comment on explanations for the large annihilation rate needed to explain the data and additionally on constraints from gamma rays, synchrotron emission, and cosmic ray antiproton measurements.

  1. Apparatus for photon activation positron annihilation analysis

    DOEpatents

    Akers, Douglas W. (Idaho Falls, ID)

    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.

  2. 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., E-mail: S.W.H.Eijt@tudelft.nl [Department of Radiation, Radionuclides and Reactors, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft (Netherlands); Al-Sawai, W.; Barbiellini, B.; Bansil, A. [Physics Department, Northeastern University, Boston, Massachusetts 02115 (United States); Gao, Y. [Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, NL-2628 BL Delft (Netherlands); Kavli Institute of Nanoscience, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, NL-2628 CJ Delft (Netherlands); Houtepen, A. J. [Department of Chemical Engineering, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, NL-2628 BL Delft (Netherlands); Mijnarends, P. E. [Department of Radiation, Radionuclides and Reactors, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft (Netherlands); Physics Department, Northeastern University, Boston, Massachusetts 02115 (United States); Huis, M. A. van [Kavli Institute of Nanoscience, Faculty of Applied Sciences, Delft University of Technology, Lorentzweg 1, NL-2628 CJ Delft (Netherlands); Ravelli, L.; Egger, W. [Institut für Angewandte Physik und Messtechnik, Universität der Bundeswehr München, Werner-Heisenberg-Weg 39, D-85579 Neubiberg (Germany); Kaprzyk, S. [Physics Department, Northeastern University, Boston, Massachusetts 02115 (United States); Academy of Mining and Metallurgy AGH, PL-30059 Kraków (Poland)

    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.

  3. 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.

  4. Positron annihilation radiation from the Galactic center region

    NASA Technical Reports Server (NTRS)

    Ramaty, R.; Lingenfelter, R. E.

    1991-01-01

    Observations show that there are two components of positron annihilation radiation from the region of the Galactic center: a variable component resulting from one or just a few compact sources at or near the Galactic center and a steady, diffuse component resulting from positron annihilation in the Galactic disk. The diffuse component is modeled using the observed longitude distributions of 70-150 MeV gamma rays, CO, and hot plasma revealed by Fe line emission. Recent results on positron annihilation in the interstellar medium are reviewed and the implications of the annihilation processes on the fraction of positrons annihilating via positronium and on the shape of the 511 keV annihilation line are discussed. The sources of diffuse Galactic positrons are also reviewed and the nature of the compact source of annihilation radiation near the Galactic center is discussed.

  5. Enhancement of direct positron annihilation due to temporal capture by an atom

    SciTech Connect

    Yamanaka, Nobuhiro; Kino, Yasushi; Takano, Yasushi; Kudo, Hiroshi; Ichimura, Atsushi [RIKEN - of Physical and Chemical Research, Wako, Saitama 351-0198 (Japan); Department of Chemistry, Tohoku University, Sendai 980-8578 (Japan); Institute of Space and Astronautical Science, Sagamihara, Kanagawa 229-8510 (Japan)

    2003-05-01

    We investigate direct positron annihilation during a collision in a gas; this is physically separable from indirect annihilation, i.e., via formation of positronium (Ps) and its decay. The cross section of direct annihilation is calculated from the evolution of a wave packet with a time width much shorter than the Ps lifetime, where the direct contribution is extracted through the time dependence of the survival probability. The cross section obtained for a hydrogen atom is found to be enhanced in energies around 10 eV well above the Ps formation threshold. This enhancement is shown to be due to temporal positron capture with electronic excitation in the atom.

  6. Dark matter annihilations and decays after the AMS-02 positron measurements

    NASA Astrophysics Data System (ADS)

    Ibarra, Alejandro; Lamperstorfer, Anna S.; Silk, Joseph

    2014-03-01

    The AMS-02 Collaboration has recently presented high-quality measurements of the cosmic electron and positron fluxes as well as the positron fraction. We use the measurements of the positron flux to derive, for the first time, limits on the dark matter annihilation cross section and lifetime for various final states. Working under the well-motivated assumption that a background positron flux exists from spallations of cosmic rays with the interstellar medium and from astrophysical sources, we find strong limits on the dark matter properties which are competitive, although slightly weaker, than those derived from the positron fraction. Specifically, for dark matter particles annihilating only into e+e- or into ?+?-, our limits on the annihilation cross section are stronger than the thermal value when the dark matter mass is smaller than 100 GeV or 60 GeV, respectively.

  7. Positron annihilation in the MuPs system

    E-print Network

    Alexei M. Frolov

    2012-07-25

    The life-time of the four-body atomic system MuPs ($\\mu^{+} e^{-}_2 e^{+}$ or muonium-positronium) against positron annihilation has been evaluated as $\\tau = \\frac{1}{\\Gamma} \\approx 4.076453 \\cdot 10^{-10}$ $sec$. Various annihilation rates for MuPs are determined to a good numerical accuracy, e.g., $\\Gamma_{2 \\gamma} \\approx$ 2.446485$\\cdot 10^{9}$ $sec^{-1}$, $\\Gamma_{3 \\gamma} \\approx$ 6.62798$\\cdot 10^{6}$ $sec^{-1}$, $\\Gamma_{4 \\gamma} \\approx$ 3.61680$\\cdot 10^{3}$ $sec^{-1}$, $\\Gamma_{5 \\gamma} \\approx$ 6.32973 $sec^{-1}$. The hyperfine structure splitting for the ground state in the MuPs system has also been evaluated as $\\Delta$ = 23.078 $MHz$.

  8. Positron annihilation spectroscopy as a probe of microscopic structure and physical aging in polymer

    SciTech Connect

    Yu, M.

    1992-01-01

    Positron annihilation is studied as a characterization method for the properties of polymers. Previous studies indicate that the orthopositronium lifetime [tau][sub 3] and intensity I[sub 3] is correlated to the free volume [open quotes]hole[close quotes] 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 disphenol-A polycarbonate shows that the free volume increases with increasing temperature, and it also obtains the transition temperatures T[sub g] and T[sub [beta

  9. Positron Annihilation Spectroscopy Of High Performance Polymer Films Under CO{sub 2} Pressure

    SciTech Connect

    Quarles, C. A. [Texas Christian University, Fort Worth TX 76109 (United States); Klaehn, John R.; Peterson, Eric S. [Idaho National Laboratory, Idaho Falls ID 83415-2208 (United States); Urban-Klaehn, Jagoda M. [Pajarito Scientific Corporation, Idaho Falls ID 83404 (United States)

    2011-06-01

    Positron annihilation Lifetime and Doppler broadening measurements are reported for six polymer films as a function of carbon dioxide (CO{sub 2}) 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. The studied polymers are found to behave differently from each other. Some polymers form positronium and others, such as the polyimide structures, do not. For those polymers 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. A few of the studied polymers exhibit changes in positron lifetime and intensity under CO{sub 2} pressure which may be described by the Henry or Langmuir sorption models, while the positron response of other polymers is rather insensitive to the CO{sub 2} pressure. The results demonstrate the usefulness of positron annihilation spectroscopy in investigating the sorption of CO{sub 2} into various polymers at pressures up to about 3 atm (45psi).

  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. Analytical evidence for quantum states in aqueous vanadium pentoxide with positron lifetime spectroscopy

    E-print Network

    L. V. Elnikova

    2010-04-26

    The possibility of registration of quantum states, such as the coalescence of droplets (tactoids) in the sol phase of aqueous vanadium pentoxide V$_2$O$_5$, with positron annihilation lifetime spectroscopy is discussed. The decrease of the long-living positronium (Ps) lifetime term in the result of the coalescence of V$_2$O$_5$ tactoids is predicted.

  12. Depth-dependent positron annihilation in different polymers

    NASA Astrophysics Data System (ADS)

    Yang, J.; Zhang, P.; Cheng, G. D.; Li, D. X.; Wu, H. B.; Li, Z. X.; Cao, X. Z.; Jia, Q. J.; Yu, R. S.; Wang, B. Y.

    2013-09-01

    Depth-dependent positron annihilation Doppler broadening measurements were conducted for polymers with different chemical compositions. Variations of the S parameter with respect to incident positron energy were observed. For pure hydrocarbons PP, HDPE and oxygen-containing polymer PC, S parameter rises with increasing positron implantation depth. While for PI and fluoropolymers like PTFE, ETFE and PVF, S parameter decreases with higher positron energy. For chlorine-containing polymer PVDC, S parameter remains nearly constant at all incident positron energies. It is suggested that these three variation trends are resulted from a competitive effect between the depth-dependent positronium formation and the influence of highly electronegative atoms on positron annihilation characteristics.

  13. Three-photon annihilation of the electron-positron pairs

    E-print Network

    A. M. Frolov

    2009-05-08

    Three-photon annihilation of the electron-positron pairs (= $(e^{-}, e^{+})-$pairs) is considered in the electron rest frame. The energy of the incident positron can be arbitrary. The analytical expression for the cross-section of three-photon annihilation of the $(e^{-},e^{+})-$pair has been derived and investigated.

  14. JOURNALDE PHYSIQUE Col/oqzie C9, su~~~p/~iiientau no 1 1-1 2, To111e34, Nouembre-Dicembre 1973, page (3-97 ON THE INFLUENCE OF DEFECTS ON POSITRON ANNIHILATION

    E-print Network

    Paris-Sud XI, Université de

    , page (3-97 ON THE INFLUENCE OF DEFECTS ON POSITRON ANNIHILATION IN ALKALI-HALIDES L. S M E D S K J E R'irradiation. Abstract. -The technique of positron annihilation by use of lifetime and angular correlation as traps for positrons by the presence of a new 1.1 ns lifetime, and that trapping probability

  15. Positron-annihilation characteristics in real solids including many-body enhancement effects

    SciTech Connect

    Chakraborty, B.

    1982-06-01

    A density functional scheme has been developed for incorporating electron-positron correlation effects into band-structure calculations of positron-annihilation characteristics. The electron and positron densities are determined from a self-consistent set of equations based on a generalized Kohn-Sham scheme. Application of this formalism to defect-free aluminum is described. The results for a two-dimensional angular-correlation spectrum and the positron lifetime show very good agreement with experiment. The method is of general applicability and can be used with any existing self-consistent band-structure scheme.

  16. Positron annihilation study on free volume of amino acid modified, starch-grafted acrylamide copolymer

    NASA Astrophysics Data System (ADS)

    Mahmoud, K. R.; Al-Sigeny, S.; Sharshar, T.; El-Hamshary, H.

    2006-05-01

    Free volume measurements using positron annihilation lifetime spectroscopy was performed for uncrosslinked and crosslinked starch-grafted polyacrylamide, and their modified amino acid samples including some of their iron(III) complexes. The measurements were performed at room temperature. The analysis of lifetime spectra yielded mostly three lifetime components. It was observed that the values of the short lifetime component ?1 are slightly higher than the lifetime associated with the self-decay of para-positronium atoms in polymers. The free volume was probed using ortho-positronium pick-off annihilation lifetime parameters. The mean free volume has also been calculated from the lifetime data. The avrage value of this parameter of the crosslinked polymer were found to be higher than those of the uncrosslinked polymer.

  17. Positron-annihilation study of voids in aSi and aSi:H

    Microsoft Academic Search

    Y. J. He; M. Hasegawa; R. Lee; S. Berko; David Adler; Ai-Lien Jung

    1986-01-01

    Angular correlation of positron-electron annihilation radiation (ACAR) experiments and positron-lifetime measurements have been performed in a-Si and a-Si:H films as a function of temperature. Positronium formation in microvoids is observed in a-Si:H, but not in a-Si. From the width of the narrow positronium ACAR components we estimate the average diameter of the microvoids to be ~20 Å. A complex temperature

  18. 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.

  19. Free volume dependent fluorescence property of PMMA composite: Positron annihilation studies

    NASA Astrophysics Data System (ADS)

    Ravindrachary, V.; Praveena, S. D.; Bhajantri, R. F.; Ismayil, Crasta, Vincent

    2013-02-01

    The free volume related fluorescence properties of chalcone chromophore [1-(4-methylphenyl)-3-(4-N, N, dimethylaminophenyl)-2-propen-1-one doped Poly(methyl methacrylate) have been studied using fluorescence spectroscopy and Positron Annihilation lifetime spectroscopy techniques. The fluorescence spectra show that the fluorescence behavior depends on the free volume dependent polymer microstructure and varies with dopant concentration with in the composite. The origin and variation of fluorescence is understood by twisted internal charge transfer state as well as free volume. The Positron annihilation study shows that the free volume related microstructure of the composite is vary with doping level.

  20. 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.

  1. 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.

  2. Positron annihilation studies of Eu and Dy doped ?'-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.

  3. M Ris-R-433 A Positron Lifetime Study of

    E-print Network

    M Risø-R-433 S A Positron Lifetime Study of Properties of Light Particles in Liquids Finn M. Jacobsen Risø National Laboratory, DK-4000 Roskilde, Denmark April 1981 #12;RISØ-R-433 A POSITRON LIFETIME. The positron lifetime technique has been used for study- ing the behaviour of the three light particles

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

    SciTech Connect

    Xiong, Bangyun; Mao, Wenfeng; Tang, Xiuqin; He, Chunqing, E-mail: hecq@whu.edu.cn [Key Laboratory of Nuclear Solid State Physics Hubei Province, School of Physics and Technology, Wuhan University, Wuhan 430072 (China)

    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 3? 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.

  5. Proposed Parameter-Free Model for Interpreting the Measured Positron Annihilation Spectra of Materials Using a Generalized Gradient Approximation

    NASA Astrophysics Data System (ADS)

    Barbiellini, Bernardo; Kuriplach, Jan

    2015-04-01

    Positron annihilation spectroscopy is often used to analyze the local electronic structure of materials of technological interest. Reliable theoretical tools are crucial to interpret the measured spectra. Here, we propose a parameter-free gradient correction scheme for a local-density approximation obtained from high-quality quantum Monte Carlo data. The results of our calculations compare favorably with positron affinity and lifetime measurements, opening new avenues for highly precise and advanced positron characterization of materials.

  6. Positron lifetime spectroscopy in thin polymer coatings

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Eftekhari, Abe; Sprinkle, Danny R.

    1990-01-01

    Polymer coatings are finding increasing applications in aerospace industry. The effectiveness of coatings depends strongly on their microstructure and adhesion to the substrates. Currently, there is no technique for adequately monitoring the quality of the coatings. We adapted positron lifetime spectroscopy for the investigation of thin coatings. Results of measurements on 0.001-in-thick polyurethane coatings on aluminum and steel substrates and thicker (0.080-in.) self-standing polyurethane discs were compared. In all cases, we find positron lifetime groups centered around 560 ps, corresponding to the presence of 0.9 A exp 3 free volume cells. However, the number of free volume cells in thin coatings is larger, suggesting that the morphology of thin coatings is different from that of bulk polyurethane. These results and their structural implications are discussed.

  7. Positron Annihilation Studies In Polymer Nano-Composites

    SciTech Connect

    Chen, H. M.; Awad, Somia; Jean, Y. C. [Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64112 (United States); Yang, J.; Lee, L. James [Department of Chemical and Bimolecular Engineering, The Ohio State University, Columbus, OH 43210 (United States)

    2011-06-01

    Positron annihilation spectroscopy coupled with a variable mono-energy positron beam has been applied to study nanoscale polymeric nanocomposites. New information about multilayer depth profiles and structures, interfacial free-volume and open space properties have been obtained in polystyrene/carbon nano fiber composites. The S parameter in Doppler Broadening Energy Spectra combined slow positron beam is used to quantitatively represent the free volume, open spaces, and interactions in the interface between polystyrene matrix and carbon nanofibers.

  8. 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.

  9. Mechanical durability of polymeric coatings studied by positron annihilation spectroscopy: correlation between cyclic loading and free volumes

    NASA Astrophysics Data System (ADS)

    Chen, H.; Peng, Q.; Huang, Y. Y.; Zhang, R.; Mallon, P. E.; Zhang, J.; Li, Y.; Wu, Y.; Richardson, J. R.; Sandreczki, T. C.; Jean, Y. C.; Suzuki, R.; Ohdaira, T.

    2002-06-01

    The mechanical durability of seven commercially polymeric coatings is investigated using slow positron beam techniques to monitor changes in sub-nanometer defects during the process of cyclic loading. Doppler broadened energy spectra and positron annihilation lifetime (PAL) measurements were performed as a function of the slow positron energy at different periods of cycling loading. The positron annihilation dada show that both S-defect parameter and o-positronium (Ps) lifetime decrease as the loading cycle increases. The results indicate a loss of free volumes due to the loss of mechanical durability by cyclic loading. A direct correlation between the loss of S-defect parameter and the period of loading cycle is observed. This is interpreted as that durability of polymeric coatings is controlled by the atomic level free volumes. It is shown that the slow positron beam is a very successful probe in detecting the very early stages of coating degradation due to mechanical processes.

  10. 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.

  11. Electron irradiation induced microstructural modifications in BaCl 2 doped PVA: A positron annihilation study

    Microsoft Academic Search

    A. Harisha; V. Ravindrachary; R. F. Bhajantri; Ismayil; Ganesh Sanjeev; Boja Poojary; Dhanadeep Dutta; P. K. Pujari

    2008-01-01

    Thin films of pure and 10wt% BaCl2 doped poly(vinyl alcohol) (PVA) were prepared by solution casting method. These films were subjected to electron irradiation for different doses ranging from 0 to 400kGy in air at room temperature. The effect of electron irradiation on the optical and free volume related microstructures of these polymer films was studied using positron annihilation lifetime

  12. 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.

  13. A study of defects in deformed FeSi alloys using positron annihilation techniques

    NASA Astrophysics Data System (ADS)

    Mostafa, Khaled M.; De Baerdemaeker, J.; Calvillo, Pablo R.; Houbaert, Y.; Segers, D.

    2008-10-01

    Steels with high amounts of silicon are used in electrical applications due to their low magnetostriction, high electrical resistivity and reduced energy losses, but they exhibit poor formability. The slow positron beam of Gent is used to investigate defects in different deformed FeSi alloys. It was found that the concentration of defects for the alloys deformed at high temperatures are different from the ones related to the alloys deformed at room temperature. These results are correlated to the results of positron annihilation lifetime spectroscopy (PALS).

  14. Positron annihilation in high-Tc superconductors

    Microsoft Academic Search

    David Singh; W. E. Pickett; R. E. Cohen; Henry Krakauer; Stephan Berko

    1989-01-01

    We report ab initio calculations of positron wave functions in the high-Tc superconductors YBa2Cu3O7, Bi2Sr2CaCu2O8, and Tl2Ba2CaCu2O8 using the general potential linearized augmented plane-wave method. The calculated positron wave functions are fairly insensitive to whether or not electron-positron correlation is included in the calculation for YBa2Cu3O7 and Tl2Ba2CaCu2O8, but the calculated positron density is quite sensitive to correlation in Bi2Sr2CaCu2O8.

  15. Positron annihilation in TiBe/sub 2/

    SciTech Connect

    Manuel, A.A.; Hoffmann, L.; Singh, A.K.; Jarlborg, T.; Peter, M.; Smith, J.L.; Fisk, Z.; Pecora, L.M.; Ehrlich, A.C.

    1988-01-01

    We report positron annihilation measurements on TiBe/sub 2/. Calculations using LMTO band structure method are also presented. The good agreement with the experimental data leads to the conclusion that the unusual magnetic properties of this compound can be well explained in terms of its electronic structure. A reconstruction of the electron-positron momentum distribution from calculated and measured 2D-ACPAR is discussed. 10 refs., 3 figs.

  16. Nuclear-charge and positron-energy dependence of the single-quantum annihilation of positrons

    Microsoft Academic Search

    J. C. Palathingal; P. Asoka-Kumar; K. G. Lynn; X. Y. Wu

    1995-01-01

    We report an experimental study of the single-quantum annihilation of positrons in a number of elements having atomic numbers between 49 and 90, utilizing a monoenergetic positron beam. Measurements were made of the differential cross sections for the forward direction for the K, L, and M atomic shells in targets of Th, Pb, Au, Hf, Gd, and In, having thicknesses

  17. Nuclear-charge and positron-energy dependence of the single-quantum annihilation of positrons

    Microsoft Academic Search

    J. C. Palathingal; P. Asoka-Kumar; K. G. Lynn; X. Y. Wu

    1995-01-01

    We report an experimental study of the single-quantum annihilation of positrons in a number of elements having atomic numbers between 49 and 90, utilizing a monoenergetic positron beam. Measurements were made of the differential cross sections for the forward direction for the [ital K], [ital L], and [ital M] atomic shells in targets of Th, Pb, Au, Hf, Gd, and

  18. Positron annihilation measurements in high-energy alpha-irradiated n-type Gallium Arsenide

    NASA Astrophysics Data System (ADS)

    Pan, Sandip; Mandal, Arunava; Roychowdhury, Anirban; SenGupta, Asmita

    2015-07-01

    Positron annihilation lifetime spectroscopy and Doppler broadening annihilation line-shape measurements have been carried out in 40-MeV alpha-irradiated n-type GaAs. After irradiation, the sample has been subjected to an isochronal annealing over temperature region of 25-800 °C with an annealing time of 30 min at each set temperature. After each annealing, the positron measurements are taken at room temperature. Formation of radiation-induced defects and their recovery with annealing temperature are investigated. The lifetime spectra of the irradiated sample have been fitted with two lifetimes. The average positron lifetime ?avg = 244 ps at room temperature after irradiation indicates the presence of defects, and the value of ?2 (262 ps) at room temperature suggests that the probable defects are mono-vacancies. Two distinct annealing stages in ?avg at 400-600 °C and at 650-800 °C are observed. The variations in line-shape parameter ( S) and defect-specific parameter ( R) during annealing in the temperature region 25-800 °C resemble the behaviour of ?avg indicating the migration of vacancies, formation of vacancy clusters and the disappearance of defects between 400 and 800 °C.

  19. Thermal evolution of boron irradiation induced defects in predoped Si revealed by positron annihilation experiments

    NASA Astrophysics Data System (ADS)

    Nambissan, P. M. G.; Bhagwat, P. V.; Kurup, M. B.

    2007-06-01

    The isochronal annealing behavior of high energy (25-72 MeV) boron ion irradiation induced defects in boron-doped silicon is monitored through measurements of positron lifetimes and three distinct defect-evolution stages are identified. The initial boron doping created a defect environment where positrons could sensitively annihilate with the boron electrons, suggesting boron-decorated Si monovacancies as potential trapping sites. The irradiation results in the dissolution of boron from these sites and positrons are then trapped by the empty divacancies of Si. Charge neutralization of divacancies through interaction with boron atoms leads to enhanced positron trapping in the initial stages of isochronal annealing. The divacancies start annealing above 673 K. However, a remarkable defect evolution stage due to the diffusion of the boron atoms beyond their initial depths of penetration is seen above 873 K and it leaves the sample with defects still present even at the highest annealing temperature 1273 K used in this work.

  20. Positron Annihilation in Molecules by Capture into Vibrational Feshbach Resonances of Infrared-Active Modes

    E-print Network

    Gribakin, Gleb

    Positron Annihilation in Molecules by Capture into Vibrational Feshbach Resonances of Infrared 2006) Enhanced positron annihilation on polyatomic molecules is a long-standing and complex problem. We report the results of calculations of resonant positron annihilation on methyl halides. A free parameter

  1. Positron annihilation at the Si/Si02 interface T. C. Leung

    E-print Network

    Rubloff, Gary W.

    Positron annihilation at the Si/Si02 interface T. C. Leung Brookhaven National Laboratory, Upton for publication 21 September 1991) Variable-energy positron annihilation depth-profiling has been applied- type silicon under conditions of negative gate bias, the positron annihilation S-factor characteristic

  2. The Galactic Positron Annihilation Radiation & The Propagation of Positrons in the Interstellar Medium

    E-print Network

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

    2009-04-06

    We consider positron propagation in the interstellar medium and show that the positrons from the beta-plus decay chains of the radioactive nuclei Ni-56, Ti-44, and Al-26, produced in Galactic supernovae, can fully account for all the features of the diffuse Galactic 511 keV annihilation radiation observed by INTEGRAL/SPI. We also predict additional measurable features that can further test the origin of positrons and provide new information on the nature of the interstellar medium.

  3. 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.

  4. Source theory analysis of electron-positron annihilation experiments

    PubMed Central

    Schwinger, Julian

    1975-01-01

    The phenomenological viewpoint already applied to deep inelastic scattering is extended to the discussion of electron-positron annihilation experiments. Some heuristic arguments lead to simple forms for the pion differential cross section that are in reasonable accord with the published experimental data in the energy interval 3-4.8 Gev. PMID:16592296

  5. Electron-Positron Radiative Annihilation : Timelike Virtual Compton Scattering

    E-print Network

    Asmita Mukherjee

    2010-10-01

    We report on a recent work proposing measurements of the deeply virtual Compton amplitude (DVCS) $\\gamma^* \\to h \\bar h \\gamma$ in the timelike kinematic domain which is accessible at electron-positron colliders via the radiative annihilation process $e^+ e^- \\to h \\bar h \\gamma$.

  6. Positron states and annihilation characteristics of surface-trapped positrons at the oxidized Cu(110) surface

    NASA Astrophysics Data System (ADS)

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

    2013-03-01

    The process by which oxide layers are formed on metal surfaces is still not well understood. In this work we present the results of theoretical studies of positron states and annihilation characteristics of surface-trapped positrons at the oxidized Cu(110) surface. An ab-initio investigation of stability and associated electronic properties of different adsorption phases of oxygen on Cu(110) has been performed on the basis of density functional theory and using DMOl3 code. The changes in the positron work function and the surface dipole moment when oxygen atoms occupy on-surface and sub-surface sites have been attributed to charge redistribution within the first two layers, buckling effects within each layer and interlayer expansion. The computed positron binding energy, positron surface state wave function, and annihilation probabilities of surface trapped positrons with relevant core electrons demonstrate their sensitivity to oxygen coverage, elemental content, atomic structure of the topmost layers of surfaces, and charge transfer effects. Theoretical results are compared with experimental data obtained from studies of oxidized transition metal surfaces using positron annihilation induced Auger electron spectroscopy. The process by which oxide layers are formed on metal surfaces is still not well understood. In this work we present the results of theoretical studies of positron states and annihilation characteristics of surface-trapped positrons at the oxidized Cu(110) surface. An ab-initio investigation of stability and associated electronic properties of different adsorption phases of oxygen on Cu(110) has been performed on the basis of density functional theory and using DMOl3 code. The changes in the positron work function and the surface dipole moment when oxygen atoms occupy on-surface and sub-surface sites have been attributed to charge redistribution within the first two layers, buckling effects within each layer and interlayer expansion. The computed positron binding energy, positron surface state wave function, and annihilation probabilities of surface trapped positrons with relevant core electrons demonstrate their sensitivity to oxygen coverage, elemental content, atomic structure of the topmost layers of surfaces, and charge transfer effects. Theoretical results are compared with experimental data obtained from studies of oxidized transition metal surfaces using positron annihilation induced Auger electron spectroscopy. This work was supported in part by the National Science Foundation Grant DMR-0907679.

  7. Detecting Positron-Atom Bound States through Resonant Annihilation V. A. Dzuba* and V. V. Flambaum

    E-print Network

    Gribakin, Gleb

    Detecting Positron-Atom Bound States through Resonant Annihilation V. A. Dzuba* and V. V. Flambaum) A method is proposed for detecting positron-atom bound states by observing enhanced positron annihilation of open-shell transition-metal atoms which are likely to bind the positron: Fe, Co, Ni, Tc, Ru, Rh, Sn, Sb

  8. 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.

  9. Annihilating dark matter and the galactic positron excess

    E-print Network

    Irit Maor

    2006-02-20

    The possibility that the Galactic dark matter is composed of neutralinos that are just above half the $Z^o$ mass is examined, in the context of the Galactic positron excess. In particular, we check if the anomalous bump in the cosmic ray positron to electron ratio at $10~GeV$ can be explained with the ``decay'' of virtual $Z^o$ bosons produced when the neutralinos annihilate. We find that the low energy behaviour of our prediction fits well the existing data. Assuming the neutralinos annihilate primarily in the distant density concentration in the Galaxy and allowing combination of older, diffused positrons with young free-streaming ones, produces a fit which is not satisfactory on its own but is significantly better than the one obtained with homogeneous injection.

  10. 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.

    2015-05-01

    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.

  11. THE GALACTIC POSITRON ANNIHILATION RADIATION AND THE PROPAGATION OF POSITRONS IN THE INTERSTELLAR MEDIUM

    SciTech Connect

    Higdon, J. C. [W. M. Keck Science Center, Claremont Colleges, Claremont, CA 91711-5916 (United States); Lingenfelter, R. E.; Rothschild, R. E. [Center for Astrophysics and Space Sciences, University of California San Diego, La Jolla, CA 92093 (United States)], E-mail: jimh@lobach.JSD.claremont.edu, E-mail: rlingenfelter@ucsd.edu, E-mail: rrothschild@ucsd.edu

    2009-06-10

    The ratio of the luminosity of diffuse 511 keV positron annihilation radiation, measured by INTEGRAL in its four years, from a Galactic 'positron bulge' (<1.5 kpc) compared to that of the disk is {approx}1.4. This ratio is roughly 4 times larger than that expected simply from the stellar bulge-to-disk ratio of {approx}0.33 of the Galactic supernovae (SNe), which are thought to be the principal source of the annihilating positrons through the decay of radionuclei made by explosive nucleosynthesis in the SNe. This large discrepancy has prompted a search for new sources. Here, however, we show that the measured 511 keV luminosity ratio can be fully understood in the context of a Galactic SN origin when the differential propagation of these {approx} MeV positrons in the various phases of the interstellar medium is taken into consideration, since these relativistic positrons must first slow down to energies {<=}10 eV before they can annihilate. Moreover, without propagation, none of the proposed positron sources, new or old, can explain the two basic properties on the Galactic annihilation radiation: the fraction of the annihilation that occurs through positronium formation and the ratio of the broad/narrow components of the 511 keV line. In particular, we show that in the neutral phases of the interstellar medium, which fill most of the disk (>3.5 kpc), the cascade of the magnetic turbulence, which scatters the positrons, is damped by ion-neutral friction, allowing positrons to stream along magnetic flux tubes. We find that nearly 1/2 of the positrons produced in the disk escape from it into the halo. On the other hand, we show that within the extended, or interstellar, bulge (<3.5 kpc), essentially all of the positrons are born in the hot plasmas which fill that volume. We find that the diffusion mean free path is long enough that only a negligible fraction annihilate there and {approx}80% of them escape down into the H II and H I envelopes of molecular clouds that lie within 1.5 kpc before they slow down and annihilate, while the remaining {approx}20% escape out into the halo and the disk beyond. This propagation accounts for the low observed annihilation radiation luminosity of the disk compared to the bulge. In addition, we show that the primary annihilation sites of the propagating positrons in both the bulge and the disk are in the warm ionized phases of the interstellar medium. Such annihilation can also account for those two basic properties of the emission, the fraction ({approx}93% {+-} 7%) of annihilation via positronium and the ratio ({approx}0.5) of broad ({approx}5.4 keV) to narrow ({approx}1.3 keV) components of the bulge 511 keV line emission. Moreover, we expect that the bulk of this broad line emission comes from the tilted disk region (0.5 < R < 1.5 kpc) with a very large broad/narrow flux ratio of {approx}6, while much of the narrow line emission comes from the inner bulge (R < 0.5 kpc) with a negligible broad/narrow flux ratio. Separate spectral analyses of the 511 keV line emission from these two regions should be able to test this prediction, and further probe the structure of the interstellar medium. Lastly, we show that the asymmetry in the inner disk annihilation line flux, which has been suggested as added evidence for new sources, can also be fully understood from positron propagation and the asymmetry in the inner spiral arms as viewed from our solar perspective without any additional sources.

  12. Investigation of free volume changes in the structure of the polymer bifocal contact lenses using positron lifetime spectroscopy PALS.

    PubMed

    Filipecki, Jacek; Kocela, Agnieszka; Korzekwa, Piotr; Filipecka, Katarzyna; Golis, Edmund; Korzekwa, Witold

    2011-01-01

    Positron annihilation lifetime spectroscopy PALS has been applied of free volume properties in bifocal contact lenses. The measurements have been made on new lenses and then after one, two, three and four weeks wear. The longest lifetime, obtained via three-component analyses of the spectra, was associated with the pick-off annihilation of ortho-positronium trapped in the free volume. After wear of the lenses changes in the ortho-positronium lifetimes and the relative intensity of the longest component were observed. These results are discussed on the basis of a free volume model. PMID:21866793

  13. Positrons from supernova and the origin of the galactic-center positron-annihilation radiation

    Microsoft Academic Search

    S. A. Colgate

    1983-01-01

    The emission of positrons from supernova ejecta was discussed in terms of the galactic center annihilation radiation. The positrons from the radioactive sequences (56)Ni(FEMADE) (56)Co(FEMADE) (56) Fe are the most numerous source from supernova. Only type 1 supenova allows a significant fraction to escape the expanding ejecta. For a neutron star model of a type 1 SN a fraction 4

  14. Coupled experimental and DFT +U investigation of positron lifetimes in UO2

    NASA Astrophysics Data System (ADS)

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

    2014-11-01

    We performed positron annihilation spectroscopy measurements on uranium dioxide irradiated with 45 MeV ? particles. The positron lifetime was measured as a function of the temperature in the 15-300 K range. The experimental results were combined with electronic structure calculations of positron lifetimes of vacancies and vacancy clusters in UO2. Neutral and charged defects consisting of from one to six vacancies were studied computationally using the DFT +U method to take into account strong correlations between the 5 f electrons of uranium. The two-component density functional theory with two different fully self-consistent schemes was used to calculate the positron lifetimes. All defects were relaxed taking into account the forces due to the creation of defects and the positron localized in the vacancy. The interpretation of the experimental observations in the light of the DFT + U results and the positron trapping model indicates that neutral VU+2 VO trivacancies (bound Schottky defects) are the predominant defects detected in the 45 MeV ? irradiated UO2 samples. Our results show that the coupling of a precise experimental work and calculations using carefully chosen assumptions is an effective method to bring further insight into the subject of irradiation induced defects in UO2.

  15. Positron Annihilation Spectroscopy Characterization of Nanostructural Features in Reactor Steels

    Microsoft Academic Search

    Stephen Glade; Brian Wirth; Palakkal Asoka-Kumar; Philip Sterne; Matthew Alinger; George Odette

    2004-01-01

    Irradiation embrittlement in nuclear reactor pressure vessel steels results from the formation of a high number density of nanometer sized copper rich precipitates and sub-nanometer defect-solute clusters. We present results of study to characterize the size and compositions of simple binary and ternary Fe-Cu-Mn model alloys and more representative Fe-Cu-Mn-Ni-Si-Mo-C reactor pressure vessel steels using positron annihilation spectroscopy (PAS). Using

  16. Enhancement of positron annihilation on molecules due to vibrational Feshbach resonances

    E-print Network

    Gribakin, Gleb

    Enhancement of positron annihilation on molecules due to vibrational Feshbach resonances G, Northern Ireland, UK Abstract The zero-range potential model is used to investigate positron collisions and annihilation with molecules. The Kr2 dimer is considered as an example. It is shown that (i) although positrons

  17. Positron annihilation studies in the field induced depletion regions of metal-oxide-semiconductor structures

    E-print Network

    Rubloff, Gary W.

    Positron annihilation studies in the field induced depletion regions of metal February 1992) The centroid shifts of positron annihilation spectra are reported from the depletion regions-gaussian positron implantation profile. Inadequacy of the present analysis scheme is evident from the derived

  18. The PAMELA positron excess from annihilations into a light boson

    SciTech Connect

    Cholis, Ilias; Goodenough, Lisa; Weiner, Neal [Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, NY 10003 (United States); Finkbeiner, Douglas P., E-mail: ijc219@nyu.edu, E-mail: dfinkbeiner@cfa.harvard.edu, E-mail: lcg261@nyu.edu, E-mail: neal.weiner@nyu.edu [Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138 (United States)

    2009-12-01

    Recently published results from the PAMELA experiment have shown conclusive evidence for an excess of positrons at high ( ? 10–100 GeV) energies, confirming earlier indications from HEAT and AMS-01. Such a signal is generally expected from dark matter annihilations. However, the hard positron spectrum and large amplitude are difficult to achieve in most conventional WIMP models. The absence of any associated excess in anti-protons is highly constraining on models with hadronic annihilation modes. We revisit an earlier proposal, wherein the dark matter annihilates into a new light (?positrons.

  19. The PAMELA positron excess from annihilations into a light boson

    NASA Astrophysics Data System (ADS)

    Cholis, Ilias; Finkbeiner, Douglas P.; Goodenough, Lisa; Weiner, Neal

    2009-12-01

    Recently published results from the PAMELA experiment have shown conclusive evidence for an excess of positrons at high ( ~ 10-100 GeV) energies, confirming earlier indications from HEAT and AMS-01. Such a signal is generally expected from dark matter annihilations. However, the hard positron spectrum and large amplitude are difficult to achieve in most conventional WIMP models. The absence of any associated excess in anti-protons is highly constraining on models with hadronic annihilation modes. We revisit an earlier proposal, wherein the dark matter annihilates into a new light (lsimGeV) boson phi, which is kinematically constrained to go to hard leptonic states, without anti-protons or ?0's. We find this provides a very good fit to the data. The light boson naturally provides a mechanism by which large cross sections can be achieved through the Sommerfeld enhancement, as was recently proposed. Depending on the mass of the WIMP, the rise may continue above 300 GeV, the extent of PAMELA's ability to discriminate between electrons and positrons.

  20. Study on the microstructure and miscibility of dynamically vulcanized EPDM/PP blend by positron annihilation

    NASA Astrophysics Data System (ADS)

    Dai, Y. Q.; Wang, B.; Wang, S. J.; Jiang, T.; Cheng, S. Y.

    2003-10-01

    Positron annihilation lifetime spectroscopy (PALS) was employed to investigate the relationship between the free volume hole properties and miscibility of dynamically vulcanized ethylene propylene diene monomer (EPDM)/polypropylene (PP) blend. The free volume hole concentration and the relative fractional free volume show negative deviation from linear additivity only when the weight percent of EPDM is over 50%. Combined with the results of dynamical mechanical thermal analysis measurements and mechanical properties tests, we found that the noncrystalline region of PP and EPDM are partially miscible and the miscibility of the blend became better when the weight percent of EPDM is over 50%.

  1. Application of positron annihilation line-shape analysis to fatigue damage for nuclear plant materials

    Microsoft Academic Search

    Noriyoshi Maeda; Noriko Nakamura; Misako Uchida; Yoshio Ohta; Kazuo Yoshida

    1996-01-01

    Positron annihilation line-shape analysis is sufficiently sensitive to detect microstructural defects such as vacancies and dislocations. We are developing a portable positron annihilation system and applying this technique to fatigue damage in type 316 stainless steel and SA508 low alloy steel. The positron annihilation technique was found to be sensitive in the early fatigue life, i.e. up to 10% of

  2. 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.

  3. Temperature dependence of positron-annihilation lifetime, free volume, conductivity, ionic mobility, and number of charge carriers in a polymer electrolyte polyethylene oxide complexed with NH{sub 4}ClO{sub 4}

    SciTech Connect

    Haldar, B.; Singru, R.M. [Department of Physics, Indian Institute of Technology, Kanpur, 208016 (India)] [Department of Physics, Indian Institute of Technology, Kanpur, 208016 (India); Maurya, K.K.; Chandra, S. [Department of Physics, Banaras Hindu University, Varanasi, 221005 (India)] [Department of Physics, Banaras Hindu University, Varanasi, 221005 (India)

    1996-09-01

    Various physical properties of the solution-cast films of the proton conducting polymer polyethylene oxide (PEO) complexed with ammonium perchlorate (NH{sub 4}ClO{sub 4}) have been studied in the temperature range 300{endash}370 K. These properties studied by us include free volume by positron lifetime spectroscopy, ionic conductivity by impedance spectroscopy, ionic mobility by transient ionic current technique, number of charge carriers, dielectric constant, etc. The hole volume and conductivity show a steep rise at {ital T}{approx_equal}{ital T}{sub {ital m}} ({approximately}333 K). It appears that the increase in free volume arises out of the increase in the size of the holes rather than an increase in their number. Although the free volume shows an increase around {ital T}{sub {ital m}}, the measured ionic mobility does not show similar behavior. The increase in the conductivity at {ital T}{sub {ital m}} is, therefore, ascribed to an increase in the number of charge carriers at {approximately}{ital T}{sub {ital m}}. A suitable dissociation model involving the dielectric constant is proposed to explain this increase. The value of the dissociation energy for PEO:NH{sub 4}ClO{sub 4} has been determined to be 2.4 eV. {copyright} {ital 1996 The American Physical Society.}

  4. Glass transition of polystyrene near the surface studied by slow-positron-annihilation spectroscopy

    Microsoft Academic Search

    Y. C. Jean; Renwu Zhang; H. Cao; Jen-Pwu Yuan; Chia-Ming Huang; B. Nielsen; P. Asoka-Kumar

    1997-01-01

    We have measured the Doppler broadening energy spectra of positron annihilation using a variable monoenergetic positron beam as a function of positron energy and of temperature in polystyrene. The S parameters from the energy spectra versus temperature show that onset temperatures significantly decrease as the positron energy decreases. This result gives direct evidence that the glass-transition temperature is suppressed at

  5. Morphology of Thermoset Polyimides by Positron Annihilation Spectroscopy

    NASA Technical Reports Server (NTRS)

    Ranganathaiah, C.; Pater, R. H.; Sprinkle, D. R.; Baugher, A. H.; Eftekhari, A.; Singh, J. J.

    1994-01-01

    Thermoset polyimides have great potential for successfully meeting tough stress and temperature challenges in the advanced aircraft development program. However, studies of structure/property relationships in these materials have not been very successful so far. Positron annihilation spectroscopy has been used to investigate free volumes and associated parameters. It has been noted that the free volume correlates well with the molecular weight, cross-link density and thermal coefficient of expansion of these materials. Currently no other techniques are available for direct measurement of these parameters. Experimental results and their interpretations will be discussed.

  6. GRIS observations of positron annihilation radiation from the Galactic center

    NASA Technical Reports Server (NTRS)

    Gehrels, N.; Barthelmy, S. D.; Teegarden, B. J.; Tueller, J.; Leventhal, M.

    1991-01-01

    In the present Gamma-Ray Imaging Spectrometer (GRIS) observations, the 511 keV line has been spectrally resolved and the Galactic plane and center components have been independently measured. These data, in combination with those from other narrow-field observations in the 1980s, support the two-component model of the positron annihilation source. A strong hard-X-ray continuum was detected in the Galactic plane observation; this 'diffuse' continuum component solves the mystery as to why wide-field instruments have detected such high continuum emission from the Galactic center, in virtue of its source contributions.

  7. Fatigue damage in superalloys determined using Doppler broadening positron annihilation

    NASA Technical Reports Server (NTRS)

    Hoeckelman, Donald; Leighly, H. P., Jr.

    1990-01-01

    Axial fatigue specimens of three superalloys, Inconel 718, Incoloy 903 and Haynes 188, were machined from solution-heat-treated material and artificially aged. They were subjected to cyclic loading for a selected number of cycles after which the S parameter was determined using Doppler broadening positron annihilation. Initially, the S parameter decreased, followed by a large increase and a subsequent decline leading to fracture. This has been interpreted as the removal of residual vacancies, the introduction of new defects by cyclic loading, and, finally, a clustering of the defects as microcracks which grow to cause failure.

  8. Defect identification in semiconductors with positron annihilation: Experiment and theory

    NASA Astrophysics Data System (ADS)

    Tuomisto, Filip; Makkonen, Ilja

    2013-10-01

    Positron annihilation spectroscopy is particularly suitable for studying vacancy-type defects in semiconductors. Combining state-of-the-art experimental and theoretical methods allows for detailed identification of the defects and their chemical surroundings. Also charge states and defect levels in the band gap are accessible. In this review the main experimental and theoretical analysis techniques are described. The usage of these methods is illustrated through examples in technologically important elemental and compound semiconductors. Future challenges include the analysis of noncrystalline materials and of transient defect-related phenomena.

  9. 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.

  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. Positron Annihilation Spectroscopy and Small Angle Neutron Scattering Characterization of Nanostructural Features in Irradiated Fe-Cu-Mn Alloys

    SciTech Connect

    Wirth, B D; Asoka-Kumar, P; Howell, R H; Odette, G R; Sterne, P A

    2001-01-01

    Radiation embrittlement of nuclear reactor pressure vessel steels results from a high number density of nanometer sized Cu-Mn-Ni rich precipitates (CRPs) and sub-nanometer matrix features, thought to be vacancy-solute cluster complexes (VSC). However, questions exist regarding both the composition of the precipitates and the defect character and composition of the matrix features. We present results of positron annihilation spectroscopy (PAS) and small angle neutron scattering (SANS) characterization of irradiated and thermally aged Fe-Cu and Fe-Cu-Mn alloys. These complementary techniques provide insight into the composition and character of both types of nanoscale features. The SANS measurements indicate populations of CRPs and VSCs in both alloys. The CRPs are coarser in the Fe-Cu alloy and the number densities of CRP and VSC increase with the addition of Mn. The PAS involved measuring both the positron lifetimes and the Doppler broadened annihilation spectra in the high momentum region to provide elemental sensitivity at the annihilation site. The spectra in Fe-Cu-Mn specimens thermally aged to peak hardness at 450 C and irradiated at 288 C are nearly identical to elemental Cu. Positron lifetime and spectrum measurements in Fe-Cu specimens irradiated at 288 C clearly show the existence of long lifetime ({approx}500 ps) open volume defects, which also contain Cu. Thus the SANS and PAS provide a self-consistent picture of nanostructures composed of CRPs and VSCs and tend to discount high Fe concentrations in the CRPs.

  12. Defects in nitride-based semiconductors probed by positron annihilation

    NASA Astrophysics Data System (ADS)

    Uedono, A.; Sumiya, M.; Ishibashi, S.; Oshima, N.; Suzuki, R.

    2014-04-01

    Point defects in InxGa1-xN grown by metal organic chemical vapor deposition were studied by a monoenergetic positron beam. Measurements of Doppler broadening spectra of the annihilation radiation as a function of incident positron energy for InxGa1-xN (x = 0.08 and 0.14) showed that vacancy-type defects were introduced with increasing InN composition. From comparisons between coincidence Doppler broadening spectra and the results calculated using the projector augmented-wave method, the major defect species was identified as the complexes between a cation vacancy and nitride vacancies. The concentration of the defects was found to be suppressed by Mg doping. An effect of Mg-doping on the positron diffusion properties in GaN and InN was also discussed. The momentum distribution of electrons at the InxGa1-xN/GaN interface was close to that in defect-free GaN or InxGa1-xN, which was attributed to the localization of positrons at the interface due to the electric field caused by polarizations.

  13. Positron Annihilations at the Galactic Center: Stringent Constraint on Positron Injection Energies

    Microsoft Academic Search

    Hasan Yüksel

    2007-01-01

    The intense 0.511 MeV gamma-ray line emission from the Galactic Center observed by INTEGRAL requires a large annihilation rate of nonrelativistic positrons. The emission is strongly concentrated at the Galactic Center (GC), in contrast to gamma-ray maps tracing nucleosynthesis (e.g., the 1.809 MeV line from decaying 26Al) or cosmic ray processes (e.g., the 1-30 MeV continuum), which reveal a bright

  14. Neutron irradiation and post annealing effect on sapphire by positron annihilation.

    PubMed

    Han, Jie-cai; Zhang, Hai-liang; Zhang, Ming-fu; Wang, Bao-yi; Li, Zhuo-xin; Xu, Cheng-hai; Guo, Huai-xin

    2010-09-01

    Sapphire single crystals grown by an improved Kyropoulos-like method are irradiated by fast neutron flux. The irradiated doses of neutron are 10(18) and 10(19)n/cm(2). The infrared transmission spectra of sapphire were studied before and after irradiation. The irradiated samples were annealed at 200, 400, 600, 800 and 1000 degrees C for 10min in ambient atmosphere. Positron annihilation studies have been carried out before and after neutron irradiation. The experimentally measured positron lifetime in the pristine specimen is 143ps. There were aluminum vacancies produced in sapphire crystals after neutron irradiation. The positron lifetime increased with the dose of neutron flux. A longer value tau(2) was found after annealing at 600 degrees C, which indicated vacancies were aggregated with each other. The second long-time component tau(2) has been found to increase with the annealing temperature. There was almost no change in peak position of the CDB spectra after neutron irradiation and isothermal annealing. The chemical environment of core in sapphire did not change greatly after neutron irradiation. PMID:20452230

  15. 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.

  16. 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.

  17. One-photon annihilation of thermal positrons with bound atomic electrons

    NASA Technical Reports Server (NTRS)

    Jung, Young-Dae

    1994-01-01

    Direct one-photon annihilation rate of positrons with a bound atomic electron is evaluated in the nonrelativistic limit. The K- and L-shell contributions are estimated including the screening and effective Coulomb repulsion effects. The annihilation rate of thermal positrons is calculated for various temperatures. The total number of one-photon annihilation events in the interstellar medium is discussed. These results provide the directional and structural information for cosmic gamma-ray sources.

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

    SciTech Connect

    Hisano, Junji; Kawasaki, Masahiro [Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582 (Japan); Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa 277-8568 (Japan); Kohri, Kazunori [Physics Department, Lancaster University, Lancaster LA1 4YB (United Kingdom); Nakayama, Kazunori [Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582 (Japan)

    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.

  19. Monte Carlo calculation of energy spectrum and spatial distribution of photons from positron annihilation

    NASA Astrophysics Data System (ADS)

    Capitani, G. P.; De Sanctis, E.; Di Giacomo, P.; Guaraldo, C.; Lucherini, V.; Polli, E.; Reolon, A. R.; Bellini, V.

    1982-12-01

    A Monte Carlo computing program has been used to calculate the photon spectrum and emittance from positron annihilation and bremsstrahlung. The positron energy spread, the energy loss and the multiple scattering in the annihilation target have been taken into account. Moreover, the positron emittance, the positron incidence angle and the finite angular acceptance of the photon collimation channel have been explicitly considered. Experimental results, obtained with a pair spectrometer and a beam profile monitor are in good agreement with calculations. In particular, it is shown that the positron emittance has a crucial influence on the shape of the photon spectrum and on the absolute value of the photon flux.

  20. 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.

  1. Higgs shifts from electron-positron annihilations near neutron stars

    E-print Network

    Wegner, Gary A

    2015-01-01

    We discuss the potential for using neutron stars to determine bounds on the Higgs-Kretschmann coupling by looking at peculiar shifts in gamma-ray spectroscopic features. In particular, we reanalyse multiple lines observed in GRB781119 detected by two gamma-ray spectrometers, and derive an upper bound on the Higgs-Kretschmann coupling that is much more constraining than the one recently obtained from white dwarfs. This calls for targeted analyses of spectra of gamma-ray bursts from more recent observatories, dedicated searches for differential shifts on electron-positron and proton-antiproton annihilation spectra in proximity of compact sources, and signals of electron and proton cyclotron lines from the same neutron star.

  2. Positron lifetime measurements by proton capture F. A. Selim,a

    E-print Network

    Collins, Gary S.

    Positron lifetime measurements by proton capture F. A. Selim,a D. P. Wells, and J. F. Harmon December 2004; published online 22 February 2005 A positron lifetime spectroscopy PLS technique coincident MeV rays, allowing positron lifetime to be measured. One quantum provides a start signal

  3. On the 511 keV emission line of positron annihilation in the Milky Way

    E-print Network

    N. Prantzos

    2008-09-15

    I review our current understanding of positron sources in the Galaxy, on the basis of the reported properties of the observed 511 keV annihilation line. It is argued here that most of the disk positrons propagate away from the disk and the resulting low surface brightness annihilation emission is currently undetectable by SPI/INTEGRAL. It is also argued that a large fraction of the disk positrons may be transported via the regular magnetic field of the Galaxy into the bulge and annihilate there. These ideas may alleviate current difficulties in interepreting INTEGRAL results in a "conventional" framework.

  4. Glass transition of polystyrene near the surface studied by slow-positron-annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Jean, Y. C.; Zhang, Renwu; Cao, H.; Yuan, Jen-Pwu; Huang, Chia-Ming; Nielsen, B.; Asoka-Kumar, P.

    1997-10-01

    We have measured the Doppler broadening energy spectra of positron annihilation using a variable monoenergetic positron beam as a function of positron energy and of temperature in polystyrene. The S parameters from the energy spectra versus temperature show that onset temperatures significantly decrease as the positron energy decreases. This result gives direct evidence that the glass-transition temperature is suppressed at a magnitude of 57 °C at the depth of 50 Å from the surface.

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

    SciTech Connect

    Amsterdamski, P.; Haensel, P. (Nicolaus Copernicus Astronomical Center, 00-716 Warszawa, Bartycka (Poland))

    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. 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.

  7. Identification of vacancy complexes in Si by positron annihilation

    NASA Astrophysics Data System (ADS)

    Saarinen, K.; Ranki, V.

    2003-10-01

    We show that the detailed atomic structure of vacancy complexes in Si can be experimentally determined by combining positron lifetime and electron momentum distribution measurements. The divacancies, vacancy-oxygen complexes (A-centres) and vacancies paired with a donor impurity, V-P and V-As (E-centres), are identified in electron-irradiated Si. The formation of native vacancy defects is observed in highly As-doped Si at the doping level of 1020 cm-3. The defects are identified as monovacancies surrounded by three As atoms. We verify the formation mechanism of these defects by annealing experiments in irradiated Si. We show that the migration of V-As pairs at 450 K leads to the formation of V-As2 complexes, which convert to V-As3 defects at 700 K. These results explain the electrical deactivation and clustering of As in epitaxial or ion-implanted Si during postgrowth heat treatment at 700 K.

  8. Deconvolution of positron annihilation coincidence Doppler broadening spectra using iterative projected Newton method with non-negativity

    E-print Network

    Soatto, Stefano

    1 Deconvolution of positron annihilation coincidence Doppler broadening spectra using iterative retrieval of the underlying positron-electron momentum distributions in the low momentum region- positron Annihilating Pairs (MDAP) [1]. In regular solids the MDAP can be used to check theoretical band

  9. Doppler broadening positron annihilation measurements of lattice defects during transformation cycling in Cu-Zn-Al alloy

    SciTech Connect

    Zhongguo Wei (China Univ. of Mining and Tech., Jiangsu (China). Dept. of Mechanical Engineering); Dazhi Yang (Dalian Univ. of Technology (China). Dept. of Materials Engineering); Wu, K.H. (Florida International Univ., Miami, FL (United States). Dept. of Mechanical Engineering)

    1993-09-01

    Positron Annihilation Techniques have been proved to be a characteristic tool in the research of lattice defects in metals and alloys. Especially, vacancy type point defects and dislocations can be detected. During recent years, researchers have completed Doppler broadening and positron lifetime measurements to study the phase transformations and behavior of vacancies in CuZnAl, CuAlNi, CuAlMn and NiTi shape memory alloys. Also, research has demonstrated that positrons are sensitive to the evolution of defects, during aging and thermal cycling. From common knowledge, the authors know that copper base shape memory alloys contain a high concentration of excess vacancies once the alloys are quenched from the high temperature single beta phase. Also, the excess vacancies play a very important role in the complicated aging effects of both the parent and martensite phases. The present work intends, by means of Doppler broadening positron annihilation measurements, to investigate the behavior of quenched-in vacancies and the evolution of defects during martensitic transformation cycling.

  10. Polarized hadron pair production from electron-positron annihilation

    NASA Astrophysics Data System (ADS)

    Pitonyak, D.; Schlegel, M.; Metz, A.

    2014-03-01

    We study the production of two almost back-to-back hadrons from the annihilation of an electron and a positron, allowing for the polarization of all particles involved. In particular, we conduct a general (model-independent) structure function decomposition of the cross section for the case e+e-??*?hahbX. Moreover, using the parton model we calculate the relevant structure functions in terms of twist-2 transverse momentum dependent (TMD) fragmentation functions (FFs). We also give results for the situation e+e-?Z*?hahbX (including ?-Z interference) within this model. This is the first time a complete framework has been presented for the examination of TMD FFs within e+e-?hahbX. We also specify certain parts of our analysis that hold for the triple-polarized semi-inclusive deep-inelastic scattering process and for di-hadron fragmentation. Furthermore, we give an explicit prescription of how our work can be translated to the Drell-Yan reaction, which provides, for the first time, full results for double-polarized Drell-Yan that include electroweak effects. We further discuss the relevance of our e+e-?hahbX results for future experiments at e+e- machines.

  11. Could electron-positron annihilation lines in the Galactic center result from pulsar winds?

    E-print Network

    W. Wang; C. S. J. Pun; K. S. Cheng

    2005-09-26

    we study the possibility of pulsar winds in the Galactic center (GC) to produce the 511 keV line. We propose that there may exist three possible scenarios of pulsar winds as the positron sources: normal pulsars; rapidly spinning strongly magnetized neutron stars in GRB progenitors; a population of millisecond pulsars in the Galactic center region. These $e^\\pm$pairs could be trapped in the region by the magnetic field in the GC, and cool through the synchrotron radiation and Coulomb interactions with the medium. The cooling timescales are lower than the diffuse timescale of positrons, so low energy positrons could annihilate directly with electrons into 511 keV photons or form positronium before annihilation. We find that normal pulsars cannot be a significant contributor to the positron sources. Although magnetars in the GC could be potential sources of positrons, their birth rate and birth locations may impose some problems for this scenario. We believe that the most likely candidate positron sources in the GC may be a population of millisecond pulsars in the GC. Our preliminary estimations predict the e$^\\pm$ annihilation rate in the GC is $> 5\\times 10^{42}$ s$^{-1}$. Therefore, the $e^\\pm$ pairs from pulsars winds can contribute significantly to the positron sources in the GC. Furthermore, since the diffusion length of positrons is short, we predict that the intensity distribution of the annihilation line should follow the distribution of millisecond pulsars, which should correlate to the mass distribution in the GC.

  12. A new way of using positron-lifetime measurements to study lattice defects

    E-print Network

    Boyer, Edmond

    39 A new way of using positron-lifetime measurements to study lattice defects Ch. Janot, B. George). Abstract. 2014 With the aim of extending positron-lifetime measurements to high temperature and avoiding surface and source contributions, experiments have been performed with positron emitting isotopes

  13. Study of bicontinuous phase in (TTAB+pentanol)/water/n-octane reverse micellar system using positron lifetime spectroscopy

    NASA Astrophysics Data System (ADS)

    Chandramani Singh, K.; Yadav, R.; Khani, P. H.

    2013-06-01

    A phase diagram of (TTAB+pentanol)/water/n-octane has been mapped by using optical method. It exhibits a reverse micellar (L2) phase extending over a wide range of concentrations of the constituents. To investigate the fine structure of the L2 phase, a series of (TTAB+pentanol)/n-octane ternary mixtures having initial concentrations of (TTAB+pentanol) (1:1) in n-octane as 35%, 50% and 65% by weight were prepared. In each of these mixtures, positron lifetime measurements were performed as a function of the concentration of water, using a standard lifetime spectrometer. At water concentrations of 11.8%, 8.5% and 8.4% by weight respectively for the above systems, the o-Ps pick-off lifetime ?3 shows an oscillatory behaviour while I3 representing the Ps formation exhibits an abrupt change. These changes in the positron annihilation parameters have been explained on the basis of onset of bicontinuity in the microemulsion phase. The positron annihilation technique thus suggests the existence of droplet-like and bicontinuous structures in the L2 phase which is otherwise considered optically to be a single phase as the system remains clear and isotropic throughout this phase. Supporting evidence has been provided by the electrical conductivity measurements performed in these systems. These results are presented in this paper.

  14. Positron Annihilations at the Galactic Center: Generating More Questions Than Answers

    E-print Network

    Hasan Yuksel

    2006-09-06

    The bulge of our Galaxy is illuminated by the 0.511 MeV gamma-ray line flux from annihilations of nonrelativistic positrons. The emission is strongly concentrated at the Galactic Center, in contrast to gamma-ray maps tracing nucleosynthesis (e.g., the 1.809 MeV line from decaying ^26Al) or cosmic ray processes (e.g., the 1-30 MeV continuum), which reveal a bright disk with a much less prominent central region. If positrons are generated at relativistic energies, higher-energy gamma rays will also be produced from inflight annihilation of positrons on ambient electrons. The comparison of the gamma-ray spectrum from inflight annihilation to the observed diffuse Galactic gamma-ray data constrains the injection energies of Galactic positrons to be less than 3 MeV.

  15. Positron annihilation spectroscopy study of rubber-carbon black composites

    NASA Astrophysics Data System (ADS)

    Jobando, Vincent Okello

    The focus of this research was to use Positron Annihilation Spectroscopy (PAS) to investigate the response of rubber and rubber-carbon black composites subjected to different physical conditions. The work examined the effect of deforming rubber and rubber filled with carbon black. The results showed that deformation of the rubber depends on whether the sample is filled with carbon black (CB) or not. CB, we propose impedes the aligning of the rubber chains during deformation. Aging of rubber was done and natural rubber was found to exhibit reversion property of its chains from a vulcanized state to un-vulcanized gum state as opposed to synthetic rubbers. This shows how vulnerable the natural rubber chains are at high temperature. We also found that heat can induce crystallization in the rubber chain network. The most common type of rubber crystallization inducement is through strain, which has been studied in detail. In our investigation, we have found that when rubber is heated and allowed to cool slowly to room temperature, its chains can align themselves in an orderly fashion many times leading to crystal growths. Heat also favors oxidation of the rubber chains, hence causing their quick degradation. We studied the effect of sulfur in the cross-linking of rubber. We found that during vulcanization, sulfur cross-links rubber chains by tying them together in a network like structure reducing the chains' mobility. The work also explored the positronium formation in liquids and some common polymers then compared the results with those found from rubber. It was found that Ps formation depends on the nature of the liquid. We found that the results for rubber were similar to those of liquids and concluded that rubber behaves more like a liquid. At room temperature, rubber is far away from its glass transition temperature hence has soft and flexible chains. Ps atom can thus dig itself a cavity within the rubber chains and live longer in it. This explanation was explored through the bubble model.

  16. Positron lifetime spectroscopy for investigation of thin polymer coatings

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Sprinkle, Danny R.; Eftekhari, Abe

    1993-01-01

    In the aerospace industry, applications for polymer coatings are increasing. They are now used for thermal control on aerospace structures and for protective insulating layers on optical and microelectronic components. However, the effectiveness of polymer coatings depends strongly on their microstructure and adhesion to the substrates. Currently, no technique exists to adequately monitor the quality of these coatings. We have adapted positron lifetime spectroscopy to investigate the quality of thin coatings. Results of measurements on thin (25-micron) polyurethane coatings on aluminum and steel substrates have been compared with measurements on thicker (0.2-cm) self-standing polyurethane discs. In all cases, we find positron lifetime groups centered around 560 psec, which corresponds to the presence of 0.9-A(exp 3) free-volume cells. However, the number of these free-volume cells in thin coatings is larger than in thick discs. This suggests that some of these cells may be located in the interfacial regions between the coatings and the substrates. These results and their structural implications are discussed in this report.

  17. 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.

  18. 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.

  19. Characterization of interfaces in Binary and Ternary Polymer Blends by Positron Lifetime Spectroscopy

    NASA Astrophysics Data System (ADS)

    Ranganathaiah, C.

    2015-06-01

    A miscible blend is a single-phase system with compact packing of the polymeric chains/segments due configuration/conformational changes upon blending. Differential Scanning Calorimetry (DSC) is the most employed method to ascertain whether the blend is miscible or immiscible. Positron Lifetime Spectroscopy (PLS) has been employed in recent times to study miscibility properties of polymer blends by monitoring the ortho-Positronium annihilation lifetimes as function of composition. However, just free volume monitoring and the DSC methods fail to provide the composition dependent miscibility of blends. To overcome this limitation, an alternative approach based on hydrodynamic interactions has been developed to derive this information using the same o-Ps lifetime measurements. This has led to the development of a new method of measuring composition dependent miscibility level in binary and ternary polymer blends. Further, the new method also provides interface characteristics for immiscible blends. The interactions between the blend components has a direct bearing on the strength of adhesion at the interface and hence the hydrodynamic interaction. Understanding the characteristic of interfaces which decides the miscibility level of the blend and their end applications is made easy by the present method. The efficacy of the present method is demonstrated for few binary and ternary blends.

  20. 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.

  1. 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.

  2. 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.

  3. A positron annihilation spectroscopic investigation of europium-doped cerium oxide nanoparticles

    NASA Astrophysics Data System (ADS)

    Thorat, Atul V.; Ghoshal, Tandra; Holmes, Justin D.; Nambissan, P. M. G.; Morris, Michael A.

    2013-12-01

    Doping in ceria (CeO2) nanoparticles with europium (Eu) of varying concentrations (0, 0.1, 0.5, ..., 50 atom%) is studied using complementary experimental techniques and novel observations were made during the investigation. The immediate observable effect was a distinct reduction in particle sizes with increasing Eu concentration attributed to the relaxation of strain introduced due to the replacement of Ce4+ ions by Eu3+ ions of larger radius. However, this general trend was reversed in the doping concentration range of 0.1-1 atom% due to the reduction of Ce4+ to Ce3+ and the formation of anion vacancies. Quantum confinement effects became evident with the increase of band gap energy when the particle sizes reduced below 7-8 nm. Positron annihilation studies indicated the presence of vacancy type defects in the form of vacancy clusters within the nanoparticles. Some positron annihilation was also seen on the surface of crystallites as a result of diffusion of thermalized positrons before annihilation. Coincidence Doppler broadening measurements indicated the annihilation of positrons with electrons of different species of atoms and the characteristic S-W plot showed a kink-like feature at the particle sizes where quantum confinement effects began.Doping in ceria (CeO2) nanoparticles with europium (Eu) of varying concentrations (0, 0.1, 0.5, ..., 50 atom%) is studied using complementary experimental techniques and novel observations were made during the investigation. The immediate observable effect was a distinct reduction in particle sizes with increasing Eu concentration attributed to the relaxation of strain introduced due to the replacement of Ce4+ ions by Eu3+ ions of larger radius. However, this general trend was reversed in the doping concentration range of 0.1-1 atom% due to the reduction of Ce4+ to Ce3+ and the formation of anion vacancies. Quantum confinement effects became evident with the increase of band gap energy when the particle sizes reduced below 7-8 nm. Positron annihilation studies indicated the presence of vacancy type defects in the form of vacancy clusters within the nanoparticles. Some positron annihilation was also seen on the surface of crystallites as a result of diffusion of thermalized positrons before annihilation. Coincidence Doppler broadening measurements indicated the annihilation of positrons with electrons of different species of atoms and the characteristic S-W plot showed a kink-like feature at the particle sizes where quantum confinement effects began. Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr03936f

  4. Nondestructive monitoring of fatigue damage evolution in austenitic stainless steel by positron-lifetime measurements

    Microsoft Academic Search

    Uwe Holzwarth; Petra Schaaff

    2004-01-01

    Positron-lifetime measurements have been performed on austenitic stainless steel during (i) stress- and (ii) strain-controlled fatigue experiments for different applied stress and strain amplitudes, respectively. For this purpose a generator-detector assembly with a 72Se\\/72As positron generator [maximum activity 25 muCi (0.9 MBq)] has been mounted on mechanical testing machines in order to measure the positron lifetime without removing the specimens

  5. Test of the second postulate of special relativity using positron annihilation

    NASA Astrophysics Data System (ADS)

    Dryzek, Jerzy; Singleton, Douglas

    2007-08-01

    An experiment to directly test the second postulate of special relativity is described. The speed of photons, resulting from the annihilation of either thermal positrons or in-flight positrons (moving with relativistic velocity), is measured using two complementary variations of the same basic experiment. For both at rest and moving positrons the constancy of the speed of light was confirmed to an accuracy of approximately 1%. This apparatus can be used in an advanced undergraduate laboratory and also used to place limits on alternative theories to special relativity that have transformations other than the Lorentz transformation.

  6. 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.

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

    SciTech Connect

    Fazleev, N. G. [Department of Physics, Box 19059, University of Texas at Arlington, Arlington Texas 76019 (United States) and Institute of Physics, Kazan Federal University, Kremlevskaya18, Kazan 420008 (Russian Federation); Weiss, A. H. [Department of Physics, Box 19059, University of Texas at Arlington, Arlington Texas 76019 (United States)

    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.

  8. The Positron Annihilation Study of Hydrogen Charged Copper and Copper-Aluminum Alloys

    Microsoft Academic Search

    Yi. Pan

    1986-01-01

    Hydrogen effects on Cu and Cu-Al alloys of varying stacking fault energies were investigated mainly by using the positron annihilation Doppler broadening technique. Samples were charged with hydrogen by the thermal hydrogen charging or, for comparison, treated at the same condition but in argon atmosphere. Hardening of Cu and Cu-Al alloys caused by hydrogen did not coincide with a narrowing

  9. EERAD3: Event shapes and jet rates in electron-positron annihilation at order ?s3

    NASA Astrophysics Data System (ADS)

    Gehrmann-De Ridder, A.; Gehrmann, T.; Glover, E. W. N.; Heinrich, G.

    2014-12-01

    The program EERAD3 computes the parton-level QCD contributions to event shapes and jet rates in electron-positron annihilation to order ?s3. For three-jet production and related observables, this corresponds to next-to-next-to-leading order corrections, and allows for precision QCD studies. We describe the program and its usage in detail.

  10. Positron annihilation studies of the f-electron character in actinides.

    PubMed

    Rusz, J; Biasini, M; Czopnik, A

    2004-10-01

    Measurement of the angular correlation of the electron-positron annihilation radiation complemented with ab initio calculations can provide decisive information about the character of f electrons in actinide compounds. Our studies of the antiferromagnet UGa3 in the paramagnetic phase produce substantial evidence that an unconstrained f-electron itinerant description applies. PMID:15524913

  11. Gamma-ray spectra of methane in the positron-electron annihilation process

    E-print Network

    Xiaoguang Ma; Feng Wang

    2013-03-20

    Bound electron contribution to the Doppler-shift of gamma-ray spectra in the positron-electron annihilation process of molecular methane has been studied in gas phase. Two accurate ab initio quantum mechanical schemes, i.e. the delocalized molecular orbital (MO) and the localized natural bond orbital (NBO) schemes, are applied to study the multi-centred methane molecule. The present ab initio calculations of methane indicate that the C-H bonds are polarized with the partial negative charge of -0.36 a.u. on the carbon atom and the partial positive charge of +0.09 a.u. on each of the hydrogen atoms. The positively charged hydrogen atoms produce repulsive Coulomb potentials to a positron. Both the MO and NBO schemes further reveal that the 2a1 electrons of methane, that is, the 2a1 electron component of the C-H bonds rather than the whole C-H bonds of methane, predominates the positron-electron annihilation gamma-ray spectra of the molecule. Electrons of a molecule which are dominant the positron-electron annihilation processes are called positrophilic electrons in the present study. It is further shown that the negative electrostatic potential (ESP) of methane facilitates with the density of the positrophilic 2a1 electrons of methane. Other valence electrons (e.g. 1t2) in the C-H bonds play a minor spectator role in the annihilation process of methane.

  12. 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.

  13. Positron annihilation in high-T/sub c/ superconductors

    SciTech Connect

    Singh, D.; Pickett, W.E.; Cohen, R.E.; Krakauer, H.; Berko, S.

    1989-05-01

    We report ab initio calculations of positron wave functions in the high-T/sub c/ superconductors YBa/sub 2/Cu/sub 3/O/sub 7/, Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/, and Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ using the general potential linearized augmented plane-wave method. The calculated positron wave functions are fairly insensitive to whether or not electron-positron correlation is included in the calculation for YBa/sub 2/Cu/sub 3/O/sub 7/ and Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/, but the calculated positron density is quite sensitive to correlation in Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/. While the positron wave function samples primarily the chain region in YBa/sub 2/Cu/sub 3/O/sub 7/, the results indicate that positrons should be good probes of the Cu-O layer-derived electronic states near the Fermi energy in Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ since a large overlap with these states is predicted.

  14. Quality of Heusler single crystals examined by depth-dependent positron annihilation techniques

    NASA Astrophysics Data System (ADS)

    Hugenschmidt, C.; Bauer, A.; Böni, P.; Ceeh, H.; Eijt, S. W. H.; Gigl, T.; Pfleiderer, C.; Piochacz, C.; Neubauer, A.; Reiner, M.; Schut, H.; Weber, J.

    2015-06-01

    Heusler compounds exhibit a wide range of different electronic ground states and are hence expected to be applicable as functional materials in novel electronic and spintronic devices. Since the growth of large and defect-free Heusler crystals is still challenging, single crystals of Fe2TiSn and Cu2MnAl were grown by the optical floating zone technique. Two positron annihilation techniques—angular correlation of annihilation radiation and Doppler broadening spectroscopy (DBS)—were applied in order to study both the electronic structure and lattice defects. Recently, we succeeded to observe clearly the anisotropy of the Fermi surface of Cu2MnAl, whereas the spectra of Fe2TiSn were disturbed by foreign phases. In order to estimate the defect concentration in different samples of Heusler compounds, the positron diffusion length was determined by DBS using a monoenergetic positron beam.

  15. Positron annihilation and TEM studies on ion irradiated Fe and Fe-Cr model alloys of ferritic/martensitic steel

    NASA Astrophysics Data System (ADS)

    Huang, S. S.; Sato, K.; Xu, Q.; Yoshiie, T.; Tsuchida, H.; Itoh, A.

    2014-12-01

    Fe ion irradiation-induced defects in F82H model alloys were studied using positron annihilation lifetime measurements. Irradiation was performed with an accelerating voltage of 6.0 MeV to a dose of 22 dpa at 593 K. In addition, a comparative study was conducted with results from our previous neutron irradiation measurements for F82H model alloys. The results showed an apparent decrease of the mobility of interstitial clusters in Fe by addition of Cr. This finding supports the configuration model of Cr on void swelling resistance in Fe-Cr alloys. A remarkable delay of defect structural evolution was also observed by ion irradiation compared to neutron irradiation because of a high damage rate.

  16. 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.

  17. Prevacancy effects in metals observed by positron annihilation

    SciTech Connect

    Smedskjaer, L.C.

    1982-03-01

    The prevacancy effects sometimes observed in high-purity, well-annealed metals, are discussed. It is concluded that these effects are extrinsic and are most likely due to positron trapping in defects. The nature of the defects is discussed, and it is pointed out that the presence of dislocations in the samples could cause prevacancy effects.

  18. 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.

  19. Positron Annihilation Spectroscopy Study of Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

    NASA Astrophysics Data System (ADS)

    Merida, David; Garcia, Jose Angel; Apiñaniz, Estibaliz; Plazaola, Fernando; Sanchez-Alarcos, Vicente; Pérez-Landazábal, Jose Ignacio; Recarte, Vicente

    We have studied the role that vacancy type defects play in the martensitic transformation of Ni-Mn-Ga ferromagnetic shape memory alloys by means of positron lifetime spectroscopy. The measurements presented in this work have been performed in five ternary alloys. Three of them transform to modulated and two to non-modulated martensitic phases. With these five samples we cover a large range in composition. Positron experiments have been performed at room temperature after subsequent isochronal annealing at different temperatures and up to a maximum temperature of 600°C. Results show a large variation of the average positron lifetime value with the isochronal annealing temperature in non-modulated samples. However, the response in the modulated samples is quite different. The results are discussed in term of different type of positron trapping defects and their evolution with the annealing temperature. The present work shows a correlation between vacancy concentration and martensitic transformation temperature of ferromagnetic shape memory alloys.

  20. Single crystal growth of Ga[subscript 2](Se[subscript x]Te[subscript 1;#8722;x])[subscript 3] semiconductors and defect studies via positron annihilation spectroscopy

    SciTech Connect

    Abdul-Jabbar, N.M.; Bourret-Courchesne, E.D.; Wirth, B.D. (UCB); (Tennessee-K); (LBNL)

    2012-12-10

    Small single crystals of Ga{sub 2}(Se{sub x}Te{sub 1-x}){sub 3} semiconductors, for x = 0.1, 0.2, 0.3, were obtained via modified Bridgman growth techniques. High resolution powder x-ray diffractometry confirms a zincblende cubic structure, with additional satellite peaks observed near the (111) Bragg line. This suggests the presence of ordered vacancy planes along the [111] direction that have been previously observed in Ga{sub 2}Te{sub 3}. Defect studies via positron annihilation spectroscopy show an average positron lifetime of {approx} 400 ps in bulk as-grown specimens. Such a large lifetime suggests that the positron annihilation sites in these materials are dominated by defects. Moreover, analyzing the electron momenta via coincidence Doppler broadening measurements suggests a strong presence of large open-volume defects, likely to be vacancy clusters or voids.

  1. Comparative study of the binary icosahedral quasicrystal Cd5.7 Yb and its crystalline 1/1 -approximant Cd6 Yb by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Sato, K.; Kobayashi, Y.; Arinuma, K.; Kanazawa, I.; Tamura, R.; Shibuya, T.; Takeuchi, S.

    2004-09-01

    Previously, we showed that the icosahedral quasicrystal Cd5.7Yb possesses similar structural vacancies to those in its cubic 1/1 -approximant Cd6Yb by positron lifetime measurements [K. Sato, H. Uchiyama, K. Arinuma, I. Kanazawa, R. Tamura, T. Shibuya, and S. Takeuchi, Phys. Rev. B 66, 052201 (2002)]. In the present paper, the local chemical environment around the structural vacancies is specifically investigated by two-detector coincident Doppler broadening spectroscopy. Essentially the same annihilation sites with Cd-rich chemical environments are identified for the two phases. This strongly suggests that the quasicrystal is composed of the same cluster as the approximant. The difference in the structural vacancy density between the two phases is examined by positron diffusion experiments using a slow positron beam. The structural vacancy density in the quasicrystal is found to be 20% lower than that in the approximant.

  2. An interpretation of the narrow positron annihilation feature from X-ray nova Muscae 1991

    NASA Technical Reports Server (NTRS)

    Chen, Wan; Gehrels, Neil; Cheng, F. H.

    1993-01-01

    The physical mechanism responsible for the narrow redshifted positron annihilation gamma-ray line from the X-ray nova Muscae 1991 is studied. The orbital inclination angle of the system is estimated and its black hole mass is constrained under the assumptions that the annihilation line centroid redshift is purely gravitational and that the line width is due to the combined effect of temperature broadening and disk rotation. The large black hole mass lower limit of 8 solar and the high binary mass ratio it implies raise a serious challenge to theoretical models of the formation and evolution of massive binaries.

  3. 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 ?-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 ? 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.

  4. Characterisation of epoxy systems by positron annihillation and near infrared spectroscopy

    SciTech Connect

    Simon, G.; Gonis, J.; Hill, A. [Monash Univ., Clayton, Victoria (Australia)] [and others

    1995-12-31

    In order to better understand highly crosslinked epoxy resins such as those used in the aerospace industry, it is necessary to use a wide range of characterisation techniques. In this study, a range of chemical, spectroscopic, mechanical and thermal techniques are used to monitor the polymerisation of a commercial tri-functional epoxy resin and the results compared. Thermal analysis is used to determine the kinetics of the cure, to mark the onset of vitrification and to determine the change in glass transition as a function of cure for this highly branched system. This is compared with torsion braid analysis, capable of determining the point of gelation and vitrification. In order to give a more quantitative understanding of the cure, near infrared spectroscopy (NIR) can be used to determine the concentration of various functional groups as a function of cure. This is easier than using the mid-IR region due to a {open_quotes}cleaner{close_quotes} region of the spectrum. Using this technique it is possible to observe when side reactions such as etherification occur (well into the glassy state), indicating that it will largely result in cyclisation. The effect of postcure on functional groups can also be determined. Positron annihilation lifetime spectroscopy (PALS), a non-destructive, molecular technique capable of measuring directly the {open_quotes}free{close_quotes} or excluded volume of the network is also used to follow the cure of the material from the gel point to vitrification. There is little change in the free volume parameters during this large part of the cure, indicating the free volumes that this technique measures (of the order of 0.5 to 1 nm) are smaller than the gross changes in free volume and mobility that occur during vitrification.

  5. Positron annihilation study of free volume in electron irradiated Zr50Cu40Al10 bulk glassy alloy: Effects of thermal relaxation before irradiation

    NASA Astrophysics Data System (ADS)

    Onodera, N.; Ishii, A.; Iwase, A.; Yokoyama, Y.; Sato, K.; Xu, Q.; Yoshiie, T.; Hori, F.

    2013-06-01

    Change in free volume reflects various properties such as hardness and ductility of glassy alloys. The electron- and ion- irradiations affect the free volume of ZrCuAl bulk glassy alloys that is associated with the mechanical properties. In this study, as-quenched Zr50Cu40Al10 bulk glassy alloys and structural relaxed one by annealing for 5 hours at 673K below glass transition temperature (Tg) were irradiated by 8 MeV electrons with a maximum fluence of about 2×1018 e-/cm2 at room temperature. The behaviour of free volume in these samples was investigated by positron annihilation techniques. X-ray diffraction measurements showed that no crystallization occurred after the irradiation. We observed the positron lifetime increased by the irradiation for both as-quenched and structural relaxed samples. The increases in positron lifetime at 2×1018 e-/cm2 were 9 psec for as-quenched and 12 psec for relaxed sample, respectively. In addition, the increase in positron lifetime with irradiation fluence was clearly different for the two types of sample. These facts imply that the thermal relaxation before irradiation of Zr50Cu40Al10 bulk glassy alloy influences the radiation effects, especially the free volume change.

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

    SciTech Connect

    Smedskjaer, L.C. [Argonne National Lab., IL (United States); Bansil, A. [Northeastern Univ., Boston, MA (United States). 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.

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

    SciTech Connect

    Smedskjaer, L.C. (Argonne National Lab., IL (United States)); Bansil, A. (Northeastern Univ., Boston, MA (United States). 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.

  8. Weathering durability of commercial polymeric coatings tested by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    Chen, H.; Peng, Q.; Huang, Y. Y.; Zhang, R.; Li, Y.; Zhang, J.; Wu, Y. C.; Richardson, J. R.; Sandreczki, T. C.; Jean, Y. C.

    2003-10-01

    A series of commercial coatings were prepared according to the industrial specifications and were exposed to Florida natural weathering and controlled UVA irradiation. The Doppler broadening energy spectra (DBES) of positron annihilation were measured as a function of incident positron energy at different periods of weathering. A significant decrease in the S parameter was observed and it was used as an indicator to test coating durability in micro-scale. Application to weathering durability of commercial polymeric coatings under natural weathering and controlled UVA irradiation is investigated by using the S parameter from the DBES.

  9. PHYSICAL REVIEW B 88, 174102 (2013) Positron lifetime measurements of hydrogen passivation of cation vacancies

    E-print Network

    McCluskey, Matthew

    2013-01-01

    of cation vacancies in yttrium aluminum oxide garnets F. A. Selim,1,* C. R. Varney,1 M. C. Tarun,1 M. C oxides based on positron lifetime measurements, infrared spectroscopy, and composition analysis. Defects were characterized in samples of yttrium aluminum garnet grown in O2 or Ar. However, no positron

  10. Z .Applied Surface Science 149 1999 97102 Unfolding positron lifetime spectra with neural networks

    E-print Network

    Pázsit, Imre

    Z .Applied Surface Science 149 1999 97­102 Unfolding positron lifetime spectra with neural networks is based on the use of artificial neural networks ANNs . By using data from simulated positron spectra: Artificial neural networks ANNs ; Amplitudes; Simulation model 1. Introduction Determination of mean

  11. Monte Carlo Simulation of Pileup Effects in the Electron-Positron Annihilation Peak

    NASA Astrophysics Data System (ADS)

    do Nascimento, Eduardo; Fernández-Varea, José M.; Vanin, Vito R.; Maidana, Nora L.

    2011-08-01

    The Monte Carlo code PENELOPE is employed to simulate a typical experimental Doppler broadening coincidence spectrum (DBCS) where the energy spectrum of the photons emitted by the positrons interacting in the sample is recorded with two HPGe detectors in coincidence. The simulated spectrum reproduces well some of the structures observed in the measured DBCS, but not the prominent tails on the low- and high-energy sides of the electron-positron annihilation peak seen in the latter. Ad hoc variations of the cross sections implemented in PENELOPE did not improve the situation. A simple parameterization of the background noise in the DBCS is proposed, and the simulated spectrum is modified to account for pileup effects using this model of the background. The resulting spectrum is in good agreement with the experiment on the high-energy side of the annihilation peak.

  12. 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.

  13. Slow positron annihilation studies of defects in metal implanted TiN coatings

    Microsoft Academic Search

    S. J. Bull; P. C. Rice-Evans; A. Saleh; A. J. Perry; J. R. Treglio

    1997-01-01

    Positron annihilation spectroscopy has been used to investigate the effects of metal ion implantation on the defect density of TiN coatings deposited onto cemented carbide substrates by chemical vapour deposition (CVD). With a range of ions, accelerated up to 70 kV, at doses up to 2.8 × 1017 ions\\/cm2, it was found that defect levels increase with dose, ion energy

  14. 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.

  15. Positron Annihilation Spectroscopy of Nanostructural Features in Model Reactor Pressure Vessel Steels

    Microsoft Academic Search

    S C Glade; B D Wirth; P Asoka-Kumar; P A Sterne; G R Odette

    2003-01-01

    Irradiation embrittlement in nuclear reactor pressure vessel steels results from the formation of a high number density of nanometer sized copper rich precipitates and sub-nanometer defect-solute clusters. We present positron annihilation spectroscopy (PAS) results to characterize the compositions and magnetic character of these defects in model A533B reactor pressure vessel steels. The results confirm the presence of copper-rich precipitates after

  16. Positron Annihilation Spectroscopy of Nanostructural Features in Model Reactor Pressure Vessel Steels

    Microsoft Academic Search

    S. C. Glade; Brian D. Wirth; P. Asoka-Kumar; P. A. Sterne; G. R. Odette

    2004-01-01

    Irradiation embrittlement in nuclear reactor pressure vessel steels results from the formation of a high number density of nanometer sized copper rich precipitates and sub-nanometer defect-solute clusters. We present positron annihilation spectroscopy (PAS) results to characterize the compositions and magnetic character of these defects in model A533B reactor pressure vessel steels. The results confirm the presence of copper-rich precipitates after

  17. State-Selective High-Energy Excitation of Nuclei by Resonant Positron Annihilation

    E-print Network

    N. A. Belov; Z. Harman

    2014-10-23

    In the annihilation of a positron with a bound atomic electron, the virtual gamma photon created may excite the atomic nucleus. We put forward this effect as a spectroscopic tool for an energy-selective excitation of nuclear transitions. This scheme can efficiently populate nuclear levels of arbitrary multipolarities in the MeV regime, including monopole transitions and giant resonances. In certain cases, it may have a higher cross sections than the conventionally used Coulomb excitation.

  18. Experimental results obtained with the positron-annihilation-radiation telescope of the Toulouse-Argonne collaboration

    Microsoft Academic Search

    J. E. Naya; P. von Ballmoos; R. K. Smither; M. Faiz; P. B. Fernandez; T. Graber; F. Albernhe; G. Vedrenne

    1996-01-01

    We present laboratory measurements obtained with a ground-based prototype of the focusing positron-annihilation-radiation telescope developed by the Toulouse-Argonne collaboration. This instrument has been designed to collect 511-keV photons from astrophysical sources when operating as a balloon borne observatory.The ground-based prototype consists of a crystal lens holding small cubes of diffracting germanium crystals and a 3×3 germanium array that detects the

  19. Dark Matter with multi-annihilation channels and AMS-02 positron excess and antiproton

    E-print Network

    Yu-Heng Chen; Kingman Cheung; Po-Yan Tseng

    2015-05-01

    AMS-02 provided the unprecedented statistics in the measurement of the positron fraction from cosmic rays. That may offer a unique opportunity to distinguish the positron spectrum coming from various dark matter (DM) annihilation channels, if DM is the source of this positron excess. Therefore, we consider the scenario that the DM can annihilate into leptonic, quark, and massive gauge boson channels simultaneously with floating branching ratios to test this hypothesis. We also study the impacts from MAX, MED, and MIN diffusion models as well as from isothermal, NFW, and Einasto DM density profiles on our results. We found that under this DM annihilation scenario it is difficult to fit the AMS-02 $\\frac{e^+}{e^++e^-}$ data while evading the PAMELA $\\bar{p}/p$ constraint, except for the combination of MED diffusion model with the Einasto density profile, where the DM mass between 450 GeV to 1.2 TeV can satisfy both data sets at 95\\% CL. Finally, we compare to the newest AMS-02 antiproton data.

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

    SciTech Connect

    Profumo, Stefano [Department of Physics, University of California, 1156 High St, Santa Cruz, CA 95064 (United States); Jeltema, Tesla E., E-mail: profumo@scipp.ucsc.edu, E-mail: tesla@ucolick.org [UCO/Lick Observatories, 1156 High St, Santa Cruz, CA 95064 (United States)

    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.

  1. 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.

  2. 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.

  3. Effect of ?-rays irradiation on Mn-Ni ferrites: Structure, magnetic properties and positron annihilation studies

    NASA Astrophysics Data System (ADS)

    Hassan, H. E.; Sharshar, T.; Hessien, M. M.; Hemeda, O. M.

    2013-06-01

    Manganese-nickel ferrites powder with general formula MnxNi1-xFe2O4 (x = 0.0, 0.2, 0.4) were synthesized through oxalate precursor route and sintered at 1000 °C. The X-ray diffraction (XRD) patterns were measured for the prepared samples to confirm the existence of single-phase structure. The crystallite size was estimated and found to be within the range 125-170 nm. To study the radiation effect on the structure and magnetic properties, a representative group of the investigated samples were irradiated by ?-rays of 60Co source with a dose of 310 kGy. The XRD spectra were performed for the irradiated samples and compared with that of the pristine samples to estimate changes in the structure. The obtained results showed that the crystallite size increased by a factor of 10-16% after gamma irradiation. The lattice parameter also was increased due to the conversion of Fe3+ (0.64 Å) to Fe2+ (0.76 Å). The formula of the cation distribution of the ferrites samples was suggested at x = 0, 0.2, 0.4 before and after irradiation. The theoretical lattice parameter, sample density and porosity were calculated and compared with that obtained from the experimental data. Good agreement was found between theoretical and experimental structural data which confirms the proposed formula of cations distribution. The hysteresis curves were measured using vibrating sample magnetometer (VSM) for the unirradiated and irradiated samples and the saturation magnetization was estimated. The obtained results showed increase in saturation magnetizations (Ms) for all the samples by irradiation due to redistribution of the cations between A and B sites and changing the net magnetic moments. Theoretical calculation of magnetic moments and saturation magnetization using the proposed cations distribution of A and B sites confirmed the experimental results. The positron annihilation lifetime spectroscopy (PALS) was used to investigate the defects and changes in electron density after irradiation. The PAL parameters (?1, I1, ?2, I2 and mean lifetime) show that the irradiation affects the size and concentration of the vacant type defects. The results reveal that there are some large voids (with radius ranged from 0.28 to 0.38 nm and mean value of 0.34 ± 0.04 nm in the studied samples). The obtained results indicate the high sensitivity of PALS technique to the enhanced structure changes due to gamma rays irradiation.

  4. The recovery of electron irradiated zinc and cadmium by positron annihilation spectroscopy

    Microsoft Academic Search

    C. Hidalgo; N. de Diego; P. Moser

    1986-01-01

    Positron lifetime and Doppler-broadening measurements have been performed in Zn and Cd specimens irradiated with 3 MeV electrons at 20 K. Isochronal annealing of the irradiation induced defects between 20 and 280 K has been studied. No evidence of three-dimensional vacancy agglomerates was found.

  5. Positron annihilation study of proton-irradiated reactor pressure vessel steels

    NASA Astrophysics Data System (ADS)

    Liu, Xiangbing; Wang, Rongshan; Ren, Ai; Huang, Ping; Wu, Yichu; Jiang, Jing; Zhang, Chonghong; Wang, Xitao

    2012-10-01

    The microstructures, irradiation-induced defects and changes of mechanical property of Chinese domestic A508-3 steels after proton irradiation were investigated by TEM, positron lifetime, slow positron beam Doppler broadening spectroscopy and hardness measurements. The defects were induced by 240 keV proton irradiation with fluences of 1.25×1017 ions cm-2 (0.26 dpa), 2.5×1017 ions cm-2 (0.5 dpa), and 5.0×1017 ions cm-2 (1.0 dpa). The TEM observation revealed that the as-received steel had typical bainitic-ferritic microstructures. It was also observed that Doppler broadening S-parameter and average lifetime increased with dose level owing to the formation of defects and voids induced by proton irradiation. The correlation between positron parameters and hardness was found.

  6. Two-dimensional angular correlation of annihilation radiation study of positron interactions with surfaces of aluminum

    SciTech Connect

    Chen, D.M.; Berko, S.; Canter, K.F.; Lynn, K.G.; Mills A.P. Jr.; Roellig, L.O.; Sferlazzo, P.; Weinert, M.; West, R.N.

    1989-03-01

    Studies were made of the interactions of positrons with three low index surfaces of aluminum ((100), (110), and (111)) at room temperature with the aluminum surface at elevated temperatures and with oxygen adsorption on the aluminum surface. Our measurements using the improved two-dimensional angular correlation of annihilation radiation technique on surfaces support three established processes: spontaneous positronium (Ps) formation and emission, positrons bound in a surface state, and surface-state positrons thermally desorbed as Ps atoms. A method has been developed to accurately separate these components. The positron surface-state annihilation spectra are nearly isotropic for all three Al surfaces, which indicates a lateral localization and is inconsistent with either the ideal image-potential-induced bound state or the physisorbed Ps state. The thermal Ps momentum distributions agree well with the thermostatistical emission mechanism. The momentum distributions of the directly emitted Ps atoms are found to be very sensitive to the surface conditions, and to reflect the electron density of states near the surfaces, thus suggesting a new surface spectroscopy, angle-resolved Ps spectroscopy.

  7. Gamma-ray spectra of methane in the positron-electron annihilation process

    E-print Network

    Ma, Xiaoguang

    2012-01-01

    Bound electron contribution to the Doppler-shift of gamma-ray spectra in the positron-electron annihilation process of molecular methane has been studied in gas phase. Two accurate ab initio quantum mechanical schemes, i.e. the delocalized molecular orbital (MO) and the localized natural bond orbital (NBO) schemes, are applied to study the multi-centred methane molecule. The present ab initio calculations of methane indicate that the C-H bonds are polarized with the partial negative charge of -0.36 a.u. on the carbon atom and the partial positive charge of +0.09 a.u. on each of the hydrogen atoms. The positively charged hydrogen atoms produce repulsive Coulomb potentials to a positron. Both the MO and NBO schemes further reveal that the 2a1 electrons of methane, that is, the 2a1 electron component of the C-H bonds rather than the whole C-H bonds of methane, predominates the positron-electron annihilation gamma-ray spectra of the molecule. Electrons of a molecule which are dominant the positron-electron annihi...

  8. Positron annihilation on defects in silicon irradiated with 15 MeV protons

    NASA Astrophysics Data System (ADS)

    Arutyunov, N. Y.; Elsayed, M.; Krause-Rehberg, R.; Emtsev, V. V.; Oganesyan, G. A.; Kozlovski, V. V.

    2013-01-01

    Microstructure and thermal stability of the radiation defects in n-FZ-Si ([P] ? 7 × 1015 cm-3) single crystals have been investigated. The radiation defects have been induced by irradiation with 15 MeV protons and studied by means of both the positron lifetime spectroscopy and low-temperature measurements of the Hall effect. At each step of the isochronal annealing over the temperature range ˜60-700?°C the positron lifetime has been measured for the temperature interval ˜30-300 K, and for samples-satellites the temperature dependences of the charge carriers and mobility have been determined over the range ˜4.2-300 K. It is argued that as-grown impurity centers influence the average positron lifetime by forming shallow (Eb ? 0.013 eV) positron states. The radiation-induced defects were also found to trap positrons into weakly bound (Eb ? 0.01 eV) states. These positron states are observed at cryogenic temperatures during the isochronal annealing up to Tanneal. = 340?°C. The stages of annealing in the temperature intervals ˜60-180?°C and ˜180-260?°C reflect the disappearance of E-centers and divacancies, respectively. Besides these defects the positrons were found to be localized at deep donor centers hidden in the process of annealing up to the temperature Tanneal. ? 300?°C. The annealing of the deep donors occurs over the temperature range ˜300-650?°C. At these centers positrons are estimated to be bound with energies Eb ? 0.096 and 0.021 eV within the temperature intervals ˜200-270 K and ˜166-66 K, respectively. The positron trapping coefficient from these defects increases from ˜1.1 × 1016 to ˜6.5 × 1017 s-1 over the temperature range ˜266-66 K, thus substantiating a cascade phonon-assisted positron trapping mechanism whose efficiency is described by ?T-3 law. It is argued that the value of activation energy of the isochronal annealing Ea ? 0.74-0.59 eV is due to dissociation of the positron traps, which is accompanied by restoration of the electrical activity of the phosphorus atoms. The data suggest that the deep donors involve a phosphorus atom and at least two vacancies. Their energy levels are at least at E > Ec - 0.24 eV in the investigated material.

  9. Microstructural evolution of ZnS during sintering monitored by optical and positron annihilation techniques

    NASA Astrophysics Data System (ADS)

    Adams, M.; Mascher, P.; Kitai, A. H.

    1995-07-01

    Positron lifetime and optical absorption techniques were employed to track the microstructural evolution of polycrystalline ZnS grown by Chemical Vapor Deposition (CVD). As grown material and material treated with Hot Isostatic Pressure (HIP) was sintered at temperatures ranging from 400 to 1000 °C for 2 18 h. A 290 ps defect lifetime could be resolved in all samples, while an additional longer lifetime (?=430 ps) was found only in samples annealed at low temperatures. This component gradually disappeared during annealing at 800 ° C. Associated with the disappearance of the long-lived component, the apparent bulk lifetime of the material changed from 235 to 215 ps. A 215±2 ps bulk parameter was also found for HIP-treated material annealed at temperatures greater than 400 ° C and hence is taken to represent the delocalized state of the positrons in ZnS. Optical absorption measurements showed that annealing at 800 ° C also caused the absorption profiles of the CVD and HIP samples to converge. The rate of the bulk lifetime transition correlates with the absorption changes. The observed sharpening of the absorption profile is attributed to a decrease in scattering from grain boundaries and voids, and a decrease in absorption from point defects. The 430 ps lifetime is believed to be due to trapping at voids and grain boundaries, while the 290 ps lifetime likely is due to a monovacancy stabilized as a small complex.

  10. A slow positron beam generator for lifetime studies

    NASA Astrophysics Data System (ADS)

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

    1989-04-01

    A slow positron beam generator using well-annealed polycrystalline tungsten moderators and a Na-22 positron source was developed. A 250 micro c source, deposited on a 2.54 micron thick aluminized mylar, is sandwiched between two (2.54 cm x 2.54 cm x 0.0127 cm) tungsten pieces. Two (2.54 cm x 2.54 cm x t cm) test polymer films insulate the two tungsten moderator pieces from the aluminized mylar source holder (t=0.00127 to 0.0127). A potential difference of 10 to 100 volts--depending on the test polymer film thickness (t)--is applied between the tungsten pieces and the source foil. Thermalized positrons diffusing out of the moderator pieces are attracted to the source foil held at an appropriate potential below the moderator pieces. These positrons have to pass through the test polymer films before they can reach the source foil. The potential difference between the moderator pieces and the aluminized mylar is so adjusted as to force the positrons to stop in the test polymer films. Thus the new generator becomes an effective source of positrons for assaying thin polymer films for their molecular morphology.

  11. 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.

  12. 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 ?-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 ?-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.

  13. Central-field model for the ? spectrum of positrons annihilating on rare-gas atoms

    NASA Astrophysics Data System (ADS)

    Cheng, Yongjun; Mitroy, J.

    2014-10-01

    A central field model is used to study the two-photon positron annihilation spectrum for the rare gas atoms He, Ne, Ar, Kr, and Xe at energies close to thermal. Correlation effects are incorporated with a semiempirical polarization potential. The ? spectrum is given, with values reported for individual subshells. The predicted full widths at half-maximum (FWHM) for all systems are typically 5 %-20 % larger than the experimental values reported using the positron trap at the University of California, San Diego, while, with the exception of neon, generally being 2 %-10 % smaller than the FWHMs measured at University College London. The detailed spectrum for xenon is reported and the likelihood of core annihilation's making a measurable contribution to the observed Doppler spectrum is discussed. The ? spectra are found to be insensitive to variations in the scattering potential and whether the target is represented by a Hartree-Fock or a Dirac-Fock wave function. The model potential used in the solution of the positron-atom Schrödinger equation provides a reasonable fit to recent total elastic cross-section measurements reported by the Australian National University and University of Trento groups.

  14. 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.

  15. Positron annihilation spectroscopy techniques applied to the study of an HPGe detector

    NASA Astrophysics Data System (ADS)

    Nascimento, E. do; Vanin, V. R.; Maidana, N. L.; Silva, T. F.; Rizzutto, M. A.; Fernández-Varea, J. M.

    2013-05-01

    Doppler Broadening Spectroscopy of the large Ge crystal of an HPGe detector was performed using positrons from pair production of 6.13 MeV ?-rays from the 19F(p,??)16O reaction. Two HPGe detectors facing opposite sides of the Ge crystal acting as target provided both coincidence and singles spectra. Changes in the shape of the annihilation peak were observed when the high voltage applied to the target detector was switched on or off, amounting to somewhat less than 20% when the areas of equivalent energy intervals in the corresponding normalized spectra are compared.

  16. Development of positron annihilation spectroscopy for investigating deuterium decorated voids in neutron-damaged tungsten

    SciTech Connect

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

    2014-10-01

    Tritium retention in plasma facing component (PFC) materials will pose an operational and safety concern for future fusion devices. Ion bombardment damages the near surface; however neutron bombardment produces lattice defects throughout the bulk of the material, thus increasing the density of trapping sites and thereby the capacity for retaining tritium by these PFCs. Experiments involving neutron irradiated PFCs are challenging, but are essential in order to accurately assess the possible inventories of hydrogen isotopes that can readily permeate and occupy the voids throughout the bulk of PFCs. Positron annihilation spectroscopy (PAS) is a useful tool in qualitatively examining damage in bulk tungsten. Positrons become trapped in voids, which enables this technique to interrogate monovacancies to large clusters. Positron-electron annihilation emits two 511 keV photons which can easily be detected. Doppler broadening (DB-PAS) of the annihilation radiation allows discriminating samples with varying void concentrations. DB-PAS is parameterized into the S-parameter, which allows for simple comparison of several samples. The present work is a continuation of a recent research [1] to develop and optimize 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 the Idaho National Laboratory. The implanted deuterium was desorbed through sample annealing, and DP-PAS was performed both before and after annealing. Results show that deuterium impregnated tungsten is identified as having a higher S-parameter. The S-parameter decreases after deuterium desorption. In the present work, the application of using DB-PAS to examine deuterium retention in tungsten is examined. [1] C.N. Taylor, M. Shimada, B.J. Merrill, M.W. Drigert, D.W. Akers, Y. Hatano, Development of positron annihilation spectroscopy for characterizing neutron irradiated tungsten, Physica Scripta, In Press 2013. Work supported by DOE ID Field Office contract DE-AC07–05ID14517.

  17. New limits on dark matter annihilation from Alpha Magnetic Spectrometer cosmic ray positron data.

    PubMed

    Bergström, Lars; Bringmann, Torsten; Cholis, Ilias; Hooper, Dan; Weniger, Christoph

    2013-10-25

    The Alpha Magnetic Spectrometer experiment onboard the International Space Station has recently provided cosmic ray electron and positron data with unprecedented precision in the range from 0.5 to 350 GeV. The observed rise in the positron fraction at energies above 10 GeV remains unexplained, with proposed solutions ranging from local pulsars to TeV-scale dark matter. Here, we make use of this high quality data to place stringent limits on dark matter with masses below ~300 GeV, annihilating or decaying to leptonic final states, essentially independent of the origin of this rise. We significantly improve on existing constraints, in some cases by up to 2 orders of magnitude. PMID:24206472

  18. Internal structure and positron annihilation in the four-body MuPs system

    E-print Network

    Alexei M. Frolov

    2015-01-07

    A large number of bound state properties of the four-body muonium-positronium system MuPs (or $\\mu^{+} e^{-}_2 e^{+}$) are determined to high accuracy. Based on these expectation values we predict that the weakly-bound four-body MuPs system has the `two-body' cluster structure Mu + Ps. The two neutral clusters Mu ($\\mu^{+} e^{-}$) and Ps ($e^{+} e^{-}$) interact with each other by the attractive van der Waals forces. By using our expectation values of the electron-positron delta-functions we evaluated the half-life $\\tau_a$ of the MuPs system against annihilation of the electron-positron pair: $\\tau_a = \\frac{1}{\\Gamma} \\approx 4.071509 \\cdot 10^{-10}$ $sec$. The hyperfine structure splitting of the ground state in the MuPs system evaluated with our expectation values is $\\Delta \\approx$ 23.064(5) $MHz$.

  19. 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.

  20. Defect Density Mapping of Shot Peened Materials Using Positron Annihilation Spectroscopy

    SciTech Connect

    Gagliardi, Marcus A.; Hunt, Alan W. [Department of Physics, Idaho State University, Pocatello, Idaho, 83209 (United States); Idaho Accelerator Center, 1500 Alvin Ricken Drive, Pocatello, Idaho, 83201 (United States)

    2009-03-10

    Shot peening is a technique used in industry to increase the fatigue life of components by creating compressive residual stresses in the near surface region. This compressive stress is pinned in the material by defects such as dislocations and monovacancies to which positrons are sensitive. Using a {sup 22}Na source S-parameter measurements were made using positron annihilation spectroscopy (PAS) on both non-peened and peened copper coupons. These measurements show that a correlation exists between copper coupons shot peened at different intensities that in principal can be used for verification of shot intensity. Finally a relative defect density map was produced to show that the shot peening uniformity can also be measured across the surface of a large component.

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

    NASA Astrophysics Data System (ADS)

    ?ížek, J.; Neslušan, M.; ?illiková, M.; Mi?ietová, 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.

  2. PALSfit: A computer program for analysing positron lifetime spectra

    E-print Network

    , Morten Eldrup, and Niels Jørgen Pedersen Risø-R-1652(EN) Risø National Laboratory for Sustainable Energy: 43 Information Service Department Risø National Laboratory for Sustainable Energy Technical) February 2009 Abstract: A Windows based computer program PALSfit has been developed for analysing positron

  3. Positron lifetime measurements as a non-destructive technique to monitor fatigue damage

    Microsoft Academic Search

    Byrne

    1975-01-01

    In the fatigue cycling of initially hard copper, self consistent ; positron lifetime and x-ray particle size measurements followed the softening ; process and revealed a new feature which may be the final development of ; microvoids before fracture. In the cyclic fatigue of initially soft 4340 steel ; closely spaced concurrent measurements of these parameters are now in progress.

  4. Miscibility and phase separation in SAN\\/PMMA blends investigated by positron lifetime measurements

    Microsoft Academic Search

    G. N. Kumaraswamy; C. Ranganathaiah; M. V. Deepa Urs; H. B. Ravikumar

    2006-01-01

    Miscibility and phase separation in SAN\\/PMMA blends have been investigated using DSC, IR spectroscopy and positron lifetime spectroscopy (PLS). Single broad glass transition observed throughout the blend compositions, may be due to overlap of two glass transitions. IR measurements clearly indicate the absence of strong interactions. This supports miscibility is due to intramolecular repulsive forces in the SAN component. On

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

    SciTech Connect

    Idczak, R., E-mail: ridczak@ifd.uni.wroc.pl; Konieczny, R.; Chojcan, J. [Institute of Experimental Physics, University of Wroc?aw, pl. M. Borna 9, 50-204 Wroc?aw (Poland)

    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??m 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.

  6. Probing defects in chemically synthesized ZnO nanostrucures by positron annihilation and photoluminescence spectroscopy

    SciTech Connect

    Chaudhuri, S. K.; Das, D. [UGC-DAE Consortium for Scientific, III/LB-8, Salt Lake, Kolkata 700 098 (India); Ghosh, Manoranjan; Raychaudhuri, A. K. [DST Unit for Nanoscience, S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-3, Salt Lake, Kolkata 700 098 (India)

    2010-09-15

    The present article describes the size induced changes in the structural arrangement of intrinsic defects present in chemically synthesized ZnO nanoparticles of various sizes. Routine x-ray diffraction and transmission electron microscopy have been performed to determine the shapes and sizes of the nanocrystalline ZnO samples. Detailed studies using positron annihilation spectroscopy reveals the presence of zinc vacancy. Whereas analysis of photoluminescence results predict the signature of charged oxygen vacancies. The size induced changes in positron parameters as well as the photoluminescence properties, has shown contrasting or nonmonotonous trends as size varies from 4 to 85 nm. Small spherical particles below a critical size ({approx}23 nm) receive more positive surface charge due to the higher occupancy of the doubly charge oxygen vacancy as compared to the bigger nanostructures where singly charged oxygen vacancy predominates. This electronic alteration has been seen to trigger yet another interesting phenomenon, described as positron confinement inside nanoparticles. Finally, based on all the results, a model of the structural arrangement of the intrinsic defects in the present samples has been reconciled.

  7. 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

  8. Centroid shift of y rays from positron annihilation iin the depletion region of metal-oxide-semiconductor structures

    E-print Network

    Rubloff, Gary W.

    Centroid shift of y rays from positron annihilation iin the depletion region of metal, New York 10598 (Received4 June 1990;acceptedfor publication 6 November 1990) Using metal, then the detector is equally likely to measureupshifted and downshifted energies.The 511 keV peakis Doppler

  9. APPLICATION OF POSITRON ANNIHILATION TO FATIGUE AND PLASTIC DAMAGE DETECTION IN SA508 AND TYPE 304 STEELS

    Microsoft Academic Search

    M. UCHIDA; K. YOSHIDA; Y. G. NAKAGAWA; A. J. ALLEN; A. D. WHAPHAM

    1992-01-01

    The positron annihilation(PA) lineshape analysis method was applied to evaluate the fatigue damage in SA508 Ierritic steel and type 304 stainless steel, and the damage distribution in a type 304 sample plastically deformed by 3-point bending. The PA was found to be effective to detect early stages of the fatigue damage (less than 25% of the cycles to failure) in

  10. 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.

  11. Solid phase transition of overpressurised helium bubbles seen from positron annihilation studies

    NASA Astrophysics Data System (ADS)

    Maji, Sanjib; Singh, Amarjeet; Nambissan, P. M. G.

    2001-03-01

    Positron lifetime and Doppler broadening measurements on highly overpressurised ( P?10 Peq) helium bubbles in Al indicated abnormal variations at low temperatures, which is being assigned to the transformation of dense helium bubbles into the solid phase, as expected from the Simon-Glatzel equation. The bubble pressure calculated using separate models for the fluid and solid phases shows consistency with the helium freezing characteristics. The solid phase transition is further confirmed by verifying an exponential variation of the ultrasonic velocity with the solid bubble pressure.

  12. Positron lifetime spectroscopic studies of nanocrystalline ZnFe2O4

    NASA Astrophysics Data System (ADS)

    Nambissan, P. M. G.; Upadhyay, C.; Verma, H. C.

    2003-05-01

    By carrying out positron lifetime measurements in zinc ferrite (ZnFe2O4) samples of various grain sizes down to 5 nm, the defect microstructures have been identified. In the bulk samples composed of grains of large sizes, positrons were trapped by monovacancies in the crystalline structure. Upon reduction of the grain sizes to nanometer dimensions, positrons get trapped selectively at either the diffused vacancies on the grain surfaces and the intergranular regions. Below about 9 nm, the grains undergo the transformation from the normal spinel structure to the inverse phase. A concomitant lattice contraction results in substantial reduction of the octahedral site volume, and hence, a fraction of the Zn2+ ions with larger ionic radius fails to occupy these sites. This leaves vacancies at the octahedral sites which then turn out to be the major trapping sites for positrons. ZnFe2O4 samples prepared through different routes were investigated, which showed similar qualitative features, although those synthesized through the hydrothermal precipitation method showed remarkably larger lifetimes for trapped positrons upon nanocrystallization in comparison to the samples prepared through the citrate route.

  13. Application of positron annihilation lineshape analysis to fatigue damage and thermal embrittlement for nuclear plant materials

    SciTech Connect

    Uchida, M.; Ohta, Y.; Nakamura, N. [Ishikawajima-Harima Heavy Industries Co., Ltd., Tokyo (Japan); Yoshida, K. [Ishikawajima-Harima Heavy Industries Co., Ltd., Kanagawa (Japan)

    1995-08-01

    Positron annihilation (PA) lineshape analysis is sensitive to detect microstructural defects such as vacancies and dislocations. The authors are developing a portable system and applying this technique to nuclear power plant material evaluations; fatigue damage in type 316 stainless steel and SA508 low alloy steel, and thermal embrittlement in duplex stainless steel. The PA technique was found to be sensitive in the early fatigue life (up to 10%), but showed a little sensitivity for later stages of the fatigue life in both type 316 stainless steel and SA508 ferritic steel. Type 316 steel showed a higher PA sensitivity than SA508 since the initial SA508 microstructure already contained a high dislocation density in the as-received state. The PA parameter increased as a fraction of aging time in CF8M samples aged at 350 C and 400 C, but didn`t change much in CF8 samples.

  14. Positron Annihilation Spectroscopy of Nanostructural Features in Model Reactor Pressure Vessel Steels

    SciTech Connect

    Glade, S C; Wirth, B D; Asoka-Kumar, P; Sterne, P A; Odette, G R

    2003-07-16

    Irradiation embrittlement in nuclear reactor pressure vessel steels results from the formation of a high number density of nanometer sized copper rich precipitates and sub-nanometer defect-solute clusters. We present positron annihilation spectroscopy (PAS) results to characterize the compositions and magnetic character of these defects in model A533B reactor pressure vessel steels. The results confirm the presence of copper-rich precipitates after irradiation. The measured orbital electron momentum spectra indicate the precipitates are alloyed with Mn and Ni. The copper precipitates larger than R {approx} 1.2 nm (from SANS measurements) are non-magnetic, which limits the possible Fe content of the precipitates to at most a few %. Notably, large vacancy clusters observed in neutron irradiated Fe-Cu alloys were not observed in the steels after irradiation.

  15. Photoluminescence and positron annihilation spectroscopy investigation of (Ge, Er) codoped Si oxides deposited by magnetron sputtering

    SciTech Connect

    Heng, C. L.; Chelomentsev, E.; Peng, Z. L.; Mascher, P. [Department of Engineering Physics and Centre for Emerging Device Technologies, McMaster University, Hamilton, Ontario L8S 4K1 (Canada); Simpson, P. J. [Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7 (Canada)

    2009-01-01

    We have investigated the nature of violet-blue emission from (Ge, Er) codoped Si oxides (Ge+Er+SiO{sub 2}) using photoluminescence (PL) and positron annihilation spectroscopy (PAS) measurements. The PL spectra and PAS analysis for a control Ge-doped SiO{sub 2} (Ge+SiO{sub 2}) indicate that Ge-associated neutral oxygen vacancies (Ge-NOV) are likely responsible for the major emission in the violet-blue band. For Ge+Er+SiO{sub 2}, both Ge-NOV and GeO color centers are believed to be responsible for the emission band. The addition of Er has a significant influence on the emission, which is discussed in terms of Er-concentration-related structural change in the Ge+Er+SiO{sub 2}.

  16. 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 success of the conference. We are grateful to all participants for their informative talks, poster presentations and fruitful discussions; the session chairs for keeping to the tight time schedule and for making sure the oral presentation sessions ran smoothly; Maria Dugdale for her time and effort in organising the social programme for the accompanying persons; the student volunteers from the Bristol Positron Group for all their help and time before, during and after the conference; the Bath positron group for helping with the organisation of the excursion and last, but not least, the University of Bristol Conference Office staff for their help with the organisation of the conference. We are also very grateful for the financial and logistical help from the University of Bristol and financial support from our sponsors and exhibitors, Ortec and Canberra. We conclude by wishing the Organising Committee of ICPA-17 all the best for a successful conference. We look forward to seeing everyone in China in 2015. Ashraf Alam, Paul Coleman, Stephen Dugdale and Mina Roussenova Guest Editors Bristol, April 2013 Local organising committeeInternational Advisory committee M A Alam, S Dugdale and M Roussenova P Coleman (UK, Chairman) University of Bristol, UK R Krause-Rehberg(Germany, Vice-chairman) P Coleman and S Townrow M A Alam (UK) University of Bath, UK G Laricchia (UK) M Charlton R Brusa (Italy) University of Swansea, UK M Doyama (Japan) G Laricchia B Ganguly (India) University College London, UK C Hugenschmidt (Germany) D Keeble Zs Kajcsos (Hungary, deceased) University of Dundee, UK Y Kobayashi (Japan) J Kuriplach (Czech Republic) P Mascher (Canada) A Mills (USA) Y Nagashima (Japan) Steering committee M Puska (Finland) M A Alam (UK, Secretary) H Schut (Netherlands) P Coleman (UK) A Seeger (Germany) B Ganguly (India) Y Shirai (Japan) Y Kobayashi (Japan) A Somoza (Argentina) P Mascher (Canada) A Stewart (Canada) H Schut (Netherlands) Z Tang (China) R Krause-Rehberg (Germany) A Weiss (USA) Sponsor logos Conference photograph

  17. 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.

  18. Investigation of vacancy defect in InP crystal by positron lifetime measurement

    Microsoft Academic Search

    Niefeng Sun; Luhong Mao; Weidong Mao; Hezou Wang; Xiang Wu; Keyun Bi; Zhengping Zhao; Weilian Guo; Xiawan Wu; Xiaolong Zhou; Bingke Chen; Yanjun Zhao; Kewu Yang; Tongnian Sun

    2008-01-01

    Positron lifetime measurements have been carried out on liquid-encapsulated Czochralski-grown undoped InP samples sliced from the middle part of ingots over the temperature range 10–300K. And at 70K, the spectra have been measured in darkness, under illumination of infrared LED, and with illumination off is one sample. The measurements at low temperature reveal different concentration of hydrogen indium vacancy complex

  19. Comparative study of the binary icosahedral quasicrystal Cd{sub 5.7}Yb and its crystalline 1/1-approximant Cd{sub 6}Yb by positron annihilation spectroscopy

    SciTech Connect

    Sato, K.; Kobayashi, Y.; Arinuma, K.; Kanazawa, I.; Tamura, R.; Shibuya, T.; Takeuchi, S. [National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan); Department of Physics, Tokyo Gakugei University, 4-1-1 Koganei, Tokyo 184-8501 (Japan); Department of Materials Science and Technology, Tokyo University of Science, Noda, Chiba 278-8501 (Japan)

    2004-09-01

    Previously, we showed that the icosahedral quasicrystal Cd{sub 5.7}Yb possesses similar structural vacancies to those in its cubic 1/1-approximant Cd{sub 6}Yb by positron lifetime measurements [K. Sato, H. Uchiyama, K. Arinuma, I. Kanazawa, R. Tamura, T. Shibuya, and S. Takeuchi, Phys. Rev. B 66, 052201 (2002)]. In the present paper, the local chemical environment around the structural vacancies is specifically investigated by two-detector coincident Doppler broadening spectroscopy. Essentially the same annihilation sites with Cd-rich chemical environments are identified for the two phases. This strongly suggests that the quasicrystal is composed of the same cluster as the approximant. The difference in the structural vacancy density between the two phases is examined by positron diffusion experiments using a slow positron beam. The structural vacancy density in the quasicrystal is found to be 20% lower than that in the approximant.

  20. Momentum density measurements by positron annihilation in metals and alloys; Recent experiments with a multicounter two-dimensional angular correlation apparatus

    Microsoft Academic Search

    S. Berko; J. Mader

    1975-01-01

    The technique of positron annihilation as applied to the study of momentum densities and Fermi surfaces is reviewed. The angular\\u000a correlation of the two annihilation photons is directly related to the momentum distribution of the positron-electron system;\\u000a breaks in this distribution reveal the size and shape of the Fermi surface. After a general introduction to the theory and\\u000a the experimental

  1. Positron lifetime in vacancy-impurity complexes (*) Section d'Etudes des Solides Irradis, Centre d'Etudes Nuclaires, Boite Postale n 6, 92260 Fontenay aux Roses, France

    E-print Network

    Boyer, Edmond

    L-547 Positron lifetime in vacancy-impurity complexes (*) C. Corbel Section d'Etudes des Solides positon. Abstract. 2014 We calculate the positron lifetime in vacancy-solute (Na, Mg, Zn) complexes in Al-550 15 DÉCEMBRE 1981,1 Classification Physics Abstracts 78.70B 1. Introduction. - The positron

  2. Radiative return at NLO and the measurement of the hadronic cross-section in electron-positron annihilation

    E-print Network

    G. Rodrigo; H. Czyz; J. H. Kuhn; M. Szopa

    2001-12-13

    Electron-positron annihilation into hadrons plus an energetic photon from initial state radiation allows the hadronic cross-section to be measured over a wide range of energies. The full next-to-leading order QED corrections for the cross-section for e^+ e^- annihilation into a real tagged photon and a virtual photon converting into hadrons are calculated where the tagged photon is radiated off the initial electron or positron. This includes virtual and soft photon corrections to the process e^+ e^- \\to \\gamma +\\gamma^* and the emission of two real hard photons: e^+ e^- \\to \\gamma + \\gamma + \\gamma^*. A Monte Carlo generator has been constructed, which incorporates these corrections and simulates the production of two charged pions or muons plus one or two photons. Predictions are presented for centre-of-mass energies between 1 and 10 GeV, corresponding to the energies of DAPHNE, CLEO-C and B-meson factories.

  3. Introduction to Positron Emission Tomography

    E-print Network

    Oakes, Terry

    Introduction to Positron Emission Tomography Positron Annihilation 180 o #1 #2 with your host, Terry Oakes #12;Positron Annihilation 180 o #1 #2 positron emitting atom neighboring atom Positron range: 1-10 mm Gamma-Ray range: 10 mm - 8 positron annihilation #12;Positron Emission Tomography

  4. Defect identification using the core-electron contribution in Doppler-broadening spectroscopy of positron-annihilation radiation

    Microsoft Academic Search

    S. Szpala; P. Asoka-Kumar; B. Nielsen; J. P. Peng; S. Hayakawa; K. G. Lynn; H.-J. Gossmann

    1996-01-01

    Reduction of background using a coincidence-detection system in Doppler-broadening spectroscopy of positron-annihilation radiation allows us to examine the contribution of high-momentum core electrons. The contribution is used as a fingerprint to identify chemical variations at a defect site. The technique is applied to study a variety of open volume defects in Si, including decorated vacancies associated with doping.

  5. Hadron Production by Electron-Positron Annihilation at 4GeV Center-of-Mass Energy

    Microsoft Academic Search

    A. Litke; G. Hanson; A. Hofmann; J. Koch; L. Law; M. E. Law; J. Leong; R. Little; R. Madaras; H. Newman; J. M. Paterson; R. Pordes; K. Strauch; G. Tarnopolsky; Richard Wilson

    1973-01-01

    We have measured the total cross section for electron-positron annihilation into three or more hadrons, with at least two charged particles in the final state. The measurement was made at a center-of-mass energy of 4 GeV with a 2pi-sr nonmagnetic detector. With 88 events detected, we obtain a model-independent lower limit on the hadron production cross section of 9.6 +\\/-

  6. A study of precipitate formation in aluminium-copper alloys by positron annihilation and transmission electron microscopy

    Microsoft Academic Search

    W. B. Gauster; W. R. Wampler

    1980-01-01

    Polycrystalline Al?Cu samples containing 0.5, 2.0 and 4.0 wt.% Cu were quenched, after homogenization, from a temperature of 823 to 200 K. Positron annihilation Doppler broadening measurements were then made after isochronal annealing steps with 30 min holding times. Transmission electron microscopy was used to study a set of similarly prepared specimens after several annealing steps. The results indicate that

  7. Study of the structure of the Rh\\/Ag surface using Positron annihilation induced Auger Electron Spectroscopy (PAES)

    Microsoft Academic Search

    G. Yang; S. Yang; J. H. Kim; K. H. Lee; A. R. Koymen; A. H. Weiss; G. A. Mulhollan

    1994-01-01

    Positron Annihilation induced Auger Electron Spectroscopy (PAES), Electron induced Auger Electron Spectroscopy (EAES), and LEED have been used to study the sandwich-like structure resulting from vapor-deposition of Rh on Ag(100). Schmitz etal. found using AES, ISS, and TDS that the Ag layer diffuses to the Rh surface upon annealing to form a structure which Rh is sandwiched between a Ag

  8. 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.

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

    SciTech Connect

    Dlubek, G. [ITA Inst. fuer Innovative Technologien GmbH, Halle (Germany)] [ITA Inst. fuer Innovative Technologien GmbH, Halle (Germany); [Martin-Luther-Univ. Halle-Wittenberg, Halle (Germany). Fachbereich Physik; Lademann, P.; Krause, H.; Krause, S.; Unger, R. [Martin-Luther-Univ. Halle-Wittenberg, Halle (Germany). Fachbereich Physik] [Martin-Luther-Univ. Halle-Wittenberg, Halle (Germany). Fachbereich Physik

    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.

  10. Zero range potential for particles interacting via Coulomb potential: application to electron positron annihilation

    E-print Network

    S. L. Yakovlev; V. A. Gradusov

    2012-06-25

    The zero range potential is constructed for a system of two particles interacting via the Coulomb potential. The singular part of the asymptote of the wave function at the origin which is caused by the common effect of the zero range potential singularity and of the Coulomb potential is explicitly calculated by using the Lippmann-Schwinger type integral equation. The singular pseudo potential is constructed from the requirement that it enforces the solution to the Coulomb Schr\\"odinger equation to possess the calculated asymptotic behavior at the origin. This pseudo potential is then used for constructing a model of the imaginary absorbing potential which allows to treat the annihilation process in positron electron collisions on the basis of the non relativistic Schr\\"odinger equation. The functional form of the pseudo potential constructed in this paper is analogous to the well known Fermi-Breit-Huang pseudo potential. The generalization of the optical theorem on the case of the imaginary absorbing potential in presence of the Coulomb force is given in terms of the partial wave series.

  11. Momentum density of vanadium: A reconstruction from two-dimensional positron-annihilation data

    SciTech Connect

    Pecora, L.M.; Ehrlich, A.C.; Manuel, A.A.; Singh, A.K.; Peter, M.; Singru, R.M.

    1988-04-15

    The momentum distribution, rho(p), in vanadium has been reconstructed from two-dimensional (2D) angular correlation of positron-annihilation radiation (ACPAR) data sets n/sup R/(p/sub x/,p/sub y/). Reconstruction has been performed by using an expansion in terms of cubic harmonics on experimental as well as theoretical data sets n/sup R/(p/sub x/,p/sub y/) for four orientations in V. The effect of statistical errors propagated during the reconstruction is also estimated. The results for rho(p) reconstructed from experiment are compared with those from theory by examining radial plots of the momentum distribution, 2D surfaces and their contour plots in various (p/sub x/,p/sub y/) planes. A satisfactory agreement is observed in the general structure shown by the experimental and theoretical distributions and the observed structure is analyzed in terms of contributions from different sheets of Fermi surface (FS) of V. The ratio of NP-bar/NH-bar of the semiaxes of the N-centered ellipsoidal hole FS sheet in V has been determined to be 1.36 from 2D ACPAR experiment, and this value agrees well with the ratio found by other experimental techniques. Present results show that a wealth of information about rho(p) can be reconstructed from n/sup R/(p/sub x/,p/sub y/) measured for a limited number of orientations.

  12. 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. [Institut fuer Physik, Martin-Luther-Universitaet Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle (Germany); ITA Institut fuer Innovative Technologien, Koethen/Halle, Wiesenring 4, D-06120 Lieskau (Germany); Yu, Yang; Krause-Rehberg, R. [Institut fuer Physik, Martin-Luther-Universitaet Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle (Germany); Beichel, W.; Bulut, S.; Pogodina, N.; Krossing, I.; Friedrich, Ch. [Freiburger Materialforschungszentrum (FMF), Albert-Ludwigs-Universitaet Freiburg, Stefan-Meier-Strasse 21, Germany and Institut fuer Anorganische Chemie, Albert-Ludwigs-Universitaet Freiburg, Albertstrasse 21, D-79104 Freiburg i. Br. (Germany)

    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.

  13. Slow Positron Annihilation in Silicide Films Formed by Solid State Interaction of Co/Ti/Si and Co/Si

    NASA Astrophysics Data System (ADS)

    Liu, Ping; Lin, Chenglu; Zhou, Zuyao; Zou, Shichang; Weng, Huiming; Han, Rongdian; Li, Bingzong

    1994-04-01

    Slow positron beam was used to investigate the solid state reaction of Co/Si and Co/Ti/Si. Variable-energy (0-20 keV) positrons were implanted into samples at different depths. The Doppler broadening of the annihilation ?-ray energy spectra measured at a number of different incident positron energies is characterized by a line-shape parameter, S. It was found that the measured S parameters are sensitive to thin film reaction and crystalline characteristics. In particular, the S parameter of epitaxial CoSi2 formed by the ternary reaction is quite different from that of the polycrystalline CoSi2 formed by direct reaction of Co with Si.

  14. Investigation of microstructural changes in polyetherether-ketone films at cryogenic temperatures by positron lifetime spectroscopy

    NASA Technical Reports Server (NTRS)

    Singh, Jag J.; Eftekhari, Abe; St.clair, Terry L.; Sprinkle, Danny R.

    1991-01-01

    Microstructural changes in Polyetherether-ketone (PEEK) films were investigated in the temperature ranges of 23 to -196 C, using Positron Lifetime Spectroscopy (PLS) technique. It was determined that the total free volume decreases by about 46 percent in amorphous PEEK samples and about 36 percent in semicrystalline PEEK samples when they are cooled down from room temperature to liquid nitrogen (LN2) temperature. If this trend in reduction in free volume with decreasing temperature continues, as expected, it is surmised that PEEK will be able to withstand cooling down to liquid hydrogen (LH2) temperature without any detrimental effect on its diffusivity for liquid hydrogen.

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

    SciTech Connect

    Hareesh, K.; Sanjeev, Ganesh [Microtron Centre, Department of Physics, Mangalore University, Mangalagangotri-574199 (India); Pandey, A. K. [Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085 (India); Meghala, D.; Ranganathaiah, C. [Department of Studies in Physics, University of Mysore, Manasagangotri-570006 (India)

    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.

  16. 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.

  17. Introduction to Positron Emission Tomography

    E-print Network

    Wisconsin at Madison, University of

    Introduction to Positron Emission Tomography with your host, Terry Oakes Positron Annihilation #1 #2 180 o T.R.Oakes Univ. WI-Madison #12;Positron Annihilation 180 o positron emitting atom neighboring atom Positron range: 1-10 mm Gamma-Ray range: 10 mm - 8 positron annihilation #2 #1 T

  18. Temperature dependence of the positron annihilation parameters in Bi2-xPbxCa2Sr2Cu3Oy and Bi2CaSr2Cu2Oy superconductors

    NASA Astrophysics Data System (ADS)

    Pujari, P. K.; Datta, T.; de, Udayan; Ghosh, B.

    1994-08-01

    We report the results of temperature dependence of S-parameter and lifetime (?) measurements on the normal and superconducting states of Bi2CaSr2Cu2Oy (Bi-2:1:2:2) and Bi2-xPbxCa2Sr2Cu3Oy (Bi-2:2:2:3) superconductors. A decrease of the S parameter as well as ? below Tc is seen for both compounds. These observations are interpreted qualitatively considering a reported positron-density-distribution calculation and invoking the local charge-transfer process from CuO2 to Bi-O planes at the onset of the superconducting transition. The possible influence of sample preparation conditions and structural changes on the temperature dependencies of the positron annihilation parameters across Tc is also discussed.

  19. 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.

  20. 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.

  1. 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.

  2. Microdefects in Al{sub 2}O{sub 3} films and interfaces revealed by positron lifetime spectroscopy

    SciTech Connect

    Xu, J.; Somieski, B.; Hulett, L.D.; Pint, B.A.; Tortorelli, P.F. [Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6142 (United States)] [Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6142 (United States); Suzuki, R.; Ohdaira, T. [Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305 (Japan)] [Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305 (Japan)

    1997-11-01

    We have studied microdefects and interfaces of Al{sub 2}O{sub 3} films on iron and nickel aluminide substrates using variable-energy positron lifetime spectroscopy. Di-vacancies, vacancy clusters, and microvoids were observed in the oxide scales. Their sizes and distributions were determined by the nature of the process used to synthesize the alumina film, and influenced by the composition of the alloy substrates. For oxide{endash}iron aluminide interfaces, positron lifetimes are longer than those for the alumina layer itself, suggesting a greater defect concentration at such sites. {copyright} {ital 1997 American Institute of Physics.}

  3. Defects in CeO2\\/SrTiO3 fabricated by automatic feeding epitaxy probed using positron annihilation

    Microsoft Academic Search

    A. Uedono; K. Shimoyama; M. Kiyohara; K. Yamabe

    2003-01-01

    Vacancy-type defects in the CeO2\\/SrTiO3 structure were studied by means of positron annihilation. Thin CeO2 films were grown on SrTiO3 substrates by molecular-beam epitaxy without using an oxidant; oxygen was supplied by diffusion from the substrate (this process is referred to as automatic feeding epitaxy). A preferential epitaxial growth of CeO2(001) on SrTiO3(001) was observed, but an increase in the

  4. Characterization of fatigue-induced free volume changes in a bulk metallic glass using positron annihilation spectroscopy

    SciTech Connect

    Vallery, R. S.; Liu, M.; Gidley, D. W.; Launey, M. E.; Kruzic, J. J. [Department of Physics, University of Michigan, Randall Laboratory, Ann Arbor, Michigan 48109 (United States); Materials Science, School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, Oregon 97331 (United States)

    2007-12-24

    Depth-profiled Doppler broadening spectroscopy of positron annihilation on the cyclic fatigue-induced fracture surfaces of three amorphous Zr{sub 44}Ti{sub 11}Ni{sub 10}Cu{sub 10}Be{sub 25} metallic glass specimens reveals the presence of a 30-50 nm layer of increased free volume that is generated by the propagating fatigue crack tip. The presence and character of this fatigue transformation zone is independent of the initial amount of bulk free volume, which was varied by structural relaxation via annealing, and the voids generated in the zone by intense cyclic deformation are distinct from those typical of the bulk.

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

    SciTech Connect

    Tang, Z., E-mail: ztang@ee.ecnu.edu.cn [Key Laboratory of Polar Materials and Devices, Ministry of Education of China, East China Normal University, Shanghai 200241 (China); Institute for Materials Research, Tohoku University, Sendai 980-8577 (Japan); Chiba, T.; Nagai, Y.; Inoue, K.; Toyama, T.; Hasegawa, M. [Institute for Materials Research, Tohoku University, Sendai 980-8577 (Japan)

    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.

  6. Correlation of mechanical stress and Doppler broadening of the positron annihilation line in Al and Al alloys

    NASA Astrophysics Data System (ADS)

    Hugenschmidt, C.; Qi, N.; Stadlbauer, M.; Schreckenbach, K.

    2009-12-01

    The plastic deformation of Al samples, of the technical alloys AlMgSi0.5, and AlMg3 was studied by Doppler-broadening spectroscopy (DBS) of the positron annihilation line. First, the defect sensitive line-shape parameter S was measured after the application of axial tensile stress, and the corresponding stress-strain curves were recorded that allowed us to correlate the strain, the mechanical stress and the S -parameter values after mechanical load quantitatively. In a next step, asymmetrically deformed Al samples were investigated with a monoenergetic positron beam by DBS in order to obtain the laterally resolved information of the stress-induced defects. It is demonstrated that the resulting two-dimensional S -parameter map (scan area 14×14mm2 , step width 1 mm) can be expressed quantitatively in terms of the locally acting stress which is responsible for the creation of the lattice defects.

  7. Structural vacancies and their local atomic environment in the Zn-Mg-Sc alloy system studied by positron annihilation spectroscopy

    SciTech Connect

    Sato, K.; Kobayashi, Y.; Takagiwa, Y.; Kanazawa, I.; Tamura, R.; Takeuchi, S. [National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565 (Japan); Department of Physics, Tokyo Gakugei University, 4-1-1 Koganei, Tokyo 184-8501 (Japan); Department of Materials Science and Technology, Tokyo University of Science, Noda, Chiba 278-8501 (Japan)

    2004-11-01

    Recently discovered P-type icosahedral quasicrystal Zn{sub 80}Mg{sub 5}Sc{sub 15} of high structural perfection and its crystalline 1/1-approximant Zn{sub 85}Sc{sub 15} are investigated by positron annihilation spectroscopy. Positrons are trapped by high concentration of the vacancy-type sites surrounded by Zn atoms in both the quasicrystal and approximant. Based on the atomic structure of the approximant, the vacancy-type sites are identified to be inside the dodecahedral 20 Zn first shell, suggesting that the quasicrystal is composed of the same cluster units as the approximant. In contrast to the proposed structure model of Zn{sub 85}Sc{sub 15} where inside of the dodecahedral first shell is empty, the present results indicate the existence of several atoms inside the dodecahedral first shell. Positron diffusion experiments using a slow positron beam reveal that the structural vacancy density in the quasicrystal is 35% lower than that in the approximant.

  8. Positron wave-function effects in the measurement of the two-dimensional angular correlation of the annihilation radiation of a spin-polarized system

    NASA Astrophysics Data System (ADS)

    Rusz, Ján; Biasini, Maurizio

    2007-06-01

    In positron annihilation studies of the electronic structure of metals, a nonuniform positron density can distort significantly the faithful detection of the Fermi surface. Recently, we have predicted a nearly complete cancellation of the aforementioned distortion in the spin-polarized two-dimensional angular correlation of annihilation radiation experiment on CrO2 [M. Biasini and J. Rusz, J. Phys.: Condens. Matter 18, L289 (2006)]. We provide a detailed investigation of the predicted effect on several transition-element- and actinide-based systems of increasing complexity, demonstrating that the cancellation effect is of rather general nature.

  9. OSSE observations of Galactic 511 keV positron annihilation radiation - Initial phase 1 results. [Oriented Scintillation Spectrometer Experiment

    NASA Technical Reports Server (NTRS)

    Purcell, W. R.; Grabelsky, D. A.; Ulmer, M. P.; Johnson, W. N.; Kinzer, R. L.; Kurfess, J. D.; Strickman, M. S.; Jung, G. V.

    1993-01-01

    The Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma-Ray Observatory (GRO) has performed numerous observations of the Galactic plane and Galactic center region to measure the distribution of Galactic 511 keV positron annihilation radiation and to search for time variability of the emission. The initial 511 keV line fluxes for the observations performed during the first 18 months of the GRO mission are presented. The 511 keV line flux for a typical Galactic center observation is (2.5 +/- 0.3) x 10 exp -4 gamma/sq cm per sec, where the quoted uncertainty represents the 1 sigma statistical uncertainty. No statistically significant time variability of the line flux has been observed; the 3 sigma upper limit to daily variations from the mean is 3 x 10 exp -4 gamma/sq cm per sec. The distribution of Galactic 511 keV positron annihilation radiation implied by the OSSE observations is discussed and compared with observations by other instruments.

  10. Electron-positron energy deposition rate from neutrino pair annihilation on the rotation axis of neutron and quark stars

    E-print Network

    Z. Kovacs; K. S. Cheng; T. Harko

    2010-09-30

    We investigate the deposition of energy due to the annihilations of neutrinos and antineutrinos on the rotation axis of rotating neutron and quark stars, respectively. The source of the neutrinos is assumed to be a neutrino-cooled accretion disk around the compact object. Under the assumption of the separability of the neutrino null geodesic equation of motion we obtain the general relativistic expression of the energy deposition rate for arbitrary stationary and axisymmetric space-times. The neutrino trajectories are obtained by using a ray tracing algorithm, based on numerically solving the Hamilton-Jacobi equation for neutrinos by reversing the proper time evolution. We obtain the energy deposition rates for several classes of rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the MIT bag model equation of state and in the CFL (Color-Flavor-Locked) phase, respectively. The electron-positron energy deposition rate on the rotation axis of rotating neutron and quark stars is studied for two accretion disk models (isothermal disk and accretion disk in thermodynamical equilibrium). Rotation and general relativistic effects modify the total annihilation rate of the neutrino-antineutrino pairs on the rotation axis of compact stellar, as measured by an observer at infinity. The differences in the equations of state for neutron and quark matter also have important effects on the spatial distribution of the energy deposition rate by neutrino-antineutrino annihilation.

  11. Application of slow positrons to coating degradation

    Microsoft Academic Search

    H. Cao; R. Zhang; H. M Chen; P. Mallon; C.-M Huang; Y. He; T. C Sandreczki; Y. C Jean; B. Nielsen; T. Friessnegg; R. Suzuki; T. Ohdaira

    2000-01-01

    Photodegradation of a polyurethane-based topcoat induced by accelerated UV irradiation is studied using Doppler broadened energy spectra (DBES) and positron annihilation lifetime (PAL) spectroscopies coupled with slow positron technique. Significant and similar variations of S-parameter and ortho-positronium intensity (I3) in coatings are observed as functions of depth and of exposure time. The decrease of S is interpreted as a result

  12. Defects in electron-irradiated GaAs studied by positron lifetime spectroscopy

    SciTech Connect

    Polity, A.; Rudolf, F.; Nagel, C.; Eichler, S.; Krause-Rehberg, R. [Fachbereich Physik der Martin-Luther-Universitaet Halle-Wittenberg, Experimentelle Physik III, Friedemann-Bach-Platz 6, D-06108 Halle/Saale (Germany)] [Fachbereich Physik der Martin-Luther-Universitaet Halle-Wittenberg, Experimentelle Physik III, Friedemann-Bach-Platz 6, D-06108 Halle/Saale (Germany)

    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}

  13. A study of Pd-Ta on Si(100) using AES, RBS and variable energy positron annihilation

    SciTech Connect

    Van Der Kolk, G.J.; Kuiper, A.E.T.; Duchateau, J.P.W.B.; Willemsen, M.; Nielsen, B.; Lynn, K.G.

    1988-01-01

    The applicability of Pd/sub x/Ta/sub 1-x/ as a diffusion barrier on Si has been investigated. For this purpose Pd/sub x/Ta/sub 1-x/ films of 200 nm thickness (x ranges from 0 to 1) were deposited on Si(100), and the reaction between over-layer and substrate was studied as a function of temperature. Interaction was found to occur at temperatures increasing with the Ta content. The as-deposited Pd/sub x/Ta/sub 1-x/ films with 0.2 less than or equal to x less than or equal to 0.6 were found to be amorphous. The amorphous phase had a higher reaction temperature than the crystalline one, causing a discontinuous step in the reaction temperature. RBS spectra revealed that for the Pd-rich compositions first a stoichiometric Pd2Si layer formed underneath a pure Ta layer. At higher temperatures TaSi2 formed at the surface. For Ta-rich compositions Pd2Si formed first as well, however, the reaction temperature was so high that Pd2Si grains formed in a Si matrix. The defect density of the Ta layer, which remained after outdiffusion of Pd, was investigated using variable energy positron annihilation. The defect concentration is very high, as deduced from the trapped positron fraction. A model is presented that describes the composition dependence of the reaction temperature. 26 refs., 6 figs., 1 tab.

  14. Electron-positron annihilation into three pions and the radiative return

    E-print Network

    Henryk Czyz; Agnieszka Grzelinska; Johann H. Kuhn; German Rodrigo

    2006-06-20

    The Monte Carlo event generator PHOKHARA, which simulates hadron and muon production at electron-positron colliders through radiative return, has been extended to final states with three pions. A model for the form factor based on generalized vector dominance has been employed, which is consistent with presently available experimental observations.

  15. Monte Carlo Simulation of Pileup Effects in the Electron-Positron Annihilation Peak

    Microsoft Academic Search

    Eduardo do Nascimento; Jose´ M. Ferna´ndez-Varea; Vito R. Vanin; Nora L. Maidana

    2011-01-01

    The Monte Carlo code PENELOPE is employed to simulate a typical experimental Doppler broadening coincidence spectrum (DBCS) where the energy spectrum of the photons emitted by the positrons interacting in the sample is recorded with two HPGe detectors in coincidence. The simulated spectrum reproduces well some of the structures observed in the measured DBCS, but not the prominent tails on

  16. 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.

  17. Early processes in positron and positronium chemistry: possible scavenging of epithermal e+ by nitrate ion in aqueous solutions

    NASA Astrophysics Data System (ADS)

    Stepanov, Serge V.; Byakov, Vsevolod M.; Duplâtre, Gilles; Zvezhinskiy, Dmitrii S.; Stepanov, Petr S.; Zaluzhnyi, Alexandr G.

    2015-06-01

    Positron ionization slowing down, formation of the positron track, reactions of e+ with track species and its interaction with a scavenger on a subpicosecond timescale, including the process of the positronium formation process are discussed. Interpretation of the positron annihilation lifetime data on positronium formation in aqueous solutions of NO?3 anions, known as efficient scavengers of the presolvated track electrons, suggests that these ions may also capture epithermal (presolvated) positrons as well.

  18. JOURNAL DE PHYSIQUE Colloque Cl, supplment au n 12, Tome 37, Dcembre 1976, page C7-101 POSITRON ANNIHILATION STUDIES IN ALKALI HALIDES

    E-print Network

    Paris-Sud XI, Université de

    ANNIHILATION STUDIES IN ALKALI HALIDES G. W. DEAN, S. DANNEFAER, B. V. SOBOLEV (*), D. P. KERR and B. G. HOGG employée pour étudier seize alkali halides. Les cristaux étaient irradiés par les rayons y. Un composant in a series of 16 alkali halides has been studied using the techniques of both lifetime and Doppler broadening

  19. 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.

  20. 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.

  1. Development of a compact and fast response detector using an Yb:Lu2O3 scintillator for lifetime sensitive positron emission tomography

    NASA Astrophysics Data System (ADS)

    Taira, Y.; Kuroda, R.; Tanaka, M.; Oshima, N.; O'Rourke, B. E.; Suzuki, R.; Toyokawa, H.; Watanabe, K.; Yanagida, T.; Yagi, H.; Yanagitani, T.

    2014-05-01

    We propose a method for obtaining three-dimensional imaging measurements of the defect distribution inside industrial materials by measuring positron lifetimes, in addition to using the imaging technique of positron emission tomography. A compact and fast response detector that uses an Yb3+-doped Lu2O3 scintillator and a photomultiplier tube was developed and tested. Yb3+ charge transfer luminescence exhibits a fast response in the ultraviolet and visible regions. The first measurement of the positron lifetime for a bulk material using an Yb:Lu2O3 scintillator was carried out. The lifetime of positrons created inside an yttria-stabilized zirconia block via pair production produced by ultrashort photon pulses was successfully measured.

  2. 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. [Positron Systems, Inc., 411 S. Fifth St., Boise, Idaho 83702 (United States)

    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.

  3. Nondestructive Induced Residual Stress Assessment in Superalloy Turbine Engine Components Using Induced Positron Annihilation (IPA)

    NASA Astrophysics Data System (ADS)

    Rideout, C. A.; Ritchie, S. J.; Denison, A.

    2007-03-01

    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.

  4. Thermally activated precipitation at deformation-induced defects in Fe-Cu and Fe-Cu-B-N alloys studied by positron annihilation spectroscopy

    NASA Astrophysics Data System (ADS)

    He, S. M.; van Dijk, N. H.; Schut, H.; Peekstok, E. R.; van der Zwaag, S.

    2010-03-01

    We have investigated the influence of deformation-induced defects on the isothermal precipitation at 550°C in as-quenched (solute-supersaturated) and annealed (solute-depleted) Fe-Cu and Fe-Cu-B-N alloys by positron annihilation spectroscopy and hardness tests. Using the coincidence Doppler broadening technique, the evolution of local environment at the positron annihilation sites (open-volume defects, Cu precipitates, and matrix) was monitored as a function of the aging time. For all samples, plastic deformation causes a pronounced change in S and W parameters signaling the formation of open-volume defects. For the as-quenched samples, aging results in a sharp decrease in the amount of open-volume defects combined with the rise of a strong copper signature, which can be attributed to preferential copper precipitation at the open-volume defects introduced by plastic deformation. In contrast, the open-volume defects of the annealed samples can only be reduced partially. Both the hardness tests and the positron annihilation spectroscopy indicate that the addition of B and N to the Fe-Cu alloy causes a significant acceleration of the precipitation in the as-quenched alloys.

  5. Positron annihilation in the Galactic Center: ''Cheshire cat'' Compton scattering and ''excess continuum''

    SciTech Connect

    Bildsten, M.L.; Zurek, W.H.

    1986-01-01

    Two separate observations of the ..gamma..-ray spectrum originating from the Galactic Center were made by HEAO-3 in the fall of 1979 and in the spring of 1980. The 2..gamma.. 511 KeV annihilation line flux decreased by a factor of three over the corresponding six month period, whereas the excess ..gamma..-ray continuum below the 511 KeV line, often interpreted as 3..gamma.. decay of orthopositronium, barely changed. This apparent discrepancy in the temporal behavior makes it difficult to associate the bulk of the excess continuum with the 3..gamma.. decay of positronium. We will show that Compton scattering of the line and high energy radiation provides a natural explanation for the surprisingly small change seen in the excess continuum. 11 refs., 2 figs.

  6. Correlation between interfacial defects and ferromagnetism of BaTiO 3 nanocrystals studied by positron annihilation

    NASA Astrophysics Data System (ADS)

    Chen, Zhi-Yuan; Chen, Z. Q.; Wang, D. D.; Wang, S. J.

    2011-10-01

    High purity BaTiO 3 nanopowders were pressed into pellets and annealed between 100 and 1200 °C. The crystal quality and grain size of the BaTiO 3 nanocrystals were characterized by X-ray diffraction measurements. Annealing induces an increase in the grain size from 44 to 82 nm with temperature increasing up to 1200 °C. XRD and Raman spectroscopy studies confirm that all the samples were single phase with a tetragonal structure after annealing at different temperatures. Positron annihilation measurements reveal large number of vacancy defects in the grain boundary region. These interfacial defects remain stable after annealing at temperatures below 400 ° C and begin to disappear rapidly above 700 °C. After annealing at 1200 °C, most of the interfacial defects have been removed. Hysteresis loops are observed for the 100 ° C annealed samples, which indicate ferromagnetism in BaTiO 3 nanocrystals. The ferromagnetism becomes a little weaker after annealing at 700 °C, and it disappears after 1200 ° C annealing. This change of ferromagnetism coincides with the defect recovery process after annealing, suggesting that ferromagnetism might originate from the interfacial defects.

  7. Study of radiation-induced degradation of RPV steels and model alloys by positron annihilation and Mössbauer spectroscopy

    NASA Astrophysics Data System (ADS)

    Zeman, Andrej; Debarberis, Luigi; Kup?a, L.'udovít; Acosta, Beatriz; Kytka, Miloš; Degmová, Jarmila

    2007-03-01

    The influence of different microstructural processes on the degradation due to radiation embrittlement has studied by positron annihilation and Mössbauer spectroscopy. The materials studied consisted of WWER-440 base (15Kh2MFA) and weld (10KhMFT) RPV steels which were neutron-irradiated at fluence levels of 0.78 × 10 24 m -2, 1.47 × 10 24 m -2 and 2.54 × 10 24 m -2; WWER-1000 base (15Kh2NMFAA) and weld (12Kh2N2MAA) irradiated at a fluence level 1.12 × 10 24 m -2; three different model alloys implanted with protons at two dose levels (up to 0.026 dpa), finally the base metal of WWER-1000 (15Kh2NMFAA) was thermally treated with the intention to simulate the P-segregation process. It has been shown possible to correlate the values of parameters obtained by such techniques and data of mechanical testing (ductile-to-brittle transition temperature and upper shelf energy).

  8. Vacancy-oxygen complexes and their optical properties in AlN epitaxial films studied by positron annihilation

    NASA Astrophysics Data System (ADS)

    Uedono, A.; Ishibashi, S.; Keller, S.; Moe, C.; Cantu, P.; Katona, T. M.; Kamber, D. S.; Wu, Y.; Letts, E.; Newman, S. A.; Nakamura, S.; Speck, J. S.; Mishra, U. K.; DenBaars, S. P.; Onuma, T.; Chichibu, S. F.

    2009-03-01

    Vacancy-type defects in AlN grown by metal-organic vapor phase epitaxy (MOVPE) and lateral epitaxial overgrowth (LEO) using halide vapor phase epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured and compared to the spectra calculated using the projector augmented-wave method. For MOVPE-AlN, the concentration of vacancy-type defects was high near the interface between AlN and the GaN buffer layer, and the defect-rich region expanded from the interface toward the surface when the NH3 flow rate increased. For the sample grown on the AlN buffer layer, however, the introduction of such defects was suppressed. For LEO-AlN, distinct deep emission peaks at 3-6 eV were observed in cathodoluminescence spectra. From a comparison between Doppler broadening spectra measured for LEO-AlN and computer simulated ones, an origin of the peaks was identified as complexes of Al vacancy (VAl) and oxygen atoms substituting nitrogen sites such as VAl(ON)n (n =3 and 4).

  9. 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.

  10. Observation of slow positron annihilation in silicide films formed by solid state interaction of Co/Ti/Si and Co/Si

    NASA Astrophysics Data System (ADS)

    Liu, Ping; Lin, Chenglu; Zhou, Zuyao; Zou, Shichang; Weng, Huiming; Han, Rongdian; Li, Bingzong

    1994-05-01

    Slow positron beam was used to investigate the solid state reaction of Co/Si and Co/Ti/Si. Variable-energy (0-20 keV) positrons were implanted into samples at different depths. The Doppler broadening of the annihilation ?-ray energy spectra measured at a number of different incident positron energies were characterized by a line-shape parameter ``S.'' It was found that the measured S parameters were sensitive to thin-film reaction and crystalline characteristics. In particular, the S parameter of epitaxial CoSi2 formed by the ternary reaction was quite different with that of the polycrystalline CoSi2 formed by direct reaction of Co with Si.

  11. Polymeric membrane studied using slow positron beam

    NASA Astrophysics Data System (ADS)

    Hung, Wei-Song; Lo, Chia-Hao; Cheng, Mei-Ling; Chen, Hongmin; Liu, Guang; Chakka, Lakshmi; Nanda, D.; Tung, Kuo-Lun; Huang, Shu-Hsien; Lee, Kueir-Rarn; Lai, Juin-Yih; Sun, Yi-Ming; Yu, Chang-Cheng; Zhang, Renwu; Jean, Y. C.

    2008-10-01

    A radioisotope slow positron beam has been built at the Chung Yuan Christian University in Taiwan for the research and development in membrane science and technology. Doppler broadening energy spectra and positron annihilation lifetime have been measured as a function of positron energy up to 30 keV in a polyamide membrane prepared by the interfacial polymerization between triethylenetetraamine (TETA) and trimesoyl chloride (TMC) on modified porous polyacrylonitrile (PAN) asymmetric membrane. The multilayer structures and free-volume depth profile for this asymmetric membrane system are obtained. Positron annihilation spectroscopy coupled with a slow beam could provide new information about size selectivity of transporting molecules and guidance for molecular designs in polymeric membranes.

  12. Confronting electron-positron annihilation into hadrons with QCD: an operator product expansion analysis

    E-print Network

    S. Bodenstein; C. A. Dominguez; S. I. Eidelman; H. Spiesberger; K. Schilcher

    2011-11-11

    Experimental data on the total cross section of $e^+ e^-$ annihilation into hadrons are confronted with QCD and the operator product expansion using finite energy sum rules. Specifically, the power corrections in the operator product expansion, i.e. the vacuum condensates, of dimension $d = 2$, 4 and 6 are determined using recent isospin $I=0+1$ data sets. Reasonably stable results are obtained which are compatible within errors with values from $\\tau$-decay. However, the rather large data uncertainties, together with the current value of the strong coupling constant, lead to very large errors in the condensates. It also appears that the separation into isovector and isoscalar pieces introduces additional uncertainties and errors. In contrast, the high precision $\\tau$-decay data of the ALEPH collaboration in the vector channel allows for a more precise determination of the condensates. This is in spite of QCD asymptotics not quite been reached at the end of the $\\tau$ spectrum. We point out that isospin violation is negligible in the integrated cross sections, unlike the case of individual channels.

  13. Electron-positron energy deposition rate from neutrino pair annihilation in the equatorial plane of rapidly rotating neutron and quark stars

    E-print Network

    Z. Kovacs; K. S. Cheng; T. Harko

    2009-11-06

    The neutrino-antineutrino annihilation into electron-positron pairs near the surface of compact general relativistic stars could play an important role in supernova explosions, neutron star collapse, or for close neutron star binaries near their last stable orbit. General relativistic effects increase the energy deposition rates due to the annihilation process. We investigate the deposition of energy and momentum due to the annihilations of neutrinos and antineutrinos in the equatorial plane of the rapidly rotating neutron and quark stars, respectively. We analyze the influence of general relativistic effects, and we obtain the general relativistic corrections to the energy and momentum deposition rates for arbitrary stationary and axisymmetric space-times. We obtain the energy and momentum deposition rates for several classes of rapidly rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the MIT bag model equation of state and in the CFL (Color-Flavor-Locked) phase, respectively. Compared to the Newtonian calculations, rotation and general relativistic effects increase the total annihilation rate measured by an observer at infinity. The differences in the equations of state for neutron and quark matter also have important effects on the spatial distribution of the energy deposition rate by neutrino-antineutrino annihilation.

  14. Radiative and nonradiative processes in strain-free AlxGa1-xN films studied by time-resolved photoluminescence and positron annihilation techniques

    NASA Astrophysics Data System (ADS)

    Onuma, Takeyoshi; Chichibu, Shigefusa F.; Uedono, Akira; Sota, Takayuki; Cantu, Pablo; Katona, Thomas M.; Keading, John F.; Keller, Stacia; Mishra, Umesh K.; Nakamura, Shuji; DenBaars, Steven P.

    2004-03-01

    Radiative and nonradiative processes in nearly strain-free AlxGa1-xN alloys were studied by means of steady-state and time-resolved (TR) photoluminescence (PL) spectroscopy, and the results were connected with that of positron annihilation measurement. The results of steady-state optical reflectance and PL measurements gave the bowing parameter b of approximately -0.82 eV. Values of the full width at half maximum (FWHM) of the near-band-edge PL peak nearly agreed with those predicted by the classical alloy broadening model. However, the Stokes-type shifts (SS) were as large as 100-250 meV and both SS and FWHM of the PL increased with the increase in x for x?0.7. Simultaneously, the luminescence redshift due to the increase in temperature T from 8 to 300 K decreased with increasing x and approached zero for x=0.5. These results indicated the presence of compositional fluctuation forming weakly bound states in the alloys, and the localized excitons tended to delocalize with the increase in T. The TRPL signals showed a biexponential decay at low temperature, and the slower component became longer with the increase in x (over 40 ns for x=0.49). Simultaneously, density or size of cation vacancies (VIII) and relative intensity of the deep-level emission over that of the near-band-edge one at 300 K increased as x increased to x=0.7. Consequently, certain trapping mechanisms associated with VIII where suggested, and excitons were then detrapped and transferred to the localized states before the radiative decay at low temperature; the increase in the slower lifetime and its dominance over the entire TRPL signal intensity with increasing x may reflect the increase of the depth and concentration of the trapping level. As the temperature was increased, the TRPL signal became single exponential due to the increasing dominance of nonradiative recombination processes in the free states, resulting in lower internal quantum efficiency (?int) with increasing x for x?0.7. Therefore, realization of AlGaN-based efficient deep-UV light emitters requires further reduction of the nonradiative defect density as well as the VIII-related trap density.

  15. 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.

  16. 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)?(+) 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)?(+) 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

  17. Study of scattering in positron emission tomography 

    E-print Network

    Aguiar, James

    1998-01-01

    The Texas A&M University positron emission tomograph (TAMU PET) is an experimental, medical-imaging instrument designed to detect gamma rays produced by positron-electron annihilation. Each annihilation yields two coincident gamma rays...

  18. Simulations of <1 0 0> edge and 1/2<1 1 1> screw dislocations in ?-iron and tungsten and positron lifetime calculations

    NASA Astrophysics Data System (ADS)

    Staikov, P.; Djourelov, N.

    2013-03-01

    Dislocations in BCC metals are of crucial importance for understanding behavior of fusion materials. In this study model positron lifetime quantum-mechanical calculations have been carried out in the two-component density functional theory (DFT) in local density approximation (LDA) for perfect iron and tungsten lattices, lattices with <1 0 0> edge and 1/2<1 1 1> screw dislocations and several cases in which dislocations interact with a vacancy, bi-vacancy and vacancies containing hydrogen or helium atoms. The core structures of the dislocations have been obtained by MD-simulations using Mendelev and Ackland potentials for iron and Finnis-Sinclair potential for tungsten. The calculated values for iron are 153 ps for edge dislocation and 124 ps for screw dislocation, while for tungsten are 161 and 130 ps, respectively. We report new results for screw dislocation showing that minor dilation of the lattice volume associated with second-order elasticity theory influences the calculated positron lifetime.

  19. 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. [ZWE FRM II, Lichtenbergstrasse 1, 85747 Garching (Germany); Physik Department E21, Technische Universitaet Muenchen, James Franck Strasse, 85748 Garching (Germany)

    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.

  20. A comparison of the doppler-broadened positron annihilation spectra of neutron irradiated Al 2O 3 and MgAl 2O 3

    NASA Astrophysics Data System (ADS)

    Jones, P. L.; Schaffer, J. P.; Cocks, F. H.; Clinard, F. W.; Hurley, G. F.

    1985-01-01

    Radiation damage studies of oxides and ceramics have become of increasing importance due to the projected use of these materials in thermonuclear fusion reactors as electronic insulators and first wall materials. In addition these materials are important in RAD waste disposal. As part of a study of the defect structure in radiation damaged ceramics Doppler-broadened positron annihilation spectra have been obtained for a series of single crystal sapphire (?-Al 2O 3) and polycrystal (1:1) and (1:2) magnesium aluminate spinel (MgO·Al 2O 3 and MgO-2Al 2O 3) samples. These samples were irradiated in EBR-II to a fluence of 3 × 10 25 n/m 2 (E > 0.1 MeV) at 740°C, and 2 × 10 26 n/m 2 (E > 0.1 MeV) at ~ 550°C respectively. Positron annihilation spectra lineshapes for the irradiated, annealed, and as-received samples of both materials were compared using S parameter analysis. These calculations were made on deconvoluted gamma ray spectra that were free of any instrumental broadening effects. In this way, absolute S parameter changes could be calculated. The observed changes in the S parameter are consistent with independent volume swelling measurements for both the ?-A1 2O 3 and the (1:2) MgAl 2O 4 samples. However, the change in S parameter measured for the (1:1) spinel is contrary to the measured volume change. This apparent anomaly indicates a predominence of interstitial as opposed to vacancy type defects in this material.

  1. Spherulites, positrons, and SAXS in PP films

    NASA Astrophysics Data System (ADS)

    Debowska, M.; Gidley, D. W.; Slusarczyk, Czeslaw; Binias, W.; Muszer, A.

    2000-09-01

    Three polypropylene films of constant thickness (approximately equals 50(mu) m) and different morphology were studied with use of an polarizing microscope, slow positron annihilation lifetime technique, wide and small angle X- ray scattering and attenuated total reflection of infrared radiation. Correlation was observed between the intensities of the intermediate components in the positron lifetime spectra; (lifetimes (tau) 2 equals 1.3 ns and (tau) 3approximately equals 2.3divided by2.8ns) and crystallinity of samples. The evident trend is for the longer-lived (tau) 3-component intensity to decrease with crystallinity while the reverse is true for the shorter, fixed (tau) 2-component. The decrease in the intensity of the longer-lived component can result from reduction of the fractional free volume. Changes of annihilation parameters with the positron penetration depth were observed that may reflect the spatial heterogeneity of samples resulting form the way of their crystallization and possible oxidation of the subsurface regions.

  2. Constraints on Resonant Dark Matter Annihilation

    E-print Network

    Backovi?, Mihailo

    2011-07-04

    Resonant dark matter annihilation drew much attention in the light of recent measurements of charged cosmic ray fluxes. Interpreting the anomalous signal in the positron fraction as a sign of dark matter annihilation in the galactic halo requires...

  3. 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. [National Renewable Energy Lab., Golden, CO (United States); Asoka-Kumar, P.; Lynn, K.G. [Brookhaven National Lab., Upton, NY (United States)

    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.

  4. Measurement of Azimuthal Modulations in the Cross-Section of Di-Pion Pairs in Di-Jet Production from Electron-Positron Annihilation

    E-print Network

    Abdesselam, A; Adamczyk, K; Aihara, H; Said, S Al; Arinstein, K; Arita, Y; Asner, D M; Aso, T; Aulchenko, V; Aushev, T; Ayad, R; Aziz, T; Babu, V; Badhrees, I; Bahinipati, S; Bakich, A M; Bala, A; Ban, Y; Bansal, V; Barberio, E; Barrett, M; Bartel, W; Bay, A; Bedny, I; Behera, P; Belhorn, M; Belous, K; Bhardwaj, V; Bhuyan, B; Bischofberger, M; Biswal, J; Bloomfield, T; Blyth, S; Bobrov, A; Bondar, A; Bonvicini, G; Bookwalter, C; Bozek, A; Bra?ko, M; Breibeck, F; Brodzicka, J; Browder, T E; ?ervenkov, D; Chang, M -C; Chang, P; Chao, Y; Chekelian, V; Chen, A; Chen, K -F; Chen, P; Cheon, B G; Chilikin, K; Chistov, R; Cho, K; Chobanova, V; Choi, S -K; Choi, Y; Cinabro, D; Crnkovic, J; Dalseno, J; Danilov, M; Di Carlo, S; Dingfelder, J; Doležal, Z; Drásal, Z; Drutskoy, A; Dubey, S; Dutta, D; Dutta, K; Eidelman, S; Epifanov, D; Esen, S; Farhat, H; Fast, J E; Feindt, M; Ferber, T; Frey, A; Frost, O; Fujikawa, M; Fulsom, B G; Gaur, V; Gabyshev, N; Ganguly, S; Garmash, A; Getzkow, D; Gillard, R; Giordano, F; Glattauer, R; Goh, Y M; Golob, B; Perdekamp, M Grosse; Grygier, J; Grzymkowska, O; Guo, H; Haba, J; Hamer, P; Han, Y L; Hara, K; Hara, T; Hasegawa, Y; Hasenbusch, J; Hayasaka, K; Hayashii, H; He, X H; Heck, M; Hedges, M; Heffernan, D; Heider, M; Heller, A; Higuchi, T; Himori, S; Horiguchi, T; Hoshi, Y; Hoshina, K; Hou, W -S; Hsiung, Y B; Hsu, C -L; Huschle, M; Hyun, H J; Igarashi, Y; Iijima, T; Imamura, M; Inami, K; Inguglia, G; Ishikawa, A; Itagaki, K; Itoh, R; Iwabuchi, M; Iwasaki, M; Iwasaki, Y; Iwashita, T; Iwata, S; Jacobs, W W; Jaegle, I; Jones, M; Joo, K K; Julius, T; Kah, D H; Kakuno, H; Kang, J H; Kang, K H; Kapusta, P; Kataoka, S U; Katayama, N; Kato, E; Kato, Y; Katrenko, P; Kawai, H; Kawasaki, T; Kichimi, H; Kiesling, C; Kim, B H; Kim, D Y; Kim, H J; Kim, J B; Kim, J H; Kim, K T; Kim, M J; Kim, S H; Kim, S K; Kim, Y J; Kinoshita, K; Kleinwort, C; Klucar, J; Ko, B R; Kobayashi, N; Koblitz, S; Kodyš, P; Koga, Y; Korpar, S; Kouzes, R T; Križan, P; Krokovny, P; Kronenbitter, B; Kuhr, T; Kumar, R; Kumita, T; Kurihara, E; Kuroki, Y; Kuzmin, A; Kvasni?ka, P; Kwon, Y -J; Lai, Y -T; Lange, J S; Lee, D H; Lee, I S; Lee, S -H; Leitgab, M; Leitner, R; Lewis, P; Li, H; Li, J; Li, X; Li, Y; Gioi, L Li; Libby, J; Limosani, A; Liu, C; Liu, Y; Liu, Z Q; Liventsev, D; Loos, A; Louvot, R; Lukin, P; MacNaughton, J; Masuda, M; Matvienko, D; Matyja, A; McOnie, S; Mikami, Y; Miyabayashi, K; Miyachi, Y; Miyake, H; Miyata, H; Miyazaki, Y; Mizuk, R; Mohanty, G B; Mohanty, S; Mohapatra, D; Moll, A; Moon, H K; Mori, T; Moser, H -G; Müller, T; Muramatsu, N; Mussa, R; Nagamine, T; Nagasaka, Y; Nakahama, Y; Nakamura, I; Nakamura, K; Nakano, E; Nakano, H; Nakano, T; Nakao, M; Nakayama, H; Nakazawa, H; Nanut, T; Natkaniec, Z; Nayak, M; Nedelkovska, E; Negishi, K; Neichi, K; Ng, C; Niebuhr, C; Niiyama, M; Nisar, N K; Nishida, S; Nishimura, K; Nitoh, O; Nozaki, T; Ogawa, A; Ogawa, S; Ohshima, T; Okuno, S; Olsen, S L; Ono, Y; Onuki, Y; Ostrowicz, W; Oswald, C; Ozaki, H; Pakhlov, P; Pakhlova, G; Pal, B; Palka, H; Panzenböck, E; Park, C -S; Park, C W; Park, H; Park, H K; Park, K S; Peak, L S; Pedlar, T K; Peng, T; Pesantez, L; Pestotnik, R; Peters, M; Petri?, M; Piilonen, L E; Poluektov, A; Prasanth, K; Prim, M; Prothmann, K; Pulvermacher, C; Purohit, M; Reisert, B; Ribežl, E; Ritter, M; Röhrken, M; Rorie, J; Rostomyan, A; Rozanska, M; Ryu, S; Sahoo, H; Saito, T; Sakai, K; Sakai, Y; Sandilya, S; Santel, D; Santelj, L; Sanuki, T; Sasao, N; Sato, Y; Savinov, V; Schneider, O; Schnell, G; Schönmeier, P; Schram, M; Schwanda, C; Schwartz, A J; Schwenker, B; Seidl, R; Sekiya, A; Semmler, D; Senyo, K; Seon, O; Seong, I S; Sevior, M E; Shang, L; Shapkin, M; Shebalin, V; Shen, C P; Shibata, T -A; Shibuya, H; Shinomiya, S; Shiu, J -G; Shwartz, B; Sibidanov, A; Simon, F; Singh, J B; Sinha, R; Smerkol, P; Sohn, Y -S; Sokolov, A; Soloviev, Y; Solovieva, E; Stani?, S; Stari?, M; Steder, M; Stypula, J; Sugihara, S; Sugiyama, A; Sumihama, M; Sumisawa, K; Sumiyoshi, T; Suzuki, K; Suzuki, S; Suzuki, S Y; Suzuki, Z; Takeichi, H; Tamponi, U; Tanaka, M; Tanaka, S; Tanida, K; Taniguchi, N; Tatishvili, G; Taylor, G N; Teramoto, Y; Tikhomirov, I; Trabelsi, K; Trusov, V; Tse, Y F; Tsuboyama, T; Uchida, M; Uchida, T; Uchida, Y; Uehara, S; Ueno, K; Uglov, T; Unno, Y; Uno, S; Urquijo, P; Ushiroda, Y; Usov, Y; Vahsen, S E; Van Hulse, C; Vanhoefer, P; Varner, G; Varvell, K E; Vervink, K; Vinokurova, A; Vorobyev, V; Vossen, A; Wagner, M N; Wang, C H; Wang, J; Wang, M -Z; Wang, P; Wang, X L; Watanabe, M; Watanabe, Y; Wedd, R; Wehle, S; White, E; Wiechczynski, J; Williams, K M; Won, E; Yabsley, B D; Yamada, S; Yamamoto, H; Yamaoka, J; Yamashita, Y; Yamauchi, M; Yashchenko, S; Ye, H; Yelton, J; Yook, Y; Yuan, C Z; Yusa, Y; Zhang, C C; Zhang, L M; Zhang, Z P; Zhao, L; Zhilich, V; Zhulanov, V; Ziegler, M; Zivko, T; Zupanc, A; Zwahlen, N; Zyukova, O

    2015-01-01

    We present an extraction of azimuthal correlations between two pairs of charged pions detected in opposite jets from electron-positron annihilation. These correlations may arise from the dependence of the di-pion fragmentation on the polarization of the parent quark in the process $e^+e^- \\rightarrow q \\bar{q}$. Due to the correlation of the quark polarizations, the cross-section of di-pion pair production, in which the pion pairs are detected in opposite jets in a dijet event, exhibits a modulation in the azimuthal angles of the planes containing the hadron pairs with respect to the production plane. The measurement of this modulation allows access to combinations of fragmentation functions that are sensitive to the quark's transverse polarization and helicity. Within our uncertainties we do not observe a significant signal from the previously unmeasured helicity dependent fragmentation function $G_1^\\perp$. This measurement uses a dataset of 938~fb$^{-1}$ collected by the Belle experiment at or near $\\sqrt{...

  5. Positron Annihilation Spectroscopy and Small Angle Neutron Scattering Characterization of the Effect of Mn on the Nanostructural Features formed in Irradiated Fe-Cu-Mn Alloys

    SciTech Connect

    Glade, S C; Wirth, B D; Asoka-Kumar, P; Odette, G R; Sterne, P A; Howell, R H

    2003-02-27

    The size, number density and composition of the nanometer defects responsible for the hardening and embrittlement in irradiated Fe-0.9wt.% Cu and Fe-0.9wt.% Cu-1.0wt% Mn model reactor pressure vessel alloys were measured using small angle neutron scattering and positron annihilation spectroscopy. These alloys were irradiated at 290 C to relatively low neutron fluences (E > 1 MeV, 6.0 x 10{sup 20} to 4.0 x 10{sup 21} n/m{sup 2}) in order to study the effect of manganese on the nucleation and growth of copper rich precipitates and secondary defect features. Copper rich precipitates were present in both alloys following irradiation. The Fe-Cu-Mn alloy had smaller precipitates and a larger number density of precipitates, suggesting Mn segregation at the iron matrix-precipitate interface which reduces the interfacial energy and in turn the driving force for coarsening. Mn also retards the precipitation kinetics and inhibits large vacancy cluster formation, suggesting a strong Mn-vacancy interaction which reduces radiation enhanced diffusion.

  6. Measurement of Azimuthal Modulations in the Cross-Section of Di-Pion Pairs in Di-Jet Production from Electron-Positron Annihilation

    E-print Network

    A. Abdesselam; I. Adachi; K. Adamczyk; H. Aihara; S. Al Said; K. Arinstein; Y. Arita; D. M. Asner; T. Aso; V. Aulchenko; T. Aushev; R. Ayad; T. Aziz; V. Babu; I. Badhrees; S. Bahinipati; A. M. Bakich; A. Bala; Y. Ban; V. Bansal; E. Barberio; M. Barrett; W. Bartel; A. Bay; I. Bedny; P. Behera; M. Belhorn; K. Belous; V. Bhardwaj; B. Bhuyan; M. Bischofberger; J. Biswal; T. Bloomfield; S. Blyth; A. Bobrov; A. Bondar; G. Bonvicini; C. Bookwalter; A. Bozek; M. Bra?ko; F. Breibeck; J. Brodzicka; T. E. Browder; D. ?ervenkov; M. -C. Chang; P. Chang; Y. Chao; V. Chekelian; A. Chen; K. -F. Chen; P. Chen; B. G. Cheon; K. Chilikin; R. Chistov; K. Cho; V. Chobanova; S. -K. Choi; Y. Choi; D. Cinabro; J. Crnkovic; J. Dalseno; M. Danilov; S. Di Carlo; J. Dingfelder; Z. Doležal; Z. Drásal; A. Drutskoy; S. Dubey; D. Dutta; K. Dutta; S. Eidelman; D. Epifanov; S. Esen; H. Farhat; J. E. Fast; M. Feindt; T. Ferber; A. Frey; O. Frost; M. Fujikawa; B. G. Fulsom; V. Gaur; N. Gabyshev; S. Ganguly; A. Garmash; D. Getzkow; R. Gillard; F. Giordano; R. Glattauer; Y. M. Goh; B. Golob; M. Grosse Perdekamp; J. Grygier; O. Grzymkowska; H. Guo; J. Haba; P. Hamer; Y. L. Han; K. Hara; T. Hara; Y. Hasegawa; J. Hasenbusch; K. Hayasaka; H. Hayashii; X. H. He; M. Heck; M. Hedges; D. Heffernan; M. Heider; A. Heller; T. Higuchi; S. Himori; T. Horiguchi; Y. Hoshi; K. Hoshina; W. -S. Hou; Y. B. Hsiung; C. -L. Hsu; M. Huschle; H. J. Hyun; Y. Igarashi; T. Iijima; M. Imamura; K. Inami; G. Inguglia; A. Ishikawa; K. Itagaki; R. Itoh; M. Iwabuchi; M. Iwasaki; Y. Iwasaki; T. Iwashita; S. Iwata; W. W. Jacobs; I. Jaegle; M. Jones; K. K. Joo; T. Julius; D. H. Kah; H. Kakuno; J. H. Kang; K. H. Kang; P. Kapusta; S. U. Kataoka; N. Katayama; E. Kato; Y. Kato; P. Katrenko; H. Kawai; T. Kawasaki; H. Kichimi; C. Kiesling; B. H. Kim; D. Y. Kim; H. J. Kim; J. B. Kim; J. H. Kim; K. T. Kim; M. J. Kim; S. H. Kim; S. K. Kim; Y. J. Kim; K. Kinoshita; C. Kleinwort; J. Klucar; B. R. Ko; N. Kobayashi; S. Koblitz; P. Kodyš; Y. Koga; S. Korpar; R. T. Kouzes; P. Križan; P. Krokovny; B. Kronenbitter; T. Kuhr; R. Kumar; T. Kumita; E. Kurihara; Y. Kuroki; A. Kuzmin; P. Kvasni?ka; Y. -J. Kwon; Y. -T. Lai; J. S. Lange; D. H. Lee; I. S. Lee; S. -H. Lee; M. Leitgab; R. Leitner; P. Lewis; H. Li; J. Li; X. Li; Y. Li; L. Li Gioi; J. Libby; A. Limosani; C. Liu; Y. Liu; Z. Q. Liu; D. Liventsev; A. Loos; R. Louvot; P. Lukin; J. MacNaughton; M. Masuda; D. Matvienko; A. Matyja; S. McOnie; Y. Mikami; K. Miyabayashi; Y. Miyachi; H. Miyake; H. Miyata; Y. Miyazaki; R. Mizuk; G. B. Mohanty; S. Mohanty; D. Mohapatra; A. Moll; H. K. Moon; T. Mori; H. -G. Moser; T. Müller; N. Muramatsu; R. Mussa; T. Nagamine; Y. Nagasaka; Y. Nakahama; I. Nakamura; K. Nakamura; E. Nakano; H. Nakano; T. Nakano; M. Nakao; H. Nakayama; H. Nakazawa; T. Nanut; Z. Natkaniec; M. Nayak; E. Nedelkovska; K. Negishi; K. Neichi; C. Ng; C. Niebuhr; M. Niiyama; N. K. Nisar; S. Nishida; K. Nishimura; O. Nitoh; T. Nozaki; A. Ogawa; S. Ogawa; T. Ohshima; S. Okuno; S. L. Olsen; Y. Ono; Y. Onuki; W. Ostrowicz; C. Oswald; H. Ozaki; P. Pakhlov; G. Pakhlova; B. Pal; H. Palka; E. Panzenböck; C. -S. Park; C. W. Park; H. Park; H. K. Park; K. S. Park; L. S. Peak; T. K. Pedlar; T. Peng; L. Pesantez; R. Pestotnik; M. Peters; M. Petri?; L. E. Piilonen; A. Poluektov; K. Prasanth; M. Prim; K. Prothmann; C. Pulvermacher; M. Purohit; B. Reisert; E. Ribežl; M. Ritter; M. Röhrken; J. Rorie; A. Rostomyan; M. Rozanska; S. Ryu; H. Sahoo; T. Saito; K. Sakai; Y. Sakai; S. Sandilya; D. Santel; L. Santelj; T. Sanuki; N. Sasao; Y. Sato; V. Savinov; O. Schneider; G. Schnell; P. Schönmeier; M. Schram; C. Schwanda; A. J. Schwartz; B. Schwenker; R. Seidl; A. Sekiya; D. Semmler; K. Senyo; O. Seon; I. S. Seong; M. E. Sevior; L. Shang; M. Shapkin; V. Shebalin; C. P. Shen; T. -A. Shibata; H. Shibuya; S. Shinomiya; J. -G. Shiu; B. Shwartz; A. Sibidanov; F. Simon; J. B. Singh; R. Sinha; P. Smerkol; Y. -S. Sohn; A. Sokolov; Y. Soloviev; E. Solovieva; S. Stani?; M. Stari?; M. Steder; J. Stypula; S. Sugihara; A. Sugiyama; M. Sumihama; K. Sumisawa; T. Sumiyoshi; K. Suzuki; S. Suzuki; S. Y. Suzuki; Z. Suzuki; H. Takeichi; U. Tamponi; M. Tanaka; S. Tanaka; K. Tanida; N. Taniguchi; G. Tatishvili; G. N. Taylor; Y. Teramoto; I. Tikhomirov; K. Trabelsi; V. Trusov; Y. F. Tse; T. Tsuboyama; M. Uchida; T. Uchida; Y. Uchida; S. Uehara; K. Ueno; T. Uglov; Y. Unno; S. Uno; P. Urquijo; Y. Ushiroda; Y. Usov; S. E. Vahsen; C. Van Hulse; P. Vanhoefer; G. Varner; K. E. Varvell; K. Vervink; A. Vinokurova; V. Vorobyev; A. Vossen; M. N. Wagner; C. H. Wang; J. Wang; M. -Z. Wang; P. Wang; X. L. Wang; M. Watanabe; Y. Watanabe; R. Wedd; S. Wehle; E. White; J. Wiechczynski; K. M. Williams; E. Won; B. D. Yabsley; S. Yamada; H. Yamamoto; J. Yamaoka; Y. Yamashita; M. Yamauchi; S. Yashchenko; H. Ye; J. Yelton; Y. Yook; C. Z. Yuan; Y. Yusa; C. C. Zhang; L. M. Zhang; Z. P. Zhang; L. Zhao; V. Zhilich; V. Zhulanov

    2015-05-29

    We present an extraction of azimuthal correlations between two pairs of charged pions detected in opposite jets from electron-positron annihilation. These correlations may arise from the dependence of the di-pion fragmentation on the polarization of the parent quark in the process $e^+e^- \\rightarrow q \\bar{q}$. Due to the correlation of the quark polarizations, the cross-section of di-pion pair production, in which the pion pairs are detected in opposite jets in a dijet event, exhibits a modulation in the azimuthal angles of the planes containing the hadron pairs with respect to the production plane. The measurement of this modulation allows access to combinations of fragmentation functions that are sensitive to the quark's transverse polarization and helicity. Within our uncertainties we do not observe a significant signal from the previously unmeasured helicity dependent fragmentation function $G_1^\\perp$. This measurement uses a dataset of 938~fb$^{-1}$ collected by the Belle experiment at or near $\\sqrt{s}\\approx10.58$ GeV.

  7. 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 ?/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%.

  8. Development of a new time and position resolving detector for the pulsed low energy positron system PLEPS

    NASA Astrophysics Data System (ADS)

    Ackermann, U.; Egger, W.; Sperr, P.; Löwe, B.; Ravelli, L.; Kögel, G.; Dollinger, G.; Jagutzki, O.

    2013-06-01

    The pulsed low energy positron system PLEPS at the Munich research reactor FRM-II is a user facility for depth resolved positron lifetime measurements. Besides positron lifetime measurements 2D-AMOC (Two Dimensional Age Momentum Correlation) experiments are also possible. 2D-AMOC provides in coincidence the lifetime of the positron and the longitudinal momentum distribution of the annihilated electron. It would be of great scientific concern to measure simultaneously the entire 3D-momentum distribution of the electron annihilating with the positron and the corresponding lifetime of the positron (4D-AMOC). To perform 4D-AMOC measurements a time and position resolving detector is required in coincidence with a pixelated Germanium detector. Therefore a time and spatially resolving detector is currently developed at our institute with envisaged time resolution of 100 ps (FWHM) and a spatial resolution of about 2.6 mm (FWHM) over an area of 12 cm2. First test measurements have been carried out with a 25 mm diameter MCP (Micro Channel Plate) image intensifier and with special delay-line anode readout for the spatial information. Up to now 178 ps (FWHM) time resolution and on average 3.4 mm (FWHM) position resolution have been achieved with BaF2 as scintillator material and a 60Co source.

  9. SMM observations of gamma-ray transients. 3: A search for a broadened, redshifted positron annihilation line from the direction of the Galactic center

    NASA Technical Reports Server (NTRS)

    Harris, Michael J.; Share, Gerald H.; Leising, Mark D.

    1994-01-01

    We have searched for 1980-1988 Solar Maximum Mission gamma-ray spectrometer data for transient emission on timescales from hours to approximately 12 days of broad gamma-ray lines at energies approximately 400 keV, which were reported by the High Energy Astronomy Observatory (HEAO) 1 and SIGMA experiments from two sources lying toward the Galactic center. The lines have been interpreted as the product of the annihilation of positrons in pair plasmas surrounding the black hole candidate 1E 1740.7-2942 and the X-ray binary 1H 1822-371. Our results from a combined exposure of approximately 1.5 x 10(exp 7)s provide no convincing evidence for transient emission of this line on any timescale between approximately 9 hr and approximately 1 yr. Our 3 sigma upper limits on the line flux during approximately 12 day intervals are characteristically 4.8 x 10(exp -3) photon/sq cm/s, while for approximately 1 day intervals our 3 sigma upper limits are characteristically 4.9 x 10(exp -3) photon/sq cm/s. These results imply a duty cycle of less than 1.3% for the transient line measured from 1H 1822-371 during a approximately 3 week interval in 1977 by HEAO 1, and a duty cycle of less than or = 0.8% for the transient line detected in 1990 and 1992 from 1E 1740.7-2942 on approximately 1 day timescales by SIGMA.

  10. Characterization of arachidate Langmuir--Blodgett films by variable energy positron beams

    SciTech Connect

    Marek, T.; Szeles, C.; Suvegh, K.; Kiss, E.; Vertes, A.; Lynn, K.G.

    1999-11-09

    Archidate Langmuir-Blodgett (LB) films of different chemical composition and number of monomolecular layers deposited on silylated silica glass substrates were studied by means of positron annihilation spectroscopy. The applied methods included the measurement of the Doppler broadening of the annihilation photopeak with variable energy positron beams and bulk positron lifetime measurements. The studied samples were 58 monomolecular layers (MML) thick Mg- and Cd-arachidate, arachidic acid (50 MML) and a series of Pb-arachidate samples with 4, 10, 20, 40, and 58 MML. The investigation showed that the variable energy positron beam technique is capable of measuring the thickness of the deposited LB films. The measured positron annihilation parameters are sensitive to the chemical composition of the films and the behavior of the films in a vacuum. The results confirmed the stability of salt base LB films in high vacuum conditions and showed the desorption of pure acid films. These investigations have also shown that a strong position trap is formed in the near-surface region of the hydrophobized substrate as a consequence of the silylation process. The results suggest that positron beams provide valuable complementary information to results obtained by other techniques.

  11. Annihilation explosions in macroscopic polyelectrons. Photon detonation

    E-print Network

    Alexei M. Frolov

    2009-09-03

    Annihilation of the electron-positron pairs in macroscopic polyelectrons is considered. It is shown that very fast collapse of the spatial area occupied by macroscopic polyelectron (or dense electron-positron plasma) produces an instant annihilation of a very large number of electron-positron pairs. This phenomenon corresponds to the so-called annihilation explosion. Annihilation of each electron-positron pair is a highly exothermic process. Therefore, in dense electron-positron plasma one can observe a very interesting phenomenon of photon detonation, i.e. a self-organized formation and propagation of the detonation wave which coincides with the annihilation wave. The photon detonation can be used in many applications, including many military and astrophysical problems.

  12. 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.

  13. Cold Positrons from Decaying Dark Matter

    SciTech Connect

    Boubekeur, Lotfi [Universitate de Valencia (Spain); Dodelson, Scott [Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States); Vives, Oscar [Universitate de Valencia (Spain)

    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.

  14. The electron beam lifetime problem in HERA

    Microsoft Academic Search

    D. R. C. Kelly; W. Bialowons; R. Brinkmann; H. Ehrlichmann; J S Kouptsidis

    1995-01-01

    The electron beam lifetime in the HERA storage ring is not only uniformly lower than the positron beam lifetime, but the electron beam lifetime curves display complicated structures not present in the smoothly behaved positron beam lifetime curves. Summaries of characterising quantities are presented for large numbers of runs in 1993 and 1994, enabling the identification of trends in the

  15. 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.

  16. A low energy positron flux generator for microstructural characterization of thin polymer films

    NASA Astrophysics Data System (ADS)

    Singh, J. J.; Eftekhari, A.; St. Clair, T. L.

    1994-06-01

    A low energy positron flux generator using well-annealed polycrystalline moderators and a Na22 positron source has been developed for microstructural characterization of thin polymer films. A 200 ?c Na22 source, deposited on a thin (2.54 ?m) aluminized mylar film, is sandwiched between two 0.0127 cm×2.54 cm×2.54 cm tungsten strips. Two identical test polymer films, whose thicknesses may range from 0.001 to 0.01 cm, insulate the two tungsten moderator strips from the aluminized source film. A potential difference of 10-100 volts, depending on the test film thickness, is applied between the tungsten strips and the aluminized source film. Thermalized positrons diffusing out of the moderator strips are attracted to, or repelled from, the source foil depending on the polarity of the potential difference between the moderator strips and the source foil. Thus, more positrons are expected to stop/annihilate in the test films when the source is at a negative potential than when it is at a positive potential. The difference in positron lifetime spectra with the source at ?(V) volts with respect to the moderator is entirely due to the positrons annihilating in the test films. Two examples showing the usefulness of the new device for monitoring the morphology of thin polymer films are presented.

  17. Portable Positron Measurement System (PPMS)

    SciTech Connect

    None

    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.

  18. Observation of positronium annihilation in the 2S state: towards a new measurement of the 1S-2S transition frequency

    E-print Network

    D. A. Cooke; P. Crivelli; J. Alnis; A. Antognini; B. Brown; S. Friedreich; A. Gabard; T. W. Haensch; K. Kirch; A. Rubbia; V. Vrankovic

    2015-03-19

    We report the first observation of the annihilation of positronium from the 2S state. Positronium (Ps) is excited with a two-photon transition from the 1S to the 2S state where its lifetime is increased by a factor of eight compared to the ground state due to the decrease in the overlap of the positron electron wavefunction. The yield of delayed annihilation photons detected as a function of laser frequency is used as a new method of detecting laser-excited Ps in the 2S state. This can be considered the first step towards a new high precision measurement of the 1S-2S Ps line.

  19. Preliminary results of a slow positron study on an epoxy polymer

    NASA Astrophysics Data System (ADS)

    Jean, Y. C.; Dai, G. H.; Shi, H.; Suzuki, R.; Kobayashi, Y.

    1994-06-01

    The positron annihilation lifetime spectra and positronium energy spectra of an epoxy polymer with a 5:2:3 equivalent ratio of DGEBA/DDA/DAB epoxy (Tg=52 °C) are measured as a function of temperature and of positron incident energies (0-5 keV). Preliminary results from these experiments show: (1) a 1-9% of long-lived o-Ps are emited from the energy surface, (2) the lifetime of o-Ps near the surface is about three times longer than that in the bulk, (3) the positron lifetime spectra obtained from the incident positron energy exceeding 2 keV are essentially the same as those in the bulk, and (4) the energies of emitted Ps are distributed from thermal to about 100 eV. These results are discussed in terms of the potential uses of slow positrons in probing the free-volume hole properties of thin films and near surfaces of polymeric materials.

  20. Total Cross Section for Hadron Production by Electron-Positron Annihilation between 2.4 and 5.0 GeV Center-of-Mass Energy

    Microsoft Academic Search

    J.-E. Augustin; A. M. Boyarski; M. Breidenbach; F. Bulos; J. T. Dakin; G. J. Feldman; G. E. Fischer; D. Fryberger; G. Hanson; B. Jean-Marie; R. R. Larsen; V. Lüth; H. L. Lynch; D. Lyon; C. C. Morehouse; J. M. Paterson; M. L. Perl; B. Richter; R. F. Schwitters; F. Vannucci; G. S. Abrams; D. Briggs; W. Chinowsky; C. E. Friedberg; G. Goldhaber; R. J. Hollebeek; J. A. Kadyk; G. H. Trilling; J. S. Whitaker; J. E. Zipse

    1975-01-01

    The total cross section for hadron production by e+e- annihilation has been measured at center-of-mass energies between 2.4 and 5.0 GeV. Aside from the very narrow resonances psi(3105) and psi(3695), the cross section varies between 32 and 17 nb over this region with structure in the vicinity of 4.1 GeV.

  1. Investigating the binding properties of porous drug delivery systems using nuclear sensors (radiotracers) and positron annihilation lifetime spectroscopy--predicting conditions for optimum performance.

    PubMed

    Mume, Eskender; Lynch, Daniel E; Uedono, Akira; Smith, Suzanne V

    2011-06-21

    Understanding how the size, charge and number of available pores in porous material influences the uptake and release properties is important for optimising their design and ultimately their application. Unfortunately there are no standard methods for screening porous materials in solution and therefore formulations must be developed for each encapsulated agent. This study investigates the potential of a library of radiotracers (nuclear sensors) for assessing the binding properties of hollow silica shell materials. Uptake and release of Cu(2+) and Co(2+) and their respective complexes with polyazacarboxylate macrocycles (dota and teta) and a series of hexa aza cages (diamsar, sarar and bis-(p-aminobenzyl)-diamsar) from the hollow silica shells was monitored using their radioisotopic analogues. Coordination chemistry of the metal (M) species, subtle alterations in the molecular architecture of ligands (Ligand) and their resultant complexes (M-Ligand) were found to significantly influence their uptake over pH 3 to 9 at room temperature. Positively charged species were selectively and rapidly (within 10 min) absorbed at pH 7 to 9. Negatively charged species were preferentially absorbed at low pH (3 to 5). Rates of release varied for each nuclear sensor, and time to establish equilibrium varied from minutes to days. The subtle changes in design of the nuclear sensors proved to be a valuable tool for determining the binding properties of porous materials. The data support the development of a library of nuclear sensors for screening porous materials for use in optimising the design of porous materials and the potential of nuclear sensors for high through-put screening of materials. PMID:21409200

  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. Positron program at the Idaho Accelerator Center

    SciTech Connect

    Stancari, Giulio [Idaho State University, Department of Physics, Pocatello, Idaho 83209 (United States) and Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606 (United States)

    2009-09-02

    Positron physics is an important part of the research activities at the Idaho Accelerator Center (IAC). With positron annihilation spectroscopy, maps of nanodefects in materials have been obtained. For this purpose, positrons are generated by radioactive decay, photoactivation, or pair production. Preliminary tests of positron sources in the MeV range based on electron linacs have also been carried out at the IAC, and an expansion of this program is planned. A similar positron beam at Jefferson Lab would greatly improve our knowledge of the inner structure of the proton. In this paper, research with positrons at the IAC is reviewed. After a description of the Center's facilities, results from positron annihilation spectroscopy are discussed, together with future plans for testing a prototype positron source for CEBAF.

  4. Stringent Constraint on Galactic Positron Production

    E-print Network

    John F. Beacom; Hasan Yuksel

    2006-08-15

    The intense 0.511 MeV gamma-ray line emission from the Galactic Center observed by INTEGRAL requires a large annihilation rate of nonrelativistic positrons. If these positrons are injected at even mildly relativistic energies, higher-energy gamma rays will also be produced. We calculate the gamma-ray spectrum due to inflight annihilation and compare to the observed diffuse Galactic gamma-ray data. Even in a simplified but conservative treatment, we find that the positron injection energies must be $\\lesssim 3$ MeV, which strongly constrains models for Galactic positron production.

  5. Microstructure of thermally grown and deposited alumina films probed with positrons

    SciTech Connect

    Somieski, B.; Hulett, L.D.; Xu, J. [Oak Ridge National Laboratory, Chemical Analytical Sciences Division, Oak Ridge, Tennessee 37831 (United States)] [Oak Ridge National Laboratory, Chemical Analytical Sciences Division, Oak Ridge, Tennessee 37831 (United States); Pint, B.A.; Tortorelli, P.F. [Oak Ridge National Laboratory, Metals Ceramics Division, Oak Ridge, Tennessee 37831 (United States)] [Oak Ridge National Laboratory, Metals Ceramics Division, Oak Ridge, Tennessee 37831 (United States); Nielsen, B.; Asoka-Kumar, P. [Brookhaven National Laboratory, Department of Physics, Upton, New York 11973 (United States)] [Brookhaven National Laboratory, Department of Physics, Upton, New York 11973 (United States); Suzuki, R.; Ohdaira, T. [Electrotechnical Laboratory, Quantum Radiation Division, Tsukuba, Ibaraki 305 (Japan)] [Electrotechnical Laboratory, Quantum Radiation Division, Tsukuba, Ibaraki 305 (Japan)

    1999-03-01

    Aluminum oxide films used for corrosion protection of iron and nickel aluminides were generated by substrate oxidation as well as plasma and physical vapor depositions. The films grown by oxidation were crystalline. The others were amorphous. Defect structures of the films were studied by positron spectroscopy techniques. Lifetimes of the positrons, and Doppler broadening of the {gamma} photons generated by their annihilation, were measured as functions of the energies with which they were injected. In this manner, densities and sizes of the defects were determined as functions of depths from the outer surfaces of the films. Alumina films generated by oxidation had high densities of open volume defects, mainly consisting of a few aggregated vacancies. In the outer regions of the films the structures of the defects did not depend on substrate compositions. Positron lifetime measurements, and the {ital S} and {ital W} parameters extracted from Doppler broadening spectra, showed uniform distributions of defects in the crystalline Al{sub 2}O{sub 3} films grown on nickel aluminide substrates, but these data indicated intermediate layers of higher defect contents at the film/substrate interfaces of oxides grown on iron aluminide substrates. Amorphous films generated by plasma and physical vapor deposition had much larger open volume defects, which caused the average lifetimes of the injected positrons to be significantly longer. The plasma deposited film exhibited a high density of large cavities. {copyright} {ital 1999} {ital The American Physical Society}

  6. Investigation of nanolevel molecular packing and its role in thermo-mechanical properties of PVA-fMWCNT composites: positron annihilation and small angle X-ray scattering studies.

    PubMed

    Sharma, S K; Prakash, J; Bahadur, J; Sudarshan, K; Maheshwari, P; Mazumder, S; Pujari, P K

    2014-01-28

    Carbon based nanofillers have shown phenomenal improvements in thermo-mechanical properties of poly vinyl alcohol (PVA) based nanocomposites depending on their interaction with PVA molecules and dispersion in the polymer matrix. In the present study, PVA based nanocomposites with amino-functionalized multi-wall carbon nanotubes (fMWCNTs, 0.2, 0.4, 0.8 and 1.0 wt%) were prepared by a simple casting method from aqueous solution. The relative increase in Young's modulus with 0.4% fMWCNTs was observed to be comparable with that for PVA-nanodiamond composite films which have been shown to have higher strength compared to nanotube and graphene oxide based nanocomposites. In order to investigate the nanolevel molecular packing (sub-nano level free volumes and nano level lamellar structure) and its role in thermal and mechanical properties, positron annihilation spectroscopy and small angle scattering have been used. The crystallinity and morphology of the samples were characterized using X-ray diffraction and scanning electron microscopy. The studies showed that interfacial interaction between PVA molecules and functionalities on the surface of fMWCNTs results in the formation of an ordered structure of PVA molecules which enhances load transfer between the PVA matrix and fMWCNTs leading to improved mechanical properties. The thermal properties of the composites were observed to be unaffected at the studied filler concentration. PMID:24296912

  7. SUSY dark matter annihilation in the Galactic halo

    NASA Astrophysics Data System (ADS)

    Berezinsky, Veniamin; Dokuchaev, Vyachelav; Eroshenko, Yury

    2015-05-01

    Neutralino annihilation in the Galactic halo is the most definite observational signature proposed for indirect registration of the SUSY Dark Matter (DM) candidate particles. The corresponding annihilation signal (in the form of gamma-rays, positrons and antiprotons) may be boosted for one or three orders of magnitude due to the clustering of cold DM particles into the small-scale and very dense self-gravitating clumps. We discuss the formation of these clumps from the initial density perturbations and their successive fate in the Galactic halo. Only a small fraction of these clumps, ? 0.1%, in each logarithmic mass interval ? log M ? 1 survives the stage of hierarchical clustering. We calculate the probability of surviving the remnants of dark matter clumps in the Galaxy by modelling the tidal destruction of the small-scale clumps by the Galactic disk and stars. It is demonstrated that a substantial fraction of clump remnants may survive through the tidal destruction during the lifetime of the Galaxy. The resulting mass spectrum of survived clumps is extended down to the mass of the core of the cosmologically produced clumps with a minimal mass. The survived dense remnants of tidally destructed clumps provide an amplification (boosting) of the annihilation signal with respect to the diffuse DM in the Galactic halo. We describe the anisotropy of clump distribution caused by the tidal destruction of clumps in the Galactic disk.

  8. What Can Positrons Contribute to High-Tc Superconductivity?

    NASA Astrophysics Data System (ADS)

    Peter, M.; Jarlborg, T.; Manuel, A. A.; Barbiellini, B.

    1993-02-01

    The positron annihilation technique has made well-known contributions to the study of Fermi surfaces in "classical" superconductors, including A15 phases where the definition of the Fermi surface has been questioned on the grounds of Anderson localization. In the case of the superconducting oxides, even more far out models were proposed, which made the clear imaging of the Fermi surface by positrons desirable. The difficulties due to the predicted weakness of the signal, and the large possibility for trapping have now been surmounted and the Fermi surface has been seen; what more can we learn from positrons? After presenting the actual situation with experiment, we will comment on enhancement and correlation and their effect on ACAR and lifetime studies. Then we explain the picture of Jarlborg and Singh of enhancement, with its recent tests for many substances. We conclude by asking the question of sensitivity of positrons to many-body effects. Ferromagnetism, antiferromagnetism, possibly charge density waves have been seen -superconductors, heavy fermions and spinons-holons would pose a problem. Stephan Berko (1924-1991) was interested in these problems and knew that better machines and better detectors would open new possibilities.

  9. Probing the ordering transformation in the alloy Cu69Zn14Al17 with positrons

    Microsoft Academic Search

    P. K. Pujari; T. Datta; K. Madangopal; J. Singh

    1993-01-01

    The ordering transformation in the Cu69Zn14Al17 shape-memory alloy has been studied with Doppler-broadened annihilation-radiation line-shape-parameter (S) and lifetime measurements. The S-parameter profile as a function of temperature, S(T), from room temperature to 825 K shows beta-->B2 and B2-->DO3 transitions in conformity with resistivity measurements. The constancy of the S(T) profile in the heating and cooling cycles is indicative of positrons

  10. Annihilation, bound state properties and photodetachment of the positronium negatively charged ion

    E-print Network

    Frolov, Alexei M

    2015-01-01

    Bound state properties of the negatively charged Ps$^{-}$ ion (or $e^{-} e^{+} e^{-}$) are discussed. The expectation values of operators which correspond to these properties have been determined with the use of the highly accurate wave functions constructed for this ion. Our best variational energy obtained for the Ps$^{-}$ ion is $E$ = -0.2620050 7023298 0107770 40035 $a.u.$ Annihilation of the electron-positron pair(s) in the negatively charged Ps$^{-}$ ion (or $e^{-} e^{+} e^{-}$) is considered in detail. The overall life-time of the Ps$^{-}$ ion against positron annihilation $\\tau_a = \\frac{1}{\\Gamma} \\approx 4.793584140 \\cdot 10^{-10}$ $sec$ is determined together with a number of different annihilation rates $\\Gamma_{n \\gamma}$, where $n$ = 1, 2, 3, 4 and 5. Photodetachment of the Ps$^{-}$ ion is considered in the long-range, asymptotic approximation. The overall accuracy of the photodetachment cross-section of the Ps$^{-}$ ion is very good for such a simple approximation.

  11. Measurement of charm meson lifetimes

    E-print Network

    Ammar, Raymond G.; Baringer, Philip S.; Bean, Alice; Besson, David Zeke; Coppage, Don; Davis, Robin E. P.; Kravchenko, I.; Kwak, Nowhan; Zhao, L.

    1999-06-01

    We report measurements of the D-0, D-,(+) and D-s(+) meson lifetimes using 3.7 fb(-1) of e(+)e(-) annihilation data collected near the Y(4S) resonance with the CLEO detector. The measured lifetimes of the D-0, D+, and ...

  12. Many-body theory of positron-atom interactions G. F. Gribakin* and J. Ludlow

    E-print Network

    Gribakin, Gleb

    Many-body theory of positron-atom interactions G. F. Gribakin* and J. Ludlow Department of Applied for the problem of positron-atom scattering and annihilation. Strong electron-positron correlations are included nonperturbatively through the calculation of the electron- positron vertex function. It corresponds to the sum

  13. Positron spectroscopy of defects in submicrocrystalline nickel after low-temperature annealing

    NASA Astrophysics Data System (ADS)

    Kuznetsov, P. V.; Mironov, Yu. P.; Tolmachev, A. I.; Bordulev, Yu. S.; Laptev, R. S.; Lider, A. M.; Korznikov, A. V.

    2015-02-01

    Using the method of measuring the positron lifetime spectra and Doppler broadening annihilation line spectroscopy, the annealing of defects in submicrocrystalline nickel produced by equal channel angular pressing has been studied. In as-prepared samples, the positrons are trapped by dislocation defects and vacancy complexes inside crystallites. The size of vacancy complexes decreases with increasing annealing temperature in the interval ? T = 20-300°C. However, at T = 360°C, the complexes start growing again. The dependence of S-parameter on W-parameter derived from the Doppler broadening spectroscopy has two parts with different inclinations to axes that correspond to different types of primary centers of positron trapping in submicrocrystalline nickel. It has been elucidated that, at recovery stage in the temperature interval ? T = 20-180°C, the main centers of positron trapping are low-angle boundaries enriched by impurities, while at in situ recrystallization stage in the temperature interval ? T = 180-360°C, the primary centers of positron trapping are low-angle boundaries.

  14. Mesopore quality determines the lifetime of hierarchically structured zeolite catalysts

    NASA Astrophysics Data System (ADS)

    Milina, Maria; Mitchell, Sharon; Crivelli, Paolo; Cooke, David; Pérez-Ramírez, Javier

    2014-05-01

    Deactivation due to coking limits the lifetime of zeolite catalysts in the production of chemicals and fuels. Superior performance can be achieved through hierarchically structuring the zeolite porosity, yet no relation has been established between the mesopore architecture and the catalyst lifetime. Here we introduce a top-down demetallation strategy to locate mesopores in different regions of MFI-type crystals with identical bulk porous and acidic properties. In contrast, well-established bottom-up strategies as carbon templating and seed silanization fail to yield materials with matching characteristics. Advanced characterization tools capable of accurately discriminating the mesopore size, distribution and connectivity are applied to corroborate the concept of mesopore quality. Positron annihilation lifetime spectroscopy proves powerful to quantify the global connectivity of the intracrystalline pore network, which, as demonstrated in the conversions of methanol or of propanal to hydrocarbons, is closely linked to the lifetime of zeolite catalysts. The findings emphasize the need to aptly tailor hierarchical materials for maximal catalytic advantage.

  15. Positron Beam Characteristics at NEPOMUC Upgrade

    NASA Astrophysics Data System (ADS)

    Hugenschmidt, C.; Ceeh, H.; Gigl, T.; Lippert, F.; Piochacz, C.; Reiner, M.; Schreckenbach, K.; Vohburger, S.; Weber, J.; Zimnik, S.

    2014-04-01

    In 2012, the new neutron induced positron source NEPOMUC upgrade was put into operation at FRMII. Major changes have been made to the source which consists of a neutron-?-converter out of Cd and a Pt foil structure for electron positron pair production and positron moderation. The new design leads to an improvement of both intensity and brightness of the mono-energetic positron beam. In addition, the application of highly enriched 113Cd as neutron-?-converter extends the lifetime of the positron source to 25 years. A new switching and remoderation device has been installed in order to allow toggling from the high-intensity primary beam to a brightness enhanced remoderated positron beam. At present, an intensity of more than 109 moderated positrons per second is achieved at NEPOMUC upgrade. The main characteristics are presented which comprise positron yield and beam profile of both the primary and the remoderated positron beam.

  16. SUSY dark matter annihilation in the Galactic halo

    E-print Network

    Berezinsky, Veniamin; Erohenko, Yury

    2015-01-01

    Neutralino annihilation in the Galactic halo is the most definite observational signature proposed for indirect registration of the SUSY Dark Matter (DM) candidate particles. The corresponding annihilation signal (in the form of gamma-rays, positrons and antiprotons) may be boosted for one or three orders of magnitude due to the clustering of cold DM particles into the small-scale and very dense self-gravitating clumps. We discuss the formation of these clumps from the initial density perturbations and their successive fate in the Galactic halo. Only a small fraction of these clumps, $\\sim0.1$%, in each logarithmic mass interval $\\Delta\\log M\\sim1$ survives the stage of hierarchical clustering. We calculate the probability of surviving the remnants of dark matter clumps in the Galaxy by modelling the tidal destruction of the small-scale clumps by the Galactic disk and stars. It is demonstrated that a substantial fraction of clump remnants may survive through the tidal destruction during the lifetime of the Ga...

  17. Transport of positrons in the interstellar medium

    E-print Network

    W. Gillard; P. Jean; A. Marcowith; K. Ferriere

    2007-02-06

    This work investigates some aspects of the transport of low-energy positrons in the interstellar medium (ISM). We consider resonance interactions with magnetohydrodynamic waves above the resonance threshold. Below the threshold, collisions take over and deflect positrons in their motion parallel to magnetic-field lines. Using Monte-Carlo simulations, we model the propagation and energy losses of positrons in the different phases of the ISM until they annihilate. We suggest that positrons produced in the disk by an old population of stars, with initial kinetic energies below 1 MeV, and propagating in the spiral magnetic field of the disk, can probably not penetrate the Galactic bulge.

  18. What is the fate of runaway positrons in tokamaks?

    DOE PAGESBeta

    Liu, Jian [University of Science and Technology of China, Hefei, Anhui (China). Center for Advanced Fusion Energy and Plasma Sciences; Qin, Hong [University of Science and Technology of China, Hefei, Anhui (China). Center for Advanced Fusion Energy and Plasma Sciences; Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States); Fisch, Nathaniel J. [Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States); Teng, Qian [Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States); Wang, Xiaogang [Peking Univ, Beijing (China). School of Physics

    2014-06-01

    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.

  19. Positronium lifetime in polymers

    E-print Network

    Camacho, Abel

    2004-01-01

    A model describing the relationship between the ortho--positronium lifetime and the volume of a void, located in a synthetic zeolite, is analyzed. Our idea, which allows us to take into account the effects of temperature, comprises the introduction of a non--hermitian term in the Hamiltonian, which accounts for the annihilation of the ortho--positronium. The predictions of the present model are also confronted against an already known experimental result.

  20. 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.

  1. 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.

  2. Unthermalized positrons in gamma ray burst sources

    NASA Technical Reports Server (NTRS)

    Tkaczyk, W.; Karakula, S.

    1992-01-01

    The spectra of the broadening 0.511 MeV annihilation line produced by high temperatures was calculated in the case of unthermalized plasma; i.e., T sub e(+) is not = T sub e(-). The flattening in the spectrum of the annihilation lines for large differences of electron and positron temperatures is a strong indication that the observed features of the hard tailed spectrum of the gamma bursts can be well described by annihilation of unthermalized positrons. It is proposed that the charge separation occurring in Eddington limited accretion onto a neutron star or the one photon pair production in strong magnetic fields as a mechanism for the production of unthermalized positrons in the sources of gamma bursts. From the best fit of experimental spectra by the model, the parameters of sources for which the regions with different plasma temperatures can exist is evaluated.

  3. Positron probing of phosphorus-vacancy complexes in silicon irradiated with 15 MeV protons

    NASA Astrophysics Data System (ADS)

    Arutyunov, N.; Emtsev, V.; Krause-Rehberg, R.; Elsayed, M.; Kessler, C.; Kozlovski, V.; Oganesyan, G.

    2015-06-01

    Defects in phosphorus-doped silicon samples of floating-zone material, n-FZ-Si(P), produced under irradiation with 15 MeV protons at room temperature are studied by positron annihilation lifetime spectroscopy over the temperature range of ? 30 K - 300 K and by low- temperature Hall effect measurements. After annealing of E-centersand divacancies, we detected for the first time high concentrations of positron traps which had not been observed earlier. These defects are isochronally annealed over the temperature interval of ? 320 °C - 700 °C they manifest themselves as electrically neutral deep donor centersin the material of n-type. A long-lived component of the positron lifetime, ?2(I2 < 60%) ? 280 ps, attributed to these centers, suggests a relaxed configuration involving two vacancies. The enthalpy and entropy of annealing of these centersare Ea ? 1.05(0.21) eV and ?Sm ? 3.1(0.6)kB, respectively. It is argued that the microstructure of the defect consists of two vacancies, VV, and one atom of phosphorus, P. The split configuration of the VPV complex is shortly discussed.

  4. Measurement of the ?c+ lifetime

    Microsoft Academic Search

    A. Mahmood; S. Csorna; I. Danko; Z. Xu; G. Bonvicini; D. Cinabro; M. Dubrovin; S. McGee; A. Bornheim; E. Lipeles; S. Pappas; A. Shapiro; W. Sun; A. Weinstein; G. Masek; H. Paar; R. Mahapatra; R. Morrison; H. Nelson; R. Briere; G. Chen; T. Ferguson; G. Tatishvili; H. Vogel; N. Adam; J. Alexander; C. Bebek; K. Berkelman; F. Blanc; V. Boisvert; D. Cassel; P. Drell; J. Duboscq; K. Ecklund; R. Ehrlich; R. Galik; L. Gibbons; B. Gittelman; S. Gray; D. Hartill; B. Heltsley; L. Hsu; C. Jones; J. Kandaswamy; D. Kreinick; A. Magerkurth; H. Mahlke-Krüger; T. Meyer; N. Mistry; E. Nordberg; M. Palmer; J. Patterson; D. Peterson; J. Pivarski; D. Riley; A. Sadoff; H. Schwarthoff; M. Shepherd; J. Thayer; D. Urner; B. Valant-Spaight; G. Viehhauser; A. Warburton; M. Weinberger; S. Athar; P. Avery; C. Prescott; H. Stoeck; J. Yelton; G. Brandenburg; A. Ershov; D. Kim; R. Wilson; K. Benslama; B. Eisenstein; J. Ernst; G. Gollin; R. Hans; I. Karliner; N. Lowrey; M. Marsh; C. Plager; C. Sedlack; M. Selen; J. Thaler; J. Williams; K. Edwards; R. Ammar; D. Besson; X. Zhao; S. Anderson; V. Frolov; Y. Kubota; S. Lee; S. Li; R. Poling; A. Smith; C. Stepaniak; J. Urheim; S. Ahmed; M. Alam; L. Jian; M. Saleem; F. Wappler; E. Eckhart; K. Gan; C. Gwon; T. Hart; K. Honscheid; D. Hufnagel; H. Kagan; R. Kass; T. Pedlar; E. von Toerne; T. Wilksen; M. Zoeller; S. Richichi; H. Severini; P. Skubic; S. Dytman; S. Nam; V. Savinov; S. Chen; J. Hinson; J. Lee; D. Miller; V. Pavlunin; E. Shibata; I. Shipsey; D. Cronin-Hennessy; A. Lyon; E. Thorndike; T. Coan; Y. Gao; F. Liu; Y. Maravin; I. Narsky; R. Stroynowski; J. Ye; M. Artuso; C. Boulahouache; K. Bukin; E. Dambasuren; T. Skwarnicki; S. Stone; J. Wang

    2001-01-01

    The Xi^+_c lifetime is measured using 9.0 fb^-1 of e+e- annihilation data\\u000acollected on and just below the Upsilon(4S) resonance with the CLEO II.V\\u000adetector at CESR. This is the first measurement of the Xi^+_c lifetime from a\\u000acollider experiment. Using an unbinned maximum likelihood fit, the Xi^+_c\\u000alifetime is measured to be 503 +\\/- 47 (stat.) +\\/- 18 (syst.)

  5. Searching for dark matter with future cosmic positron experiments

    SciTech Connect

    Hooper, Dan; Silk, Joseph [Astrophysics Department, University of Oxford, OX1 3RH Oxford (United Kingdom)

    2005-04-15

    Dark matter particles annihilating in the Galactic halo can provide a flux of positrons potentially observable in upcoming experiments, such as PAMELA and AMS-02. We discuss the spectral features which may be associated with dark matter annihilation in the positron spectrum and assess the prospects for observing such features in future experiments. Although we focus on some specific dark matter candidates, neutralinos and Kaluza-Klein states, we carry out our study in a model independent fashion.

  6. 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.

  7. Positron clouds within thunderstorms

    E-print Network

    Dwyer, Joseph R; Hazelton, Bryna J; Grefenstette, Brian W; Kelley, Nicole A; Lowell, Alexander W; Schaal, Meagan M; Rassoul, Hamid K

    2015-01-01

    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 seconds apart, each lasting approximately 0.2 seconds. The enhancements, which were about 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 kilometer across. It is not clear how the positron clouds were created within the thunderstorm, but it is possible they were ca...

  8. Studies of Oxidation of the Cu(100) Surface Using Low Energy Positrons

    SciTech Connect

    Fazleev, N. G. [University of Texas at Arlington, Arlington Texas 76019 (United States); Kazan State University, Kazan 420008 (Russian Federation); Maddox, W. B.; Nadesalingam, M.; Rajeshwar, K.; Weiss, A. H. [University of Texas at Arlington, Arlington Texas 76019 (United States)

    2009-03-10

    Changes in the surface of an oxidized Cu(100) single crystal resulting from vacuum annealing have been investigated using positron annihilation induced Auger electron spectroscopy (PAES). PAES measurements show a large increase in the intensity of the positron annihilation induced Cu M{sub 2,3}VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 deg. C. The intensity then decreases monotonically as the annealing temperature is increased to {approx}600 deg. C. Experimental PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface, surface reconstructions, and electron-positron correlations effects. Possible explanation for the observed behavior of the intensity of positron annihilation induced Cu M{sub 2,3}VV Auger peak with changes of the annealing temperature is proposed.

  9. Measurement of the Lambda(+)(c) lifetime

    E-print Network

    Ammar, Raymond G.; Bean, Alice; Besson, David Zeke; Davis, Robin E. P.; Kwak, Nowhan; Zhao, X.

    2001-03-01

    The Lambda (+)(c) lifetime is measured using 9.0 fb(-1) of e(+)e(-) annihilation data collected on or just below the Y (4S) resonance with the CLEO ILV detector at CESR. Using an unbinned maximum likelihood fit, the Lambda (+)(c) lifetime...

  10. 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.

  11. Positron drift in molecular hydrogen

    Microsoft Academic Search

    N. Bose; D. A. L. Paul; J.-S. Tsai

    1981-01-01

    Measurements of positron drift in H2 gas have been made in the pressure range 10-200 Torr between 0 and 4 Td. A shape analysis gives a universal function W\\/Zeff. Using the annihilation parameter Zeff=13.6, the drift speed W increases linearly up to 2.9*105 cm s-1 at 1 Td. Above 1.4 Td there is evidence for positronium formation. These are the

  12. Positron beam studies of solids and surfaces: A summary

    NASA Astrophysics Data System (ADS)

    Coleman, P. G.

    2006-02-01

    A personal overview is given of the advances in positron beam studies of solids and surfaces presented at the 10th International Workshop on Positron Beams, held in Doha, Qatar, in March 2005. Solids studied include semiconductors, metals, alloys and insulators, as well as biophysical systems. Surface studies focussed on positron annihilation-induced Auger electron spectroscopy (PAES), but interesting applications of positron-surface interactions in fields as diverse as semiconductor technology and studies of the interstellar medium serve to illustrate once again the breadth of scientific endeavour covered by slow positron beam investigations.

  13. Positron moderation and detection for positronic atoms

    NASA Astrophysics Data System (ADS)

    Fardad, Abolfazl

    An apparatus is under development for H--+* production, atoms consisting of a positron bound in a Rydberg state to an H-- ion. High energy e+ from radioactive N2211a are slowed (moderated) to eV energies in solid neon and captured in a Penning trap. The procedure to deposit the neon is optimized, resulting in a 1.5% efficiency for moderating high energy e +. Neutral H--+* atoms with ˜100 eV will be produced from these trapped e+ and exit the trap, hitting a metal surface where the e+ annihilates. Back-to-back annihilation gamma photons (Egamma ? 0.511 MeV) detected in coincidence, at the expected energy are the fingerprint for H--+* production. A N2211a test source mocks H--+* experiments with ˜2.7% of the e+ emitting disintegrations detected. This high efficiency, with a background rate of ˜2.8 events/hour is achieved by surrounding the detectors with lead and cosmic ray detectors.

  14. Reduction of the trapping of positrons in dislocated single crystals of iron when charged with hydrogen

    SciTech Connect

    Park, Y.K.; Waber, J.T.; Snead, C.L. Jr.

    1985-01-01

    The positron annihilation measurement was carried out with the pure iron single crystals deformed in various ways before and after hydrogen permeation. The positron trapping intensity was reduced more in the screw dislocation than in the edge dislocation by hydrogen charging. The trap occupancy by hydrogen was very close to the fraction of the reduction in positron trapping intensity.

  15. The Energy Spectrum of Positrons from the Decay of the mu-Meson

    Microsoft Academic Search

    H. J. Bramson; A. M. Seifert; W. W. Jr. Havens

    1952-01-01

    The energy of the positrons from 301pi-->mu-->beta decays have been determined in electron sensitive emulsions by measuring the multiple scattering of the positron tracks. The annihilation properties of the charged particle emitted in the decay of the mu+ meson confirm its identity as a positron. This spectrum has a maximum about 36 Mev and a nonzero cutoff at the high

  16. 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.

  17. 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.

  18. Probing the ordering transformation in the alloy Cu69Zn14Al17 with positrons

    NASA Astrophysics Data System (ADS)

    Pujari, P. K.; Datta, T.; Madangopal, K.; Singh, J.

    1993-05-01

    The ordering transformation in the Cu69Zn14Al17 shape-memory alloy has been studied with Doppler-broadened annihilation-radiation line-shape-parameter (S) and lifetime measurements. The S-parameter profile as a function of temperature, S(T), from room temperature to 825 K shows ?-->B2 and B2-->DO3 transitions in conformity with resistivity measurements. The constancy of the S(T) profile in the heating and cooling cycles is indicative of positrons sampling the equilibrium vacancies. Results obtained from both measurement techniques are not indicative of the presence of divacancies in the disordered (?) state. Analysis of the S(T) profile in terms of a trapping model over the entire temperature range shows a temperature dependence for the vacancy-formation energy, probably due to the ordering process. The results are discussed.

  19. POSITRON LIFETI ME SPECTRA IN MOLECULAR SUBSTANCES 232 broadening of the electron-density distribution aGects

    E-print Network

    Stillinger, Frank

    POSITRON LIFETI ME SPECTRA IN MOLECULAR SUBSTANCES 232 broadening of the electron 8 with the annihilation of positrons which did not form Ps, be- cause of the magnitude of v between a molecule and a positron on one hand and a positronium atom on the other. The small values of v

  20. Positron-gamma angular correlation measurements on the decays of 84Rb, 74As, 124I and 102Rh

    Microsoft Academic Search

    A. de Beer; J. H. Stuivenberg; B. Meindersma; J. Blok

    1969-01-01

    Positron-gamma angular correlations have been measured using a plastic scintillator to detect the positrons. The disturbance of the positron spectrum due to Compton scattering of the annihilation quanta in the plastic has been avoided by counting these quanta simultaneously in two large NaI(Tl) crystals. Measurements were performed on 2- --> 2+ nonunique first order forbidden transitions from the ground states

  1. 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

  2. Radiative proton-antiproton annihilation to a lepton pair

    SciTech Connect

    Ahmadov, A. I. [Joint Institute for Nuclear Research, Dubna (Russian Federation); Institute of Physics, Azerbaijan National Academy of Sciences, Baku (Azerbaijan); Bytev, V. V.; Kuraev, E. A. [Joint Institute for Nuclear Research, Dubna (Russian Federation); Tomasi-Gustafsson, E. [CEA, IRFU, SPhN, Saclay, 91191 Gif-sur-Yvette Cedex, France, and CNRS/IN2P3, Institut de Physique Nucleaire, UMR 8608, 91405 Orsay (France)

    2010-11-01

    The annihilation of proton and antiproton to an electron-positron pair, including radiative corrections due to the emission of virtual and real photons is considered. The results are generalized to leading and next-to leading approximations. The relevant distributions are derived and numerical applications are given in the kinematical range accessible to the PANDA experiment at the FAIR facility.

  3. Galactic secondary positron flux at the Earth

    E-print Network

    Delahaye, T; Fornengo, N; Lavalle, J; Lineros, R; Salati, P; Taillet, R

    2008-01-01

    CONTEXT: Current measurements of the positron cosmic rays exhibit a bump around 10 GeV which is still hardly explained by standard secondary astrophysical processes, i.e. spallation of cosmic rays off the interstellar gas. Many scenarios have been invoked as potential solutions to this excess, among them being some additional primary positrons originating from dark matter annihilation in the Galaxy. AIMS: While the PAMELA satellite is about to yield much more precise data, it is of paramount importance to theoretically constrain the expected secondary positron flux before any putative interpretation of the observations. Moskalenko and Strong (1998) provided a precise estimate some years ago, but using rather old parameterizations for the nuclear cross sections, and without giving the theoretical uncertainties affecting their predictions. METHODS: We reestimate the secondary positron flux by using and comparing different up-to-date nuclear cross sections and, and by considering an independent model of cosmic r...

  4. 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.

  5. Hypernovae and light dark matter as possible Galactic positron sources

    E-print Network

    S. Schanne; M. Casse; J. Paul; B. Cordier

    2005-07-25

    The electron-positron annihilation source in the Galactic center region has recently been observed with INTEGRAL/SPI, which shows that this 511 keV source is strong and its extension is consistent with the Galactic bulge geometry. The positron production rate, estimated to more than 10$^{43}$ per second, is very high and raises a challenging question about the nature of the Galactic positron source. Commonly considered astrophysical positron injectors, namely type Ia supernovae are rare events and fall short to explain the observed positron production rate. In this paper, we study the possibility of Galactic positron production by hypernovae events, exemplified by the recently observed SN2003dh/GRB030329, an asymmetric explosion of a Wolf-Rayet star associated with a gamma-ray burst. In these kinds of events, the ejected material becomes quickly transparent to positrons, which spread out in the interstellar medium. Non radioactive processes, such as decays of heavy dark matter particles (neutralinos) predicted by most extensions of the standard model of particle physics, could also produce positrons as byproducts. However they are expected to be accompanied by a large flux of high-energy gamma-rays, which were not observed by EGRET and ground based Tcherenkov experiments. In this context we explore the possibility of direct positron production by annihilation of light dark matter particles.

  6. 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.

  7. 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.

  8. Exciton dynamics and annihilation in WS2 2D semiconductors

    NASA Astrophysics Data System (ADS)

    Yuan, Long; Huang, Libai

    2015-04-01

    We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 +/- 37 ps, 401 +/- 25 ps, and 332 +/- 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 +/- 0.02, (6.00 +/- 1.09) × 10-3 and (1.88 +/- 0.47) × 10-3 cm2 s-1, respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons.We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 +/- 37 ps, 401 +/- 25 ps, and 332 +/- 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 +/- 0.02, (6.00 +/- 1.09) × 10-3 and (1.88 +/- 0.47) × 10-3 cm2 s-1, respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons. Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nr00383k

  9. Positron annihilation studies of Bi 2CaSr 2Cu 2O x and Bi 1.6Pb 0.4Ca 2Sr 2Cu 3O y in the region of the superconducting transition

    NASA Astrophysics Data System (ADS)

    Pujari, P. K.; Datta, T.; Manohar, S. B.; Prakash, Satya; Sastry, P. V. P. S. S.; Yakhmi, J. V.; Iyer, R. M.

    1990-03-01

    Doppler broadened annihilation radiation (DBAR) spectral parameters have been reported- for the first time- between 77 K and 300 K, for several Bi-based oxide superconductors, viz. A: single phase (2122) Bi 2CaSr 2Cu 2O x with Tc=85 K (R=0), B: a mixed phase lead doped sample containing both 2122 and 2223 with a nominal composition Bi 1.6Pb 0.4Ca 2Sr 2Cu 3O y, and, C: another 2122+2223 sample with same nominal composition as that of B but synthesised under a different heat-treatment schedule so as to yield a Tc=85 K (R=0). Analyses of these spectra using PAACFIT program yielded two components, of which the intensity of the narrow component, I N, and, the width of the broad component, T B, were seen to be the only temperature dependent parameters. At the onset of superconducting transition both T B and I N were seen to increase to a maximum value and decrease on further cooling. A double peak structure in T B vs temperature profile were observed in sample B and C, similar to one reported by us in Tl-Ca-Ba-Cu-O systems. In addition, presence of a magnetic field (1 KG) yielded no significant change in the DBAR spectral parameters. The results are discussed.

  10. 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.

  11. Local dark matter clumps and the cosmic ray positron excess

    E-print Network

    Daniel T. Cumberbatch; Joseph Silk

    2006-07-24

    It has been proposed that the excess in cosmic ray positrons at approximately 8 GeV, observed on both flights of the HEAT balloon experiment, may be associated with the annihilation of dark matter within the Milky Way halo. In this paper we demonstrate how the self-annihilation of neutralino dark matter within local substructure can account for this excess, and estimate the annihilation cross-section for several benchmark minimal supersymmetric (MSSM) models. We also demonstrate the effect on the permitted parameter space as tidal stripping effects and destruction of substructure by mergers becomes increasingly severe.

  12. 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.

  13. 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

    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.

  14. Implantation profile of 22Na continuous energy spectrum positrons in silicon

    Microsoft Academic Search

    P. J. Foster; P. Mascher; A. P. Knights; P. G. Coleman

    2007-01-01

    The implantation profile of positrons emitted from a continuous energy spectrum source of 22Na in close proximity to a silicon target is modeled. The primary motivation is the use of positron lifetime spectroscopy to characterize layers of defects such as those created by ion irradiation, usually deemed accessible only to techniques which utilize slow positrons. The model combines the Makhov

  15. Electron emission due to positronium annihilation in solid Ar, Kr, and Xe

    NASA Astrophysics Data System (ADS)

    Gullikson, E. M.; Mills, A. P., Jr.

    1989-03-01

    We present measurements of positron and electron emission by solid rare-gas surfaces implanted with low-energy positrons. The electron yield exhibits a 0.5-eV-wide peak at a positron energy just above the threshold for positronium (Ps) formation, but below the electron-hole-pair threshold. A similar peak in the fraction of the incident positrons annihilating in the sample leads us to conclude that (1) slow Ps has an anomalously short diffusion length possibly due to self-trapping, and (2) energetic electrons are liberated from Ps atoms following annihilation of the positrons with valence electrons. The energy spectrum of the emitted electrons should contain information about the internal Ps wave function in the solids.

  16. 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.

  17. 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 ?-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.

  18. Vacancy-type defects induced by grinding of Si wafers studied by monoenergetic positron beams

    NASA Astrophysics Data System (ADS)

    Uedono, Akira; Mizushima, Yoriko; Kim, Youngsuk; Nakamura, Tomoji; Ohba, Takayuki; Yoshihara, Nakaaki; Oshima, Nagayasu; Suzuki, Ryoichi

    2014-10-01

    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.

  19. Vacancy-type defects induced by grinding of Si wafers studied by monoenergetic positron beams

    SciTech Connect

    Uedono, Akira; Yoshihara, Nakaaki [Division of Applied Physics, Faculty of Pure and Applied Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573 (Japan); Mizushima, Yoriko [Devices and Materials Labs Fujitsu Laboratories Ltd., Atsugi, Kanagawa 243-0197 (Japan); ICE Cube Center, Tokyo Institute of Technology, Yokohama 226-8503 (Japan); Kim, Youngsuk [ICE Cube Center, Tokyo Institute of Technology, Yokohama 226-8503 (Japan); Disco Corporation, Ota, Tokyo 143-8580 (Japan); Nakamura, Tomoji [Devices and Materials Labs Fujitsu Laboratories Ltd., Atsugi, Kanagawa 243-0197 (Japan); Ohba, Takayuki [ICE Cube Center, Tokyo Institute of Technology, Yokohama 226-8503 (Japan); Oshima, Nagayasu; Suzuki, Ryoichi [Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568 (Japan)

    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.

  20. Modeling Positron Transport in Gaseous and Soft-condensed Systems with Kinetic Theory and Monte Carlo

    NASA Astrophysics Data System (ADS)

    Boyle, G.; Tattersall, W.; Robson, R. E.; White, Ron; Dujko, S.; Petrovic, Z. Lj.; Brunger, M. J.; Sullivan, J. P.; Buckman, S. J.; Garcia, G.

    2013-09-01

    An accurate quantitative understanding of the behavior of positrons in gaseous and soft-condensed systems is important for many technological applications as well as to fundamental physics research. Optimizing Positron Emission Tomography (PET) technology and understanding the associated radiation damage requires knowledge of how positrons interact with matter prior to annihilation. Modeling techniques developed for electrons can also be employed to model positrons, and these techniques can also be extended to account for the structural properties of the medium. Two complementary approaches have been implemented in the present work: kinetic theory and Monte Carlo simulations. Kinetic theory is based on the multi-term Boltzmann equation, which has recently been modified to include the positron-specific interaction processes of annihilation and positronium formation. Simultaneously, a Monte Carlo simulation code has been developed that can likewise incorporate positron-specific processes. An accurate quantitative understanding of the behavior of positrons in gaseous and soft-condensed systems is important for many technological applications as well as to fundamental physics research. Optimizing Positron Emission Tomography (PET) technology and understanding the associated radiation damage requires knowledge of how positrons interact with matter prior to annihilation. Modeling techniques developed for electrons can also be employed to model positrons, and these techniques can also be extended to account for the structural properties of the medium. Two complementary approaches have been implemented in the present work: kinetic theory and Monte Carlo simulations. Kinetic theory is based on the multi-term Boltzmann equation, which has recently been modified to include the positron-specific interaction processes of annihilation and positronium formation. Simultaneously, a Monte Carlo simulation code has been developed that can likewise incorporate positron-specific processes. Funding support from ARC (CoE and DP schemes).

  1. Positron on positroniumlike surface state on Al(100)

    SciTech Connect

    Lynn, K.G.; Mills, A.P.; West, R.N.; Berko, S.; Canter, K.F.; Roellig, L.O.

    1985-04-15

    Using a high-intensity beam of 200-eV positrons we have measured the two-dimensional angular correlation of the 2..gamma.. annihilation radiation from a clean Al(100) surface. The momentum distribution identified with the positron surface state has a nearly isotropic conical shape and a 7.1 +- 0.5-mrad full width at half maximum. The data are not consistent with a simple interpretation based on either the usual model of a positron bound in a surface state by its ''image-correlation potential'' or a positronium atom weakly bound to the surface.

  2. Positron or positronium-like surface state on Al(100)

    SciTech Connect

    Lynn, K.G.; Mills, A.P. Jr.; West, R.N.; Berko, S.; Canter, K.F.; Roellig, L.O.

    1985-04-15

    Using a high-intensity beam of 200-eV positrons the authors have measured the two-dimensional angular correlation of the 2..gamma.. annihilation radiation from a clean Al(100) surface. The momentum distribution identified with the positron surface state has a nearly isotropic conical shape and a (7.1 +/- 0.5)-mrad full width at half maximum. The data are not consistent with a simple interpretation based on either the usual model of a positron bound in a surface state by its image-correlation potential or a positronium atom weakly bound to the surface. 15 references, 3 figures.

  3. Positron states on the Cs/Cu(100) surface

    SciTech Connect

    Koeymen, A.R.; Lee, K.H.; Mehl, D.; Weiss, A. (Physics Department, University of Texas at Arlington, Texas (USA)); Jensen, K.O. (School of Mathematics and Physics, University of East Anglia, Norwich NR4 7TJ (United Kingdom))

    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.

  4. The development of a positron ionization gauge

    SciTech Connect

    Jacobsen, F.; Strongin, M.; Ruckman, M. [Brookhaven National Lab., Upton, NY (United States). Dept. of Physics; Wiess, A. [Univ. of Texas, Arlington, TX (United States). Dept. of Physics; Turner, W.C. [Superconducting Super Collider Lab., Dallas, TX (United States)

    1994-01-01

    The authors present a method by which gas pressure (density) can be measured by positrons. The process to monitor is the formation of positronium, Ps, via electron capture by the e{sup +} from the rest gas molecules. The Ps signal which is proportional to the gas density is obtained from the annihilation photons which are emitted when the Ps atom decays. By this method it is not necessary to have access to the vacuum system in question other than having the possibility of passing a positron beam through it. Also the present method is fully UHV compatible. In its simplest version pressures below 10{sup {minus}8} torr (at room temperature) can be measured within a reasonable time. Techniques are discussed which will significantly improve the sensitivity of the present ionization gauge. The specific reason for designing and using the positron ionization gauge is to be able to measure the pressure inside the Superconducting Super Collider beam tubing during simulated operation.

  5. Dark matter annihilation and the PAMELA, FERMI, and ATIC anomalies

    SciTech Connect

    El Zant, A. A.; Okada, H. [Centre for Theoretical Physics, British University in Egypt, El Sherouk City, Postal No, 11837, Post Office Box 43 (Egypt); Khalil, S. [Centre for Theoretical Physics, British University in Egypt, El Sherouk City, Postal No, 11837, Post Office Box 43 (Egypt); Department of Mathematics, Ain Shams University, Faculty of Science, Cairo, 11566 (Egypt)

    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.

  6. A constraint on the pair-density ratio (Z+) in an electron-positron pair wind

    NASA Technical Reports Server (NTRS)

    Moscoso, M. D.; Wheeler, J. C.

    1994-01-01

    We derive a constraint on the pair density ratio, z(sub +) = n(sub +)/n(sub p), in an electron-positron pair wind flowing away from the central region of an accretion disk around a compact object under the assumption of a coupling between electrons, positrons, and protons. The minimum rate at which positrons are injected into the annihilation volume is given by the observed annihilation flux per unit volume. This rate is then used to determine a minimum mass loss rate per unit area, M(dot)(sub *) for a given pair density ratio at the base of the streamline. The requirement that M(dot)(sub *) less than M(dot)(sub *)(sub Edd) (the mean Eddington mass loss rate per unit area) then places a lower limit on the pair density ratio, z(sub +,)(sub min). A positron annihilation line was observed in Nova Muscae 1991 by GRANAT/SIGMA. The narrow width and redshift of the line suggest that the pair production and annihilation regions are physically distinct. We hypothesize that an electron-positron pair wind transports the pairs from the production to the annihilation region and calculate z(sub +),(sub min). We then determine constraints on the physical parameters on the pair production region by comparing z(sub +),(sub min) with previous studies of two-temperature and one-temperature accretion disks with electron-positron pairs.

  7. Exciton dynamics and annihilation in WS2 2D semiconductors.

    PubMed

    Yuan, Long; Huang, Libai

    2015-04-01

    We systematically investigate the exciton dynamics in monolayered, bilayered, and trilayered WS2 two-dimensional (2D) crystals by time-resolved photoluminescence (TRPL) spectroscopy. The exciton lifetime when free of exciton annihilation was determined to be 806 ± 37 ps, 401 ± 25 ps, and 332 ± 19 ps for WS2 monolayer, bilayer, and trilayer, respectively. By measuring the fluorescence quantum yields, we also establish the radiative and nonradiative lifetimes of the direct and indirect excitons. The exciton decay in monolayered WS2 exhibits a strong excitation density-dependence, which can be described using an exciton-exciton annihilation (two-particle Auger recombination) model. The exciton-exciton annihilation rate for monolayered, bilayered, and trilayered WS2 was determined to be 0.41 ± 0.02, (6.00 ± 1.09) × 10(-3) and (1.88 ± 0.47) × 10(-3) cm(2) s(-1), respectively. Notably, the exciton-exciton annihilation rate is two orders of magnitude faster in the monolayer than in the bilayer and trilayer. We attribute the much slower exciton-exciton annihilation rate in the bilayer and trilayer to reduced many-body interaction and phonon-assisted exciton-exciton annihilation of indirect excitons. PMID:25826397

  8. Evidence for a positron bound state on the surface of a topological insulator

    NASA Astrophysics Data System (ADS)

    Shastry, K.; Weiss, A. H.; Barbiellini, B.; Assaf, B. A.; Lim, Z. H.; Joglekar, P. V.; Heiman, D.

    2015-06-01

    We describe experiments aimed at probing the sticking of positrons to the surfaces of topological insulators using the Positron Annihilation induced Auger Electron Spectrometer (PAES). A magnetically guided beam was used to deposit positrons at the surface of Bi2Te2Se sample at energy of ?2eV. Peaks observed in the energy spectra and intensities of electrons emitted as a result of positron annihilation showed peaks at energies corresponding to Auger peaks in Bi, Teand Se providing clear evidence of Auger emission associated with the annihilation of positrons in a surface bound state. Theoretical estimates of the binding energy of this state are compared with estimates obtained by measuring the incident beam energy threshold for secondary electron emission and the temperature dependence positronium(Ps) emission. The experiments provide strong evidence for the existence of a positron bound state at the surface of Bi2Te2Se and indicate the practicality of using positron annihilation to selectively probe the critically important top most layer of topological insulator system.

  9. 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

    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.

  10. Automation of variable low energy positron beam experiments

    NASA Astrophysics Data System (ADS)

    Jayapandian, J.; Kumaresan, M.; Amarendra, G.; Venugopal Rao, G.; Purniah, B.; Viswanathan, B.

    2000-04-01

    By exploiting the special BIOS interrupt (INT 1CH) of PC in conjunction with a compatible high-voltage controller card and menu-driven control program, we report here the automation of variable low-energy positron beam experiments. The beam experiment consists of monitoring the Doppler broadening lineshape parameters corresponding to the annihilation 511 keV ?-ray at various positron beam implantation energies. The variation and monitoring of the sample high voltage, which determines positron beam energy, is carried out using a controller add-on card coupled to a 0-30 kV high-voltage unit. The design features of this controller card are discussed. This controller card is housed in a PC, which also houses a multichannel analyser (MCA) card. The MCA stores the Doppler energy spectrum of the annihilation ?-ray. The interactive control program, written in Turbo C, carries out the assigned tasks. The design features of the automation and results are presented.

  11. Evidence Of Dark Matter Annihilations In The WMAP Haze

    E-print Network

    Dan Hooper; Douglas P. Finkbeiner; Gregory Dobler

    2007-05-24

    The WMAP experiment has revealed an excess of microwave emission from the region around the center of our Galaxy. It has been suggested that this signal, known as the ``WMAP Haze'', could be synchrotron emission from relativistic electrons and positrons generated in dark matter annihilations. In this letter, we revisit this possibility. We find that the angular distribution of the WMAP Haze matches the prediction for dark matter annihilations with a cusped density profile, $\\rho(r) \\propto r^{-1.2}$ in the inner kiloparsecs. Comparing the intensity in different WMAP frequency bands, we find that a wide range of possible WIMP annihilation modes are consistent with the spectrum of the haze for a WIMP with a mass in the 100 GeV to multi-TeV range. Most interestingly, we find that to generate the observed intensity of the haze, the dark matter annihilation cross section is required to be approximately equal to the value needed for a thermal relic, $\\sigma v \\sim 3 \\times 10^{-26}$ cm$^3$/s. No boost factors are required. If dark matter annihilations are in fact responsible for the WMAP Haze, and the slope of the halo profile continues into the inner Galaxy, GLAST is expected to detect gamma rays from the dark matter annihilations in the Galactic Center if the WIMP mass is less than several hundred GeV.

  12. Neutrinos from WIMP annihilations

    E-print Network

    Mattias Blennow

    2007-10-08

    We make an improved analysis on the flow of neutrinos originating from WIMP annihilations inside the Sun and the Earth. We treat both neutrino interaction and oscillation effects in a consistent framework. Our numerical simulations are performed in an event based setting, which is useful for both theoretical studies and for creating neutrino telescope Monte Carlos. We find that the flow of muon-type neutrinos is enhanced or suppressed depending on the dominant WIMP annihilation channel.

  13. Electroweak fragmentation functions for dark matter annihilation

    E-print Network

    Leila Ali Cavasonza; Michael Krämer; Mathieu Pellen

    2014-10-02

    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.

  14. Search for WIMP Annihilation with the AMS-01 Electron Spectrum

    NASA Astrophysics Data System (ADS)

    Xiao, S.; Carosi, G.; Fisher, P.; Rybka, G.; Zhou, F.

    2008-12-01

    Dark matter neutralinos' self-annihilation can give rise to anomalous features in the spectrum of electrons, positrons, protons and antiprotons in cosmic rays. We search for such features in the electron spectrum at Earth with data taken by AMS precursor flight on the Space Shuttle in 1998. Limits are placed in the parameter space of certain supersymmetric models. Propagation uncertainty is specifically addressed by studying the boron-to-carbon ratio.

  15. Energy spectrum of neutrinos produced in e + , e ? pair annihilation

    Microsoft Academic Search

    A. Masani; G. Silvestro

    1964-01-01

    Summary  A calculation is made of the energy spectrum of neutrinos and antineutrinos produced in stars by electron-positron pair annihilation,\\u000a on the basis of Feynman and Gell-Mann’s theory of weak interactions, in the electron density range from 1.6·103 to 1.6·106 g\\/cm3 and temperature range from 5·108 to 1010 °K. It is seen that the maximum of the neutrino spectrum ranges from

  16. Fluxes and spectra of quasimonochromatic annihilation photons for studying E1 giant resonances in nuclei

    SciTech Connect

    Dzhilavyan, L. Z., E-mail: dzhil@cpc.inr.ac.ru [Russian Academy of Sciences, Institute for Nuclear Research (Russian Federation)

    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.

  17. Non-thermal X-rays from the Ophiuchus galaxy cluster and dark matter annihilation

    E-print Network

    Stefano Profumo

    2008-05-14

    We investigate a scenario where the recently discovered non-thermal hard X-ray emission from the Ophiuchus cluster originates from inverse Compton scattering of energetic electrons and positrons produced in weakly interacting dark matter pair annihilations. We show that this scenario can account for both the X-ray and the radio emission, provided the average magnetic field is of the order of 0.1 microGauss. We demonstrate that GLAST will conclusively test the dark matter annihilation hypothesis. Depending on the particle dark matter model, GLAST might even detect the monochromatic line produced by dark matter pair annihilation into two photons.

  18. Study of near surface defects in He-implanted stainless steels by monoenergetic positron beam

    NASA Astrophysics Data System (ADS)

    Uedono, A.; Tanigawa, S.; Sakairi, H.

    1990-10-01

    Variable-energy (0-30 keV) positron beam studies have been carried out on 70 keV He-implanted fusion reactor candidate stainless steels, JPCA (Japanese Prime Candidate Alloy). Doppler broadening profiles of the positron annihilation as a function of incident positron energy were shown to be quite sensitive to defects introduced by He-implantation. A fitting model neglecting positron diffusion into the defects was used to analyze parameterized momentum data. The variation of the defect profile as a function of isochronal annealing temperature showed a clear correlation with the release rate of He measured by a quadrupole mass spectrometer.

  19. Annihilation radiation from a hot e/+/-e/-/ plasma. [gamma ray bursts

    NASA Technical Reports Server (NTRS)

    Ramaty, R.; Meszaros, P.

    1981-01-01

    The radiation from electron-positron annihilations in a plasma of temperature above 10 to the 8th K is investigated as a possible source of the emission line at energies between 400 and 460 keV frequently seen in gamma-ray bursts. The annihilation rate and luminosity of an optically thin electron-positron plasma and the energy distribution of the resulting annihilation radiation are calculated by the use of a Monte Carlo technique as functions of temperature. Results indicate the annihilation spectrum to be peaked at an energy of 0.511 MeV plus a temperature-dependent blueshift, and the annihilation line to be significantly temperature-broadened. The widths of the observed burst emission lines set an upper limit of 3 x 10 to the 8th K on the temperature of any pair annihilation region in burst sources, which is considerably lower than the typical kinetic temperatures of the radiating particles. It is thus inferred that either the annihilation region is nonthermal, or spatially distinct from the burst site.

  20. Electron emission from surfaces resulting from low energy positron bombardment

    NASA Astrophysics Data System (ADS)

    Mukherjee, Saurabh

    Measurements of the secondary electron energy spectra resulting from very low energy positron bombardment of a polycrystalline Au and Cu (100) surfaces are presented that provide evidence for a single step transition from an unbound scattering state to an image potential bound state. The primary positron energy threshold for secondary electron emission and energy cutoff of the positron induced secondary electron energy peak are consistent with an Auger like process in which an incident positron make a transition from a scattering state to a surface-image potential bound while transferring all of the energy difference to an outgoing secondary electron. We term this process: the Auger mediated quantum sticking effect (AQSE). The intensities of the positron induced secondary electron peak are used to estimate the probability of this process as a function of incident positron energy. Positron annihilation induced Auger spectra (PAES) of Cu and Au are presented that are free of all primary beam induced secondary electron background. This background was eliminated by setting the positron beam energy below AQSE threshold. The background free PAES spectra obtained include the first measurements of the low energy tail of CVV Auger transitions all the way down to zero kinetic energy. The integrated intensity of this tail is several times larger than Auger peak itself which provides strong evidence for multi-electron Auger processes.

  1. Studies of defects in the near-surface region and at interfaces using low energy positron beams

    SciTech Connect

    Asoka-Kumar, P. [Brookhaven National Lab., Upton, NY (United States). Dept. of Physics

    1995-11-01

    Positron Annihilation Spectroscopy (PAS) is a powerful probe to study open-volume defects in solids. Its success is due to the propensity of positrons to seek out low-density regions of a solid, such as vacancies and voids, and the emissions of gamma rays from their annihilations that carry information about the local electronic environment. The development of low-energy positron beams allows probing of defects to depths of few microns, and can successfully characterize defects in the near-surface and interface regions of several technologically important systems. This review focuses on recent studies conducted on semiconductor-based systems.

  2. Power Corrections in Electron-Positron Annihilation: Experimental Review

    E-print Network

    Stefan Kluth

    2006-06-20

    Experimental studies of power corrections with e+e- data are reviewed. An overview of the available data for jet and event shape observables is given and recent analyses based on the Dokshitzer-Marchesini-Webber (DMW) model of power corrections are summarised. The studies involve both distributions of the observables and their mean values. The agreement between perturbative QCD combined with DMW power corrections and the data is generally good, and the few exceptions are discussed. The use of low energy data sets highlights deficiencies in the existing calculations for some observables. A study of the finiteness of the physical strong coupling at low energies using hadronic $\\tau$ decays is shown.

  3. 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

  4. Cosmic Positron Signature from Dark Matter in the Littlest Higgs Model with T-parity

    E-print Network

    Masaki Asano; Shigeki Matsumoto; Nobuchika Okada; Yasuhiro Okada

    2006-11-22

    We calculate the flux of cosmic positrons from the dark matter annihilation in the littlest Higgs model with T-parity. The dark matter annihilates mainly into weak gauge bosons in the halo, and high energy positrons are produced through leptonic and hadronic decays of the bosons. We investigate a possibility to detect the positron signal in upcoming experiments such as PAMELA and AMS-02. We found that the dark matter signal can be distinguished from the background in the PAMELA experiment when the dark matter mass is less than 120 GeV and the signal flux is enhanced due to a small scale clustering of dark matter. Furthermore, the signal from the dark matter annihilation can be detected in the AMS-02 experiment, even if such enhancement does not exist. We also discuss the invisible width of the Higgs boson in this model.

  5. The Isotropic Radio Background and Annihilating Dark Matter

    SciTech Connect

    Hooper, Dan [Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States); Belikov, Alexander V. [Institut d'Astrophysique (France); Jeltema, Tesla E. [Univ. of California, Santa Cruz, CA (United States); Linden, Tim [Univ. of California, Santa Cruz, CA (United States); Profumo, Stefano [Univ. of California, Santa Cruz, CA (United States); Slatyer, Tracy R. [Princeton Univ., Princeton, NJ (United States)

    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.

  6. Positron binding: A positron-density viewpoint

    SciTech Connect

    Baruah, T.; Zope, R.R.; Kshirsagar, A.; Pathak, R.K. (Department of Physics, University of Poona, Ganeshkhind, Pune 411007, Maharashtra (India))

    1994-09-01

    The binding of a single positron to a many-electron system (in the field of a fixed nuclear configuration) is viewed in terms of positron and electron densities. Following the treatments due to Levy and co-workers [Phys. Rev. A 32, 2010 (1985); 30, 2745 (1984)], certain general integral conditions and also an exact differential equation for the positron density are derived. It is demonstrated further that within a density-functional approach, decent estimates to the positron affinity for negative ions can be obtained by invoking the Slater transition state concept. Finally, the redistribution in the electron part brought about by transiently binding the positron to the negative ion is discussed at the density-matrix level.

  7. 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.

  8. Brane annihilations during inflation

    Microsoft Academic Search

    Diana Battefeld; Thorsten Battefeld; Hassan Firouzjahi; Nima Khosravi

    2010-01-01

    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

  9. Spin-polarization of an electro-static positron beam

    NASA Astrophysics Data System (ADS)

    Kawasuso, A.; Maekawa, M.

    2008-10-01

    We constructed an electro-static positron beam apparatus. We fabricated a simple spin-polarimeter composed of a permanent magnet with a surface magnetic field of 0.65 T and an iron pole piece. The longitudinal spin-polarization of the positron beam was determined to be 0.3 by analyzing the magnetic field dependence of the Doppler broadening of annihilation radiation from a fused silica specimen. The effect of spin rotation was examined using an iron poly-crystal and a simple E × B filter.

  10. Leptophilic Dark Matter and AMS-02 Cosmic-ray Positron Flux

    E-print Network

    Qing-Hong Cao; Chuan-Ren Chen; Ti Gong

    2014-09-25

    With the measurement of positron flux published recently by AMS-02 collaboration, we show how the leptophilic dark matter fits the observation. We obtain the percentages of different products of dark matter annihilation that can best describe the flux of high energy positrons observed by AMS. We show that dark matter annihilates predominantly into $\\tau\\tau$ pair, while both $ee$ and $\\mu\\mu$ final states should be less than $20\\%$. When gauge boson final states are included, the best branching ratio of needed $\\tau\\tau$ mode reduces.

  11. Measurement of positron spin polarization by using the Doppler broadening method

    NASA Astrophysics Data System (ADS)

    Nagai, Yasuyoshi; Nagashima, Yasuyuki; Kim, Jaehong; Itoh, Yoshiko; Hyodo, Toshio

    2000-08-01

    A new positron spin polarimeter based on the measurement of the Doppler broadening of annihilation radiation is described. The polarization is determined from the magnetic field dependence of the S-parameter of the Doppler broadening spectra for amorphous SiO 2. In order to examine the sensitivity of this polarimeter, the spin polarization of the positrons from a 22Na source is measured and compared with the value theoretically expected. The result is also compared with the polarization measured by using the angular correlation of annihilation radiation method.

  12. Annihilation, bound state properties and photodetachment of the positronium negatively charged ion

    NASA Astrophysics Data System (ADS)

    Frolov, Alexei M.

    2015-04-01

    Bound state properties of the negatively charged Ps- ion (or e-e+e-) are discussed. The expectation values of operators which correspond to these properties have been determined with the use of the highly accurate wave functions constructed for this ion. Our best variational energy obtained for the Ps- ion is E = -0.2620050 7023298 0107770 40051 a.u. Annihilation of the electron-positron pair(s) in the negatively charged Ps- ion (or e-e+e-) is considered in detail. By using accurate values for a number annihilation rates ?n?, where n = 1, 2, 3, 4 and 5, we evaluated the half-life ?a of the Ps- ion against positron annihilation (?a = 1/? ? 4.793584140 × 10-10 s). Photodetachment of the Ps- ion is considered in the long-range, asymptotic approximation. The overall accuracy of our photodetachment cross-section of the Ps- ion is very good for such a simple approximation.

  13. High energy positrons and the WMAP haze from exciting dark matter

    SciTech Connect

    Cholis, Ilias; Goodenough, Lisa; Weiner, Neal [Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, New York 10003 (United States)

    2009-06-15

    We consider the signals of positrons and electrons from 'exciting' dark matter annihilation. Because of the light (m{sub {phi}} < or approx. 1 GeV) force carrier {phi} into which the dark matter states can annihilate, the electrons and positrons are generally very boosted, yielding a hard spectrum, in addition to the low energy positrons needed for INTEGRAL observations of the Galactic center. We consider the relevance of this scenario for HEAT, PAMELA, and the WMAP 'haze', focusing on light (m{sub {phi}} < or approx. 2m{sub {pi}}) {phi} bosons, and find that significant signals can be found for all three, although significant signals generally require high dark matter densities. We find that measurements of the positron fraction are generally insensitive to the halo model, but do suffer significant astrophysical uncertainties. We discuss the implications for upcoming PAMELA results.

  14. High energy positrons and the WMAP haze from exciting dark matter

    NASA Astrophysics Data System (ADS)

    Cholis, Ilias; Goodenough, Lisa; Weiner, Neal

    2009-06-01

    We consider the signals of positrons and electrons from “exciting” dark matter annihilation. Because of the light (m??1GeV) force carrier ? into which the dark matter states can annihilate, the electrons and positrons are generally very boosted, yielding a hard spectrum, in addition to the low energy positrons needed for INTEGRAL observations of the Galactic center. We consider the relevance of this scenario for HEAT, PAMELA, and the WMAP “haze,” focusing on light (m??2m?) ? bosons, and find that significant signals can be found for all three, although significant signals generally require high dark matter densities. We find that measurements of the positron fraction are generally insensitive to the halo model, but do suffer significant astrophysical uncertainties. We discuss the implications for upcoming PAMELA results.

  15. Remarks on calculation of positron flux from galactic dark matter

    SciTech Connect

    Perelstein, Maxim; Shakya, Bibhushan [Newman Laboratory of Elementary Particle Physics, Cornell University, Ithaca, New York 14853 (United States)

    2010-08-15

    Energetic positrons produced in annihilation or decay of dark matter particles in the Milky Way can serve as an important indirect signature of dark matter. Computing the positron flux expected in a given dark matter model involves solving transport equations, which account for interaction of positrons with matter and galactic magnetic fields. Existing calculations solve the equations inside the diffusion zone, where galactic magnetic fields confine positrons, and assume vanishing positron density on the boundaries of this zone. However, in many models, a substantial fraction of the dark matter halo lies outside the diffusion zone. Positrons produced there can then enter the diffusion zone and get trapped, potentially reaching the Earth and increasing the expected flux. We calculate this enhancement for a variety of models. We also evaluate the expected enhancement of the flux of energetic photons produced by the inverse Compton scattering of the extra positrons on starlight and cosmic microwave background. We find maximal flux enhancements of order 20% in both cases.

  16. Microstructural characterization of thin polymer films using Langley low energy positron flux generator

    NASA Technical Reports Server (NTRS)

    Singh, Jag. J.

    1992-01-01

    We have developed a highly efficient scheme for generating high fluxes of slow positrons. These positrons have been successfully used to measure lifetimes in thin test films. The lifetime data have been used to develop two structure-property models for the test films. The first model relates the free volume cell size to the molecular weight of the polymer repeat unit. The second model relates the free volume fraction to the dielectric constant of the polymer film.

  17. Exciton annihilation studies in poly(p-phenylene vinylene)

    SciTech Connect

    Valencia, V.S.; Kepler, R.G.; Jacobs, S.J.; Beeson, P.M. [Sandia National Labs., Albuquerque, NM (United States); Allemond, P.M. [Donnelly Corp., Tucson, AZ (United States)

    1995-08-01

    To verify the excitonic nature of the light-emitting state in PPV, fluorescence intensities and decay lifetimes were investigated as a function of excitation intensity. The results agree with the behavior predicted by the molecular exciton model. In particular, exciton-exciton annihilation causes the fluorescence intensity to saturate and the fluorescence lifetime to shorten at high exciton densities. In addition, the exciton annihilation, and thus diffusion, coefficients are found to be relatively large, even at low temperatures, indicating that exciton migration is important in PPV. These results indicate that the fluorescent (photoluminescent) state in PPV is excitonic in nature. The results argue against the band model where high mobility at reduced temperatures is not expected because the light-emitting species, neutral bipolarons, are associated with large lattice distortions.

  18. Optimization of drift bias in an UHV based pulsed positron beam system

    SciTech Connect

    Anto, C. Varghese; Rajaraman, R.; Rao, G. Venugopal; Abhaya, S.; Parimala, J.; Amarendra, G. [Materials Physics Division, Material Science Group, I GCAR, Kalpakkam - 603102, T.N (India); Materials Physics Division, Material Science Group, I GCAR, Kalpakkam - 603102, T.N. (India)

    2012-06-05

    We report here the design of ultra high vacuum (UHV) compatible pulsed positron beam lifetime system, which combines the principles of a conventional slow positron beam and RF based pulsing scheme. The mechanical design and construction of the UHV system to house the beam has been completed and it has been tested for a vacuum of {approx} 10{sup -10} mbar. The voltages applied to the drift tube as a function of positron energies have been optimized using SIMION.

  19. Positron trapping kinetics in thermally generated vacancy donor complexes in highly As-doped silicon

    Microsoft Academic Search

    K. Kuitunen; K. Saarinen; F. Tuomisto

    2007-01-01

    We have measured positron lifetime and Doppler broadening in highly As-doped silicon containing thermally generated V-As3 defect complexes (vacancy is surrounded by three arsenic atoms). We observe positron detrapping from the V-As3 defect complex and determine the binding energy of 0.27eV of a positron to the complex. The results explain why 85% of the thermal vacancies formed in highly As-doped

  20. New constraints from PAMELA anti-proton data on annihilating and decaying dark matter

    SciTech Connect

    Cholis, Ilias, E-mail: ilias.cholis@sissa.it [International School for Advanced Studies (SISSA), via Bonomea 265, 34136 Trieste (Italy)

    2011-09-01

    Recently the PAMELA experiment has released its updated anti-proton flux and anti-proton to proton flux ratio data up to energies of ? 200GeV. With no clear excess of cosmic ray anti-protons at high energies, one can extend constraints on the production of anti-protons from dark matter. In this letter, we consider both the cases of dark matter annihilating and decaying into standard model particles that produce significant numbers of anti-protons. We provide two sets of constraints on the annihilation cross-sections/decay lifetimes. In the one set of constraints we ignore any source of anti-protons other than dark matter, which give the highest allowed cross-sections/inverse lifetimes. In the other set we include also anti-protons produced in collisions of cosmic rays with interstellar medium nuclei, getting tighter but more realistic constraints on the annihilation cross-sections/decay lifetimes.

  1. New Constraints from PAMELA anti-proton data on Annihilating and Decaying Dark Matter

    E-print Network

    Cholis, Ilias

    2010-01-01

    Recently the PAMELA experiment has released its updated anti-proton flux and anti-proton to proton flux ratio data up to energies of ~200GeV. With no clear excess of cosmic ray anti-protons at high energies, one can extend constraints on the production of anti-protons from dark matter. In this letter, we consider both the cases of dark matter annihilating and decaying into standard model particles that produce significant numbers of anti-protons. We provide two sets of constraints on the annihilation cross-sections/decay lifetimes. In the one set of constraints we ignore any source of anti-protons other than dark matter, which give the highest allowed cross-sections/inverse lifetimes. In the other set we include also anti-protons produced in collisions of cosmic rays with interstellar medium nuclei, getting tighter but more realistic constraints on the annihilation cross-sections/decay lifetimes.

  2. New Constraints from PAMELA anti-proton data on Annihilating and Decaying Dark Matter

    E-print Network

    Ilias Cholis

    2011-09-27

    Recently the PAMELA experiment has released its updated anti-proton flux and anti-proton to proton flux ratio data up to energies of ~200GeV. With no clear excess of cosmic ray anti-protons at high energies, one can extend constraints on the production of anti-protons from dark matter. In this letter, we consider both the cases of dark matter annihilating and decaying into standard model particles that produce significant numbers of anti-protons. We provide two sets of constraints on the annihilation cross-sections/decay lifetimes. In the one set of constraints we ignore any source of anti-protons other than dark matter, which give the highest allowed cross-sections/inverse lifetimes. In the other set we include also anti-protons produced in collisions of cosmic rays with interstellar medium nuclei, getting tighter but more realistic constraints on the annihilation cross-sections/decay lifetimes.

  3. Elastic positron-cadmium scattering at low energies

    NASA Astrophysics Data System (ADS)

    Bromley, M. W. J.; Mitroy, J.

    2010-05-01

    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+-Cd system. The s-wave binding energy is estimated to be 126±42 meV, with a scattering length of Ascat=(14.2±2.1)a0, while the threshold annihilation parameter, Zeff, was 93.9±26.5. The p-wave phase shift exhibits a weak shape resonance that results in a peak Zeff of 91±17 at a collision energy of about 490±50 meV.

  4. Status and Perspectives for a Slow Positron Beam Facility at the HH-NIPNE Bucharest

    Microsoft Academic Search

    Mihai Straticiuc; Liviu Stefan Craciun; Olimpiu Constantinescu; Ionica Alina Ghita; Cristina Ionescu; Petru Mihai Racolta; Angela Vasilescu; Viorel Braic; Catalin Zoita; Adrian Kiss; Dionezie Bojin

    2009-01-01

    The development of a positron annihilation spectroscopy laboratory at the HH-NIPNE Bucharest-to be used for material studies and applications was started in the last 10 years. In the framework of a national research project extended over the last 3 years, was designed a low energy positron accelerator, as a high-vacuum dedicated beam line with two options: a 25 mCi 22NaCl

  5. Positron and transmission electron microscopy study of precipitation phenomena in an Al-Li-Zr alloy

    Microsoft Academic Search

    M. D. Aycirex; N. de Diego; A. Dupasquier; J. Del Río; A. Somoza

    1996-01-01

    Positron annihilation spectroscopy has been used to follow the precipitation processes in an aluminium alloy containing 12.5at.% Li and 0.04at.% Zr. Complementary techniques such as optical and electron microscopy and micro-hardness have also been used to characterize the samples. From the data it is confirmed that positrons are trapped at the ?? precipitates and is inferred that they are extremely

  6. 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.

  7. The scattering of low energy positrons by helium

    NASA Technical Reports Server (NTRS)

    Humberston, J. W.

    1973-01-01

    Kohn's variational method is used to calculate the positron-helium scattering length and low energy S-wave phase shifts for a quite realistic Hylleraas type of helium function containing an electron-electron correlation term. The zero energy wavefunction is used to calculate the value of the annihilation rate parameter Z sub eff. All the results are significantly different from those for Drachman's helium model B, but are in better agreement with the available experimental data.

  8. Single and couple doping ZnO nanocrystals characterized by positron techniques

    NASA Astrophysics Data System (ADS)

    Pasang, Tenzin; Namratha, Keerthiraj; Guagliardo, Paul; Byrappa, Kullaiah; Ranganathaiah, Chikkakuntappa; Samarin, S.; Williams, J. F.

    2015-04-01

    Zinc oxide (ZnO) nanocrystals have been synthesized using a mild hydrothermal process using low temperatures and pressures with the advantages of free growth catalyst, low cost and alternative technology. Positron annihilation lifetime spectroscopy and coincidence Doppler broadening (CDB) spectroscopic methods have been used to investigate the roles of single- and co-dopants and native defects of the ZnO nanocrystals controlled by the synthesis process. It is shown that single Ag1+ and Pd2+ dopants occupy interstitial sites of the ZnO lattice and single Ru3+ doping replaces Zn vacancies substitutionally with a significant effect on the CDB momentum ratio curves when compared using ZnO as the reference spectrum. The co-doping of the ZnO lattice with (Sn4+ + Co2+) shows similar CDB ratios as Ru3+ single-doping. Also co-doping with (Ag1+ + Pd2+) or (Ag1+ + W6+) shows significant decreases in the band gap energy up to about 12.6% compared to single doping. The momentum ratio curves, referenced to undoped ZnO, indicate dopants in interstitial and substitutional sites. The presence of transition metal ions interstitially will trap electrons which resist the recombination of electrons and in turn affect the conductivity of the material.

  9. Positron-rubidium scattering

    NASA Technical Reports Server (NTRS)

    Mceachran, R. P.; Horbatsch, M.; Stauffer, A. D.

    1990-01-01

    A 5-state close-coupling calculation (5s-5p-4d-6s-6p) was carried out for positron-Rb scattering in the energy range 3.7 to 28.0 eV. In contrast to the results of similar close-coupling calculations for positron-Na and positron-K scattering the (effective) total integrated cross section has an energy dependence which is contrary to recent experimental measurements.

  10. Scintillators for positron emission tomography

    SciTech Connect

    Moses, W.W.; Derenzo, S.E.

    1995-09-01

    Like most applications that utilize scintillators for gamma detection, Positron Emission Tomography (PET) desires materials with high light output, short decay time, and excellent stopping power that are also inexpensive, mechanically rugged, and chemically inert. Realizing that this ``ultimate`` scintillator may not exist, this paper evaluates the relative importance of these qualities and describes their impact on the imaging performance of PET. The most important PET scintillator quality is the ability to absorb 511 keV photons in a small volume, which affects the spatial resolution of the camera. The dominant factor is a short attenuation length ({le} 1.5 cm is required), although a high photoelectric fraction is also important (> 30% is desired). The next most important quality is a short decay time, which affects both the dead time and the coincidence timing resolution. Detection rates for single 511 keV photons can be extremely high, so decay times {le} 500 ns are essential to avoid dead time losses. In addition, positron annihilations are identified by time coincidence so {le}5 ns fwhm coincidence pair timing resolution is required to identify events with narrow coincidence windows, reducing contamination due to accidental coincidences. Current trends in PET cameras are toward septaless, ``fully-3D`` cameras, which have significantly higher count rates than conventional 2-D cameras and so place higher demands on scintillator decay time. Light output affects energy resolution, and thus the ability of the camera to identify and reject events where the initial 511 keV photon has undergone Compton scatter in the patient. The scatter to true event fraction is much higher in fully-3D cameras than in 2-D cameras, so future PET cameras would benefit from scintillators with a 511 keV energy resolution < 10--12% fwhm.

  11. Bepcii Positron Source

    NASA Astrophysics Data System (ADS)

    Pei, Guoxi; Sun, Yaolin; Liu, Jintong; Chi, Yunlong; Liu, Yuncheng; Liu, Nianzong

    2013-10-01

    BEPCII- an upgrade project of the Beijing Electron Positron Collider (BEPC) is a factory type of e+e- collider. The fundamental requirements for its injector 1inac are the beam energy of 1.89GeV for on-energy injection and a 40mA positron beam current at the linac end with a low beam emittance of 1.6?m and a low energy spread of ±0.5% so as to guarantee a higher injection rate (?50mA/min) to the storage ring. Since the positron flux is proportional to the primary electron beam power on the target, we will increase the electron gun current from 4A to 10A by using a new electmn gun system and increase the primary electron energy from 120MeV to 240MeV. The positron souree itself is an extremely important system for producing more positrons, including a positron converter target chamber, a 12kA flux modulator, the 7-m focusing module with DC power supplies and the support. The new positron production linac from the electron gun to the positron source has been installed into the tunnel. In what follows, we will emphasize the positron source design, manufacture and tests.

  12. Nuclear medicine and positron emission tomography: An overview

    SciTech Connect

    McCarthy, T.J.; Schwarz, S.W.; Welch, M.J. (Washington Univ. Medical School, St. Louis, MO (United States). Mallinckrodt Inst. of Radiology)

    1994-10-01

    Nuclear medicine is the field of medical practice that involves the oral or intravenous administration of radioactive materials for use in diagnosis and therapy. The majority of radiopharmaceutical available are used for diagnostic purposes. These involve the determination of organ function, shape, or position from an image of the radioactivity distribution within an organ or at a location within the body. After administration, the radiopharmaceutical localizes within an organ or target tissue due to its biological or physiologic characteristics. This diagnostic capability is usually the result of the emission of gamma radiation from the radiopharmaceutical localized within an organ. This allows for external detection and imaging using a special type of camera known as a gamma camera. When a positron-emitting radionuclide decays, a positron (positive electron) is emitted from the nucleus. The positron then annihilates with an electron, resulting in the release of energy in the form of two 511-KeV [gamma]-rays at 180[degree] to one another. The energy of these photons is sufficient to pass through tissue. Thus, placing a series of detectors around the patient allows technicians to monitor the emission of both of the photons that result from a single positron annihilation. this ultimately allows an accurate quantification of the distribution of radioactivity in the body not possible when only a single [gamma]-ray is emitted.

  13. Electron emission from surfaces resulting from low energy positron bombardment

    NASA Astrophysics Data System (ADS)

    Mukherjee, S.; Shastry, K.; Weiss, A. H.

    2009-03-01

    Measurements of the energy distribution of electrons resulting from very low energy positron bombardment of a polycrystalline Au and Cu(100) surfaces provide evidence for a single step transition from an unbound scattering state to an image potential bound state. The primary positron energy threshold for secondary electron emission and cutoff in the secondary electron energy spectra are consistent with a process in which an incident positrons make a transition from a scattering state to a surface-image potential bound while transferring all of the energy difference to an outgoing secondary electron. Estimates of the probability of this process as a function of incident positron energy are also presented. Background free Auger spectra of the MVV transition in Cu and the OVV transition in Au were obtained by setting the incident positron beam energy below the secondary electron emission threshold. Auger electron emission resulted from the annihilation of surface state positrons with core electrons. The low energy tail associated with the low energy CVV Auger transitions in Cu and Au were found to have integrated intensity several times larger than Auger peak providing strong evidence for multi-electron Auger processes.

  14. PAMELA Positron Excess as a Signal from the Hidden Sector

    E-print Network

    Daniel Feldman; Zuowei Liu; Pran Nath

    2009-03-14

    The recent positron excess observed in the PAMELA satellite experiment strengthens previous experimental findings. We give here an analysis of this excess in the framework of the Stueckelberg extension of the standard model which includes an extra $U(1)_X$ gauge field and matter in the hidden sector. Such matter can produce the right amount of dark matter consistent with the WMAP constraints. Assuming the hidden sector matter to be Dirac fermions it is shown that their annihilation can produce the positron excess with the right positron energy dependence seen in the HEAT, AMS and the PAMELA experiments. Further test of the proposed model can come at the Large Hadron Collider. The predictions of the $\\bar p/p$ flux ratio also fit the data.

  15. PAMELA positron excess as a signal from the hidden sector

    NASA Astrophysics Data System (ADS)

    Feldman, Daniel; Liu, Zuowei; Nath, Pran

    2009-03-01

    The recent positron excess observed in the PAMELA satellite experiment strengthens previous experimental findings. We give here an analysis of this excess in the framework of the Stueckelberg extension of the standard model which includes an extra U(1)X gauge field and matter in the hidden sector. Such matter can produce the right amount of dark matter consistent with the Wilkinson Microwave Anisotropy Probe (WMAP) constraints. Assuming the hidden sector matter to be Dirac fermions it is shown that their annihilation can produce the positron excess with the right positron energy dependence seen in the HEAT, AMS, and the PAMELA experiments. The predictions of the p¯/p flux ratio also fit the data.

  16. 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.

  17. List-mode maximum-likelihood reconstruction applied to positron emission mammography (PEM) with irregular sampling

    Microsoft Academic Search

    Ronald H. Huesman; Gregory J. Klein; William W. Moses; Jinyi Qi; Bryan W. Reutter; Patrick R. G. Virador

    2000-01-01

    Presents a preliminary study of list-mode likelihood reconstruction of images for a rectangular positron emission tomograph (PET) specifically designed to image the human breast. The prospective device consists of small arrays of scintillation crystals for which depth of interaction is estimated. Except in very rare instances, the number of annihilation events detected is expected to be far less than the

  18. Fluorescence lifetime measurement from a designated single-bunch in the BEPC II colliding mode

    Microsoft Academic Search

    Shuai-Shuai Sun; Guang-Lei Xu; Ge Lei; Yan Huang; Zhen-Hua Gao; Zhi-Yin Zhang; Ye Tao

    2011-01-01

    Fluorescence lifetime measurement in the time domain requires excitation from a well separated single bunch using synchrotron light sources. In the colliding mode of the Beijing Electron Positron Collider II (BEPCII), a hybrid filling pattern was realized such that a single bunch was placed in the middle of a large gap between two multi-bunch groups. Detection of fluorescence lifetime, based

  19. Intense low energy positron beams

    SciTech Connect

    Lynn, K.G.; Jacobsen, F.M.

    1993-12-31

    Intense positron beams are under development or being considered at several laboratories. Already today a few accelerator based high intensity, low brightness e{sup +} beams exist producing of the order of 10{sup 8} {minus} 10{sup 9} e{sup +}/sec. Several laboratories are aiming at high intensity, high brightness e{sup +} beams with intensities greater than 10{sup 9} e{sup +}/sec and current densities of the order of 10{sup 13} {minus} 10{sup 14} e{sup +} sec{sup {minus}} {sup 1}cm{sup {minus}2}. Intense e{sup +} beams can be realized in two ways (or in a combination thereof) either through a development of more efficient B{sup +} moderators or by increasing the available activity of B{sup +} particles. In this review we shall mainly concentrate on the latter approach. In atomic physics the main trust for these developments is to be able to measure differential and high energy cross-sections in e{sup +} collisions with atoms and molecules. Within solid state physics high intensity, high brightness e{sup +} beams are in demand in areas such as the re-emission e{sup +} microscope, two dimensional angular correlation of annihilation radiation, low energy e{sup +} diffraction and other fields. Intense e{sup +} beams are also important for the development of positronium beams, as well as exotic experiments such as Bose condensation and Ps liquid studies.

  20. Positron trapping kinetics in thermally generated vacancy donor complexes in highly As-doped silicon

    NASA Astrophysics Data System (ADS)

    Kuitunen, K.; Saarinen, K.; Tuomisto, F.

    2007-01-01

    We have measured positron lifetime and Doppler broadening in highly As-doped silicon containing thermally generated V-As3 defect complexes (vacancy is surrounded by three arsenic atoms). We observe positron detrapping from the V-As3 defect complex and determine the binding energy of 0.27eV of a positron to the complex. The results explain why 85% of the thermal vacancies formed in highly As-doped Si at temperatures over 700K are invisible to positron measurements at elevated temperatures.

  1. Positrons for linear colliders

    SciTech Connect

    Ecklund, S.

    1987-11-01

    The requirements of a positron source for a linear collider are briefly reviewed, followed by methods of positron production and production of photons by electromagnetic cascade showers. Cross sections for the electromagnetic cascade shower processes of positron-electron pair production and Compton scattering are compared. A program used for Monte Carlo analysis of electromagnetic cascades is briefly discussed, and positron distributions obtained from several runs of the program are discussed. Photons from synchrotron radiation and from channeling are also mentioned briefly, as well as positron collection, transverse focusing techniques, and longitudinal capture. Computer ray tracing is then briefly discussed, followed by space-charge effects and thermal heating and stress due to showers. (LEW)

  2. Characterization of the surface layer of LB-films using a slow positron beam

    NASA Astrophysics Data System (ADS)

    Koshimizu, M.; Asai, K.; Ishigure, K.; Iwai, T.; Shibata, H.; Ito, Y.

    2000-06-01

    Langmuir-Blodgett films were studied using a variable energy slow-positron beam. We measured the energy spectra of positron annihilation radiation for Cd and Mg eicosanoid films and obtained the V- and S-parameters as a function of the incident positron energy, E. In the V-E curves of Cd eicosanoid films, there were dips at the positron energy whose mean implantation depth corresponding to the first and second Cd 2+ layers from the surface. These dips are interpreted as the result of inhibition of Ps formation by the Cd 2+ ions. The S-parameter was found to be sensitive to chemical composition of the film and also to possible structural change due to heat treatment. Our results suggest that positron beams provide valuable information about the microstructure of the Langmuir-Blodgett films.

  3. Determination of interfacial states in solid heterostructures using a variable-energy positron beam

    DOEpatents

    Asoka kumar, Palakkal P. V. (Coram, NY); Lynn, Kelvin G. (Center Moriches, NY)

    1993-01-01

    A method and means is provided for characterizing interfacial electron states in solid heterostructures using a variable energy positron beam to probe the solid heterostructure. The method includes the steps of directing a positron beam having a selected energy level at a point on the solid heterostructure so that the positron beam penetrates into the solid heterostructure and causes positrons to collide with the electrons at an interface of the solid heterostructure. The number and energy of gamma rays emitted from the solid heterostructure as a result of the annihilation of positrons with electrons at the interface are detected. The data is quantified as a function of the Doppler broadening of the photopeak about the 511 keV line created by the annihilation of the positrons and electrons at the interface, preferably, as an S-parameter function; and a normalized S-parameter function of the data is obtained. The function of data obtained is compared with a corresponding function of the Doppler broadening of the annihilation photopeak about 511 keV for a positron beam having a second energy level directed at the same material making up a portion of the solid heterostructure. The comparison of these functions facilitates characterization of the interfacial states of electrons in the solid heterostructure at points corresponding to the penetration of positrons having the particular energy levels into the interface of the solid heterostructure. Accordingly, the invention provides a variable-energy non-destructive probe of solid heterostructures, such as SiO.sub.2 /Si, MOS or other semiconductor devices.

  4. Fermi surface and momentum space densities of ferromagnetic Gd using the positron technique

    SciTech Connect

    Hoffman, K.R.

    1984-01-01

    Using the angular correlation of annihilation radiation (ACAR) technique, the positron-sampled momentum density and momentum space spin density of the electrons in gadolinium were investigated. The positron tends to annihilate with electrons whose spin is anti-parallel to its own. For a ferromagnetic sample, that implies that one can preferentially sample the majority and minority electrons by obtaining distributions with the saturating magnetic field parallel and antiparallel to the positron's polarization. Using the theory developed by Berko and Mills, the momentum density and momentum space spin density were obtained from the measured distributions. With the point slit configuration, two dimensional projections of the densities were obtained. Each two dimensional projection corresponds to an integral through the three dimensional density. The apparatus was described previously by Berko, Haghgooie, and Mader, and employs discrete NaI scintillators. The reconstruction of the three dimensional momentum density and momentum space spin density provides the most Fermi surface information.

  5. On the use of positrons to probe magnetic versus electrostatic turbulence

    SciTech Connect

    Stambaugh, R.D.

    1990-10-01

    Kwon, et al. have shown that runaway electron (positron) diffusion is produced by magnetic turbulence and unaffected by electrostatic turbulence. By measuring the diffusion coefficient of positrons at runaway energies (0.1-2 MeV) as a function of radius for two discrete positron energies, the radial correlation length W of the turbulence can be extracted. Then if the thermal electrons are in a weak turbulence regime, the thermal electron diffusion coefficient from magnetic fluctuations alone can be calculated and compared to values from other techniques. We propose to inject charged energetic positrons (100--2000 keV) in few msec bursts from radioactive sources by means of their curvature drift when trapped in toroidal field ripples. The energetic positrons will diffuse over 60--600 msec time scales. At any time the radial profile of the positrons can be sampled by injecting a small solid pellet. A fraction of all the positrons on a flux surface will annihilate in the pellet as it passes that flux surface. The time dependent 0.511 MeV {gamma}-ray signal then can be unfolded into the positron radial profile and the positron diffusion coefficient determined from the time evolution of those profiles. 8 refs.

  6. Brane annihilations during inflation

    SciTech Connect

    Battefeld, Diana; Battefeld, Thorsten [Department of Physics, Princeton University, Princeton, NJ 08544 (United States); Firouzjahi, Hassan; Khosravi, Nima, E-mail: dbattefe@princeton.edu, E-mail: tbattefe@princeton.edu, E-mail: firouz@ipm.ir, E-mail: nima@ipm.ir [School of Physics, Institute for Research in Fundamental Sciences (IPM), P.O. Box 19395-5531, Tehran (Iran, Islamic Republic of)

    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.

  7. Carrier lifetimes in silicon

    Microsoft Academic Search

    Dieter K. Schroder

    1997-01-01

    Carrier lifetimes in semiconductors are being rediscovered by the Si IC community, because the lifetime is a very effective parameter to characterize the purity of a material or device. It has become a process and equipment characterization parameter. The various recombination mechanisms are discussed and the concept of recombination and generation lifetime is presented. We show that surface recombination\\/generation plays

  8. Positron Collection in Linear Collider

    SciTech Connect

    Batygin, Y.K.; /SLAC

    2005-12-08

    In the Linear Collider design, the positron capture system includes a positron production target, followed by an adiabatic matching device (solenoid), a pre-linac with solenoidal focusing, and a linac with quadrupole focusing before injection into a positron damping ring. Two schemes for positron production have been studied: (1) a conventional approach with high-energy electron beam interacting with a high-Z target and (2) polarized positron production using polarized photons generated in a helical undulator by a primary collider electron beam which then interact with a positron production target. The capture efficiencies for both schemes are compared. Various parameters affecting the positron capture are analyzed.

  9. Positron diffusion in Si

    SciTech Connect

    Nielsen, B.; Lynn, K.G.; Vehanen, A.; Schultz, P.J.

    1985-06-01

    Positron diffusion in Si(100) and Si(111) has been studied using a variable energy positron beam. The positron diffusion coefficient is found to be D/sub +/ = 2.7 +- 0.3 cm/sup 2//sec using a Makhov-type positron implantation profile, which is demonstrated to fit the data more reliably than the more commonly applied exponential profile. The diffusion related parameter, E/sub 0/, which results from the exponential profile, is found to be 4.2 +- 0.2 keV, significantly longer than previously reported values. A drastic reduction in E/sub 0/ is found after annealing the sample at 1300 K, showing that previously reported low values of E/sub 0/ are probably associated with the thermal history of the sample.

  10. Positron-emission tomography

    SciTech Connect

    Ter-Pogossian, M.M.; Raichle, M.E.; Sobel, B.E.

    1980-10-01

    Positron-emission tomography (PET) combines early biochemical assessment of pathology achieved by nuclear medicine with the precise localization achieved by computerized image reconstruction. In this technique a chemical compound with the desired biological activity is labeled with a radioactive isotope that decays by emitting a positron, or positive electron. With suitable interpretation PET images can provide a noninvasive, regional assessment of many biochemical processes that are essential to the functioning of the organ that is being visualized.

  11. The source and distribution of Galactic positrons

    NASA Technical Reports Server (NTRS)

    Purcell, W. R.; Dixon, D. D.; Cheng, L.-X.; Leventhal, M.; Kinzer, R. L.; Kurfess, J. D.; Skibo, J. G.; Smith, D. M.; Tueller, J.

    1997-01-01

    The oriented scintillation spectrometer experiment (OSSE) observations of the Galactic plane and the Galactic center region were combined with observations acquired with other instruments in order to produce a map of the Galactic 511 keV annihilation radiation. Two mapping techniques were applied to the data: the maximum entropy method, and the basis pursuit inversion method. The resulting maps are qualitatively similar and show evidence for a central bulge and a weak galactic disk component. The weak disk is consistent with that expected from positrons produced by the decay of radioactive Al-26 in the interstellar medium. Both maps suggest an enhanced region of emission near l = -4 deg, b = 7 deg, with a flux of approximately 50 percent of that of the bulge. The existence of this emission appears significant, although the location is not well determined. The source of this enhanced emission is presently unknown.

  12. Positron trapping at divacancies in thin polycrystalline CdTe films deposited on glass

    NASA Astrophysics Data System (ADS)

    Liszkay, L.; Corbel, C.; Baroux, L.; Hautojärvi, P.; Bayhan, M.; Brinkman, A. W.; Tatarenko, S.

    1994-03-01

    We have performed positron annihilation experiments on CdTe films grown by vacuum evaporation at 220 °C on both plain glass and indium-tin-oxide-coated glass substrates. By checking the linearity of the valence annihilation parameter S versus the core annihilation parameter W we introduce a method to analyze the data which directly shows that the same vacancy defect can be present in all the films. By comparing the core annihilation parameter at the defect to that at the VCd vacancy we can identify this defect as the divacancy VCd-VTe. Its concentration in the films decreases from about 1018 to less than 1016 cm-3 after annealing in air at 400 °C for about 30 min. Chlorine doping seems to stabilize the divacancies.

  13. Eternal annihilations of light photinos

    NASA Astrophysics Data System (ADS)

    Kolb, Edward W.; Riotto, Antonio

    1996-09-01

    In a class of low-energy supersymmetry models the photino is a natural dark-matter candidate. We investigate the effects of post-freeze-out photino annihilations that generate electromagnetic cascades and lead to photodestruction of 4He and subsequent overproduction of D and 3He. We also generalize our analysis of electromagnetic showers to include those from a generic dark-matter component whose relic abundance is not determined by the self-annihilation cross section.

  14. Compton-backscattered annihilation radiation from the Galactic Center region

    NASA Astrophysics Data System (ADS)

    Smith, D. M.; Lin, R. P.; Feffer, P.; Slassi, S.; Hurley, K.; Matteson, J.; Bowman, H. B.; Pelling, R. M.; Briggs, M.; Gruber, D.; Peterson, L. E.; Lingenfelter, R. E.; von Ballmoos, P.; Malet, I.; Niel, M.; Vedrenne, G.; Durouchoux, P.; Wallyn, P.; Chapuis, C.; Cork, C.; Landis, D.; Luke, P.; Madden, N.; Malone, D.; Pehl, R.

    1993-09-01

    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.

  15. Soft gamma-ray background and light Dark Matter annihilation

    E-print Network

    Yann Rasera; Romain Teyssier; Patrick Sizun; Michel Casse; Pierre Fayet; Bertrand Cordier; Jacques Paul

    2006-05-02

    The bulk of the extragalactic background between 10 keV and 10 GeV is likely to be explained by the emission of Seyfert galaxies, type Ia supernovae, and blazars. However, as revealed by the INTEGRAL satellite, the bulge of our galaxy is an intense source of a 511 keV gamma-ray line, indicating the production of a large number of positrons that annihilate. The origin of the latter is debated, and they could be produced, in particular, by the (S- or P-wave) annihilation of light Dark Matter particles into e+e-. In any case, the cumulated effect of similar sources at all redshifts could lead to a new background of hard X-ray and soft gamma-ray photons. On the basis of the hierarchical model of galaxy formation, we compute analytically the SNIa contribution to the background, and add it to Seyfert and blazars emission models. We find that any extra contribution to this unresolved background at 511 keV should be lower than about 4 keV/cm^2/s/sr. We also estimate analytically the extragalactic background due to Dark Matter annihilation, increasing the accuracy of the earlier computations. Indeed, we take into account the large positron escape fraction from low mass dark matter halos, unable to confine a dense and magnetized interstellar medium. Our new background estimate turns out to be one order of magnitude lower, so that the hypothesis of a light Dark Matter candidate remains compatible with the observed extragalactic background for a wider range of particle masses and cross-sections.

  16. Soft gamma-ray background and light dark matter annihilation

    SciTech Connect

    Rasera, Yann; Teyssier, Romain; Sizun, Patrick; Casse, Michel; Fayet, Pierre; Cordier, Bertrand; Paul, Jacques [CEA Saclay, Bat. 709, 91191 Gif-sur-Yvette (France); CEA Saclay, Bat. 709, 91191 Gif-sur-Yvette, France and Institut d'Astrophysique de Paris, 98bis Bd Arago, 75014 Paris (France); Laboratoire de Physique Theorique de l'ENS, UMR 8549 CNRS, 24 rue Lhomond, 75231 Paris Cedex 05 (France); CEA Saclay, Bat. 709, 91191 Gif-sur-Yvette (France)

    2006-05-15

    The bulk of the extragalactic background between 10 keV and 10 GeV is likely to be explained by the emission of Seyfert galaxies, type Ia supernovae, and blazars. However, as revealed by the INTEGRAL satellite, the bulge of our galaxy is an intense source of a 511 keV gamma-ray line, indicating the production of a large number of positrons that annihilate. The origin of the latter is debated, and they could be produced, in particular, by the (S- or P-wave) annihilation of light dark matter particles into e{sup +}e{sup -}. In any case, the cumulated effect of similar sources at all redshifts could lead to a new background of hard x-ray and soft gamma-ray photons. On the basis of the hierarchical model of galaxy formation, we compute analytically the SNIa contribution to the background, and add it to Seyfert and blazars emission models. Confronting these expected contributions to observation, we find that any extra contribution to this unresolved background around 511 keV should be lower than about 4 keV cm{sup -2} s{sup -1} sr{sup -1}. We also estimate analytically the extragalactic background due to dark matter annihilation, increasing the accuracy of the earlier computations. Indeed, we take into account the large positron escape fraction from low mass dark matter halos, unable to confine a dense and magnetized interstellar medium. Our new background estimate turns out to be 1 order of magnitude lower so that the hypothesis of a light dark matter candidate remains compatible with the observed extragalactic background for a wider range of particle masses and cross sections.

  17. Herwig++ 1.0: an event generator for e+e- annihilation

    NASA Astrophysics Data System (ADS)

    Gieseke, Stefan; Ribon, Alberto; Seymour, Michael H.; Stephens, Philip; Webber, Bryan

    2004-02-01

    Results from the new Monte Carlo event generator Herwig++ are presented. This first version simulates Hadron Emission Reactions With Interfering Gluons in electron-positron annihilation. The parton shower evolution is carried out using new evolution variables suited to describing radiation from heavy quarks as well as light partons. The partonic final state is fragmented into hadrons by means of an improved cluster hadronization model. The results are compared with a wide variety of data from LEP and SLC.

  18. High sensitivity of positrons to oxygen vacancies and to copper-oxygen chain disorder in YBa2Cu3O(7-x)

    Microsoft Academic Search

    E. C. von Stetten; S. Berko; X. S. Li; R. R. Lee; J. Brynestad; D. Singh; H. Krakauer; W. Pickett; R. Cohen

    1988-01-01

    Temperature-dependent positron-electron momentum densities have been studied by two-dimensional angular correlation of annihilation radiation from 10 to 320 K in YBa2Cu3O(7-x) samples. The positron ground-state charge density, computed by the linearized augmented-plane-wave method, indicates that in YBa2Cu3O7 delocalized positrons sample preferentially the linear copper-oxygen chains. Positron localization due to disorder in these chains is invoked to explain the striking differences

  19. 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.

  20. ELECTRON-POSITRON FLOWS AROUND MAGNETARS

    SciTech Connect

    Beloborodov, Andrei M., E-mail: amb@phys.columbia.edu [Physics Department and Columbia Astrophysics Laboratory, Columbia University, 538 West 120th Street New York, NY 10027 (United States)

    2013-11-10

    The twisted magnetospheres of magnetars must sustain a persistent flow of electron-positron plasma. The flow dynamics is controlled by the radiation field around the hot neutron star. The problem of plasma motion in the self-consistent radiation field is solved using the method of virtual beams. The plasma and radiation exchange momentum via resonant scattering and self-organize into the 'radiatively locked' outflow with a well-defined, decreasing Lorentz factor. There is an extended zone around the magnetar where the plasma flow is ultra-relativistic; its Lorentz factor is self-regulated so that it can marginally scatter thermal photons. The flow becomes slow and opaque in an outer equatorial zone, where the decelerated plasma accumulates and annihilates; this region serves as a reflector for the thermal photons emitted by the neutron star. The e {sup ±} flow carries electric current, which is sustained by a moderate induced electric field. The electric field maintains a separation between the electron and positron velocities, against the will of the radiation field. The two-stream instability is then inevitable, and the induced turbulence can generate low-frequency emission. In particular, radio emission may escape around the magnetic dipole axis of the star. Most of the flow energy is converted to hard X-ray emission, which is examined in an accompanying paper.

  1. Status of an R&D project of a positron gun at “Horia Hulubei” NIPNE Bucharest

    NASA Astrophysics Data System (ADS)

    Vasilescu, Angela; Craciun, L.; Ghita, Ionica A.; Constantinescu, O.; Constantin, F.; Chiojdeanu, Catalina; Zoita, C. N.; Kiss, A.; Bojin, D.; Racolta, P. M.

    2008-10-01

    A new positron gun (PG) will enable high sensitivity measurements in applications of positron annihilation spectroscopy in Romania. Some data concerning the design of a modular system for focussing, transport and acceleration of mono-energetic positrons in the range 0.8-50 keV have been obtained and experimenting on moderators and CDBS was performed. We present a short overview of the present status of the project and preliminary results from Coincidence Doppler Broadening Spectroscopy with a 22NaCl source, on Al samples. The entire positron gun system will be designed as a high-vacuum dedicated extension operating with two options: a 50 mCi 22NaCl source and in-line with the NIPNE cyclotron or a new intense compact cyclotron.

  2. Alternative positron-target design for electron-positron colliders

    SciTech Connect

    Donahue, R.J. (Lawrence Berkeley Lab., CA (United States)); Nelson, W.R. (Stanford Linear Accelerator Center, Menlo Park, CA (United States))

    1991-04-01

    Current electron-positron linear colliders are limited in luminosity by the number of positrons which can be generated from targets presently used. This paper examines the possibility of using an alternate wire-target geometry for the production of positrons via an electron-induced electromagnetic cascade shower. 39 refs., 38 figs., 5 tabs.

  3. Dark matter for excess of AMS-02 positrons and antiprotons

    E-print Network

    Chen, Chuan-Hung; Nomura, Takaaki

    2015-01-01

    We propose a dark matter explanation to simultaneously account for the excess of antiproton-to-proton and positron power spectra observed in the AMS-02 experiment while having the right dark matter relic abundance and satisfying the current direct search bounds. We extend the Higgs triplet model with a hidden gauge symmetry of $SU(2)_X$ that is broken to $Z_3$ by a quadruplet scalar field, rendering the associated gauge bosons stable weakly-interacting massive particle dark matter candidates. By coupling the complex Higgs triplet and the $SU(2)_X$ quadruplet, the dark matter candidates can annihilate into triplet Higgs bosons each of which in turn decays into lepton or gauge boson final states. Such a mechanism gives rise to correct excess of positrons and antiprotons with an appropriate choice of the triplet vacuum expectation value. Besides, the model provides a link between neutrino mass and dark matter phenomenology.

  4. Dark matter for excess of AMS-02 positrons and antiprotons

    E-print Network

    Chuan-Hung Chen; Cheng-Wei Chiang; Takaaki Nomura

    2015-04-29

    We propose a dark matter explanation to simultaneously account for the excess of antiproton-to-proton and positron power spectra observed in the AMS-02 experiment while having the right dark matter relic abundance and satisfying the current direct search bounds. We extend the Higgs triplet model with a hidden gauge symmetry of $SU(2)_X$ that is broken to $Z_3$ by a quadruplet scalar field, rendering the associated gauge bosons stable weakly-interacting massive particle dark matter candidates. By coupling the complex Higgs triplet and the $SU(2)_X$ quadruplet, the dark matter candidates can annihilate into triplet Higgs bosons each of which in turn decays into lepton or gauge boson final states. Such a mechanism gives rise to correct excess of positrons and antiprotons with an appropriate choice of the triplet vacuum expectation value. Besides, the model provides a link between neutrino mass and dark matter phenomenology.

  5. Radioactive Positron Emitter Production by Energetic Alpha Particles in Solar Flares

    NASA Astrophysics Data System (ADS)

    Murphy, R. J.; Kozlovsky, B.; Share, G. H.

    2014-12-01

    Measurements of the 0.511 MeV positron-annihilation line from solar flares are used to explore the flare process in general and ion acceleration in particular. In flares, positrons are produced primarily by the decay of radioactive positron-emitting isotopes resulting from nuclear interactions of flare-accelerated ions with ambient solar material. Kozlovsky et al. provided ion-energy-dependent production cross sections for 67 positron emitters evaluated from their threshold energies (some <1 MeV nucleon-1) to a GeV nucleon-1, incorporating them into a computer code for calculating positron-emitter production. Adequate cross-section measurements were available for proton reactions, but not for ?-particle reactions where only crude estimates were possible. Here we re-evaluate the ?-particle cross sections using new measurements and nuclear reaction codes. In typical large gamma-ray line flares, proton reactions dominate positron production, but ?-particle reactions will dominate for steeper accelerated-ion spectra because of their relatively low threshold energies. With the accelerated-3He reactions added previously, the code is now reliable for calculating positron production from any distribution of accelerated-ion energies, not just those of typical flares. We have made the code available in the online version of the Journal. We investigate which reactions, projectiles, and ion energies contribute to positron production. We calculate ratios of the annihilation-line fluence to fluences of other gamma-ray lines. Such ratios can be used in interpreting flare data and in determining which nuclear radiation is most sensitive for revealing acceleration of low-energy ions at the Sun.

  6. RADIOACTIVE POSITRON EMITTER PRODUCTION BY ENERGETIC ALPHA PARTICLES IN SOLAR FLARES

    SciTech Connect

    Murphy, R. J. [Code 7650, Naval Research Laboratory, Washington, DC 20375 (United States); Kozlovsky, B. [Tel Aviv University, Tel Aviv (Israel); Share, G. H., E-mail: murphy@ssd5.nrl.navy.mil, E-mail: benz@wise.tau.ac.il, E-mail: share@astro.umd.edu [University of Maryland, College Park, MD 20742 (United States)

    2015-01-01

    Measurements of the 0.511 MeV positron-annihilation line from solar flares are used to explore the flare process in general and ion acceleration in particular. In flares, positrons are produced primarily by the decay of radioactive positron-emitting isotopes resulting from nuclear interactions of flare-accelerated ions with ambient solar material. Kozlovsky et al. provided ion-energy-dependent production cross sections for 67 positron emitters evaluated from their threshold energies (some <1 MeV nucleon{sup –1}) to a GeV nucleon{sup –1}, incorporating them into a computer code for calculating positron-emitter production. Adequate cross-section measurements were available for proton reactions, but not for ?-particle reactions where only crude estimates were possible. Here we re-evaluate the ?-particle cross sections using new measurements and nuclear reaction codes. In typical large gamma-ray line flares, proton reactions dominate positron production, but ?-particle reactions will dominate for steeper accelerated-ion spectra because of their relatively low threshold energies. With the accelerated-{sup 3}He reactions added previously, the code is now reliable for calculating positron production from any distribution of accelerated-ion energies, not just those of typical flares. We have made the code available in the online version of the Journal. We investigate which reactions, projectiles, and ion energies contribute to positron production. We calculate ratios of the annihilation-line fluence to fluences of other gamma-ray lines. Such ratios can be used in interpreting flare data and in determining which nuclear radiation is most sensitive for revealing acceleration of low-energy ions at the Sun.

  7. Lifetimes of Nuclear Isomers

    Microsoft Academic Search

    Steven A. Moszkowski

    1953-01-01

    Lifetimes of nuclear isomers for gamma-transitions are calculated theoretically on the basis of various independent particle models; e.g., single proton, single neutron, and states of several particles (Sec. II). The calculations of this paper are essentially restricted to the most common type of transition viz., multipole order equal to spin change. The lifetime is expressed in terms of a matrix

  8. Why positron physics is fun

    Microsoft Academic Search

    Richard J. Drachman

    1996-01-01

    In this Review I will describe some properties of the positron (``antimatter'') in interaction with ordinary matter at low energies, in order to explain why positron physics inspires such devotion and enthusiasm in its practitioners. Positron scattering is much like electron scattering, may involve unusual bound states and resonances, often satisfies simple dispersion relations, and can usually be analyzed without

  9. Neutron Lifetime Measurements

    SciTech Connect

    Nico, J. S. [National Institute of Standards and Technology, Physics Laboratory, Gaithersburg, MD 20899 (United States)

    2006-11-17

    Precision measurements of neutron beta decay address basic questions in nuclear and particle physics, astrophysics, and cosmology. As the simplest semileptonic decay system, the free neutron plays an important role in understanding the physics of the weak interaction, and improving the precision of the neutron lifetime is fundamental to testing the validity of the theory. The neutron lifetime also directly affects the relative abundance of primordial helium in big bang nucleosynthesis. There are two distinct strategies for measuring the lifetime. Experiments using cold neutrons measure the absolute specific activity of a beam of neutrons by counting decay protons; experiments using confined, ultracold neutrons determine the lifetime by counting neutrons that remain after some elapsed time. The status of the recent lifetime measurements using both of these techniques is discussed.

  10. Nanohole formation in TEOS-HMDSO hybrid CVD films elucidated by positron beams

    NASA Astrophysics Data System (ADS)

    Ito, K.; Oshima, N.; Yabuuchi, A.; O'Rourke, B. E.

    2014-04-01

    Silicon-oxide-backboned hybrid thin films with thicknesses around 600 nm were fabricated through plasma enhanced chemical vapor deposition from tetraethyl orthosilicate mixed with hexamethyldisiloxane as precursors, and their subnano-scaled holes, generated by annealing at 560 °C, were investigated by means of the positron lifetime technique with a pulsed, low-energy positron beam. The hole dimension was quantified from the ortho-positronium lifetime for the as-prepared and annealed films with different compositions. The effect of the heat treatment and the precursor composition on the subnano holes was discussed.

  11. Constraint on dark matter annihilation with dark star formation using Fermi extragalactic diffuse gamma-ray background data

    SciTech Connect

    Yuan, Qiang [Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China); Yue, Bin; Chen, Xuelei [National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 (China); Zhang, Bing, E-mail: yuanq@mail.ihep.ac.cn, E-mail: yuebin@bao.ac.cn, E-mail: zhang@physics.unlv.edu, E-mail: xuelei@cosmology.bao.ac.cn [Department of Physics and Astronomy, University of Nevada Las Vegas, Las Vegas, NV 89154 (United States)

    2011-04-01

    It has been proposed that during the formation of the first generation stars there might be a ''dark star'' phase in which the power of the star comes from dark matter annihilation. The adiabatic contraction process to form the dark star would result in a highly concentrated density profile of the host halo at the same time, which may give enhanced indirect detection signals of dark matter. In this work we investigate the extragalactic ?-ray background from dark matter annihilation with such a dark star formation scenario, and employ the isotropic ?-ray data from Fermi-LAT to constrain the model parameters of dark matter. The results suffer from large uncertainties of both the formation rate of the first generation stars and the subsequent evolution effects of the host halos of the dark stars. We find, in the most optimistic case for ?-ray production via dark matter annihilation, the expected extragalactic ?-ray flux will be enhanced by 1-2 orders of magnitude. In such a case, the annihilation cross section of the supersymmetric dark matter can be constrained to the thermal production level, and the leptonic dark matter model which is proposed to explain the positron/electron excesses can be well excluded. Conversely, if the positron/electron excesses are of a dark matter annihilation origin, then the early Universe environment is such that no dark star can form.

  12. Positron excitation of neon

    NASA Technical Reports Server (NTRS)

    Parcell, L. A.; Mceachran, R. P.; Stauffer, A. D.

    1990-01-01

    The differential and total cross section for the excitation of the 3s1P10 and 3p1P1 states of neon by positron impact were calculated using a distorted-wave approximation. The results agree well with experimental conclusions.

  13. Positron trapping at thermal vacancies in highly As-doped Si

    NASA Astrophysics Data System (ADS)

    Pennanen, K.; Ranki, V.; Saarinen, K.

    2006-04-01

    Using positron annihilation measurements we observed the formation of thermal vacancies in highly As-doped Si. The vacancies form already at 700 K and upon cooling down the vacancies form stable vacancy-impurity complexes such as V-As3. Only a fraction of thermal vacancies is observed during equilibrium temperature measurements, but the quenching to room temperature reveals that the concentration of thermally generated V-As3 defects is an order of magnitude larger. This mismatch is explained by positron detrapping from V-As3 at high temperatures.

  14. The ``accumulation effect'' of positrons in the stack of foils, detected by measurements of the positron implantation profile

    NASA Astrophysics Data System (ADS)

    Dryzek, Jerzy; Siemek, Krzysztof

    2013-12-01

    The profiles of positrons implanted from the radioactive source 22Na into a stack of foils and plates are the subject of our experimental and theoretical studies. The measurements were performed using the depth scanning of positron implantation profile method, and the theoretical calculations using the phenomenological multi-scattering model (MSM). Several stacks consisting of silver, gold and aluminum foils, and titanium and germanium plates were investigated. We notice that the MSM describes well the experimental profiles; however when the stack consisting of silver and gold foils, the backscattering and linear absorption coefficients differ significantly from those reported in the literature. We suggest the energy dependency of the backscattering coefficient for silver and gold. In the stacks which comprise titanium and germanium plates, there were observed the features, which indicate the presence of the "accumulation effect" in the experimental implantation profile. This effect was previously detected in implantation profiles in Monte Carlo simulations using the GEANT4 tool kit, and it consists in higher localization of positrons close the interface. We suppose that this effect can be essential for positron annihilation in any heterogeneous materials.

  15. Theoretical electron-positron zone-reduced momentum density for YBa sub 2 Cu sub 3 O sub 7 : Fermi surface and wave-function effects

    SciTech Connect

    Singh, D.; Pickett, W.E. (Naval Research Laboratory, Washington, DC (USA)); von Stetten, E.C.; Berko, S. (Department of Physics, Brandeis University, Waltham, MA (USA))

    1990-08-01

    Using the linearized augmented-plane-wave (LAPW) -calculated electron and positron charge densities for YBa{sub 2}Cu{sub 3}O{sub 7}, the Brillouin-zone-reduced electron-positron momentum density is computed and the zone-reduced two-dimensional angular correlation of annihilation radiation (2D ACAR) spectrum is produced. The calculations show that the relative weights of the Fermi-surface discontinuities are substantially altered due to the positron preferentially sampling the Cu-O chain region. In addition, the reduced 2D ACAR spectrum contains large {ital k}-dependent wave-function effects. The theoretical zone-reduced 2D ACAR spectrum is compared to the several existing experimental spectra. It is concluded that, at present, positron-annihilation experiments do not provide consistent and clear evidence for the existence and shapes of Fermi surfaces in YBa{sub 2}Cu{sub 3}O{sub 7}.

  16. Possible evidence for dark matter annihilations from the excess microwave emission around the center of the Galaxy seen by the Wilkinson Microwave Anisotropy Probe

    Microsoft Academic Search

    Dan Hooper; Douglas P. Finkbeiner; Gregory Dobler

    2007-01-01

    The Wilkinson Microwave Anisotropy Probe (WMAP) experiment has revealed an excess of microwave emission from the region around the center of our Galaxy. It has been suggested that this signal, known as the ``WMAP haze,'' could be synchrotron emission from relativistic electrons and positrons generated in dark matter annihilations. In this article, we revisit this possibility. We find that the

  17. The trigger detector for APEX: An array of position-sensitive NaI(Tl) detectors for the imaging of positrons from heavy-ion collisions

    Microsoft Academic Search

    N. I. Kaloskamis; K. C. Chan; A. A. Chishti; J. S. Greenberg; C. J. Lister; S. J. Freedman; M. Wolanski; J. Last; B. Utts

    1993-01-01

    We describe a 24-element position-sensitive cylindrical NaI(Tl) detector which allows positron imaging with an on-axis position resolution of 3 cm FWHM and annihilation radiation energy resolution of 13% FWHM. Two such detectors will provide the trigger for the APEX experiment at Argonne National Laboratory, which is exploring anomalous positron and electron production observed in heavy-ion collisions. Present address: CENG, 85X-38041

  18. Radio Signal Constraints on Galactic Dark Matter Annihilation

    SciTech Connect

    Borriello, E. [Universita 'Federico II', Dipartimento di Scienze Fisiche, Napoli (Italy); Cuoco, A. [Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, Bygn. 1520 8000 Aarhus (Denmark); Mangano, G. [INFN Sezione di Napoli, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli (Italy); Miele, G. [Universita 'Federico II', Dipartimento di Scienze Fisiche, Napoli (Italy); INFN Sezione di Napoli, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli (Italy); Moura, C. A. [INFN Sezione di Napoli, Complesso Universitario di Monte S. Angelo, Via Cintia, I-80126 Napoli (Italy); Instituto de Fisica 'Gleb Wataghin'-UNICAMP, 13083-970, Campinas, Sao Paulo (Brazil)

    2010-06-23

    Annihilation of Dark Matter usually produces together with gamma rays comparable amounts of electrons and positrons. The e{sup +}e{sup -} gyrating in the galactic magnetic field then produce secondary synchrotron radiation which thus provides an indirect mean to constrain the DM signal itself. To this purpose, we calculate the radio emission from the galactic halo as well as from its expected substructures and we then compare it with the measured diffuse radio background. We employ a multi-frequency approach using data in the relevant frequency range 100 MHz-100 GHz, as well as the WMAP Haze data at 23 GHz. The derived constraints are of the order <{sigma}{sub A}{sup v}> = 10{sup -24} cm{sup 3}s{sup -1} for a DM mass m{sub {kappa}}= 100 GeV even though sensibly depending on the astrophysical uncertainties.

  19. The Neutron Lifetime

    E-print Network

    F. E. Wietfeldt

    2014-11-13

    The decay of the free neutron into a proton, electron, and antineutrino is the prototype semileptonic weak decay and the simplest example of nuclear beta decay. The nucleon vector and axial vector weak coupling constants G_V and G_A determine the neutron lifetime as well as the strengths of weak interaction processes involving free neutrons and protons that are important in astrophysics, cosmology, solar physics and neutrino detection. In combination with a neutron decay angular correlation measurement, the neutron lifetime can be used to determine the first element of the CKM matrix Vud. Unfortunately the two main experimental methods for measuring the neutron lifetime currently disagree by almost 4 sigma. I will present a brief review of the status of the neutron lifetime and prospects for the future.

  20. New positron spectral features from supersymmetric dark matter: A way to explain the PAMELA data?

    Microsoft Academic Search

    Lars Bergstroem; Torsten Bringmann; Joakim Edsjoe

    2008-01-01

    The space-borne antimatter experiment PAMELA has recently reported a surprising rise in the positron to electron ratio at high energies. It has also recently been found that electromagnetic radiative corrections in some cases may boost the gamma-ray yield from supersymmetric dark-matter annihilations in the galactic halo by up to 3 or 4 orders of magnitude, providing distinct spectral signatures for

  1. Single negative W positive W production by positron electron yields W plus hadrons

    Microsoft Academic Search

    F. M. Renard; M. Talon

    1977-01-01

    It is shown that the annihilation reaction electron positron yields positive and negative omega mesons can be an interesting source of single negative and positive omega-mesons production for a total center of mass energy of greater than or equal to 65 GeV at a rate of 10 to the minus 36th power to 10 to the minus 35th power sq

  2. A statistical analysis of count normalization methods used in positron-emission tomography

    SciTech Connect

    Holmes, T.J.; Ficke, D.C.; Snyder, D.L.

    1984-02-01

    As part of the Positron-Emission Tomography (PET) reconstruction process, annihilation counts are normalized for photon absorption, detector efficiency and detector-pair duty-cycle. Several normalization methods of time-of-flight and conventional systems are analyzed mathematically for count bias and variance. The results of the study have some implications on hardware and software complexity and on image noise and distortion.

  3. Positron trapping at thermal vacancies in highly As-doped Si

    Microsoft Academic Search

    K. Pennanen; V. Ranki; K. Saarinen

    2006-01-01

    Using positron annihilation measurements we observed the formation of thermal vacancies in highly As-doped Si. The vacancies form already at 700K and upon cooling down the vacancies form stable vacancy-impurity complexes such as V–As3. Only a fraction of thermal vacancies is observed during equilibrium temperature measurements, but the quenching to room temperature reveals that the concentration of thermally generated V–As3

  4. Positron scattering from tetrahydrofuran

    NASA Astrophysics Data System (ADS)

    Brunger, M. J.; Chiari, L.; Tattersall, W.; Anderson, E.; Machacek, J.; Makochekanwa, C.; Sullivan, J.; Buckman, S. J.

    2012-10-01

    We present recent experimental results for positron scattering from tetrahydrofuran. Being a model for the deoxyribose sugar rings in the nucleic acids backbone, tetrahydrofuran is of particular interest for investigating radiation damage in biomolecular systems. The measurements on this species were carried out using the atomic and molecular trap-based positron beamline at The Australian National University with an energy resolution of ˜60 meV. Total cross sections and integral cross sections for the positronium formation, elastic and inelastic (direct ionization and electronic excitation) scattering channels are presented over the energy range of 1-190 eV. Low-energy elastic differential cross sections are also presented at selected energies between 1 eV and 25 eV. A fairly good agreement is found with the total cross section results from the only existing previous experimental investigation on this target species by the Trento group.

  5. Neutron LifetimeNeutron Lifetime IUCF Colloquium April 13,

    E-print Network

    Steyerl, Albert

    Neutron LifetimeNeutron Lifetime IUCF Colloquium April 13, 2007 Albert Steyerl Department 940 878.5±0.8 885.7±0.8 new result neutronlifetime(),s year world average Neutron lifetime data #12 world average Neutron lifetime data A. Serebrov et al. 2005Storage of ultra-cold neutrons878.5 ±±±± 0

  6. Excesses in cosmic ray positron and electron spectra from a nearby clump of neutralino dark matter

    SciTech Connect

    Hooper, Dan [Theoretical Astrophysics, Fermi National Accelerator Laboratory, Batavia, Illinois (United States); Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois (United States); Stebbins, Albert; Zurek, Kathryn M. [Theoretical Astrophysics, Fermi National Accelerator Laboratory, Batavia, Illinois (United States)

    2009-05-15

    In this letter, we suggest that a nearby clump of 600-1000 GeV neutralinos may be responsible for the excesses recently observed in the cosmic ray positron and electron spectra by the PAMELA and ATIC experiments. Although neutralino dark matter annihilating throughout the halo of the Milky Way is predicted to produce a softer spectrum than is observed, and violate constraints from cosmic ray antiproton measurements, a large nearby (within 1-2 kiloparsecs of the Solar System) clump of annihilating neutralinos can lead to a spectrum which is consistent with PAMELA and ATIC, while also producing an acceptable antiproton flux. Furthermore, the presence of a large dark matter clump can potentially accommodate the very large annihilation rate required to produce the PAMELA and ATIC signals. We estimate the probability of a sufficiently large clump being present to be {approx}10{sup -3} or less.

  7. A Comprehensive Search for Dark Matter Annihilation in Dwarf Galaxies

    E-print Network

    Alex Geringer-Sameth; Savvas M. Koushiappas; Matthew G. Walker

    2015-03-11

    We present a new formalism designed to discover dark matter annihilation occurring in the Milky Way's dwarf galaxies. The statistical framework extracts all available information in the data by simultaneously combining observations of all the dwarf galaxies and incorporating the impact of particle physics properties, the distribution of dark matter in the dwarfs, and the detector response. The method performs maximally powerful frequentist searches and produces confidence limits on particle physics parameters. Probability distributions of test statistics under various hypotheses are constructed exactly, without relying on large sample approximations. The derived limits have proper coverage by construction and claims of detection are not biased by imperfect background modeling. We implement this formalism using data from the Fermi Gamma-ray Space Telescope to search for an annihilation signal in the complete sample of Milky Way dwarfs whose dark matter distributions can be reliably determined. We find that the observed data is consistent with background for each of the dwarf galaxies individually as well as in a joint analysis. The strongest constraints are at small dark matter particle masses. Taking the median of the systematic uncertainty in dwarf density profiles, the cross section upper limits are below the pure s-wave weak scale relic abundance value (2.2 x 10^-26 cm^3/s) for dark matter masses below 26 GeV (for annihilation into b quarks), 29 GeV (tau leptons), 35 GeV (up, down, strange, and charm quarks and gluons), 6 GeV (electrons/positrons), and 114 GeV (two-photon final state). For dark matter particle masses less than 1 TeV, these represent the strongest limits obtained to date using dwarf galaxies.

  8. Accelerator based coal positron beams

    SciTech Connect

    Schoessow, P.; Simpson, J.

    1987-01-01

    Cold positron beams produced using solid state moderators have been used profitably for condensed matter and positronium research. The low emittance and energy spread of these beams make the technique attractive as a potential positron source for future linear colliders, reducing or eliminating the need for damping rings. However, the intensities attained so far fall short of the requirements of a high energy linear collider. (approx.10/sup 11/ positrons/pulse at 10 kHz was taken as the positron flux necessary for a linear collider-B anti B factory). This report briefly reviews the state of the art in accelerator produced coal positron beams and indicates some areas in which yields might be improved. The discussion here is limited to electroproduced positrons.

  9. Comparison of measurements of ionization cross-section for the CVV Auger transition from Ge(100) and Ag(100) over the energy range 150eV-750eV utilizing a positron beam and an electron beam.

    NASA Astrophysics Data System (ADS)

    Venkataraman, Ranjani; Jung, E.; Starnes, S.; Chen, W.; Weiss, A.

    1997-03-01

    We present measurements of the M_4,5N_1N2 Auger transition from the Ge(100) surface, using incident positron and electron beam energies of 375eV. The positron induced Auger signal can be expected to have contributions from positron annihilation with core electrons as well as from ionization of electrons through collisions. In the case of electron bombardment, only collisional ionization is operant. This hypothesis is confirmed with the observation of a decrease in the positron induced Auger signal when the sample temperature is raised. The elevated temperature results in thermal desorption of the surface trapped positrons as positronium before they can contribute to the annihilation induced part of the Auger signal. Measurements for N_2,3VV transition from the Ag(100) surface utilizing various positron and electron beam energies will be discussed. In Ag(100) the complete disappearance of the annihilation induced contributions to the Auger signal is to be expected at 510 degC, which will make it easier to measure the annihilation induced and collisional cross-sections separately.

  10. High sensitivity of positrons to oxygen vacancies and to copper-oxygen chain disorder in YBa/sub 2/Cu/sub 3/O/sub 7/. sqrt. /sub x/

    SciTech Connect

    von Stetten, E.C.; Berko, S.; Li, X.S.; Lee, R.R.; Brynestad, J.; Singh, D.; Krakauer, H.; Pickett, W.E.; Cohen, R.E.

    1988-05-23

    Temperature-dependent positron-electron momentum densities have been studied by two-dimensional angular correlation of annihilation radiation from 10 to 320 K in YBa/sub 2/Cu/sub 3/O/sub 7/..sqrt../sub x/ samples. The positron ground-state charge density, computed by the linearized augmented plane-wave method, indicates that in YBa/sub 2/Cu/sub 3/O/sub 7/ delocalized positrons sample preferentially the linear copper-oxygen chains. Positron localization due to disorder in these chains is invoked to explain the striking differences observed between superconducting (xapprox. =0.02) and nonsuperconducting (xapprox. =0.70) samples

  11. Texas Intense Positron Source (TIPS)

    Microsoft Academic Search

    D. O'Kelly

    2003-01-01

    The Texas Intense Positron Source (TIPS) is a state of the art variable energy positron beam under construction at the Nuclear Engineering Teaching Laboratory (NETL). Projected intensities on the order of the order of 10^7 e+\\/second using ^64Cu as the positron source are expected. Owing to is short half-life (t1\\/2 12.8 hrs), plans are to produce the ^64Cu isotope on-site

  12. Measurement of alphas from hadron jets in e+e- annihilation at &surd;s of 29 GeV

    Microsoft Academic Search

    W. T. Ford; N. Qi; A. L. Read; J. G. Smith; T. Camporesi; R. de Sangro; I. Peruzzi; M. Piccolo; R. B. Hurst; J. Pyrlik; J. P. Venuti; R. Weinstein; M. W. Gettner; G. P. Goderre; J. C. Sleeman; E. von Goeler; G. B. Chadwick; R. E. Leedy; R. L. Messner; L. J. Moss; F. Muller; H. N. Nelson; D. M. Ritson; L. J. Rosenberg; D. E. Wiser; R. W. Zdarko; D. E. Groom; P. G. Verdini; H. R. Band; M. C. Delfino; J. R. Johnson; T. L. Lavine; T. Maruyama; R. Prepost

    1989-01-01

    A study of the lateral development of jets of hadrons produced in electron-positron annihilation has been used to determine the strong coupling constant alphas. Data were obtained with the MAC detector at the SLAC e+e- storage ring PEP at &surd;s =29 GeV. Based on the parton calculations of Gottschalk and Shatz, a value for alphas of 0.133+\\/-0.005(stat)+\\/-0.009(syst) has been determined

  13. Multiple gamma lines from semi-annihilation

    SciTech Connect

    D'Eramo, Francesco [Department of Physics, University of California, Berkeley, CA 94720 (United States); McCullough, Matthew; Thaler, Jesse, E-mail: fraderamo@berkeley.edu, E-mail: mccull@mit.edu, E-mail: jthaler@mit.edu [Center for Theoretical Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)

    2013-04-01

    Hints in the Fermi data for a 130 GeV gamma line from the galactic center have ignited interest in potential gamma line signatures of dark matter. Explanations of this line based on dark matter annihilation face a parametric tension since they often rely on large enhancements of loop-suppressed cross sections. In this paper, we pursue an alternative possibility that dark matter gamma lines could arise from ''semi-annihilation'' among multiple dark sector states. The semi-annihilation reaction ?{sub i}?{sub j} ? ?{sub k}? with a single final state photon is typically enhanced relative to ordinary annihilation ?{sub i}?-bar {sub i} ? ?? into photon pairs. Semi-annihilation allows for a wide range of dark matter masses compared to the fixed mass value required by annihilation, opening the possibility to explain potential dark matter signatures at higher energies. The most striking prediction of semi-annihilation is the presence of multiple gamma lines, with as many as order N{sup 3} lines possible for N dark sector states, allowing for dark sector spectroscopy. A smoking gun signature arises in the simplest case of degenerate dark matter, where a strong semi-annihilation line at 130 GeV would be accompanied by a weaker annihilation line at 173 GeV. As a proof of principle, we construct two explicit models of dark matter semi-annihilation, one based on non-Abelian vector dark matter and the other based on retrofitting Rayleigh dark matter.

  14. Tracking in Antiproton Annihilation Experiments

    E-print Network

    O. N. Hartmann

    2006-02-02

    A major ingredient of the planned new accelerator complex FAIR, to be constructed at the GSI, Darmstadt, Germany, is the availability of antiproton beams with high quality and intensity. Among the experiments which will make use of this opportunity is PANDA, a dedicated experiment to study antiproton annihilations on nucleons and nuclei. This article gives an overview on the foreseen techniques to perform charged particle tracking in the high rate environment of this experiment.

  15. The ATLAS Positron Experiment -- APEX

    SciTech Connect

    Ahmad, I.; Back, B.B.; Betts, R.R.; Dunford, R.; Kutschera, W.; Rhein, M.D.; Schiffer, J.P.; Wilt, P.; Wuosmaa, A. [Argonne National Lab., IL (United States). Physics Div.; Austin, S.M.; Kashy, E.; Winfield, J.S.; Yurkon, J.E. [Michigan State Univ., East Lansing, MI (United States). NSCL; Bazin, D. [GANIL, Caen (France); Calaprice, F.P.; Young, A. [Princeton Univ., NJ (United States). Physics Dept.; Chan, K.C.; Chisti, A.; Chowhury, P.; Greenberg, J.S.; Kaloskamis, N.; Lister, C.J. [Yale Univ., New Haven, CT (United States). Wright Nuclear Structure Lab.; Fox, J.D.; Roa, E. [Florida State Univ., Tallahassee, FL (United States). Physics Dept.; Freedman, S.; Maier, M.R. [Lawrence Berkeley Lab., CA (United States); Freer, M. [Univ. of Birmingham (United Kingdom). Dept. of Physics; Gazes, S. [Univ. of Chicago, IL (United States). Dept. of Physics; Hallin, A.L.; Liu, M. [Queen`s Univ., Kingston, Ontario (Canada). Physics Dept.; Happ, T. [GSI, Darmstadt (Germany); Perera, A.; Wolfs, F.L.H. [Univ. of Rochester, NY (United States). NSRL; Trainor, T. [Univ. of Washington, Seattle, WA (United States). Nuclear Physics Lab.; Wolanski, M. [Argonne National Lab., IL (United States). Physics Div.]|[Univ. of Chicago, IL (United States). Dept. of Physics

    1994-03-01

    APEX -- the ATLAS Positron Experiment -- is designed to measure electrons and positrons emitted in heavy-ion collisions. Its scientific goal is to gain insight into the puzzling positron-line phenomena observed at the GSI Darmstadt. It is in operation at the ATLAS accelerator at Argonne National Lab. The assembly of the apparatus is finished and beginning 1993 the first positrons produced in heavy-ion collisions were observed. The first full scale experiment was carried out in December 1993, and the data are currently being analyzed. In this paper, the principles of operation are explained and a status report on the experiment is given.

  16. Constraining the origin of the rising cosmic ray positron fraction with the boron-to-carbon ratio

    E-print Network

    Ilias Cholis; Dan Hooper

    2014-03-04

    The rapid rise in the cosmic ray positron fraction above 10 GeV, as measured by PAMELA and AMS, suggests the existence of nearby primary sources of high energy positrons, such as pulsars or annihilating/decaying dark matter. In contrast, the spectrum of secondary positrons produced through the collisions of cosmic rays in the interstellar medium is predicted to fall rapidly with energy, and thus is unable to account for the observed rise. It has been proposed, however, that secondary positrons could be produced and then accelerated in nearby supernova remnants, potentially explaining the observed rise, without the need of primary positron sources. Yet, if secondary positrons are accelerated in such shocks, other secondary cosmic ray species (such as boron nuclei, and antiprotons) will also be accelerated, leading to rises in the boron-to-carbon and antiproton-to-proton ratios. The measurements of the boron-to-carbon ratio by the PAMELA and AMS collaborations, however, show no sign of such a rise. With this new data in hand, we revisit the secondary acceleration scenario for the rising positron fraction. Assuming that the same supernova remnants accelerate both light nuclei (protons, helium) and heavier cosmic ray species, we find that no more than ~25% of the observed rise in the positron fraction can result from this mechanism (at the 95% confidence level)

  17. The Intense Slow Positron Beam Facility at the NC State University PULSTAR Reactor

    SciTech Connect

    Hawari, Ayman I.; Moxom, Jeremy; Hathaway, Alfred G.; Brown, Benjamin [Nuclear Engineering/Nuclear Reactor Program, North Carolina State University, P.O. Box 7909, Raleigh NC 27695 (United States); Gidley, David W.; Vallery, Richard [Physics Department, University of Michigan, 450 Church Street, Ann Arbor MI 48109 (United States); Xu, Jun [Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge TN 37831 (United States)

    2009-03-10

    An intense slow positron beam is in its early stages of operation at the 1-MW open-pool PULSTAR research reactor at North Carolina State University. The positron beam line is installed in a beam port that has a 30-cmx30-cm cross sectional view of the core. The positrons are created in a tungsten converter/moderator by pair-production using gamma rays produced in the reactor core and by neutron capture reactions in cadmium cladding surrounding the tungsten. Upon moderation, slow ({approx}3 eV) positrons that are emitted from the moderator are electrostatically extracted, focused and magnetically guided until they exit the reactor biological shield with 1-keV energy, approximately 3-cm beam diameter and an intensity exceeding 6x10{sup 8} positrons per second. A magnetic beam switch and transport system has been installed and tested that directs the beam into one of two spectrometers. The spectrometers are designed to implement state-of-the-art PALS and DBS techniques to perform positron and positronium annihilation studies of nanophases in matter.

  18. CONSTRAINTS ON DARK MATTER ANNIHILATION IN CLUSTERS OF GALAXIES FROM DIFFUSE RADIO EMISSION

    SciTech Connect

    Storm, Emma; Jeltema, Tesla E.; Profumo, Stefano [Department of Physics, University of California, 1156 High St., Santa Cruz, CA 95064 (United States); Rudnick, Lawrence [Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455 (United States)

    2013-05-10

    Annihilation of dark matter can result in the production of stable Standard Model particles including electrons and positrons that, in the presence of magnetic fields, lose energy via synchrotron radiation, observable as radio emission. Galaxy clusters are excellent targets to search for or to constrain the rate of dark matter annihilation, as they are both massive and dark matter dominated. In this study, we place limits on dark matter annihilation in a sample of nearby clusters using upper limits on the diffuse radio emission, low levels of observed diffuse emission, or detections of radio mini-halos. We find that the strongest limits on the annihilation cross section are better than limits derived from the non-detection of clusters in the gamma-ray band by a factor of {approx}3 or more when the same annihilation channel and substructure model, but different best-case clusters, are compared. The limits on the cross section depend on the assumed amount of substructure, varying by as much as two orders of magnitude for increasingly optimistic substructure models as compared to a smooth Navarro-Frenk-White profile. In our most optimistic case, using the results of the Phoenix Project, we find that the derived limits reach below the thermal relic cross section of 3 Multiplication-Sign 10{sup -26} cm{sup 3} s{sup -1} for dark matter masses as large as 400 GeV, for the b b-bar annihilation channel. We discuss uncertainties due to the limited available data on the magnetic field structure of individual clusters. We also report the discovery of diffuse radio emission from the central 30-40 kpc regions of the groups M49 and NGC 4636.

  19. A field-assisted moderator for low-energy positron beams

    NASA Astrophysics Data System (ADS)

    Beling, C. D.; Simpson, R. I.; Charlton, M.; Jacobsen, F. M.; Griffith, T. C.; Moriarty, P.; Fung, S.

    1987-01-01

    A new positron field-assisted (FA) moderator based on the drift of positrons across a cooled silicon crystal is proposed. Using estimates for both the ? + implantation profile and attainable drift velocities, the efficiency of drift to a slow e+ emitting surface is calculated using a diffusion equation which incorporates terms describing positron drift and annihilation. It is conjectured that efficiencies of up to 10% can be achieved. The use of epitaxially grown metallic suicide contacts to facilitate the application of the electric field is described and the consequences of using such contacts are fully discussed. Applications of the FA transmission mode moderator described here to produce timed brightness enhanced beams are briefly discussed.

  20. Dipole-interacting fermionic dark matter in positron, antiproton, and gamma-ray channels

    NASA Astrophysics Data System (ADS)

    Heo, Jae Ho; Kim, C. S.

    2013-01-01

    Cosmic ray signals from dipole-interacting dark matter annihilation are considered in the positron, antiproton, and photon channels. The predicted signals in the positron channel could nicely account for the excess of positron fraction from Fermi LAT, PAMELA, HEAT, and AMS-01 experiments for the dark matter mass larger than 100 GeV with a boost (enhancement) factor of 30-80. No excess of antiproton over proton ratio at the experiments also gives a severe restriction for this scenario. With the boost factors, the predicted signals from Galactic halo and signals as monoenergetic gamma-ray lines (monochromatic photons) for the region close to the Galactic center are investigated. The gamma-ray excess of recent tentative analyses based on Fermi LAT data and the potential probe of the monochromatic lines at a planned experiment, AMS-02, are also considered.