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1

Position-resolved Positron Annihilation Lifetime Spectroscopy  

NASA Astrophysics Data System (ADS)

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.

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

2013-06-01

2

Investigation of nanocrystalline CoFe2O4 by positron annihilation lifetime spectroscopy  

Microsoft Academic Search

Nanoparticles of cobalt ferrite prepared by the co-precipitation method with crystallite size varying from 4.7 to 41 nm have been characterized by positron annihilation lifetime spectroscopy. Three lifetime components are fitted to the lifetime data. The shortest lifetime component is attributed to the delocalized positron lifetime shortened by defect trapping. The intermediate lifetime is assigned to the positron annihilation in

S. Bandyopadhyay; A. Roy; D. Das; S. S. Ghugre; J. Ghose

2003-01-01

3

Study of Chemical Carcinogens by Positron Annihilation Lifetime Spectroscopy  

NASA Astrophysics Data System (ADS)

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.

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

2013-11-01

4

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

NASA Astrophysics Data System (ADS)

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

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

2011-08-01

5

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

PubMed

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

Sebe, István; Szabó, Barnabás; Zelkó, Romána

2012-01-01

6

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

NASA Astrophysics Data System (ADS)

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.

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

2014-12-01

7

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

PubMed

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. PMID:25554275

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

2014-12-01

8

Positron annihilation lifetime study of interfaces in ternary polymer blends  

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

9

Systematics in positron annihilation lifetime analysis of high (Tc) superconducting transitions  

NASA Astrophysics Data System (ADS)

The lifetimes and momentum distributions of annihilating electron-positron pairs are changed in high Tc superconductors cooled below the superconducting transition. Since the initial observation of this phenomena is YBCO the observation of these effects was reproduced and extended to other high (Tc) systems. There is a general consistency of results in most families of superconductors. An exception to this is YBCO which shows strong sample dependence. These consistent features have led to a model relating the pairing of the positive charge carriers and the positron distribution as one explanation for the lifetime changes.

Howell, R. H.; Radousky, H. B.; Wachs, A. L.; Fluss, M. J.; Turchi, P. E. A.; Jean, Y. C.; Sundar, C. S.; Chu, C. W.; Shelton, R. N.; Hinks, D. G.

1989-06-01

10

Kinetics of natural aging in Al-Mg-Si alloys studied by positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

The process of natural aging in pure ternary Al-Mg-Si alloys was studied by positron annihilation lifetime spectroscopy in real time in order to clarify the sequence and kinetics of clustering and precipitation. It was found that natural aging takes place in at least five stages in these alloys, four of which were directly observed. This is interpreted as the result of complex interactions between vacancies and solute atoms or clusters. One of the early stages of positron lifetime evolution coincides with a clustering process observed by differential scanning calorimetry (DSC) and involves the formation of a positron trap with ˜0.200 ns lifetime. In later stages, a positron trap with a higher lifetime develops in coincidence with the DSC signal of a second clustering reaction. Mg governs both the kinetics and the lifetime change in this stage. Within the first 10 min after quenching, a period of nearly constant positron lifetime was found for those Mg-rich alloys that later show an insufficient hardness response to artificial aging, the so-called “negative effect.” The various processes observed could be described by two effective activation energies that were found by varying the aging temperature from 10 to 37°C.

Banhart, J.; Lay, M. D. H.; Chang, C. S. T.; Hill, A. J.

2011-01-01

11

Vacancy profile in reverse osmosis membranes studied by positron annihilation lifetime measurements and molecular dynamics simulations  

NASA Astrophysics Data System (ADS)

The positron annihilation technique using a slow positron beam can be used for the study of the vacancy profiles in typical reverse osmosis (RO) membranes. In this study, the vacancy profile in the polyamide membrane that exhibits a high permselectivity between ions and water was studied using the positron annihilation technique and molecular dynamics simulations. Ortho-positronium (o-Ps) lifetimes in the surface region of the membranes were evaluated by using a slow positron beam. The diffusion behavior of Na+ and water in the polyamides was simulated by molecular dynamics (MD) methods using the TSUBAME2 supercomputer at the Tokyo Institute of Technology and discussed with the vacancy profile probed by the o-Ps. The results suggested that the large hydration size of Na+ compared to the vacancy size in the polyamides contributes to the increased diffusivity selectivity of water/Na+ that is related to the NaCl desalination performance of the membrane. Both the hydration size of the ions and the vacancy size appeared to be significant parameters to discuss the diffusivity selectivity of water/ions in typical polyamide membranes.

Shimazu, A.; Goto, H.; Shintani, T.; Hirose, M.; Suzuki, R.; Kobayashi, Y.

2013-06-01

12

Ion implantation induced defects in Fe-Cr alloys studied by conventional positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

The influence of chromium on the radiation damage resistance of the iron based alloys has been studied using conventional positron annihilation lifetime spectroscopy (PALS). Experimental data evaluation has been supported by the former theoretical calculation of positron lifetimes in the studied materials and well-defined types of defects. For this purpose, density functional theory (DFT) computation method has been applied. The spectrum of used 22Na positron source was decomposed into discrete fractions to better calculate efficiency of near surface layers study. For the experimental simulation of a-radiation and obtaining of defined cascade collisions in the materials, helium implantation was used. Different level of the implanted dose (6.24×1017 - 3.12×1018 cm-2) corresponds to local damage up to 90 DPA acquired in thin <1 ?m region. Experimental measurement has been performed using the PALS technique on the four different Fe-Cr binary alloys (2.36; 4.62; 8.39; 11.62 wt% of Cr). The results showed that chromium has a significant effect on the size and density of the implanted defects and specific Cr content should prevent the vacancy clusters formation.

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

2011-01-01

13

Voids in mixed-cation silicate glasses: Studies by positron annihilation lifetime and Fourier transform infrared spectroscopies  

NASA Astrophysics Data System (ADS)

PALS in comparison with FTIR studies have been applied to investigate the structure of different oxide glasses. Three components of the positron lifetime ? (?1 para- and ?3 ortho-positronium and ?2 intermediate lifetime component) and their intensities were obtained. The results of the calculation of mean values of positron lifetimes for the investigated glasses showed the existence of a long-living component on the positron annihilation lifetime spectra. From the Tao-Eldrup formula we can estimate the size of free volume. On the basis of the measurements we can conclude that the size and fraction of free volume reaches the biggest value for the fused silica glass. The degree of network polymerisation increases void size.

Reben, M.; Golis, E.; Filipecki, J.; Sitarz, M.; Kotynia, K.; Jele?, P.; Grelowska, I.

2014-08-01

14

Formation Energy in Al-Mg Alloy by Positron Annihilation Lifetime Technique (PALT)  

NASA Astrophysics Data System (ADS)

The propose of the present work is to study the interaction of positrons with quenched-in defects and clustered atoms to estimate formation enthalpy in series 50xx of commercial Al-Mg alloys, namely, 5049, 5051,5052 and 5083 at various concentrations: 1.9, 2.09, 2.46 and 4.44 wt % of Mg, respectively. Typically additional impurities were mainly Si, Fe, Cu, Cr and Ti. The monvacancy formation energy of Al-Mg alloys was measured from a trapping model analysis of the T-dependence of the positron lifetime.

Abedl-Rahman, Mamduh; Badawi, Emad A.; Hassan, Essmat Mahmoud; Yahya, Gamal

2002-09-01

15

Vacancy migration process in F82H and Fe-Cr binary alloy using positron annihilation lifetime measurement  

NASA Astrophysics Data System (ADS)

Microstructral evolution of electron-irradiated F82H and Fe-8%Cr at 77 K was studied using positron annihilation lifetime measurements. Irradiation-induced vacancies started to migrate at 300 K and 180 K in F82H and Fe-8%Cr, respectively. Solute Cr atoms did not suppress vacancy migration, but they made di-vacancies more stable. Microvoids were not formed by annealing. In F82H, solute atoms acted as trapping site of irradiation-induced defects and annihilation of vacancies and interstitials was facilitated. Pre-existing dislocations and precipitates were also their sinks. These lead to the suppression of microvoids formation. In Fe-8%Cr, small vacancy-type dislocation loops were formed by isochronal annealing test.

Sato, K.; Xu, Q.; Hamaguchi, D.; Huang, S. S.; Yoshiie, T.

2013-06-01

16

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

17

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

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

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

1996-01-01

18

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)

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.

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

2015-01-01

19

Positron annihilation lifetime spectroscopy (PALS) and small angle x-ray scattering (SAXS) of self-assembled amphiphiles  

NASA Astrophysics Data System (ADS)

Self-assembled amphiphile systems are utilized in a wide variety of applications including drug delivery and energy storage. Nano-scale physical and chemical interactions govern the packing of self-assembled amphiphilic molecules, resulting in thermodynamically stable phases of defined geometries. Possible phases include micellar, hexagonal, cubic, lamellar and sponge phases. The internal nano-structure of the amphiphile self assembly materials play an important role in the properties of these systems and their application. To date small angle x-ray scattering (SAXS) has been the most common technique used to characterise their structure. Positron annihilation lifetime spectroscopy (PALS) offers a possible alternative technique as it is sensitive to both the internal cavities and the intermolecular forces and in combination with SAXS, may provide more detailed structural information such as trends with composition and temperature variations. The phase behaviour of a bulk phytantriol sample, consisting of 33 % w/w water was explored using PALS, and it was found that PALS was sensitive to phase transitions from bicontinuous cubic (Pn3m) to reversed hexagonal (H II) to reversed micellar (L II) phases. These boundaries agreed well with SAXS data. Trends observed for the PALS parameters ?3 and I3 as a function of temperature largely supports the concept that the ortho-positronium is annihilating in the organic regions of the self-assembled structure. However, further investigation is required. We have also developed an innovative data analysis technique to analyse PALS spectra for pore information, with the aim of minimising operator bias and error, which leads to better quantitative comparison of PALS results between laboratories.

Dong, Aurelia W.; Pascual-Izarra, Carlos; Dong, Yao-Da; Pas, Steven J.; Hill, Anita J.; Boyd, Ben J.; Drummond, Calum

2007-12-01

20

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

SciTech Connect

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.

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

21

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

PubMed

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

Szabó, Barnabás; Kállai, Nikolett; Tóth, Gerg?; Hetényi, Gergely; Zelkó, Romána

2014-02-01

22

The annihilation of galactic positrons  

NASA Technical Reports Server (NTRS)

The probabilities of various channels of galactic positron annihilation were evaluated and the spectrum of the resulting radiation was calculated. The narrow width (FWHM less than 3.2 keV) of the 0.511 MeV line observed from the galactic center implies that a large fraction of positrons should annihilate in a medium of temperature less than 100,000 K and ionization fraction greater than 0.05. HII regions at the galactic center could be possible sites of annihilation.

Bussard, R.; Rematy, R.

1978-01-01

23

Positron lifetime setup based on DRS4 evaluation board  

NASA Astrophysics Data System (ADS)

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.

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

2014-04-01

24

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

PubMed

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) > Q2?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

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

2015-01-21

25

Positron annihilation induced Auger electron spectroscopy  

NASA Technical Reports Server (NTRS)

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.

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

1990-01-01

26

Nondestructive examination using neutron activated positron annihilation  

DOEpatents

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

Akers, Douglas W. (Idaho Falls, ID); Denison, Arthur B. (Idaho Falls, ID)

2001-01-01

27

Positron annihilation in gamma-ray bursts  

NASA Technical Reports Server (NTRS)

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

Harding, Alice K.

1990-01-01

28

Microstructural Characterization of Thin Polyimide Films by Positron Lifetime Spectroscopy  

NASA Technical Reports Server (NTRS)

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.

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

1996-01-01

29

Application of positron annihilation and Raman spectroscopies to the study of perovskite type materials  

Microsoft Academic Search

Defect properties of perovskite type materials, Ba3B'Nb2O9 (where B'=Mg, Zn, or Co), with near-stoichiometric compositions were studied by positron annihilation and Raman spectroscopies. Theoretical simulations of stoichiometric perovskites revealed a dependence of the positron bulk lifetime on the degree of ordering. In Ba3MgNb2O9 (BMN) the positron bulk lifetime for a completely disordered structure is 195 ps versus 237 ps for

D. Grebennikov; O. Ovchar; A. Belous; P. Mascher

2010-01-01

30

Positron creation and annihilation in tokamak plasmas with runaway electrons.  

PubMed

It is shown that electron-positron pair production is expected to occur in post-disruption plasmas in large tokamaks, including JET and JT-60U, where up to about 10(14) positrons may be created in collisions between multi-MeV runaway electrons and thermal particles. If the loop voltage is large enough, they are accelerated and form a beam of long-lived runaway positrons in the direction opposite to that of the electrons; if the loop voltage is smaller, the positrons have a lifetime of a few hundred ms, in which they are slowed down to energies comparable to that of the cool ( less, similar 10 eV) background plasma before being annihilated. PMID:12689298

Helander, P; Ward, D J

2003-04-01

31

Positron lifetimes of polycrystalline metals: A positron source correction study  

NASA Astrophysics Data System (ADS)

Positron lifetime measurements have been made on a series of polycrystalline metals with atomic numbers varying from 13 to 82, using positron sources deposited on Ni, Al, and Kapton support foils. Bulk positron lifetimes for Al, Ni, Zr, In, and Pb are reported. The results support previous assignments of monovacancy lifetimes for Ag, Ta, and Pb. A method for obtaining the extrinsic positron lifetime components due to the radionuclide source is developed, and the experimentally determined source lifetime components are studied as a function of the sample atomic number. The Z dependence of the source foil component intensities were fitted using known expressions for positron backscatter and multiple scattering within the source sample geometry, neglecting backscatter from the foil. The foil material positron absorption coefficients were obtained and found to be comparable with the results of previous transmission experiments.

McGuire, S.; Keeble, D. J.

2006-11-01

32

Moisture dependence of positron annihilation rates in molecular substances  

NASA Technical Reports Server (NTRS)

Positron annihilation rates have been studied in polymers and graphite-polymer composites as a function of their moisture content. The annihilation rates have been found to increase linearly with increasing moisture content in epoxies and polyamides, whereas no definite trends have been observed in polyimides. These experimental results have been used as the basis for the calculation of moisture content of several polymeric test specimens. For example, the directly measured moisture content of a Kevlar specimen was 45.5 + or - 5.0% of saturation value, whereas the moisture content on the basis of the decrease in positron lifetime was calculated to be 46.5 + or - 3.5%. Similarly, the directly measured moisture content of a graphite-epoxy composite (55 v/o fiber) was 19.2 + or - 0.6% of saturation value as opposed to a calculated value of 16.0 + or - 5.0%.

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

1982-01-01

33

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

PubMed Central

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

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

2013-01-01

34

Vacancy-type defects in cold-worked iron studied using positron annihilation techniques  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime (PAL) and coincidence Doppler broadening of the annihilation radiation (CDBAR) measurements have been investigated on cold-worked iron with different percentages of a deformation up to 40%. The PAL spectra were analyzed into two lifetime components. The shorter lifetimes were explicitly smaller than the monovacancy lifetimes, which were possibly caused by deformation-induced dislocations and vacancy-impurity complexes. The longer lifetimes showed that, the deformation introduces also larger vacancy clusters (consisting of five to ten vacancies) as a result of a cold-working. The behavior of the S-parameter (extracted from CDBAR) with the rolling deformation was analogous to the mean positron lifetime. It increased at up to 5% rolling deformation and leveled off above this percentage of the rolling deformation. The CDBAR ratio curve with respect to pure iron showed that, above the 5% rolling deformation the nature of the defects trapping the positron changed.

Mohamed, Hamdy F. M.; Kwon, Junhyun; Kim, Yong-Min; Kim, Whungwhoe

2007-05-01

35

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

SciTech Connect

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.

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

36

Apparatus for photon activation positron annihilation analysis  

DOEpatents

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.

Akers, Douglas W. (Idaho Falls, ID)

2007-06-12

37

Positron scattering and annihilation from hydrogenlike ions  

SciTech Connect

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

Novikov, S.A.; Bromley, M.W.J.; Mitroy, J. [Faculty of TIE, Charles Darwin University, Darwin NT 0909 (Australia)

2004-05-01

38

Positron annihilation radiation from solar flares  

NASA Technical Reports Server (NTRS)

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

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

1983-01-01

39

Study of Co-containing Ni-Mn-Ga by positron annihilation  

NASA Astrophysics Data System (ADS)

Positron annihilation spectroscopy measurements have been carried out in the ferromagnetic shape memory alloy Ni-Mn-Ga-Co. Positron experiments have been performed at room temperature after subsequent isochronal annealing up to 600 °C. Positron lifetime results have been compared with Differential Scanning Analysis experiments performed in the same samples. Experiments show a large variation of the average positron lifetime values with the annealing temperature of the samples, indicating a clear relation between the concentration of vacancies and the properties of the martensitic transformations of these alloys.

Mérida, D.; García, J. A.; Plazaola, F.; Sanchez-Alarcos, V.; Pérez-Landazábal, J. I.; Recarte, V.

2011-01-01

40

Positron Annihilation Spectroscopy Of High Performance Polymer Films Under CO2 Pressure  

NASA Astrophysics Data System (ADS)

Positron annihilation Lifetime and Doppler broadening measurements are reported for six polymer films as a function of carbon dioxide (CO2) 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 CO2 pressure which may be described by the Henry or Langmuir sorption models, while the positron response of other polymers is rather insensitive to the CO2 pressure. The results demonstrate the usefulness of positron annihilation spectroscopy in investigating the sorption of CO2 into various polymers at pressures up to about 3 atm (45psi).

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

2011-06-01

41

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

E-print Network

The possibility of registration of quantum states, such as the coalescence of droplets in the sol phase of aqueous vanadium pentoxide V$_2$O$_5$, with positron annihilation lifetime spectroscopy is discussed. The decrease of positronium lifetime in the result of the coalescence is explaned.

Elnikova, L V

2009-01-01

42

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

E-print Network

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.

L. V. Elnikova

2010-04-26

43

Positron annihilation study of the Mg-Zn -Y alloys with long period stacking ordered (LPSO) structures  

NASA Astrophysics Data System (ADS)

The Mg-Zn-Y alloys with long period stacking ordered (LPSO) structures have been studied by positron annihilation lifetime (PAL), coincidence Doppler broadening (CDB) and atom probe tomography (APT). The positron lifetime for all the Mg-Zn-Y alloys is in a range of 221~225 ps, very close to the positron lifetime for pure Mg bulk, 222 ps. Low temperature measurements of the positron lifetime also give no evidence for shallow positron trapping sites in the LPSO phases. The CDB shows that most of the positrons are annihilated with electrons of Mg. These results suggest that sub-nano scale open volumes, which were expected to exist in the Zn/Y enriched layers synchronized with stacking faults of the LPSO phases by the first principles calculations, are not present.

Xu, H. X.; Inoue, K.; Nagai, Y.; Toyama, T.; Matsukawa, Y.; Kuramoto, A.; Egusa, D.; Abe, E.; Ye, B. J.

2013-06-01

44

Depth-dependent positron annihilation in different polymers  

NASA Astrophysics Data System (ADS)

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.

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

45

Resolvability of defect ensembles with positron annihilation studies  

SciTech Connect

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

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

1984-11-12

46

Moisture dependence of positron annihilation spectra in nylon-6  

NASA Technical Reports Server (NTRS)

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.

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

1984-01-01

47

Resonances and Bound States in Positron Annihilation on Molecules  

NASA Astrophysics Data System (ADS)

Positron annihilation is important in such diverse areas as study of metabolic processes in the human brain and the characterization of materials. Annihilation on molecules has been a subject of keen interest for decades. In particular, annihilation rates can be orders of magnitude greater than those expected for simple collisions. Recent results put our understanding of many aspects of this long-standing problem on a firm footing. We now understand that the annihilation proceeds by vibrational Feshbach resonances (VFR). A prerequisite for the existence of these VFR is that the positron binds to the target. The annihilation energy spectra provide the best measures to date of positron binding energies. Predictions of a new theory of VFR-enhanced annihilation in small molecules (methyl halides) [1] show excellent, quantitative agreement with experiment. New data and analyses for larger molecules (e.g., hydrocarbons with more than two carbon atoms) show that annihilation rates depend strongly on the number of vibrational degrees of freedom but, surprisingly, only weakly on positron binding energy. This places important constraints on theories of annihilation in these molecules. Results for second bound (i.e., positronically excited) states and overtone and combination-mode VFR, as well as outstanding questions, will also be discussed. This work is done in collaboration with Jason Young. [1] G. F. Gribakin and C. M. R. Lee, Phys. Rev. Lett. 97, 193201 (2006).

Surko, C. M.

2007-10-01

48

Temperature dependence of positron annihilation at dislocations in Pb(Cd)  

NASA Astrophysics Data System (ADS)

We have measured the temperature dependence of positron lifetimes in a Pb96.1Cd3.9 alloy having a dislocation density large enough to significantly affect the e+e- annihilation spectra. These spectra were resolved into two unconstrained lifetimes in addition to a sourceterm lifetime. The longer of the two lifetimes, which is attributed to positrons localized by traps associated with dislocations, decreases with increasing sample temperature T at about 0.1 psec K-1 for 90<=T<=300 K. These data represent the first reported observation of a temperature-dependent lifetime of positrons localized at dislocation traps. It is proposed that the temperature dependence is principally due to atoms penetrating deeper into the traps as T is raised. We suggest that these atomic incursions also effect a negative contribution to the slope of the line-shape parameter S(T). Analysis of the data in terms of the two-state trapping model reveals (i) that both the trapping rate per unit dislocation density and the probability of annihilation occurring at the dislocation traps for constant dislocation density are weakly temperature dependent, and (ii) that during recovery anneals the total trapping rate decreases markedly, whereas little change is exhibited by the lifetime of the trapped positron.

Hu, C.-K.; Gruzalski, G. R.

1983-01-01

49

Positron annihilation characteristics in mesostructural silica films with various porosities  

SciTech Connect

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.

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

50

Positron lifetime spectroscopy in thin polymer coatings  

NASA Technical Reports Server (NTRS)

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.

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

1990-01-01

51

Positron Annihilation Studies In Polymer Nano-Composites  

SciTech Connect

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.

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

52

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

NASA Astrophysics Data System (ADS)

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.

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

53

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

NASA Astrophysics Data System (ADS)

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

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

54

First-principles calculation of positron states and annihilation parameters for group-III nitrides  

NASA Astrophysics Data System (ADS)

We calculated theoretically positron states and annihilation parameters for VGa-C complexes in GaN. Positron lifetimes and Doppler-broadening S-W parameters were evaluated. The obtained results are compared other various types of defects in group-III nitrides. A random alloy In0.5Ga0.5N and its multilayer system together with GaN are investigated also. The results of the present study are expected to be useful for interpretation of experimental results on practical materials.

Ishibashi, S.; Uedono, A.

2014-04-01

55

Positron annihilation induced Auger electron spetroscopy and its implementation at accelerator based low energy positron factories  

NASA Astrophysics Data System (ADS)

Positron annihilation induced auger electron spectroscopy (PAES) makes use of a beam of low energy positrons to excite Auger transitions by annihilating core electrons. The large secondary electron background usually present in Auger spectra can be eliminated by setting the positron beam energy well below the Auger electron energy. This allows true Auger lineshapes to be obtained. Further, because the positron is localized just outside the surface before it annihilates, PAES is extremely sensitive to the topmost atomic layer. Recent PAES results obtained at the University of Texas at Arlington will be presented. In addition, the use of high resolution energy analyzers with multichannel particle detection schemes to prevent problems due to the high data rates associated with accelerator based positron beams will be discussed.

Weiss, Alex; Köymen, Ali R.; Mehl, David; Lee, Kuen-Ho; Yang, Gimo; Jensen, Kjeld

1991-05-01

56

Positron annihilation study of acryl amide/poly (metha acrylic acid) membrane  

NASA Astrophysics Data System (ADS)

Gamma irradiation posses a serious role for casting the membranes. Acryl amide /poly (methacrylic acid) membrane was synthesized under ?-radiation effect. The structure of the membrane was characterized by FTIR, thermo-gravimetric analysis and the scanning electron microscope. The properties of the membranes were also investigated in terms of proton conductivity and positron annihilation lifetime (PAL) parameters. On the basis of the values of the long-lived components in the lifetime spectra, the size of the free volume and their intensity were calculated. The positron lifetime study on these irradiated casted membranes shows that the cross-linking and degradation within the membrane matrix affect the free volume content and hence the microstructure.

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

2011-01-01

57

Positron Annihilation Studies in Search of Fine Precipitates in Fe-9Cr alloys  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime studies were carried out on cold worked pure Fe and Fe-9Cr alloy subjected to isochronal annealing in the temperature range from 300 to 1323 K. The measured lifetimes of Fe-9Cr alloy showed three distinct annealing stages as compared to pure Fe viz., initial annealing of defects, a plateau between 623 K and 873 K and noticeable increase beyond 1123 K. The second annealing stage is likely due to the formation of chromium rich nanoclusters. Third annealing stage beyond 1123 K is attributed to highly defected martensitic phase formation during cooling from y-phase.

Babu, S. Hari; Rajaraman, R.; Govindaraj, R.; Amarendra, G.; Sundar, C. S.

2011-07-01

58

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

SciTech Connect

The emission of positrons from supernova ejecta is dicussed in terms of the galactic-center annihilation radiation. The positrons from the radioactive sequences /sup 56/Ni..-->../sup 56/Co..-->../sup 56/Fe are the most numerous source from supernova. Only type I supernova will allow a significant fraction to escape the expanding ejecta. For a neutron star model of a type I SN a fraction 4 x 10/sup -3/ of the escaped positron is enough to create the observed several year fluctuation of the annihilation radiation. The likelihood of this model is discussed in terms of other astrophysical evidence as well as the type I SN light curve.

Colgate, S.A.

1983-03-17

59

Moisture determination in composite materials using positron lifetime techniques  

NASA Technical Reports Server (NTRS)

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.

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

1980-01-01

60

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

NASA Astrophysics Data System (ADS)

Radiation damage and other defect studies of materials are limited to thin samples because of inherent limitations of well-established techniques such as diffraction methods and traditional positron annihilation spectroscopy (PAS) [P. Hautojarvi, et al., Positrons in Solids, Springer, Berlin, 1979, K.G. Lynn, et al., Appl. Phys. Lett. 47 (1985) 239]. This limitation has greatly hampered industrial and in-situ applications. ISU has developed new methods that use pair-production to produce positrons throughout the volume of thick samples [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427, F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D.P. Wells, et al., Nucl. Instr. and Meth. B. 241 (2005) 262]. Unlike prior work at other laboratories that use bremsstrahlung beams to create positron beams (via pair-production) that are then directed at a sample of interest, we produce electron-positron pairs directly in samples of interest, and eliminate the intermediate step of a positron beam and its attendant penetrability limitations. Our methods include accelerator-based bremsstrahlung-induced pair-production in the sample for positron annihilation energy spectroscopy measurements (PAES), coincident proton-capture gamma-rays (where one of the gammas is used for pair-production in the sample) for positron annihilation lifetime spectroscopy (PALS), or photo-nuclear activation of samples for either type of measurement. The positrons subsequently annihilate with sample electrons, emitting coincident 511 keV gamma-rays [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427, F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D.P. Wells, et al., Nucl. Instr. and Meth. B. 241 (2005) 262]. These gamma-ray photons are then either measured with a high-resolution germanium detector (PAES) or fast scintillators (PALS) and subsequently analyzed using standard positron data analysis methods. The high penetrability of few MeV photons allows one to study defects and characterize materials in thick samples up to hundreds of g/cm2 (approximately a meter in steel), a thickness that is completely inaccessible by any other non-destructive technique. We have demonstrated the proof-of-principle of these techniques to probe tensile strain in thick steel alloys and other metals, to measure positron lifetimes in bulk samples of lead, copper and aluminium with positron lifetime spectra that are free of the surface and source background lifetimes that complicate conventional positron lifetime measurements, and demonstrated the activation technique for damage studies of copper and single-crystal iron [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 192 (2002) 197, F.A. Selim, D.P. Wells, et al., Nucl. Instru. Meth. A 495 (2002) 154, F.A. Selim, et al., J. Rad. Phys. Chem. 68 (2004) 427]. We have also demonstrated the potential application of these techniques to 3-D imaging of defect density in thick structural materials [F.A. Selim, D.P. Wells, et al., Nucl. Instr. and Meth. B 241 (2005) 253, A.W. Hunt, D.P. Wells, et al., Nucl. Instr. and Meth. B. 241 (2005) 262].

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

2006-06-01

61

Recent progress in understanding positron annihilation on molecules  

NASA Astrophysics Data System (ADS)

Annihilation at positron energies in the range of the molecular vibrational modes is dominated by large-amplitude vibrational Feshbach resonances (VFR) in which the positron attaches to the molecule.footnotetextG. F. Gribakin, J. A. Young, C. M. Surko, Rev. Mod. Phys. 82, 2557 (2010). Recently, a broad spectrum of enhanced annihilation has been discovered and is observed in the spectra of many, if not most, molecules.footnotetextA. C. L. Jones, et al., Phys. Rev. Lett., 108, 093201 (2012). This spectral component, known as statistical multimode resonant annihilation (SMRA), dominates the spectra in small molecules with relatively large binding energies, such as CCl4 and CBr4. Incorporation of an SMRA spectral component has allowed for a more accurate probe of VFR magnitudes and is providing insight into the process of intramolecular vibrational redistribution (IVR), through which VFRs can be either enhanced or suppressed.

Jones, A. C. L.; Danielson, J. R.; Natisin, M. R.; Surko, C. M.

2012-10-01

62

Positron annihilation response and broadband dielectric spectroscopy: salol.  

PubMed

A phenomenological analysis of the ortho-positronium (o-Ps) annihilation from positron annihilation lifetime spectroscopy (PALS) and the dynamics from broadband dielectric spectroscopy (BDS) are reported on a small molecular glass former of intermediate H-bonding and fragility: salol. The dielectric spectra extend over a very broad frequency range of about 2 × 10(-2)-3.5 × 10(11) Hz, providing information on the ?-relaxation, the secondary relaxation giving rise to the excess wing, and the shallow high-frequency minimum in the micro- to milli-meter wave range. A number of empirical correlations between the o-Ps lifetime, ?(3)(T), and the various spectral and relaxation features have been observed. Thus, the phenomenological evaluation of the ?(3)(T) dependence of the PALS response of the amorphous sample reveals three characteristic PALS temperatures: T(g)(PALS), T(b1)(L) = 1.15T(g)(PALS) and T(b2)(L) = 1.25T(g)(PALS), which are discussed in relation to similar findings for some typical small molecular vdW- and H-bonded glass formers. A slighter change of the slope at T(b1)(L) appears to be related to the transition from excess wing to the primary ?-process-dominated behavior, with the secondary process dominating in the deeply supercooled liquid state below T(b1)(L). The high-temperature plateau effect in the ? (3)(T) plot occurs at T(b2)(L) and agrees with the characteristic Stickel temperature, T(B)(ST), marking a qualitative change of the primary ? process, but it does not follow the relation T(b2)(L) < T(?) [?(3)(T(b2)) < ?(?)]. Both effects at T(b1)(L) and T(b2)(L) correlate with two crossovers in the spectral shape and related non-exponentiality parameter of the structural relaxation, ? (KWW). Finally, the application of the two-order parameter (TOP) model to the structural relaxation as represented by the primary ? relaxation times from BDS leads to the characteristic TOP temperature, T(m)(c), close to T(b1) from PALS. Within this model the phenomenological interpretation is offered based on changes in the probability of occurrence of solid-like and liquid-like domains to explain the dynamic as well as PALS responses. In summary, all the empirical correlations support further very close connections between the PALS response and the dielectric relaxation behavior in small molecule glass formers. PMID:21947898

Bartoš, J; Iskrová, M; Köhler, M; Wehn, R; Sauša, O; Lunkenheimer, P; Krištiak, J; Loidl, A

2011-09-01

63

Positron annihilation studies on the electronic structure of Ni1-xCux alloys  

NASA Astrophysics Data System (ADS)

The entire concentration range of ?-phase Ni1-xCux alloys has been studied by Angular Correlation and Positron Lifetime methods. The results show inapplicability of the Rigid Band Model to description of the electronic structure of this alloy. Linearity of annihilation rate ?(x) as well as of parameters of correlation curves leading to linearity of s-like and d-like electron numbers with x confirms our earlier expectation as to the character of annihilation parameters for split-band alloys with constant Fermi energy. On the ground of the present results and the previous ones for palladium alloys it was postulated that the behaviour of the bulk annihilation rate in binary alloys can predict which type of the electronic structure, split-band or VCA, is the appropriate one in the given case.

Debowska, E.; Rudzinska-Girulska, J.

1989-06-01

64

Defect evolution during the phase transition of hexagonal nickel sulfide studied by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

The defect structures of the hexagonal nickel sulfide with different Ni contents (Ni1-?S) have been investigated mainly by the positron annihilation lifetime spectroscopy and the temperature-dependent positron Doppler broadening technique. Combining with theoretical calculations, it is convincingly found that there is a high concentration of Ni-monovacancies in Ni1-?S not only in the low temperature (LT) phase but also in the high temperature (HT) phase. The defect concentration and the trapping functionality for positrons have almost no changes before and after the phase transition. Besides, the transition temperature decreases and the thermal hysteresis width of the phase transition narrows down with the reduction of Ni contents, since the existence of Ni-monovacancies can lower the energy requirement of the transformation and thus promote the transition occurrence.

Fan, Shaojuan; Zhang, Jie; Xiao, Chong; Li, Zhou; Li, Qiang; Xie, Yi; Ye, Bangjiao

2015-01-01

65

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

NASA Astrophysics Data System (ADS)

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.

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

2014-11-01

66

Positron annihilation spectroscopy of biological tissue in 11C irradiation  

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

67

Defect evolution during annealing of deformed FeSi alloys studied by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

High silicon steel is widely used in electrical appliances. Alloying iron with silicon improves its magnetic performance. A silicon content up to 6.5 wt. % gives excellent magnetic properties such as high saturation magnetization, near zero magnetostriction and low iron loss in high frequencies. Their workability is greatly reduced by the appearance of ordered structures, namely B2 and D03, as soon as the Si content becomes higher than 3.5 wt. %. This limits the mass production by conventional rolling to this maximum percentage of Si. In this work a series of FeSi (7.5 wt. % Si) samples with different degrees of deformation are investigated with positron annihilation spectroscopy and optical microscopy (OM). The influence of annealing on the concentration of defects of different deformed FeSi alloys has been investigated by positron annihilation lifetime spectroscopy and Doppler broadening of the annihilation radiation. OM is used to investigate the microstructure of deformed samples before and after annealing. The values of the S parameter present a decrease for all studied FeSi alloys with the increase of the annealing temperature, being attributed to a decrease of the concentration of defects. A sudden increase of the S-parameter value at 600 °C was observed for all samples, which could be related to the change of the ordering of the FeSi alloys at that temperature. At 700 °C, the values of the S parameter decreased drastically and starting from 900 °C, they became constant. The microstructures of the alloys, investigated by OM, show that recrystallization is completed at 900 °C and the samples are mainly free of defects, which is in agreement with the positron annihilation lifetime data.

Mostafa, K. M.; Cámara, F. González; Petrov, Roumen; Calvillo, P. Rodríguez; De Grave, E.; Segers, D.; Houbaert, Y.

2011-04-01

68

Photon-induced positron annihilation for standoff bomb detection  

Microsoft Academic Search

We describe an approach to detect improvised explosive devices (IEDs) by using photon-induced positron annihilation radiation (PIPAR). This system relies on back-scattered ? photons from the target and surrounding objects following exposure to high energy X-rays from a betatron. In this work we simulate the use of Bremsstrahlung source operating at 3.5MeV, with a scintillation detector, working in PIPAR mode,

D. A. Bradley; S. Hashim; J. Cabello; K. Wells; W. L. Dunn

2010-01-01

69

Using the method of positron annihilation for detecting defects in structural alloys caused by fatigue  

SciTech Connect

This paper examines the possibilities of using the method ofpositron annihilation for detecting fatigue defects in structural alloys with various types of crystal structure. The parameter of the positron annihilation process most suitable for the inspection was determined.

Arefev, K.P.; Boev, O.V.; Chernitsyn, A.I.; Polukhin, Y.E.; Vordb'ev, S.A.

1986-12-01

70

Behaviour of vacancies in dilute Fe-Re alloys: a positron annihilation study  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy was used in a room temperature study of the influence of heat treatment on behaviour of vacancies in Fe0.97Re0.03 and Fe0.94Re0.06 alloys. In this experiment, the vacancies were created during the formation and further mechanical processing of the iron systems under consideration so the lifetime spectra of positrons were collected at least twice. The first samples were taken just after the melting process in an arc furnace, and the second ones were taken for the specimens annealed at 1,270 K and then cold-rolled at room temperature. After that, the spectra were measured for all studied samples after annealing at some temperatures gradually increasing from 300 to 1,270 K. It was found that vacancy-Re pairs are the dominant type of structural defects in alloys just after the melting process. In the case of alloys after a cold rolling process, the dominant type of structural defects is vacancies associated with edge dislocations. Moreover, for cold-rolled samples annealed at 473-573 K, the growth of the vacancy clusters associated with edge dislocations is observed by an increase in the mean positron lifetime. Finally, at temperatures above 573 K, vacancy clusters associated with edge dislocations as well as vacancy-Re pairs become unstable, and freely migrating vacancies sink at grain boundaries.

Idczak, R.; Konieczny, R.

2014-12-01

71

Statistical Multimode Resonant Annihilation of Positrons on Molecules  

NASA Astrophysics Data System (ADS)

Annihilation at positron energies in the range of the molecular vibrational modes is dominated by large-amplitude vibrational Feshbach resonances (VFR) in which the positron attaches to the molecule.ootnotetextG. F. Gribakin, J. A. Young, C. M. Surko, Rev. Mod. Phys. 82, 2557 (2010). In small molecules, there is a quantitative description of the annihilation rates, Zeff, due to the VFR.ootnotetextG. F. Gribakin, C. M. R. Lee, Phys. Rev. Lett. 97, 193201 (2006). Here we focus on a broad spectrum of enhanced annihilation that is observed in the spectra of many, if not most, molecules.ootnotetextA. C. L. Jones, et al., Phys. Rev. Lett., in press (2012). This spectral component, for example, dominates the spectra in small molecules with relatively large binding energies, such as CCl4 and CBr4. A model that assumes excitation and escape from a statistically complete ensemble of multimode vibrations is presentedootnotetextG. F. Gribakin, C. M. R. Lee, European Phys. J. D 51, 51 (2009). that reproduces key features of the data. Related issues of intramolecular vibrational redistribution (IVR), and the effects of escape channels on the primary VFRs will also be discussed.

Natisin, M. R.; Jones, A. C. L.; Danielson, J. R.; Surko, C. M.; Gribakin, G. F.

2012-06-01

72

Fatigue damage in superalloys determined using Doppler broadening positron annihilation  

NASA Technical Reports Server (NTRS)

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.

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

1990-01-01

73

Development of positron annihilation spectroscopy for characterizing neutron irradiated tungsten  

SciTech Connect

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

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

2013-05-01

74

Electron-positron annihilation lines and decaying sterile neutrinos  

NASA Astrophysics Data System (ADS)

If massive sterile neutrinos exist, their decays into photons and/or electron-positron pairs may give rise to observable consequences. We consider the possibility that MeV sterile neutrino decays lead to the diffuse positron annihilation line in the Milky Way center, and we thus obtain bounds on the sterile neutrino decay rate ? e ?10-28 s-1 from relevant astrophysical/cosmological data. Also, we expect a soft gamma flux of 1.2×10-4-9.7×10-4 ph cm-2 s-1 from the Milky Way center which shows up as a small MeV bump in the background photon spectrum. Furthermore, we estimate the flux of active neutrinos produced by sterile neutrino decays to be 0.02-0.1 cm-2 s-1 passing through the earth.

Chan, M. H.; Chu, M.-C.

2012-04-01

75

Moisture dependence of positron lifetime in Kevlar-49  

NASA Technical Reports Server (NTRS)

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.

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

1984-01-01

76

Defects in nitride-based semiconductors probed by positron annihilation  

NASA Astrophysics Data System (ADS)

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.

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

2014-04-01

77

Nonstoichiometry accommodation in SrTiO3 thin films studied by positron annihilation and electron microscopy  

NASA Astrophysics Data System (ADS)

Accommodation of nonstoichiometry in SrTiO3 pulsed laser deposited (PLD) films was investigated using positron annihilation lifetime spectroscopy and (scanning) transmission electron microscopy. Increasing PLD laser fluence changed the stoichiometry from Ti to Sr deficient. Cation vacancy defects were detected, and the concentration ratio of Sr to Ti vacancies, [VSr]/[VTi], was observed to increase systematically in the Sr-deficient region, although no change in the electron microscopy lattice images was detected. Increasing Ti deficiency resulted in the accommodation of SrO layers in planar defects, and in the formation of vacancy cluster defects. A change from VTi to VSr defect positron trapping was also detected.

Keeble, D. J.; Wicklein, S.; Jin, L.; Jia, C. L.; Egger, W.; Dittmann, R.

2013-05-01

78

Calculation of the Doppler broadening of the electron-positron annihilation radiation in defect-free bulk materials  

SciTech Connect

Results of a calculation of the Doppler broadening of the positron-electron annihilation radiation and positron lifetimes in a large number of elemental defect-free materials are presented. A simple scheme based on the method of superimposed atoms is used for these calculations. Calculated values of the Doppler broadening are compared with experimental data for a number of elemental materials, and qualitative agreement is obtained. These results provide a database which can be used for characterizing materials and identifying impurity-vacancy complexes. (c) 2000 The American Physical Society.

Ghosh, V. J. [Brookhaven National Laboratory, Upton, New York 11973 (United States)] [Brookhaven National Laboratory, Upton, New York 11973 (United States); Alatalo, M. [Helsinki Institute of Physics and Laboratory of Physics, P.O. Box 1100, 02015, University of Helsinki, Helsinki, (Finland)] [Helsinki Institute of Physics and Laboratory of Physics, P.O. Box 1100, 02015, University of Helsinki, Helsinki, (Finland); Asoka-Kumar, P. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States)] [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States); Nielsen, B. [Brookhaven National Laboratory, Upton, New York 11973 (United States)] [Brookhaven National Laboratory, Upton, New York 11973 (United States); Lynn, K. G. [Materials Research Center, Washington State University, Pullman, Washington 99613 (United States)] [Materials Research Center, Washington State University, Pullman, Washington 99613 (United States); Kruseman, A. C. [Interfaculty Reactor Institute, Delft University of Technology, 2629 JB Delft, The Netherlands (Netherlands)] [Interfaculty Reactor Institute, Delft University of Technology, 2629 JB Delft, The Netherlands (Netherlands); Mijnarends, P. E. [Interfaculty Reactor Institute, Delft University of Technology, 2629 JB Delft, The Netherlands (Netherlands)] [Interfaculty Reactor Institute, Delft University of Technology, 2629 JB Delft, The Netherlands (Netherlands)

2000-04-15

79

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

PubMed

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

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

2010-09-01

80

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

PubMed

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

Dryzek, E; Juszy?ska, E; Zaleski, R; Jasi?ska, B; Gorgol, M; Massalska-Arod?, M

2013-08-01

81

Positron lifetime calculation for the elements of the periodic table  

Microsoft Academic Search

Theoretical positron lifetime values have been calculated systematically for most of the elements of the periodic table. Self-consistent and non-self-consistent schemes have been used for the calculation of the electronic structure in the solid, as well as different parametrizations for the positron enhancement factor and correlation energy. The results obtained have been studied and compared with experimental data, confirming the

J. M. Campillo Robles; E. Ogando; F. Plazaola

2007-01-01

82

The Influence of Heat Treatment and Mechanical Work on the Lifetimes of Positrons in Fe, Pd and Pd-H  

NASA Astrophysics Data System (ADS)

The influence of annealing temperature and stretching with external forces on lifetimes of positrons in Fe, Pd and Pd-H0.05 was investigated. It was found that in all of the examined cases the best fitting of the obtained spectrum of lifetimes of positrons to the theoretical model can be achieved with the assumption that the spectra consist of two components of lifetimes ?1 and ?2 and intensities I1 and I2, respectively. In rolled Fe and Pd, at a high degree of cold work, all the positrons annihilate from the trapping state in two types of defects of clearly different lifetimes. It was observed that dislocations in Fe become active in temperatures exceeding 923 K and in Pd-above 623 K. Deformation through one-axis stretching of Fe, Pd and PdH0.05 within the range of proportional elastic stresses, causes a decrease in lifetimes of positrons in defects of larger sizes. A hysteresis of component ?2 was observed at returning from the stressed state to the free one. Within the range of plastic deformations, changes in the long life component of the spectrum of positron lifetimes correlate with the changes in the length resulting from stretching with external forces.

Pietrzak, R.; Szatanik, R.; Smiatek, W.

83

Microstructural Characterization of Polymers by Positron Lifetime Spectroscopy  

NASA Technical Reports Server (NTRS)

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.

Singh, Jag J.

1996-01-01

84

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

SciTech Connect

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.

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

85

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

SciTech Connect

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

Mitroy, J. [Faculty of Technology, Charles Darwin University, Darwin, Northern Territory 0909 (Australia)

2005-12-15

86

Design concept and modeling of a new Positron Identification by Coincident Annihilation Photons (PICAP) system  

Microsoft Academic Search

The Positron Identification by Coincident Annihilation Photons (PICAP) system is a new design concept to measure moderate energy (?few MeV) positrons and negatrons in space. Positron measurements in this energy range would open a window on cosmic ray propagation in the Galaxy, Solar modulation of cosmic rays (particularly charge-sign effects) and Solar energetic particles. Electrons of both charge signs are

J. J. Connell; J. R. Kalainoff; C. Lopate

2008-01-01

87

Glucose and water diffusion kinetics study in a fluorosilicone acrylate contact lens material by positron lifetime spectroscopy  

Microsoft Academic Search

A comparative study of water and glucose sorption in a rigid gas-permeable contact lens polymer, Fluoroperm 92, has been carried out using positron annihilation lifetime spectroscopy, gravimetric analysis and refractive index measurements. The water sorption in FP92 is non-Fickian, whereas for glucose sorption it changes to Fickian. Glucose molecules reduce the interaction of water molecules with the polymer, resulting in

M. V. Deepa Urs; C. Ranganathaiah

2007-01-01

88

Defects and hyperfine interactions in binary Fe-Al alloys studied by positron annihilation and Mössbauer spectroscopies  

NASA Astrophysics Data System (ADS)

The defects, the behavior of 3d electrons and the hyperfine interactions in binary Fe-Al alloys with different Al contents have been studied by measurements of positron lifetime spectra, coincidence Doppler broadening spectra of positron annihilation radiation and Mössbauer spectra. The results show that on increasing the Al content in Fe-Al alloys, the mean positron lifetime of the alloys increase, while the mean electron density of the alloys decrease. The increase of Al content in binary Fe-Al alloys will decrease the amount of unpaired 3d electrons; as a consequence the probability of positron annihilation with 3d electrons and the hyperfine field decrease rapidly. Mössbauer spectra of binary Fe-Al alloys with Al content less than 25 at.% show discrete sextets and these alloys make a ferromagnetic contribution at room temperature. The Mössbauer spectrum of Fe70Al30 shows a broad singlet. As Al content higher than 40 at.%, the Mössbauer spectra of these alloys are singlet, that is, the alloys are paramagnetic. The behavior of a 3d electron and its effect on the hyperfine field of the binary Fe-Al alloy has been discussed.

Deng, Wen; Sun, Xiao-Xiang; Tan, Shao-Xi; Li, Yu-Xia; Xiong, Ding-Kang; Huang, Yu-Yang

2013-12-01

89

Positron annihilation induced Auger electron emission from silicon carbide surfaces  

NASA Astrophysics Data System (ADS)

Silicon carbide (SiC) in monocrystalline, hexagonal polytype form is a very interesting material for a wide class of novel applications in electronics. SiC fiber-reinforced SiC matrix composite materials (SiC/SiC) are considered to be the attractive candidates as materials for advanced energy systems, such as high performance combustion systems, fuel-flexible gasification systems, fuel cell / turbine hybrid systems, nuclear fusion reactors, and high temperature gas-cooled fission reactors. Positron Annihilation induced Auger Electron Spectroscopy (PAES) is an established tool to characterize the top most atomic surface layer of solids. Here, PAES has been used to study the surface of 6H-SiC after annealing under different thermal and ambient conditions. In addition, results of investigating the surface of a composite consisting of sintered SiC Nanopowder and fibres of pyrolytic carbon are presented and discussed. This research supported by the Welch Foundation under Y-1100.

Mukherjee, S.; Nadesalingam, M.; Brauer, G.; Nozawa, T.; Kohyama, A.; Weiss, A. H.

2006-03-01

90

Photon-induced positron annihilation for standoff bomb detection  

NASA Astrophysics Data System (ADS)

We describe an approach to detect improvised explosive devices (IEDs) by using photon-induced positron annihilation radiation (PIPAR). This system relies on back-scattered ? photons from the target and surrounding objects following exposure to high energy X-rays from a betatron. In this work we simulate the use of Bremsstrahlung source operating at 3.5 MeV, with a scintillation detector, working in PIPAR mode, in order to reduce noise produced by undesired back-scattering from the surrounding objects. In this paper, we describe the basic imaging method and preliminary results on simulating a suitable betatron source. Two types of X-ray filters copper (Cu) and aluminium (Al), have been used in the simulation to observe their differences in the deposited energy spectrum in the iron target. It was found that the use of iron target in conjunction with 2 mm Al filter is capable of detecting annihilation ? photons. An initiated experiment with an interlaced source also shows promise.

Bradley, D. A.; Hashim, S.; Cabello, J.; Wells, K.; Dunn, W. L.

2010-07-01

91

Free volume in Zr-based bulk glassy alloys studied by positron annihilation techniques  

NASA Astrophysics Data System (ADS)

The eutectic Zr50Cu40Al10 bulk glassy (BG) alloys, which have a good glass forming ability, have been investigated for various properties, such as toughness, electronic resistivity and so on. Recently, the hypoeutectic Zr60Cu30Al10 BG alloys have been paid attention, because they exhibit no degradation of ductility and toughness after the isothermal annealing below the glass transition temperature (Tg). Although the toughness correlates with the free volume to be considered, the free volume of the hypoeutectic BG alloy has not been assessed in detail so far. In order to study the free volume in each BG alloy, therefore, positron annihilation lifetime and coincidence Doppler broadening (CDB) measurements have been performed for Zr60Cu30Al10 (hypoeutectic) and Zr55Cu35Al10 (middle-composition) and Zr50Cu40Al10 (eutectic) BG alloys. The value of the positron lifetime for hypoeutectic BG alloy is virtually the same. Additionally, the CDB ratio curve for hypoeutectic alloy dose not match that for eutectic alloy in an electron momentum region around 0.015 m0c, indicating that the fraction of Zr atoms around the free volume in hypoeutectic BG alloy is greater than that in eutectic BG alloy. Moreover, the CDB results show that the local structure around free volume for hypoeutectic BG alloy is different from that for eutectic BG alloy.

Ishii, A.; Iwase, A.; Yokoyama, Y.; Konno, T. J.; Kawasuso, Y.; Yabu-uchi, A.; Maekawa, M.; Hori, F.

2010-04-01

92

Development of positron annihilation spectroscopy for characterizing neutron irradiated tungsten  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

93

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

NASA Technical Reports Server (NTRS)

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

Turner, Michael S.; Wilczek, Frank

1989-01-01

94

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

SciTech Connect

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.

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

1990-10-15

95

Low-temperature positron annihilation study of B+-ion implanted PMMA  

NASA Astrophysics Data System (ADS)

Temperature dependent positron annihilation lifetime spectroscopy (PALS) measurements in the range of 50-300 K are carried out to study positronium formation in 40 KeV B+-ion implanted polymethylmethacrylate (B:PMMA) with two ion doses of 3.13 × 1015 and 3.75 × 1016 ions/cm2. The investigated samples show the various temperature trends of ortho-positronium (o-Ps) lifetime ?3 and intensity I3 in PMMA before and after ion implantation. Two transitions in the vicinity of ˜150 and ˜250 K, ascribed to ? and ? transitions, respectively, are observed in the PMMA and B:PMMA samples in consistent with reference data for pristine sample. The obtained results are compared with room temperature PALS study of PMMA with different molecular weight (Mw) which known from literature. It is found that B+-ion implantation leads to decreasing Mw in PMMA at lower ion dose. At higher ion dose the local destruction of polymeric structure follows to broadening of lifetime distribution (hole size distribution).

Kavetskyy, T. S.; Tsmots, V. M.; Voloshanska, S. Ya.; Šauša, O.; Nuzhdin, V. I.; Valeev, V. F.; Osin, Y. N.; Stepanov, A. L.

2014-08-01

96

Positron Annihilation Induced Auger Electron Spectroscopic Studies Of Reconstructed Semiconductor Surfaces  

NASA Astrophysics Data System (ADS)

The positron annihilation induced Auger spectrum from GaAs(100) displays six As and three Ga Auger peaks below 110 eV corresponding to M4,5VV, M2M4V, M2,3M4,5M4,5 Auger transitions for As and M2,3M4,5M4,5 Auger transitions for Ga. The integrated Auger peak intensities have been used to obtain experimental annihilation probabilities of surface trapped positrons with As 3p and 3d and Ga 3p core level electrons. PAES data is analyzed by performing calculations of positron surface and bulk states and annihilation characteristics of surface trapped positrons with relevant Ga and As core level electrons for both Ga- and As-rich (100) surfaces of GaAs, ideally terminated, non-reconstructed and with (2×8), (2×4), and (4×4) reconstructions. The orientation-dependent variations of the atomic and electron densities associated with reconstructions are found to affect localization of the positron wave function at the surface. Computed positron binding energy, work function, and annihilation characteristics demonstrate their sensitivity both to chemical composition and atomic structure of the topmost layers of the surface. Theoretical annihilation probabilities of surface trapped positrons with As 3d, 3p, and Ga 3p core level electrons are compared with the ones estimated from the measured Auger peak intensities.

Fazleev, N. G.; Reed, J. A.; Starnes, S. G.; Weiss, A. H.

2011-06-01

97

Polarized hadron pair production from electron-positron annihilation  

E-print Network

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 $e^+e^- \\to \\gamma^* \\to h_ah_bX$. 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 $e^+e^- \\to Z^* \\to h_ah_bX$ (including $\\gamma$-$Z$ interference) within this model. This is the first time a complete framework has been presented for the examination of TMD FFs within $e^+e^-\\to h_ah_bX$. 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 $e^+e^-\\to h_ah_bX$ results for future experiments at $e^+e^-$ machines.

D. Pitonyak; M. Schlegel; A. Metz

2014-03-17

98

Timelike Virtual Compton Scattering from Electron-Positron Radiative Annihilation  

SciTech Connect

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

Afanasev, Andrei; /Hampton U. /Jefferson Lab; Brodsky, Stanley J.; /SLAC; Carlson, Carl E.; /William-Mary Coll.; Mukherjee, Asmita; /Indian Inst. Tech., Mumbai

2009-03-31

99

Timelike Virtual Compton Scattering from Electron-Positron Radiative Annihilation  

SciTech Connect

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

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

2009-01-01

100

Positron probes of the Ge(1 0 0) surface: The effects of surface reconstructions and electron positron correlations on positron trapping and annihilation characteristics  

NASA Astrophysics Data System (ADS)

Positron annihilation induced Auger electron spectroscopy (PAES) has been applied to study the Ge(1 0 0) surface. The high-resolution PAES spectrum from the Ge(1 0 0) surface displays several strong Auger peaks corresponding to M 4,5N 1N 2,3, M 2,3M 4,5M 4,5, M 2,3M 4,5V and M 1M 4,5M 4,5 Auger transitions. The integrated peak intensities of Auger transitions are used to obtain experimental annihilation probabilities for the Ge 3d and 3p core level electrons. These experimental results are analyzed by performing calculations of positron surface states and annihilation characteristics of surface trapped positrons with relevant Ge core-level electrons for the non-reconstructed and reconstructed Ge(1 0 0)-p(2 × 1), Ge(1 0 0)-p(2 × 2) and Ge(1 0 0)-c(4 × 2) surfaces. It is found that the positron surface state wave function extends into the Ge lattice in the regions where atoms are displaced from their ideal terminated positions due to reconstructions. Estimates of the positron binding energy and the positron annihilation characteristics reveal their sensitivity to the specific atomic structure of the topmost layers of Ge(1 0 0). A comparison with PAES data reveals an agreement with theoretical core annihilation probabilities for the Auger transitions considered.

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

2007-08-01

101

A positron lifetime study of lanthanum and niobium doped Pb(Zr0.6Ti0.4)O3  

NASA Astrophysics Data System (ADS)

A study of vacancy-related defects in lanthanum and niobium doped PbZr0.6Ti0.4O3 with dopant concentrations of 0-6 and 0-4mol%, respectively has been performed using positron annihilation spectroscopy X-ray diffraction, and photoelectron spectroscopy. Positron lifetime as well as coincidence annihilation radiation Doppler line broadening measurements were carried out. It was found that the samples exhibit vacancylike defects that act as positron traps. Two main defect lifetime components were found in both sample sets one at ?150ps and one at ?300ps. These defect trapping sites can be attributed to single oxygen vacancies and A-site vacancies, respectively. Doppler line broadening measurements, however, do not show significant changes as a function of dopant concentrations in terms of shape S and wing W parameters.

Gottschalk, S.; Hahn, H.; Balogh, A. G.; Puff, W.; Kungl, H.; Hoffmann, M. J.

2004-12-01

102

Free volume study of 70 MeV carbon induced modification in polymers through positron annihilation  

NASA Astrophysics Data System (ADS)

Free volume properties of polymers have strong correlation with macroscopic properties and can be modified by radiations and ions. Positron annihilation lifetime spectroscopy (PALS) provide direct information about the dimension, content and size distribution of free volume holes in polymers. CR-39 (DOP), a polycarbonate widely used as ion track detector and polyamide Nylon-6, a high performance plastic having a unique combination of superior mechanical, electrical, chemical and thermal properties were irradiated with 70 MeV C 5+ ion beam to different fluences ranging from 10 11 to 3.7 × 10 13 ions/cm 2. The results of characterization by PALS are reported here. o-Ps lifetime and the average free volume for both the polymers are found to decrease with fluence, indicating the facilitation of cross-linking. The results are interpreted in terms of change in the free volume. Maximum change of 9.7% in average free volume was observed in PN-6 irradiated to 3.7 × 10 13 ions/cm 2.

Kumar, Rajesh; Ali, S. A.; Mahur, A. K.; Das, D.; Naqvi, A. H.; Virk, H. S.; Prasad, Rajendra

2006-03-01

103

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

PubMed

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

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

2013-02-01

104

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

NASA Astrophysics Data System (ADS)

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.

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

105

Characterization of defects in (ZnMg)Se compounds by positron annihilation and photoluminescence  

NASA Astrophysics Data System (ADS)

Defect characterization of as-grown Zn1-xMgxSe mixed crystals (0?x<0.6) and the effect of Zn vapor annealing has been studied by positron lifetime and photoluminescence measurements. We obtain both experimental and theoretical evidence that the bulk lifetime of free positrons increases linearly with Mg alloying. The average positron lifetime increases with temperature indicating that both vacancies and negative ions trap positrons. The decompositions of the lifetime spectra show that the vacancy has the characteristic positron lifetime of 325 ps. The comparison with theoretical calculations indicate that the lifetime 325 ps corresponds either to divacancies relaxed inwards or to monovacancies strongly relaxed outwards. We consider the latter identification more likely and attribute the positron lifetime 325 ps to Zn vacancy or a complex involving VZn. The vacancy concentration is almost independent of Mg content above x=0.2 but decreases strongly at x=0.56. The Zn vapor annealing decreases the concentration of Zn vacancies. The intensity of the green photoluminescence correlates with the concentration of VZn both as a function of Mg alloying and Zn vapor annealing. We thus conclude that the electron levels of the Zn vacancy are involved in the optical transition leading to the green photoluminescence.

Plazaola, F.; Saarinen, K.; Dobrzynski, L.; Reniewicz, H.; Firszt, F.; Szatkowski, J.; Meczynska, H.; Legowski, S.; Chabik, S.

2000-08-01

106

Advanced fitting algorithms for analysing positron annihilation lifetime spectra  

NASA Astrophysics Data System (ADS)

The most common way to analyse PALS spectra involves fitting a parameter-dependent model to the experimental data. Traditionally, this fit involves local non-linear optimisation routines that depend on a reasonable initial guess for the searched parameters. This, together with the fact that very different sets of parameters may yield indistinguishably good fits for a given experimental spectrum, gives rise to ambiguities in the data analysis in most but the simplest cases. In order to alleviate these difficulties, a computer program named PAScual was developed that incorporates 2 advanced algorithms to provide a robust fitting tool: on the one hand, it incorporates a global non-linear optimisation routine based on the Simulated Annealing algorithm and, on the other hand, it yields information on the reliability of the results by means of a Markov Chain Monte-Carlo Bayesian Inference method. In this work the methods used in PAScual are described and tested against both simulated and experimental spectra, comparing the results with those from the well-established program LTv9. The examples focus on the type of complex data that results from the study of self-assembled amphiphile materials containing co-existing aqueous and hydrocarbon regions.

Pascual-Izarra, Carlos; Dong, Aurelia W.; Pas, Steven J.; Hill, Anita J.; Boyd, Ben J.; Drummond, Calum J.

2009-05-01

107

Energies 0.16 and 0.53 ev for Dislocation Recovery in AlSi11.35Mg0.23 by Positron Lifetime  

NASA Astrophysics Data System (ADS)

The recovery behavior of 20% plastically deformed AlSi11.35Mg0.23 in various stages of isochronal annealing is investigated by positron lifetime (LT). Experimental results show that the positron mean lifetime is a function of annealing temperature. The lifetime of the positron annihilating in a perfect lattice is 187.3 ps. It is 229.8 ps in a 20% deformed one. There are two regions in the isochronal annealing, one of them is related to the point defect and the other to the dislocation. The activation enthalpy for the dislocation is calculated from the isothermal study in the dislocation region from 575-675 K by slow and fast cooling and it is 0.16 ± 0.02 and 0.53 ± 0.06 eV, respectively.

Abdel-Rahman, M. A.; Abdallah, M. S.; Badawi, Emad A.

108

Positron Annihilation Induced Auger Electron Spectroscopic Studies Of Reconstructed Semiconductor Surfaces  

Microsoft Academic Search

The positron annihilation induced Auger spectrum from GaAs(100) displays six As and three Ga Auger peaks below 110 eV corresponding to M4,5VV,M2M4V,M2,3M4,5M4,5 Auger transitions for As and M2,3M4,5M4,5 Auger transitions for Ga. The integrated Auger peak intensities have been used to obtain experimental annihilation probabilities of surface trapped positrons with As 3p and 3d and Ga 3p core level electrons.

N. G. Fazleev; J. A. Reed; S. G. Starnes; A. H. Weiss

2011-01-01

109

Positron Annihilation Induced Auger Electron Spectroscopic Studies Of Reconstructed Semiconductor Surfaces  

Microsoft Academic Search

The positron annihilation induced Auger spectrum from GaAs(100) displays six As and three Ga Auger peaks below 110 eV corresponding to M4,5VV, M2M4V, M2,3M4,5M4,5 Auger transitions for As and M2,3M4,5M4,5 Auger transitions for Ga. The integrated Auger peak intensities have been used to obtain experimental annihilation probabilities of surface trapped positrons with As 3p and 3d and Ga 3p core

N. G. Fazleev; J. A. Reed; S. G. Starnes; A. H. Weiss

2011-01-01

110

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

PubMed

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

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

2009-07-17

111

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

SciTech Connect

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.

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

1987-09-01

112

Study on Momentum Density of Electrons and Fermi Surface in Niobium by Positron Annihilation  

NASA Astrophysics Data System (ADS)

The three dimensional electron-positron momentum density in niobium has been reconstructed from measurements of two dimensional angular correlation of positron annihilation radiations (2D-ACAR) followed by the image reconstruction technique based on a direct Fourier transformation. We determined the position of the Fermi surface sheets; \\varGamma-centered hole octahedron, multiply connected jungle-gym arms and N-centered hole ellipsoids. The Fermi surface topology is in good agreement with the theory.

Kubota, Takeshi; Kondo, Hitoshi; Watanabe, Kazuhiro; Murakami, Yasukazu; Cho, Yang-Koo; Tanigawa, Shoichiro; Kawano, Takao; Bahng, Gun-Woong

1990-12-01

113

Positron annihilation study of thermoelastic martensite stabilization in a thermally cycled CuZnAl alloy  

NASA Astrophysics Data System (ADS)

The thermally cycled samples of Cu-23at%Zn-10at%Al have been studied by means of positron annihilation. It is shown that vacancies and their movement seem to be the leading factors resulting in thermoelastic martensite stabilization based on the fact that stabilization was no longer present and the great decrease of positron parameters after the samples in martensite were thermally cycled.

Jing-cheng, Wang; Fu-qiang, You; Zi-chang, Shao; Jin-lin, Qi; Xue-feng, Tang

1998-10-01

114

Correlation of soft magnetic properties with free volume and medium range ordering in metallic glasses probed by fluctuation microscopy and positron annihilation technique  

NASA Astrophysics Data System (ADS)

Amorphous ribbons of different thicknesses of Co64.5Fe3.5Si16B14Ni2 alloy were synthesized using the melt spinning technique by varying wheel speed. The effect of cooling rate on the ribbon thickness and their soft magnetic properties have been studied. The amorphous structure has been characterized in terms of structural free volume and medium range order (MRO) by positron annihilation spectroscopy and fluctuation electron microscopy techniques. Positron lifetime spectra of amorphous samples showed two lifetime components. The first component was found to be correlated with MRO whereas, the second lifetime component was found to be associated with nanovoid type of defects, and the second component was strongly dependent on processing conditions. It could be established that the coercivity of the amorphous samples produced by the rapid solidification technique mainly depends on the defects formed during processing rather than change induced in MRO.

Srivastava, A. P.; Srivastava, D.; Sudarshan, K.; Sharma, S. K.; Pujari, P. K.; Majumdar, B.; Suresh, K. G.; Dey, G. K.

2012-08-01

115

Radiation defects induced by helium implantation in gold-based alloys investigated by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

The formation of gas bubbles in metallic materials may result in drastic degradation of in-service properties. In order to investigate this effect in high density and medium-low melting temperature ( T-M ) alloys, positron annihilation spectroscopy measurements were performed on helium-implanted gold-silver solid solutions after isochronal annealing treatments. Three recovery stages are observed, attributed to the migration and elimination of defects not stabilized by helium atoms, helium bubble nucleation and bubble growth. Similarities with other metals are found for the recovery stages involving bubble nucleation and growth processes. Lifetime measurements indicate that He implantation leads to the formation of small and over-pressurized bubbles that generate internal stresses in the material. A comprehensive picture is drawn for possible mechanisms of helium bubble evolution. Two values of activation energy (0.26 and 0.53 eV) are determined below and above 0.7 T-M , respectively, from the variation of the helium bubble radius during the bubble growth stage. The migration and coalescence mechanism, which accounts for these very low activation energies, controls the helium bubble growth.

Thome, T.; Grynszpan, R. I.

2006-06-01

116

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

NASA Astrophysics Data System (ADS)

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.

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

2013-04-01

117

Positron Annihilation Studies of the f-Electron Character in Actinides  

NASA Astrophysics Data System (ADS)

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.

Rusz, J.; Biasini, M.; Czopnik, A.

2004-10-01

118

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

PubMed

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

Rusz, J; Biasini, M; Czopnik, A

2004-10-01

119

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

PubMed

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 Ce(4+) ions by Eu(3+) 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 Ce(4+) to Ce(3+) 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. PMID:24247546

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

2014-01-01

120

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

Microsoft Academic Search

Five Type 304L stainless steel specimens were subjected to incrementally increasing values of plastic strain. At each value of strain, the associated static stress was recorded and the specimen was subjected to positron annihilation spectroscopy (PAS) using the Doppler Broadening method. A calibration curve for the ‘S’ parameter as a function of stress was developed based on the five specimens.

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

2011-01-01

121

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

PubMed

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

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

2014-08-14

122

Quenched-in vacancies in Fe3Al based alloys: a positron annihilation study  

NASA Astrophysics Data System (ADS)

In the present contribution, high-resolution positron lifetime spectroscopy and slow positron implantation spectroscopy are used to characterize defects in Fe75.99Al24.01 and Fe71.98Al28.02 alloys. In order to facilitate defect identification, we also perform a theoretical study of basic vacancy-like defects in three phases of the Fe3Al system: ordered (D03), short-range ordered (B2) and disordered (A2). Positron characteristics, i.e. positron lifetime and positron binding energy to defects are calculated from the first principles for various defect configurations. The results are discussed in the context of experimental data obtained here and available in literature.

Melikhova, O.; ?ížek, J.; Kuriplach, J.; Procházka, I.; Luká?, F.; Cieslar, M.; Anwand, W.; Brauer, G.

2011-01-01

123

Study of positron trapping at quantum-dot like Cu particles on the surface of Fe using Positron-Annihilation Induced Auger Electron Spectroscopy  

NASA Astrophysics Data System (ADS)

Annihilation studies in bulk Fe-Cu alloy samples have demonstrated the trapping of positrons in quantum-dot like Cu nano-particles embedded in Fe. Here we report on studies of sputtered surfaces of these materials using Positron annihilation induced Auger electron spectroscopy (PAES). In PAES energetic electron emission results from Auger transitions initiated by annihilation of core electrons with positrons trapped in a surface-state. PAES measurements on high purity Cu and Fe foils have been performed in order to provide data for calibration of the PAES intensities of the low energy Auger peaks of Fe and Cu. Measurements were then performed on the surface of the Fe-Cu alloy. The results indicated an enhancement of the PAES signal from Cu suggesting that positron trapping is occurring near centers of high Cu concentration leading to enhanced annihilation with Cu.

Nadesalingam, Manori P.; Kim, Seogkyu; Fazleev, Nail; Fry, John.; Weiss, Alex; Hasegawa, Masayuki; Nagai, Y.

2003-03-01

124

Study of defects in Fe-Re and Fe-Mo alloys by the Mössbauer and positron annihilation spectroscopies  

NASA Astrophysics Data System (ADS)

The room temperature positron annihilation lifetime spectra and 57Fe Mössbauer spectra were measured for pure Fe as well as for iron-based Fe1-xRex and Fe1-xMox solid solutions with x in the range 0.01?x?0.05. The measurements were performed in order to learn more about creation of structural defects during formation and further mechanical processing of the iron systems under consideration. The spectra 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 in the annealed samples positrons live much shorter than in the not annealed ones which suggest that the latter samples are more defected as it could be expected. Moreover simultaneous analysis of the positron and Mössbauer data shows that cold rolling leads to creation of two types of defects. It seems that they are dislocations and vacancies. Finally from the Mössbauer data it follows that vacancies are located mainly in the vicinity of non-iron atoms, Re or Mo. This speaks in favour of the suggestion that in iron matrix the impurities mentioned above and vacancies interact attractively which supports the known from the literature, theoretical calculations on the Mo-vacancy interaction in iron.

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

2012-10-01

125

Two-photon positron-electron annihilation in a strong magnetic field  

NASA Astrophysics Data System (ADS)

We consider the two-photon positron and electron annihilation in flight, it means the annihilating particles exhibit the middly relativistic momenta in a super strong magnetic field. Such particles are present in the corona of pulsars and magnetars. The paper presents how the total emission rate for the two-photon process is affected not only by magnetic field but also by the relativistic momentum of the annihilating particles. We found that the momenta influence significantly the total emission rate and the directions of the emitted photons. Additionally, the total emission for the two-photon process is comparable to that for the one-photon process at the momentum of annihilating particles of about m0, where m0 is the electron mass, and the magnetic field close to the critical Schwinger value of 4.41 × 1013 G. The latter is reported as a main annihilation channel in a super strong magnetic field. We calculated also the energetic spectra of annihilating photons emitted, which are also affected by the magnetic field and the momenta of the annihilating particles.

Lewicka, Sylwia; Dryzek, Jerzy

2013-12-01

126

Positron probes of the Ge(1 0 0) surface: The effects of surface reconstructions and electron positron correlations on positron trapping and annihilation characteristics  

Microsoft Academic Search

Positron annihilation induced Auger electron spectroscopy (PAES) has been applied to study the Ge(1 0 0) surface. The high-resolution PAES spectrum from the Ge(1 0 0) surface displays several strong Auger peaks corresponding to M4,5N1N2,3, M2,3M4,5M4,5, M2,3M4,5V and M1M4,5M4,5 Auger transitions. The integrated peak intensities of Auger transitions are used to obtain experimental annihilation probabilities for the Ge 3d and

N. G. Fazleev; E. Jung; A. H. Weiss

2007-01-01

127

Positron probes of the Ge(1 0 0) surface: The effects of surface reconstructions and electron–positron correlations on positron trapping and annihilation characteristics  

Microsoft Academic Search

Positron annihilation induced Auger electron spectroscopy (PAES) has been applied to study the Ge(100) surface. The high-resolution PAES spectrum from the Ge(100) surface displays several strong Auger peaks corresponding to M4,5N1N2,3, M2,3M4,5M4,5, M2,3M4,5V and M1M4,5M4,5 Auger transitions. The integrated peak intensities of Auger transitions are used to obtain experimental annihilation probabilities for the Ge 3d and 3p core level electrons.

N. G. Fazleev; E. Jung; A. H. Weiss

2007-01-01

128

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

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.

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

2012-12-10

129

Free volume investigation of imidazolium ionic liquids from positron lifetime spectroscopy  

E-print Network

In this work, relationships between the free volume and various fundamental physical properties (density, surface tension and transport properties) of ionic liquids were investigated. Two imidazolium ionic liquids 1-butyl-3-methylimidazolium tris(pentafluoroethyl)trifluoro phosphate ([C4MIM][FAP]) and 1-butyl-3-methylimidazolium bis[bis(pentafluoroethyl)phosphinyl]imide ([C4MIM][FPI]) were measured by positron annihilation lifetime spectroscopy (PALS). Changes of the ortho-positronium lifetime (o-Ps) with different states (amorphous and crystalline) were depicted as completely as possible. The mean local free (hole) volume was calculated from the o-Ps lifetime in amorphous state for the samples. Comparison between and specific volume obtained from the temperature dependent mass density gave the specific hole densities Nf and the occupied volumes Vocc. Thermal expansion of hole volume was compared with molecular volume VM of [C4MIM][FAP] and [C4MIM][FPI] as well as five other ionic liquids from our previous works, a monotonically increasing correlation between the two quantities was displayed. Hole volume of [C4MIM][FAP] sample from PALS experiment was compared with the result from surface tension according to F\\"urth hole theory, good agreement exhibited. The free volume obtained from this work was applied to Cohen-Turnbull fitting of viscosity for [C4MIM][FPI] sample. The influence of the free volume to transport properties was investigated by the comparison of /VM with the viscosity and conductivity for various ionic liquids. Correlation between the free volume and the molecular volume of ionic liquids were explained by a schematic free volume model.

Yang Yu; Dana Bejan; Reinhard Krause-Rehberg

2014-04-01

130

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

E-print Network

molecules at sub-Ps-threshold pos- itron energies 5 , and rapid increase of the fragmentation to small changes in the molecular structure 4 , large ionization-fragmentation cross sections for organic, by comparison with Eq. 1 , we have a r0 2 cZeff /v, 2 where v is the positron velocity. Accordingly, the spin

Gribakin, Gleb

131

Recent progress in annihilation related studies by slow positrons  

Microsoft Academic Search

The field of slow-positron physics has expanded significantly in the last few years to include particles and atomic physics but has been most extensive in those associated with condensed matter or material science. This can primarily be attributed to the development of more efficient moderators. These moderators have been associated with both laboratory- and facility-based beams. In this paper I

Lynn

1989-01-01

132

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

NASA Astrophysics Data System (ADS)

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.

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

133

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

NASA Astrophysics Data System (ADS)

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.

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

2013-06-01

134

Study on Cd vacancy in CdZnTe Crystal by Positron Annihilation Technology  

NASA Astrophysics Data System (ADS)

Cd vacancies in cadmium zinc telluride(CdZnTe) crystals have an important effect on the crystal properties. In this paper, position distribution and concentration change of Cd vacancy in CdZnTe crystal grown by the temperature gradient solution growth (TGSG) were investigated by positron annihilation technology (PAT), which was based on the potential energy distribution and probability density of the positron in the crystal. The results showed that, the density of Cd vacancy increased obviously from the first-to-freeze to stable growth of the ingots, while decreased along the radial direction of the ingots.

Liu, Weiwei; Min, Jiahua; Liang, Xiaoyan; Zhang, Jijun; Sun, Xiaoxiang; Wang, Linjun; Ran, An; Ye, Bangjiao

2013-03-01

135

Application of Positron Annihilation Spectroscopy to the Study of Defects in Perovskite Type Materials  

Microsoft Academic Search

Formation of defects in ceramics BaA1\\/3Nb2\\/3O3 (where A=Mg, Co or Zn) suitable for microwave applications was analyzed by using positron annihilation spectroscopy and X-ray diffraction techniques. The dependence of the microwave properties of perovskite ceramics on the defect structure of ANb2O6 (A = Mg, Co or Zn ) that was used as a precursor during the sintering process prompted us

Dmytro Grebennikov

2007-01-01

136

Hadron-jet production in high-energy electron-positron annihilation  

Microsoft Academic Search

A simple model is developed for the conversion of an initially produced quark-antiquark pair into hadron jets in electron-positron annihilation. The model assumes that only two quark flavors (u and d) are produced, both quarks have the same mass, and all final-state hadrons are pions. In the model, hadrons are produced in two oppositely directed jets in the center-of-momentum (c.m.)

T. Mongan

1979-01-01

137

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

NASA Technical Reports Server (NTRS)

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.

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

138

Quantification of Stress History in Type 304L Stainless Steel Using Positron Annihilation Spectroscopy  

SciTech Connect

Five type 304L stainless steel specimens were subjected to incrementally increasing values of plastic strain. At each value of strain, the associated static stress was recorded and the specimen was subjected to Positron Annihilation Spectroscopy (PAS) using the Doppler Broadening method. A calibration curve for the ‘S’ parameter as a function of stress was developed based on the five specimens. Seven different specimens (blind specimens labeled B1-B7) of 304L stainless steel were subjected to values of stress inducing plastic deformation. The values of stress ranged from 310-517 MPa. The seven specimens were subjected to Positron Annihilation Spectroscopy post loading using the Doppler Broadening method, and the results were compared against the developed curve from the previous five specimens to determine feasibility of applying the curve to materials in order to non-destructively quantify stress history in materials based only on the ‘S’ parameter extracted from the Positron Annihilation Spectroscopy. Results for the calibration set of specimens indicated that calibration development is possible.

Thomas W. Walters

2011-04-01

139

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

SciTech Connect

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.

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

140

Studies Of Oxidation And Thermal Reduction Of The Cu(100) Surface Using Positron Annihilation Induced Auger Electron Spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation induced Auger electron spectroscopy (PAES) measurements from the surface of an oxidized Cu(100) single crystal show a large increase in the intensity of the annihilation induced Cu M2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 °C. The PAES intensity then decreases monotonically as the annealing temperature is increased to ˜550 °C. Experimental positron annihilation probabilities with Cu 3p and O 1s core electrons are estimated from the measured intensities of the positron annihilation induced Cu M2,3VV and O KLL Auger transitions. PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of the surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface and various surface structures associated with low and high oxygen coverages. The variations in atomic structure and chemical composition of the topmost layers of the oxidized Cu(100) surface are found to affect localization and spatial extent of the positron surface state wave function. The computed positron binding energy and annihilation characteristics reveal their sensitivity to charge transfer effects, atomic structure and chemical composition of the topmost layers of the oxidized Cu(100) surface. Theoretical positron annihilation probabilities with Cu 3p and O 1s core electrons computed for the oxidized Cu(100) surface are compared with experimental ones. The obtained results provide a demonstration of thermal reduction of the copper oxide surface after annealing at 300 °C followed by re-oxidation of the Cu(100) surface at higher annealing temperatures presumably due to diffusion of subsurface oxygen to the surface.

Fazleev, N. G.; Nadesalingam, M. P.; Maddox, W.; Weiss, A. H.

2011-06-01

141

Study of ageing in Al-Mg-Si alloys by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

In many common Al-Mg-Si alloys (6000 series) intermediate storage at or near ‘room temperature’ after solutionising leads to pronounced changes of the precipitation kinetics during the ensuing artificial ageing step at ?180 °C. This is not only an annoyance in production, but also a challenge for researchers. We studied the kinetics of natural ‘room temperature’ ageing (NA) in Al-Mg-Si alloys by means of various different techniques, namely electrical resistivity and hardness measurement, thermoanalysis and positron lifetime and Doppler broadening (DB) spectroscopy to identify the stages in which the negative effect of NA on artificial ageing might appear. Positron lifetime measurements were carried out in a fast mode, allowing us to measure average lifetimes in below 1 min. DB measurements were carried out with a single detector and a 68Ge positron source by employing high momentum analysis. The various measurements show that NA is much more complex than anticipated and at least four different stages can be distinguished. The nature of these stages cannot be given with certainty, but a possible sequence includes vacancy diffusion to individual solute atoms, nucleation of solute clusters, Mg agglomeration to clusters and coarsening or ordering of such clusters. Positron lifetime measurements after more complex ageing treatments involving storage at 0 °C, 20 °C and 180 °C have also been carried out and help to understand the mechanisms involved.

Banhart, J.; Liu, M.; Yong, Y.; Liang, Z.; Chang, C. S. T.; Elsayed, M.; Lay, M. D. H.

2012-07-01

142

Microdefects and 3 d electrons in ordered B2-FeAl alloys investigated by positron annihilation techniques  

NASA Astrophysics Data System (ADS)

Microdefects and 3 d electrons in B2-FeAl alloys with different chemical composition, single crystal of Fe and cold-rolled Fe has been studied by positron lifetime and coincidence Doppler broadening spectroscopy. The coincidence Doppler broadening spectrum of the single crystal of Fe shows the highest 3 d electron signal in the spectra of all tested samples. The 3 d electron signal in the spectrum of Fe 50Al 50 alloy is much lower than that of the cold-rolled Fe. This indicates that some of the 3 d electrons of Fe atoms and 3 p electrons of Al atoms in B2-FeAl alloy are localized to form strong covalent bonds, thus decreasing the probability of positron annihilation with 3 d electrons of Fe atoms. With the increase of Al content in B2-FeAl alloys, the 3 d electron signal in the spectrum of the alloy decreases, while the open volume of defect increases.

Yuyang, Huang; Yanqiong, Lu; Yanyan, Zhu; Yuxia, Li; Wen, Deng

2009-09-01

143

The 511 keV emission from positron annihilation in the Galaxy  

SciTech Connect

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

Prantzos, N.; Boehm, C.; Bykov, A. M.; Diehl, R.; Ferriere, K.; Guessoum, N.; Jean, P.; Knoedlseder, J.; Marcowith, A.; Moskalenko, I. V.; Strong, A.; Weidenspointner, G. [CNRS, UMR7095, UMPC and Institut d'Astrophysique de Paris, F-75014, Paris (France) and LAPP, 9 Chemin de Bellevue, BP 110 F-74941 Annecy-le-Vieux (France); A. F. Ioffe Institute of Physics and Technology, Russian Academy of Sciences, 194021, St. Petersburg (Russian Federation); Max Planck Institut fuer Extraterrestrische Physik, D-85741 Garching (Germany); Laboratoire d'Astrophysique de Toulouse-Tarbes, Universite de Toulouse, CNRS, 14 avenue Edouard Belin, F-31400 Toulouse (France); American University of Sharjah, College of Arts and Sciences/Physics Department, P.O. Box 26666, Sharjah (United Arab Emirates); CESR, Universite de Toulouse, CNRS, 9, Avenue du Colonel Roche, Boite Postal 4346, F-31028 Toulouse Cedex 4 (France); L.U.P.M., Universite Montpellier II, CNRS, Place Eugene Bataillon, F-34095 Montpellier (France); Hansen Experimental Physics Laboratory and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, California 94305 (United States); Max Planck Institut fuer Extraterrestrische Physik, D-85741, Garching (Germany); Max Planck Institut fur Extraterrestrische Physik, Garching, D-85741 Germany, and MPI Halbleiterlabor, Otto-Hahn-Ring 6, D-81739 Muenchen (Germany)

2011-07-01

144

Study of defects in electron irradiated CuInSe{sub 2} by positron lifetime spectroscopy  

SciTech Connect

CuInSe{sub 2} was studied in the as-grown state and after low-temperature (4 K) 2 MeV electron irradiation. The positron bulk lifetime of 235 ps was measured for the unirradiated sample. The positron bulk lifetime was theoretically calculated and is in good agreement with the experimental value. In addition, the defect-related lifetimes for mono-, di-, and trivacancies are theoretically determined. An increased average positron lifetime indicated after electron irradiation the appearance of open-volume defects, most probably of divacancy type. The disappearance of this defect was observed during annealing below 250 K. Other defects were formed leading to a divacancy signal at least stable up to 600 K in the temperature range above 450 K. {copyright} {ital 1998 American Institute of Physics.}

Polity, A.; Krause-Rehberg, R.; Staab, T.E. [Fachbereich Physik, Universitaet Halle, D-06099 Halle (Saale) (Germany)] [Fachbereich Physik, Universitaet Halle, D-06099 Halle (Saale) (Germany); Puska, M.J. [Laboratory of Physics, Helsinki University of Technology, 02150 Espoo (Finland)] [Laboratory of Physics, Helsinki University of Technology, 02150 Espoo (Finland); Klais, J.; Moeller, H.J. [Fachbereich Physik, Technische Universitaet Bergakademie Freiberg, D-09599 Freiberg (Germany)] [Fachbereich Physik, Technische Universitaet Bergakademie Freiberg, D-09599 Freiberg (Germany); Meyer, B.K. [Fachbereich Physik, Justus--Liebig--Universitaet Giessen, D-35392 Giessen (Germany)] [Fachbereich Physik, Justus--Liebig--Universitaet Giessen, D-35392 Giessen (Germany)

1998-01-01

145

Positron annihilation and Moessbauer effect studies of ln-Pb and Cu-Ni binary alloys  

NASA Astrophysics Data System (ADS)

The alloy systems In(1-x)Pb(x) with x=0.00, 0.05, 0.20, 0.50, 0.80, 1.00 and Cu(1-x)Ni(x) with x=0.00, 0.10, 0.15, 0.17, 0.25, 0.30, 0.40, 0.60, 0.80 and 1.00 were studied using the positron annihilation method. Moessbauer spectroscopy was used for the In-Pb system studies. The composition dependence of the h parameter obtained from the angular distribution curves indicates a preferred annihilation of positron at the Pb atomic site in In-Pb alloys and at the Ni atomic site in Cu-Ni alloys. Additionally, temperature dependent measurements were carried out on In-20% Pb in order to evaluate the vacancy formation energy. The obtained value was 0.48 + or - 0.03 eV. The low momentum component (attributed to the contribution of the conduction electrons in the annihilation process) observed from the momentum distribution was correlated with the isomer shift obtained from the Moessbauer effect studies.

Ani, K. A.; Dezsi, I.; Balogh, A.; Kajcsos, Z.; Nagy, D. L.

1981-01-01

146

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

NASA Astrophysics Data System (ADS)

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

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

2009-02-01

147

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

NASA Astrophysics Data System (ADS)

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.

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

2013-07-01

148

Measurement of the hadronic cross section in electron-positron annihilation  

SciTech Connect

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.

Clearwater, S.

1983-11-01

149

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

NASA Astrophysics Data System (ADS)

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.

Cheng, Yongjun; Mitroy, J.

2014-10-01

150

Studies of the Ge(100) Surface Using a Low Energy Positron Beam: The Effects of Surface Reconstructions on Positron Trapping and Annihilation Characteristics  

NASA Astrophysics Data System (ADS)

Positron annihilation induced Auger electron spectroscopy (PAES) has been applied to study the Ge(100) surface. The PAES spectrum from the Ge(100) surface displays several strong Auger peaks corresponding to M4,5N1N2,3 , M2,3M4,5M4,5 , M2,3M4,5V, and M1M4,5M4,5 Auger transitions. The integrated peak intensities of Auger transitions are used to obtain experimental annihilation probabilities for the Ge 3d and 3p core level electrons. The experimental results are analyzed by performing calculations of positron surface states and annihilation characteristics of surface trapped positrons with relevant Ge core-level electrons for the reconstructed Ge(100)-p(2x1), Ge(100)-p(2x2), and Ge(100)-c(4x2) surfaces. Estimates of positron binding energy, work function, and annihilation characteristics reveal their sensitivity to surface reconstruction of the topmost layers of clean Ge(100). These results are compared to the ones obtained for the reconstructed Si(100)-(2x1) and Si(100)-p(2x2) surfaces. A comparison with PAES data reveals an agreement with theoretical core annihilation probabilities for the Auger transitions considered.

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

2008-03-01

151

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

SciTech Connect

The subject of cosmic ray antiproton production is reexamined by considering other choices for the nature of the Majorana fermion chi other than the photino considered in a previous article. The calculations are extended to include cosmic-ray positrons and cosmic gamma rays as annihilation products. Taking chi to be a generic higgsino or simply a heavy Majorana neutrino with standard couplings to the Z-zero boson allows the previous interpretation of the cosmic antiproton data to be maintained. In this case also, the annihilation cross section can be calculated independently of unknown particle physics parameters. Whereas the relic density of photinos with the choice of parameters in the previous paper turned out to be only a few percent of the closure density, the corresponding value for Omega in the generic higgsino or Majorana case is about 0.2, in excellent agreement with the value associated with galaxies and one which is sufficient to give the halo mass. 52 references.

Rudaz, S.; Stecker, F.W.

1988-02-01

152

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

NASA Technical Reports Server (NTRS)

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

Rudaz, S.; Stecker, F. W.

1988-01-01

153

Positron annihilation study of the Fermi surface of Ni2MnGa  

NASA Astrophysics Data System (ADS)

The Fermi surface of the ferromagnetic shape-memory alloy Ni2MnGa has been determined experimentally with two-dimensional angular correlation of electron-positron annihilation radiation. Our results are supported by first principles electronic structure calculations. The measured electron occupancy within the Brillouin zone is consistent with the existence of two nesting features present in the Fermi surfaces calculated in previous studies. The nesting vectors of the calculated Fermi surface match the modulation of the pre-martensitic intermediate structure and that of the martensitic structure.

Haynes, T. D.; Watts, R. J.; Laverock, J.; Major, Zs; Alam, M. A.; Taylor, J. W.; Duffy, J. A.; Dugdale, S. B.

2012-03-01

154

The effect of vacancies on the microwave surface resistance of niobium revealed by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Using variable-energy positron annihilation spectroscopy, we demonstrate that a different near-surface vacancy concentration accompanies drastic differences in surface resistance of superconducting niobium cavities for particle acceleration. Our data suggest that vacuum baking at 120 °C leads to the doping of a near-surface layer with vacancy-hydrogen complexes, and that higher vacancy-type defect concentration distinguishes electropolished from chemically etched cavities. Our findings may help to explain a strong dependence of cavity performance on heat and chemical treatments, and may be of interest to other physics fields including cavity quantum electrodynamics (QED), microresonators, and single photon detectors.

Romanenko, A.; Edwardson, C. J.; Coleman, P. G.; Simpson, P. J.

2013-06-01

155

Microstructure evolution of heavily deformed AA5083 Al-Mg alloy studied by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Commercial Al-Mg alloy AA5083 was severely deformed with thickness reduction from 50 mm to 1 mm by hot rolling plus cold rolling. Isochronal annealing studies of this heavily deformed Al-Mg alloy were investigated by positron annihilation and Vickers microhardness as well as X-ray diffraction measurements. Positrons are almost fully trapped at dislocations in the as deformed sample. This suggests that deformation introduces large amounts of dislocations. The Vickers microhardness also increases from 71 Hv to 155 Hv after deformation. Most of the deformation induced dislocations are annealed out at 250 °C. At the same time, the microhardness also decreases to about 90 Hv after annealing at 200-250 °C and remain stable after further annealing up to 500 °C. This indicates that hardening of the AA5083 alloy is primarily due to dislocations. Dislocation-solute interaction may be also responsible for the strengthening of hardness.

Zou, B.; Chen, Z. Q.; Liu, C. H.; Chen, J. H.

2014-03-01

156

Microstructural Characterization of Ferritic/Martensitic Steels by Positron Annihilation Spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation (PA) studies are carried on two ferritc/martensitic steels (modified 9Cr-1Mo and EUROFER97) and Fe-9Cr binary alloy. Normalized modified 9Cr-1Mo steel is subjected to isochronal heat treatments between 300 K - 1273 K with PA studies at different intervals. Due to changes in the concentration of positron trapping open volume defects associated, different stages of the microstructural changes are identified. These results are corroborated with scanning electron microscopy studies. In EUROFER97 steel, the relative increase in precipitate number density due to the additional cold-work (after normalization) is brought out. In binary Fe-9Cr alloy, which is the model alloy of 9Cr ferritic/martensitic steels, the effect of dislocations on Cr segregation is brought out by studying alloys with two different initial conditions of varying dislocation densities.

Babu, S. Hari; Amarendra, G.; Rajaraman, R.; Sundar, C. S.

2013-06-01

157

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

158

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

SciTech Connect

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.

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

159

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

PubMed

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

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

2013-10-25

160

Positron annihilation in Al single crystals from 85 mK to 300K  

Microsoft Academic Search

Doppler-broadening and positron lifetime data have been taken for well annealed ultra-pure Al(110) crystals from 85 mK to 300K. The Doppler-broadening data are independent of specimen temperature from 85 mK to 77K, while they show an increase between 77K and 300K consistent with the linear slope observed previously by Hood and Schultz (1979). Over the whole temperature range studied, the

P. J. Schultz; A. Vehanen; W. Thomlinson; K. G. Lynn; I. K. MacKenzie

1983-01-01

161

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

NASA Astrophysics Data System (ADS)

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.

?ížek, J.; Neslušan, M.; ?illiková, M.; Mi?ietová, A.; Melikhova, O.

2014-11-01

162

Defect characterization of ZnBeSe solid solutions by means of positron annihilation and photoluminescence techniques  

NASA Astrophysics Data System (ADS)

Defect characterization of as-grown Zn1-xBexSe compound semiconductors was studied by positron lifetime and photoluminescence measurements. We obtain both experimental and theoretical evidence that the bulk lifetime of free positrons decreases linearly with Be alloying. The average positron lifetime increases with the temperature, indicating that both vacancies and negative ions trap positrons. The decomposition of the lifetime spectra shows that the positron lifetime of the vacancy decreases with an increase in Be content, as predicted by theoretical calculations. The concentration of vacancies and negative ions obtained from positron measurements follows the same trend with Be alloying. A comparison of positron measurements with theoretical calculations and photoluminescence experiments in Zn vapor annealed samples indicates that the vacancy corresponds to a Be vacancy. Besides, the negative ions trapping positrons at low temperatures have been attributed to Be interstitial atoms. The intensity of the B photoluminescence band correlates with the concentration of Be vacancies and Be interstitial atoms. We thus conclude that the electron levels of those defects are involved in the optical transition that leads to the B photoluminescence band.

Plazaola, F.; Flyktman, J.; Saarinen, K.; Dobrzynski, L.; Firszt, F.; Legowski, S.; Meczynska, H.; Paszkowicz, W.; Reniewicz, H.

2003-08-01

163

Configuration interaction calculations of potential curves and annihilation rates for positronic complexes of alkali monoxides.  

PubMed

Ab initio multireference single- and double-excitation configuration interaction calculations have been carried out to compute the potential curves and annihilation rates (ARs) of positronic molecular complexes of a series of alkali monoxides. The dissociation limit for the lowest states of these systems consists of the positive alkali ion ground state (M(+)) and the OPs (e(+)O(-)) complex formed by attaching the positron to O(-), even though the ground state of the corresponding neutral molecule always correlates with uncharged fragments (M+O). The positron affinity of the neutral oxide (2)Pi state is greater than that of (2)Sigma(+) in each case, so that the e(+)MO ground state always has (3,1)Pi symmetry, despite the fact that both KO and RbO have (2)Sigma(+) ground states. The bonding in the positronic systems is highly ionic at all internuclear distances and this causes their ARs to decrease gradually as the positive alkali ion approaches the OPs fragment. PMID:19778100

Buenker, Robert J; Liebermann, Heinz-Peter

2009-09-21

164

Precipitation Behavior Investigated Through Positron Annihilation in Sc-doped Al-6Mg Followed by the Effects of Zr-addition  

NASA Astrophysics Data System (ADS)

Phase decomposition in Al-6Mg alloy doped with Sc up to 0.6 wt.% was first investigated through positron lifetime and coincidence Doppler broadening spectroscopic (CDBS) measurements. The results varied significantly with the degrees of doping and heat treatment conditions due to the entrapment and annihilation of positrons at vacancies and lattice irregularities like coherent and semi-coherent precipitate-matrix interfaces. Sc-vacancy complexes helped in fine scale precipitation of Al3Sc during the annealing. The substructure stabilization is effected more at low annealing temperature and shorter annealing times. The precipitation behaviour in 0.2 wt.% Zr-doped Al-6Mg-0.4Sc alloy under different annealing conditions was also studied. Although Sc has better diffusivity in Al-6Mg than Zr, the latter appeared to be an ideal additive to generate new precipitates of the form Al3Sc1-xZrx and the differences are reflected in the positron lifetimes and CDBS ratio curves. Transmission electron microscopy showed spheroidal precipitates with complete absence of facets, implying the modification of the surface morphology of the precipitates.

Sen, R.; Nambissan, P. M. G.; Mitra, M. K.; Banerjee, M. K.

165

Proton-irradiation-induced damage in Fe-0.3 wt.%Cu alloys characterized by positron annihilation and nanoindentation  

NASA Astrophysics Data System (ADS)

Proton irradiation in combination with positron annihilation and nanoindentation offers the possibility to characterize irradiation damage in a range of dose. Proton irradiation experiments for Fe-0.3 wt.%Cu alloys were carried out at about 100 °C with an energy of 240 keV. Slow positron beam Doppler broadening spectroscopy was used to detect the depth dependence of microstructure evolution. The constant stiffness measurement (CSM) with a diamond Berkovich indenter was used to obtain the depth profile of hardness. The results showed that S-parameter in the analysis of positron annihilation increased with dose after irradiation owing to open-volume defects induced by proton irradiation. For specimens irradiated with different dose, hardness values exceeded that of un-irradiated alloys. The correlation between positron parameters and hardness was found. The hardness of any dpa was also calculated by Kasada method.

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

2013-07-01

166

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

SciTech Connect

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

Walters, Thomas W., E-mail: Thomas.Walters@inl.gov [Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID, 83415-6188 (United States); Walters, Leon C. [Argonne National Laboratory, 9700 Cass Ave., Argonne, IL, 60439 (United States); Schoen, Marco P.; Naidu, D. Subbaram [Idaho State University, 921 S. 8th Avenue, Pocatello, ID, 83201 (United States); Dickerson, Charles [Positron Systems, Inc., 1500 Alvin Ricken Dr., Pocatello, ID, 83201-2783 (United States); Perrenoud, Ben C. [Idaho National Laboratory, P.O. Box 1625, Idaho Falls, ID, 83415-6188 (United States)

2011-04-15

167

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

E-print Network

Z .Applied Surface Science 149 1999 97­102 Unfolding positron lifetime spectra with neural networks developed, a trained network could be a very effective and efficient alternative to the existing methods, with very short identification times. q 1999 Elsevier Science B.V. All rights reserved. Z .Keywords

Pázsit, Imre

168

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

NASA Astrophysics Data System (ADS)

The room temperature positron annihilation lifetime spectra and 57Fe Mössbauer spectra were measured for pure Fe as well as for iron-based Fe1-xRex, Fe1-xOsx, Fe1-xMox, and Fe1-xCrx 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.

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

2014-03-01

169

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

SciTech Connect

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.

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

170

A Positron Annihilation Study of Corrosion of Aluminum and Aluminum Alloy by NaOH  

NASA Astrophysics Data System (ADS)

Corrosion of fully-annealed pure aluminum and a continuous-cast AA2037 aluminum alloy (solutionized and water quenched) in a 1M NaOH solution for various periods of time were analyzed with positron beam-based Doppler broadening spectroscopy. By varying the energy of the incident positron beam, corrosion-induced defects at different depths from the surface were detected. It was found that the Doppler-broadened annihilation line-width parameter was significantly increased near the surface of pure aluminum after corrosion, probably due to the interaction between positrons and nanometer-sized voids formed near the aluminum surface during corrosion. Examination by atomic force microscopy indicated that many pits were formed on the aluminum surface after corrosion. In contrast, a significant decrease in the line-width parameter was observed in AA2037 alloy after corrosion and interpreted as being caused by copper enrichment at the metal-oxide interface during corrosion; such enrichment at large cavity sites was confirmed by energy dispersion spectrometry.

Wu, Y. C.; Zhai, T.; Coleman, P. G.

2012-08-01

171

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

PubMed

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

Hugenschmidt, Christoph; Ceeh, Hubert

2014-08-01

172

On the sensitivity of positron annihilation signals to alloy homogeneity in InxGa1-xN  

NASA Astrophysics Data System (ADS)

We present results of theoretical calculations of positron annihilation signals in InGaN alloys with and without vacancies. We demonstrate the sensitivity of the signals to the different configurations of the In/Ga atoms in In1-xGaxN supercells.

Tuomisto, F.; Norrman, V.; Makkonen, I.

2014-04-01

173

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

174

Free volume change in Zr50Cu40Al10 glassy alloy by the annealing studied by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

It is well known that structural relaxation in a bulk metallic glass affects some properties such as viscosity, electrical conductivity, and ductility. The free volume in glassy alloy, which is open volume retained by rapidly solidification from liquid state, has a significant roll for those properties. In order to discuss the nature of free volume in a Zr50Cu40Al10 bulk metallic glass, positron lifetime measurements have been performed for this metallic glass before and after annealing, and all positron lifetime spectra have been decomposed into multi components as a size distribution function by use of CONTIN-PALS II program. The positron lifetime distribution for this metallic glass has a broad spectrum comparing to crystal metal including a single vacancy, and its average lifetime corresponds to the free volume size. This width of positron lifetime (free volume size) distribution decreases by the annealing at 673 K. This change of the positron lifetime distribution can be attributed to a free volume relaxation.

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

2011-01-01

175

Positron annihilation studies of electron-irradiated F82H model alloys  

NASA Astrophysics Data System (ADS)

Irradiation-induced vacancy defects in F82H model alloys were studied using positron lifetime measurements. It is shown that the presence of Cr and carbides in iron-based alloys prevents the formation of vacancy clusters. Divacancies are stabilized at room temperature by adding Cr to Fe. The suppression of void swelling by Cr can be explained by existing models. During the incubation period, elements such as W, V, and Ta have little effect on the evolution of point defect clusters in Fe-Cr based alloys those do not contain C. M23C6 plays an important role on void swelling resistance.

Huang, S. S.; Yoshiie, T.; Xu, Q.; Sato, K.; Troev, T. D.

2013-09-01

176

The wide lower temperature internal friction peak and its positron annihilation study  

NASA Astrophysics Data System (ADS)

A wide lower temperature internal friction (mechanical loss) peak was observed during heating and cooling processes in dilute Al-Mg solid solutions deformed by cold-rolling. By carefully decomposing using a nonlinear fitting program, the peak has a fine structure and is actually made up of three sub-peaks. After considering the behaviour of the three sub-peaks during the annealing process, we propose the `vacancy mechanism' on the basis of interaction between dislocations and point defects for the origin of the three peaks. The thought was further supported by the corresponding positron lifetime spectrum study.

Zou, Q. D.; Fang, Q. F.; Zhou, X. Y.; Gong, Z. F.; Yang, B. Z.

1998-06-01

177

Studies of the Ge(100) Surface Using a Low Energy Positron Beam: The Effects of Surface Reconstructions on Positron Trapping and Annihilation Characteristics  

Microsoft Academic Search

Positron annihilation induced Auger electron spectroscopy (PAES) has been applied to study the Ge(100) surface. The PAES spectrum from the Ge(100) surface displays several strong Auger peaks corresponding to M4,5N1N2,3 , M2,3M4,5M4,5 , M2,3M4,5V, and M1M4,5M4,5 Auger transitions. The integrated peak intensities of Auger transitions are used to obtain experimental annihilation probabilities for the Ge 3d and 3p core level

N. G. Fazleev; A. H. Weiss

2008-01-01

178

Surface modification of mica due to titanium sputtering as studied by positron annihilation.  

PubMed

Study was conducted of surface structural modification of layered sheets of mica that had been subjected to titanium (Ti) sputter coating, use being made of a variable but mono energetic slow positron beam. Comparison has been drawn with the structure of pure muscovite mica sheet. Doppler broadening (DB) of the energy of annihilation radiation has been used as a parameter to assay changes in the surface characteristics within a depth profile of 500 angstroms. In support of these investigations, X-ray reflectivity data was used to assay titanium film thickness and the electron density profile of the film on mica. Finally, changes involving the surface micro-morphology of the system have been corroborated by scanning electron microscopy. PMID:16458012

Ganguly, Bichitra; Djourelov, Nikolay; Suzuki, Takenori; Kundu, Srinanda

2006-06-01

179

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

E-print Network

We discuss 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 contribution of the annihilation interaction in the quantum hydrodynamic equations and in spectrum of waves in magnetized electron-positron plasmas. We consider propagation of waves parallel and perpendicular to an external magnetic field. We also consider oblique propagation of longitudinal waves. We derive set of quantum kinetic equations for electron-positron plasmas with the Darwin and annihilation interactions. We apply the kinetic theory for the linear wave behavior in absence of external fields. We calculate 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 electron-positron plasmas including the Darwin and annihilation interactions. Existence of conserving helicity in electron-positron quantum plasmas of spinning particles with the Darwin and annihilation interactions is demonstrated. We show that annihilation interaction plays an important role in quantum electron-positron plasmas giving contribution of the same magnitude as the spin-spin interaction.

Pavel A. Andreev

2014-04-18

180

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

NASA Technical Reports Server (NTRS)

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

Ray, Asit K.

1995-01-01

181

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

E-print Network

We discuss 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 contribution of the annihilation interaction in the quantum hydrodynamic equations and in spectrum of waves in magnetized electron-positron plasmas. We consider propagation of waves parallel and perpendicular to an external magnetic field. We also consider oblique propagation of longitudinal waves. We derive set of quantum kinetic equations for electron-positron plasmas with the Darwin and annihilation interactions. We apply the kinetic theory for the linear wave behavior in absence of external fields. We calculate contribution of the Darwin...

Andreev, Pavel A

2014-01-01

182

Six-quark decays of off-shell WW pairs via parton-level QCD in electron-positron annihilations  

E-print Network

We study the decays into six quarks of off-shell pairs of W bosons produced in electron-positron annihilations, through the O(as^2) order in the strong coupling constant. We give explicit helicity amplitude formulae. We present numerical results in the context of phenomenological analyses of relevance at LEP2 and future Linear Colliders: such as MW determinations, New Physics and Higgs searches.

S. Moretti

2000-02-09

183

Vacancy analysis in a Ni-Nb-Zr-H glassy alloy by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

The positron lifetimes of Ni36Nb24Zr40 and (Ni0.36Nb0.24Zr0.40)90H10 glassy alloys are almost the same but longer than those of pure Zr, Nb, and Ni crystals, indicating that they have higher density of vacancies with smaller size than in crystals. The coincidence Doppler broadening spectrum for both specimens shows that the contribution of Ni around the vacancies is lower than that of Zr and Nb, suggesting that hydrogen atoms favour to exist between Ni atoms comprising neighboring distorted icosahedral Zr5Ni5Nb3 clusters. Thus, these results provide a substitute model of quantum dot tunneling along Ni-H-Ni atomic bond arrays among the clusters.

Fukuhara, Mikio

2012-02-01

185

Calculations of gamma-ray spectral profiles of linear alkanes in the positron annihilation process  

E-print Network

The positron-electron annihilation gamma-ray spectra of linear alkanes CnH2n+2 (n=1-12) have been studied systematically. A profile quality (PQ) parameter, is introduced to assess the agreement between the obtained theoretical profiles and the experimental measurements in the entire region of energy shift of the spectra. Together with the Doppler shift of the gamma-ray spectra, the two parameters,PQ and Doppler shift, are able to provide a more comprehensive assessment of the calculated gamma-ray spectra with respect to available experiment. Applying the recently developed docking model, the present study determines the positrophilic electrons for individual alkanes from which the gamma-ray spectral profiles are calculated. The results achieve an excellent agreement with experiment, not only with respect to the Doppler shift, but also with respect to the gamma-ray profiles in the photon energy region up to 5 keV. The study further calculates the gamma-ray spectra of other linear alkanes in the series without ...

Ma, X G

2014-01-01

186

The influence of microstructure on the sintering process in crystalline metal powders investigated by positron lifetime spectroscopy: III. Nickel reduction powder  

NASA Astrophysics Data System (ADS)

The sintering process in compacts of nickel reduction powder is investigated by positron lifetime spectroscopy. Additionally, the lifetime data obtained are compared to the data on the recovery and recrystallization after plastic deformation caused by cold rolling or pressing, and on the annealing out of vacancy clusters caused by low-temperature electron irradiation. Due to the small grain sizes inside the powder particles at lower temperature, positrons annihilate at grain boundaries, leading to vacancy-cluster-like signals. At intermediate temperature, a surface signal is detected 0953-8984/11/7/011/img7. This is in agreement with an effective powder-particle size of 0953-8984/11/7/011/img8 estimated from scanning electron microscopy. When sintering starts, i.e. above 0953-8984/11/7/011/img9, the only detected defect signal, besides a small surface signal, stems from large-angle grain boundaries. At the intensive-shrinkage stage, there are contributions from different shrinkage mechanisms. The experimentally observed shrinkage rates can be explained by Coble creep which seems to dominate the material transport. Nabarro-Herring and dislocation creep play only minor roles in this system.

Staab, T. E. M.; Krause-Rehberg, R.; Vetter, B.; Kieback, B.

1999-02-01

187

Photo-degradation of Lexan polycarbonate studied using positron lifetime spectroscopy  

SciTech Connect

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.

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

188

Temperature dependence of free volume of polyacrylamide gels studied by positron lifetime measurements  

NASA Astrophysics Data System (ADS)

Changes of positronium (Ps) cavity radii in polyacrylamide and poly( N-isopropylacrylamide) gels were studied from 120 to 300 K by positron lifetime technique and it has been shown that the Ps cavity radius in the hydrogels changes by three or four stages. Temperature dependence of the Ps cavity radius exhibits variations similar to common polymers around the glass transition temperature. Hydrophilicity of the polymer chains significantly affects the Ps cavity radius just below 273 K. These results suggest an important role of free volume on the state of water in hydrogels.

Ito, Kenji; Ujihira, Yusuke; Yamashita, Takashi; Horie, Kazuyuki

2000-06-01

189

Positron Annihilation And Electrical Conductivity Studies Of Cu Substitution Effect In Li0.5-0.5xCuxFe2.5-0.5xO4 Spinel Ferrite  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime (PAL), and electrical conductivity studies were carried out on a series of polycrystalline spinel ferrite with composition Li0.5-0.5xCuxFe2.5-0.5xO4 where (x = 0.2, 0.4, 0.6, 0.8, 1). The radii of the tetrahedral A-sites (rA) and the octahedral B-sites (rB) were studied as a function of composition. The infrared spectra have been analyzed in the frequency range 200-1000 cm-1 and two bands were observed. The bands around 589 cm-1 (?A) and 400 cm-1 (?B) were assigned to the tetrahedral and octahedral complexes, respectively. The compositional dependence of the dc electrical conductivity ?dc showed a maximum at x = 0.4 then decreases with increasing Cu2+ ions content. For lifetime measurements, a conventional fast -fast coincidence system was used. The variations in positron lifetime parameters with Cu2+ ions substitution are attributed to positron trapping in A- and B- sites.

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

2011-06-01

190

Vacancy-impurity pairs in relaxed Si1-xGex layers studied by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation spectroscopy was applied to study relaxed P-doped n -type and undoped Si1-xGex layers with x up to 0.30. The as-grown SiGe layers were found to be defect free and annihilation parameters in a random SiGe alloy could be represented as superpositions of annihilations in bulk Si and Ge. A 2MeV proton irradiation with a 1.6×1015cm-2 fluence was used to produce saturated positron trapping in monovacancy related defects in the n -type layers. The defects were identified as V-P pairs, the E center. The distribution of Si and Ge atoms surrounding the E center was the same as in the host lattice. The process leading to the formation of V-P pairs therefore does not seem to have a significant preference for either Si or Ge atoms. In undoped Si1-xGex we find that a similar irradiation produces a low concentration of divacancies or larger vacancy defects and found no evidence of monovacancies surrounded by several Ge atoms.

Rummukainen, M.; Slotte, J.; Saarinen, K.; Radamson, H. H.; Hållstedt, J.; Kuznetsov, A. Yu.

2006-04-01

191

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

NASA Technical Reports Server (NTRS)

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.

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

1996-01-01

192

The effect of carbon on the evolution of vacancy defects in electron-irradiated nickel studied by positron annihilation  

NASA Astrophysics Data System (ADS)

The effect of carbon atoms on the evolution of vacancy defects in nickel doped with controlled amounts of carbon impurities under electron irradiation at 300 and 423 K was investigated using positron annihilation spectroscopy. It has been found that migrating on stage III of annealing (around 350 K) vacancies are captured by immobile carbon atoms resulting in vacancy-carbon (V-C) pair formation. It was shown that the V-C pair is highly symmetric, i.e. C atom being located near the vacancy center. This pair cannot trap a positron. The dissociation of the V-C pairs was observed after the annealing above 450 K. Realized vacancies form vacancy clusters, which become unstable at 500 K and complete anneal out to 600 K. Under irradiation at elevated temperature (423 K), at which vacancies are mobile, the variation of annihilation parameters from the bulk values are not observed in Ni-C alloy. This phenomenon was connected with the formation during irradiation of the V-C pairs, which do not bind positrons.

Druzhkov, A. P.; Perminov, D. A.; Arbuzov, V. L.

2013-03-01

193

Electronic structure calculations of positron lifetimes in SiC: Self-consistent schemes and relaxation effect  

NASA Astrophysics Data System (ADS)

We present electronic structure calculations of positron lifetimes in various neutral and negative monovacancies in 3C and 6H silicon carbide. Self-consistent positron lifetime calculation schemes were used and full defect relaxation due to the creation of the vacancy and the presence of the positron was considered. Formation energies of the various possible charges of the defects were also calculated to predict their detectability in PAS. Lifetimes between 170 ps and 195 ps for VC and between 222 ps and 227 ps for VSi were obtained. Based on these results we propose new interpretations of the experimental PAS signals observed in n-type 3C and 6H-SiC samples.

Wiktor, J.; Jomard, G.; Bertolus, M.

2014-05-01

194

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

NASA Technical Reports Server (NTRS)

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.

Singh, Jag J.; Eftekhari, Abe

1996-01-01

195

Vacancy defects in a stress-corrosion-cracked Type 304 stainless steel investigated by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

To reveal vacancy formation during the stress corrosion cracking (SCC), three factors influencing SCC in Type 304 stainless steels—sensitization heat treatment, corrosion treatment and tensile plastic deformation—were investigated by means of positron annihilation spectroscopy. Vacancy defects induced by the sensitization heat treatment and by tensile deformation were identified as monovacancies. These monovacancies were annealed within the same temperature range in which light water reactors are operated (280-320 °C). The above results allow us to conclude that such vacancy defects play an important role in high-temperature-water SCC crack propagation.

Yabuuchi, A.; Maekawa, M.; Kawasuso, A.

2011-12-01

196

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

SciTech Connect

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.

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

197

Positron Annihilation Studies of Defects in Ti-6Al-4 V Subjected to Heat Treatments and Rolling  

NASA Astrophysics Data System (ADS)

Positron lifetime and coincidence Doppler broadening (CDBS) measurements were made on samples of Ti-6Al-4 V alloy subjected to different mechanical and thermal treatments. The results indicated the presence of point defects in moderate concentrations. The as-received sample was initially heat treated at 1343K for 30 minutes and slow-cooled to extract reference values. Even when subjected to heat treatment at 1343K and fast-quenched, the inherent defect structures did not show substantial reconfiguration in size or concentration. Yet in a separate case of heat treatment at the same temperature and duration followed by hot-rolling to 50% deformation, the deformation-induced defects were found retained in it on quenching. Soaking at the elevated temperature did not produce any significant difference. In another sample, when cold-rolling was performed after heat-treatment and fast-quenching, we found the retention followed by room-temperature migration and coalescence of the generated defects with the existing ones. The measured positron lifetimes suggested the existence of defects of smaller sizes and/or dislocations. In another sample of the alloy with identical treatments at 1248K (below the ? to ? transition), the positron lifetimes were slightly larger in magnitude.

Sultana, Nashrin; Nambissan, P. M. G.; Datta, S.; Banerjee, M. K.

198

Measurement of the spectra of low energy electrons resulting from Auger transitions induced by the annihilation of low energy positrons implanted at The Ag (100) surface  

NASA Astrophysics Data System (ADS)

A few percent of positrons bound to a solid surface annihilate with core electrons resulting in highly excited atoms containing core holes. These core holes may be filled in an auto-ionizing process in which a less tightly bound electron drops into the hole and the energy difference transferred to an outgoing "Auger electron." Because the core holes are created by annihilation and not impact it is possible to use very low energy positron beams to obtain annihilation induced Auger signals. The Auger signals so obtained have little or none of the large impact induced secondary electron background that interferes with measurements of the low energy Auger spectra obtained using the much higher incident energies necessary when using electron or photon beams. Here we present the results of measurements of the energy spectrum of low energy electrons emitted as a result of Positron Annihilation Induce Auger Electron Emission [1] from a clean Ag (100) surface. The measurements were performed using the University of Texas Arlington Time of Flight Positron Annihilation induced Auger Electron Spectrometer (T-O-F-PAES) System [2]. A strong double peak was observed at ˜35eV corresponding to the N2VV and N3VV Auger transitions in agreement with previous PAES studies [3].

Shastry, Karthik; Joglekar, Prasad; Weiss, A. H.; Fazleev, N. G.

2013-04-01

199

A comparative study of defects in LiF implanted with 100 KeV Al, Mg and Ar by slow Positron Annihilation Spectroscopy  

NASA Astrophysics Data System (ADS)

The fluence-dependent evolution of defects in LiF implanted at room temperature with Al+, Mg+, Ar+ ions is measured using the slow positron beam technique. Samples were implanted with 100 KeV ions to doses of 1015 and 1016 cm-2. Doppler broadened spectra of positron annihilation as a function of positron incident energy are measured and are analyzed by the computer program VEPFIT. The depth profiles of the defect distribution are presented and discussed as a function of dose and nature of the implanted ions. The Positron Annihilation Spectroscopy (PAS) results are correlated with optical absorption measurements on the crystals, and are shown to provide important information about the spatial extent of defects in ion-implanted samples.

Sendezera, E. J.; Davidson, A. T.; Kozakiewicz, A. G.; Anwand, W.; Brauer, G.; Nicht, E. H.

200

Positron States in Si1-xGex Alloys  

NASA Astrophysics Data System (ADS)

Electron-positron momentum densities along different crystallographic directions and positron bulk lifetime in Si1-xGex alloys have been investigated within the pseudopotential formalism employing the independent particle model. Special attention has been given to the effect of the deviation of the alloy lattice parameters from Vegard's rule on the studied quantities. It is found that using Vegard's law leads to an underestimation of the total positron annihilation rate indicating therefore an overestimation of the positron bulk lifetime. This result could not be checked using the Siethoff relation (H. Siethoff, Phys. Stat. Sol. B205, R3 (1998)). Moreover, this relation predicts a monotonic dependence of the positron bulk lifetime on the alloy composition which disagrees with the positron lifetime measurement.

Bouarissa, N.

201

Study on Defects in H+ion Implanted B2 type Fe-Al Alloy using Slow Positron Beam  

NASA Astrophysics Data System (ADS)

Fe48-at.%Al alloy were implanted with 50 keV H+ ions to the fluence of 3×1016 and 1×1018/cm2 at room temperature. Positron annihilation Doppler broadening and lifetime measurements for these alloys have been carried out using slow positron beam apparatus with an energy range of 0.2 to 30.2 keV. The positron annihilation S-parameter decreased by H+ ion irradiation. Also the positron lifetimes for hydrogen deposited region in the alloy decreased by the irradiation. These results show that implanted H atoms were trapped by vacancy type defects.

Komagata, S.; Kawasuso, A.; Yabuuchi, A.; Maekawa, M.; Batchulun, C.; Yasuda, K.; Ishigami, R.; Kume, K.; Iwase, A.; Hori, F.

202

Positron annihilation studies of the AlO{sub x}/SiO{sub 2}/Si interface in solar cell structures  

SciTech Connect

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

Edwardson, C. J.; Coleman, P. G. [Department of Physics, University of Bath, Bath BA2 7AY (United Kingdom); Li, T.-T. A.; Cuevas, A. [College of Engineering and Computer Science, Australian National University, Canberra, ACT 0200 (Australia); Ruffell, S. [Research School of Physics and Engineering, Australian National University, Canberra, ACT 0200 (Australia)

2012-03-01

203

Role of intermetallic nanoparticles in radiation damage of austenitic Fe-Ni-based alloys studied by positron annihilation  

NASA Astrophysics Data System (ADS)

The effect of homogeneous distributed nanosized (1-8 nm) intermetallic particles on the evolution of the vacancy defects in fcc Fe-Ni-based alloys during electron irradiation at temperatures 300, 423 and 573 K was studied by the positron annihilation spectroscopy. It was shown that coherent intermetallic nanoparticles like Ni 3Al(Ti) inhibited accumulation of the sub-nanometer vacancy clusters. The efficiency of this process depends on type, size and density of nanoparticles and it increases at the elevated irradiation temperatures. It is also observed that in Fe-Ni-Al alloy radiation-induced nucleation of intermetallic particles of Ni 3Al type occurs during irradiation at 573 K. In addition, possible mechanisms of the absorption of point defects by nanoparticles are discussed.

Perminov, D. A.; Druzhkov, A. P.; Arbuzov, V. L.

2011-07-01

204

Electron and positron densities and the temperature dependence of the positron lifetime in a vacancy in aluminum  

Microsoft Academic Search

The results of an augmented-plane-wave calculation of the electron and positron density distributions in a vacancy in Al are reported. The environment of the vacancy was simulated by a super cell with a volume 27 times that of the primative unit cell. These results show that jellium-model calculations overestimate the electron density in a vacancy. The variation of the positron

Raju Gupta; R. W. Siegel

1977-01-01

205

Optimization of BaF2 positron-lifetime spectrometer geometry based on the Geant4 simulations  

NASA Astrophysics Data System (ADS)

Incorrect choice of measuring experimental geometry and energy windows for a positron-lifetime spectrometer with BaF2 scintillators can lead to a significant distortion in a measured spectrum. Contribution of the distorted events for a non-optimal geometry may exceed 50%. It reduces the measured lifetimes of the spectral components and redistributes their intensities. The Geant4 simulation allows to estimate an influence of the measuring geometry on the spectrum distortion and to choose the most appropriate energy windows. The optimal geometry with a lead absorber between detectors suppresses contribution of the distorted coincidences down to 1% providing sufficiently high count rate of true events.

Dubov, L. Yu.; Grafutin, V. I.; Funtikov, Yu. V.; Shtotsky, Yu. V.; Elnikova, L. V.

2014-09-01

206

The Evolution of Defects in Deformed Cu-Ni-Si Alloys during Isochronal Annealing Studied by Positron Annihilation  

NASA Astrophysics Data System (ADS)

The effect of isochronal annealing on the deformation-induced defects in pure Cu and Cu-Ni-Si alloys is studied by positron annihilation spectroscopy. For the cold-rolled Cu, annealing up to 900°C causes a gradual recovery of the deformation-induced defects and monotonous decrease of the hardness. This indicates that its hardening is mainly related with defects such as dislocations. However, for the hot-rolled and quenched Cu-Ni-Si alloy, although there is a partial recovery of defects after annealing below 500°C, formation of additional defects is observed after annealing above 500°C. The hardness of Cu-Ni-Si alloy has a maximum value after annealing at 500°C, which suggests that the hardening of Cu-Ni-Si alloy is not due to defects, but primarily due to the precipitation formed during annealing. Further annealing of the Cu-Ni-Si alloy above 500°C results in over-aging effect and the precipitates lose coherence with the host matrix, which leads to positron trapping by vacancy clusters in the incoherent interface region.

Qi, Ning; Jia, Yan-Lin; Liu, Hui-Qun; Yi, Dan-Qing; Chen, Zhi-Quan

2012-12-01

207

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

Microsoft Academic Search

Compacts of tungsten powder with five different powder-particle sizes (from 0953-8984\\/11\\/7\\/010\\/img7 to 0953-8984\\/11\\/7\\/010\\/img8) are subjected to pressureless sintering. We investigate the change in microstructure during the sintering process by positron lifetime spectroscopy. So as to be able to distinguish between defects having the same positron lifetime, we investigate their kinetics when the sample is annealed. In particular, we consider the

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

1999-01-01

208

Isochronal annealing studies of deformed FeSi alloys using positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

A series of FeSi samples were deformed to a thickness reduction of 16%. They were isochronally annealed for one hour at different temperatures and characterized by the Doppler broadening of the annihilation radiation (DBAR) measured at room temperature. Optical microscopy (OM) is used to investigate the microstructure of the deformed samples before and after annealing. The S parameter data show a decrease with the increase of the annealing temperature. At 973 K a significant decrease sets in. The microstructures of the alloys, investigated by OM, show that recrystallization is completed at 1173 K.

Mostafa, K. M.; Gonzalez Camara, F.; Petrov, Roumen; Rodriguez Calvillo, P.; De Baerdemaeker, J.; De Grave, E.; Segers, D.; Houbaert, Y.

2011-07-01

209

Fermi surface and conduction electrons of Na 0.64WO 3 by two-dimensional angular correlation of positron annihilation radiation  

NASA Astrophysics Data System (ADS)

Two-dimensional angular correlation of positron annihilation radiation (2-D ACAR) form a Na 0.64WO 3 single crystal has been measured with a 64 detector 2-D ACAR apparatus. The results show that the Fermi surface of this compound has a jungle-gym like structure similar to that of ReO 3 and that the conduction electrons have strong t2g character.

Akahane, T.; Hoffmann, K. R.; Chiba, T.; Berko, S.

1985-06-01

210

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

NASA Astrophysics Data System (ADS)

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.

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

2007-03-01

211

Positron-annihilation study of the aging kinetics of AlCu-based alloys. II. Ag microalloying  

NASA Astrophysics Data System (ADS)

Positron-annihilation spectroscopy has been applied for studying the effects of microalloying additions of Ag to Al-4%Cu and Al-4%Cu-0.3%Mg (wt %) alloys on the aging kinetics and on the association of vacancies to solute elements. The results show that: (i) in Al-Cu, the addition of Ag increases (from 0.32 to 0.61 eV) the effective activation energy that controls the formation of solute aggregates; (ii) on the contrary, in the Al-Cu-Mg alloy, the activation energy is decreased (from 0.65 to 0.22 eV); (iii) in Al-Cu and in Al-4%Cu-0.3%Ag (wt %) alloys the solute aggregates (clusters or GP zones) formed after long aging at temperatures below 70 °C do not contain vacancies; (iv) in Al-Cu-Mg, the addition of Ag enhances the formation of co-clusters (or GPB zones) containing Mg and vacancies; (v) the formation of Mg-rich aggregates is enhanced by Ag also at a high aging temperature (180 °C), and this leads to a better stabilization of the structure produced by artificial aging. The above effects are interpreted as due to the tendency of Ag atoms to bind vacancies in the Al-Cu and vacancy-Mg complexes in Al-Cu-Mg.

Somoza, A.; Dupasquier, A.; Polmear, I. J.; Folegati, P.; Ferragut, R.

2000-06-01

212

Effect of phosphorus on vacancy-type defect behaviour in electron-irradiated Ni studied by positron annihilation  

NASA Astrophysics Data System (ADS)

Very dilute Ni-P system (containing 50-240 appm phosphorus) irradiated by 5 MeV electrons at various temperatures (270-543 K) was studied by positron annihilation spectroscopy (PAS) and the electrical resistivity measurements. Under irradiation at 270 K (below stage III in Ni), the accumulation of the monovacancies in the Ni-P system is 1.5-2.0 times greater than that in pure Ni irradiated in the same conditions. This fact attests to the strong interaction between P atoms and self-interstitial atoms (SIAs). As a result of the non-mobile SIA-P complexes formation, the mutual recombination of point defects is suppressed and the vacancy accumulation is, respectively, enhanced. During post-irradiation annealing, the vacancy migration induces the transport process of the phosphorus atoms and leads to the formation of the vacancy clusters decorated with P atoms. The annealing behaviour of the defect structures in Ni-P systems after irradiation at enhanced temperatures was also studied. The influence of phosphorus on the formation and further evolution of the vacancy aggregates decrease with increasing of the irradiation temperature.

Druzhkov, A. P.; Danilov, S. E.; Perminov, D. A.; Arbuzov, V. L.

2015-02-01

213

Effect of post oxidation anneal on VUV radiation-hardness of the Si/SiO{sub 2} system studied by positron annihilation spectroscopy  

SciTech Connect

The effect of a post oxidation anneal at 1,000 C in a N{sub 2} ambient of the thermally grown Si/SiO{sub 2} system was investigated using vacuum ultraviolet irradiation for determining the generation of interface traps of the Al metallized system in combination with positron annihilation spectroscopy to characterize the structure of the oxide network. A correlation was found between the generation of interface traps and the S parameter of the positron trapping sites in the oxide close to the Si. It appears likely that the positrons are trapped in the larger near-interfacial oxide network interstices. These interstices could act as scavengers for the metastable intermediate (atomic hydrogen or excitons) involved in the generation of the interface traps.

Clement, M.; Nijs, J.M.M. de; Veen, A. van; Schut, H.; Balk, P. [Delft Univ. of Technology (Netherlands)] [Delft Univ. of Technology (Netherlands)

1995-12-01

214

Influence of mg Content on Trapping Efficiency in Al-Mg System by Positron Annihilation Spectroscopy  

NASA Astrophysics Data System (ADS)

PLT and Doppler broadening S-parameter were used for Al-Mg alloy, namely 5005, 5051, 5052 and 5083. The trapping efficiency was estimated for the pervious alloys as 2.42 × 109, 2.29 × 109, 2.24 × 109 and 2.27 × 109 s-1 cm3, respectively and the trapping cross-section was estimated to be as 2.66 × 10-16, 2.14 × 10-16, 2.10 × 10-16 and 3.51 × 10-16 cm2, respectively. It is clear that the mean lifetime and S-parameter have the same behavior as a function of deformation degree and saturated at the same value of thickness reduction.

Badawi, Emad; Abdel-Rahman, M. A.; Abdelhamed, M. O.

215

Free volumes in bulk nanocrystalline metals studied by the complementary techniques of positron annihilation and dilatometry  

PubMed Central

Free-volume type defects, such as vacancies, vacancy-agglomerates, dislocations, and grain boundaries represent a key parameter in the properties of ultrafine-grained and nanocrystalline materials. Such free-volume type defects are introduced in high excess concentration during the processes of structural refinement by severe plastic deformation. The direct method of time-differential dilatometry is applied in the present work to determine the total amount and the kinetics of free volume by measuring the irreversible length change upon annealing of bulk nanocrystalline metals (Fe, Cu, Ni) prepared by high-pressure torsion (HPT). In the case of HPT-deformed Ni and Cu, distinct substages of the length change upon linear heating occur due to the loss of grain boundaries in the wake of crystallite growth. The data on dilatometric length change can be directly related to the fast annealing of free-volume type defects studied by in situ Doppler broadening measurements performed at the high-intensity positron beam of the FRM II (Garching, Munich, Germany). PMID:23471443

Würschum, Roland; Oberdorfer, Bernd; Steyskal, Eva-Maria; Sprengel, Wolfgang; Puff, Werner; Pikart, Philip; Hugenschmidt, Christoph; Pippan, Reinhard

2012-01-01

216

Positron annihilation Doppler broadening spectroscopy study on Fe-ion irradiated NHS steel  

NASA Astrophysics Data System (ADS)

In order to study the evolution of irradiation-induced vacancy-type defects at different irradiation fluences and temperatures, a new type of ferritic/martensitic (F/M) steel named NHS (Novel High Silicon) was irradiated by 3.25 MeV Fe-ion at room temperature and 723 K to fluences of 4.3 × 1015 and 1.7 × 1016 ions/cm2. After irradiation, vacancy-type defects were investigated with variable-energy positron beam Doppler broadening spectra. Energetic Fe-ions produced a large number of vacancy-type defects in the NHS steel, but one single main type of vacancy-type defect was observed in both unirradiated and irradiated samples. The concentration of vacancy-type defects decreased with increasing temperature. With the increase of irradiation fluence, the concentration of vacancy-type defects increased in the sample irradiated at RT, whereas for the sample irradiated at 723 K, it decreased. The enhanced recombination between vacancies and excess interstitial Fe atoms from deeper layers, and high diffusion rate of self-interstitial atoms further improved by diffusion via grain boundary and dislocations at high temperature, are thought to be the main reasons for the reversed trend of vacancy-type defects between the samples irradiated at RT and 723 K.

Zhu, Huiping; Wang, Zhiguang; Gao, Xing; Cui, Minghuan; Li, Bingsheng; Sun, Jianrong; Yao, Cunfeng; Wei, Kongfang; Shen, Tielong; Pang, Lilong; Zhu, Yabin; Li, Yuanfei; Wang, Ji; Song, Peng; Zhang, Peng; Cao, Xingzhong

2015-02-01

217

Nanocluster-associated vacancies in nanocluster-strengthened ferritic steel as seen via positron-lifetime spectroscopy  

SciTech Connect

Nanocluster-strengthened ferritic alloys are promising as structural materials because of their excellent high-temperature strength and radiation-damage resistance. Recently, Fu et al. [Phys. Rev. Lett. 99, 225502 (2007)] predicted that vacancies play an essential role in the formation and stabilization of nanoclusters in these materials. Positron-lifetime spectroscopy has been used to test this theoretical prediction in a nanocluster-strengthened Fe-based alloy. Nanoclusters (2-4 nm in diameter) containing Ti, Y, and O have been observed in a mechanically alloyed ferritic steel by atom-probe tomography. Vacancy clusters containing four to six vacancies have also been found in this material. In contrast, no vacancy clusters were detected in similar alloys containing no nanoclusters. These results indicate that vacancies are a vital component of the nanoclusters in these alloys.

Xu, Jun [ORNL; Liu, C.T. [University of Tennessee, Knoxville (UTK); Miller, M [Oak Ridge National Laboratory (ORNL); Chen, Hongmin [University of Missouri

2009-01-01

218

The Effect of Alloying with Magnesium on the Annealing Behavior of Aluminum Alloys Studied by Positron Lifetime Technique  

NASA Astrophysics Data System (ADS)

The migration enthalpy Hivm for point defects and dislocations is estimated by using positron lifetime technique; point defects and dislocations are produced as a result of plastic deformation at room temperature (RT) for the decomposition sequence, namely 5005, 5052 and 5083, of commercial Al-Mg systems. The results show that Hivm for the three systems increases as the Mg content is increased to u1=0.34±0.09 eV, u2=0.39±0.12 eV, and u3=0.42±0.08 eV for the point defect state, and u1=1.12±0.08 eV and u2=1.37±0.13 eV for the dislocation state to 5005 and 5052, respectively. All the data are analyzed in terms of the two state trapping model.

Abdel-Hamed, M. O.

219

Ab initio study of the positronation of the CaO and SrO molecules including calculation of annihilation rates.  

PubMed

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

Buenker, Robert J; Liebermann, Heinz-Peter

2012-07-15

220

Study of scattering in positron emission tomography  

E-print Network

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...

Aguiar, James

2012-06-07

221

To the problem of positron states in metal-insulator nanosandwiches  

NASA Astrophysics Data System (ADS)

The potential profiles, wave functions, energies of surface subbands, and lifetimes of positrons in aluminum nanofilms bordering insulators (solid inert gases and SiO2) have been calculated self-consistently in the previously proposed models. The size effects and the influence of the effective masses of electrons and positrons on the energy and annihilation characteristics in systems with double potential wells formed by image potentials have been investigated. The possibility of localizing a positronium atom in nanosandwiches has been discussed.

Babich, A. V.; Vakula, P. V.; Pogosov, V. V.

2015-01-01

222

Positron annihilation study of vacancy-type defects in fast-neutron-irradiated MgO·nAl2O3  

NASA Astrophysics Data System (ADS)

The positron lifetimes of fast-neutron-irradiated MgO·nAl2O3 single crystals were measured to investigate the formation of cation vacancies. Al monovacancy was possibly observed in samples irradiated by fast neutrons at ultra-low temperatures. Additionally, vacancy-oxygen complex centers were possibly observed in samples irradiated at higher temperatures and fast neutron fluences. Coincidence Doppler broadening (CDB) spectra were measured to obtain information regarding the vicinity of vacancy-type defects. A peak at approximately 11 × 10-3 m0c was observed, which may be due to the presence of oxygen atoms in the neighborhood of the vacancies.

Rahman, Abu Zayed Mohammad Saliqur; Li, Zhuoxin; Cao, Xingzhong; Wang, Baoyi; Wei, Long; Xu, Qiu; Atobe, Kozo

2014-09-01

223

Structural effects induced by 2.5 MeV proton beam on zeolite 4A: Positron annihilation and X-ray diffraction study  

NASA Astrophysics Data System (ADS)

Structural changes induced by 2.5 MeV proton beam in zeolite 4A were studied at fluences of 1.1016, 8.1016 and 2.1017 ion/cm2. Positron lifetime (LT) measurements were performed in vacuum after a heat treatment to remove the water from samples. X-ray diffraction method (XRD) was also applied to determine the changes in the crystallinity of zeolite. Gradual transformation of the crystalline structure to amorphous one can be observed in dependence of the implanted doses. Combination of LT and XRD provides a consistent view on structural effects induced by H+ beam in zeolite 4A.

Tuyen, L. A.; Szilágyi, E.; Kótai, E.; Lázár, K.; Bottyán, L.; Dung, T. Q.; Cuong, L. C.; Khiem, D. D.; Phuc, P. T.; Nguyen, L. L.; Hue, P. T.; Hue, N. T. N.; Tao, C. V.; Chuong, H. D.

2015-01-01

224

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)

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.

Staikov, P.; Djourelov, N.

2013-03-01

225

Effect of heterogeneous distributed intermetallic precipitates on accumulation of vacancy-like defects in irradiated Fe-Ni-based alloys studied by positron annihilation  

NASA Astrophysics Data System (ADS)

The effect of heterogeneous distributed intermetallic particles on interaction of point defects with dislocations in cold-worked Fe-Ni-based alloys during electron irradiation was studied by positron annihilation. It was shown that vacancy accumulation occurs in cold-worked alloys during irradiation in spite of high dislocation density. It is caused by the presence of areas with lower dislocation density in alloys. At room temperature, the vacancies are not trapped by dislocations totally because a low their mobility. At elevated temperatures, vacancies may leave dislocations and form VCs. In cold-worked Fe-Ni-Ti alloy the accumulation of defects during irradiation is essentially lowered in comparison with cold worked Fe-Ni alloy. Ni3Ti precipitates, which are present on dislocations, reduce the efficiency of interaction of dislocations with interstitial atoms and, thereby, enhance the mutual recombination of point defects.

Perminov, D. A.; Druzhkov, A. P.; Arbuzov, V. L.

2013-06-01

226

Formation of isolated Zn vacancies in ZnO single crystals by absorption of ultraviolet radiation: a combined study using positron annihilation, photoluminescence, and mass spectroscopy.  

PubMed

Positron annihilation spectra reveal isolated zinc vacancy (V(Zn)) creation in single-crystal ZnO exposed to 193-nm radiation at 100 mJ/cm(2) fluence. The appearance of a photoluminescence excitation peak at 3.18 eV in irradiated ZnO is attributed to an electronic transition from the V(Zn) acceptor level at ~100 meV to the conduction band. The observed V(Zn) density profile and hyperthermal Zn(+) ion emission support zinc vacancy-interstitial Frenkel pair creation by exciting a wide 6.34 eV Zn-O antibonding state at 193-nm photon-a novel photoelectronic process for controlled V(Zn) creation in ZnO. PMID:23863026

Khan, Enamul H; Weber, Marc H; McCluskey, Matthew D

2013-07-01

227

The effects of vacuum annealing on the top-most layer of 6H-SiC measured by Positron annihilation induced Auger Spectroscopy  

NASA Astrophysics Data System (ADS)

Silicon Carbide (SiC) in monocrystalline, hexagonal polytype form is a very interesting material for a wide class of novel application in electronics. The wide range of the band gap offered by different polytype with very little lattice mismatch can be utilized to grow smooth heterojunctions. Till now it has not been achieved and hence the surface characterization of such crystals is critical. Positron Annihilation induced Auger Electron Spectroscopy (PAES) is an established tool to characterize the top most atomic surface layer of solids. Here, PAES has been used to study the surface of 6H- SiC after annealing under different thermal and ambient conditions. The PAES measurements indicate that top-most atomic layer becomes C rich after vacuum annealing at 800 C. In additional a large chemical shift in the Si peak of approximately -12 eV was observed with PAES.

Mukherjee, S.; Nadesalingam, M.; Davis, B.; Brauer, G.; Weiss, A. H.

2007-03-01

228

Positron annihilation study of structural relaxation and crystallization in a metallic glass Al80Fe8Mo7Ni5 alloy  

NASA Astrophysics Data System (ADS)

Kinetic transformations induced in the metallic glass Al80Fe8Mo7Ni5 by thermal treatments from the amorphous to the crystallized state, following the variation of the Doppler broadening of the positron annihilation line-shaped S-and W-parameters were reported. The behavior of the annihilation line-shape shows reversible and non-reversible contributions. Crystallization of the sample appears as a three-stage process. The corresponding crystallization process consists of only one activation enthalpy, Eai. The mean value obtained for Eai is: Eam=(3.65±0.14) eV. In-situ high-temperature X-ray diffraction showed that the crystallization was a complex involving growth of the Al7Fe8Mo crystalline phase along with crystallization of the amorphous matrix Al phase at T ?500 °C. Electron micrograph with image analysis of the scanning electron microscopy pictures confirmed the presence of fcc-Al and Al7Fe8Mo crystalline phase.

Abo-Elsoud, M.

2013-04-01

229

Influence of Pb impurities on He-bubble formation and growth in Al studied by positron annihilation and transmission electron microscopy  

NASA Astrophysics Data System (ADS)

Positron-lifetime and transmission-electron-microscopy measurements are reported for He (4.5 at. ppm), in pure Al, and Pb-contaminated (~1 at. ppm) Al:0.5 wt. % Ni alloy. The observed He-bubble growth in pure Al was found to follow at t1/5 dependence, i.e., was in good agreement with previously published data. The Pb-contaminated alloy exhibited accelerated He agglomerate and bubble growth. Bubbles observed by transmission electron microscopy in the Pb-contaminated alloy were found to have precipitates attached to them. No precipitates were observed that were not associated with bubbles and no bubbles observed that did not have a precipitate attached. Agglomerate and bubble radii were calculated using the positron-lifetime data. Again the results for pure Al were found to be in good agreement with previously published data. In the Pb-contaminated alloy agglomeration and coalescence occurred at temperatures as low as ~80 °C, while accrual of free volume started at ~180 °C. An increase in bubble growth rate, which occurred between ~350° and ~400 °C, was tentatively associated with the melting of Pb precipitates.

Usmar, S. G.; Wright, R. N.

1992-07-01

230

Gamma-ray spectroscopy of the galactic center region: Confirmation of the time-variability of the positron annihilation line  

NASA Technical Reports Server (NTRS)

The GSFC Low-Energy Gamma-Ray Spectrometer observed the region of the galactic center during a balloon flight from Alice Springs, Australia, on 1981 November 20. No significant excess over background was evident in the 511 keV annihilation line. A 98 percent confidence upper limit is derived for this line of 1.2 x .001 photons/sq. cm-s. Continuum emission was detected above 100 keV with a best-fitting power law spectrum.

Paciesas, W. S.; Cline, T. L.; Teegarden, B. J.; Tueller, J.; Durouchoux, P.; Hameury, J. M.

1982-01-01

231

Positron Diffusion in Metals  

Microsoft Academic Search

From positron lifetime measurements in seven powders of Fe, Co, Ni, and W, the positron diffusion constant in metals at 300°K is found to be D+=(1.0+\\/-0.5)×10-2 cm2\\/sec. This agrees with a theory based on positron-electron scattering. Evidence is presented that positrons are trapped in metal surface states.

Robert Paulin; Roger Ripon; Werner Brandt

1973-01-01

232

Positron annihilation study of effects of Ti and plastic deformation on defect accumulation and annealing in electron-irradiated austenitic steels and alloys  

NASA Astrophysics Data System (ADS)

Positron annihilation spectroscopy was used to study the formation and annealing of vacancy clusters in 16Cr15Ni3Mo austenitic steels and Fe-36%Ni model alloys and in the same compounds containing 1.02 and 2.5 wt% titanium respectively. Defects were induced by electron (5 MeV) irradiation at 270-573 K. How a developed initial dislocation structure influenced the accumulation and annealing of vacancy defects in these steels and alloys was analyzed. It was shown that vacancies interacted with titanium atoms. As a result, the Ti-containing steels and alloys exposed to radiation at temperatures from 270 to 423 K had an enhanced concentration of fine vacancy clusters decorated with titanium, which were thermally stable up to 450 K. A high initial dislocation density in the deformed steels and alloys led to a several-fold decrease in the concentration of vacancy clusters as compared to their concentration in the solution annealed state. The formation of fine TiC particles in Ti-modified deformed steel was monitored at the annealing temperatures from 850 to 1070 K.

Druzhkov, A. P.; Arbuzov, V. L.; Perminov, D. A.

2005-05-01

233

Positron-Annihilation Studies of Frank-Kasper-Type Quasicrystals of the Mg-Zn-Ga-Al and the Mg-Zn-Rare-Earth System  

NASA Astrophysics Data System (ADS)

We have performed positron-annihilation experiments for a Frank-Kasper (F-K)-type icosahedral quasicrystal, 2/1 and 1/1 approximant crystals of Mg-Zn-Ga-Al, and the stable F-K-type icosahedral quasicrystals of Mg-Zn-RE (RE=rare-earth metal, Y, Ho and Gd) alloys. It is shown that a high density of vacant centers in triacontahedral clusters exists in both icosahedral and approximant phases of Mg-Zn-Ga-Al. Furthermore, in the present study, we report a very interesting finding that stable non-Al-based quasicrystals such as the icosahedral quasicrystal Mg-Zn-RE system also contain a high density of structural vacancies, similar to the stable Al-based quasicrystals. In particular, it is estimated experimentally that the structural vacancy densities for icosahedral quasicrystal Mg34Zn58Y8 and Mg40Zn52Ho8 are 3 × 1020 and 4 × 1020 cm-3, respectively. These results provide important information on the structural modeling of the stable icosahedral quasicrystal Mg-Zn-RE, which is a recent research topic of the icosahedral quasicrystals.

Hamada, Eisaku; Sato, Kiminori; Takahashi, Yasufumi; Uchiyama, Hidefumi; Kanazawa, Ikuzo; Oshima, Nagayasu; Suzuki, Takenori; Nakata, Masataka; Yoshida, Tomoyoshi; Takeuchi, Shin

2001-01-01

234

Study of defects in an electroresistive Au/La2/3Sr1/3MnO3/SrTiO3(001) heterostructure by positron annihilation  

NASA Astrophysics Data System (ADS)

Defects in an ultrathin Au/La2/3Sr1/3MnO3/SrTiO3 (Au/LSMO/STO) heterostructure displaying electroresistive behavior were studied using variable energy positron annihilation spectroscopy. Vacancy-like defects were found to be the dominant positron traps in the LSMO and STO thin perovskite oxides with a number density >1017 cm-3 and 2 × 1017 cm-3 in the STO substrate. High defect density was revealed by strong positron trapping at the Au/LSMO interface. Oxygen deficiency in LSMO would be the main source of these traps. Besides, a low density of sub-nano voids of ˜6 Å was found in the substrate and in the thin LSMO/STO films.

Ferragut, R.; Dupaquier, A.; Brivio, S.; Bertacco, R.; Egger, W.

2011-09-01

235

In 3+ substitution effects and defect distribution in Li 0.25Mg 0.5Mn 0.1Fe 2.15-xIn xO 4 studied by positron annihilation and Mössbauer spectroscopy  

NASA Astrophysics Data System (ADS)

The ferrite series Li 0.25Mg 0.5Mn 0.1Fe 2.15-xIn xO 4 ( x=0, 0.3, 0.6, 0.9) have been studied through positron lifetime and Mössbauer spectroscopic techniques. The changes in positron lifetime parameters are attributed to positron trapping in tetrahedral and octahedral sites at the different stages of In 3+ ion substitution. Positrons initially get trapped in the vacancies in the octahedral sites in the undoped sample. With doping by In 3+ ions upto x=0.3, tetrahedral vacancies dominate as positron trapping sites. With further doping, some of the octahedral sites become vacancies and trap positrons. Incorporation of In 3+ ions in the place of Fe 3+ ions in the series has also introduced magnetic dilution in both the tetrahedral (A) and octahedral (B) sites. Knowledge of the variation in the magnetic properties of these ferrites with increase in concentration of In 3+ ions, as derived from the Mössbauer parameters, suggests a correlation of such a variation with the defect concentration in these samples.

Ghosh, Sanjukta; Nambissan, P. M. G.; Bhattacharya, R.

2004-11-01

236

Construction of concentration density profile across the interface in SAN/EVA immiscible blend from positron lifetime parameters  

NASA Astrophysics Data System (ADS)

The interface width determination through the construction of composition density profile across the interface in an immiscible binary polymer blend using ortho-positronium lifetime parameters is described in this paper. The distribution of free volume and hence the hydrodynamic interaction parameter has been evaluated for this purpose making use of the CONTIN routine analysis of the lifetime spectra. The results showed the broad free volume distribution and narrow interface width were reminiscent of lack of interaction between SAN and EVA the constituents.

Ramya, P.; Meghala, D.; Pasang, T.; Ranganathaiah, C.

2013-02-01

237

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)

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.

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

1994-01-01

238

Positron Physics  

NASA Astrophysics Data System (ADS)

This book provides a comprehensive and up-to-date account of the field of low energy positrons and positronium within atomic and molecular physics. It begins with an introduction to the field, discussing the background to low energy positron beams, and then covers topics such as total scattering cross sections, elastic scattering, positronium formation, excitation and ionization, annihilation and positronium interactions. Each chapter contains a blend of theory and experiment, giving a balanced treatment of all the topics. The book will be useful for graduate students and researchers in physics and chemistry. It is ideal for those wishing to gain rapid, in-depth knowledge of this unique branch of atomic physics.

Charlton, M.; Humberston, J. W.

2000-12-01

239

The behavior of 3d electrons and defects in TiAl-based alloys containing V and Cu studied by positron annihilation  

NASA Astrophysics Data System (ADS)

Information of defects and 3d electrons in transition metals (Ti, V, Cu) and TiAl-based alloys (Ti50Al50, Ti50Al48V2, Ti50Al48Cu2) can be extracted from the positron lifetime and coincidence Doppler broadening spectra. The results show that the 3d electron signals for the transition metals Ti, V and Cu increase with the number of 3d electrons. The 3d electron signal and the electron density for binary TiAl alloy are relatively low due to the (Ti)3d-(Al)3p interactions. The addition of V and Cu atoms to TiAl alloy leads to the increase in the electron densities in bulk and the defects on grain boundaries simultaneously, as well as the enhancement of the 3d electron signal. The 3d electron signal in the spectrum of Ti50Al48Cu2 alloy is higher than that of Ti50Al48V2 alloy.

Deng, Wen; Chen, Zhenying; Jiang, Haifeng; Sun, Shunping; Zhu, Yingying; Huang, Yuyang

2008-09-01

240

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)

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.

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

1998-01-01

241

Inhibition and enhancement of positronium formation in aqueous solutions from the Doppler broadening of annihilation peaks  

SciTech Connect

The Doppler broadened positron annihilation line shapes have been determined in aqueous solutions of various inhibiting and enhancing agents of positronium (Ps) formation : H/sup +//sub aq/, NO/sup -//sub 3/, S2/sup -/, SCN/sup -/ and the halide ions. The results confirm that the inhibition is made through electron scavenging by H/sup +//sub aq/ and NO/sup -//sub 3/, and through positron capture, to form a bound state, by the other ions. The characteristics and yields of the various positron-containing species in the solutions are inferred from the deconvoluted experimental line shapes. On referring to the lifetime spectroscopy experimental data, it is found that the yields of positron bound states are lower than expected, and the formation of excited states is hypothesized to account for the discrepancy.

Duplatre, G.; Abbe, J.C.; Maddock, A.G.; Haessler, A.

1980-01-01

242

Temperature dependence of positron trapping at grain boundaries  

NASA Astrophysics Data System (ADS)

Positron lifetime spectra were measured for fine-grained samples of the superplastic alloy Al - 5 wt% Ca - 5 wt% Zn at temperatures from 10 to 295 K. The lifetime attributed to annihilation from traps at the grain interfaces was found to increase with the temperature, while the corresponding intensity was observed to decrease. The quantitative analysis of the experimental results according to the diffusion-trapping model (Dupasquier et al 1993 Phys. Rev. B 48 9235) leads to the following conclusions: (a) the positron diffusion coefficient in the alloy matrix (a solid solution of Zn and Ca in Al) is limited by positron - phonon scattering as well as by positron - impurity interaction; (b) the phonon-associated term in the reciprocal of the diffusion coefficient is dominant at room temperature and scales at other temperatures with the same power law as holds for pure Al (Soininen et al 1990 Phys. Rev. B 41 6277); (c) the term associated with positron - impurity scattering is small except at very low temperatures, but the positron - impurity interaction seems to give a localization effect that is more important than the scattering; and (d) the specific trapping rate at the interface has a negative temperature dependence, as expected for trapping mediated by a precursor shallow state.

Aina, S.; Dupasquier, A.; Folegati, P.; DeDiego, N.; del Rio, J.; Somoza, A.; Valli, M.

1997-08-01

243

Microwave irradiation induced modifications on the interfaces in SAN/EVA/PVC and PVAc/BPA/PVP ternary polymer blends: Positron lifetime study  

NASA Astrophysics Data System (ADS)

Ternary polymer blends of poly(styrene-co-acrylonitrile)/poly(ethylene-co-vinylacetate)/poly(vinyl chloride) (SAN/EVA/PVC) and poly(vinyl acetate)/bisphenol A/polyvinylpyrrolidone (PVAc/BPA/PVP) with different compositions have been prepared by solvent casting method and characterized by positron lifetime spectroscopy and differential scanning calorimetry DSC. Phase modifications have been induced by irradiating the blends with microwave radiation. These changes have been monitored by measuring the free-volume content in the blends. The results clearly show improved interactions between the constituent polymers of the blends upon microwave irradiation. However, the free-volume data and DSC measurements are found to be inadequate to reveal the changes at the interfaces and the interfaces determine the final properties of the blend. For this we have used hydrodynamic interaction (?ij) approach developed by us to measure strength of hydrodynamic interaction at the interfaces. These results show that microwave irradiation stabilizes the interfaces if the blend contains strong polar groups. SAN/EVA/PVC blend shows an increased effective hydrodynamic interaction from -3.18 to -4.85 at composition 50/35/15 upon microwave irradiation and PVAc/BPA/PVP blend shows an increased effective hydrodynamic interaction from -3.81 to -7.57 at composition 20/50/30 after irradiation.

Dinesh, Meghala; Chikkakuntappa, Ranganathaiah

2013-09-01

244

High-field penning-malmberg trap: confinement properties and use in positron accumulation  

SciTech Connect

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.

Hartley, J.H.

1997-09-01

245

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

Microsoft Academic Search

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.

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

246

Implications of the first AMS-02 measurement for dark matter annihilation and decay  

NASA Astrophysics Data System (ADS)

In light of the first measurement of the positron fraction by the AMS-02 experiment, we perform a detailed global analysis on the interpretation of the latest data of PAMELA, Fermi-LAT, and AMS-02 in terms of dark matter (DM) annihilation and decay in various propagation models. The allowed regions for the DM particle mass and annihilation cross section or decay life-time are obtained for channels with leptonic final states: 2e, 2?, 2?, 4e, 4? and 4?. We show that for the conventional astrophysical background the AMS-02 positron fraction data alone favour a DM particle mass ~ 500(800) GeV if DM particles annihilate dominantly into 2?(4?) final states, which is significantly lower than that favoured by the Fermi-LAT data of the total flux of electrons and positrons. The allowed regions by the two experiments do not overlap at a high confidence level (99.99999%C.L.). We consider a number of propagation models with different halo height Zh, diffusion parameters D0 and ?1/2, and power indices of primary nucleon sources ?p1/p2. The normalization and the slope of the electron background are also allowed to vary. We find that the tension between the two experiments can be only slightly reduced in the propagation model with large Zh and D0. The consistency of fit is improved for annihilation channels with 2? and 4? final states which favour TeV scale DM particle with large cross sections above ~ 10-23 cm3s-1. In all the considered leptonic channels, the current data favour the scenario of DM annihilation over DM decay. In the decay scenario, the charge asymmetric DM decay is slightly favoured.

Jin, Hong-Bo; Wu, Yue-Liang; Zhou, Yu-Feng

2013-11-01

247

Positron production by pulsars  

NASA Technical Reports Server (NTRS)

Calculations based on two specific pulsar models show that in either case the 511 keV gamma-ray line from the galactic center can be explained as the as the result of the annihilation of positrons produced by pulsars.

Sturrock, P.; Baker, K. B.

1979-01-01

248

Cross-Section for Double Charmonium Production in Electron-Positron Annihilation at Energy ? {s} = 10.6 GeV  

NASA Astrophysics Data System (ADS)

In this work we study the process e++e-?J/?+?c at energy ? {s} = 10.6 GeV observed recently at B-factories whose measurements were made by Babar and Belle groups. We calculate the cross-section for this process in the Bethe-Salpeter formalism under Covariant Instantaneous Anstaz. To simplify our calculation, the heavy quark approximation is employed in the quark and gluon propagators. In the exclusive process of e+e- annihilation into two heavy quarkonia, the cross-section calculated in this scenario is compatible with the experimental data of Babar and Belle.

Mengesha, Elias; Bhatnagar, Shashank

249

Surface and bulk investigations at the high intensity positron beam facility NEPOMUC  

NASA Astrophysics Data System (ADS)

The NEutron-induced POsitron source MUniCh (NEPOMUC) at the research reactor FRM II delivers a low-energy positron beam ( E = 15-1000 eV) of high intensity in the range between 4 × 10 7 and 5 × 10 8 moderated positrons per second. At present four experimental facilities are in operation at NEPOMUC: a coincident Doppler-broadening spectrometer (CDBS) for defect spectroscopy and investigations of the chemical vicinity of defects, a positron annihilation-induced Auger-electron spectrometer (PAES) for surface studies and an apparatus for the production of the negatively charged positronium ion Ps -. Recently, the pulsed low-energy positron system (PLEPS) has been connected to the NEPOMUC beam line, and first positron lifetime spectra were recorded within short measurement times. A positron remoderation unit which is operated with a tungsten single crystal in back reflection geometry has been implemented in order to improve the beam brilliance. An overview of NEPOMUC's status, experimental results and recent developments at the running spectrometers are presented.

Hugenschmidt, C.; Dollinger, G.; Egger, W.; Kögel, G.; Löwe, B.; Mayer, J.; Pikart, P.; Piochacz, C.; Repper, R.; Schreckenbach, K.; Sperr, P.; Stadlbauer, M.

2008-10-01

250

Precise Determination of the Strong Coupling Constant at NNLO in QCD from the Three-Jet Rate in Electron-Positron Annihilation at LEP  

SciTech Connect

We present the first determination of the strong coupling constant from the three-jet rate in e{sup +}e{sup -} annihilation at LEP, based on a next-to-next-to-leading-order (NNLO) perturbative QCD prediction. More precisely, we extract {alpha}{sub s}(M{sub Z}) by fitting perturbative QCD predictions at O({alpha}{sub s}{sup 3}) to data from the ALEPH experiment at LEP. Over a large range of the jet-resolution parameter y{sub cut}, this observable is characterized by small nonperturbative corrections and an excellent stability under renormalization scale variation. We find {alpha}{sub s}(M{sub Z})=0.1175+-0.0020(expt)+-0.0015(theor), which is more accurate than the values of {alpha}{sub s}(M{sub Z}) from e{sup +}e{sup -} event-shape data currently used in the world average.

Dissertori, G. [Institute for Particle Physics, ETH Zurich, CH-8093 Zurich (Switzerland); Gehrmann-DeRidder, A. [Institute for Theoretical Physics, ETH Zurich, CH-8093 Zurich (Switzerland); Gehrmann, T. [Institut fuer Theoretische Physik, Universitaet Zuerich, CH-8057 Zuerich (Switzerland); Glover, E. W. N.; Heinrich, G. [Institute for Particle Physics Phenomenology, Department of Physics, University of Durham, Durham, DH1 3LE (United Kingdom); Stenzel, H. [II. Physikalisches Institut, Justus-Liebig Universitaet Giessen, D-35392 Giessen (Germany)

2010-02-19

251

Effects of post-irradiation annealing and re-irradiation on microstructure in surveillance test specimens of the Loviisa-1 reactor studied by atom probe tomography and positron annihilation  

NASA Astrophysics Data System (ADS)

This paper presents a microstructural study of a surveillance test specimen from the Loviisa-1 reactor in Finland, which is a Russian-type pressurized water reactor (VVER-440), after initial irradiation to a neutron fluence of 2.5 × 1019 n/cm2 (E > 1 MeV), post-irradiation annealing at 475 °C for 100 h and re-irradiation to three different fluences up to 2.7 × 1019 n/cm2. Atom probe tomography (APT) and positron annihilation spectroscopy (PAS) were used to characterize the test specimens. APT results showed the formation of Cu-rich solute clusters (SCs) during the initial irradiation and their subsequent coarsening during annealing. After re-irradiation, a small number of SCs formed once again. The hardening due to the SCs was estimated using the Russell-Brown model based on the APT results, and was in good agreement with the measured hardening after the initial irradiation and post-irradiation annealing. In contrast, during the first-step of re-irradiation, the estimated hardening due to the SCs was smaller than the measured hardening. This suggested that the hardening after re-irradiation was due to some microstructure other than the observed SCs. This difference was attributed to newly-formed matrix defects during re-irradiation, which was supported by the PAS results. However in subsequent steps of re-irradiation, the hardening was almost constant.

Toyama, T.; Kuramoto, A.; Nagai, Y.; Inoue, K.; Nozawa, Y.; Shimizu, Y.; Matsukawa, Y.; Hasegawa, M.; Valo, M.

2014-06-01

252

The intense slow positron beam facility at the PULSTAR reactor and applications in nano-materials study  

NASA Astrophysics Data System (ADS)

An intense slow positron beam has been established at the PULSTAR nuclear research reactor of North Carolina State University. The slow positrons are generated by pair production in a tungsten moderator from gammarays produced in the reactor core and by neutron capture reactions in cadmium. The moderated positrons are electrostatically extracted and magnetically guided out of the region near the core. Subsequently, the positrons are used in two spectrometers that are capable of performing positron annihilation lifetime spectroscopy (PALS) and positron Doppler broadening spectroscopy (DBS) to probe the defect and free volume properties of materials. One of the spectrometers (e+-PALS) utilizes an rf buncher to produce a pulsed beam and has a timing resolution of 277 ps. The second spectrometer (Ps-PALS) uses a secondary electron timing technique and is dedicated to positronium lifetime measurements with an approximately 1 ns timing resolution. PALS measurements have been conducted in the e+-PALS spectrometer on a series of nano-materials including organic photovoltaic thin films, membranes for filtration, and polymeric fibers. These studies have resulted in understanding some critical issues related to the development of the examined nano-materials.

Liu, Ming; Moxom, Jeremy; Hawari, Ayman I.; Gidley, David W.

2013-04-01

253

The intense slow positron beam facility at the PULSTAR reactor and applications in nano-materials study  

SciTech Connect

An intense slow positron beam has been established at the PULSTAR nuclear research reactor of North Carolina State University. The slow positrons are generated by pair production in a tungsten moderator from gammarays produced in the reactor core and by neutron capture reactions in cadmium. The moderated positrons are electrostatically extracted and magnetically guided out of the region near the core. Subsequently, the positrons are used in two spectrometers that are capable of performing positron annihilation lifetime spectroscopy (PALS) and positron Doppler broadening spectroscopy (DBS) to probe the defect and free volume properties of materials. One of the spectrometers (e{sup +}-PALS) utilizes an rf buncher to produce a pulsed beam and has a timing resolution of 277 ps. The second spectrometer (Ps-PALS) uses a secondary electron timing technique and is dedicated to positronium lifetime measurements with an approximately 1 ns timing resolution. PALS measurements have been conducted in the e{sup +}-PALS spectrometer on a series of nano-materials including organic photovoltaic thin films, membranes for filtration, and polymeric fibers. These studies have resulted in understanding some critical issues related to the development of the examined nano-materials.

Liu, Ming; Moxom, Jeremy; Hawari, Ayman I. [Nuclear Reactor Program, Department of Nuclear Engineering, North Carolina State University, P.O. Box 7909, Raleigh, NC 27695 (United States); Gidley, David W. [Department of Physics, University of Michigan, 450 Church Street, Ann Arbor MI 48109 (United States)

2013-04-19

254

Screening of a positron in an inhomogeneous electron gas  

Microsoft Academic Search

Most calculations of positron annihilation characteristics in metals avoid direct determination of the effect of electron-positron interaction and use heuristic approaches such as the local density, generalized gradient, or weighted density approximations, benefitting in this way of enhancement factors calculated for an electron gas. As shown in the paper these approaches lead to quite different values of the local annihilation

H. Stachowiak; E. Boronski

2001-01-01

255

Positrons as imaging agents and probes in nanotechnology  

NASA Astrophysics Data System (ADS)

Positron emission tomography (PET) tracks a positron emitting radiopharmaceutical injected into the body and generates a 3-dimensional image of its location. Introduced in the early 70s, it has now developed into a powerful medical diagnostic tool for routine clinical use as well as in drug development. Unrivalled as a highly sensitive, specific and non-invasive imaging tool, PET unfortunately lacks the resolution of Computer Tomography (CT) and Magnetic Resonance Imaging (MRI). As the resolution of PET depends significantly on the energy of the positron incorporated in the radiopharmaceutical and its interaction with its surrounding tissue, there is growing interest in expanding our understanding of how positrons interact at the atomic and molecular level. A better understanding of these interactions will contribute to improving the resolution of PET and assist in the design of better imaging agents. Positrons are also used in Positron Annihilation Lifetime Spectroscopy (PALS) to determine electron density and or presence and incidence of micro- and mesopores (0.1 to 10 nm) in materials. The control of porosity in engineered materials is crucial for applications such as controlled release or air and water resistant films. Equally important to the design of nano and microtechnologies, is our understanding of the microenvironments within these pores and on surfaces. Hence as radiopharmaceuticals are designed to track disease, nuclear probes (radioactive molecules) are synthesized to investigate the chemical properties within these pores. This article will give a brief overview of the present role of positrons in imaging as well as explore its potential to contribute in the engineering of new materials to the marketplace.

Smith, Suzanne V.

2009-09-01

256

A calorimetric measurement of the strong coupling constant in electron-positron annihilation at a center-of-mass energy of 91.6 GeV  

SciTech Connect

In this work, a measurement of the strong coupling constant {alpha}{sub s} in e{sup +}e{sup {minus}} annihilation at a center-of-mass energy of 91.6 GeV is presented. The measurement was performed with the SLD at the Stanford Linear Collider facility located at the Stanford Linear Accelerator Center in California. The procedure used consisted of measuring the rate of hard gluon radiation from the primary quarks in a sample of 9,878 hadronic events. After defining the asymptotic manifestation of partons as `jets`, various phenomenological models were used to correct for the hadronization process. A value for the QCD scale parameter {Lambda}{sub bar MS}, defined in the {sub bar MS} renormalization convention with 5 active quark flavors, was then obtained by a direct fit to O({alpha}{sub s}{sup 2}) calculations. The value of {alpha}{sub s} obtained was {alpha}{sub s}(M{sub z0}) = 0.122 {plus_minus} 0.004 {sub {minus}0.007} {sup +0.008} where the uncertainties are experimental (combined statistical and systematic) and theoretical (systematic) respectively. Equivalently, {Lambda}{sub bar MS} = 0.28 {sub {minus}0.10}{sup +0.16} GeV where the experimental and theoretical uncertainties have been combined.

Martirena, S.G.

1994-04-01

257

Low Energy Positron Interactions with Biological Molecules  

NASA Astrophysics Data System (ADS)

There is some experimental evidence that positrons can produce distinctive molecular fragmentation patterns. It is known that tuning the incident positron energy to near resonance with molecule vibrations can strongly enhance the positron annihilation probability for a molecule. This suggests that fragmentation induced by slow positrons may provide valuable complementary information to existing techniques for identification and study of proteins. In order to study this concept, we are developing a general quantum method for reliably calculating the density distribution for positrons bound to large biological molecules using NEO/GAMESS. We find that the outer molecular orbitals as well as the higher p orbitals on the O atoms contribute heavily to the total annihilation rate. Using the basis sets and approximations we have tested to predict where annihilation occurs can ultimately help us understand the resulting fragmentation patterns of larger biological molecules.

Wanniarachchi, Indika; Morgan, Caroline; Schlegel, Bernhard; Kedziora, Gary; Burgrraf, Larry; Pak, Michael; Hammes-Schiffer, Sharon

2012-10-01

258

Descriptions of positron defect analysis capabilities  

SciTech Connect

A series of descriptive papers and graphics appropriate for distribution to potential collaborators has been assembled. These describe the capabilities for defect analysis using positron annihilation spectroscopy. The application of positrons to problems in the polymer and semiconductor industries is addressed.

Howell, R.H.

1994-10-01

259

What is the fate of runaway positrons in tokamaks?  

DOE PAGESBeta

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.

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

2014-04-01

260

What is the fate of runaway positrons in tokamaks?  

SciTech Connect

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.

Liu, Jian [Department of Modern Physics and Collaborative Innovation Center for Advanced Fusion Energy and Plasma Sciences, University of Science and Technology of China, Hefei, Anhui 230026 (China); Qin, Hong, E-mail: hongqin@ustc.edu.cn [Department of Modern Physics and Collaborative Innovation Center for Advanced Fusion Energy and Plasma Sciences, University of Science and Technology of China, Hefei, Anhui 230026 (China); Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543 (United States); Fisch, Nathaniel J.; Teng, Qian [Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543 (United States); Wang, Xiaogang [School of Physics, Peking University, Beijing 100871 (China)

2014-06-15

261

Transport of positrons in the interstellar medium  

E-print Network

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.

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

2007-02-06

262

A measurement of the D 0 lifetime  

Microsoft Academic Search

We have determined the D 0 lifetime from reconstructed vertices of D 0 mesons produced in e + e - annihilations at an average center of mass energy of 42.2 GeV. From fifteen events the D 0 lifetime was determined to be (4.3{-1.4\\/+2.0}±0.8)×10-13s.

M. Althoff; W. Braunschweig; R. Gerhards; F. J. Kirschfink; H.-U. Martyn; P. Rosskamp; W. Wallraff; B. Bock; J. Eisenmann; H. M. Fischer; H. Hartmann; A. Jocksch; H. Kolanoski; V. Mertens; R. Wedemeyer; B. Foster; E. Bernardi; Y. Eisenberg; A. Eskreys; K. Gather; H. Hultschig; P. Joos; B. Klima; H. Kowalski; A. Ladage; B. Löhr; D. Lüke; P. Mättig; D. Notz; D. Revel; E. Ronat; W. Schütte; D. Trines; T. Tymieniecka; R. Walczak; G. Wolf; W. Zeuner; E. Hilger; T. Kracht; H. L. Krasemann; J. Krüger; E. Lohrmann; G. Poelz; K.-U. Pösnecker; D. M. Binnie; P. J. Dornan; D. A. Garbutt; C. Jenkins; W. G. Jones; J. K. Sedgbeer; D. Su; J. Thomas; W. A. T. Wan Abdullah; F. Barreiro; E. Ros; M. G. Bowler; P. Bull; R. J. Cashmore; P. Dauncey; R. Devenish; G. Heath; D. J. Mellor; P. Ratoff; J. M. Yelton; S. L. Lloyd; G. E. Forden; J. C. Hart; D. K. Hasell; D. H. Saxon; S. Brandt; M. Holder; L. Labarga; B. Neumann; U. Karshon; G. Mikenberg; R. Mir; A. Montag; G. Yekutieli; A. Shapira; G. Baranko; A. Caldwell; M. Cherney; D. Demille; J. M. Izen; J. Messersmith; S. Ritz; D. Strom; M. Takashima; E. Wicklund; Sau Lan Wu; G. Zobernig

1986-01-01

263

A measurement of the D 0 lifetime  

Microsoft Academic Search

We have determined theD0 lifetime from reconstructed vertices ofD0 mesons produced ine+e- annihilations at an average center of mass energy of 42.2 GeV. From fifteen events theD0 lifetime was determined to be (4.3-1.4+2.0±0.8)×10-13s.

M. Althoff; W. Braunschweig; R. Gerhards; F. J. Kirschfink; H.-U. Martyn; P. Rosskamp; W. Wallraff; B. Bock; J. Eisenmann; H. M. Fischer; H. Hartmann; A. Jocksch; H. Kolanoski; V. Mertens; R. Wedemeyer; B. Foster; E. Bernardi; Y. Eisenberg; A. Eskreys; K. Gather; H. Hultschig; P. Joos; B. Klima; H. Kowalski; A. Ladage; B. Löhr; D. Lüke; P. Mättig; D. Notz; D. Revel; E. Ronat; W. Schütte; D. Trines; T. Tymieniecka; R. Walczak; G. Wolf; W. Zeuner; E. Hilger; T. Kracht; H. L. Krasemann; J. Krüger; E. Lohrmann; G. Poelz; K.-U. Pösnecker; D. M. Binnie; P. J. Dornan; D. A. Garbutt; C. Jenkins; W. G. Jones; J. K. Sedgbeer; D. Su; J. Thomas; W. A. T. Wan Abdullah; F. Barreiro; E. Ros; M. G. Bowler; P. Bull; R. J. Cashmore; P. Dauncey; R. Devenish; G. Heath; D. J. Mellor; P. Ratoff; J. M. Yelton; S. L. Lloyd; G. E. Forden; J. C. Hart; D. K. Hasell; D. H. Saxon; S. Brandt; M. Holder; L. Labarga; B. Neumann; U. Karshon; G. Mikenberg; R. Mir; A. Montag; G. Yekutieli; A. Shapira; G. Baranko; A. Caldwell; M. Cherney; D. DeMille; J. M. Izen; J. Messersmith; S. Ritz; D. Strom; M. Takashima; E. Wicklund; Sau Lan Wu; G. Zobernig

1986-01-01

264

Positron trapping at grain boundaries  

SciTech Connect

The standard positron trapping model has often been applied, as a simple approximation, to the interpretation of positron lifetime spectra in situations of diffusion-controlled trapping. This paper shows that this approximation is not sufficiently accurate, and presents a model based on the correct solution of the diffusion equation, in the version appropriate for studying positron trapping at grain boundaries. The model is used for the analysis of new experimental data on positron lifetime spectra in a fine-grained Al-Ca-Zn alloy. Previous results on similar systems are also discussed and reinterpreted. The analysis yields effective diffusion coefficients not far from the values known for the base metals of the alloys.

Dupasquier, A. (Dipartimento di Fisica del Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano (Italy)); Romero, R.; Somoza, A. (Instituto de Fisica de Materiales Tandil, Facultad de Ciencias Exactas, Universidad Nacional del Centro de la Provincia de Buenos Aires, Pinto 399, 7000 Tandil (Argentina))

1993-10-01

265

Searching for dark matter with future cosmic positron experiments  

SciTech Connect

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.

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

2005-04-15

266

Positron microscopy  

SciTech Connect

The negative work function property that some materials have for positrons make possible the development of positron reemission microscopy (PRM). Because of the low energies with which the positrons are emitted, some unique applications, such as the imaging of defects, can be made. The history of the concept of PRM, and its present state of development will be reviewed. The potential of positron microprobe techniques will be discussed also.

Hulett, L.D. Jr.; Xu, J.

1995-02-01

267

Energy spectra of electrons and positrons emitted from surfaces as a result of low energy positron bombardment  

NASA Astrophysics Data System (ADS)

Recent measurements of the energy spectra of electrons and positrons emitted from metal and semiconductor surfaces as a result of bombardment with low energy electrons and positrons are reviewed. The high resolution positron re-emission spectra show a close correspondence with that part of the electron induced electron spectrum termed 'redistributed primaries' while positron impact-induced secondary electrons show a close correspondence with that part of the electron induced electron spectrum termed 'true secondaries'. Positron re-emission data from a SiC(0001) surface indicate that this surface has a negative positron workfunction. Peaks in the positron induced electron spectra for Ge(100) and SiC(0001) corresponding to Auger electrons resulting from the annihilation of positrons trapped in a surface state with core electrons in the surface layer are identified and discussed. Additional evidence for the existence of a positron surface state on SiC is provided by data indicating thermal desorption of positronium (Ps).

Weiss, Alex; Jung, E.; Kim, J. H.; Nangia, A.; Venkataraman, R.; Starnes, S.; Brauer, G.

1997-05-01

268

Positron spectroscopy in atomic and solid state physics  

Microsoft Academic Search

Basic research problems in Atomic Physics with positrons (total cross sections, Ramsauer minima, positronium formation, selective ionization) and some benchmark measurements in Solid State Physics using positron annihilation (He-created nano-voids in Si, Oxygen precipitates in Si, low ? materials) are discussed.

G. P. Karwasz; R. S. Brusa; A. Zecca

2003-01-01

269

VOLUME 79, NUMBER 1 P H Y S I C A L R E V I E W L E T T E R S 7 JULY 1997 Positron Annihilation with Inner-Shell Electrons in Noble Gas Atoms  

E-print Network

with Inner-Shell Electrons in Noble Gas Atoms Koji Iwata,1 G. F. Gribakin,2 R. G. Greaves,1 and C. M. Surko1 on inner-shell electrons are studied quantitatively in an isolated atomic system for the first time electrons. In the work presented here, we detect inner-shell annihilation by measuring the mo- mentum

Gribakin, Gleb

270

Dark matter annihilation in the universe  

NASA Astrophysics Data System (ADS)

The astronomical dark matter is an essential component of the Universe and yet its nature is still unresolved. It could be made of neutral and massive elementary particles which are their own antimatter partners. These dark matter species undergo mutual annihilations whose effects are briefly reviewed in this article. Dark matter annihilation plays a key role at early times as it sets the relic abundance of the particles once they have decoupled from the primordial plasma. A weak annihilation cross section naturally leads to a cosmological abundance in agreement with observations. Dark matter species subsequently annihilate — or decay — during Big Bang nucleosynthesis and could play havoc with the light element abundances unless they offer a possible solution to the 7Li problem. They could also reionize the intergalactic medium after recombination and leave visible imprints in the cosmic microwave background. But one of the most exciting aspects of the question lies in the possibility to indirectly detect the dark matter species through the rare antimatter particles — antiprotons, positrons and antideuterons — which they produce as they currently annihilate inside the galactic halo. Finally, the effects of dark matter annihilation on stars is discussed.

Salati, Pierre

2014-05-01

271

Positron beam studies of solids and surfaces: A summary  

NASA Astrophysics Data System (ADS)

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.

Coleman, P. G.

2006-02-01

272

Measurement of the tau lepton lifetime  

Microsoft Academic Search

We measure the ? lepton lifetime with ?+?? pairs in which one or both of the ?'s decays to three charged particles. The data were collected with the CLEO II detector operating at the electron-positron collider CESR at energies on and near the Y(4S). We use displacements of the three-track vertices to determine the ? lifetime. The results is ??

R. Balest; B. H Behrens; K. Cho; M. Daoudi; W. T Ford; M. Lohner; P. Rankin; J. Roy; J. G Smith; J. P Alexander; C. Bebek; B. E Berger; K. Berkelman; K. Bloom; David G Cassel; H. A Cho; D. M Coffman; D. S Crowcroft; M. Dickson; P. S Drell; D. J Dumas; R. Ehrlich; R. Elia; P. Gaidarev; R. S Galik; B. Gittelman; S. W Gray; D. L Hartill; B. K Heltsley; C. D Jones; S. L Jones; J. Kandaswamy; N. Katayama; P. C Kim; D. L Kreinick; Lee T; Liu Y; G. S Ludwig; J. Masui; J. Mevissen; N. B Mistry; C. R Ng; E. Nordberg; J. R Patterson; D. Peterson; D. Riley; A. Soffer; C. Ward; P. Avery; C. Prescott; Yang S; J. Yelton; G. Brandenburg; R. A Briere; Liu T; M. Saulnier; R. Wilson; H. Yamamoto; T. E Browder; Li F; J. L Rodriguez; T. Bergfeld; B. I. Eisenstein; J. Ernst; G. E Gladding; G. D Gollin; I. Karliner; M. Palmer; M. Selen; J. J. Thaler; K. W Edwards; K. W McLean; M. Ogg; A. Bellerive; D. I. Britton; R. Janicek; B. D. Macfarlane; P. M Patel; B. Spaan; A. J Sadoff; R. Ammar; P. Baringer; A. Bean; D. Besson; D. Coppage; N. Copty; R. Davis; N. Hancock; S. Kotov; I V Kravchenko; N. Kwak; S. Anderson; Y. Kubota; M. Lattery; J. J O'Neill; S. Patton; R. Poling; T. Riehle; A. Smith; V. Savinov; M. S Alam; S. B Athar; I. J Kim; Z. Ling; A. H Mahmood; H. Severini; C. R Sun; S. Timm; F. Wappler; J. E Duboscq; R. Fulton; D. Fujino; K. K. Gan; K. Honscheid; H. Kagan; R. Kass; J. Lee; M. Sung; A. Undrus; C. White; R. Wanke; A. Wolf; M. M. Zoeller; B. Nemati; S. J Richichi; W. R Ross; P. Skubic; M. Wood; M. Bishai; J. Fast; E. Gerndt; J. W Hinson; D. H Miller; E. I. Shibata; I. P. J Shipsey; M. Yurko; L. Gibbons; S. D Johnson; Y. Kwon; S. Roberts; E. H Thorndike; C. P Jessop; K. Lingel; H. Marsiske; M. L Perl; S. F Schaffner; R. Wang; T. E Coan; V. Fadeyev; I. Korolkov; Y. Maravin; I. Narsky; V. Shelkov; R. Stroynowski; J. Staeck; I. Volobouev; J. Ye; M. Artuso; A. Efimov; M. Gao; M. Goldberg; R. Greene; D. He; S. Kopp; G. C Moneti; Y. Mukhin; T. Skwarnicki; S. Stone; Xing X; J. Bartelt; S. E Csorna; V. Jain; S. Marka; A. Freyberger; D. Gibaut; K. Kinoshita; I. C Lai; P. Pomianowski; S. Schrenk; G. Bonvicini; D. Cinabro; B. Barish; M. Chadha; Chan S; G. Eigen; J. S Miller; C O'Grady; M. Schmidtler; J. Urheim; A. J Weinstein; F. Würthwein; D. M Asner; M. Athanas; D. W Bliss; W. S Brower; G San Martin; H. P Paar; J. Gronberg; C. M Korte; D. J Lange; R. Kutschke; S. Menary; R. J Morrison; S. Nakanishi; H. N Nelson; T. K Nelson; C. Qiao; J. D Richman; D. Roberts; A. Ryd; H. Tajima; M. S Witherell; K Kleinknecht; G Quast; J Raab; B Renk; H G Sander; P Van Gemmeren; C Zeitnitz; Jean-Jacques Aubert; A M Bencheikh; C Benchouk; A Bonissent; G Bujosa; D Calvet; J Carr; C A Diaconu; F Etienne; M Thulasidas; D Nicod; P Payre; D Rousseau; M Talby; I Abt; R W Assmann; C Bauer; Walter Blum; D Brown; H Dietl; Friedrich Dydak; G Ganis; C Gotzhein; K Jakobs; H Kroha; G Lütjens; Gerhard Lutz; W Männer; H G Moser; R H Richter; A Rosado-Schlosser; S Schael; Ronald Settles; H C J Seywerd; R Saint-Denis; G Wolf; R Alemany; J Boucrot; O Callot; A Cordier; F Courault; M Davier; L Duflot; J F Grivaz; P Heusse; M Jacquet; D W Kim; F R Le Diberder; J Lefrançois; A M Lutz; G Musolino; I A Nikolic; H J Park; I C Park; M H Schune; S Simion; J J Veillet; I Videau; D Abbaneo; P Azzurri; G Bagliesi; G Batignani; S Bettarini; C Bozzi; G Calderini; M Carpinelli; M A Ciocci; V Ciulli; R Dell'Orso; R Fantechi; I Ferrante; F Fidecaro; L Foà; F Forti; A Giassi; M A Giorgi; A Gregorio; F Ligabue; A Lusiani; P S Marrocchesi; A Messineo; G Rizzo; G Sanguinetti; A Sciabà; P Spagnolo; Jack Steinberger; Roberto Tenchini; G Tonelli; G Triggiani; C Vannini; P G Verdini; J Walsh; A P Betteridge; G A Blair; L M Bryant; F Cerutti; Y Gao; M G Green; D L Johnson; T Medcalf; L M Mir; P Perrodo; J A Strong; V Bertin; David R Botterill; R W Clifft; T R Edgecock; S Haywood; M Edwards; P Maley; P R Norton; J C Thompson; B Bloch-Devaux; P Colas; S Emery; Witold Kozanecki; E Lançon; M C Lemaire; E Locci; B Marx; P Pérez; J Rander; J F Renardy; A Roussarie; J P Schuller; J Schwindling; A Trabelsi; B Vallage; R P Johnson; H Y Kim; A M Litke; M A McNeil; G Taylor; A Beddall; C N Booth; R Boswell; S L Cartwright; F Combley; I Dawson; A Köksal; M Letho; W M Newton; C Rankin; L F Thompson; A Böhrer; S Brandt; G D Cowan; E Feigl; Claus Grupen; G Lutters; J A Minguet-Rodríguez; F Rivera; P Saraiva; L Smolik; F Stephan; M Apollonio; L Bosisio; R Della Marina; G Giannini; B Gobbo; F Ragusa; J E Rothberg; S R Wasserbaech; S R Armstrong; L Bellantoni; P Elmer; Z Feng; D P S Ferguson; S González; J Grahl; J L Harton; O J Hayes; H Hu; P A McNamara; J M Nachtman; W Orejudos; Y B Pan; Y Saadi; M Schmitt; I J Scott; V Sharma; J Turk; A M Walsh; Wu Sau Lan; X Wu; J M Yamartino; M Zheng; G Zobernig

1996-01-01

273

Annihilation physics of exotic galactic dark matter particles  

NASA Technical Reports Server (NTRS)

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.

Stecker, F. W.

1990-01-01

274

Diffusion constant and surface states of positrons in metals  

Microsoft Academic Search

Positron lifetime spectra and angular correlation curves for seven fine-grained powders of Fe, Co, Ni, and W are analyzed.\\u000a From the lifetime data, the positron diffusion constant in metals atT=300K was found to beD\\u000a +=(1.00.5)10?2 cm2 sec?1. Evidence is presented that positrons are trapped in metal surface states.

Robert Paulin; Roger Ripon; Werner Brandt

1974-01-01

275

Performance of an analytical positron range modelling approach in the context of whole body small animal and clinical PET  

Microsoft Academic Search

Monte Carlo simulation codes that model positron interactions along its tortuous path are expected to be accurate but slow. A simpler and potentially faster approach is to model positron range from analytical annihilation density distributions. We have implemented and validated such a method. 1D annihilation density distributions for different isotope-media combinations were fitted with Gaussian functions and described by simple

Wencke Lehnert; Marie-Claude Gregoire; Anthonin Reilhac; Steven R. Meikle

2011-01-01

276

Positrons in the Galaxy: Their Births, Marriages and Deaths  

NASA Technical Reports Server (NTRS)

High energy (approximately GeV) positrons are seen within cosmic rays and observation of a narrow line at 511 keV shows that positrons are annihilating in the galaxy after slowing down to approximately keV energies or less. Our state of knowledge of the origin of these positrons, of the formation of positronium 'atoms', and of the circumstances of their annihilation or escape from the galaxy are reviewed and the question of whether the two phenomena are linked is discussed.

Skinner, Gerald K.

2010-01-01

277

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

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

Chieng, Norman; Cicerone, Marcus T.; Zhong, Qin; Liu, Ming; Pikal, Michael J.

2013-01-01

278

Positron-molecule bound states and positive ion production  

NASA Technical Reports Server (NTRS)

The interaction was studied of low energy positrons with large molecules such as alkanes. These data provide evidencce for the existence of long lived resonances and bound states of positrons with neutral molecules. The formation process and the nature of these resonances are discussed. The positive ions produced when a positron annihilates with an electron in one of these resonances were observed and this positive ion formation process is discussed. A review is presented of the current state of the understanding of these positron-molecule resonances and the resulting positive ion formation. A number of outstanding issues in this area is also discussed.

Leventhal, M.; Passner, A.; Surko, C. M.

1990-01-01

279

Positron research at the University of Texas at Austin  

NASA Astrophysics Data System (ADS)

The objective of the research presented in this dissertation is to advance the applications of positron annihilation research. An intense positron beam facility was designed and constructed, and a method was developed to better analyze the defect structure of solids. The Texas Reactor-based Intense Positron beam facility (TRIP) was designed to provide a monoenergetic/monodirectional beam of at least 108 e +/sec on a sample. This increase in beam intensity will enhance many positron research techniques both in atomic physics and materials science. The TRIP facility, the result of a collaboration between UT Austin and UT Arlington, is being developed around the concept of multiple scattering of positrons from solid krypton. A large area copper source will be irradiated in a beam port of the 1 MW TRIGA Mark-II research reactor at UT Austin. The source will form the bottom face of a cube with the remaining faces made of tungsten. All surfaces will be cooled down to 22 K and coated with krypton. The top face of the cube has a 1 cm diameter hole to allow for the passage of positrons. The fast beta particles emitted from the ?+ decay of 64Cu will be moderated while passing through the krypton. The non- moderated positrons will lose their energy while interacting with the remaining walls. The positrons will be removed from the box by an electric field and electrostatically delivered to the sample. The work on the TRIP facility is ongoing. The results of early measurements at UT Arlington have shown that the facility will be able to achieve its goals. The method developed to better analyze the positron depth profiling (PDP) experiments uses the difference spectra of the measured Doppler broadened annihilation peaks. The difference spectra, which are obtained by subtracting the bulk peak shape from the peaks recorded for each incident positron energy, enhance the differences of the observed peaks, while removing the bulk annihilation term from the multi-state annihilation models. Knowledge of the energy dependent surface and defect annihilation probabilities enables one to directly find the depth dependent annihilation probabilities, which are a function of defect density. This method was applied to silicon samples implanted with various doses and energies of boron or boron fluoride. The PDP measurements were performed at the BNL positron beam facility. The results show good agreement between measurements and theoretical predictions.

Goktepeli, Sinan

280

Time evolution of positron affinity trapping at embedded nanoparticles by age-momentum correlation  

NASA Astrophysics Data System (ADS)

A positron annihilation age-momentum correlation (AMOC) technique using a digital oscilloscope with high time resolution enabled us to directly estimate positron trapping rate to nano- and subnanoparticles embedded in materials by observing the annihilation time evolution of the momentum distribution of affinity-trapped positron-electron pairs. As a representative case we successfully apply the present technique to (sub)nano Cu particles embedded in an Fe-Cu dilute alloy after thermal aging. This enhances the ability of the positron annihilation method as a quantitative tool to detect ultrafine embedded particles which are difficult to observe by other techniques. We also show that the AMOC measurements give chemical information on the embedded particles through the positron trapping kinetic behavior.

Inoue, K.; Nagai, Y.; Tang, Z.; Toyama, T.; Hosoda, Y.; Tsuto, A.; Hasegawa, M.

2011-03-01

281

Gamma-Ray background spectrum and annihilation rate in the baryon-symmetric big-bang cosmology  

NASA Technical Reports Server (NTRS)

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.

Puget, J. L.

1973-01-01

282

Positron Physics  

NASA Technical Reports Server (NTRS)

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.

Drachman, Richard J.

2003-01-01

283

Microstructural Characterization of Polymers with Positrons  

NASA Technical Reports Server (NTRS)

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.

Singh, Jag J.

1997-01-01

284

Use of positrons to study transport in tokamak plasmas (invited)  

SciTech Connect

It now appears feasible to deposit positrons (e-italic/sup +/) in a tokamak plasma by injecting bursts of neutral positronium atoms (e-italic/sup +/e-italic/sup -/), which are then ionized by the plasma. The annihilation time of these positrons in the plasma is long compared with typical particle containment times. Thus the subsequent transport of the positrons can be studied by monitoring the time dependence of the annihilation, gamma radiation produced when the positrons strike a limiter. This paper discusses the design of such an experiment, the kinds of data which can be obtained, and the physics questions which this experiment might address. This diagnostic technique could also be useful in studying transport in other magnetic confinement devices such as reversed-field pinches and magnetic mirrors.

Surko, C.M.; Leventhal, M.; Crane, W.S.; Passner, A.; Wysocki, F.; Murphy, T.J.; Strachan, J.; Rowan, W.L.

1986-08-01

285

One-photon pair annihilation in magnetized relativistic plasmas  

NASA Technical Reports Server (NTRS)

In supersonic magnetic fields, electron-positron pairs may annihilate into single photons producing spectral features above 1 MeV. The paper calculates the exact one-photon annihilation rate in the general case where pairs may annihilate from excited Landau states, extending the previous studies which were restricted to pairs in the ground state. Asymptotic expressions for annihilation spectra and rates in the limit of large pair quantum numbers are also derived. It is found that the rate of annihilation from excited states can exceed the rate from the ground state by orders of magnitude in fields less than about 2 x 10 to the 12th G. This allows one-photon annihilation to be competitive with the two-photon process at typical neutron star field strengths. Annihilation spectra from a Maxwellian pair plasma at transrelativistic temperatures show fine structure near threshold on a scale (h/2pi)omega sub B as the result of contributions from individual pair states, which blend into a smooth continuum at higher energies.

Harding, A. K.

1986-01-01

286

Profiling multilayer structures with monoenergetic positrons  

SciTech Connect

Variable-energy (0--25 keV) positron stopping and annihilation behavior is studied in a multilayer structure, which has subsequent (approx.3000-A-thick) ZnS and Al/sub 2/O/sub 3/ layers on a glass substrate. Direct information on positron slowing-down properties is obtained. The positron implantation profile is shown to possess the shape of a derivative of a Gaussian function, in contrast to the more commonly used exponential profile. The mean positron penetration depth varies with incident positron energy E (in keV) as z-bar = (4.0(3) ..mu..g/cm/sup 2/)(E)/sup 1.62(5)/. The feasibility of the present technique for depth profiling of heterogeneous samples is considered. The accuracy of determining the positions of the interfaces is typically less than 100 A in the present system. The mobility of positrons in ZnS and Al/sub 2/O/sub 3/ layers is observed to be very low. This corresponds to positron trapping into structural defects with a relatively high concentration.

Vehanen, A.; Saarinen, K.; Hautojaervi, P.; Huomo, H.

1987-04-01

287

Cosmic Ray Positrons from Cosmic Strings  

E-print Network

We study the spectrum of cosmic ray positrons produced by a scaling distribution of non-superconducting cosmic strings. In this scenario, the positrons are produced from the jets which form from the cosmic string cusp annihilation process. The spectral shape is a robust feature of our scenario, and is in good agreement with the results from the recent PAMELA and ATIC experiments. In particular, the model predicts a sharp upper cutoff in the spectrum, and a flux which rises as the upper cutoff is approached. The energy at which the flux peaks is determined by the initial jet energy. The amplitude of the flux can be adjusted by changing the cosmic string tension and also depends on the cusp annihilation efficiency.

Brandenberger, Robert; Xue, Wei; Zhang, Xinmin

2009-01-01

288

Cosmic Ray Positrons from Cosmic Strings  

E-print Network

We study the spectrum of cosmic ray positrons produced by a scaling distribution of non-superconducting cosmic strings. In this scenario, the positrons are produced from the jets which form from the cosmic string cusp annihilation process. The spectral shape is a robust feature of our scenario, and is in good agreement with the results from the recent PAMELA and ATIC experiments. In particular, the model predicts a sharp upper cutoff in the spectrum, and a flux which rises as the upper cutoff is approached. The energy at which the flux peaks is determined by the initial jet energy. The amplitude of the flux can be adjusted by changing the cosmic string tension and also depends on the cusp annihilation efficiency.

Robert Brandenberger; Yi-Fu Cai; Wei Xue; Xinmin Zhang

2009-01-22

289

Low energy positrons at the Ga-rich (100) surface of GaAs  

Microsoft Academic Search

We present results of theoretical studies of positron surface states and annihilation characteristics at the Ga-rich (100) surface of GaAs with different reconstructions. Calculations are based on a treatment of a positron as a single charged particle trapped in a \\

Nail G. Fazleev; John L. Fry; Shannon G. Starnes; Alex H. Weiss

2000-01-01

290

Dark Matter Annihilation and the PAMELA, FERMI and ATIC Anomalies  

E-print Network

If dark matter (DM) 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 DM abundances that are incompatible with astrophysical observations. We explore possible resolutions to this apparent conflict in terms of non-standard cosmological scenarios; plausibly allowing for large cross sections, while maintaining relic abundances in accord with current observations.

A. A. El-Zant; S. Khalil; H. Okada

2012-03-14

291

QSO Lifetimes  

E-print Network

The QSO lifetime t_Q is one of the most fundamental quantities for understanding black hole and QSO evolution, yet it remains uncertain by several orders of magnitude. If t_Q is long, then only a small fraction of galaxies went through a luminous QSO phase. In contrast, a short lifetime would require most galaxies today to have undergone a QSO phase in their youth. The current best estimates or constraints on t_Q from black hole demographics and the radiative properties of QSOs vary from at least 10^6 to 10^8 years. This broad range still allows both possibilities: that QSOs were either a rare or a common stage of galaxy evolution. These constraints also do not rule out the possibility that QSO activity is episodic, with individual active periods much shorter than the total active lifetime. In the next few years a variety of additional observational constraints on the lifetimes of QSOs will become available, including clustering measurements and the proximity effect. These new constraints can potentially determine t_Q to within a factor of 3 and therefore answer one of the most fundamental questions in black hole evolution: Do they shine as they grow? This precision will also test the viability of our current model for accretion physics, specifically the radiative efficiency and need for super-Eddington luminosities to explain the black hole population.

Paul Martini

2003-04-01

292

Annealing properties of open volumes in HfSiO{sub x} and HfAlO{sub x} gate dielectrics studied using monoenergetic positron beams  

SciTech Connect

Thin Hf{sub 0.6}Si{sub 0.4}O{sub x} and Hf{sub 0.3}Al{sub 0.7}O{sub x} films fabricated by metal-organic chemical-vapor deposition and atomic-layer-deposition techniques were characterized using monoenergetic positron beams. Measurements of the Doppler broadening spectra of annihilation radiation and the lifetime spectra of positions indicated that positrons annihilated from the trapped state by open volumes that exist intrinsically in amorphous structures of the films. For HfSiO{sub x}, the mean size of the open volumes and their size distribution decreased with increasing postdeposition annealing (PDA) temperature. For HfAlO{sub x}, although the overall behavior of the open volumes in response to annealing was similar to that for HfSiO{sub x}, PDA caused a separation of the mean size of the open volumes. When this separation occurred, the value of the line-shape parameter S increased, suggesting an oxygen deficiency in the amorphous matrix. This fragmentation of the amorphous matrix can be suppressed by decreasing the annealing time.

Uedono, A.; Ikeuchi, K.; Yamabe, K.; Ohdaira, T.; Muramatsu, M.; Suzuki, R.; Hamid, A. S.; Chikyow, T.; Torii, K.; Yamada, K. [Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573 (Japan); National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568 (Japan); Nanomaterials Laboratory, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003 (Japan); Semiconductor Leading Edge Technologies, Inc. (Selete), 34 Miyukigaoka, Tsukuba, Ibaraki 305-8501 (Japan); Nano Technology Research Laboratory, Waseda University, 513, Waseda-Tsurumaki, Shinjuku, Tokyo 16-0041 (Japan)

2005-07-15

293

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

SciTech Connect

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.

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

294

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

NASA Astrophysics Data System (ADS)

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.

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

2014-10-01

295

In-situ (C)DBS at high temperatures at the NEPOMUC positron beam line  

NASA Astrophysics Data System (ADS)

We report on the current status of the sample heating stage in the CDB-spectrometer at the NEPOMUC positron beam line. The currently installed new sample heating is described in detail and various design aspects are discussed briefly. As an exemplary application, the positron diffusion at high temperatures in Ge was investigated by a depth dependent evaluation of both the Doppler broadening of the annihilation line and free Ps annihilation at the surface. It was confirmed that the temperature dependence of the positron diffusion is extraordinarily strong above 670 K.

Reiner, Markus; Pikart, Philip; Hugenschmidt, Christoph

2013-06-01

296

Positron production using a 1.7 MV pelletron accelerator  

SciTech Connect

We report the foremost phase of a fourth generation positron source, being constructed at the Federal University of Rio de Janeiro. Positron yields are reported by making use of the {sup 19}F(p,{alpha}e{sup +}e{sup -}){sup 16}O reaction, where the fluorine target is in the form of a CaF{sub 2} pellet. Positron production has been observed by detecting 511 keV annihilation gamma rays emerging from the irradiated CaF{sub 2} target.

Alcantara, K. F.; Santos, A. C. F. [Instituto de Fisica, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, Rio de Janeiro, RJ 21941-972 (Brazil); Crivelli, P. [Institute for Particle Physics, ETH Zurich (Switzerland)

2013-04-19

297

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

NASA Technical Reports Server (NTRS)

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.

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

1994-01-01

298

Temperature effects in positronium formation and lifetime in solutions of nonpolar organic solvents  

NASA Astrophysics Data System (ADS)

The temperature dependence of the positron annihilation lifetime (PAL) spectra of several solutions are examined. Four solvents are investigated: n-hexane, cyclohexane, squalane, and an equivolume mixture of n-hexane and squalane. Solutes in the study are nitrobenzene, nitrocyclohexane, and hexafluorobenzene. Arrhenius plots of lifetimes are made from 10 °C to 50 °C. The temperature dependence of the PAL spectra of nitrobenzene solutions in a magnetic field varying up to 15 kG are also presented. Anomalous behavior is observed for the solvents n-hexane and squalane, both neat and with nitrobenzene as a solute. In addition, it is found that the behavior of neat and nitrobenzene solutions of the n-hexane-squalane mixture is not intermediate between those involving n-hexane and squalane separately. We conclude that there is a change in the mechanism for positronium formation from n-hexane to squalane which is not accounted for by changes in free volume. Nitrocyclohexane is found to enhance positronium formation in squalane. This is the first example of antiinhibition by an aliphatic solute known to the authors.

Diehl, D. A.; Schrader, D. M.

1991-06-01

299

NUCLEAR PHYSICS: Lifetime of Antibaryon Bound in Finite Nuclei  

NASA Astrophysics Data System (ADS)

The study of finite nuclei containing antibaryon(s) in addition to nucleons is an interesting topic in nuclear physics. The calculation of the lifetime of an antibaryon embedded in a nucleus was performed in the framework of the standard quantum field theory. It was shown that the annihilation probability of the antibaryon in nuclei is strongly dependent on the effective masses of mesons involved in the annihilation channels. The contribution of the Dirac sea to the annihilation probability makes the lifetime of the antibaryon short. If the Dirac sea effect is neglected, the lifetime of the bound antibaryon tends to be longer with the nuclear density increasing. Particularly, when the nuclear density is larger than a critical value, the antibaryon may exist stably in a nucleus.

Chen, Xi; Li, Ning; Yao, Hai-Bo; Qin, Xu-Ming; Wu, Shi-Shu

2010-01-01

300

Positron studies in catalysis research. Final report, September 1993-- May 1995  

SciTech Connect

During the past 20 months, we have completed our positron microscope and performed several studies in our nonmicroscopic depth-profiling positron spectrometer which should ultimately be applicable to catalysis. These studies involve using depth-profiled positron spectrometers to observe the growth dynamics of metal silicides on silicon substrates and to observe defects in glassy polymer surfaces and thin films, and the use of bulk positron lifetime measurements to observe pore-size variations in zeolites.

NONE

1996-05-01

301

Measurement of the B0-meson lifetime  

Microsoft Academic Search

We report a measurement of the lifetime of the B0 meson based upon 29-GeV e+e- annihilation data taken with the Mark II detector at the SLAC storage ring PEP. The B0 mesons are tagged by their decays into D*-e+nu and D*-mu+nu, where the D*- is tagged by its decay into pi-D¯ 0. We reconstruct the decay vertices of 15 B0-meson

S. R. Wagner; D. A. Hinshaw; R. A. Ong; A. Snyder; G. Abrams; C. E. Adolphsen; C. Akerlof; J. P. Alexander; M. Alvarez; D. Amidei; A. R. Baden; J. Ballam; B. C. Barish; T. Barklow; B. A. Barnett; J. Bartelt; D. Blockus; G. Bonvicini; A. Boyarski; J. Boyer; B. Brabson; A. Breakstone; J. M. Brom; F. Bulos; P. R. Burchat; D. L. Burke; F. Butler; F. Calvino; R. J. Cence; J. Chapman; D. Cords; D. P. Coupal; H. C. Destaebler; D. E. Dorfan; J. M. Dorfan; P. S. Drell; G. J. Feldman; E. Fernandez; R. C. Field; W. T. Ford; C. Fordham; R. Frey; D. Fujino; K. K. Gan; G. Gidal; L. Gladney; T. Glanzman; M. S. Gold; G. Goldhaber; A. Green; P. Grosse-Wiesmann; J. Haggerty; G. Hanson; R. Harr; F. A. Harris; C. M. Hawkes; K. Hayes; D. Herrup; C. A. Heusch; T. Himel; R. J. Hollebeek; D. Hutchinson; J. Hylen; W. R. Innes; M. Jaffre; J. A. Jaros; I. Juricic; J. A. Kadyk; D. Karlen; J. Kent; S. R. Klein; W. Koska; W. Kozanecki; A. J. Lankford; R. R. Larsen; B. W. Leclaire; M. E. Levi; A. M. Litke; N. S. Lockyer; V. Lüth; J. A. Matthews; D. I. Meyer; B. D. Milliken; K. C. Moffeit; L. Müller; J. Nash; M. E. Nelson; D. Nitz; H. Ogren; K. F. O'shaughnessy; S. I. Parker; C. Peck; M. L. Perl; A. Petersen; M. Petradza; F. C. Porter; P. Rankin; B. Richter; K. Riles; P. C. Rowson; D. R. Rust; H. F. Sadrozinski; T. Schaad; T. L. Schalk; H. Schellman; W. B. Schmidke; A. S. Schwarz; A. Seiden; P. D. Sheldon; J. G. Smith; E. Soderstrom; D. P. Stoker; R. Stroynowski; R. Thun; G. H. Trilling; R. Tschirhart; R. van Kooten; H. Veltman; P. Voruganti; P. Weber; A. J. Weinstein; S. Weisz; S. L. White; E. Wicklund; A. J. Weir; D. R. Wood; D. Y. Wu; J. M. Yelton

1990-01-01

302

Use of positrons to probe magnetic versus electrostatic turbulence  

NASA Astrophysics Data System (ADS)

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 the 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 millisecond bursts from radioactive sources by means of their curvature drift when trapped in toroidal field ripples. The energetic positrons will diffuse over 60-600 ms 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 ?-ray signal then can be unfolded into the positron radial profile and the positron diffusion coefficient determined from the time evolution of those profiles.

Stambaugh, R. D.

1990-10-01

303

Positron emulator for commissioning ILC positron source.  

SciTech Connect

The gamma-ray based positron source [1] is an attractive option for the International Linear Collider (ILC) positron option because of the less stringent target requirement and the possibility to provide polarization. The drawback is the fact that the positron accelerator system including positron linac and damping ring can not be fully commissioned until the electron accelerator system is commissioned at high energies (> 150 GeV). In this paper, we discuss a scheme that would use a low energy ({approx} 100 MeV) electron beam scattered through a set of carefully selected targets to mimic phase space distribution of the positron beam. The positron 'emulator' can be used to effectively commission the positron accelerator system including the capture optics, the linac, and even the damping ring before the ILC colliding electron beam is ready. Simulation results using EGS4 for beam scattering and PARMELA for beam dynamics are presented.

Wang, H.; Liu, W.; Gai, W.; Kim, K.-J.; High Energy Physics

2005-01-01

304

Models of pair annihilation in 1E 1740.7-2942 and the HEAO 1 A-4 annihilation source  

NASA Technical Reports Server (NTRS)

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.

Maciolek-Niedzwiecki, Andrzej; Zdziarski, Andrzej

1994-01-01

305

Flavored co-annihilations  

NASA Astrophysics Data System (ADS)

Neutralino dark matter in supersymmetric models is revisited in the presence of flavor violation in the soft supersymmetry breaking sector. We focus on flavor violation in the sleptonic sector and study the implications for the co-annihilation regions. Flavor violation is introduced by a single {widetilde{? }_R} - {widetilde{tau }_R} insertion in the slepton mass matrix. Limits on this insertion from BR(? ? ? + ?) are weak in some regions of the parameter space where cancellations happen within the amplitudes. We look for overlaps in parameter space where both the co-annihilation condition as well as the cancellations within the amplitudes occur. In mSUGRA, such overlap regions are not existent, whereas they are present in models with non-universal Higgs boundary conditions (NUHM). The effect of flavor violation is two fold: (a) it shifts the co-annihilation regions towards lighter neutralino masses (b) the co-annihilation cross sections would be modified with the inclusion of flavor violating diagrams which can contribute significantly. Even if flavor violation is within the presently allowed limits, this is sufficient to modify the thermally averaged cross-sections by about (10-15)% in mSUGRA and (20-30)% in NUHM, depending on the parameter space. In the overlap regions, the flavor violating cross sections become comparable and in some cases even dominant to the flavor conserving ones. A comparative study of the channels is presented for mSUGRA and NUHM cases.

Chowdhury, Debtosh; Garani, Raghuveer; Vempati, Sudhir K.

2012-06-01

306

A Monte Carlo model to produce baryons in e + e ? annihilation  

Microsoft Academic Search

A simple model is described extending the Field-Feynman model to baryon production in quark fragmentation. The model predicts baryon baryon correlations within jets and in opposite jets produced in electron-positron annihilation. Existing data is well described by the model.

Thomas Meyer

1982-01-01

307

Electron emission from surfaces resulting from low energy positron bombardment  

NASA Astrophysics Data System (ADS)

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.

Mukherjee, Saurabh

308

Test facility for a neutron induced positron source  

NASA Astrophysics Data System (ADS)

At the research reactor FRM of the Technical University of Munich, an in-pile intensive slow positron source was developed. Neutron capture gamma rays created by 113Cd(n, ?) 114Cd convert into e +-e - pairs by gamma absorption in tungsten foils. Moderated positrons are emitted from the foil surface and accelerated by electric lenses to a few keV. The positron beam is guided to a remoderation stage outside the reactor pool by a magnetic solenoid. In order to test various improvements of the experimental set-up the positron source was positioned at the accessible end of an external neutron guide outside the reactor building. To minimize background, a curved beam tube was installed at the end of the beamline. A germanium detector was used to measure the annihilation signal of the incoming positrons. This facility allows to optimize acceleration potentials and guide fields, as well as settings of earth magnetic compensation. With a continuous flux of 4×10 7 neutrons/cm 2s, the intensity of the primary positron beam was about 10 2 slow positrons per second. Recently, the profile of the positron beam, as well as photo electrons emitted from the tungsten foils, were studied by a space-resolving micro channel plate detector combined with a fluorescent screen and a CCD-camera.

Straßer, B.; Springer, M.; Hugenschmidt, C.; Schreckenbach, K.

1999-08-01

309

Power Corrections in Electron-Positron Annihilation: Experimental Review  

E-print Network

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.

Stefan Kluth

2006-06-20

310

Quark flavor identification in electron-positron annihilation  

SciTech Connect

The theoretical issues relevant to inclusive muon analysis, the MAC detector and its data flow structure, the identification of muons in hadronic events and the measurement of their momenta, and the selection of events so as to minimize background are described. Experimental results are presented describing the fragmentation of heavy quarks into hadrons, the semimuonic branching fractions of the heavy quarks, the asymmetry in the angular distribution of the heavy quarks, and the invariant mass and charged multiplicity of heavy quark jets. In addition, lower limits are set on the masses of certain proposed particles that are expected to decay semileptonically. Finally, events containing two muons are analyzed in order to investigate the possibility of mixing in the B-B system and whether the b might form its own SU(2) singlet.

Kaye, H.S.

1983-09-01

311

Positron annihilation induced Auger electron emission from silicon carbide surfaces  

Microsoft Academic Search

Silicon carbide (SiC) in monocrystalline, hexagonal polytype form is a very interesting material for a wide class of novel applications in electronics. SiC fiber-reinforced SiC matrix composite materials (SiC\\/SiC) are considered to be the attractive candidates as materials for advanced energy systems, such as high performance combustion systems, fuel-flexible gasification systems, fuel cell \\/ turbine hybrid systems, nuclear fusion reactors,

S. Mukherjee; M. Nadesalingam; G. Brauer; T. Nozawa; A. Kohyama; A. H. Weiss

2006-01-01

312

The multi-scattering model for calculations of positron spatial distribution in the multilayer stacks, useful for conventional positron measurements  

NASA Astrophysics Data System (ADS)

The spatial distribution of positrons emitted from radioactive isotopes into stacks or layered samples is a subject of the presented report. It was found that Monte Carlo (MC) simulations using GEANT4 code are not able to describe correctly the experimental data of the positron fractions in stacks. The mathematical model was proposed for calculations of the implantation profile or positron fractions in separated layers or foils being components of a stack. The model takes into account only two processes, i.e., the positron absorption and backscattering at interfaces. The mathematical formulas were applied in the computer program called LYS-1 (layers profile analysis). The theoretical predictions of the model were in the good agreement with the results of the MC simulations for the semi infinite sample. The experimental verifications of the model were performed on the symmetrical and non-symmetrical stacks of different foils. The good agreement between the experimental and calculated fractions of positrons in components of a stack was achieved. Also the experimental implantation profile obtained using the depth scanning of positron implantation technique is very well described by the theoretical profile obtained within the proposed model. The LYS-1 program allows us also to calculate the fraction of positrons which annihilate in the source, which can be useful in the positron spectroscopy.

Dryzek, Jerzy; Siemek, Krzysztof

2013-08-01

313

The multi-scattering model for calculations of positron spatial distribution in the multilayer stacks, useful for conventional positron measurements  

SciTech Connect

The spatial distribution of positrons emitted from radioactive isotopes into stacks or layered samples is a subject of the presented report. It was found that Monte Carlo (MC) simulations using GEANT4 code are not able to describe correctly the experimental data of the positron fractions in stacks. The mathematical model was proposed for calculations of the implantation profile or positron fractions in separated layers or foils being components of a stack. The model takes into account only two processes, i.e., the positron absorption and backscattering at interfaces. The mathematical formulas were applied in the computer program called LYS-1 (layers profile analysis). The theoretical predictions of the model were in the good agreement with the results of the MC simulations for the semi infinite sample. The experimental verifications of the model were performed on the symmetrical and non-symmetrical stacks of different foils. The good agreement between the experimental and calculated fractions of positrons in components of a stack was achieved. Also the experimental implantation profile obtained using the depth scanning of positron implantation technique is very well described by the theoretical profile obtained within the proposed model. The LYS-1 program allows us also to calculate the fraction of positrons which annihilate in the source, which can be useful in the positron spectroscopy.

Dryzek, Jerzy [Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków (Poland) [Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków (Poland); Institute of Physics, Opole University, ul. Oleska 48, 45-052 Opole (Poland); Siemek, Krzysztof [Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków (Poland)] [Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków (Poland)

2013-08-21

314

Positronic complexes with unnatural parity  

SciTech Connect

The structure of the unnatural parity states of PsH, LiPs, NaPs, and KPs are investigated with the configuration interaction and stochastic variational methods. The binding energies (in hartree) are found to be 8.17x10{sup -4}, 4.42x10{sup -4}, 15.14x10{sup -4}, and 21.80x10{sup -4}, respectively. These states are constructed by first coupling the two electrons into a configuration which is predominantly {sup 3}P{sup e}, and then adding a p-wave positron. All the active particles are in states in which the relative angular momentum between any pair of particles is at least L=1. The LiPs state is Borromean since there are no three-body bound subsystems (of the correct symmetry) of the (Li{sup +}, e{sup -}, e{sup -}, e{sup +}) particles that make up the system. The dominant decay mode of these states will be radiative decay into a configuration that autoionizes or undergoes positron annihilation.

Bromley, M. W. J.; Mitroy, J.; Varga, K. [Department of Physics and Computational Science Research Center, San Diego State University, San Diego, California 92182 (United States); ARC Center for Anti-matter Studies, Faculty of Technology, Charles Darwin University, Darwin NT 0909 (Australia); Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 (United States)

2007-06-15

315

An asymmetric distribution of positrons in the Galactic disk revealed by gamma-rays.  

PubMed

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

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

316

The annihilation diagram in three-body D-meson decay  

Microsoft Academic Search

We discuss some features of three-body decays of the D meson which are puzzling from the standpoint of the annihilation diagram. As a result, we (1) provide an upper bound on the lifetime ratio of D's, tau+D\\/tau0D < 2.5 +\\/- 3.4 and (2) argue that the puzzles are resolved, even if somewhat inelegantly, if final state interactions generate the annihilation

John F. Donoghue; Barry R. Holstein

1981-01-01

317

Cosmic-Ray Positrons: Are There Primary Sources?  

E-print Network

Cosmic rays at the Earth include a secondary component originating in collisions of primary particles with the diffuse interstellar gas. The secondary cosmic rays are relatively rare but carry important information on the Galactic propagation of the primary particles. The secondary component includes a small fraction of antimatter particles, positrons and antiprotons. In addition, positrons and antiprotons may also come from unusual sources and possibly provide insight into new physics. For instance, the annihilation of heavy supersymmetric dark matter particles within the Galactic halo could lead to positrons or antiprotons with distinctive energy signatures. With the High-Energy Antimatter Telescope (HEAT) balloon-borne instrument, we have measured the abundances of positrons and electrons at energies between 1 and 50 GeV. The data suggest that indeed a small additional antimatter component may be present that cannot be explained by a purely secondary production mechanism. Here we describe the signature of the effect and discuss its possible origin.

Stephane Coutu; Steven W. Barwick; James J. Beatty; Amit Bhattacharyya; Chuck R. Bower; Christopher J. Chaput; Georgia A. de Nolfo; Michael A. DuVernois; Allan Labrador; Shawn P. McKee; Dietrich Muller; James A. Musser; Scott L. Nutter; Eric Schneider; Simon P. Swordy; Gregory Tarle; Andrew D. Tomasch; Eric Torbet

1999-02-10

318

Exciton annihilation studies in poly(p-phenylene vinylene)  

SciTech Connect

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.

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

319

Annihilating asymmetric dark matter  

NASA Astrophysics Data System (ADS)

The relic abundance of particle and antiparticle dark matter (DM) need not be vastly different in thermal asymmetric dark matter (ADM) models. By considering the effect of a primordial asymmetry on the thermal Boltzmann evolution of coupled DM and anti-DM, we derive the requisite annihilation cross section. This is used in conjunction with cosmic microwave background and Fermi-LAT gamma-ray data to impose a limit on the number density of anti-DM particles surviving thermal freeze-out. When the extended gamma-ray emission from the Galactic Center is reanalyzed in a thermal ADM framework, we find that annihilation into ? leptons prefer anti-DM number densities 1%-4% that of DM while the b -quark channel prefers 50%-100%.

Bell, Nicole F.; Horiuchi, Shunsaku; Shoemaker, Ian M.

2015-01-01

320

Bismuth germanate as a potential scintillation detector in positron cameras.  

PubMed

Timing and energy resolutions of the bismuth germanate (Bi4Ge3O12) scintillation crystals were studied, with particular respect to a positron-camera application. In comparison with the NaI(Tl) system, the detection efficiency for annihilation radiation is more than triple, and coincidence detection efficiency is more than ten times as good. This paper explores the properties of the new scintillator material and their bearing on the spatial resolution and the efficiency of coincidence detection in positron cameras with stationary ring detectors. PMID:874173

Cho, Z H; Farukhi, M R

1977-08-01

321

Real-time tumor tracking using implanted positron emission markers: Concept and simulation study  

SciTech Connect

The delivery accuracy of radiation therapy for pulmonary and abdominal tumors suffers from tumor motion due to respiration. Respiratory gating should be applied to avoid the use of a large target volume margin that results in a substantial dose to the surrounding normal tissue. Precise respiratory gating requires the exact spatial position of the tumor to be determined in real time during treatment. Usually, fiducial markers are implanted inside or next to the tumor to provide both accurate patient setup and real-time tumor tracking. However, current tumor tracking systems require either substantial x-ray exposure to the patient or large fiducial markers that limit the value of their application for pulmonary tumors. We propose a real-time tumor tracking system using implanted positron emission markers (PeTrack). Each marker will be labeled with low activity positron emitting isotopes, such as {sup 124}I, {sup 74}As, or {sup 84}Rb. These isotopes have half-lives comparable to the duration of radiation therapy (from a few days to a few weeks). The size of the proposed PeTrack marker will be 0.5-0.8 mm, which is approximately one-half the size of markers currently employed in other techniques. By detecting annihilation gammas using position-sensitive detectors, multiple positron emission markers can be tracked in real time. A multimarker localization algorithm was developed using an Expectation-Maximization clustering technique. A Monte Carlo simulation model was developed for the PeTrack system. Patient dose, detector sensitivity, and scatter fraction were evaluated. Depending on the isotope, the lifetime dose from a 3.7 MBq PeTrack marker was determined to be 0.7-5.0 Gy at 10 mm from the marker. At the center of the field of view (FOV), the sensitivity of the PeTrack system was 240-320 counts/s per 1 MBq marker activity within a 30 cm thick patient. The sensitivity was reduced by 45% when the marker was near the edge of the FOV. The scatter fraction ranged from 12% ({sup 124}I,{sup 74}As) to 16% ({sup 84}Rb). In addition, four markers (labeled with {sup 124}I) inside a 30 cm diameter water phantom were simulated to evaluate the feasibility of the multimarker localization algorithm. Localization was considered successful if a marker was localized to within 2 mm from its true location. The success rate of marker localization was found to depend on the number of annihilation events used and the error in the initial estimate of the marker position. By detecting 250 positron annihilation events from 4 markers (average of 62 events per marker), the marker success rates for initial errors of {+-}5, {+-}10, and {+-}15 mm were 99.9%, 99.6%, and 92.4%, respectively. Moreover, the average localization error was 0.55 ({+-}0.27) mm, which was independent of initial error. The computing time for localizing four markers was less than 20 ms (Pentium 4, 2.8 GHz processor, 512 MB memory). In conclusion, preliminary results demonstrate that the PeTrack technique can potentially provide real-time tumor tracking with low doses associated with the marker's activity. Furthermore, the small size of PeTrack markers is expected to facilitate implantation and reduce patient risk.

Xu Tong; Wong, Jerry T.; Shikhaliev, Polad M.; Ducote, Justin L.; Al-Ghazi, Muthana S.; Molloi, Sabee [Department of Radiological Sciences University of California, Irvine, California 92697 (United States); Department of Radiation Oncology, University of California, Irvine, California 92697 (United States); Department of Radiological Sciences, University of California, Irvine, California 92697 (United States)

2006-07-15

322

Elastic positron-cadmium scattering at low energies  

SciTech Connect

The elastic and annihilation cross sections for positron-cadmium scattering are reported up to the positronium-formation threshold (at 2.2 eV). The low-energy phase shifts for the elastic scattering of positrons from cadmium were derived from the bound and pseudostate energies of a very large basis configuration-interaction calculation of the e{sup +}-Cd system. The s-wave binding energy is estimated to be 126{+-}42 meV, with a scattering length of A{sub scat}=(14.2{+-}2.1)a{sub 0}, while the threshold annihilation parameter, Z{sub eff}, was 93.9{+-}26.5. The p-wave phase shift exhibits a weak shape resonance that results in a peak Z{sub eff} of 91{+-}17 at a collision energy of about 490{+-}50 meV.

Bromley, M. W. J.; Mitroy, J. [Department of Physics and Computational Science Research Center, San Diego State University, San Diego, California 92182 (United States); ARC Centre for Antimatter-Matter Studies and School of Engineering, Charles Darwin University, Darwin, Northern Territory 0909 (Australia)

2010-05-15

323

Measuring the efficiency of the Texas A&M Positron Emission Tomograph  

E-print Network

. John A. Mclntyre The Texas A&M University Positron Emission Tomograph (TAMU PET) detects the gamma rays which are emitted in positron-electron annihilation by using plastic scintillators and optical fibers to code the light given off... to determine the location of a. gamma ray interaction. The efficiency at which a PET detects scintillstions, in part, determines the clarity of the images produced. This detection efficiency was measured for the TAMU PET by determining the number...

Loewer, Wesley Blake

2012-06-07

324

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

NASA Technical Reports Server (NTRS)

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.

Singh, Jag. J.

1992-01-01

325

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

SciTech Connect

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.

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

326

Pair annihilation into neutrinos in strong magnetic fields.  

NASA Technical Reports Server (NTRS)

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.

Canuto, V.; Fassio-Canuto, L.

1973-01-01

327

Vacancy-type defects in InxGa1-xN grown on GaN templates probed using monoenergetic positron beams  

NASA Astrophysics Data System (ADS)

Native defects in InxGa1-xN layers grown by metalorganic chemical vapor deposition were studied using monoenergetic positron beams. Measurements of Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons for a 200-nm-thick In0.13Ga0.87N layer showed that vacancy-type defects were introduced by InN alloying, and the major species of such defects was identified as complexes between a cation vacancy and nitrogen vacancies. The presence of the defects correlated with lattice relaxation of the In0.13Ga0.87N layer and the increase in photon emissions from donor-acceptor-pair recombination. The species of native defects in In0.06Ga0.94N layers was the same but its concentration was decreased by decreasing the InN composition. With the layer thickness increased from 120 nm to 360 nm, a defect-rich region was introduced in the subsurface region (<160 nm), which can be associated with layer growth with the relaxation of compressive stress.

Uedono, Akira; Watanabe, Tomohito; Kimura, Shogo; Zhang, Yang; Lozac'h, Mickael; Sang, Liwen; Ishibashi, Shoji; Oshima, Nagayasu; Suzuki, Ryoichi; Sumiya, Masatomo

2013-11-01

328

Vacancy-type defects in In{sub x}Ga{sub 1?x}N grown on GaN templates probed using monoenergetic positron beams  

SciTech Connect

Native defects in In{sub x}Ga{sub 1?x}N layers grown by metalorganic chemical vapor deposition were studied using monoenergetic positron beams. Measurements of Doppler broadening spectra of the annihilation radiation and lifetime spectra of positrons for a 200-nm-thick In{sub 0.13}Ga{sub 0.87}N layer showed that vacancy-type defects were introduced by InN alloying, and the major species of such defects was identified as complexes between a cation vacancy and nitrogen vacancies. The presence of the defects correlated with lattice relaxation of the In{sub 0.13}Ga{sub 0.87}N layer and the increase in photon emissions from donor-acceptor-pair recombination. The species of native defects in In{sub 0.06}Ga{sub 0.94}N layers was the same but its concentration was decreased by decreasing the InN composition. With the layer thickness increased from 120?nm to 360?nm, a defect-rich region was introduced in the subsurface region (<160?nm), which can be associated with layer growth with the relaxation of compressive stress.

Uedono, Akira [Division of Applied Physics, Faculty of Pure and Applied Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573 (Japan); Watanabe, Tomohito; Kimura, Shogo; Zhang, Yang; Lozac'h, Mickael [Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573 (Japan); Sang, Liwen; Sumiya, Masatomo [Wide Bandgap Material Group, National Institute for Materials Science, Tsukuba 305-0044 (Japan); Ishibashi, Shoji [Nanosystem Research Institute (NRI) “RICS,” National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568 (Japan); Oshima, Nagayasu; Suzuki, Ryoichi [Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8568 (Japan)

2013-11-14

329

Monte Carlo modelling of positron transport in real world applications  

NASA Astrophysics Data System (ADS)

Due to the unstable nature of positrons and their short lifetime, it is difficult to obtain high positron particle densities. This is why the Monte Carlo simulation technique, as a swarm method, is very suitable for modelling most of the current positron applications involving gaseous and liquid media. The ongoing work on the measurements of cross-sections for positron interactions with atoms and molecules and swarm calculations for positrons in gasses led to the establishment of good cross-section sets for positron interaction with gasses commonly used in real-world applications. Using the standard Monte Carlo technique and codes that can follow both low- (down to thermal energy) and high- (up to keV) energy particles, we are able to model different systems directly applicable to existing experimental setups and techniques. This paper reviews the results on modelling Surko-type positron buffer gas traps, application of the rotating wall technique and simulation of positron tracks in water vapor as a substitute for human tissue, and pinpoints the challenges in and advantages of applying Monte Carlo simulations to these systems.

Marjanovi?, S.; Bankovi?, A.; Šuvakov, M.; Petrovi?, Z. Lj

2014-05-01

330

Search for resonant states in positron-electron scattering using a positron gas target  

SciTech Connect

Narrow correlated positron-electron peaks discovered in superheavy nuclear collisions may be signatures for previously undetected neutral particle-like objects having masses of 1--2 MeV/c{sup 2}. We have designed an experiment to definitively test this hypothesis by searching for resonant states formed directly in the scattering of monoenergetic electrons incident on a gas of cold positrons confined magnetically in a Malmberg trap. This technique will provide a hundred-fold improvement in sensitivity to e{sup +}e{sup {minus}} resonances compared to previous positron-beam, thin-foil scattering experiments. Together with a recoil-shadow technique, this experiment will explore a five decade range in neutral-particle lifetimes (10{sup {minus}13}s to 10{sup {minus}8}s) which cannot be probed directly to other methods. 14 refs., 2 figs., 1 tab.

Cowan, T.E.; Howell, R.H.; Rohatgi, R.R. (Lawrence Livermore National Lab., CA (USA)); Fajans, J. (California Univ., Berkeley, CA (USA). Dept. of Physics)

1990-10-15

331

Electron emission from surfaces resulting from low energy positron bombardment  

NASA Astrophysics Data System (ADS)

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.

Mukherjee, S.; Shastry, K.; Weiss, A. H.

2009-03-01

332

Gamma-ray spectra of methane in the positron-electron annihilationprocess  

NASA Astrophysics Data System (ADS)

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 delocalised molecular orbital (MO) and the localised 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 polarised 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.

Ma, Xiaoguang; Wang, Feng

2013-04-01

333

Bepcii Positron Source  

NASA Astrophysics Data System (ADS)

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.

Pei, Guoxi; Sun, Yaolin; Liu, Jintong; Chi, Yunlong; Liu, Yuncheng; Liu, Nianzong

2013-10-01

334

Radiopharmaceutical chemistry for positron emission tomography.  

PubMed

Molecular imaging is an emerging technology that allows the visualization of interactions between molecular probes and biological targets. Molecules that either direct or are subject to homeostatic controls in biological systems could be labeled with the appropriate radioisotopes for the quantitative measurement of selected molecular interactions during normal tissue homeostasis and again after perturbations of the normal state. In particular, positron emission tomography (PET) offers picomolar sensitivity and is a fully translational technique that requires specific probes radiolabeled with a usually short-lived positron-emitting radionuclide. PET has provided the capability of measuring biological processes at the molecular and metabolic levels in vivo by the detection of the gamma rays formed as a result of the annihilation of the positrons emitted. Despite the great wealth of information that such probes can provide, the potential of PET strongly depends on the availability of suitable PET radiotracers. However, the development of new imaging probes for PET is far from trivial and radiochemistry is a major limiting factor for the field of PET. In this review, we provided an overview of the most common chemical approaches for the synthesis of PET-labeled molecules and highlighted the most recent developments and trends. The discussed PET radionuclides include ¹¹C (t?(/)?=20.4min), ¹³N (t?(/)?=9.9min), ¹?O (t?(/)?=2min), ??Ga (t?(/)?=68min), ¹?F (t?(/)?=109.8min), ??Cu (t?(/)?=12.7h), and ¹²?I (t?(/)?=4.12d). PMID:20854860

Li, Zibo; Conti, Peter S

2010-08-30

335

Feasibility study for positron emission mammography.  

PubMed

A feasibility study is presented for a small, low-cost, dedicated device for positron emission mammography. Two detector arrays above and below the breast would be placed in a conventional mammography unit. These detectors are sensitive to positron annihilation radiation, and are connected to a coincidence circuit and a multiplane image memory. Images of the distribution of positron-emitting isotope are obtained in real time by incrementing the memory location at the intersection of each line of response. Monte Carlo simulations of a breast phantom are compared with actual scans of this phantom in a conventional PET scanner. The simulations and experimental data are used to predict the performance of the proposed system. Spatial resolution experiments using very narrow bismuth germanate BGO crystals suggest that spatial resolutions of about 2 mm should be possible. The efficiency of the proposed device is about ten times that of a conventional brain scanner. The scatter fraction is greater, but the scattered radiation has a very flat distribution. By designing the device to fit in an existing mammography unit, conventional mammograms can be taken after the injection of the radio-pharmaceutical allowing exact registration of the emission and conventional mammographic images. PMID:8058019

Thompson, C J; Murthy, K; Weinberg, I N; Mako, F

1994-04-01

336

Positron Momentum Calculation of Defects in ?-Fe and Fe-Cr Containing Hydrogen and Helium  

NASA Astrophysics Data System (ADS)

Lattice defects that contain hydrogen and helium in Fe and Fe-Cr have been studied by positron model calculations using Density Functional Theory (DFT) in its Local Density Approximation (LDA) implementation. It was found that the value of positron lifetime for perfect Fe lattice is 101.6 ps. and for mono vacancy is 176.6 ps. which a close to the experimental values. The calculated values of Positron Life-Time (PLT) for Fe-2.5% wt Cr is 104.7 ps., for Fe-5% wt Cr is 127.6 ps., for Fe-8% wt Cr is 128.9 ps. and for Fe-9% Cr is 128.2 ps. A correlation between the positron lifetime and H and He atom concentration has been established. The model calculations for PLT and Coincidence-Doppler-Broadening (CDB) for Fe-Cr containing vacations in the presence of different numbers of hydrogen and helium atoms will be presented.

Popov, E.; Staykov, P.; Berovski, K.; Peneva, S.; Djourelov, N.; Troev, T.

2014-06-01

337

M Ris-R-433 A Positron Lifetime Study of  

E-print Network

. The experimen^s have been per- formed as function of temperature, e.g., from just above the melting point; TEMPERATURE DEPENDENCE; TIME RESOLUTION; VISCOSITY. UDC 539.124.6 : 539.189.2 : 546.22'161-14 ISBN 87 is that it has led to the acknowledgment that matter and anti-matter exist together in the Universe. To every

338

Determination of interfacial states in solid heterostructures using a variable-energy positron beam  

DOEpatents

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.

Asokakumar, P.P.V.; Lynn, K.G.

1993-04-06

339

Impact of nitridation on open volumes in HfSiO{sub x} studied using monoenergetic positron beams  

SciTech Connect

The effects of nitridation on open volumes in thin HfSiO{sub x} films fabricated by metal-organic chemical vapor deposition were studied using monoenergetic positron beams. It was found that positrons were annihilated from the trapped state by open volumes that exist intrinsically in amorphous HfSiO{sub x} structures. In an as-deposited film, the positrons were annihilated from two different types of open volume. After plasma nitridation, the probability of positrons trapped by larger open volumes decreased, which is attributed to nitride trapped by such regions. The mean size of the open volumes increased after annealing, suggesting expansion of the open volumes due to the incorporation of nitride into the HfSiO{sub x} matrix.

Uedono, A.; Ikeuchi, K.; Otsuka, T.; Yamabe, K.; Eguchi, K.; Takayanagi, M.; Ohdaira, T.; Muramatsu, M.; Suzuki, R.; Hamid, A.S.; Chikyow, T. [Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573 (Japan); Semiconductor Technology Academic Research Center, 17-2, Shin Yokohama 3-chome, Kohoku, Yokohama 222-0033 (Japan); National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568 (Japan); Nanomaterials Laboratory, National Institute for Materials Science, 3-13 Sakura, Tsukuba, Ibaraki 305-0003 (Japan)

2006-04-24

340

Models for the positive latitude e{-}e{+} annihilation feature  

NASA Astrophysics Data System (ADS)

Galactic maps of e-e+ annihilation radiation based on CGRO-OSSE, SMM and TGRS data have indicated the existence of an extended component at positive Galactic latitudes (l~ -2 deg, b~ 7 deg), in addition to the emission from the galactic bulge and disk (Purcell et al. \\cite{Purcell97}; Cheng et al. \\cite{Cheng97}; Milne et al. \\cite{Milne00}; Milne et al. \\cite{Milne01}). This Positive Latitude Enhancement (PLE) was first attributed to an ``annihilation fountain" in the Galactic center (Dermer & Skibo \\cite{Dermer97}) but has since been the object of several models. After discussing the observational evidence for the PLE, we investigate various models for the PLE: besides the scenarios proposed in the literature, we have introduced a number of models requiring relatively modest positron rates due to a local origin of the e-e+ emission (local galactic-, solar system-, earth- and spacecraft-environment origins). The various scenarios for the PLE are constrained in the light of the latest OSSE-SMM-TGRS data analysis results: we have looked at the possible positron production mechanisms as well as the annihilation conditions in the different physical environments (temperature and dust grain content) proposed for the positive-latitude region. By constraining those parameters, based on the recent limits for the line width and the positronium fraction, we found that some of the models can essentially be discarded. A number of other scenarios will have to await further measurements and maps, such as will be possible with INTEGRAL's SPI and IBIS instruments. We present a table/checklist of model-falsification criteria.

von Ballmoos, P.; Guessoum, N.; Jean, P.; Knödlseder, J.

2003-01-01

341

Precise tests of QCD in e{sup +}e{sup {minus}} annihilation  

SciTech Connect

A pedagogical review is given of precise tests of QCD in electron-positron annihilation. Emphasis is placed on measurements that have served to establish QCD as the correct theory of strong interactions, as well as measurements of the coupling parameter {alpha}{sub s}. An outlook is given for future important tests at a high-energy e{sup +}e{sup {minus}} collider.

Burrows, P.N.

1997-03-01

342

Leptonic dark matter annihilation in the evolving universe: constraints and implications  

SciTech Connect

The cosmic electron and positron excesses have been explained as possible dark matter (DM) annihilation products. In this work we investigate the possible effects of such a DM annihilation scenario during the evolution history of the Universe. We first calculate the extragalactic ?-ray background (EGRB), which is produced through the final state radiation of DM annihilation to charged leptons and the inverse Compton scattering between electrons/positrons and the cosmic microwave background. The DM halo profile and the minimal halo mass, which are not yet well determined from the current N-body simulations, are constrained by the EGRB data from EGRET and Fermi telescopes. Then we discuss the impact of such leptonic DM models on cosmic evolution, such as the reionization and heating of intergalactic medium, neutral Hydrogen 21 cm signal and suppression of structure formation. We show that the impact on the Hydrogen 21 cm signal might show interesting signatures of DM annihilation, but the influence on star formation is not remarkable. Future observations of the 21 cm signals could be used to place new constraints on the properties of DM.

Yuan, Qiang; Bi, Xiao-Jun [Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Science, Beijing 100049, P.R.China (China); Yue, Bin; Chen, Xuelei [National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, P.R.China (China); Zhang, Xinmin, E-mail: yuanq@mail.ihep.ac.cn, E-mail: yuebin@bao.ac.cn, E-mail: bixj@mail.ihep.ac.cn, E-mail: xuelei@cosmology.bao.ac.cn, E-mail: xmzhang@mail.ihep.ac.cn [Theoretical Physics Division, Institute of High Energy Physics, Chinese Academy of Science, Beijing 100049, P.R.China (China)

2010-10-01

343

The INTEGRAL View Of The 511 keV Annihilation Line In Our Galaxy  

NASA Astrophysics Data System (ADS)

It is well know from theory and laboratory practice that an electron-positron pair can annihilate into a couple of 511 keV (the electron rest mass) gamma ray photons. The first detection of 511 keV photons from the Galactic center region dates back to early seventies. Soon after, a continuum gamma ray emission due to 3 gamma ortho-positronium decay was also measured. A 511 keV line in the Galactic gamma ray emission gives a unique proof that a large number of positrons are injected in the astrophysical environments, but nowadays we still do not know where these particles are generated. Positrons can be generated by a number of processes, in particular beta+ decays of unstable isotopes produced by stars and supernovae and energetic outflows from compact objects, but the few claimed detections of a 511 keV line from compact galactic sources are quite controversial. This fact could be explained by propagation of positrons in the intergalactic medium before they annihilate away from the birth place. The measure made with the spectrometer SPI aboard INTernational Gamma RAy Laboratory (INTEGRAL), launched on October 17 2002, confirms that about 10E43 positrons per second annihilate in the bulge of our Galaxy. Moreover, there is some evidence of an asymmetry of the 511 keV emission along the Galactic longitude, possibly correlated with the spacial distribution of the hard X (E > 20 keV) Low Mass X-ray Binaries detected by the imager IBIS aboard INTEGRAL. With IBIS, using about 5 years of observations, we find no evidence of 511 keV point sources. With an exposure of 10 Ms, in the center of the Galaxy we estimate a 1.6 x 10E-04 ph/cm2/s flux 2 sigma upper limit; a similar limit is given in a wide area in the Galactic center region with similar exposures.

De Cesare, G.

2011-09-01

344

Possibilities with pulsed polarized high density slow positrons  

NASA Astrophysics Data System (ADS)

A particularly bright and intense polarized slow positron beam could be formed from isotopically enriched 79Kr produced at a reactor. After moderation with solid Ne, accumulation, compression, and bunching, this type of positron beam would enable a number of experiments including: (1) Long term storage of a neutral polarized electron-positron plasma in a cold box; (2) Pulsed e+ ACAR with a pulsed magnet to measure Fermi surfaces of paramagnetic metals; (3) Single shot measurements of positron annihilation in laser-imploding plasmas; (4) Study of a spin-polarized positronium gas at a density around that of ordinary air to produce a Ps Bose-Einstein condensate at room temperature; (5) High energy polarized positron channelling experiments to study polarized electron spatial wave functions in ferromagnets; and (6) Study of supersonic free expansion spin polarized BEC Ps jets formed from, for example, 1011 m=1 triplet Ps atoms created within an open ended 1 ?m diameter cylindrical cavity 100 ?m in length.

Mills, A. P., Jr.

2014-04-01

345

The source and distribution of Galactic positrons  

NASA Technical Reports Server (NTRS)

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.

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

346

Customer Lifetime Value Measurement  

Microsoft Academic Search

The measurement of customer lifetime value is important because it is used as a metric in evaluating decisions in the context of customer relationship management. For a firm, it is important to form some expectations as to the lifetime value of each customer at the time a customer starts doing business with the firm, and at each purchase by the

Sharad Borle; Siddharth S. Singh; Dipak C. Jain

2008-01-01

347

Positron diffusion in Si  

SciTech Connect

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.

Nielsen, B.; Lynn, K.G.; Vehanen, A.; Schultz, P.J.

1985-06-01

348

Neutron Lifetime Measurements  

SciTech Connect

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.

Nico, J. S. [National Institute of Standards and Technology, Physics Laboratory, Gaithersburg, MD 20899 (United States)

2006-11-17

349

Semi-annihilation of dark matter  

E-print Network

We show that the thermal relic abundance of dark matter can be affected by a new type of reaction: semi-annihilation. Semi-annihilation takes the schematic form ..., where psi i are stable dark matter particles and phi is ...

D’Eramo, Francesco

350

Constraint on dark matter annihilation with dark star formation using Fermi extragalactic diffuse gamma-ray background data  

SciTech Connect

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.

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

351

About the creation of proton-antiproton pair at electron-positron collider in the energy range of ?(3770) mass  

NASA Astrophysics Data System (ADS)

The process of electron-positron annihilation into proton-antiproton pair is considered within the vicinity of ?(3770) resonance. The interference between the pure electromagnetic intermediate state and the ?(3770) state is evaluated. It is shown that this interference is destructive and the relative phase between these two contributions is large (?0?250°).

Ahmadov, A. I.; Bystritskiy, Yu. M.; Kuraev, E. A.; Wang, P.

2014-11-01

352

Positron production scenarios and the angular profile of the galactic center 511-keV line  

E-print Network

The observed angular profile of the 511-keV photon excess from the Milky Way galactic center can allow us to select among combinations of various dark matter and other positron production mechanisms with various models for the dark matter distribution. We find that a relic decay scenario gives too flat an angular distribution for any dark matter distribution in our survey, but that a dark matter-dark matter collisional scenario, or a scenario that involves particles emitted from a localized central source producing positrons some distance out, can match the observed galactic center angular profile if the dark matter distribution is neither too flat nor too cuspy. Additionally, positron migration or diffusion before annihilation broadens the angular profile to an extent that an average migration of more than half a kiloparsec is not viable with most dark matter distributions. The observed angular profile is also consistent with the occurrence of transient events in the past, followed by isotropic positron diff...

Abidin, Zainul; Carlson, Carl E

2010-01-01

353

Positron production scenarios and the angular profile of the galactic center 511-keV line  

E-print Network

The observed angular profile of the 511-keV photon excess from the Milky Way galactic center can allow us to select among combinations of various dark matter and other positron production mechanisms with various models for the dark matter distribution. We find that a relic decay scenario gives too flat an angular distribution for any dark matter distribution in our survey, but that a dark matter-dark matter collisional scenario, or a scenario that involves particles emitted from a localized central source producing positrons some distance out, can match the observed galactic center angular profile if the dark matter distribution is neither too flat nor too cuspy. Additionally, positron migration or diffusion before annihilation broadens the angular profile to an extent that an average migration of more than half a kiloparsec is not viable with most dark matter distributions. The observed angular profile is also consistent with the occurrence of transient events in the past, followed by isotropic positron diffusion.

Zainul Abidin; Andrei Afanasev; Carl E. Carlson

2010-06-28

354

A Relativistic Electron-Positron Outflow from a Tepid Fireball  

E-print Network

Through detailed numerical simulations, we demonstrate that relativistic outflows (Lorentz factor $\\Gamma \\sim 7$) of electron-positron pairs can be produced by radiative acceleration even when the flow starts from a nearly pair equilibrium state at subrelativistic temperatures. Contrary to the expectation that pairs annihilate during an expansion stage for such low temperatures, we find that most pairs can survive for the situations obtained in our previous work. This is because in the outflow-generating region the dynamical timescale is short enough even though the fireball is optically thick to scattering. Several problems that should be solved to apply to actual active galactic nucleus jets are discussed.

Katsuaki Asano; Fumio Takahara

2008-12-11

355

Lifetime and performance of NSLS storage rings  

SciTech Connect

The performance of synchrotron light sources is measured primarily in terms of beam lifetime, beam size, and the recovery of normal operation after a section of the machine has been brought to atmospheric pressure. The beam lifetime and the beam size depend on the following phenomena: Beam gas interaction which can be either elastic or inelastic scattering on residual gas nuclei or electrons. With the exception of low energy machines, this phenomenon represents the main limiting factor on lifetime; Beam interaction with trapped ions causing both beam loss and defocussing. Residual gas molecules are ionized both by circulating beam and synchrotron radiation. The cross sections for both processes are comparable. The effects of this phenomenon are most troublesome at low energies. The problem can be eliminated by switching to positron beams. Installing clearing electrodes has also been successful; Intrabeam scattering (Touschek effect) is caused by Coulomb scattering among electrons of the same bunch as they execute betatron oscillations. The Touschek effect is strongly dependent on energy and in general is a problem only in low energy machines; and Various instabilities causing both slow and fast beam decay which have been observed in both NSLS rings. A special case due to dust particles that fall into the electron beam is commonly observed in early stages of conditioning. Coherent collective instabilities will not be discussed in this paper. 19 refs., 4 figs., 1 tab.

Halama, H.J.

1988-01-01

356

Alternative positron-target design for electron-positron colliders  

SciTech Connect

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.

Donahue, R.J. (Lawrence Berkeley Lab., CA (United States)); Nelson, W.R. (Stanford Linear Accelerator Center, Menlo Park, CA (United States))

1991-04-01

357

Initial results on positron confinement in a magnetospheric configuration  

NASA Astrophysics Data System (ADS)

Creation of positron-electron plasma in a laboratory is an interesting and challenging subject, which may open many scientific applications. Although single-component plasma is stably confined in linear traps, for example Penning-Malmberg trap, it is not straightforward to simultaneously confine electrons and positrons as plasma. Toroidal geometries have advantages for solving this problem. For this purpose, studies on toroidal non-neutral plasma have been conducted in the levitated magnetospheric configuration, RT-1. Stable confinement and self-organization of toroidal non-neutral plasma was realized in RT-1; rigid-rotating pure electron plasma is confined for more than 300s [Z. Yoshida et al., PRL 104, 235004 (2010)]. As the initial step toward the formation of magnetospheric antimatter plasmas, we installed a 1MBq Na-22 radiation source in RT-1. Annihilation gamma-rays were observed by a NaI(TI) scintillator detector, for the estimation of basic injection and confinement properties of positrons in the magnetospheric configuration. Numerical analysis of positron orbits in RT-1 and the initial experimental results will be presented.

Saitoh, Haruhiko; Yoshida, Zensho; Yano, Yoshihisa; Morikawa, Junji

2011-11-01

358

The “accumulation effect” of positrons in the stack of foils, detected by measurements of the positron implantation profile  

SciTech Connect

The profiles of positrons implanted from the radioactive source {sup 22}Na 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.

Dryzek, Jerzy [Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków (Poland); Institute of Physics, Opole University, ul. Oleska 48, 45-052 Opole (Poland); Siemek, Krzysztof [Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków (Poland)

2013-12-14

359

Multiple gamma lines from semi-annihilation  

SciTech Connect

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.

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

360

The Japanese Positron Factory  

NASA Astrophysics Data System (ADS)

The Positron Factory has been planned at Japan Atomic Energy Research Institute (JAERI). The factory is expected to produce linac-based monoenergetic positron beams having world-highest intensities of more than 1010e+/sec, which will be applied for R&D of materials science, biotechnology and basic physics & chemistry. In this article, results of the design studies are demonstrated for the following essential components of the facilities: 1) Conceptual design of a high-power electron linac with 100 MeV in beam energy and 100 kW in averaged beam power, 2) Performance tests of the RF window in the high-power klystron and of the electron beam window, 3) Development of a self-driven rotating electron-to-positron converter and the performance tests, 4) Proposal of multi-channel beam generation system for monoenergetic positrons, with a series of moderator assemblies based on a newly developed Monte Carlo simulation and the demonstrative experiment, 5) Proposal of highly efficient moderator structures, 6) Conceptual design of a local shield to suppress the surrounding radiation and activation levels.

Okada, S.; Sunaga, H.; Kaneko, H.; Takizawa, H.; Kawasuso, A.; Yotsumoto, K.; Tanaka, R.

1999-06-01

361

Search for psi(2S) production in e+e- annihilations at 4.03 GeV  

Microsoft Academic Search

A search is performed for the production of the psi(2S) in e+e- annihilation at a center-of-mass energy of 4.03 GeV using the BES detector operated at the Beijing Electron Positron Collider (BEPC). The kinematic features of the reconstructed psi(2S) signal are consistent with its being produced only in association with an energetic photon resulting from initial state radiation (ISR). Limits

J. Z. Bai; J. G. Bian; I. Blum; Z. W. Chai; G. P. Chen; H. F. Chen; J. Chen; Y. B. Chen; Y. Q. Chen; B. S. Cheng; X. Z. Cui; H. L. Ding; L. Y. Ding; L. Y. Dong; Z. Z. Du; W. Dunwoodie; S. Feng; C. S. Gao; M. L. Gao; S. Q. Gao; P. Gratton; J. H. Gu; S. D. Gu; W. X. Gu; Y. F. Gu; Y. N. Guo; S. W. Han; Y. Han; F. A. Harris; J. He; M. He; D. G. Hitlin; G. Y. Hu; H. M. Hu; J. L. Hu; Q. H. Hu; T. Hu; X. Q. Hu; J. D. Huang; Y. Z. Huang; J. M. Izen; C. H. Jiang; Y. Jin; Z. J. Ke; M. H. Kelsey; B. K. Kim; D. Kong; Y. F. Lai; P. F. Lang; A. Lankford; C. G. Li; D. Li; H. B. Li; J. Li; P. Q. Li; R. B. Li; W. Li; W. G. Li; X. H. Li; X. N. Li; H. M. Liu; J. Liu; R. G. Liu; Y. Liu; X. C. Lou; B. Lowery; F. Lu; J. G. Lu; J. Y. Lu; L. C. Lu; C. H. Luo; A. M. Ma; E. C. Ma; J. M. Ma; R. Malchow; M. Mandelkern; H. S. Mao; Z. P. Mao; X. C. Meng; J. Nie; S. L. Olsen; J. Oyang; D. Paluselli; L. J. Pan; J. Panetta; F. Porter; N. D. Qi; X. R. Qi; C. D. Qian; J. F. Qiu; Y. H. Qu; Y. K. Que; G. Rong; M. Schernau; B. Schmid; J. Schultz; Y. Y. Shao; B. W. Shen; D. L. Shen; H. Shen; X. Y. Shen; H. Y. Sheng; H. Z. Shi; X. F. Song; J. Standifird; D. Stoker; F. Sun; H. S. Sun; S. Q. Tang; W. Toki; G. L. Tong; F. Wang; L. S. Wang; L. Z. Wang; M. Wang; Meng Wang; P. Wang; S. M. Wang; T. J. Wang; Y. Y. Wang; M. Weaver; C. L. Wei; Y. G. Wu; D. M. Xi; X. M. Xia; P. P. Xie; Y. Xie; W. J. Xiong; C. C. Xu; G. F. Xu; S. T. Xue; J. Yan; W. G. Yan; C. M. Yang; C. Y. Yang; J. Yang; W. Yang; X. F. Yang; M. H. Ye; S. W. Ye; Y. X. Ye; K. Yi; C. S. Yu; C. X. Yu; Y. H. Yu; Z. Q. Yu; Z. T. Yu; C. Z. Yuan; Y. Yuan; B. Y. Zhang; C. C. Zhang; D. H. Zhang; Dehong Zhang; H. L. Zhang; J. Zhang; L. S. Zhang; Q. J. Zhang; S. Q. Zhang; X. Y. Zhang; Y. Zhang; D. X. Zhao; H. W. Zhao; J. W. Zhao; M. Zhao; W. R. Zhao; J. P. Zheng; L. S. Zheng; Z. P. Zheng; G. P. Zhou; H. S. Zhou; L. Zhou; Q. M. Zhu; Y. C. Zhu; B. A. Zhuang

1998-01-01

362

Radiative lifetimes of Tb  

SciTech Connect

Radiative lifetimes measured by time-resolved laser-induced fluorescence are reported for 40 odd-parity levels and 36 even-parity levels of singly ionized terbium. The odd-parity levels range in energy from 29000 to 40000 cm{minus}1 and those of even-parity from 21000 to 37000 cm{minus}1. These lifetimes, with one exception, are accurate to {+-}5%. They will provide an absolute scale for accurate atomic-transition probabilities in Tb II (the second spectrum of terbium). {copyright} 2001 Optical Society of America

Den Hartog, E. A.; Fedchak, J. A.; Lawler, J. E.

2001-06-01

363

Convergence of configuration-interaction single-center calculations of positron-atom interactions  

SciTech Connect

The configuration interaction (CI) method using orbitals centered on the nucleus has recently been applied to calculate the interactions of positrons interacting with atoms. Computational investigations of the convergence properties of binding energy, phase shift, and annihilation rate with respect to the maximum angular momentum of the orbital basis for the e{sup +}Cu and PsH bound states, and the e{sup +}-H scattering system were completed. The annihilation rates converge very slowly with angular momentum, and moreover the convergence with radial basis dimension appears to be slower for high angular momentum. A number of methods of completing the partial wave sum are compared; an approach based on a {delta}X{sub J}=a(J+(1/2)){sup -n}+b(J+(1/2)){sup -(n+1)} form [with n=4 for phase shift (or energy) and n=2 for the annihilation rate] seems to be preferred on considerations of utility and underlying physical justification.

Mitroy, J.; Bromley, M. W. J. [Faculty of Technology, Charles Darwin University, Darwin NT 0909 (Australia); Department of Physics, San Diego State University, San Diego CA 92182 (United States)

2006-05-15

364

CONSTRAINTS ON DARK MATTER ANNIHILATION IN CLUSTERS OF GALAXIES FROM DIFFUSE RADIO EMISSION  

SciTech Connect

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.

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

365

Our Allotted Lifetimes  

ERIC Educational Resources Information Center

It is suggested that measured by the internal clock of heartbeats or breathing, all mammals live a similar lifespan. This is based on the fact that mammals, regardless of size, breathe about 200 million times in their lifetime at a rate of 1 breath for every 4 heartbeats. (AJ)

Gould, Stephen Jay

1977-01-01

366

Dipole moment bounds on dark matter annihilation  

NASA Astrophysics Data System (ADS)

We consider constraints on simplified models in which scalar dark matter annihilates to light charged leptons through the exchange of charged mediators. We find that loop diagrams will contribute corrections to the magnetic and electric dipole moments of the light charged leptons, and experimental constraints on these corrections place significant bounds on the dark matter annihilation cross section. In particular, annihilation to electrons with an observable cross section would be ruled out, while annihilation to muons is only permitted if the dominant contributions arise from CP-violating interactions.

Fukushima, Keita; Kumar, Jason

2013-09-01

367

Positron range in PET imaging: an alternative approach for assessing and correcting the blurring  

NASA Astrophysics Data System (ADS)

Positron range impairs resolution in PET imaging, especially for high-energy emitters and for small-animal PET. De-blurring in image reconstruction is possible if the blurring distribution is known. Furthermore, the percentage of annihilation events within a given distance from the point of positron emission is relevant for assessing statistical noise. This paper aims to determine the positron range distribution relevant for blurring for seven medically relevant PET isotopes, 18F, 11C, 13N, 15O, 68Ga, 62Cu and 82Rb, and derive empirical formulas for the distributions. This paper focuses on allowed-decay isotopes. It is argued that blurring at the detection level should not be described by the positron range r, but instead the 2D projected distance ? (equal to the closest distance between decay and line of response). To determine these 2D distributions, results from a dedicated positron track-structure Monte Carlo code, Electron and POsitron TRANsport (EPOTRAN), were used. Materials other than water were studied with PENELOPE. The radial cumulative probability distribution G2D(?) and the radial probability density distribution g2D(?) were determined. G2D(?) could be approximated by the empirical function 1 - exp(-A?2 - B?), where A = 0.0266 (Emean)-1.716 and B = 0.1119 (Emean)-1.934, with Emean being the mean positron energy in MeV and ? in mm. The radial density distribution g2D(?) could be approximated by differentiation of G2D(?). Distributions in other media were very similar to water. The positron range is important for improved resolution in PET imaging. Relevant distributions for the positron range have been derived for seven isotopes. Distributions for other allowed-decay isotopes may be estimated with the above formulas.

Jødal, L.; Le Loirec, C.; Champion, C.

2012-06-01

368

WMAP Microwave Emission Interpreted as Dark Matter Annihilation in the Inner Galaxy  

E-print Network

Excess microwave emission observed in the inner Galaxy (inner ~1 kpc) is consistent with synchrotron emission from highly relativistic electron-positron pairs produced by dark matter particle annihilation. More conventional sources for this emission, such as free-free (thermal bremsstrahlung), thermal dust, spinning dust, and the softer Galactic synchrotron traced by low-frequency surveys, have been ruled out. The total power observed in the range 23 =2x10^{-26} cm^3/s, and an 1/r dark matter mass profile truncated in the inner Galaxy, and find this scenario to be consistent with current data.

Douglas P. Finkbeiner

2004-09-02

369

A field-assisted moderator for low-energy positron beams  

NASA Astrophysics Data System (ADS)

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.

Beling, C. D.; Simpson, R. I.; Charlton, M.; Jacobsen, F. M.; Griffith, T. C.; Moriarty, P.; Fung, S.

1987-01-01

370

The Intense Slow Positron Beam Facility at the NC State University PULSTAR Reactor  

NASA Astrophysics Data System (ADS)

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-cm×30-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 (˜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 6×108 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 mater.

Hawari, Ayman I.; Gidley, David W.; Xu, Jun; Moxom, Jeremy; Hathaway, Alfred G.; Brown, Benjamin; Vallery, Richard

2009-03-01

371

Nonlinear positron acoustic solitary waves  

SciTech Connect

The problem of nonlinear positron acoustic solitary waves involving the dynamics of mobile cold positrons is addressed. A theoretical work is presented to show their existence and possible realization in a simple four-component plasma model. The results should be useful for the understanding of the localized structures that may occur in space and laboratory plasmas as new sources of cold positrons are now well developed.

Tribeche, Mouloud; Aoutou, Kamel; Younsi, Smain; Amour, Rabia [Plasma Physics Group, Theoretical Physics Laboratory, Faculty of Sciences-Physics, University of Bab-Ezzouar, USTHB, B.P. 32, El Alia, Algiers 16111 (Algeria)

2009-07-15

372

Muon lifetime measurement  

NASA Astrophysics Data System (ADS)

A simple experimental setup to measure the muon lifetime is presented. The muon detector consists of a sealed container with liquid scintillator coupled to a 2.5'' photomultiplier (PMT). A home-made electronics module controlled by the parallel port of a personal computer (PC) digitizes the time interval between two consecutive PMT pulses in a time window of 25.6 ?s. The muon lifetime is obtained by analysing thousands of double-pulse events in which the first pulse corresponds to a cosmic ray muon that stops inside the detector and the second to the decay electron coming from the weak decay of the muon. The background noise comes from random coincidences of pulses due to muons crossing the detector within the same time window. The PC is used as the data adquisition (DAQ) and data analysis computer. In addition to the muon lifetime, the charge ratio of cosmic ray muons and the capture rate of negative muons by carbon nuclei can be measured if the number of events is sufficiently high.

Villaseñor, Luis

1998-02-01

373

The ATLAS Positron Experiment -- APEX  

SciTech Connect

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.

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

374

Laser Created Relativistic Positron Jets  

SciTech Connect

Electron-positron jets with MeV temperature are thought to be present in a wide variety of astrophysical phenomena such as active galaxies, quasars, gamma ray bursts and black holes. They have now been created in the laboratory in a controlled fashion by irradiating a gold target with an intense picosecond duration laser pulse. About 10{sup 11} MeV positrons are emitted from the rear surface of the target in a 15 to 22-degree cone for a duration comparable to the laser pulse. These positron jets are quasi-monoenergetic (E/{delta}E {approx} 5) with peak energies controllable from 3-19 MeV. They have temperatures from 1-4 MeV in the beam frame in both the longitudinal and transverse directions. Positron production has been studied extensively in recent decades at low energies (sub-MeV) in areas related to surface science, positron emission tomography, basic antimatter science such as antihydrogen experiments, Bose-Einstein condensed positronium, and basic plasma physics. However, the experimental tools to produce very high temperature positrons and high-flux positron jets needed to simulate astrophysical positron conditions have so far been absent. The MeV temperature jets of positrons and electrons produced in our experiments offer a first step to evaluate the physics models used to explain some of the most energetic phenomena in the universe.

Chen, H; Wilks, S C; Meyerhofer, D D; Bonlie, J; Chen, C D; Chen, S N; Courtois, C; Elberson, L; Gregori, G; Kruer, W; Landoas, O; Mithen, J; Murphy, C; Nilson, P; Price, D; Scheider, M; Shepherd, R; Stoeckl, C; Tabak, M; Tommasini, R; Beiersdorder, P

2009-10-08

375

The Sprint to Lifetime Sports  

ERIC Educational Resources Information Center

Describes the trend in high school physical education programs toward lifetime sports, defined by the author as physical activities that will serve the interests of students for a lifetime. Included are a special report on program costs and a model of a performance-based lifetime sports program. (Author/DN)

Ernst, Leonard

1973-01-01

376

Implications of the AMS-02 positron fraction in cosmic rays  

NASA Astrophysics Data System (ADS)

The AMS-02 collaboration has just released its first result of the cosmic positron fraction e+/(e-+e+) with high precision up to ?350 GeV. The AMS-02 result shows the same trend with the previous PAMELA result, which requires extra electron/positron sources on top of the conventional cosmic ray background, either from astrophysical sources or from dark matter annihilation/decay. In this paper we try to figure out the nature of the extra sources by fitting to the AMS-02 e+/(e-+e+) data, as well as the electron and proton spectra by PAMELA and the (e-+e+) spectrum by Fermi and HESS. We adopt the GALPROP package to calculate the propagation of the Galactic cosmic rays and the Markov Chain Monte Carlo sampler to do the fit. We find that under the conventional assumptions about the background and the extra source of the e-+e+, we cannot fit the AMS-02 and Fermi/HESS data well simultaneously. The AMS-02 data require less electrons/positrons from the extra sources than that required by Fermi/HESS. It may indicate that the model needs to be refined or the data between these experiments have systematic uncertainties. The pulsar scenario generally fits the data better than the DM scenario. Furthermore, the constraints from ?-rays also disfavor the DM scenario to explain the cosmic ray lepton data.

Yuan, Qiang; Bi, Xiao-Jun; Chen, Guo-Ming; Guo, Yi-Qing; Lin, Su-Jie; Zhang, Xinmin

2015-01-01

377

Exciton annihilation and diffusion in semiconducting polymers  

NASA Astrophysics Data System (ADS)

We show that time-resolved luminescence measurements at high excitation densities can be used to study exciton annihilation and diffusion, and report the results of such measurements on films of P3HT and MEH-PPV. The results fit to an exciton-exciton annihilation model with a time independent annihilation rate ?, which was measured to be ? = (2.8+/-0.5)×10 -8 cm 3s -1 in MEH-PPV and ? = (5.2+/-1)×10 -10 cm 3s -1 in P3HT. This implies much faster diffusion in MEHPPV. Assuming a value of 1 nm for the annihilation radius we evaluated the diffusion length for pristine P3HT in one direction to be 3.2 nm. Annealing of P3HT was found to increase the annihilation rate to (1.1+/-0.2)×10 -9 cm 3s -1 and the diffusion length to 4.7 nm.

Shaw, P. E.; Lewis, A. J.; Ruseckas, A.; Samuel, I. D. W.

2006-08-01

378

Positronium formation and lifetimes in n-higher aliphatic alcohols  

NASA Astrophysics Data System (ADS)

Positron lifetime measurements have been carried out for five solid and liquid n-higher aliphatic alcohols (dodecanol, tetradecanol, hexadecanol, eicosanol and docosanol) in wide temperature ranges. Positronium formation processes in the studied alcohols are analysed in terms of the spur reaction model. Among others, the localisation mechanims of e -, e + in dipole traps in liquid and in solid rotator phases and the influence of the changes of the electron-positron critical distance and their thermalization ranges on Ps formation probability are discussed. Free volume distribution, defect generation, crystal impurities and phase transition dynamics influence additionally the Ps fraction in solid alcohols as in many other molecular crystals. The Ps trapping mechanism in defects (vacancies and other stable or unstable lattice voids) of the rotator phases is also discussed.

Lenc, R.

1989-03-01

379

Positron Emission Tomography (PET)  

SciTech Connect

Positron emission tomography (PET) assesses biochemical processes in the living subject, producing images of function rather than form. Using PET, physicians are able to obtain not the anatomical information provided by other medical imaging techniques, but pictures of physiological activity. In metaphoric terms, traditional imaging methods supply a map of the body's roadways, its, anatomy; PET shows the traffic along those paths, its biochemistry. This document discusses the principles of PET, the radiopharmaceuticals in PET, PET research, clinical applications of PET, the cost of PET, training of individuals for PET, the role of the United States Department of Energy in PET, and the futures of PET. 22 figs.

Welch, M.J.

1990-01-01

380

Quantum positron acoustic waves  

NASA Astrophysics Data System (ADS)

Nonlinear quantum positron-acoustic (QPA) waves are investigated for the first time, within the theoretical framework of the quantum hydrodynamic model. In the small but finite amplitude limit, both deformed Korteweg-de Vries and generalized Korteweg-de Vries equations governing, respectively, the dynamics of QPA solitary waves and double-layers are derived. Moreover, a full finite amplitude analysis is undertaken, and a numerical integration of the obtained highly nonlinear equations is carried out. The results complement our previously published results on this problem.

Metref, Hassina; Tribeche, Mouloud

2014-12-01

381

Positron trapping at vacancies in electron-irradiated Si at low temperatures  

SciTech Connect

Experimental results on positron trapping at vacancies in electron-irradiated silicon are presented. The positron lifetimes 273 +- 3 and 248 +- 2 ps in pure Si and heavily-phosphorus-doped Si ((P) = 10/sup 20/ cm/sup -3/) are assigned to a negative monovacancy V/sup -/ and a negative vacancy-phosphorus pair (V-P)/sup -/, respectively. In pure Si, positron trapping displays a strong negative temperature dependence, and the specific trapping rate reaches very large values (10/sup 17//sup --/10/sup 18/ s/sup -1/) at low temperatures. In Si:P the trapping rate is independent of temperature. These different temperature behaviors are attributed to different positron-trapping mechanisms, a cascade of one-phonon transitions in pure Si, and an Auger process in Si:P.

Maekinen, J.; Corbel, C.; Hautojaervi, P.; Moser, P.; Pierre, F.

1989-05-15

382

Applying two-photon excitation fluorescence lifetime imaging microscopy to study photosynthesis in plant leaves.  

PubMed

This study investigates to which extent two-photon excitation (TPE) fluorescence lifetime imaging microscopy can be applied to study picosecond fluorescence kinetics of individual chloroplasts in leaves. Using femtosecond 860 nm excitation pulses, fluorescence lifetimes can be measured in leaves of Arabidopsis thaliana and Alocasia wentii under excitation-annihilation free conditions, both for the F (0)- and the F (m)-state. The corresponding average lifetimes are approximately 250 ps and approximately 1.5 ns, respectively, similar to those of isolated chloroplasts. These values appear to be the same for chloroplasts in the top, middle, and bottom layer of the leaves. With the spatial resolution of approximately 500 nm in the focal (xy) plane and 2 microm in the z direction, it appears to be impossible to fully resolve the grana stacks and stroma lamellae, but variations in the fluorescence lifetimes, and thus of the composition on a pixel-to-pixel base can be observed. PMID:19468857

Broess, Koen; Borst, Jan Willem; van Amerongen, Herbert

2009-05-01

383

Vacancy clusters introduced by CF4-based plasma treatment in GaN probed with a monoenergetic positron beam  

NASA Astrophysics Data System (ADS)

Vacancy-type defects introduced by CF4-plasma treatment in GaN grown on Si substrates have been studied using a monoenergetic positron beam. By positron annihilation spectroscopy, it was found that vacancies were introduced below the surface (?30 nm), and this region expanded to a depth of 50 nm after 400 °C annealing. The major species of such defects were identified as vacancy clusters coupled with fluorine. The charge transition between these defects and optically active native defects was studied. The defects introduced by plasma treatment were found to introduce deep levels into the bandgap of GaN and act as nonradiative recombination centers.

Uedono, Akira; Yoshihara, Nakaaki; Zhang, Yuhao; Sun, Min; Piedra, Daniel; Fujishima, Tatsuya; Ishibashi, Shoji; Sumiya, Masatomo; Laboutin, Oleg; Johnson, Wayne; Palacios, Tomás

2014-12-01

384

Cyclotrons and positron emitting radiopharmaceuticals  

SciTech Connect

The state of the art of Positron Emission Tomography (PET) technology as related to cyclotron use and radiopharmaceutical production is reviewed. The paper discusses available small cyclotrons, the positron emitters which can be produced and the yields possible, target design, and radiopharmaceutical development and application. 97 refs., 12 tabs. (ACR)

Wolf, A.P.; Fowler, J.S.

1984-01-01

385

B Lifetimes and Mixing  

SciTech Connect

The Tevatron experiments, CDF and D0, have produced a wealth of new B-physics results since the start of Run II in 2001. We've observed new B-hadrons, seen new effects, and increased many-fold the precision with which we know the properties of b-quark systems. In these proceedings, we will discuss two of the most fruitful areas in the Tevatron B-physics program: lifetimes and mixing. We'll examine the experimental issues driving these analyses, present a summary of the latest results, and discuss prospects for the future.

Evans, Harold G.; /Indiana U.

2009-05-01

386

Positron driven plasma wakefields  

NASA Astrophysics Data System (ADS)

The LHC is producing high-energy, high-charge proton bunches (1e11 protons at 1-7 TeV each) that could be used to accelerate ``witness'' electron bunches to TeV range eneregies via a plasma wakefield accelerator (PWFA). Simulations [1] suggest that a proton ``drive'' bunch is able to excite large wakefields if the bunch size is on the order of 100 ?m; however, the LHC paramters are currently on the 1 cm scale. SLAC'S FACET is able to supply positorn bunchs with the ideal parameters for driving a PWFA. Although at lower energy (2e10 positrons at 23 GeV each), initial simiulations in QuickPIC show that the physics of a positron drive bunch is very similar to that of a proton drive bunch. Differences in the physics arise from the mass difference: slower dephasing but faster transverse bunch evolution. Other considerations include driver head erosion and purity of the wakefield ion column. The physics of positive drivers for PWFA and the viability of this scheme for future high-energy colliders will be investigated at SLAC's FACET.[4pt] [1] Caldwell, et al. Nature Physics 5, 363 (2009).[0pt] [2] C.H. Huang, et al., J. Comp. Phys., 217(2), 658, (2006).

Pinkerton, S.; Shi, Y.; Huang, C.; An, W.; Mori, W. B.; Muggli, P.

2010-11-01

387

Vacancy-type defects in InxGa1-xN alloys probed using a monoenergetic positron beam  

NASA Astrophysics Data System (ADS)

Native defects in InxGa1-xN grown by plasma-assisted molecular beam epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured, and these were compared with results obtained using first-principles calculation. The defect concentration increased with increasing In composition x and reached the maximum at x = 0.44˜0.56. A clear correlation between the line-width of photoluminescence and the defect concentration was obtained. The major defect species detected by positron annihilation was identified as cation vacancies coupled with multiple nitrogen vacancies (VNs), and their introduction mechanism is discussed in terms of the strain energy due to bond-length/angle distortions and the suppression of the VN formation energy by neighboring In atoms.

Uedono, A.; Ishibashi, S.; Watanabe, T.; Wang, X. Q.; Liu, S. T.; Chen, G.; Sang, L. W.; Sumiya, M.; Shen, B.

2012-07-01

388

Distributions of positron-emitting nuclei in proton and carbon-ion therapy studied with GEANT4  

Microsoft Academic Search

Depth distributions of positron-emitting nuclei in PMMA phantoms are calculated within a Monte Carlo model for heavy-ion therapy (MCHIT) based on the GEANT4 toolkit (version 8.0). The calculated total production rates of 11C, 10C and 15O nuclei are compared with experimental data and with corresponding results of the FLUKA and POSGEN codes. The distributions of e+ annihilation points are obtained

Igor Pshenichnov; Igor Mishustin; Walter Greiner

2006-01-01

389

Quantum chemical study of simple positronic systems using explicitly correlated Gaussian functions - PsH and PsLi+  

NASA Astrophysics Data System (ADS)

The electronic structure of positronium hydride has been studied using explicitly correlated Gaussian functions. The resulting energy constitutes new upper bound to the exact nonrelativistic energy of PsH within the Born-Oppenheimer approximation. The two photon annihilation rate was computed using the optimized wave function. Preliminary results for the positron bonded with the lithium atom indicate the stability of this system against the dissociation into Li+ cation and Ps atom.

Strasburger, K.; Chojnacki, H.

1998-02-01

390

Multiple gamma lines from semi-annihilation  

E-print Network

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 ...

D'Eramo, Francesco

391

Positron emission mammography imaging  

SciTech Connect

This paper examines current trends in Positron Emission Mammography (PEM) instrumentation and the performance tradeoffs inherent in them. The most common geometry is a pair of parallel planes of detector modules. They subtend a larger solid angle around the breast than conventional PET cameras, and so have both higher efficiency and lower cost. Extensions to this geometry include encircling the breast, measuring the depth of interaction (DOI), and dual-modality imaging (PEM and x-ray mammography, as well as PEM and x-ray guided biopsy). The ultimate utility of PEM may not be decided by instrument performance, but by biological and medical factors, such as the patient to patient variation in radiotracer uptake or the as yet undetermined role of PEM in breast cancer diagnosis and treatment.

Moses, William W.

2003-10-02

392

Collective versus independent annihilation of transhydrogen'' antinuclei  

SciTech Connect

Annihilation of heavy'' antinuclei, such as {ital {bar d}}, {ital {bar t}}, or {bar {alpha}} in nuclear matter could serve to probe whether the constituent antinucleons annihilate independently of each other or whether a single fireball is formed. In the latter case the total meson multiplicity distribution should have a lower mean and a larger relative width than in the trivial case. The meson spectrum would be correspondingly harder.

Friedlander, E.M.; Plumer, M. (Nuclear Science Division, Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, California 94720 (USA))

1989-11-01

393

Cosmic Ray Positrons from Pulsars  

NASA Technical Reports Server (NTRS)

Pulsars are potential Galactic sources of positrons through pair cascades in their magnetospheres. There are, however, many uncertainties in establishing their contribution to the local primary positron flux. Among these are the local density of pulsars, the cascade pair multiplicities that determine the injection rate of positrons from the pulsar, the acceleration of the injected particles by the pulsar wind termination shock, their rate of escape from the pulsar wind nebula, and their propagation through the interstellar medium. I will discuss these issues in the context of what we are learning from the new Fermi pulsar detections and discoveries.

Harding, Alice K.

2010-01-01

394

Positron emission particle tracking using a modular positron camera  

NASA Astrophysics Data System (ADS)

The technique of positron emission particle tracking (PEPT), developed at Birmingham in the early 1990s, enables a radioactively labelled tracer particle to be accurately tracked as it moves between the detectors of a "positron camera". In 1999 the original Birmingham positron camera, which consisted of a pair of MWPCs, was replaced by a system comprising two NaI(Tl) gamma camera heads operating in coincidence. This system has been successfully used for PEPT studies of a wide range of granular and fluid flow processes. More recently a modular positron camera has been developed using a number of the bismuth germanate (BGO) block detectors from standard PET scanners (CTI ECAT 930 and 950 series). This camera has flexible geometry, is transportable, and is capable of delivering high data rates. This paper presents simple models of its performance, and initial experience of its use in a range of geometries and applications.

Parker, D. J.; Leadbeater, T. W.; Fan, X.; Hausard, M. N.; Ingram, A.; Yang, Z.

2009-06-01

395

Future directions in high energy electron-positron experimentation  

SciTech Connect

In this report, the possibilities of studying particle physics at the TeV scale with high energy electron-positron linear colliders are discussed. A status report on the SLC and the MARK II program is given to provide some insights on the feasibility of experiments at linear colliders. The technical issues in going from SLC to the development of TeV colliders are briefly discussed. Some of the elements of the e/sup +/e/sup -/ experimental environment which differentiate it from that in hadron colliders and give examples of processes particularly well suited to attack by e/sup +/e/sup -/ annihilation are summarized. Finally, some concluding remarks are given. 8 refs., 10 figs., 2 tabs.

Trilling, G.H.

1988-09-01

396

Slow positron study of positronium formation and diffusion in crystalline and amorphous ice  

SciTech Connect

The aim of the present work has been to study the interaction of low energy positrons and of Ps with a molecular solid. As a first example we have chosen ice which has been extensively investigated both by conventional positron annihilation methods as well as by many other techniques. Unlike in metals, positrons injected into most molecular solids may have a certain probability to form Ps in the bulk of the material. The process by which Ps is formed in condensed molecular materials has been a matter of strong debate. According to the Ore model of Ps formation, which is recognized as being valid for low density gases, the positron during its slowing down picks off an electron from a molecule with which it simultaneously forms Ps. For energies below E/sub L/ = I-6.8 eV, where I is the lowest ionisation energy and 6.8 eV is the Ps binding energy in vacuum, this process is not energetically possible. Above an energy E/sub u/ approx. I the model predicts a reduction of the yield of stable Ps, because of rapid slowing down of the positron by ionisation or by splitting up again of a Ps atom which may have been formed. The energy range E/sub L/ less than or equal to E approximately less than E/sub u/ is the so-called Ore gap in gases. In the present work both crystalline and amorphous ice have been studied using positrons of energies in the range 0 to 4.5 keV. The experimental methods used were determination of the yields of 3..gamma..-annihilations as well as Doppler broadening measurements in the temperature range 45 to 150 K. The main results obtained were a determination of the Ps diffusion constant, and a demonstration that in the present experiments one contribution to the total Ps yield comes from positrons with typical Ore gap energies, and another contribution, due to spur processes, from positrons with initial energies of several hundred eV. 57 references, 9 figures.

Lynn, K.G.; Eldrup, M.; Vehanen, A.; Schultz, P.J.

1984-10-01

397

Performance as annihilation or integration?  

PubMed

What happens when the analyst has the impression of being annihilated by the patient? Analysts have a tendency to use more general, i.e. simplifying, constructions such as destructiveness, psychosis or death instinct as explanatory models. In the authors' view, these constructions in the end evade rather than mirror clinical reality. More recent research points to promising possibilities of differentiation, e.g. psychotic mechanisms which are--as yet undiscussed--based on Freud's notion of the partial 'rent in the relation between ego and external world'. These findings emphasize the restitutive function of a symptom or disturbance, i.e. destruction of a relationship which hinders the therapeutic process and which is not understood initially, instead of solely stressing the destructive meaning in a tabooing gesture. The concept of performance attempts to replace simplifying models with a discriminant process, and will be preliminarily defined and explained in delineation to terms already in use such as acting out, enactment, and role responsiveness. The authors explore the question of how the perception of unthought certainty in the performance can either be recognized as a blueprint, i.e. organizing activity, or as the destruction of the relationship so that a new one can emerge. The evidence from a detailed clinical example shows that many treatments can fail at this point and demonstrates how an understanding of performance in this sense offers a chance for integrating processes that otherwise impede treatment. PMID:17908672

Danckwardt, Joachim F; Wegner, Peter

2007-10-01

398

Positron Spectra of Co56  

Microsoft Academic Search

The positron spectrum of Co56 has been carefully studied with a magnetic spectrometer. Two positron groups were observed. The maximum energies and intensities of the two groups are 1.464+\\/-0.015 Mev and 0.440+\\/-0.030 Mev and >=90% and <=10%, respectively. No evidence for any other groups was found. In particular, an upper limit of 1% was set for the presence of any

J. H. Hamilton; L. M. Langer; D. R. Smith

1961-01-01

399

Dark matter annihilation effects on the first stars  

E-print Network

We study the effects of WIMP dark matter (DM) on the collapse and evolution of the first stars in the Universe. Using a stellar evolution code, we follow the pre-Main Sequence (MS) phase of a grid of metal-free stars with masses in the range 5-600 solar mass forming in the centre of a 1e6 solar mass halo at redhisft z=20. DM particles of the parent halo are accreted in the proto-stellar interior by adiabatic contraction and scattering/capture processes, reaching central densities of order 1e12 GeV/cm3 at radii of the order of 10 AU. Energy release from annihilation reactions can effectively counteract the gravitational collapse, in agreement with results from other groups. We find this stalling phase (known as "dark" star) is transients and lasts from 2.1e3 yr (M=600 solar mass) to 1.8e4 yr (M=9 solar mass). Later in the evolution, DM scattering/capture rate becomes high enough that energy deposition from annihilations significantly alters the pre-MS evolution of the star in a way that depends on DM (i) velocity dispersion, (ii) density, (iii) elastic scattering cross section with baryons. For our fiducial set of parameters (10 km/s, 1e11 GeV/cm3, 1e-38 cm2) we find that the evolution of stars of mass lower than 40 solar masses "freezes" on the HR diagram before reaching the ZAMS. Stars with bigger masses manage to ignite nuclear reactions; however, DM "burning" prolonges their lifetimes by a factor 2 (5) for a 600 (40) solar mass star.

F. Iocco; A. Bressan; E. Ripamonti; R. Schneider; A. Ferrara; P. Marigo

2008-05-26

400

Observation of e+e- Annihilation into the C=+1 Hadronic Final States ?0?0 and ??0  

NASA Astrophysics Data System (ADS)

We report the first observation of e+e- annihilation into states of positive C parity, namely, ?0?0 and ??0. The two states are observed in the ?+?-?+?- and K+K-?+?- final states, respectively, in a data sample of 225fb-1 collected by the BABAR experiment at the Positron-Electron Project II e+e- storage rings at energies near s=10.58GeV. The distributions of cos??*, where ?* is the center-of-mass polar angle of the ? meson or the forward ?0 meson, suggest production by two-virtual-photon annihilation. We measure cross sections within the range |cos??*|<0.8 of ?(e+e-??0?0)=20.7±0.7(stat)±2.7(syst)fb and ?(e+e-???0)=5.7±0.5(stat)±0.8(syst)fb.

Aubert, B.; Barate, R.; Bona, M.; Boutigny, D.; Couderc, F.; Karyotakis, Y.; Lees, J. P.; Poireau, V.; Tisserand, V.; Zghiche, A.; Grauges, E.; Palano, A.; Chen, J. C.; Qi, N. D.; Rong, G.; Wang, P.; Zhu, Y. S.; Eigen, G.; Ofte, I.; Stugu, B.; Abrams, G. S.; Battaglia, M.; Brown, D. N.; Button-Shafer, J.; Cahn, R. N.; Charles, E.; Gill, M. S.; Groysman, Y.; Jacobsen, R. G.; Kadyk, J. A.; Kerth, L. T.; Kolomensky, Yu. G.; Kukartsev, G.; Lynch, G.; Mir, L. M.; Oddone, P. J.; Orimoto, T. J.; Pripstein, M.; Roe, N. A.; Ronan, M. T.; Wenzel, W. A.; Del Amo Sanchez, P.; Barrett, M.; Ford, K. E.; Harrison, T. J.; Hart, A. J.; Hawkes, C. M.; Morgan, S. E.; Watson, A. T.; Goetzen, K.; Held, T.; Koch, H.; Lewandowski, B.; Pelizaeus, M.; Peters, K.; Schroeder, T.; Steinke, M.; Boyd, J. T.; Burke, J. P.; Cottingham, W. N.; Walker, D.; Cuhadar-Donszelmann, T.; Fulsom, B. G.; Hearty, C.; Knecht, N. S.; Mattison, T. S.; McKenna, J. A.; Khan, A.; Kyberd, P.; Saleem, M.; Sherwood, D. J.; Teodorescu, L.; Blinov, V. E.; Bukin, A. D.; Druzhinin, V. P.; Golubev, V. B.; Onuchin, A. P.; Serednyakov, S. I.; Skovpen, Yu. I.; Solodov, E. P.; Todyshev, K. Yu; Best, D. S.; Bondioli, M.; Bruinsma, M.; Chao, M.; Curry, S.; Eschrich, I.; Kirkby, D.; Lankford, A. J.; Lund, P.; Mandelkern, M.; Mommsen, R. K.; Roethel, W.; Stoker, D. P.; Abachi, S.; Buchanan, C.; Foulkes, S. D.; Gary, J. W.; Long, O.; Shen, B. C.; Wang, K.; Zhang, L.; Hadavand, H. K.; Hill, E. J.; Paar, H. P.; Rahatlou, S.; Sharma, V.; Berryhill, J. W.; Campagnari, C.; Cunha, A.; Dahmes, B.; Hong, T. M.; Kovalskyi, D.; Richman, J. D.; Beck, T. W.; Eisner, A. M.; Flacco, C. J.; Heusch, C. A.; Kroseberg, J.; Lockman, W. S.; Nesom, G.; Schalk, T.; Schumm, B. A.; Seiden, A.; Spradlin, P.; Williams, D. C.; Wilson, M. G.; Albert, J.; Chen, E.; Dvoretskii, A.; Fang, F.; Hitlin, D. G.; Narsky, I.; Piatenko, T.; Porter, F. C.; Ryd, A.; Samuel, A.; Mancinelli, G.; Meadows, B. T.; Sokoloff, M. D.; Blanc, F.; Bloom, P. C.; Chen, S.; Ford, W. T.; Hirschauer, J. F.; Kreisel, A.; Nauenberg, U.; Olivas, A.; Ruddick, W. O.; Smith, J. G.; Ulmer, K. A.; Wagner, S. R.; Zhang, J.; Chen, A.; Eckhart, E. A.; Soffer, A.; Toki, W. H.; Wilson, R. J.; Winklmeier, F.; Zeng, Q.; Altenburg, D. D.; Feltresi, E.; Hauke, A.; Jasper, H.; Petzold, A.; Spaan, B.; Brandt, T.; Klose, V.; Lacker, H. M.; Mader, W. F.; Nogowski, R.; Schubert, J.; Schubert, K. R.; Schwierz, R.; Sundermann, J. E.; Volk, A.; Bernard, D.; Bonneaud, G. R.; Grenier, P.; Latour, E.; Thiebaux, Ch.; Verderi, M.; Bard, D. J.; Clark, P. J.; Gradl, W.; Muheim, F.; Playfer, S.; Robertson, A. I.; Xie, Y.; Andreotti, M.; Bettoni, D.; Bozzi, C.; Calabrese, R.; Cibinetto, G.; Luppi, E.; Negrini, M.; Petrella, A.; Piemontese, L.; Prencipe, E.; Anulli, F.; Baldini-Ferroli, R.; Calcaterra, A.; de Sangro, R.; Finocchiaro, G.; Pacetti, S.; Patteri, P.; Peruzzi, I. M.; Piccolo, M.; Rama, M.; Zallo, A.; Buzzo, A.; Capra, R.; Contri, R.; Lo Vetere, M.; Macri, M. M.; Monge, M. R.; Passaggio, S.; Patrignani, C.; Robutti, E.; Santroni, A.; Tosi, S.; Brandenburg, G.; Chaisanguanthum, K. S.; Morii, M.; Wu, J.; Dubitzky, R. S.; Marks, J.; Schenk, S.; Uwer, U.; Bhimji, W.; Bowerman, D. A.; Dauncey, P. D.; Egede, U.; Flack, R. L.; Nash, J. A.; Nikolich, M. B.; Panduro Vazquez, W.; Chai, X.; Charles, M. J.; Mallik, U.; Meyer, N. T.; Ziegler, V.; Cochran, J.; Crawley, H. B.; Dong, L.; Eyges, V.; Meyer, W. T.; Prell, S.; Rosenberg, E. I.; Rubin, A. E.; Gritsan, A. V.; Fritsch, M.; Schott, G.; Arnaud, N.; Davier, M.; Grosdidier, G.; Höcker, A.; Le Diberder, F.; Lepeltier, V.; Lutz, A. M.; Oyanguren, A.; Pruvot, S.; Rodier, S.; Roudeau, P.; Schune, M. H.; Stocchi, A.; Wang, W. F.; Wormser, G.; Cheng, C. H.; Lange, D. J.; Wright, D. M.; Chavez, C. A.; Forster, I. J.; Fry, J. R.; Gabathuler, E.; Gamet, R.; George, K. A.; Hutchcroft, D. E.; Payne, D. J.; Schofield, K. C.; Touramanis, C.; Bevan, A. J.; di Lodovico, F.; Menges, W.; Sacco, R.; Cowan, G.; Flaecher, H. U.; Hopkins, D. A.; Jackson, P. S.; McMahon, T. R.; Ricciardi, S.; Salvatore, F.; Wren, A. C.; Brown, D. N.; Davis, C. L.; Allison, J.; Barlow, N. R.; Barlow, R. J.; Chia, Y. M.; Edgar, C. L.; Lafferty, G. D.; Naisbit, M. T.; Williams, J. C.; Yi, J. I.; Chen, C.; Hulsbergen, W. D.; Jawahery, A.; Lae, C. K.; Roberts, D. A.; Simi, G.; Blaylock, G.; Dallapiccola, C.; Hertzbach, S. S.; Li, X.; Moore, T. B.; Saremi, S.; Staengle, H.; Cowan, R.; Sciolla, G.; Sekula, S. J.; Spitznagel, M.; Taylor, F.; Yamamoto, R. K.; Kim, H.; Patel, P. M.; Robertson, S. H.; Lazzaro, A.; Lombardo, V.; Palombo, F.; Bauer, J. M.; Cremaldi, L.; Eschenburg, V.; Godang, R.; Kroeger, R.; Sanders, D. A.; Summers, D. J.; Zhao, H. W.; Brunet, S.; Côté, D.; Taras, P.; Viaud, F. B.; Nicholson, H.; Cavallo, N.; de Nardo, G.

2006-09-01

401

Electromagnetic cascades in the magnetosphere of a very young pulsar - A model for the positron production near the Galactic center  

NASA Technical Reports Server (NTRS)

A detailed model for positron production by a young pulsar is presented. It is shown that electromagnetic cascades can develop in a young pulsar's magnetosphere, and the model results are applied to the pulsar which is hypothesized to lie near the Galactic center. It is found that such a pulsar would be expected to produce relatively low energy electron-positron pairs with an efficiency rating high enough to explain the observed luminosity of the Galactic center annihilation line. Virtually all of the gamma ray continuum radiation produced in the cascades would be beamed along the magnetic poles of the neutron star, and therefore probably would not be observed from earth. Some observational predictions generated by the proposed model for the Galactic center positron source are given.

Mastichiadis, Apostolos; Brecher, Kenneth; Marscher, Alan P.

1987-01-01

402

Constraining the origin of the rising cosmic ray positron fraction with the boron-to-carbon ratio  

E-print Network

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 ne...

Cholis, Ilias

2013-01-01

403

The cosmic-ray positron excess from a local Dark Matter over-density  

NASA Astrophysics Data System (ADS)

We show that the cosmic-ray positron excess measured by PAMELA and AMS could be induced by Dark Matter annihilations in a local over-density. In such a context leptophilic DM is not needed and good fits to positron data, in agreement with antiproton and gamma-ray measurements, are obtained for DM annihilations to WW, hh, ZZ, ttbar, bbbar, qqbar channels. The classic Dark Matter candidates, such as the pure supersymmetric Wino with standard thermal annihilation cross-section, can fit the positron excess, without invoking any additional assumption on Dark Matter properties. The pink band is the diffuse isotropic gamma-ray background, as extracted from the FERMI Collaboration [42]. The slightly higher gray band is extracted by us from FERMI data, following a simpler procedure. We subtracted known point-like sources and reduced the Galactic gamma-ray background by restricting the observation region to high Galactic latitudes, |b|>60°. We do not show the expected astrophysical gamma-ray background because we do not know any reliable estimate of it.We here neglected the Inverse Compton gamma-ray flux, because it is strongly reduced with respect to the standard scenario, where it is problematic, by our assumptions that the e+ excess is just local.Finally, we point out that, while the main features of our results have been explained with simple approximations, our numerical results have been derived from a full numerical study where we have taken into account energy losses for e± and other small effects. In Figs. 4 and 5 we also plotted the contributions to the gamma-ray and antiproton fluxes coming from regions of the Milky Way outside from the dominant local over-density. We see that such contribution is so small that the analysis would remain unchanged in presence of a moderate Bpart˜10, or even larger.

Hektor, Andi; Raidal, Martti; Strumia, Alessandro; Tempel, Elmo

2014-01-01

404

Search for Neutrinos from Annihilating Dark Matter in the Direction of the Galactic Center with the 40-String IceCube Neutrino Observatory  

E-print Network

A search for muon neutrinos from dark matter annihilations in the Galactic Center region has been performed with the 40-string configuration of the IceCube Neutrino Observatory using data collected in 367 days of live-time starting in April 2008. The observed fluxes were consistent with the atmospheric background expectations. Upper limits on the self-annihilation cross-section are obtained for dark matter particle masses ranging from 100 GeV to 10 TeV. In the case of decaying dark matter, lower limits on the lifetime have been determined for masses between 200 GeV and 20 TeV.

Abbasi, R; Ackermann, M; Adams, J; Aguilar, J A; Ahlers, M; Altmann, D; Andeen, K; Auffenberg, J; Bai, X; Baker, M; Barwick, S W; Baum, V; Bay, R; Beattie, K; Beatty, J J; Bechet, S; Tjus, J Becker; Becker, K -H; Bell, M; Benabderrahmane, M L; BenZvi, S; Berdermann, J; Berghaus, P; Berley, D; Bernardini, E; Bertrand, D; Besson, D Z; Bindig, D; Bissok, M; Blaufuss, E; Blumenthal, J; Boersma, D J; Bohm, C; Bose, D; Böser, S; Botner, O; Brayeur, L; Brown, A M; Bruijn, R; Brunner, J; Buitink, S; Carson, M; Casey, J; Casier, M; Chirkin, D; Christy, B; Clevermann, F; Cohen, S; Cowen, D F; Silva, A H Cruz; Danninger, M; Daughhetee, J; Davis, J C; De Clercq, C; Descamps, F; Desiati, P; Uiterweerd, G de Vries; DeYoung, T; Díaz-Vélez, J C; Dreyer, J; Dumm, J P; Dunkman, M; Eagan, R; Eisch, J; Ellsworth, R W; Engdegård, O; Euler, S; Evenson, P A; Fadiran, O; Fazely, A R; Fedynitch, A; Feintzeig, J; Feusels, T; Filimonov, K; Finley, C; Fischer-Wasels, T; Flis, S; Franckowiak, A; Franke, R; Frantzen, K; Fuchs, T; Gaisser, T K; Gallagher, J; Gerhardt, L; Gladstone, L; Glüsenkamp, T; Goldschmidt, A; Goodman, J A; Góra, D; Grant, D; Groß, A; Grullon, S; Gurtner, M; Ha, C; Ismail, A Haj; Hallgren, A; Halzen, F; Hanson, K; Heereman, D; Heimann, P; Heinen, D; Helbing, K; Hellauer, R; Hickford, S; Hill, G C; Hoffman, K D; Hoffmann, R; Homeier, A; Hoshina, K; Hülß, J -P; Huelsnitz, W; Hulth, P O; Hultqvist, K; Hussain, S; Ishihara, A; Jacobi, E; Jacobsen, J; Japaridze, G S; Jlelati, O; Kappes, A; Karg, T; Karle, A; Kiryluk, J; Kislat, F; Kläs, J; Klein, S R; Köhne, J -H; Kohnen, G; Kolanoski, H; Köpke, L; Kopper, C; Kopper, S; Koskinen, D J; Kowalski, M; Krasberg, M; Kroll, G; Kunnen, J; Kurahashi, N; Kuwabara, T; Labare, M; Laihem, K; Landsman, H; Larson, M J; Lauer, R; Lesiak-Bzdak, M; Lünemann, J; Madsen, J; Maruyama, R; Mase, K; Matis, H S; McNally, F; Meagher, K; Merck, M; Mészáros, P; Meures, T; Miarecki, S; Middell, E; Milke, N; Miller, J; Mohrmann, L; Montaruli, T; Morse, R; Movit, S M; Nahnhauer, R; Naumann, U; Nowicki, S C; Nygren, D R; Obertacke, A; Odrowski, S; Olivas, A; Olivo, M; O'Murchadha, A; Panknin, S; Paul, L; Pepper, J A; Heros, C Pérez de los; Pieloth, D; Pirk, N; Posselt, J; Price, P B; Przybylski, G T; Rädel, L; Rawlins, K; Redl, P; Resconi, E; Rhode, W; Ribordy, M; Richman, M; Riedel, B; Rodrigues, J P; Rothmaier, F; Rott, C; Ruhe, T; Ruzybayev, B; Ryckbosch, D; Saba, S M; Salameh, T; Sander, H -G; Santander, M; Sarkar, S; Schatto, K; Scheel, M; Scheriau, F; Schmidt, T; Schmitz, M; Schoenen, S; Schöneberg, S; Schönherr, L; Schönwald, A; Schukraft, A; Schulte, L; Schulz, O; Seckel, D; Seo, S H; Sestayo, Y; Seunarine, S; Smith, M W E; Soiron, M; Soldin, D; Spiczak, G M; Spiering, C; Stamatikos, M; Stanev, T; Stasik, A; Stezelberger, T; Stokstad, R G; Stößl, A; Strahler, E A; Ström, R; Sullivan, G W; Taavola, H; Taboada, I; Tamburro, A; Ter-Antonyan, S; Tilav, S; Toale, P A; Toscano, S; Usner, M; van der Drift, D; van Eijndhoven, N; Van Overloop, A; van Santen, J; Vehring, M; Voge, M; Walck, C; Waldenmaier, T; Wallraff, M; Walter, M; Wasserman, R; Weaver, Ch; Wendt, C; Westerhoff, S; Whitehorn, N; Wiebe, K; Wiebusch, C H; Williams, D R; Wissing, H; Wolf, M; Wood, T R; Woschnagg, K; Xu, C; Xu, D L; Xu, X W; Yanez, J P; Yodh, G; Yoshida, S; Zarzhitsky, P; Ziemann, J; Zilles, A; Zoll, M

2012-01-01

405

Cu Precipitates in Fe Ion Irradiated Fe-Cu Alloys Studied Using Positron Techniques  

NASA Astrophysics Data System (ADS)

In this paper, we summarized our recent experimental results on Fe-Cu model alloys irradiated by Fe ion. Two kinds of Fe-Cu alloys with 0.3%Cu and 0.6%Cu were prepared and irradiated by 2.5 MeV Fe ion at 573 K. Irradiation dose is 0.1 dpa and 1.2 dpa for each type alloy respectively. Positron annihilation techniques of slow positron beam were used to investigate the irradiation induced defects. Results show that the S parameters are higher in the specimens with high irradiation dose, but the intensity of Cu peaks of CDB is lower. It indicates that the precipitation of Cu atoms formed easily as lower irradiation dose.

Cao, X. Z.; Zhang, P.; Xu, Q.; Sato, K.; Tsuchida, H.; Cheng, G. D.; Wu, H. B.; Jiang, X. P.; Yu, R. S.; Wang, B. Y.; Wei, L.

2013-06-01

406

Corrosion-related defects in Zircaloys: a preliminary study with slow positron beam  

NASA Astrophysics Data System (ADS)

Corrosion-related microstructure and defects in Zircaloy-4 and N18 alloys were investigated by variable energy positron annihilation spectroscopy. The specimens were corroded in 0.01mol/L LiOH aqueous solution at 360 °C/18.6 MPa and in super heated steam at 400 °C/10.3 MPa, respectively. Defect profiles were analyzed by measuring the S parameter as a function of incident positron energy from 0.25 to 27 keV. Results indicated that Zircaloys corroded in LiOH aqueous solution contained more defects in the oxide layer than that in superheated steam, which implies that formation of defects in oxide layer may relate to the effects of Li+ ions in corrosion solution.

Zhu, Z. J.; Yao, M. Y.; Xue, X. D.; Wu, Y. C.; Zhou, B. X.

2014-04-01

407

Corrosion of iron and stainless steels studied using slow positron beam technique  

NASA Astrophysics Data System (ADS)

Corrosion-related defects of pure iron, and AISI 304 and 316 stainless steels were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation. Defect profiles of the S parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ˜1 ?m depth) were analyzed. When the sample is treated with various anodic potentials (0-4 V), a significant increase in the S parameter in pure iron is observed. This corrosion process shows the presence of defects and voids of a few nanometers near the surface. However, a significant decrease of the S parameter in AISI 304 and 316 stainless steels after corrosion is observed, which is interpreted as dissolution of passivity oxide film. Different defect results are observed between pure iron and stainless steels.

Wu, Y. C.; Zhang, R.; Chen, H.; Li, Y.; Zhang, J.; Zhu, D.-M.; Jean, Y. C.

2003-10-01

408

Large-dimension configuration-interaction calculations of positron binding to the group-II atoms  

SciTech Connect

The configuration-interaction (CI) method is applied to the calculation of the structures of a number of positron binding systems, including e{sup +}Be, e{sup +}Mg, e{sup +}Ca, and e{sup +}Sr. These calculations were carried out in orbital spaces containing about 200 electron and 200 positron orbitals up to l=12. Despite the very large dimensions, the binding energy and annihilation rate converge slowly with l, and the final values do contain an appreciable correction obtained by extrapolating the calculation to the l{yields}{infinity} limit. The binding energies were 0.00317 hartree for e{sup +}Be, 0.0170 hartree for e{sup +}Mg, 0.0189 hartree for e{sup +}Ca, and 0.0131 hartree for e{sup +}Sr.

Bromley, M. W. J.; Mitroy, J. [Department of Physics, San Diego State University, San Diego, California 92182 (United States); Faculty of Technology, Charles Darwin University, Darwin NT 0909 (Australia)

2006-03-15

409

Cosmic positron excess: is the dark matter solution a good bet?  

E-print Network

The recent observation by the PAMELA satellite of a rising positron fraction up to $\\sim$ 100 GeV has triggered a considerable amount of putative interpretations in terms of dark matter (DM) annihilation or decay. Here, we make a critical reassessment of such a possibility, recalling the elementary conditions with respect to the standard astrophysical background that would make it likely, showing that they are not fulfilled. Likewise, we argue that, as now well accepted, DM would need somewhat contrived properties to contribute significantly to the observed positron signal, even when including e.g. clumpiness effects. This means that most of natural DM candidates arising in particle physics beyond the standard model are not expected to be observed in the cosmic antimatter spectrum, unfortunately. However, this does not prevent them from remaining excellent DM candidates, this only points towards the crucial need of developing much more complex detection strategies (multimessenger, multiwavelength, multiscale ...

Lavalle, Julien

2009-01-01

410

Modelling Positron Transport in Biological Media -- Towards a Positron Dosimetry  

NASA Astrophysics Data System (ADS)

We have a combined program of measurement, analysis and modeling for positron interactions with biologically relevant molecules. The measurement involves direct determination of positron scattering cross sections including, most importantly, positronium formation, which are then combined into a set of cross sections and, together with energy loss spectra, serve as the input to a number of modeling approaches. These include both Monte Carlo and Boltzmann approaches and, ultimately, they aim to model interactions in liquid systems in order to best approximate tissue equivalents. Results for the archetypical system, water, will be presented as well as preliminary studies on other biologically relevant molecules.

Petrovic, Z.; Garcia, G.; White, R.; Brunger, M.; Sullivan, J.; Buckman, S.

2011-11-01

411

On Sunspot and Starspot Lifetimes  

NASA Astrophysics Data System (ADS)

We consider the lifetimes of spots on the Sun and other stars from the standpoint of magnetic diffusion. While normal magnetic diffusivity predicts lifetimes of sunspots that are too large by at least two orders of magnitude, turbulent magnetic diffusivity accounts for both the functional form of the solar empirical spot-lifetime relation and for the observed sunspot lifetimes, provided that the relevant diffusion length is the supergranule size. Applying this relation to other stars, the value of turbulent diffusivity depends almost entirely on supergranule size, with very weak dependence on other variables such as magnetic field strength and density. Overall, the best observational data for other stars is consistent with the extension of the solar relation, provided that stellar supergranule sizes for some stars are significantly larger than they are on the Sun.

Bradshaw, S. J.; Hartigan, P.

2014-11-01

412

PREFACE 12th International Workshop on Slow Positron Beam Techniques  

NASA Astrophysics Data System (ADS)

Preface These proceedings arose from the 12th International Workshop on Slow Positron Beam Techniques (SLOPOS12), which was held on Magnetic Island, North Queensland, Australia, between 1-6th August 2010. Meetings in the SLOPOS series are held (roughly) every three years and have now been held on (almost) all continents, indicating the truly international nature of the field. SLOPOS12 marked the second time that the Workshop had been held in the southern hemisphere, and the first time in Australia. SLOPOS12 attracted 122 delegates from 16 countries. Most encouraging was the attendance of 28 student delegates, and that about half of the overall delegates were early career researchers - a good sign for the future of our field. We also enjoyed the company of more than a dozen partners and families of delegates. In a slight departure from previous SLOPOS meetings, the International Advisory Committee approved a broader scope of scientific topics for inclusion in the program for the 2010 Workshop. This broader scope was intended to capture the applications of positrons in atomic, molecular and biomedical areas and was encapsulated in the byeline for SLOPOS-12: The 12th International Workshop on Slow Positron Beam Techniques for Solids, Surfaces, Atoms and Molecules. The scientific and social program for the meeting ran over 6 days with delegates gathering on Sunday August 1st and departing on August 6th. The scientific program included plenary, invited, contributed and student lectures, the latter being the subject of a student prize. In all there were 53 oral presentations during the week. There were also two poster sessions, with 63 posters exhibited, and a prize was awarded for the best poster by a student delegate. The standard of the student presentations, both oral and posters, was outstanding, so much so that the judging panel recommended an additional number of prizes be awarded. Topics that were the focus of invited presentations and contributed papers at SLOPOS-12 included: Positron Interactions with Surfaces Positron Beam and Detector Technology Positron Interactions with Atoms and Molecules Positronium Science Defects and Vacancies in Materials Porosity and Open Volume in Materials Antimatter in Biomedical Science Anti-hydrogen Studies Positron Transport Annihilation On a sad note, delegates paid tribute to the contributions of one of our colleagues, Chris Beling, who tragically passed away shortly before the meeting. Chris' contributions to positron science and to the education of young scientists were noted in a number of the invited presentations. It is an honour for our community to begin these proceedings with a short tribute to Chris' life by Professor Paul Coleman. The Workshop could not have occurred without the generous support of our sponsors: The ARC Centre for Antimatter-Matter Studies, The Australian National University, Flinders University, James Cook University, The Institute of Physics (UK) and the Australian Government's Department of Innovation, Industry, Science and Research. It would also not have been possible without the hard work of the Local and International Organising Committees and the friendly and efficient staff at the All Seasons Resort, Magnetic Island. We are most grateful for the on-site assistance of Gillian Drew, the CAMS student and postdoc team, the financial wizardry of Chris Kalos, and the post-Workshop editorial assistance of Julia Wee and Adam Edwards. Finally we would like to thank all of the attendees at SLOPOS12 for their scientific contributions to the Workshop, and for the warm spirit of engagement which characterised the scientific discussions and social occasions. SLOPOS13 will be held in Germany in 2013 and we all look forward to the occasion. Stephen Buckman, James Sullivan and Ronald White(Guest Editors) Local Organising CommitteeInternational Committee Stephen Buckman (Chair, ANU, Canberra)G Amarendra (India) James Sullivan (Secretary, ANU, Canberra)M-F Barthe (France) Ronald White (JCU, Townsville)C Beling (Hong Kong) Jim Williams (UWA, Perth)R Brusa (Italy) Suz

Buckman, Stephen; Sullivan, James; White, Ronald

2011-01-01

413

Positron emitter labeled enzyme inhibitors  

SciTech Connect

This invention involves a new strategy for imaging and mapping enzyme activity in the living human and animal body using positron emitter-labeled suicide enzyme inactivators or inhibitors which become covalently bound to the enzyme as a result of enzymatic catalysis. Two such suicide inactivators for monoamine oxidase have been labeled with carbon-11 and used to map the enzyme subtypes in the living human and animal body using PET. By using positron emission tomography to image the distribution of radioactivity produced by the body penetrating radiation emitted by carbon-11, a map of functionally active monoamine oxidase activity is obtained. Clorgyline and L-deprenyl are suicide enzyme inhibitors and irreversibly inhibit monoamine oxidase. When these inhibitors are labeled with carbon-11 they provide selective probes for monoamine oxidase localization and reactivity in vivo using positron emission tomography.

Fowler, J.S.; MacGregor, R.R.; Wolf, A.P.; Langstrom, B.

1990-04-03

414

Short and medium range order in two-component silica glasses by positron annihilation spectroscopy  

SciTech Connect

The dependence of chemical composition on the average sizes of subnanometer-scale intrinsic structural open spaces surrounded by glass random networks in two-component silica-based glasses was investigated systematically using positronium (Ps) confined in the open spaces. The average sizes of the open spaces for SiO{sub 2}-B{sub 2}O{sub 3} and SiO{sub 2}-GeO{sub 2} glasses are only slightly dependent on the chemical compositions because the B{sub 2}O{sub 3} and GeO{sub 2} are glass network formers that are incorporated into the glass network of the base SiO{sub 2}. However, the open space sizes for all SiO{sub 2}-R{sub 2}O (R?=?Li, Na, K) glasses, where R{sub 2}O is a glass network modifier that occupies the open spaces, decrease rapidly with an increase in the R{sub 2}O concentration. Despite the large difference in the ionic radii of the alkali metal (R) atoms, the open space sizes decrease similarly for all the alkali metal atoms studied. This dependence of the chemical composition on the open space sizes in SiO{sub 2}-R{sub 2}O observed by Ps shows that the alkali metal atoms do not randomly occupy the structural open spaces, but filling of the open spaces by R{sub 2}O proceeds selectively from the larger to the smaller open spaces as the R{sub 2}O concentrations are increased.

Inoue, K.; Kataoka, H.; Nagai, Y. [The Oarai Center, Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313 (Japan); Hasegawa, M. [Institute for Materials Research, Tohoku University, Sendai 980-8577 (Japan); Kobayashi, Y. [The Oarai Center, Institute for Materials Research, Tohoku University, Oarai, Ibaraki 311-1313 (Japan); Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology, Umezono, Tsukuba, Ibaraki 305-8568 (Japan)

2014-05-28

415

21 CFR 892.1110 - Positron camera.  

Code of Federal Regulations, 2011 CFR

... Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES RADIOLOGY DEVICES Diagnostic Devices § 892.1110 Positron camera. (a) Identification. A positron camera is a...

2011-04-01

416

21 CFR 892.1110 - Positron camera.  

Code of Federal Regulations, 2013 CFR

... Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES RADIOLOGY DEVICES Diagnostic Devices § 892.1110 Positron camera. (a) Identification. A positron camera is a...

2013-04-01

417

21 CFR 892.1110 - Positron camera.  

Code of Federal Regulations, 2014 CFR

... Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES RADIOLOGY DEVICES Diagnostic Devices § 892.1110 Positron camera. (a) Identification. A positron camera is a...

2014-04-01

418

21 CFR 892.1110 - Positron camera.  

Code of Federal Regulations, 2010 CFR

... Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES RADIOLOGY DEVICES Diagnostic Devices § 892.1110 Positron camera. (a) Identification. A positron camera is a...

2010-04-01

419

21 CFR 892.1110 - Positron camera.  

Code of Federal Regulations, 2012 CFR

... Food and Drugs FOOD AND DRUG ADMINISTRATION, DEPARTMENT OF HEALTH AND HUMAN SERVICES (CONTINUED) MEDICAL DEVICES RADIOLOGY DEVICES Diagnostic Devices § 892.1110 Positron camera. (a) Identification. A positron camera is a...

2012-04-01

420

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

NASA Astrophysics Data System (ADS)

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 source and in line with the NIPNE-cyclotron or a new intense compact cyclotron. The construction of the beam line was planned as a sequence of modules: source- moderator system; magnetical filter for fast positrons in order to select the positrons energies in the range 0.8-1 keV; a modular system for focusing, transport and acceleration of monoenergetic positrons in the energy range 0.8-50 keV and a CDBS analysis chamber. The moderator proposed-is tungsten as a foil of about 3 ?m prepared at the Optoelectronics Institute were put into a thermal treatment vacuum chamber and bombarded with electrons from a 100 W electron gun After the treatment, they were tested for changes of elemental composition of the surface and structure at the Polytechnic University. The structure tests were performed on a DRON 3 M diffractometer, with a Co tube (?K? = 1.7903 A)-the angular regions studied were around 34° (1 0 0) and 69° (2 0 0). In the present time, the trajectories of the positron are going to be simulated with dedicated software (an ion and electron optics simulator). For the coincidence measurements (CDBS) set-up we used a home-made 22NaCl source, by separation without carrier from a metallic Mg target irradiated with 12 MeV protons and separated by columnar cation exchange. A home- made biparametric system for CDBS measurements will be reported, also.

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

2009-03-01

421

Status and Perspectives for a Slow Positron Beam Facility at the HH—NIPNE Bucharest  

NASA Astrophysics Data System (ADS)

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 source and in line with the NIPNE-cyclotron or a new intense compact cyclotron. The construction of the beam line was planned as a sequence of modules: source- moderator system; magnetical filter for fast positrons in order to select the positrons energies in the range 0.8-1 keV; a modular system for focusing, transport and acceleration of monoenergetic positrons in the energy range 0.8-50 keV and a CDBS analysis chamber. The moderator proposed—is tungsten as a foil of about 3 ?m prepared at the Optoelectronics Institute were put into a thermal treatment vacuum chamber and bombarded with electrons from a 100 W electron gun After the treatment, they were tested for changes of elemental composition of the surface and structure at the Polytechnic University. The structure tests were performed on a DRON 3 M diffractometer, with a Co tube (?K? = 1.7903 A)—the angular regions studied were around 34° (1 0 0) and 69° (2 0 0). In the present time, the trajectories of the positron are going to be simulated with dedicated software (an ion and electron optics simulator). For the coincidence measurements (CDBS) set-up we used a home-made 22NaCl source, by separation without carrier from a metallic Mg target irradiated with 12 MeV protons and separated by columnar cation exchange. A home- made biparametric system for CDBS measurements will be reported, also.

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

2009-03-01

422

Positron binding to alcohol molecules  

NASA Astrophysics Data System (ADS)

Results are presented for positron binding to a selection of molecules containing the hydroxyl functional group. These molecules, which span in the range of carbon atoms from 1 (methanol) to 4 (1-butanol), have moderate permanent dipole moments ranging from about 1.4 to 2.4 D. The dependence of the binding energy on the magnitude of the molecular dipole polarizability and static dipole moment is studied. An effect that appears to be due to the localization of the bound positron is discussed.

Jones, A. C. L.; Danielson, J. R.; Gosselin, J. J.; Natisin, M. R.; Surko, C. M.

2012-01-01

423

Magnetic Field Effects: Triplet-Charge Annihilation versus Triplet-Triplet Annihilation in Organic Semiconductors  

NASA Astrophysics Data System (ADS)

Triplet-charge reaction and triplet-triplet annihilation are two important processes in generating magnetic field effects in organic semiconductors. This presentation reports experimental studies on triplet-charge annihilation (TCA) versus triplet-triplet annihilation (TTA) in organic semiconductors. Specifically, we separately adjust the triplet-charge and triplet-triplet interactions towards the generation of TCA and TTA by changing triplet density, charge confinement, and charge/exciton ratio based on organic light-emitting diodes. We then use magnetic field effects of electroluminescence (MFEEL) to study the TCA and TTA through spin interactions. We observe that the electroluminescence can clearly show negative response to applied magnetic field when triplets and charges are simultaneously confined within close proximity. On contrast, the electroluminescence only exhibits positive MFEEL when triplets are confined within close proximity. Therefore, it can be concluded that the TCA is a major process to annihilate triplets through Coulomb interaction in organic semiconductors.

Hu, Bin; Shao, Ming; Yan, Liang; Li, Mingxing

2012-02-01

424

Searches for WIMP Annihilation with GLAST  

SciTech Connect

We describe signatures for WIMP annihilation in the gamma ray sky which can be observed by the GLAST mission, scheduled for launch in 2007. We review the search regions, which range from galactic substructure in the Milky Way all the way out to cosmological sources.

Wai, L.; /SLAC; ,

2005-06-21

425

Long range correlations in e + e - annihilation?  

Microsoft Academic Search

We calculate long range multiplicity correlations in e + e - annihilation. Perturbative, probabilistic models predict vanishing multiplicity correlations between two opposite jets, in contrast to the results for non-perturbative models. The former still lead to strong long range correlations within one jet.

J. H. Kühn; H. Schneider

1981-01-01

426

Gamma ray lines from dark matter annihilation  

SciTech Connect

If direct annihilation of dark matter particles into a pair of photons occurs in the galactic halo, a narrow {gamma}-ray line can be discovered at future {gamma}-ray detectors sensitive to the GeV region. The signals predicted by different dark matter candidates are analyzed. 16 refs., 3 figs.

Giudice, G.F.

1989-08-01

427

Confronting recent AMS-02 positron fraction and Fermi-LAT Extragalactic Gamma-Ray Background measurements with gravitino dark matter  

E-print Network

The positron fraction measured by the space-based detectors PAMELA, Fermi-LAT and AMS-02 presents anomalous behaviour as energy increase. In particular AMS-02 observations provide compelling evidence for a new source of positrons and electrons. Its origin is unknown, it can be non-exotic (e.g. pulsars), be dark matter or maybe a mixture. We prove the gravitino of R-parity violating supersymmetric models as this source. As the gravitino is a spin 3/2 particle, it offers particular decay channels. We compute the electron, positron and gamma-ray fluxes produced by each gravitino decay channel as it would be detected at the Earth's position. Combining the flux from the different decay modes we can fit AMS-02 measurements of the positron fraction, as well as the electron and positron fluxes, with a gravitino dark matter mass in the range $1-2$ TeV and lifetime of $\\sim 1.0-0.8\\times 10^{26}$ s. The high statistics measurement of electron and positron fluxes, and the flattering in the behaviour of the positron frac...

Carquin, Edson; Gomez-Vargas, German A; Panes, Boris; Viaux, Nicolas

2015-01-01

428

Constraining dark matter capture and annihilation cross sections by searching for neutrino signature from the Earth's core  

NASA Astrophysics Data System (ADS)

We study the sensitivity of IceCube/DeepCore detector to dark matter annihilations in the Earth's core. We focus on annihilation modes ?????¯, ?+?-, bb¯, and W+W-. Both track and cascade events are considered in our analysis. By fixing the dark matter annihilation cross section ???? at some nominal values, we study the sensitivity of the IceCube/DeepCore detector to dark matter spin-independent cross section ?pSI for m? ranging from few tens of GeV to 10 TeV. This sensitivity is compared with the existing IceCube 79-string constraint on the same cross section, which was obtained by searching for dark matter annihilations in the Sun. We compare this sensitivity to dark matter direct detection results as well, in particular the XENON100 (2012) limit and the parameter regions preferred by DAMA and CRESST-II experiments. We also present IceCube/DeepCore sensitivity to ???? as a function of m? by fixing ?pSI at XENON100 (2012) and XENON1T limits, respectively. This sensitivity is compared with the preferred dark matter parameter range derived from the combined fitting to PAMELA and AMS02 positron fraction data. We conclude that the search for dark matter annihilations in the Earth's core provides competitive constraints on ?pSI and ???? in the case of low-mass dark matter. Particularly, the expected constraint on ?pSI for 5 years of data taking in IceCube/DeepCore is more stringent than the current IceCube 79-string limit mentioned above.

Lee, Fei-Fan; Lin, Guey-Lin; Tsai, Yue-Lin Sming

2014-01-01

429

Vacancy-type defects in In{sub x}Ga{sub 1-x}N alloys probed using a monoenergetic positron beam  

SciTech Connect

Native defects in In{sub x}Ga{sub 1-x}N grown by plasma-assisted molecular beam epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured, and these were compared with results obtained using first-principles calculation. The defect concentration increased with increasing In composition x and reached the maximum at x = 0.44{approx}0.56. A clear correlation between the line-width of photoluminescence and the defect concentration was obtained. The major defect species detected by positron annihilation was identified as cation vacancies coupled with multiple nitrogen vacancies (V{sub N}s), and their introduction mechanism is discussed in terms of the strain energy due to bond-length/angle distortions and the suppression of the V{sub N} formation energy by neighboring In atoms.

Uedono, A.; Watanabe, T. [Division of Applied Physics, Faculty of Pure and Applied Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573 (Japan); Ishibashi, S. [Nanosystem Research Institute (NRI) 'RICS,' National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568 (Japan); Wang, X. Q.; Liu, S. T.; Chen, G.; Shen, B. [State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871 (China); Sang, L. W.; Sumiya, M. [Wide Bandgap Material Group, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044 (Japan)

2012-07-01

430

Lifetimes of lunar satellite orbits  

NASA Technical Reports Server (NTRS)

The Space Exploration Initiative has generated a renewed interest in lunar mission planning. The lunar missions currently under study, unlike the Apollo missions, involve long stay times. Several lunar gravity models have been formulated, but mission planners do not have enough confidence in the proposed models to conduct detailed studies of missions with long stay times. In this report, a particular lunar gravitational model, the Ferrari 5 x 5 model, was chosen to determine the lifetimes for 100-km and 300-km perilune altitude, near-circular parking orbits. The need to analyze orbital lifetimes for a large number of initial orbital parameters was the motivation for the formulation of a simplified gravitational model from the original model. Using this model, orbital lifetimes were found to be heavily dependent on the initial conditions of the nearly circular orbits, particularly the initial inclination and argument of perilune. This selected model yielded lifetime predictions of less than 40 days for some orbits, and other orbits had lifetimes exceeding a year. Although inconsistencies and limitations are inherent in all existing lunar gravity models, primarily because of a lack of information about the far side of the moon, the methods presented in this analysis are suitable for incorporating the moon's nonspherical gravitational effects on the preliminary design level for future lunar mission planning.

Meyer, Kurt W.; Buglia, James J.; Desai, Prasun N.

1994-01-01

431

Heat Deposition in Positron Sources for ILC  

SciTech Connect

In the International Linear Collider (ILC) positron source, multi-GeV electrons or multi-MeV photons impinge on a metal target to produce the needed positrons in the resulting electromagnetic showers. The incoming beam power is hundreds of kilowatts. Various computer programs -- such as FLUKA or MARS -- can calculate how the incoming beam showers in the target and can track the particle showers through the positron source system. Most of the incoming energy ends up as heat in the various positron source elements. This paper presents results from such calculations and their impact on the design of a positron source for the ILC.

Bharadwaj, V.; Pitthan, R.; Sheppard, J.; Vincke, H.; Wang, J.W.; /SLAC

2006-03-15

432

Experimental verification of proton beam monitoring in a human body by use of activity image of positron-emitting nuclei generated by nuclear fragmentation reaction  

Microsoft Academic Search

Proton therapy is a form of radiotherapy that enables concentration of dose on a tumor by use of a scanned or modulated Bragg\\u000a peak. Therefore, it is very important to evaluate the proton-irradiated volume accurately. The proton-irradiated volume can\\u000a be confirmed by detection of pair-annihilation gamma rays from positron-emitting nuclei generated by the nuclear fragmentation\\u000a reaction of the incident protons

Teiji Nishio; Aya Miyatake; Kazumasa Inoue; Tomoko Gomi-Miyagishi; Ryosuke Kohno; Satoru Kameoka; Keiichi Nakagawa; Takashi Ogino

2008-01-01

433

Positron microanalysis with high intensity beams  

SciTech Connect

One of the more common applications for a high intensity slow positron facility will be microanalysis of solid materials. In the first section of this paper some examples are given of procedures that can be developed. Since most of the attendees of this workshop are experts in positron spectroscopy, comprehensive descriptions will be omitted. With the exception of positron emission microscopy, most of the procedures will be based on those already in common use with broad beams. The utility of the methods have all been demonstrated, but material scientists use very few of them because positron microbeams are not generally available. A high intensity positron facility will make microbeams easier to obtain and partially alleviate this situation. All microanalysis techniques listed below will have a common requirement, which is the ability to locate the microscopic detail or area of interest and to focus the positron beam exclusively on it. The last section of this paper is a suggestion of how a high intensity positron facility might be designed so as to have this capability built in. The method will involve locating the specimen by scanning it with the microbeam of positrons and inducing a secondary electron image that will immediately reveal whether or not the positron beam is striking the proper portion of the specimen. This scanning positron microscope' will be a somewhat prosaic analog of the conventional SEM. It will, however, be an indispensable utility that will enhance the practicality of positron microanalysis techniques. 6 refs., 1 fig.

Hulett, L.D. Jr.; Donohue, D.L.

1990-01-01

434

High-resolution positron Q-value measurements and nuclear-structure studies far from the stability line. Progress report  

SciTech Connect

Extensive data analysis and theoretical analysis has been done to complete the extensive decay scheme investigation of /sup 206/ /sup 208/Fr and the level structures of /sup 206/ /sup 208/Rn. A final version of a journal article is presented in preprint form. Extensive Monte Carlo calculations have been made to correct the end point energies of