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1

Tomographic Positron Annihilation Lifetime Spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy serves as a perfect tool for studies of open-volume defects in solid materials such as vacancies, vacancy agglomerates, and dislocations. Moreover, structures in porous media can be investigated ranging from 0.3 nm to 30 nm employing the variation of the Positronium lifetime with the pore size. While lifetime measurements close to the material's surface can be performed at positron-beam installations bulk materials, fluids, bio-materials or composite structures cannot or only destructively accessed by positron beams. Targeting those problems, a new method of non-destructive positron annihilation lifetime spectroscopy has been developed which features even a 3-dimensional tomographic reconstruction of the spatial lifetime distribution. A beam of intense bremsstrahlung is provided by the superconducting electron linear accelerator ELBE (Electron Linear Accelerator with high Brilliance and low Emittance) at Helmholtz-Zentrum Dresden-Rossendorf. Since the generation of bremsstrahlung and the transport to the sample preserves the sharp timing of the electron beam, positrons generated inside the entire sample volume by pair production feature a sharp start time stamp for lifetime studies. In addition to the existing technique of in-situ production of positrons inside large (cm3) bulk samples using high-energy photons up to 16 MeV from bremsstrahlung production, granular position-sensitive photon detectors have been employed. The detector system will be described and results for experiments using samples with increasing complexity will be presented. The Lu2SiO5:Ce scintillation crystals allow resolving the total energy to 5.1 % (root-mean-square, RMS) and the annihilation lifetime to 225 ps (RMS). 3-dimensional annihilation lifetime maps have been created in an offline-analysis employing well-known techniques from PET.

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

2014-04-01

2

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

3

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

E-print Network

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

Mills, Allen P.

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

NEUTRON DAMAGE IN REACTOR PRESSURE-VESSEL STEEL EXAMINED WITH POSITRON ANNIHILATION LIFETIME SPECTROSCOPY  

E-print Network

NEUTRON DAMAGE IN REACTOR PRESSURE-VESSEL STEEL EXAMINED WITH POSITRON ANNIHILATION LIFETIME-vessel steels. We irradiated samples ofASTM A508 nuclear reactor pressure-vessel steel to fast neutron 17 2 (PALS) to study the effects of neutron damage in the steels on positron lifetimes. Non

Motta, Arthur T.

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

Study of PRIMAVERA steel samples by positron annihilation spectroscopy technique II - Lifetime measurements  

NASA Astrophysics Data System (ADS)

In the present article, a positron annihilation lifetime technique was used for the study of VVER-440/230 weld materials, manufactured in the frame of the international PRIMAVERA project on microstructural investigation of the irradiated WWER-440 reactor pressure vessel steel. The present results complement our previous report of positron angular correlation experiments and provide in-depth characterization of vacancy type defects behavior under irradiation and thermal treatment. The results give new insight into the previously published atom probe tomography and angular correlation of annihilation radiation studies. The measurements do not show any association of phosphorus or its segregation to the open volume defects investigated by positron annihilation spectroscopy. The embrittlement effects related to the phosphorus seem to be effectively annealed-out during 475 °C thermal treatment and the post annealing microstructure and mechanical properties of the material are consequently affected mostly by agglomerations of vacancy clusters coarsened during thermal treatment.

Krsjak, V.; Grafutin, V.; Ilyukhina, O.; Burcl, R.; Ballesteros, A.; Hähner, P.

2012-02-01

10

Void structure in silica glass with different fictive temperatures observed with positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

We examine voids in silica glasses with different fictive temperatures using positron annihilation lifetime spectroscopy. The pick-off annihilation lifetime of ortho-positronium increases with the fictive temperature, Tf, indicating that the void size increases. High Tf leads to high density and low degree of network polymerization so that increasing void size means that the density fluctuation of the silica glass increases with high Tf. Assuming that such density fluctuation causes light scattering, the previously reported Tf dependence of the Rayleigh scattering coefficient can be well explained by the change in void size.

Ono, Madoka; Hara, Kenta; Fujinami, Masanori; Ito, Setsuro

2012-10-01

11

Information on individual interfaces in ternary polymer blends from positron annihilation lifetime studies  

NASA Astrophysics Data System (ADS)

Positron Annihilation Lifetime Spectroscopy has been used to determine the free volume content in the ternary blends of SAN/EVA/PVC. The method of deriving hydrodynamic interaction parameter (?) in binary polymer blends was modified for ternary polymer blend system characterized by three distinct interfaces. Each interface characterized, is associated with an ? and its assertion for the ternary blend are compared with available literature data.

Meghala, D.; Ramya, P.; Pasang, T.; Ravikumar, H. B.; Ranganathaiah, C.

2012-06-01

12

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

13

Free volumes studies in Thymoquinone and Carvone ?-cyclodextrin nanoparticles by positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy is used to study free volume in ?-cyclodextrin with the encapsulation of thymoquinone and S-carvone, in samples covering the guest to host fraction range from 1:0.1 to 1:1. The results clearly indicate the presence of long lifetime components related to Ps-formation. Although the behavior of the two guests is different, in both cases the results indicate the formation of 1:1 cyclodextrin inclusion compounds. Data show that the addition of carvone to ?-cyclodextrin results in a decrease of the o-Ps lifetime corresponding to a reduction of the average radius of cavities from 2.41 Å to 2.29 Å, whereas the addition of thymoquine decreases the radius from 2.57 Å to 2.35 Å. In turn, the intensity varied from 20.55 to 19.20% and from 20.83 to 0.41%, respectively.

Ferreira Marques, M. F.; Gordo, P. M.; Moreira da Silva, A.

2013-06-01

14

Effect of rare earth substitutions on some physical properties of Mn–Zn ferrite studied by positron annihilation lifetime spectroscopy  

Microsoft Academic Search

Mn–Zn ferrite substituted with rare earth ions have been investigated by means of positron annihilation lifetime spectroscopy (PALS). The variations of lifetime parameters ?av, I2, and ? with ionic radius of rare earth ions, grain size and electrical resistivity for all samples have been studied. The intergranular pores increase with increasing the ionic radius of rare earth ions and grain

A. M. Samy; N. Mostafa; E. Gomaa

2006-01-01

15

Positron annihilation lifetime changes across the structural phase transition in nanocrystalline Fe{sub 2}O{sub 3}  

SciTech Connect

Nanocrystalline samples of {gamma}-Fe{sub 2}O{sub 3} were prepared through chemical routes and the particles were allowed to grow by thermal treatment at different temperatures. {gamma}-Fe{sub 2}O{sub 3} transformed to the {alpha} phase at a temperature relatively lower than that reported for coarse-grained samples. Positron lifetimes were measured in both the phases of the samples and a significant reduction was observed across the transformation. This is attributed to the expected reduction in the unit cell volume and the nature of orientation of the nearest-neighboring atoms around the positron trapping sites. Positrons are basically annihilating from the octahedral vacancy sites in {gamma}-Fe{sub 2}O{sub 3} and the lifetime decreases as the nearest-neighboring atoms increasingly influence the annihilation probability of the positrons when the sample changes to the {alpha} phase. The reduction in the positron lifetime has helped to envisage the onset of the transformation earlier than that indicated by x-ray diffraction. The positron lifetimes in the intergranular region increased due to the increase in free volume associated with the atoms on the grain surfaces when the grain size reduced. Doppler broadening measurements of the annihilation {gamma}-ray spectral line shapes were also carried out and the results supported these findings.

Chakrabarti, S.; Chaudhuri, S.; Nambissan, P.M.G. [Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032 (India); Nuclear and Atomic Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064 (India)

2005-02-01

16

Effect of OPS Dispersion Method on the Free Volume of Polyurethane by Positron Annihilation Lifetime Spectroscopy (PALS)  

Microsoft Academic Search

The free volume of octaphenyl polyhedral oligomeric silsesquioxane (OPS) modified polyurethane prepared from different dispersion method was investigated using Positron annihilation lifetime spectroscopy (PALS). The relationships of free volume and mechanical property were analyzed. Our results showed that OPS decreased the original free volume hole size and fraction, but varied with the dispersion method. For the sonication dispersion samples, the

Ke-Mei Cheng; Chunrong Tian; Yu Du; Fang He; Jian-Hua Wang; Jie Sun

2012-01-01

17

Positron annihilation lifetime spectroscopy of mechanically milled protein fibre powders and their free volume aspects  

NASA Astrophysics Data System (ADS)

The present study reports the fabrication of ultra-fine powders from animal protein fibres such as cashmere guard hair, merino wool and eri silk along with their free volume aspects. The respectively mechanically cleaned, scoured and degummed cashmere guard hair, wool and silk fibres were converted into dry powders by a process sequence: Chopping, Attritor Milling, and Spray Drying. The fabricated protein fibre powders were characterised by scanning electron microscope, particle size distribution and positron annihilation lifetime spectroscopy (PALS). The PALS results indicated that the average free volume size in protein fibres increased on their wet mechanical milling with a decrease in the corresponding intensities leading to a resultant decrease in their fractional free volumes.

Patil, K.; Sellaiyan, S.; Rajkhowa, R.; Tsuzuki, T.; Lin, T.; Smith, S. V.; Wang, X.; Uedono, A.

2013-06-01

18

Nano-free volume characterization by positron annihilation lifetime technique in flame-retardant poly (vinyl chloride) after thermal treatment  

NASA Astrophysics Data System (ADS)

The flammability tests are performed on flame-retardant poly (vinyl chloride) (FRPVC) material that has been used in cable insulation and jacketing construction for multi-purpose reactor (MPR) at Atomic Energy Authority of Egypt, as well as carbon-black FRPVC (CB-FRPVC) material produced by Egyptian Electrical Cable Company (EECC). The temperature variation of thermal conductivity, thermal expansion coefficients, and nano-size free volumes by means of positron annihilation lifetime (PAL) technique are determined. Correlation of positron annihilation and thermal conductivity has been discussed in terms of phonons as the main heat carriers.

Mohsen, M.; Mostafa, N.; Rashad, S. M.; Ayoub, A.; Salem, E. F.

2007-02-01

19

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

SciTech Connect

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

Taira, Y.; Toyokawa, H.; Kuroda, R. [Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568 (Japan); Yamamoto, N. [Nagoya University Synchrotron Radiation Research Center, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603 (Japan); Adachi, M.; Tanaka, S.; Katoh, M. [UVSOR, Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji-cho, Okazaki, Aichi 444-8585 (Japan)

2013-05-15

20

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

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

21

Clay particles - potential of positron annihilation lifetime spectroscopy (PALS) for studying interlayer spacing  

NASA Astrophysics Data System (ADS)

Characterisation of clays is generally achieved by traditional methods, such as X-ray diffraction (XRD) and transmission electron microscopy (TEM). However, clays are often difficult to characterise due to lack of long-range order, thus these tools are not always reliable. Because interlayer spacing in clays can be adjusted to house molecules, there is growing interest to use these materials for drug delivery. Positron annihilation lifetime spectroscopy (PALS) was examined as an alternative tool to characterise a series of well-known clays. XRD of two layered double hydroxides; MgAl-LDH and MgGd-LDH, natural hectorite, fluoromica and laponite, and their PALS spectra were compared. XRD data was used to calculate the interlayer d- spacing in these materials and results show a decrease in interlayer spacing as the heavy metal ions are substituted for those of large ionic radii. Similar results were obtained for PALS data. This preliminary study suggests PALS has potential as a routine tool for characterising clay particles. Further work will examine the sensitivity and reliability of PALS to percent of metal doping and hydration in clay microstructure.

Fong, N.; Guagliardo, P.; Williams, J.; Musumeci, A.; Martin, D.; Smith, S. V.

2011-01-01

22

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

SciTech Connect

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

MacQueen, R.C.

1992-01-01

23

Positron annihilation in polypropylene studied by lifetime and coincidence Doppler-broadening spectroscopy  

NASA Astrophysics Data System (ADS)

The momentum density distributions (MDDs) of electrons taking part in the annihilation processes in polypropylene (PP) have been measured by coincidence Doppler-broadening spectroscopy. MDDs at the beginning of measurements to those at the saturation level of Ps formation have been compared in order to follow the possible changes in concentration of carbonyl groups (CG). A high initial CG concentration in PP has been observed, while for antioxidant-containing PP no significant presence of CG has been detected, and no changes have been observed during positron irradiation.

Djourelov, N.; He, C.; Suzuki, T.; Shantarovich, V. P.; Ito, Y.; Kondo, K.; Ito, Y.

2003-12-01

24

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

25

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

26

Phase dependence of positron annihilation in tristearin.  

NASA Technical Reports Server (NTRS)

Positron lifetime measurements have been used to investigate the polymorphism of tristearin. The results show that both positronium formation and annihilation are sensitive to the phase changes in tristearin.

Walker, W. W.; Merritt, W. G.; Cole, G. D.

1972-01-01

27

Room-temperature microstructural evolution of electroplated Cu studied by focused ion beam and positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

Focused ion beam (FIB) microscopy was used to obtain the time dependent transformation fraction and positron annihilation lifetime spectroscopy (PALS) was employed to analyze the vacancy-type defects in electroplated copper (Cu) during room-temperature microstructrual evolution. It was found that PALS is more sensitive than FIB to show the room-temperature microstructual evolution of electroplated Cu at the first stage of self-annealing. The majority of defects in electroplated Cu are dislocation-bound vacancies and vacancy clusters. Both the size and the concentration of the defects are similar for the two samples at the completion of electroplating. During the incubation time, the mean size of vacancy-type defects increases. After the onset of visible grain growth, the size of vacancy-type defects decreases. The detail evolutions of defects differ with the two samples. The role of the evolution of codeposited species was also discussed.

Yin, K. B.; Xia, Y. D.; Zhang, W. Q.; Wang, Q. J.; Zhao, X. N.; Li, A. D.; Liu, Z. G.; Hao, X. P.; Wei, L.; Chan, C. Y.; Cheung, K. L.; Bayes, M. W.; Yee, K. W.

2008-03-01

28

Positron annihilation processes update  

NASA Technical Reports Server (NTRS)

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

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

1997-01-01

29

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

30

Tracking of the micro-structural changes of levonorgestrel-releasing intrauterine system by positron annihilation lifetime spectroscopy.  

PubMed

The morphology and the micro-structural changes of levonorgestrel-releasing intrauterine systems (IUSs) were studied in relation to the duration of their application. The morphology of the removed IUSs was examined without pre-treatment by scanning electron microscopy. The micro-structural changes of the different layers of IUSs were tracked by positron annihilation lifetime spectroscopy. Besides the previously found incrustation formation, the free volume of the hormone containing reservoir was remarkably increased after 3 years of application, thus increasing the real volume of the core of the systems. Although the free volume of the membrane encasing the core was not significantly changed in the course of the application, as a result of the core expansion, microcracks could be formed on the membrane surface. Along these cracks, deposits of different compositions can be formed, causing inflammatory complications and influencing the drug release of IUSs. Stability tests in combination with micro-structural screening of such IUSs could be required during their development phase to avoid the undesired side effects. PMID:20638215

Patai, Kálmán; Szente, Virág; Süvegh, Károly; Zelkó, Romána

2010-12-01

31

Detection of Atomic Scale Changes in the Free Volume Void Size of Three-Dimensional Colorectal Cancer Cell Culture Using Positron Annihilation Lifetime Spectroscopy  

PubMed Central

Positron annihilation lifetime spectroscopy (PALS) provides a direct measurement of the free volume void sizes in polymers and biological systems. This free volume is critical in explaining and understanding physical and mechanical properties of polymers. Moreover, PALS has been recently proposed as a potential tool in detecting cancer at early stages, probing the differences in the subnanometer scale free volume voids between cancerous/healthy skin samples of the same patient. Despite several investigations on free volume in complex cancerous tissues, no positron annihilation studies of living cancer cell cultures have been reported. We demonstrate that PALS can be applied to the study in human living 3D cell cultures. The technique is also capable to detect atomic scale changes in the size of the free volume voids due to the biological responses to TGF-?. PALS may be developed to characterize the effect of different culture conditions in the free volume voids of cells grown in vitro. PMID:24392097

Castellanos-Rubio, Ainara; Merida, David; Garcia, Jose Angel; Plaza-Izurieta, Leticia; Fernandez-Jimenez, Nora; Plazaola, Fernando; Bilbao, Jose Ramon

2014-01-01

32

Microstructure of Celluloses Studied by Positron Annihilation  

NASA Astrophysics Data System (ADS)

Positron Annihilation Lifetime (PAL) measurements have been performed on Avicel and Whatman celluloses and their ball-milled derivatives as well as cellulose acetates. Correlations between crystallinity and fractional microsctructures found. Combined studies using PAL and FTIR on deuterated celluloses and moisture sorptions will be presented.

Cao, Huimin; Lou, Yongming; Jean, Renwu; Pekarovicova, Y. C.; Venditti, Alexandra

1997-03-01

33

Positron annihilation spectroscopy to study nanoprecipitations in aluminum alloys  

NASA Astrophysics Data System (ADS)

Positron Annihilation Spectroscopy has been used to study the microstructure in aluminum alloy AA7075-T6 after RRA (retrogression and re-aging) heat treatment. Nano-precipitates act as traps for thermalised positrons. The measured positron lifetime is sensitive to the local electron density at the annihilation site while the Doppler broadening of the annihilation line is sensitive to the chemical environment of the annihilation site. The combination of both methods was used to study changes of nano-precipitates, during the Retrogression and Re-aging (RRA) process. Results are compared to previous published isochronal annealing experiments of an Al Cu model alloy.

Meyendorf, Norbert; Dlubek, Gunter; Surkov, Alexander

2004-07-01

34

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

35

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

NASA Astrophysics Data System (ADS)

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

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

2012-10-01

36

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) > Q(2?Pn3m) (?4? 2.2 ns) are observed for the phytantriol-water system, with the addition of VitEA yielding a gradual increase in ? from ?? 2.2 ns for cubosomes to ?? 3.5 ns for hexosomes. The dynamic chain packing at higher temperatures and in the L2 and H2 phases is qualitatively less "viscous", consistent with rheological measurements. This information offers increased understanding of the relationship between internal nanostructure and species permeability. PMID:25459998

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

2015-01-21

37

Applications of positron annihilation spectroscopy in materials research  

NASA Technical Reports Server (NTRS)

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

Singh, Jag J.

1988-01-01

38

THEORY OF POSITRON ANNIHILATION ON MOLECULES  

E-print Network

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

Gribakin, Gleb

39

Positron annihilation spectroscopy with magnetically analyzed beams  

NASA Technical Reports Server (NTRS)

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

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

1982-01-01

40

Positron annihilation in the interstellar medium  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

41

Positron Annihilation Measurements of High Temperature Superconductors  

NASA Astrophysics Data System (ADS)

The temperature dependence of positron annihilation parameters has been measured for basic YBCO, Dy-doped, and Pr-doped superconducting compounds. The physical properties, such as crystal structure, electrical resistance, and critical temperature, have been studied for all samples. In the basic YBCO and Dy-doped samples, the defect -related lifetime component tau_{2 } was approximately constant from room temperature to above the critical temperature and then showed a step -like decrease in the temperature range 90K { ~} 40K. No significant temperature dependence was found in the short- and long-lifetime components, tau_{1} and tau_{3}. The x-ray diffraction data showed that the crystal structure of these two samples was almost the same. These results indicated that the electronic structure changed below the critical temperature. No transition was observed in the Pr-doped YBCO sample. The advanced computer program "PFPOSFIT" for positron lifetime analysis was modified to run on the UNIX system of the University of Utah. The destruction of superconductivity with Pr doping may be due to mechanisms such as hole filling or hole localization of the charge carriers and may be related to the valence state of the Pr ion. One-parameter analyses like the positron mean lifetime parameter and the Doppler line shape parameter S also have been studied. It was found that a transition in Doppler line shape parameter S was associated with the superconducting transition temperature in basic YBCO, Dy -doped, and 0.5 Pr-doped samples, whereas no transition was observed in the nonsuperconducting Pr-doped sample. The Doppler results indicate that the average electron momentum at the annihilation sites increases as temperature is lowered across the superconducting transition range and that electronic structure change plays an important role in high temperature superconductivity.

Jung, Kang

1995-01-01

42

Location and size of nanoscale free-volume holes in crosslinked- polytetrafluoroethylene-based graft-type polymer electrolyte membranes determined by positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

The location and size of nanoscale free-volume holes (nanoholes) in graft-type polymer electrolyte membranes (PEMs), which were prepared by radiation-induced graft polymerization (grafting) of styrene into crosslinked-polytetrafluoroethylene (cPTFE) films and subsequent sulfonation, were investigated using positron annihilation lifetime (PAL) spectroscopy. The PAL spectra of the PEMs indicated the existence of two types of ortho-positronium (o-Ps) species, corresponding to nanoholes with volumes of 0.11 and 0.38 nm3. A comparison of the PAL data of the PEMs with that of the precursor original cPTFE and polystyrene-grafted films demonstrated the probability that the smaller holes were located in both the PTFE crystalline phases and the poly(styrene sulfonic acid) graft regions, whereas the larger holes are potentially localized in the PTFE amorphous phases. Taking into account both the size and the location of the nanoholes, it was concluded that gas transport through the larger holes in the amorphous PTFE phases was dominant over permeation through the smaller holes in the PTFE crystals and grafted regions.

Sawada, Shin-ichi; Yabuuchi, Atsushi; Maekawa, Masaki; Kawasuso, Atsuo; Maekawa, Yasunari

2013-06-01

43

Positron annihilation studies of thermoplastic LCP composites  

SciTech Connect

The time-dependent decrease in ortho-positronium (o-Ps) intensity detected in polymers during long-term positron annihilation lifetime (PALS) experiments has been extensively studied and modeled. This decrease is temperature dependent, polymer specific, source strength dependent and is noted to saturate as a function of source exposure time. Larger decreases and smaller saturation values are exhibited at lower temperatures. Additionally, at sufficiently high temperatures, a negligible decrease is detected. The time dependent decrease is recoverable at elevated temperatures. In contrast, the o-Ps lifetime component remains constant and independent of exposure time. Increasing the exposed sample volume by continuously varying the source-sample orientation dramatically decreases the reduction of o-Ps intensity.

Naslund, R.A.; Jones, P.L.

1995-08-03

44

Distribution of Positron Annihilation Radiation  

E-print Network

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

Peter A. Milne

2006-02-28

45

Method for photon activation positron annihilation analysis  

DOEpatents

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

Akers, Douglas W.

2006-06-06

46

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

47

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

48

Positron annihilation on large molecules Koji Iwata,1,  

E-print Network

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

Gribakin, Gleb

49

Positron annihilation Investigations of defects in copper alloys selected for nuclear fusion technology  

Microsoft Academic Search

A positron annihilation spectroscopy (PAS) based on positron lifetime measurements, using the pulsed low energy positron system (PLEPS), is used to investigate defects create by hydrogen implantation and thermal treat of copper alloys. These alloys are designated for the use in the international thermonuclear experimental reactor (ITER). The results show that the changes in the microstructure of selected copper alloys

V Slugen; J Kuriplach; P Ballo; P Domonkos; G Kögel; P Sperr; W Egger; W Triftshäuser; V. M Domankova; P Kovac; I Vavra; S Stancek; M Petriska; A Zeman

2004-01-01

50

Galactic annihilation emission from nucleosynthesis positrons  

NASA Astrophysics Data System (ADS)

Context. The Galaxy hosts a widespread population of low-energy positrons revealed by successive generations of gamma-ray telescopes through a bright annihilation emission from the bulge region, with a fainter contribution from the inner disk. The exact origin of these particles remains currently unknown. Aims: We estimate the contribution to the annihilation signal of positrons generated in the decay of radioactive 26Al, 56Ni and 44Ti. Methods: We adapted the GALPROP propagation code to simulate the transport and annihilation of radioactivity positrons in a model of our Galaxy. Using plausible source spatial distributions, we explored several possible propagation scenarios to account for the large uncertainties on the transport of ~MeV positrons in the interstellar medium. We then compared the predicted intensity distributions to the INTEGRAL/SPI observations. Results: We obtain similar intensity distributions with small bulge-to-disk ratios, even for extreme large-scale transport prescriptions. At least half of the positrons annihilate close to their sources, even when they are allowed to travel far away. In the high-diffusion, ballistic case, up to 40% of them escape the Galaxy. In proportion, this affects bulge positrons more than disk positrons because they are injected further off the plane in a tenuous medium, while disk positrons are mostly injected in the dense molecular ring. The predicted intensity distributions are fully consistent with the observed longitudinally-extended disk-like emission, but the transport scenario cannot be strongly constrained by the current data. Conclusions: Nucleosynthesis positrons alone cannot account for the observed annihilation emission in the frame of our model. An additional component is needed to explain the strong bulge contribution, and the latter is very likely concentrated in the central regions if positrons have initial energies in the 100 keV - 1 MeV range.

Martin, P.; Strong, A. W.; Jean, P.; Alexis, A.; Diehl, R.

2012-07-01

51

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

52

Positron annihilation in gamma-ray bursts  

NASA Astrophysics Data System (ADS)

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. 1012 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

53

On the method of positron lifetime measurement  

NASA Technical Reports Server (NTRS)

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

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

1983-01-01

54

High sensitivity of positron annihilation to thermal oxidation of polyethylene  

NASA Astrophysics Data System (ADS)

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

Ito, Kenji; Kobayashi, Yoshinori; Nanasawa, Atsushi

2003-01-01

55

Positron annihilation with core and valence electrons  

E-print Network

$\\gamma$-ray spectra for positron annihilation with the core and valence electrons of the noble gas atoms Ar, Kr and Xe is calculated within the framework of diagrammatic many-body theory. The effect of positron-atom and short-range positron-electron correlations on the annihilation process is examined in detail. Short-range correlations, which are described through non-local corrections to the vertex of the annihilation amplitude, are found to significantly enhance the spectra for annihilation on the core orbitals. For Ar, Kr and Xe, the core contributions to the annihilation rate are found to be 0.55\\%, 1.5\\% and 2.2\\% respectively, their small values reflecting the difficulty for the positron to probe distances close to the nucleus. Importantly however, the core subshells have a broad momentum distribution and markedly contribute to the annihilation spectra at Doppler energy shifts $\\gtrsim3$\\,keV, and even dominate the spectra of Kr and Xe at shifts $\\gtrsim5$\\,keV. Their inclusion brings the theoretical ...

Green, D G

2015-01-01

56

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

57

Positron annihilation at the Galactic center  

NASA Technical Reports Server (NTRS)

The efficiency of production of positrons by gamma-gamma interaction under conditions which may occur in the Galactic center. Their annihilation line and three-photon annihilation continuum contribute to the emitted radiation. If a geometrical model is assumed, it will be possible to determine the parameters of the gamma-ray source region. By comparing the computed spectra to observations, a rough estimate is made using extant data.

Carrigan, B. J.; Katz, J. I.

1987-01-01

58

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

E-print Network

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

Gribakin, Gleb

59

Resonant positron annihilation in ammonia G. F. Gribakin  

E-print Network

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

Gribakin, Gleb

60

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

E-print Network

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

Gribakin, Gleb

61

Slow positron beam generator for lifetime studies  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

62

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

63

Early Stages of Precipitation Process in Al-(Mn-)Sc-Zr Alloy Characterized by Positron Annihilation  

NASA Astrophysics Data System (ADS)

Thermal effects on the precipitation stages in as-cast Al-0.70 at. pct Mn-0.15 at. pct Sc-0.05 at. pct Zr alloy were studied. The role of lattice defects was elucidated by positron annihilation spectroscopy (lifetime and coincidence Doppler broadening) enabling investigation of solutes clustering at the atomic scale. This technique has never been used in the Al-Sc- and/or Al-Zr-based alloys so far. Studies by positron annihilation were combined with resistometry, hardness measurements, and microstructure observations. Positrons trapped at defects are preferentially annihilated by Sc electrons. Lifetime of trapped positrons indicates that Sc atoms segregate at dislocations. Maximum fraction of positrons annihilated by Sc electrons occurring at 453 K (180 °C) suggests that clustering of Sc bound with vacancies takes place. It is followed by peak of this fraction at 573 K (300 °C). A rise of the contribution of trapped positrons annihilated by Zr electrons starting at 513 K (240 °C) and attaining maximum also at 573 K (300 °C) confirms that Zr participates in precipitation of the Al3Sc particles already at these temperatures. The pronounced hardening at 573 K (300 °C) has its nature in the precipitation of the Al3Sc particles with a Zr-rich shell. The contribution of trapped positrons annihilated by Mn electrons was found to be negligible.

Vlach, Martin; Cizek, Jakub; Melikhova, Oksana; Stulikova, Ivana; Smola, Bohumil; Kekule, Tomas; Kudrnova, Hana; Gemma, Ryota; Neubert, Volkmar

2015-04-01

64

Application of positron annihilation in materials science  

SciTech Connect

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

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

1984-05-01

65

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

66

Positron deep level transient spectroscopy — a new application of positron annihilation to semiconductor physics  

NASA Astrophysics Data System (ADS)

Recent positron mobility and lifetime measurements made on ac-biased metal on semi-insulating GaAs junctions, which have identified the native EL2 defect through a determination of the characteristic ionization energy of the donor level, are reviewed. It is shown that these measurements point towards a new spectroscopy, tentatively named positron-DLTS (deep level transient spectroscopy), that is the direct complement to conventional DLTS in that it monitors transients in the electric field of the depletion region rather than the inversely related depletion width, as deep levels undergo ionization. In this new spectroscopy, which may be applied to doped material by use of a suitable positron beam, electric field transients are monitored through the Doppler shift of the annihilation radiation resulting from the drift velocity of the positron in the depletion region. Two useful extensions of the new spectroscopy beyond conventional capacitance-DLTS are suggested. The first is that in some instances information on the microstructure of the defect causing the deep level may be inferred from the sensitivity of the positron to vacancy defects of negative and neutral charge states. The second is that the positron annihilation technique is intrinsically much faster than conventional DLTS with the capability of observing transients some 10 6 times faster, thus allowing deep levels (and even shallow levels) to be investigated without problems associated with carrier freeze-out.

Beling, C. D.; Fung, S.; Au, H. L.; Ling, C. C.; Reddy, C. V.; Deng, A. H.; Panda, B. K.

1997-05-01

67

The role of positronium decoherence in positron annihilation in matter  

NASA Astrophysics Data System (ADS)

A small difference between the energies of the para-positronium (p-Ps) and ortho-positronium (o-Ps) states suggests the possibility of the superposition of p-Ps and o-Ps during the formation of positronium (Ps) from pre-Ps, terminating its migration in the matter in a void. It is shown that such a superposition decoheres in the basis of p-Ps and o-Ps. The decoherence time scale estimated here motivates a correction in the precise analysis of the positron annihilation lifetime spectra. More generally, the superposited Ps state should contribute to the theory of the evolution of positronium in matter.

Pietrow, M.; S?omski, P.

2011-10-01

68

Positron annihilation radiation from the Galactic center region  

NASA Technical Reports Server (NTRS)

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

Ramaty, R.; Lingenfelter, R. E.

1991-01-01

69

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

E-print Network

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

Alejandro Ibarra; Anna S. Lamperstorfer; Joseph Silk

2014-03-27

70

Positron annihilation in the MuPs system  

E-print Network

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

Alexei M. Frolov

2012-07-25

71

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

SciTech Connect

Positron annihilation Lifetime and Doppler broadening measurements are reported for six polymer films as a function of carbon dioxide (CO{sub 2}) absolute pressure ranging from 0 to 45 psi. Since the polymer films were thin and did not absorb all positrons, corrections were made in the lifetime analysis for the absorption of positrons in the positron source and sample holder using the Monte Carlo transport code MCNP. The studied polymers are found to behave differently from each other. Some polymers form positronium and others, such as the polyimide structures, do not. For those polymers that form positronium an interpretation in terms of free volume is possible; for those that don't form positronium, further work is needed to determine how best to describe the behavior in terms of the bulk positron annihilation parameters. A few of the studied polymers exhibit changes in positron lifetime and intensity under CO{sub 2} pressure which may be described by the Henry or Langmuir sorption models, while the positron response of other polymers is rather insensitive to the CO{sub 2} pressure. The results demonstrate the usefulness of positron annihilation spectroscopy in investigating the sorption of CO{sub 2} into various polymers at pressures up to about 3 atm (45psi).

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

2011-06-01

72

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

NASA Astrophysics Data System (ADS)

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

Lee, C. Y.

2014-12-01

73

Positron lifetime spectrometer using a DC positron beam  

DOEpatents

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

Xu, Jun; Moxom, Jeremy

2003-10-21

74

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

75

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

76

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

77

Three-photon annihilation of the electron-positron pairs  

E-print Network

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

A. M. Frolov

2009-05-08

78

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

E-print Network

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

Paris-Sud XI, Université de

79

Positron annihilation characterization of Fe-Y2O3 composite powder after mechanical alloying and heat treatment.  

PubMed

Fe-1 wt% Y2O3 composite powders were mechanically alloyed for 12 hr and then heat-treated at 1050 degrees C for 1 hr. Positron annihilation lifetime and coincidence Doppler broadening measurements are in qualitative agreement with X-ray diffraction studies, indicating that in the as-mixed Fe-1Y2O3 composite, up to approximately equal to 70% of the annihilations occur at vacancy clusters; a small fraction annihilates in its matrix. In the case of mechanically alloyed composite, up to approximately equal to 60% of the positrons annihilate at vacancy clusters. Some annihilations also occur in dislocations. In the heat-treated Fe-1Y2O3, positrons primarily annihilate at yttria precipitates, while a small fraction annihilates in the matrix. PMID:22630026

Lee, Jae Hoon

2012-02-01

80

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

81

Direct identification of As vacancies in GaAs using positron annihilation calibrated by scanning tunneling microscopy  

NASA Astrophysics Data System (ADS)

We report on the identification of native vacancies in GaAs by positron annihilation with a special emphasis on As vacancy-related defects. In annealed highly Si-doped GaAs, we observe a neutral vacancy defect with a positron lifetime ? of 280-285 ps and a high intensity of the core annihilation, in contrast to Ga vacancies which exhibit a lifetime of ~260 ps and a lower intensity of the core annihilation. This defect is identified by scanning tunneling microscopy measurements to be an As vacancy SiGa donor complex. We find that the same defect is also present in low n-doped GaAs, where it was earlier assigned to a neutral As vacancy. The high positron lifetime is explained by a large outward lattice relaxation. Theoretical calculations of the momentum distribution employing free atomic wave functions are in good agreement with the experimental results, provided only relative changes are considered and an outward lattice relaxation is included which yields the experimental positron lifetime. These calculations also yield annihilation parameters for Ga vacancies, in good agreement with the experiment. Our results demonstrate that vacancies in both sublattices of GaAs can directly and unambiguously be identified by positron annihilation once the annihilation characteristics are calibrated by a complementary method. On the basis of this identification, the abundance of As vacancies in GaAs is discussed in terms of stoichiometry and formation energies.

Gebauer, J.; Krause-Rehberg, R.; Domke, C.; Ebert, Ph.; Urban, K.; Staab, T. E.

2001-01-01

82

Defect structure and dielectric properties of Bi-based pyrochlores probed by positron annihilation  

Microsoft Academic Search

Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been performed to identify structural defects of the bismuth based pyrochlore systems with generic formula (Bi1.5Zn0.5)(Zn0.5?x\\/3TixNb1.5?2x\\/3)O7 (x=0, 0.25, 0.5,1.0, 1.5). We found that all studied compounds contain substantial amount of the lattice vacancy defects, the variation of the annihilation lifetime suggests that the defects structure undergoes significant changes. The complex

Huiling Du; Xi Yao; Xianfeng Zhang; Huimin Weng

2006-01-01

83

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

84

M Ris-R-433 A Positron Lifetime Study of  

E-print Network

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

85

A study of defect states in neutron-irradiated zirconium using positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation spectroscopy has been used to examine specimens of iodide-purity zirconium, irradiated with neutrons in the temperature range 355 K to 725 K. The mean positron lifetimes and Doppler broadening lineshape parameters from the irradiated specimens showed significant increases over values for annealed material. A two-component analysis of the lifetime spectra indicated saturation trapping at vacancies and dislocations for the lower irradiation temperatures. No evidence was found for the presence of trapped components with long lifetimes (? 300 ps), indicative of three-dimensional vacancy clusters. Recovery of the 355 K irradiation damage during annealing was interpreted in terms of the annihilation of vacancies at sinks (dislocations and grain boundaries) and recovery of the irradiation-induced dislocation structure.

Carpenter, G. J. C.; Easterday, H. T.; McKee, B. T. A.

1983-06-01

86

Direct positron annihilation and positronium formation in thermal plasmas  

NASA Technical Reports Server (NTRS)

In the present evaluation of the rate of direct positron annihilation with electrons in the nonrelativistic limit, general analytic expressions are given for the radiative recombination of positrons to form positronium. Formulae are derived for the radiative capture to bound states of atomic hydrogen, and the connection between the two problems is demonstrated. Annihilation from excited states of positronium is considered, and it is estimated that 90 percent of the annihilations occur from the ground 1s state for both ortho and para positronium following radiative capture and cascade. A convenient form is given for the photodissociation cross section of positronium.

Gould, Robert J.

1989-01-01

87

Positron-annihilation studies of neutral and negatively charged As vacancies in GaAs  

NASA Astrophysics Data System (ADS)

The electronic properties of native monovacancy defects were investigated using techniques based on positron annihilation in n-type Si-doped GaAs. The positron lifetime measurements as a function of temperature have shown the thermal ionization of native As vacancies from negative to neutral charge state: V-As-->V0As. By combining the results of positron lifetime and two-dimensional angular-correlation-of-annihilation-radiation (2D-ACAR) experiments, the 2D-ACAR distributions of neutral and negative arsenic vacancies were separated from the partial bulk contribution. We found that 2D-ACAR distributions of the As vacancies are different from those of the bulk, and we also observed differences between the distributions of V0As and V-As. When compared to the negative charge state V-As, the neutral charge state V0As is characterized by a narrower momentum distribution from valence electron annihilations and a lower high-momentum tail from core electrons. These findings lead us to conclude that the electronic and ionic structure of the As vacancies depend strongly on their ionization. We also infer that V-As has a smaller open volume than V0As. This conclusion is in agreement with previous positron lifetime results and molecular-dynamics calculations.

Ambigapathy, R.; Manuel, A. A.; Hautojärvi, P.; Saarinen, K.; Corbel, C.

1994-07-01

88

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

89

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

90

Analysis of positron lifetime spectra in polymers  

NASA Technical Reports Server (NTRS)

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

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

1988-01-01

91

Investigation of the Microstructure of Epoxy Resin\\/MWNTs Nanocomposite by the Positron Annihilation Technique  

Microsoft Academic Search

Diethylenetriamine (DETA) was non-covalently wrapped onto the surface of multi-walled carbon nanotubes (MWNTs). The surface properties of MWNTs were investigated by Fourier transform-infrared spectra (FTIR) and X-ray diffraction (XRD). Influence of MWNTs content on the free volume of epoxy\\/MWNTs nanocomposites was investigated by positron annihilation lifetime spectroscopy (PALS). As a result, the amine group was successfully wrapped onto the surface

Ya-Ping Zheng; Jiao-Xia Zhang; Pei-Ying Yu; Wei Shi; Bo Wang

2010-01-01

92

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

Microsoft Academic Search

Results of positron annihilation lifetime spectra of beta-cyclodextrin and beta-cyclodextrin complexed with benzyl salicylate,benzyl acetate, or ethyl salicylate in air and vacuum were used to determine the fraction of beta-cyclodextrin which remains uncomplexed in the benzyl salicylate\\/beta-cyclodextrin 1:2 molar ratio inclusion complex. The intensity of the longest-lived component in vacuum was shown to decrease when the beta-cyclodextrin cavity was filled

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

1996-01-01

93

Characterizing free volumes and layer structures in polymeric membranes using slow positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation spectroscopy coupled with a newly built slow positron beam at National University of Singapore has been used to study the free volume, pore, and depth profile (0 - 10 ?m) in cellulose acetate polymeric membrane at the bottom and top sides of membranes for ionic separation in water purification applications. The S and R parameters from Doppler broadening energy of annihilation radiation representing free volumes (0.1-1 nm size) and pores (>1 nm-?m) as a function of depth have been analyzed into multilayers, i.e. skin dense, transition, and porous layers, respectively. The top side of membrane has large free volumes and pores and the bottom side has a skin dense layer, which plays a key role in membrane performance. Positron annihilation lifetime results provide additional information about free-volume size and distribution at the atomic and molecular scale in polymeric membrane systems. Doppler broadening energy and lifetime spectroscopies coupled with a variable mono-energy slow positron beam are sensitive and novel techniques for characterization of polymeric membrane in separation applications.

Jean, Y. C.; Chen, Hongmin; Zhang, Sui; Chen, Hangzheng; Lee, L. James; Awad, Somia; Huang, James; Lau, Cher Hon; Wang, Huan; Li, Fuyun; Chung, Tai-Shung

2011-01-01

94

Finite-size effects on band structure of CdS nanocrystallites studied by positron annihilation  

SciTech Connect

Quantum confinement effects in nanocrystalline CdS were studied using positrons as spectroscopic probes to explore the defect characteristics. The lifetime of positrons annihilating at the vacancy clusters on nanocrystalline grain surfaces increased remarkably consequent to the onset of such finite-size effects. The Doppler broadened line shape was also found to reflect rather sensitively such distinct changes in the electron momentum redistribution scanned by the positrons, owing to the widening of the band gap. The nanocrystalline sizes of the samples used were confirmed from x-ray diffraction and high resolution transmission electron microscopy and the optical absorption results supported the quantum size effects. Positron annihilation results indicated distinct qualitative changes between CdS nanorods and the bulk sample, notwithstanding the identical x-ray diffraction pattern and close resemblance of the optical absorption spectra. The results are promising in the event of positron annihilation being proved to be a very successful tool for the study of such finite-size effects in semiconductor nanoparticles.

Kar, Soumitra; Biswas, Subhajit; Chaudhuri, Subhadra; Nambissan, P.M.G. [Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata 700032 (India); Nuclear and Atomic Physics Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064 (India)

2005-08-15

95

Annihilation in positron-atom collisions - A new approach  

NASA Technical Reports Server (NTRS)

Two identities are derived for the exact scattering wave function by using the method of Hiller, Sucher, and Feinberg. It is shown that the resulting identities can be used to improve the calculation of positron annihilation cross sections whenever an approximate positron-atom scattering wave function is known. In addition, the advantages of the method are illustrated with a static model for e(+)-H scattering and then with a more realistic polarized orbital approximation.

Drachman, R. J.; Sucher, J.

1979-01-01

96

Positron annihilation in TiBe/sub 2/  

SciTech Connect

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

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

1988-01-01

97

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

E-print Network

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

Gribakin, Gleb

98

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

E-print Network

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

Rubloff, Gary W.

99

Positron annihilation spectroscopy investigation of vacancy clusters in silicon carbide: Combining experiments and electronic structure calculations  

NASA Astrophysics Data System (ADS)

The temperature dependence of the point defects in 6H-SiC induced by 12-MeV proton irradiation was studied by means of isochronal annealing followed by both positron annihilation spectroscopy and electron paramagnetic resonance measurements. The formation energies and positron lifetimes of various vacancy clusters were calculated to help in the interpretation of the experiments. The combination of the experiments and calculations enabled the identification of a negative silicon vacancy, with the lifetime of 218 ps, which is annealed between 400 ?C and 700 ?C. This process involves vacancy migration and formation of the VC+VSi cluster, with a lifetime of 235 ps. In addition, our calculations confirm the identification of several clusters proposed in previous experimental studies.

Wiktor, Julia; Kerbiriou, Xavier; Jomard, Gérald; Esnouf, Stéphane; Barthe, Marie-France; Bertolus, Marjorie

2014-04-01

100

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

SciTech Connect

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. [Materials Physics Division, Indira Gandhi Center for Atomic Research, Kalpakkam-603 102 (India)

2011-07-15

101

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

E-print Network

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

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

2009-04-06

102

Electron-Positron Radiative Annihilation : Timelike Virtual Compton Scattering  

E-print Network

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

Asmita Mukherjee

2010-10-01

103

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

E-print Network

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

Gribakin, Gleb

104

Annihilating dark matter and the galactic positron excess  

E-print Network

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

Irit Maor

2006-02-20

105

Monte Carlo investigation of positron annihilation in medical positron emission tomography  

NASA Astrophysics Data System (ADS)

A number of Monte Carlo codes are available for simulating positron emission tomography (PET), however, physics approximations differ. A number of radiation processes are deemed negligible, some without rigorous investigation. Some PET literature quantify approximations to be valid, without citing the data source. The radiation source is the first step in Monte Carlo simulations, for some codes this is 511 keV photons 180° apart, not polyenergetic positrons with radiation histories of their own. Without prior assumptions, we investigated electron-positron annihilation under clinical PET conditions. Just before annihilation, we tallied the positron energy and position. Right after annihilation, we tallied the energy and separation angle of photon pairs. When comparing PET textbooks with theory, PENELOPE and EGSnrc, only the latter three agreed. From 10 6 radiation histories, a positron source of 15O in a chest phantom annihilated at as high as 1.58 MeV, producing photons with energies 0.30-2.20 MeV, 79-180° apart. From 10 6 radiation histories, an 18F positron source in a head phantom annihilated at energies as high as 0.56 MeV, producing 0.33-1.18 MeV photons 109-180° apart. 2.5% and 0.8% annihilation events occurred inflight in the chest and the head phantoms, respectively. PET textbooks typically either do not mention any deviation from 180°, or state a deviation of 0.25° or 0.5°. Our findings are founded on the well-established Heitler cross-sections and relativistic kinematics, both adopted unanimously by PENELOPE, EGSnrc and GEANT4. Our results highlight the effects of annihilation in-flight, a process sometimes forgotten within the PET community.

Chin, P. W.; Spyrou, N. M.

2007-09-01

106

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

107

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

108

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

Microsoft Academic Search

Positron annihilation, especially the angular correlation of annihilation radiation, is a powerful tool for investigating the electronic spectra of ordered as well as defected materials. The tendency of positrons to trap at vacancy-type defects should enable this technique to study the local environment of such defects. However, we need to develop a theoretical basis for calculating the two-photon annihilation momentum

A. Bansil; R. Prasad; R. Benedek

1988-01-01

109

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

110

Enhancement of positron annihilation on molecules due to vibrational Feshbach resonances  

E-print Network

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

Gribakin, Gleb

111

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

E-print Network

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

Rubloff, Gary W.

112

Positron lifetime calculation for the elements of the periodic table.  

PubMed

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 theoretical trends. As is known, positron lifetimes in bulk show a periodic behaviour with atomic number. These calculations also confirm that monovacancy lifetimes follow the same behaviour. The effects of enhancement factors used in calculations have been commented upon. Finally, we have analysed the effects that f and d electrons have on positron lifetimes. PMID:21690967

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

2007-04-30

113

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

114

PALSfit: A computer program for analysing positron lifetime spectra  

E-print Network

PALSfit: A computer program for analysing positron lifetime spectra Peter Kirkegaard, Jens V. Olsen. Olsen1) , Morten Eldrup2) , and Niels Jørgen Pedersen2) Title: PALSfit: A computer program for analysing) February 2009 Abstract: A Windows based computer program PALSfit has been developed for analysing positron

115

Resonant versus nonresonant nuclear excitation of 115 In by positron annihilation  

E-print Network

Resonant versus nonresonant nuclear excitation of 115 In by positron annihilation D. B. Cassidy,1 the radiationless annihi- lation of a positron with a K-shell electron using a monoenergetic positron beam of nuclear excitation of 115 In using the broad spectrum of positrons from the beta decay of 64 Cu. Together

Golovchenko, Jene A.

116

Positron lifetime studies in thermoplastic polyimide test specimens  

NASA Technical Reports Server (NTRS)

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

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

1982-01-01

117

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

NASA Astrophysics Data System (ADS)

Molecular charge-transfer complexes of the tetramethylethylenediamine (TMEDA) with picric acid (Pi-OH), benzene-1,4-diol (QL), tin(IV) tetrachloride (SnCl4), iodine, bromine, and zinc chloride (ZnCl2) have been synthesized and investigated by elemental and thermal analysis, electronic, infrared, Raman and proton-NMR, energy-dispersive X-ray spectroscopy, X-ray powder diffraction and positron annihilation lifetime spectroscopy, and scanning electron microscopy. In this work, three types of acceptors ?-acceptors (Pi-OH and QL), ?-acceptors (iodine and bromine), and vacant orbital acceptors (SnCl4 and ZnCl2) were covered. The results of elemental analysis indicated that the CT complexes were formed with ratios 1:1 and 1:2 for QL, SnCl4, and ZnCl2 acceptors and iodine, Pi-OH, and Br2 acceptors, respectively. The type of chelating between the TMEDA donor and the mentioned acceptors depends upon the behavior of both items. The positron annihilation lifetime parameters were found to be dependent on the structure, electronic configuration, and the power of acceptors. The correlation between these parameters and the molecular weight and biological activities of studied complexes was also observed. Regarding the electrical properties, the AC conductivity and the dielectric coefficients were measured as a function of frequency at room temperature. The TMEDA charge-transfer complexes were screened against antibacterial (Escherichia coli, Staphylococcus aureus, Bacillus subtilis, and Pseudomonas aeruginosa) and antifungal (Aspergillus flavus and Candida albicans) activities.

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

118

Morphology of Thermoset Polyimides by Positron Annihilation Spectroscopy  

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

119

GRIS observations of positron annihilation radiation from the Galactic center  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

120

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

121

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

122

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

123

Assessment of positron annihilation as a potential non-destructive examination technique  

Microsoft Academic Search

The positron annihilation technique can provide a sensitive measure of defect density in metals. In this program the technique has been used to monitor defects generated during plastic deformation by cold work or fatigue cycling. The primary goals have been: (1) to assess the degree of sensitivity of the technique; (2) to correlate positron annihilation readings with observed microstructural changes

W. B. Jones; J. A. Van Den Avyle; W. B. Gauster; W. R. Wampler

1979-01-01

124

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

125

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

126

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

127

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

NASA Astrophysics Data System (ADS)

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.

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

2013-08-01

128

Solid-solid phase transitions and molecular motions in long-chain paraffins studied by positron annihilation techniques  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime and Doppler-broadening measurements were performed on n-dotriacontane (n-C32H66) and n-tritriacontane (n-C33H68) between 80 K and their melting points. The annihilation parameters plotted as a function of temperature show irregularities above the room temperature, which reflect solid-solid phase transitions in these materials. The transition temperatures derived from the positron annihilation measurements are in excellent agreement with literature values determined by other techniques. In the case of n-C32H66 additional phase transitions not reported previously in the literature were observed. The lifetime vs temperature curves could be reasonably well interpreted within the framework of the free volume model taking into account the effect of special types of molecular motions and structural defects. On the basis of our measurements, linear dependence of ortho-positronium (o-Ps) lifetime on the size of the cavity around o-Ps could be expected. The proposed empirical relationship proved to fit well to lifetime data for molecular crystals as a function of their molecular volume. The o-Ps yield showed an abrupt decrease with increasing temperature at 190 and 165 K in the case of n-C32H66 and n-C33H68, respectively. This effect was attributed to the onset of rotation of the methyl end groups of the paraffin chains.

Lévay, B.; Lalovi?, M.; Ache, H. J.

1989-03-01

129

Positron lifetime measurements in chiral nematic liquid crystals  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

130

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

131

One-photon annihilation of thermal positrons with bound atomic electrons  

NASA Technical Reports Server (NTRS)

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

Jung, Young-Dae

1994-01-01

132

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

133

Structural transition in rare earth doped zirconium oxide: A positron annihilation study  

SciTech Connect

Graphical abstract: New microstructural analysis and phase transition of rare earth doped mixed oxide compounds such as: Sm{sub 2?x}Dy{sub x}Zr{sub 2}O{sub 7} (where x = 0.0 ? x ? 2.0) that are potentially useful as solid oxide fuels, ionic conductors, optoelectronic materials and most importantly as radiation resistant host for high level rad-waste disposal, structural transition in the system is reported through positron annihilation spectroscopy as there is an indication in the X-ray diffraction analysis. Highlights: ? Zirconium oxide material doped with rare earth ions. ? The method of positron annihilation spectroscopy suggests a phase transition in the system. ? The crystal structure transformation from pure pyrochlore to defect fluorite type of structure is shown by X-ray diffraction results. -- Abstract: A series of compounds with the general composition Sm{sub 2?x}Dy{sub x}Zr{sub 2}O{sub 7} (where 0 ? x ? 2.0) were synthesized by chemical route and characterized by powder X-ray diffraction (XRD) analysis. The rare earth ion namely Sm{sup +3} in the compound was gradually replaced with another smaller and heavier ion, Dy{sup +3} of the 4f series, there by resulting in order–disorder structural transition, which has been studied by positron annihilation lifetime and Doppler broadening spectroscopy. This study reveals the subtle electronic micro environmental changes in the pyrochlore lattice (prevalent due to the oxygen vacancy in anti-site defect structure of the compound) toward its transformation to defect fluorite structure as found in Dy{sub 2}Zr{sub 2}O{sub 7}. A comparison of the changes perceived with PAS as compared to XRD analysis is critically assayed.

Chakraborty, Keka [Applied Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085 (India)] [Applied Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085 (India); Bisoi, Abhijit [Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064 (India)] [Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064 (India); Ganguly, Bichitra Nandi, E-mail: bichitra.ganguly@saha.ac.in [Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064 (India); Grover, Vinita; Sayed, Farheen Nasir; Tyagi, A.K. [Applied Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085 (India)] [Applied Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085 (India)

2012-11-15

134

Li-doped MgO as catalysts for oxidative coupling of methane: A positron annihilation study  

NASA Astrophysics Data System (ADS)

Magnesium oxides intentionally doped with lithium (with a maximum Li content of 40 tool%) for use as catalysts for oxidative coupling of methane were characterized by means of positron annihilation. The positron lifetime spectra, which could be reasonably well interpreted within the framework of the well-known trapping model, depend on the amount of Li doping of the MgO suggesting that positrons are trapped at dispersed small Li 2CO 3 precipitates. Very similar dependencies on lithium doping of the C 2 selectivity and the positron trapping rate ? imply an intimate relationship between the concentration of [Li] 0-centers (also referred to as [Li +O -] centers) and the selective activity of Li/MgO during catalytic reactions.

Dai, G. H.; Yan, Q. J.; Wang, Y.; Liu, Q. S.

1991-08-01

135

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

136

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

137

The influence of several parameters on the lifetimes and intensities of positron lifetime spectra of metals  

Microsoft Academic Search

The influence of the instrumental time-resolution function and the source-surface spectrum on the lifetimes and intensities\\u000a of positron lifetime spectra is studied by a computer simulation of such spectra. The prompt curve is described not by a sum\\u000a of Gaussian curves but by the convolution of a Gaussian curve with a double-sided exponential. It is shown that the fitted\\u000a lifetimes

W. Puff

1979-01-01

138

Low energy positron flux generator for lifetime studies in thin films  

Microsoft Academic Search

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

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

1991-01-01

139

Positron Annihilation Spectroscopy on Nitride-Based Semiconductors  

NASA Astrophysics Data System (ADS)

Positron annihilation is a nondestructive tool for investigating vacancy-type defects in materials. Detectable defects are monovacancies to vacancy clusters, and there is no restriction of sample temperature or conductivity. Using this technique, we studied native defects in (AlInGa)N grown by metalorganic vapor phase epitaxy and plasma-assisted molecular beam epitaxy. For InxGa1-xN, the defect concentration increased with increasing In composition x and reached a maximum at x = 0.44-0.56. The major defect species was identified as cation vacancies coupled with multiple nitrogen vacancies. For AlxGa1-xN, the vacancy-type defects started to be introduced at above x = 0.54 and their concentration increased with increasing x. The observed behavior of point defects was discussed in terms of the transition of the growth mode.

Uedono, Akira; Ishibashi, Shoji; Oshima, Nagayasu; Suzuki, Ryoichi

2013-08-01

140

Higgs shifts from electron-positron annihilations near neutron stars  

E-print Network

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

Wegner, Gary A

2015-01-01

141

Fragmentation production of charmed hadrons in electron-positron annihilation  

SciTech Connect

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

Novoselov, A. A., E-mail: alexn@cern.c [Moscow Institute for Physics and Technology (State University) (Russian Federation)

2010-10-15

142

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

143

Positron annihilation study on surface structure of biological samples implanted by ions with low energy  

NASA Astrophysics Data System (ADS)

The organic materials of biological samples, such as lima bean and peanut, were implanted respectively by nitrogen ions with an energy of 100 keV and vanadium ions with an energy of 200 keV. The positron annihilation lifetime spectra of implanted and non-implanted samples were compared with each other especially in ?3 and I3. The experimental results showed that before implantation there were many small holes with diameters of 0.48 and 0.7 nm in lima bean and peanut, respectively. After ion implantation, the size of holes would be changed because of organism cross-linking and scission. The effective penetration range of implantation of ions with low energy into biological samples is about 200 µm.

Lu, Ting; Yu, Wei-zhong; Zhou, Hong-yu; Zhu, Guang-hua; Wang, Xin-fu; Wang, Chao

2001-02-01

144

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

SciTech Connect

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

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

2011-12-01

145

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

E-print Network

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

Collins, Gary S.

146

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

147

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

148

Positron annihilation studies of silicon-based materials  

NASA Astrophysics Data System (ADS)

Positron Annihilation Spectroscopy (PAS) is used as a defect-profiling tool in the characterization of Si-based materials. PAS, in conjunction with variable energy positron beams, is a non-destructive depth-profiling probe, ideally suited for studying thin films, multi-layered structures, and buried interfaces. Its sensitivity to open-volume defects covers a wide range of defect sizes and concentrations, and surpasses that of most other techniques. This dissertation presents PAS investigations of electrical, chemical and mechanical properties of a number of advanced materials for future use by the semiconductor industry. Among the subjects of this work are: hydrogenated amorphous silicon (a-Si:H) for use in solar cells and flat-panel displays; low dielectric constant materials (low-k) for interlayer dielectrics; and thin-gate transistors, focusing on the defects at the Si/SiO 2 interface, which limit the device reliability. Results from extensive research on various possibilities to enhance the PAS capability by increasing its efficiency are presented in the appendices. The recognition of different dangling bond defects for low defect densities is achieved in these first PAS studies of void-free a-Si:H. Direct evidence of the existence of dopant-defect complexes is obtained for the first time. This research lays the foundation for future studies of the role of the impurities in light- and thermal degradation of a-Si:H PAS was applied to the characterization of porous low-k dielectrics. The annihilation observables are correlated with the dielectric properties of the material and their preparation conditions. PAS is the only non-destructive local k-probe, and the only tool for measuring void densities and sizes. The method is also sensitive to the chemical environment of the voids, seen during oxidation, water absorption, and forming gas anneal. Industrial research, partially based on these results, is currently in progress at IBM. A decade-old controversy, involving different models of defect states at Si/SiO2 interfaces, has been resolved. The two-defect model was confirmed and previous results were reevaluated. Research in this area will promote the use of PAS as an on-line diagnostic tool in the manufacturing of integrated circuits.

Petkov, Mihail Petkov

149

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

E-print Network

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

Soatto, Stefano

150

Polarized hadron pair production from electron-positron annihilation  

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

151

Oxidation of uranium studied by gravimetric and positron annihilation techniques  

SciTech Connect

Oxidation of uranium under various environmental conditions, and at relatively low temperature was investigated to fulfill the following objectives: (1) to try to determine the validity and usefulness of published data; (2) to investigate the oxidation of uranium from very low (<<0.13 kPa (1 Torr)) to relatively high (approx. 20 kPa (150 Torr)) water vapor pressures in the presence and absence of oxygen (approx. 1 vppM to approx. 63.73 kPa (479 Torr)), and to determine the pressure dependence of the reaction rate constant in these ranges; (3) to determine the temperature dependence (60/sup 0/ to 180/sup 0/C) of the reaction rate constant under the environmental condition defined under (2); (4) to investigate the effect of impurities in the metal on the reaction rate; (5) to provide an interpretation of pressure and temperature dependences based on theoretical and experimental studies of the electrical conductivity and defect structure of uranium oxides; and (6) to identify experimentally the solid state defects controlling the oxidation process, under a given set of environmental conditions, with the help of positron annihilation techniques. Progress up to date on the six defined objectives are presented.

Colmenares, C.; Howell, R.; McCreary, T.

1981-04-01

152

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

E-print Network

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

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

2005-09-26

153

POLARIZATION PHENOMENA IN THE ONE-QUANTUM ANNIHILATION OF POSITRONS AND THE PHOTOELECTRIC EFFECT  

Microsoft Academic Search

Cross sections are derived for the one-quantum annihilation of ; longitudinally-polarized positrons and for the photoelectric effect with ; longitudinally-polarized photons. Simple expressions, quantitatively reliable ; for light elements, are obtained by considering only K-shell electrons and ; describing the outgoing electron (incoming positron) by plane waves. In both ; cases, the incoming and outgoing particles have predominantly the same

Kirk McVoy

1957-01-01

154

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

E-print Network

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

Hasan Yuksel

2006-09-06

155

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

156

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

157

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

NASA Astrophysics Data System (ADS)

Aims: We want to estimate whether the positrons produced by the ?+-decay of 26Al, 44Ti, and 56Ni synthesised in massive stars and supernovae are sufficient to explain the 511 keV annihilation emission observed in our Galaxy. Such a possibility has often been put forward in the past. In a previous study, we showed that nucleosynthesis positrons cannot explain the full annihilation emission. Here, we extend this work using an improved propagation model. Methods: We developed a Monte Carlo Galactic propagation code for ~MeV positrons in which the Galactic interstellar medium, the Galactic magnetic field, and the propagation are finely described. This code allows us to simulate the spatial distribution of the 511 keV annihilation emission. We tested several Galactic magnetic fields models and several positron escape fractions from type-Ia supernova for 56Ni positrons to account for the large uncertainties in these two parameters. We considered the collisional/ballistic transport mode and then compared the simulated 511 keV intensity spatial distributions to the INTEGRAL/SPI data. Results: Regardless of the Galactic magnetic field configuration and the escape fraction chosen for 56Ni positrons, the 511 keV intensity distributions are very similar. The main reason is that ~MeV positrons do not propagate very far away from their birth sites in our model. The direct comparison to the data does not allow us to constrain the Galactic magnetic field configuration and the escape fraction for 56Ni positrons. In any case, nucleosynthesis positrons produced in steady state cannot explain the full annihilation emission. The comparison to the data shows that (a) the annihilation emission from the Galactic disk can be accounted for; (b) the strongly peaked annihilation emission from the inner Galactic bulge can be explained by positrons annihilating in the central molecular zone, but this seems to require more positron sources than the population of massive stars and type Ia supernovae usually assumed for this region; (c) the more extended emission from the Galactic bulge cannot be explained. We show that a delayed 511 keV emission from a transient source, such as a starburst episode or a recent activity of Sgr A*, occurring between 0.3 and 10 Myr ago and producing between 1057 and 1060 sub-MeV positrons could explain this extended component, and potentially contribute to the inner bulge signal.

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

2014-04-01

158

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

Microsoft Academic Search

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

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

2003-01-01

159

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

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

160

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.

161

Positron lifetime spectroscopy for investigation of thin polymer coatings  

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

162

Silicide phase formation in Ni/Si system: Depth-resolved positron annihilation and Rutherford backscattering study  

SciTech Connect

Silicidation in Ni/Si thin-film junction has been investigated using depth-resolved positron annihilation spectroscopy (PAS) and Rutherford backscattering spectrometry (RBS). Identification of various silicide phases from an analysis of the positron annihilation parameters is consistent with the RBS results. Absence of vacancy defects in the silicide region is clearly brought out by PAS00.

Abhaya, S.; Amarendra, G.; Panigrahi, B.K.; Nair, K.G.M. [Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102 (India)

2006-02-01

163

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

NASA Astrophysics Data System (ADS)

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

Weinzierl, Stefan

2011-02-01

164

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

165

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

NASA Technical Reports Server (NTRS)

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

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

2008-01-01

166

? -Ray Spectra and Enhancement Factors for Positron Annihilation with Core Electrons  

NASA Astrophysics Data System (ADS)

Many-body theory is developed to calculate the ? spectra for positron annihilation in noble-gas atoms. Inclusion of electron-positron correlation effects and core annihilation gives spectra in excellent agreement with experiment [K. Iwata et al., Phys. Rev. Lett. 79, 39 (1997)]. The calculated correlation enhancement factors ?n l for individual electron orbitals n l are found to scale with the ionization energy In l (in eV), as ?n l=1 +?{A /In l }+(B /In l)?, where A ?40 eV , B ?24 eV , and ? ?2.3 .

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

2015-03-01

167

Production of muon pairs in annihilation of high-energy positrons with resting electrons  

E-print Network

A Monte Carlo generator for the electromagnetic production of muon pairs by annihilation of high-energy positrons with atomic electrons is described. The computer code is designed as a standard electromagnetic process for GEANT4. The formulas and algorithms are described and illustrated in detail. The code has been applied to study muon production by high energy positrons in a spoiler. The results are presented and compared to muon pair production by gamma conversion.

Burkhardt, H; Kokoulin, R P

2003-01-01

168

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

169

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

170

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

SciTech Connect

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

2013-04-19

171

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

E-print Network

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

Xiaoguang Ma; Feng Wang

2013-03-20

172

NNLO predictions for event shapes and jet rates in electron-positron annihilation  

E-print Network

The strong coupling constant is a fundamental parameter of nature. It can be extracted from experiments measuring three-jet events in electron-positron annihilation. For this extraction precise theoretical calculations for jet rates and event shapes are needed. In this talk I will discuss the NNLO calculation for these observables.

Stefan Weinzierl

2010-01-08

173

Angular Correlation of Photons from Positron Annihilation in Copper-Nickel Alloys  

Microsoft Academic Search

Angular correlation measurements of radiation from positrons annihilating in copper-nickel alloys have been carried out with crossed-slit geometry using single crystals containing less than 73 at % Ni. The neck radius of the Fermi surface decreases linearly with increasing nickel content, but the neck does not detach from the hexagonal zone face even at the highest nickel content. No marked

Masayuki Hasegawa; Toshiharu Suzuki; Makoto Hirabayashi

1974-01-01

174

Measurement of the Positron Annihilation Induced Auger Electron Spectrum from Ag(100)  

NASA Astrophysics Data System (ADS)

Research has demonstrated that Positron Annihilation Induced Auger Spectroscopy (PAES) can be used to probe the top-most atomic layer of surfaces and to obtain Auger spectra that are completely free of beam-impact induced secondary background. The high degree of surface selectivity in PAES is a result of the fact that positrons implanted at low energies are trapped with high efficiency at an image-correlation potential well at the surface resulting in almost all of the positrons annihilating with atoms in the top-most layer. Secondary electrons associated with the impact of the incident positrons can be eliminated by a suitable choice of an incident beam energy. In this paper we present the results of measurements of the energy spectrum of electrons emitted as a result of positron annihilation induced Auger electron emission from a clean Ag(100) surface using a series of incident beam energies ranging from 20 eV down to 2 eV. A peak in the spectrum was observed at ~40 eV corresponding to the N2,3VV Auger transition in agreement with previous PAES studies. This peak was accompanied by an even larger low energy tail which persisted even at the lowest beam energies. Our results for Ag(100) are consistent with previous studies of Cu and Au and indicate that a significant fraction of electrons leaving the sample are emitted in the low energy tail and suggest a strong mechanism for energy sharing in the Auger process.

Joglekar, P.; Shastry, K.; Fazleev, N. G.; Weiss, A. H.

2013-06-01

175

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

176

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

NASA Astrophysics Data System (ADS)

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

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

2015-02-01

177

Positron annihilation study of Pd contacts on impurity-doped GaN  

SciTech Connect

Pd contacts on both n-type and p-type GaN were studied using positron annihilation spectroscopy, and the results were used to interpret the role of Ga vacancies on the band bending below the contacts. The concentration of Ga vacancy in Si-doped GaN was higher than that in the Mg-doped one. In Si-doped GaN, implanted positrons were annihilated at the nearer surface region and the interface of Pd/n-type GaN was detected by positrons clearly shifted toward the surface of Pd. This suggests that Ga vacancies could act as an interface state, pinning the Fermi level at the interface of Pd with GaN, leading to the production of a negative electric field below the interface. {copyright} 2001 American Institute of Physics.

Lee, Jong-Lam; Kim, Jong Kyu; Weber, Marc H.; Lynn, Kelvin G.

2001-06-25

178

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

179

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

180

Study of PRIMAVERA steel samples by a positron annihilation spectroscopy technique  

NASA Astrophysics Data System (ADS)

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

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

2010-11-01

181

The Galactic Positron Annihilation Medium: Is Ionized Helium A Viable Candidate?  

NASA Astrophysics Data System (ADS)

We consider the possibility that the 511 keV electron-positron annihilation line coming from the direction of the galactic center could be the result of thermal positrons annihilating via Ps formation from charge exchange with singly ionized helium.[Fatuzzo:2000] We present an analysis of the galactic line profiles, comparing the observed line shape to the expected line shape for the proposed annihilation mechanism at various temperatures. We calculate the FWHM by both Monte Carlo and analytical techniques, using empirically derived Ps formation, ionization, and excitation cross sections for the e^+- He^+ interactions. It is found that the FWHM of the annihilation spectrum for this process shows a logarithmic temperature dependence. We conclude that ionized helium is an unlikely annihilation medium for a reasonable ambient electron temperature, given that the calculated FWHM exceeds the upper maximum of the error bars of the weighted average of the line widths of all observations known to the authors. [Fatuzzo:2000] M. Fatuzzo, F. Melia, and J. Rafelski, Astrophys. J. 549, 293 (2000)

Franking, Matthew

2005-04-01

182

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

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

183

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

184

Calculation of the positron annihilation rate in PsH with the positronic extension of the explicitly correlated nuclear-electronic orbital method.  

PubMed

The nuclear-electronic orbital explicitly correlated Hartree-Fock (NEO-XCHF) method is modified and extended to study electron-positron quantum systems. The NEO-XCHF method is more computationally efficient than the explicitly correlated methods previously applied to positron systems because only the electron-positron dynamical correlation is treated explicitly in NEO-XCHF. As a result, the form of the wave function is much simpler with fewer parameters, and the variational optimization of the molecular orbital parameters is performed through an iterative scheme rather than a stochastic optimization. The NEO-XCHF approach is used to calculate the positron annihilation rate for positronium hydride (PsH). The resulting annihilation rate for PsH is within 20% of the most accurate values available and is calculated at a fraction of the computational cost. These results suggest that qualitatively accurate positron annihilation rates can be calculated treating only electron-positron correlation explicitly, leading to significant computational savings by neglecting electron-electron dynamical correlation. Thus, the NEO-XCHF approach could potentially enable the calculation of qualitatively accurate positron annihilation rates for larger positron systems. PMID:19281179

Pak, Michael V; Chakraborty, Arindam; Hammes-Schiffer, Sharon

2009-04-23

185

Positron Lifetime Measurements in Semi-Interpenetrating Polymer Networks  

NASA Astrophysics Data System (ADS)

Physical interlocking is generally believed to be the dominant force that controls the morphology of a Semi-Interpenetrating Polymer Network (S-IPN). However, in our recent study (J. J. Singh, R. H. Pater, and A. Eftekhari: Nuclear Instruments and Methods - NIMB (To be published)) of S-IPN's prepared from thermosetting LaRC-RP46 and thermoplastic LaRC-IA polyimides, it was discovered that a strong electrostatic interaction between the constituent molecules enhances phase mixing. In order to further test this hypothesis, we replaced thermoplastic LaRC-IA by LaRC-SI. LaRC-SI has a rigid backbone which should prevent it from coming close to LaRCT-RP46 molecular chain, thereby minimizing electrostatic influences. Positron lifetime measurements were made in the new class of S-IPN samples. Experimental values of the free volume and the dielectric constant indicate that they are merely solid solutions of their constituents, without any measurable electrostatic effects. However, the density values of these materials suggest a slight repulsion (or enhanced separation) between the constituent chains.

Singh, Jag J.; Pater, Ruth H.; Sprinkle, Danny R.

1998-03-01

186

Molecular motion and relaxation below glass transition temperature in poly (methyl methacrylate) studied by positron annihilation  

NASA Astrophysics Data System (ADS)

In this paper, we present the study of local molecular motions in poly (methyl methacrylate) (PMMA) below glass transition temperature by measuring the ortho-positronium (o-Ps) intensity. Two series of experiments were performed: (1) the PMMA sample was irradiated by 22Na positron source with elongated time at room temperature, 225 K and 16 K, respectively, and positron lifetime spectra were measured as a function of irradiation time and (2) Positron lifetime and Doppler broadening spectra were measured as a function of temperature from 16 to 350 K after positron irradiation at 16 K for more than 350 h. While the o-Ps lifetime always shows no change with elapsed time, decrease and increase of o-Ps intensity I3 are observed at 225 K and 16 K, which are interpreted as the result of positron irradiation-induced free radicals and trapped electrons, respectively. With temperature increasing from 16 K, there is a continuous drop of I3 beginning at around 100 K. This is due to some local group movements such as the ester and main chain methyl group rotations, which lead to the detrapping of accumulated electrons. These local motions do not need additional free volume, so we observed no change of the o-Ps lifetime. Some other structural relaxations such as ?-relaxation are also observed and discussed.

Qi, N.; Chen, Z. Q.; Uedono, A.

2015-03-01

187

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

NASA Astrophysics Data System (ADS)

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

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

2011-08-01

188

NLO results for five, six and seven jets in electron-positron annihilation  

E-print Network

We present next-to-leading order corrections in the leading colour approximation for jet rates in electron-positron annihilation up to seven jets. The results for the two-, three-, and four-jet rates agree with known results. The NLO jet rates have been known previously only up to five jets. The results for the six- and seven-jet rate are new. The results are obtained by a new and efficient method based on subtraction and numerical integration.

Sebastian Becker; Daniel Goetz; Christian Reuschle; Christopher Schwan; Stefan Weinzierl

2011-11-07

189

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

E-print Network

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

N. A. Belov; Z. Harman

2014-10-23

190

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

NASA Astrophysics Data System (ADS)

Positrounium annihilation lifetime spectroscopy (PALS) utilizing a focused low energy beam of positrons to control implantation depth enables the analysis of very thin films. Beam-PALS was used to study confinement and interfacial effects on polymer mobility in ultra-thin polycarbonate films and to characterize nanoporous structures of polymeric low dielectric constant (low-k ) thin films. Three complementary techniques were used to address the apparent discrepancies in recent polymer film results. Beam-PALS (probing positronium nanovoid lifetime, tau), specular X-ray reflectivity (SXR, monitoring film thickness, h) and incoherent neutron scattering (INS, characterizing mean-square atomic displacements, ) were combined to study the thermophysical properties of Bisphenol-A polycarbonate (PC) ultra-thin films (60 A to ˜1000 A) supported on an oxidized silicon wafer surface. As h decreased the concomitant reduction in thermal expansion coefficients of h, tau and < u2>, as well as the decreased amplitudes of < u2>, indicated that thin film confinement produces suppressed molecular mobility in PC. These films were modeled with an immobile interface layer ranging from 38 A to 130 A, depending on the measurement technique and the temperature range. No clear trends in the apparent glass transition temperature (Tg) emerged from these techniques, thus rendering Tg shifts inconclusive and of less fundamental importance. Beam-PALS was also applied to characterize several generations of porous SiLK (Trade Mark of Dow Chemical) low-k films to reveal the size, size distribution, interconnectivity and possible morphology of the engineered nanopores. The dependence of these properties on porogen loading/porosity was carefully analyzed and compared, when possible, with results from other techniques such as small-angle X-ray scattering and AFM. Unique to the PALS technique is the ability to quantify the pore interconnection length. Porous SiLK (V9), U.2 and Y were found to have progressively smaller pores, (from ˜5 nm in diameter to less than 2 nm) with shorter interconnection length. The observed pore structure evolution with processing temperature further demonstrates the capability of PALS in monitoring the fabrication and integration of porous low-k materials. The concomitant improvement of pore characterization techniques is an essential and enabling aspect of meeting future low- k materials and integration challenges.

Peng, Huagen

191

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

NASA Technical Reports Server (NTRS)

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

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

2005-01-01

192

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

193

Multimessenger constraints on dark matter annihilation into electron-positron pairs  

NASA Astrophysics Data System (ADS)

We investigate the production of electrons and positrons in the Milky Way within the context of dark matter annihilation. Upper limits on the relevant cross-section are obtained by combining observational data at different wavelengths (from Haslam, WMAP and Fermi all-sky intensity maps) with recent measurements of the electron and positron spectra in the solar neighbourhood by PAMELA, Fermi and HESS. We consider synchrotron emission in the radio and microwave bands, as well as inverse Compton scattering and final-state radiation at gamma-ray energies. According to our results, the dark matter annihilation cross-section into electron-positron pairs should not be higher than the canonical value for a thermal relic if the mass of the dark matter candidate is smaller than a few GeV. In addition, we also derive a stringent upper limit on the inner logarithmic slope ? of the density profile of the Milky Way dark matter halo (? < 1 if mdm < 5 GeV, ? < 1.3 if mdm < 100 GeV and ? < 1.5 if mdm < 2 TeV) assuming that e± =3 × 10-26 cm3 s-1. A logarithmic slope steeper than ? ˜ 1.5 is hardly compatible with a thermal relic lighter than ˜1 TeV, regardless of the dominant annihilation channel.

Wechakama, M.; Ascasibar, Y.

2014-03-01

194

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

195

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 is based on the use of artificial neural networks ANNs . By using data from simulated positron spectra: Artificial neural networks ANNs ; Amplitudes; Simulation model 1. Introduction Determination of mean

Pázsit, Imre

196

Positron annihilation and ion beam analysis of ion-bombardment-induced hydrogen release and oxidation of ultra high molecular weight polyethylene  

NASA Astrophysics Data System (ADS)

Ultra high molecular weight polyethylene was bombarded with He + and Ar + ions to fluences ranging from 10 13 to 2×10 16 ions/cm 2. Rutherford backscattering and nuclear reaction analysis were applied to study mechanism of oxygen uptake and hydrogen release induced by ion beam bombardment. The influence of ion bombardment on positron annihilation lifetime parameters is also discussed. Hydrogen release was observed with increasing ion dose and was correlated to the ion stopping power. An important effect observed, was the rapid oxidation of samples after exposure to air.

Al-Qaradawi, I. Y.; Madi, N. K.; Turos, A.; Abdul-Kader, A. M.

2007-02-01

197

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

E-print Network

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

Ma, Xiaoguang

2012-01-01

198

Positron annihilation study of proton-irradiated reactor pressure vessel steels  

NASA Astrophysics Data System (ADS)

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

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

2012-10-01

199

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

PubMed

Microstructure and thermal stability of the radiation defects in n-FZ-Si ([P] ? 7 × 10(15) cm(-3)) single crystals have been investigated. The radiation defects have been induced by irradiation with 15 MeV protons and studied by means of both the positron lifetime spectroscopy and low-temperature measurements of the Hall effect. At each step of the isochronal annealing over the temperature range ?60-700?°C the positron lifetime has been measured for the temperature interval ?30-300 K, and for samples-satellites the temperature dependences of the charge carriers and mobility have been determined over the range ?4.2-300 K. It is argued that as-grown impurity centers influence the average positron lifetime by forming shallow (E(b) ? 0.013 eV) positron states. The radiation-induced defects were also found to trap positrons into weakly bound (E(b) ? 0.01 eV) states. These positron states are observed at cryogenic temperatures during the isochronal annealing up to T(anneal.) = 340?°C. The stages of annealing in the temperature intervals ?60-180?°C and ?180-260?°C reflect the disappearance of E-centers and divacancies, respectively. Besides these defects the positrons were found to be localized at deep donor centers hidden in the process of annealing up to the temperature T(anneal.) ? 300?°C. The annealing of the deep donors occurs over the temperature range ?300-650?°C. At these centers positrons are estimated to be bound with energies E(b) ? 0.096 and 0.021 eV within the temperature intervals ?200-270 K and ?166-66 K, respectively. The positron trapping coefficient from these defects increases from ?1.1 × 10(16) to ?6.5 × 10(17) s(-1) over the temperature range ?266-66 K, thus substantiating a cascade phonon-assisted positron trapping mechanism whose efficiency is described by ?T(-3) law. It is argued that the value of activation energy of the isochronal annealing E(a) ? 0.74-0.59 eV is due to dissociation of the positron traps, which is accompanied by restoration of the electrical activity of the phosphorus atoms. The data suggest that the deep donors involve a phosphorus atom and at least two vacancies. Their energy levels are at least at E > E(c) - 0.24 eV in the investigated material. PMID:23221281

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

2013-01-23

200

Positron annihilation on defects in silicon irradiated with 15 MeV protons  

NASA Astrophysics Data System (ADS)

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

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

2013-01-01

201

Quark Flavor Identification in Electron-Positron Annihilation.  

NASA Astrophysics Data System (ADS)

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

Kaye, Harold Stephen

202

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

203

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

204

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

205

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

206

A slow positron beam generator for lifetime studies  

NASA Technical Reports Server (NTRS)

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

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

1989-01-01

207

Enhanced Dark Matter Annihilation Rate for Positron and Electron Excesses from Q-Ball Decay  

SciTech Connect

We show that Q-ball decay in Affleck-Dine baryogenesis models can account for dark matter when the annihilation cross section is sufficiently enhanced to explain the positron and electron excesses observed by PAMELA, ATIC, and PPB-BETS. For Affleck-Dine baryogenesis along a d=6 flat direction, the reheating temperature is approximately 30 GeV and the Q-ball decay temperature is in the range of 10-100 MeV. The lightest supersymmetric particles produced by Q-ball decay annihilate down to the observed dark matter density if the cross section is enhanced by a factor approx10{sup 3} relative to the thermal relic cross section.

McDonald, John [Cosmology and Astroparticle Physics Group, University of Lancaster, Lancaster LA1 4YB (United Kingdom)

2009-10-09

208

Enhanced dark matter annihilation rate for positron and electron excesses from Q-ball decay.  

PubMed

We show that Q-ball decay in Affleck-Dine baryogenesis models can account for dark matter when the annihilation cross section is sufficiently enhanced to explain the positron and electron excesses observed by PAMELA, ATIC, and PPB-BETS. For Affleck-Dine baryogenesis along a d=6 flat direction, the reheating temperature is approximately 30 GeV and the Q-ball decay temperature is in the range of 10-100 MeV. The lightest supersymmetric particles produced by Q-ball decay annihilate down to the observed dark matter density if the cross section is enhanced by a factor approximately 10(3) relative to the thermal relic cross section. PMID:19905616

McDonald, John

2009-10-01

209

Possible New Well-Logging Tool Using Positron Annihilation Spectroscopy to Detect TOC in Source Rocks  

NASA Astrophysics Data System (ADS)

Possible New Well-Logging Tool Using Positron Annihilation Spectroscopy to Detect Total Organic Carbon (TOC) in Source Rocks PATTERSON, C., Department of Geology, Department of Physics, Texas Christian University, QUARLES, C.A., Department of Physics, Texas Christian University, Fort Worth, Texas, BREYER, J.A., Department of Geology, Texas Christian University, Fort, Worth, Texas. The positron produces two gamma rays upon annihilation with an electron. Depending on the momentum of the electron, the two resulting photons are shifted from the initial electron rest mass energy by the Doppler effect. We measure the distribution of gamma ray energies produced by annihilation on a petroleum source rock core. Core from the Mitchell Energy well T.P. Sims 2 of the Barnett Shale located in Wise County, Texas, is under study. Apparatus for the experiment consists of an Ortec Ge detector. The source used for the experiment is Ge68, which undergoes beta decay and produces the positrons that penetrate the core. It is placed on the middle of the core and covered with a small, annealed NiCu plate to prevent unnecessary background from the positrons annihilating with electrons other than in the core. Distance between the source and the detector is fixed at 6.75 inches. Measurements were made in specific locations at 2 inch increments for approximately an hour and a half where the predetermined Total Organic Carbon (TOC) values were made. Future studies involve an overall correlation of the core between experimental readings and TOC, including corrections for changes in grain size and lithology. Additional research has shown no distinct correlation between grain size and distribution of energies across the targeted spectrum. Additional corrections should be made for the decay in activity of the source. Future research also includes the determination for optimum time and distance for the source from the core. A long-term goal for the experiment is to develop an effective down-hole tool to determine TOC in potential source rocks, thereby minimizing time coring and geochemical analysis.

Patterson, Casey; Quarles, C. A.; Breyer, J. A.

2001-10-01

210

Characterization of silicon implanted SiO{sub 2} layers using positron annihilation spectroscopy  

SciTech Connect

Silicon implanted thermal SiO{sub 2} layers were studied using depth-resolved positron annihilation spectroscopy (PAS) and transmission electron microscopy (TEM). TEM observations show the presence of silicon nanocrystals (Si{sub nc}) in the region between 200 nm and 200 nm. The defect annealing behavior is studied by means of PAS. For 1000{degrees}C annealed samples at a depth for which Si{sub nc} are observed, a distinctive PAS signal is detected and ascribed to the Si{sub nc}/SiO{sub 2} interface.

Ghislotti, G.; Nielsen, B.; Asoka-Kumar, P.; Lynn, K.G.; Szeles, C. [Brookhaven National Lab., Upton, NY (United States); Bottani, C.E. [Politecnico di Milano, Milan (Italy); Bertoni, S.; Cerofolini, G.F.; Meda, L. [Montedison SpA, Novara (Italy). Ist. di Ricerche G. Donegani

1995-08-01

211

Positron-annihilation measurements of vacancy formation in Ni and Ni(Ge)  

SciTech Connect

Vacancy formation in Ni and in dilute Ni(Ge) alloys was studied under thermal equilibrium conditions using positron-annihilation Doppler broadening. A monovacancy formation enthalpy of 1.8 +- 0.1 eV was determined for pure Ni; combining this result with that from previous tracer self-diffusion measurements, a monovacancy migration enthalpy of 1.1 +- 0.1 eV was also deduced. Analysis of the vacancy formation measurements in Ni(0.3 at.% Ge) and Ni(1 at.% Ge) yielded a value for the vacancy-Ge binding enthalpy of 0.20 +- 0.04 eV.

Smedskjaer, L.C.; Fluss, M.J.; Legnini, D.G.; Chason, M.K.; Siegel, R.W.

1982-03-01

212

Effects of proton irradiation on Chinese domestic RPV steels by variable energy positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

The effects of proton irradiation on Chinese domestic reactor pressure vessel (RPV) A508-3 steels have been investigated by variable energy positron annihilation spectroscopy. The samples were irradiated using 110 keV protons at doses ranging from 0.2 to 2.0 ×1017 cm-2 at room temperature. Defect profiles were analyzed by measuring the S parameter as a function of incident positron energy from 0.25 to 26keV, corresponding to mean depths of up to ~940nm. It is interesting to note that the S-parameter increases rapidly with increasing dose, which implies that matrix damage has a direct relation to dose-dependent effects in microstructural evolution.

Jiang, J.; Wu, Y. C.; Wang, S. J.; Liu, X. B.; Wang, R. S.

2013-06-01

213

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

214

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

215

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

E-print Network

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

Alexei M. Frolov

2015-01-07

216

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

SciTech Connect

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

Umlor, M.T.; Keeble, D.J. [Michigan Technological Univ., Houghton, MI (United States). Dept. of Physics; Asoka-Kumar, P.; Lynn, K.G. [Brookhaven National Lab., Upton, NY (United States); Cooke, P.W. [Geo-Centers, Inc., Eatontown, NJ (United States). Fort Monmouth Operation

1994-08-01

217

Positron lifetime measurements of hydrogen passivation of cation vacancies in yttrium aluminum oxide garnets  

NASA Astrophysics Data System (ADS)

A charge compensation mechanism is proposed for cation vacancy defects in complex oxides based on positron lifetime measurements, infrared spectroscopy, and composition analysis. Defects were characterized in samples of yttrium aluminum garnet grown in O2 or Ar. However, no positron trapping was detected in samples grown in H2. This is attributed to decoration of cation vacancies with hydrogen, thereby passivating charges of vacancies that otherwise function as positron traps. Infrared spectroscopy gave direct evidence of the presence of hydrogen. Passivation of cation vacancies with hydrogen is proposed as an important mechanism for charge compensation in the defect physics of oxides.

Selim, F. A.; Varney, C. R.; Tarun, M. C.; Rowe, M. C.; Collins, G. S.; McCluskey, M. D.

2013-11-01

218

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

219

Configuration interaction calculations of annihilation rates for positronic complexes of alkali hydrides  

NASA Astrophysics Data System (ADS)

Ab initio multireference single- and double-excitation configuration interaction (MRD-CI) wave functions have been employed to compute the annihilation rates (AR) of positronic molecular complexes of four alkali hydrides. The first step in these calculations is the evaluation of integrals of the two-particle Dirac delta function ?+- over pairs of electronic and positronic basis functions. MRD-CI wave functions calculated with the same basis are then employed to obtain expectation values of the ?+- operator ( Zeff), which in turn are proportional to the corresponding annihilation rates (AR) of the associated many-particle states. The importance of removing near-linear dependencies in the basis sets employed is stressed as well as the advisability of placing diffuse (small-exponent) functions in the basis only at the most electronegative center of the molecule. A tendency to underestimate the Zeff values is noted because of the impracticality of including sufficiently high-l basis functions in the basis for general molecular systems. However, comparison with the relatively accurate values for the four-electron e +LiH complex obtained by Quantum Monte-Carlo (QMC) and other methods indicates that the fractional error is nearly constant over a large range of internuclear distance, consistent with the expectation that missing correlation effects in the MRD-CI treatment are predominantly atomic in nature. A scaling procedure based on the asymptotic ?+- value, which is the same for all four alkali hydrides, is then shown to produce good agreement with the QMC AR data for e +LiH. The same procedure has been applied to the ?+- values for the positronic complexes of the heavier alkali hydrides for which no other theoretical results are available. Trends in the variation of the AR results with bond distance are discussed.

Buenker, Robert J.; Liebermann, Heinz-Peter

2009-03-01

220

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

221

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

SciTech Connect

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

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

2010-09-15

222

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

223

Positron annihilation study of divacancies in silicon illuminated by monochromatic light  

NASA Astrophysics Data System (ADS)

The charge of divacancies in undoped float-zone silicon has been investigated as a function of photon energies between 0.2 and 1.2 eV by means of positron lifetime spectroscopy. For photon energies between 0.25 and 0.65 eV and above 0.75 eV, negatively charged vacancies were formed, whereas in the 0.65 to 0.75 eV range there was no effect from illumination. The results are explainable on the basis of Watkins and Corbett's model of divacancy [Phys. Rev. 138, A543 (1965)]. Evidence was found that the radioactive positron source significantly reduces the stability of the photon-induced population of negatively charged divacancies, an effect which is invoked to explain the apparently anomalous results reported by Kawasuso and Okada [Jpn. J. Appl. Phys., Part 1 36, 605 (1997)].

Dannefaer, S.; Avalos, V.

1999-07-01

224

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

SciTech Connect

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

Edwardson, C. J.; Coleman, P. G. [Department of Physics, University of Bath, Bath BA2 7AY (United Kingdom); El Mubarek, H. A. W.; Gandy, A. S. [School of Electrical and Electronic Engineering, University of Manchester, Manchester, M13 9PL (United Kingdom)

2012-04-01

225

Positrons from supernovae  

NASA Technical Reports Server (NTRS)

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

Chan, Kai-Wing; Lingenfelter, Richard E.

1993-01-01

226

Predictions for Transverse-Momentum Dependence in Electron-Positron Annihilation  

NASA Astrophysics Data System (ADS)

We calculate the transverse momentum dependence of back-to-back production of one hadron and one jet in electron-positron annihilation, i.e., e+e- ? h jet X, at Q2 = 100 GeV2. We use the parameters of the transverse-momentum-dependent (TMD) fragmentation functions we recently extracted from HERMES data at 2.4 GeV2. We apply TMD evolution according to two different approaches and using different parameters for the so-called nonperturbative part of TMD evolution. We discuss the differences in our predictions and how experimental measurements could discriminate between different implementations of TMD evolution and different choices of nonperturbative parameters.

Bacchetta, Alessandro; García-Echevarría, Miguel; Radici, Marco; Signori, Andrea

2015-02-01

227

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

SciTech Connect

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

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

1995-08-01

228

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

229

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

NASA Astrophysics Data System (ADS)

The 16th International Conference on Positron Annihilation (ICPA-16) was held at the University of Bristol, United Kingdom during 19-24 August, 2012. This triennial conference is the foremost gathering of the Positron Annihilation Physics community and it was hosted in the UK for the first time since the series of meetings first started back in 1965. The University of Bristol, the Alma Mater of Paul Dirac, is situated at the heart of the city, and it has established a worldwide reputation in research and teaching. Many of the topics which were discussed during ICPA-16 form an integral part of the research themes in the schools of Physics, Chemistry and Engineering of this University. ICPA-16 attracted a diverse audience, both from academic and industrial institutions, with over 200 participants from 29 countries. It continued the long held tradition of showcasing novel research in the field of positron annihilation and a total of 170 papers were presented as talks and posters. The papers reported studies of metallic and semi-conducting solids, polymers and soft matter, porous materials, surfaces and interfaces, as well as advances in experimental, analytical and biomedical applications. The high quality of the presented work, coupled with the enthusiastic exchange of ideas, provided an invaluable forum, especially for younger researchers and postgraduate students. The excellence of student presentations was acknowledged by the award of prizes for the best student posters, which were received by David Billington (University of Bristol, UK), Moussa Sidibe (CEMHTI, France) and Hongxia Xu (Tohoku University, Japan). All papers published in the Conference Proceedings were reviewed by ICPA-16 participants. We are indebted to all reviewers who contributed their time and intellectual resources, allowing the refereeing and editing process to move smoothly toward the compilation of the Proceedings. Our sincere thanks and gratitude go to everyone who contributed to the success of the conference. We are grateful to all participants for their informative talks, poster presentations and fruitful discussions; the session chairs for keeping to the tight time schedule and for making sure the oral presentation sessions ran smoothly; Maria Dugdale for her time and effort in organising the social programme for the accompanying persons; the student volunteers from the Bristol Positron Group for all their help and time before, during and after the conference; the Bath positron group for helping with the organisation of the excursion and last, but not least, the University of Bristol Conference Office staff for their help with the organisation of the conference. We are also very grateful for the financial and logistical help from the University of Bristol and financial support from our sponsors and exhibitors, Ortec and Canberra. We conclude by wishing the Organising Committee of ICPA-17 all the best for a successful conference. We look forward to seeing everyone in China in 2015. Ashraf Alam, Paul Coleman, Stephen Dugdale and Mina Roussenova Guest Editors Bristol, April 2013 Local organising committeeInternational Advisory committee M A Alam, S Dugdale and M Roussenova P Coleman (UK, Chairman) University of Bristol, UK R Krause-Rehberg(Germany, Vice-chairman) P Coleman and S Townrow M A Alam (UK) University of Bath, UK G Laricchia (UK) M Charlton R Brusa (Italy) University of Swansea, UK M Doyama (Japan) G Laricchia B Ganguly (India) University College London, UK C Hugenschmidt (Germany) D Keeble Zs Kajcsos (Hungary, deceased) University of Dundee, UK Y Kobayashi (Japan) J Kuriplach (Czech Republic) P Mascher (Canada) A Mills (USA) Y Nagashima (Japan) Steering committee M Puska (Finland) M A Alam (UK, Secretary) H Schut (Netherlands) P Coleman (UK) A Seeger (Germany) B Ganguly (India) Y Shirai (Japan) Y Kobayashi (Japan) A Somoza (Argentina) P Mascher (Canada) A Stewart (Canada) H Schut (Netherlands) Z Tang (China) R Krause-Rehberg (Germany) A Weiss (USA) Sponsor logos Conference photograph

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

2013-06-01

230

Multimessenger constraints on the annihilating dark matter interpretation of the positron excess  

SciTech Connect

The rise in the energy spectrum of the positron ratio, observed by the PAMELA satellite above 10 GeV, and other cosmic-ray measurements, have been interpreted as a possible signature of dark matter annihilation in the Galaxy. However, the large number of free parameters, and the large astrophysical uncertainties, make it difficult to draw conclusive statements about the viability of this scenario. Here, we perform a multiwavelength, multimessenger analysis, that combines in a consistent way the constraints arising from different astrophysical observations. We show that if standard assumptions are made for the distribution of dark matter (we build models on the recent Via Lactea II and Aquarius simulations) and the propagation of cosmic rays, current dark matter models cannot explain the observed positron flux without exceeding the observed fluxes of antiprotons or gamma-ray and radio photons. To visualize the multimessenger constraints, we introduce 'star plots', a graphical method that shows in the same plot theoretical predictions and observational constraints for different messengers and wavelengths.

Pato, Miguel; Pieri, Lidia; Bertone, Gianfranco [Institut d'Astrophysique de Paris, 98bis bd Arago, 75014, Paris (France); Dipartimento di Fisica, Universita degli Studi di Padova, Via Marzolo 8, I-35131, Padova (Italy) and Universite Paris Diderot-Paris 7, 10 rue Alice Domon et Leonie Duquet 75205, Paris (France); Institut d'Astrophysique de Paris, 98bis bd Arago, 75014, Paris (France); Institut d'Astrophysique de Paris, UMR7095-CNRS, and Universite Pierre et Marie Curie, 98bis bd Arago, 75014, Paris (France)

2009-11-15

231

Spectroscopy of positron annihilation gamma rays from laser-exited media  

NASA Astrophysics Data System (ADS)

Motivated by calculations for gamma ray yields and results of positron beam measurements from laser irradiated high Z targets [1], a Gamma-ray Crystal Spectrometer (GCS) was built by Artep Inc. and fielded at the Titan laser facility of LLNL. The spectrometer is equipped with heavy shielding around a cylindrically bent Ge crystal in a transmission geometry. The Bremsstrahlung continuum and the 511 keV annihilation gamma rays are dispersed by the Ge(440) crystal and detected by an image plate placed on the Rowland circle. The gamma rays originate inside the thick target material (1 to 3 mm Au disks) where positrons are produced in the intense field of the high energy (350 J) short pulse (10 ps) laser irradiation. In addition to the spectrometer, two different electronic detection systems also recorded the gamma ray spectra using the single hit per pixel technique. The first gamma ray spectra recorded with the crystal spectrometer and the electronic detectors will be reported. [4pt] [1] Hui Chen et al., PRL 105, 015003 (2010)

Szabo, C. I.; Feldman, U.; Seely, J.; Hudson, L.; Chen, Hui; Tommasini, R.; Hazi, A.; Shepherd, R.; Zulick, C.; Dollar, F.; Falk, K.; Murphy, C. D.

2010-11-01

232

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

233

Electron-Positron Annihilation Into Dirac Magnetic Monopole and Antimonopole:. the String Ambiguity and the Discrete Symmetries  

NASA Astrophysics Data System (ADS)

We address the problem of string arbitrariness in the quantum field theory of Dirac magnetic monopoles. Different prescriptions are shown to yield different physical results. The constraints due to the discrete symmetries (C and P) are derived for the process of electron-positron annihilation into the monopole-antimonopole pair. In the case of the annihilation through the one-photon channel, the production of spin-0 monopoles is absolutely forbidden; spin-1/2 monopole and antimonopole should have the same helicities (or, equivalently, the monopole-antimonopole state should be p-wave 1P1).

Ignatiev, A. Yu.; Joshi, G. C.

234

Further search for selectivity of positron annihilation in the skin and cancerous systems  

NASA Astrophysics Data System (ADS)

Positronium annihilation lifetime (PAL) spectroscopy and Doppler broadening energy spectra (DBES) have been used to search for selectivity and sensitivity for cancerous skin samples with and without cancer. This study is to further explore the melanoma cancerous system and other different types of skin samples. We found that the S parameter in melanoma skin samples cut at 0.39 mm depth from the same patient's skin is smaller than near the skin surface. However in 10 melanoma samples from different patients, the S parameters vary significantly. Similarly, among 10 normal skin samples without cancer, the S parameters also vary largely among different patients. To understand the sensitivity of PAS as a tool to detect cancer formation at the early stage, we propose a controlled and systematic study of in vivo experiments using UV-induced cancer skin from living animals.

Liu, Guang; Chen, Hongmin; Chakka, Lakshmi; Cheng, Mei-Ling; Gadzia, Joseph E.; Suzuki, R.; Ohdaira, T.; Oshima, N.; Jean, Y. C.

2008-10-01

235

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

NASA Technical Reports Server (NTRS)

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

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

1978-01-01

236

Positronium hydrides and the Ps{sub 2} molecule: Bound-state properties, positron annihilation rates, and hyperfine structure  

SciTech Connect

The bound-state spectra of the positronium hydrides {sup {infinity}}HPs, TPs, DPs, {sup 1}HPs, and MuPs are considered. The properties of the bound ground S states (L=0) in these systems and the Ps{sub 2} molecule have been determined by extensive variational calculations. The hyperfine structure of these states is also investigated. The positron annihilation rates have been evaluated for the positronium hydrides, the Ps{sub 2} molecule, and the Ps{sup {minus}} ion and compared. The positron annihilation rates {Gamma}{sub n{gamma}} (where n{ge}2) in the positronium hydrides are significantly closer to those in the Ps{sup {minus}} ion (the three-body system) than in the Ps{sub 2} molecule. {copyright} {ital 1997} {ital The American Physical Society}

Frolov, A.M.; Smith, V.H. Jr. [Department of Chemistry, Queens University, Kingston, Ontario, K7L3N6 (CANADA)] [Department of Chemistry, Queens University, Kingston, Ontario, K7L3N6 (CANADA)

1997-04-01

237

Implications of an astrophysical interpretation of PAMELA and Fermi-LAT data for future searches of a positron signal from dark matter annihilations  

E-print Network

The recent data from PAMELA and Fermi-LAT can be interpreted as evidence of new astrophysical sources of high energy positrons. In that case, such astrophysical positrons constitute an additional background against the positrons from dark matter annihilation. In this paper, we study the effect of that background on the prospects for the detection of a positron dark matter signal in future experiments. In particular, we determine the new regions in the (mass, $\\sigmav$) plane that are detectable by the AMS-02 experiment for several dark matter scenarios and different propagation models. We find that, due to the increased background, these regions feature annihilation rates that are up to a factor or three larger than those obtained for the conventional background. That is, an astrophysical interpretation of the present data by PAMELA and Fermi-LAT implies that the detection of positrons from dark matter annihilation is slightly more challenging than previously believed.

Ki-Young Choi; Carlos E. Yaguna

2009-12-17

238

Measurement of the Total Cross Section and Energy - Correlations for Electron-Positron Annihilation Into Hadrons at 29 GEV  

Microsoft Academic Search

This work describes measurements of the total cross section and the energy-energy correlation cross section for hadronic events produced in electron-positron annihilation at a center-of-mass energy of 29 GeV. The performance of the MAC detector at PEP, featuring total absorption calorimetry and charged particle tracking over nearly the full solid angle, is examined and found to meet the original design

Brian Keith Heltsley

1983-01-01

239

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

Microsoft Academic Search

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

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

1973-01-01

240

On Sommerfeld’s approximation in high energy photoelectric effect and one quantum annihilation of positrons in the K -shell  

Microsoft Academic Search

Summary  The angular distribution and the total cross-section for high energy photoelectric effect and one quantum annihilation of\\u000a positrons in theK-shell have been calculated with the use of the Sommerfeld-Maue approximation for the wave function of the scattering state\\u000a of the fermion. The error in this procedure is of relative order (a\\/?), wherea =Z\\/137 and ? is the energy of the

H. Banerjee

1958-01-01

241

Interference suppression in the two-photon annihilation of an electron positron pair in the light wave field  

Microsoft Academic Search

Nonresonant two-photon annihilation of an electron-positron pair in the field of a moderately strong circularly polarized wave is investigated theoretically. The partial cross-section can be summed over all processes of emission and absorption of photons. It is shown that all essentially quantum contributions caused by the Bunkin-Fedorov quantum parameter are compensated for both Feynman diagrams and are not compensated for

O. I. Denisenko; S. P. Roshchupkin; A. I. Voroshilo

2006-01-01

242

Introduction to Positron Emission Tomography  

E-print Network

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

Oakes, Terry

243

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

E-print Network

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

Boyer, Edmond

244

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

E-print Network

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

S. L. Yakovlev; V. A. Gradusov

2012-06-25

245

Measurement of the Neutral D Meson Lifetime (charm, Tasso, Electron-Positron  

NASA Astrophysics Data System (ADS)

The lifetime of the D('0) meson was determined from reconstructed vertices of D('0) mesons produced in e('+)e('-) annihilation. The experiment was performed at the e('+)e('-) storage ring PETRA using the TASSO detector, which included a high resoltuion vertex detector designed for the study of short lived particles. The D('0) mesons were selected from a data sample based on an integrated luminosity of 49 pb('-1) containing 8620 hadronic events whose mean center of mass energy of is 42.2 GeV. The D('0) mesons were identified in the reaction D*('+) (--->) D('0) (pi)('+) with the D('0) decaying into K('-)(pi)('+), K(' -)(pi)('+)(pi)('0), and K('-)(pi)('+)(pi)('-)(pi)('+). From fifteen events, the D('0) lifetime was determined by the method of maximum likelihood to be. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). in good agreement with the existing measurements.

Strom, David Mattill

246

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

SciTech Connect

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

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

2010-09-28

247

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

248

Photo-degradation of Lexan polycarbonate studied using positron lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

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

2013-02-01

249

Native vacancies in semi-insulating GaAs observed by positron lifetime spectroscopy under photoexcitation  

NASA Astrophysics Data System (ADS)

We have performed positron lifetime experiments under monochromatic illumination in undoped semi-insulating GaAs. A negative vacancy, identified as the Ga vacancy, is observed in darkness. Illumination with 1.42 eV photons below 150 K reveals another type of vacancy, identified as the As vacancy. The As vacancy has a negative charge state above the ionization level at 50+/-5 meV below the conduction band. This level offers a microscopic explanation to the optical near-band-edge absorption. The concentrations of both Ga and As vacancies are between 1015 and 1016 cm-3.

Saarinen, K.; Kuisma, S.; Hautojärvi, P.; Corbel, C.; Leberre, C.

1993-05-01

250

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

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

251

Prompt Electron Production in Electron-Positron Annihilations at 29 GEV.  

NASA Astrophysics Data System (ADS)

We have studied the production of prompt electrons in high statistics sample (118 pb('-1)) of multihadron events produced in electron positron annihilations at 29 Gev. The experiment was performed using the DELCO facility on the PEP storage ring at SLAC. Electron identification was done primarily with a large acceptance threshold Cerenkov counter. Both the momentum and the transverse momentum spectra are measured in terms of a differential cross section for electrons having momenta in the range 0.5 < P < 5.5 Gev/c. We measure the inclusive cross section in this momentum range to be 35.8 (+OR-) 3.1 pb. The final distribution of candidates in the P - P(,(PERP)) plane are fit well using a Monte Carlo having a combination of bottom and charm quark decays with the semielectronic branching ratios of (15.0 (+OR-) 2.9)% and (8.9 (+OR-) 1.4)%. We observe no evidence of electron production from new sources and determine a cross section upper limit of 11.6 pb (90% CL) for this process. We find that the fragmentation functions are hard for both b and c quarks, characterized by the values < z(,b) > = 0.77 + 0.05 and < z(,c) > = 0.68 (+OR -) 0.06, where z is the fraction of the heavy quark's energy that is retained by the primary hadron containing the heavy quark. The fragmentation function is fit well by the form. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). Finally, we observe events having two electrons with an inclusive cross section of 2.8 (+OR-) 1.3 pb for this process, which is consistent with the rate expected from the measured semielectronic rates.

Koop, Dale E.

252

Introduction to Positron Emission Tomography  

E-print Network

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

Wisconsin at Madison, University of

253

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

254

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

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

1994-01-01

255

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

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

1994-01-01

256

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

257

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

258

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

259

Ohmic contact formation mechanism of nonalloyed Pd contacts to p-type GaN observed by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

The change of the Fermi energy level at the interface of Pd/p-type GaN by surface treatment was investigated using positron annihilation spectroscopy, and the results were used to provide interpretation of the electrical properties of the contact. Changes in the positron parameters at the interface in the aqua regia-treated GaN are more pronounced than that in the HCl-treated one. This provides evidence that the surface treatment with aqua regia prior to Pd metal deposition removes surface oxides, resulting in the shift of the Fermi level position from a middle of the bandgap to near the valence band. Thus, the barrier for hole injection from metal to p-type GaN is lowered, subsequent to the decrease of contact resistivity by two orders of magnitude.

Lee, Jong-Lam; Weber, Marc; Kim, Jong Kyu; Lee, Jae Won; Park, Yong Jo; Kim, Taeil; Lynn, Kelvin

1999-04-01

260

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

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

261

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

E-print Network

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

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

2010-09-30

262

Microstructural characterization of semi-interpenetrating polymer networks by positron lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

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™-RP46 and thermoplastic LaRC™-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

263

Sensitiveness of the ratio between monovacancy and bulk positron lifetimes to the approximations used in the calculations: Periodic behaviour  

NASA Astrophysics Data System (ADS)

Positron lifetimes have been calculated in bulk and monovacancies 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 parameterizations for the positron enhancement factor and correlation energy. The ratio between the monovacancy and bulk lifetimes has been analyzed. This ratio shows a periodic behaviour with atomic number in all the calculation methods and it is in agreement with selected experimental data. The ratio shows, in contradiction to previous assumptions, sensitiveness to the approximations used in the calculations. This extensive work has allowed us to study and enlighten features of the theory and computing methods broadly used nowadays in simulating, studying and understanding positronic parameters.

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

2012-07-01

264

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

265

Probing the nanostructure of polymers via cryogenic Positron Annihilation Lifetime Spectroscopy (PALS)  

NASA Astrophysics Data System (ADS)

A new method is proposed for the evaluation of the hole volume in amorphous polymers based upon PALS data measured over a temperature of 110 to 480 K. Extrapolation of the "open hole" volume to 0 K allows its separation into that attributed to the segmental motions of the polymer chains (dynamic) and that due to inefficient packing (static). The dynamic hole volume is correlated to thermodynamic volume/density fluctuations and its temperature dependencies are in good agreement with SAXS data. Several thermosetting epoxy materials are also studied over a similar temperature range with the "open hole" volume being separated into its dynamic and static components. How these two components affect diffusional properties of these systems is examined in detail. It is also shown that the o-Ps can localize in a nearly 100material (PET), we therefore conclude that PALS measures more than the "free volume" necessary for segmental motion. Work supported by the Air Force Office of Scientific Research (AFOSR) grant # F49620-95-1-0037.

Bolan, B. A.; Soles, C. L.; Hristov, H. A.; Gidley, D. W.; Yee, A. F.

1996-03-01

266

Vacancy defects in photoexcited GaAs studied by positron two-dimensional angular correlation of annihilation radiation  

NASA Astrophysics Data System (ADS)

The positron two-dimensional angular correlation of annihilation radiation (2D-ACAR) technique has been coupled with optical excitation to study the native point defects in semi-insulating GaAs. The As vacancy was observed below ~170 K when illuminated with 1.41+/-0.07 eV photons. The temperature dependence of the 2D-ACAR spectra, with and without illumination, was studied. Data were also collected at 25 K as a function of the intensity of infrared light. The 2D-ACAR spectra reflect the e+-e- pair momentum distribution at or near the vacancy and provides symmetry and electronic structure information, which can be used as a unique defect signature for the vacancy.

Peng, J. P.; Lynn, K. G.; Umlor, M. T.; Keeble, D. J.; Harshman, D. R.

1994-10-01

267

Reaction of Positronium with Proton in Ultra-stable Y Zeolite Studied by Positrons  

NASA Astrophysics Data System (ADS)

The positron annihilation lifetime spectra were measured as a function of the heating temperature for the ultrastable Y (USY) zeolites. The lifetime spectra were well resolved into five lifetime components, and the longer lifetimes ?3, ?4 and ?5 are assigned to ortho-positronium annihilating in the ?-cage, super-cage and the second pore in USY zeolite, respectively. The interesting observation is that the annihilation rate ?5 (the reciprocal of the ?5) of the fifth component increases and its intensity I5 decreases with the increasing heating temperature. The result is interpreted in terms of the increase in proton and the reaction between the positronium and proton in USY zeolite after heated.

Jun, Zhu; Li, Ma; Shao-Jie, Wang; Xi-Hui, Luo

2000-02-01

268

Thermally- and irradiation-induced sulphur aggregation in Ni-S ageing system studied by positron annihilation  

NASA Astrophysics Data System (ADS)

Pure nickel and very dilute Ni-S systems (containing 50-340 at.ppm sulphur), irradiated by 5 MeV electrons at 270 K, were studied by positron annihilation spectroscopy (PAS) and the electrical resistivity measurements. The ultrafine S-rich precipitates (clusters), which are formed by thermal ageing at about 650 K in Ni-S, are observed clearly by PAS. The PAS data were interpreted by assuming that the positrons localize in the volume of the “defect-free” S-rich clusters due to affinity-induced confinement. Under irradiation, the accumulation of the vacancies in the Ni-S system is several times greater than that in pure Ni. This fact testifies the strong interaction between S atoms and self-interstitial atoms. During post-irradiation annealing, the formation and recovery process of the S-rich clusters are revealed: (i) the mobile vacancy-sulphur complexes, which are formed above 350 K, aggregate into nanovoids; (ii) above 550 K, the dissociation of vacancies from the nanovoids decorated with S-atoms leads to the formation of the S-rich clusters; and (iii) the S-rich clusters anneal out at about 870 K.

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

2014-06-01

269

Positron annihilation spectroscopy study of high-voltage polydimethylsiloxane (PDMS) insulators  

NASA Astrophysics Data System (ADS)

Polydimethylsiloxane (PDMS) compounds are used in outdoor high-voltage applications. Pure PDMS and compounds containing silica and aluminum trihydrate are studied using a mono-energetic positron beam. Large changes in the S parameter profiles as a function of depth from the surface are observed after samples are exposed to corona discharge. In addition, there are significant changes in the profiles on increasing relaxation time after corona treatment. Evidence is found for a silica-like layer at the surface after corona treatment. It is shown that the positron beam technique can provide important information on the mechanism of hydrophobicity loss and recovery that the samples show after corona treatment.

Mallon, P. E.; Greyling, C. J.; Vosloo, W.; Jean, Y. C.

2003-10-01

270

Study of Gadolinium-doped cerium oxide by XRD, TG-DTA, impedance analysis, and positron lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

X-ray diffraction, thermogravimetry differential thermal analysis, impedance analysis, and positron lifetime spectroscopy were conducted for Gadolinium-doped cerium oxides (GDC) prepared using oxalate coprecipitation method. XRD revealed the fluorite structure indicating that gadolinium is successfully doped into cerium oxide. Prior to sintering, the vacancy-sized free volume and nanovoid were observed at grain boundaries. The vacancy-sized free volumes shrank with increasing sintering temperatures and finally got dominant.

Ohta, S.; Kosaka, T.; Sato, K.

2010-04-01

271

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

NASA Technical Reports Server (NTRS)

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

Ruderman, Malvin

1993-01-01

272

Positronics of subnanometer atomistic imperfections in solids as a high-informative structure characterization tool  

NASA Astrophysics Data System (ADS)

Methodological possibilities of positron annihilation lifetime (PAL) spectroscopy applied to characterize different types of nanomaterials treated within three-term fitting procedure are critically reconsidered. In contrast to conventional three-term analysis based on admixed positron- and positronium-trapping modes, the process of nanostructurization is considered as substitutional positron-positronium trapping within the same host matrix. Developed formalism allows estimate interfacial void volumes responsible for positron trapping and characteristic bulk positron lifetimes in nanoparticle-affected inhomogeneous media. This algorithm was well justified at the example of thermally induced nanostructurization occurring in 80GeSe2-20Ga2Se3 glass.

Shpotyuk, Oleh; Filipecki, Jacek; Ingram, Adam; Golovchak, Roman; Vakiv, Mykola; Klym, Halyna; Balitska, Valentyna; Shpotyuk, Mykhaylo; Kozdras, Andrzej

2015-02-01

273

Electron-positron annihilation into three pions and the radiative return  

E-print Network

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

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

2006-06-20

274

Electroweak corrections to three-jet production in electron-positron annihilation  

E-print Network

We compute the electroweak ${\\cal O}(\\alpha^3\\alpha_s)$ corrections to three-jet production and related event-shape observables at electron-positron colliders. We properly account for the experimental photon isolation criteria and for the corrections to the total hadronic cross section. Corrections to the three-jet rate and to normalised event-shape distributions turn out to be at the few-per-cent level.

Ansgar Denner; Stefan Dittmaier; Thomas Gehrmann; Christian Kurz

2009-08-03

275

Lambda production in electron-positron annihilation at 29 GeV  

SciTech Connect

The inclusive cross-secton for the production of the singly-strange baryons lambda and anti lambda, along with the differential cross-sections in momentum and energy, are measured by e/sup +/e/sup -/ annihilation at a center-of-mass energy of 29GeV. The charged decay mode lambda ..-->.. p..pi.. is used in a search for polarization. Such a polarization may be used as a check of CP invariance in lambda production. The sample of events with two detected decays is analyzed for correlations in production angle. 43 refs., 44 figs.

Baden, A.R.

1986-08-01

276

Nanostructural evolution in surveillance test specimens of a commercial nuclear reactor pressure vessel studied by three-dimensional atom probe and positron annihilation  

Microsoft Academic Search

The nanostructural evolution of irradiation-induced Cu-rich nanoprecipitates (CRNPs) and vacancy clusters in surveillance test specimens of in-service commercial nuclear reactor pressure vessel steel welds of Doel-1 and Doel-2 are revealed by combining the three-dimensional local electrode atom probe and positron annihilation techniques. In both medium (0.13wt.%) and high (0.30wt.%) Cu welds, the CRNPs are found to form readily at the

T. Toyama; Y. Nagai; Z. Tang; M. Hasegawa; A. Almazouzi; E. van Walle; R. Gerard

2007-01-01

277

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

278

Investigation of free volume and the interfacial, and toughening behavior for epoxy resin/rubber composites by positron annihilation  

NASA Astrophysics Data System (ADS)

An epoxy resin (EP) matrix has been modified with carboxyl-randomized butadiene-acrylonitrile (CRBN) rubber and hydroxyl-terminated butadiene-acrylonitrile rubber (HTBN). When the rubber content was low (?5%), the free volume size varied slowly, meanwhile, the free volume size of EP/HTBN system was smaller than EP/CRBN system. With further addition of rubber component (?7%), the free volume size of the composite increased quickly. In this case, the free volume size of EP/HTBN system was larger than EP/CRBN system. The intermediate positron lifetime component ( I2) was affected by the structure of the interface layer between rubber particles and EP matrix. The different free volume properties could be attributed to the compatibility between rubber and EP.

Minfeng, Zeng; Xudong, Sun; Huiquan, Xiao; Genzhong, Ji; Xuewen, Jiang; Baoyi, Wang; Chenze, Qi

2008-03-01

279

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

SciTech Connect

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

2007-03-21

280

Positron spectroscopy for materials characterization  

SciTech Connect

One of the more active areas of research on materials involves the observation and characterization of defects. The discovery of positron localization in vacancy-type defects in solids in the 1960's initiated a vast number of experimental and theoretical investigations which continue to this day. Traditional positron annihilation spectroscopic techniques, including lifetime studies, angular correlation, and Doppler broadening of annihilation radiation, are still being applied to new problems in the bulk properties of simple metals and their alloys. In addition new techniques based on tunable sources of monoenergetic positron beams have, in the last 5 years, expanded the horizons to studies of surfaces, thin films, and interfaces. In the present paper we briefly review these experimental techniques, illustrating with some of the important accomplishments of the field. 40 refs., 19 figs.

Schultz, P.J.; Snead, C.L. Jr.

1988-01-01

281

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

282

Diffuse 0.511 MeV line emission and the distribution of positron annihilation in the galaxy  

NASA Technical Reports Server (NTRS)

We model the distribution in the Galaxy of 0.511 MeV line emission and the implied positron annihilation using plausible distributions based on observations at other photon energies. We use 0.511 MeV line observations from the Galactic center and from directions away from the Galactic center to normalize the two-dimensional 0.511 MeV sky maps. We find that in order to understand all of the available data it is necessary to invoke the presence of a time-variable component of 0.511 MeV line emission in addition to an underlying diffuse component. The large 0.511 MeV fluxes observed with broad field of view detectors, such as the SMM gamma-ray spectrometer, can be reconciled with the small fluxes seen with narrow field-of-view instruments (GRIS and OSSE) from directions away from the Galactic center if the diffuse emission follows the distribution of Galactic novae for which the recently detected 0.511 MeV line emission from the Galactic center with OSSE is predominantly of diffuse origin. For future GRIS and HEXAGONE observations from the direction of the Galactic center we predict a minimum 0.511 MeV line flux of (6.0 +/- 0.9) x 10 exp -4 photons/sq cm/s.

Skibo, J. G.; Ramaty, R.; Leventhal, M.

1992-01-01

283

Helium-Implantation-Induced Damage in NHS Steel Investigated by Slow-Positron Annihilation Spectroscopy  

NASA Astrophysics Data System (ADS)

Evolutions of defects and helium contained defects produced by atomic displacement and helium deposition with helium implantation at different temperatures in novel high silicon (NHS) steel are investigated by a slow positron beam. Differences of the defect information among samples implanted by helium to a fluence of 1 × 1017 ions/cm2 at room temperature, 300°C, 450°C and 750°C are discussed. It is found that the mobility of vacancies and vacancy clusters, a recombination of vacancy-type defects and the formation of the He-V complex lead to the occurrence of these differences. At high temperature irradiations, a change of the diffusion mechanism of He atoms/He bubbles might be one of the reasons for the change of the S-parameter.

Li, Yuan-Fei; Shen, Tie-Long; Gao, Xing; Gao, Ning; Yao, Cun-Feng; Sun, Jian-Rong; Wei, Kong-Fang; Li, Bing-Sheng; Zhang, Peng; Cao, Xing-Zhong; Zhu, Ya-Bin; Pang, Li-Long; Cui, Ming-Huan; Chang, Hai-Long; Wang, Ji; Zhu, Hui-Ping; Wang, Dong; Song, Peng; Sheng, Yan-Bin; Zhang, Hong-Peng; Hu, Bi-Tao; Wang, Zhi-Guang

2014-03-01

284

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

285

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

E-print Network

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

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

2009-11-06

286

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

E-print Network

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

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

2011-11-11

287

Two-dimensional analysis of positron age-momentum correlation (AMOC) data  

Microsoft Academic Search

By measuring the individual positron lifetimes (= positron ages) together with the energies of one of the annihilation quanta in a triple-coincidence set-up (age-momentum correlation, AMOC), time-resolved information on the evolution of the positron states may be obtained. The present paper describes a data evaluation procedure that makes full use of the time-resolved information provided by the AMOC relief (number

A. Siegle; H. Stoll; P. Castellaz; J. Major; H. Schneider; A. Seeger

1997-01-01

288

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

289

Heavy Quark Jets from Electron-Positron Annihilation at 29 GEV.  

NASA Astrophysics Data System (ADS)

We have studied the production of prompt leptons (e,(mu)) in hadronic events from e('+)e('-) annihilation at a center-of-mass energy of 29 Gev with the TPC detector at SLAC. The total integrated luminosity was 77 pb('-1). The lepton p and p(,t) distributions were well described by a combination of bottom and charm quark decays. We derived neutral axial vector couplings of a(b quark) = -1.1 (+OR -) 0.9 (+OR-) 0.3 and a(c quark) = 1.9 (+OR-) 1.0 (+OR-) 0.4 from the forward-backward asymmetries in good agreement with the Glashow-Weinberg Salam model. In addition, we compared these events with the average multi- hadron events in order to examine differences between jets with different original quark types. This was the first measurement of the differential cross section for (pi)('(+OR-)) and the mean multiplicities for different species of hadrons in different quark events. We found an average of 13.8 (+OR-) 1.0 (pi)('(+OR-)), 1.7 (+OR-) 0.6 K('(+OR-)) and 0.7 (+OR-) 0.6 p('(+OR-)) per b event, 10.8 (+OR-) 0.9 (pi)('(+OR-)), 2.0 (+OR-) 0.5 K('(+OR-)) and 0.6 (+OR-) 0.5 p('(+OR-)) per c event, and 10.1 (+OR -) 0.9 (pi)('(+OR-)), 0.9 (+OR-) 0.4 K('(+OR-)), and 0.6 (+OR-) 0.4 p('(+OR-)) per u,d,s event, to be com- pared with 10.7 (+OR-) 0.6 (pi)('(+OR-)), 1.4 (+OR-) 0.1 K('(+OR -)) and 0.6 (+OR-) 0.1 p('(+OR-)) per average event. We found a significant difference between the (pi)('(+OR-)) and hadron multiplicities in b(')b events versus light quark events, and a significant difference between the K('(+OR-)) multiplicities in heavy quark events versus light quark events. We also found the corresponding 'non -leading' multiplicities of charged particles accompanying the pair of heavy. hadrons, and then determined that bottom and charm hadrons fragment with mean energy fractions of. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI).

Lu, Xing-Qi.

290

Experimental Tests of QCD and Fragmentation Models in Electron-Positron Annihilation.  

NASA Astrophysics Data System (ADS)

This thesis presents a detailed study of the hadronic data obtained with the MARK J detector from e^+e ^- annihilation. The predictions of several new combinations of Monte Carlo parton generators and fragmentation schemes have been tuned and compared directly to the data. A study, performed in the limit of perfect detector resolution and efficiency, has shown that the string effect cannot be attributed to soft gluon coherence effects occurring at the soft perturbative stage of the event evolution. It is found that all the fragmentation schemes employing the string formalism are unable to correctly describe the form of the Thrust and Energy-Energy Correlation distributions obtained with the high energy data above 40 GeV center-of-mass energy. The problems encountered have their origins in the modelling of the fragmentation process rather than in the perturbative QCD aspects of the models. All the models investigated, however, are capable of describing a board range of distributions at the lower center-of-mass energies. No evidence for the presence of the string effect has been found in the MARK J data. The new fragmentation models have been used to extract values for the strong coupling constant from the data. The systematic uncertainties assigned to previous alpha_{s} measurements have been re-assessed in the light of this study, and it has been found that previous error assignments require no adjustment. A recent 2^{nd} order calculation for the three-jet partial cross section has been investigated and converted into a form where it can be directly compared to previous calculations. The calculations are found to be in excellent agreement, and it is concluded that the systematic errors that should be assigned to alpha_{s} measurements due to theoretical uncertainties are negligible. Motivated by the observation of an excess number of low thrust inclusive muon events at the highest PETRA energies, the MARK J hadronic data above 46.3 GeV has been compared to a lower energy data sample. A brief study of the characteristics to be expected at LEP energies for events originating in the production of top quarks has been made, and a method for isolating a top-enriched sample has been investigated. (Abstract shortened with permission of author.).

Stone, Howard David

291

General relativistic ray-tracing algorithm for the determination of the electron-positron energy deposition rate from neutrino pair annihilation around rotating neutron and quark stars  

NASA Astrophysics Data System (ADS)

We present a full general relativistic numerical code for estimating the energy-momentum deposition rate (EMDR) from neutrino pair annihilation (?). The source of the neutrinos is assumed to be a neutrino-cooled accretion disc around neutron and quark stars. We calculate the neutrino trajectories by using a ray-tracing algorithm with the general relativistic Hamilton's equations for neutrinos and derive the spatial distribution of the EMDR due to the annihilations of neutrinos and antineutrinos around rotating neutron and quark stars. We obtain the EMDR for several classes of rotating neutron stars, described by different equations of state of the neutron matter, and for quark stars, described by the Massachusetts Institute of Technology (MIT) bag model equation of state and in the colour-flavour-locked (CFL) phase. The distribution of the total annihilation rate of the neutrino-antineutrino pairs around rotating neutron and quark stars is studied for isothermal discs and accretion discs in thermodynamical equilibrium. We demonstrate both the differences in the equations of state for neutron and quark matter and rotation with the general relativistic effects significantly modify the EMDR of the electrons and positrons generated by the neutrino-antineutrino pair annihilation around compact stellar objects, as measured at infinity.

Kovács, Z.; Harko, T.

2011-11-01

292

Mass energy-transfer and mass energy-absorption coefficients, including in-flight positron annihilation for photon energies 1 keV to 100 MeV  

SciTech Connect

Mass energy-transfer (Mu(sub u)/Rho) and mass energy-absorption coefficients (Mu(sub en)/Rho) are tabulated in units of (sqcm/g) for photon energies between 1 keV and 100 MeV for 29 elements (Z = 1-92), and 14 mixtures and compounds of general dosimetric interest. Cross sections for photo-effect, incoherent scattering, pair and triplet production are those compiled or generated by the National Institute of Standards and Technology (NIST) (formerly the National Bureau of Standards). Corrections are included for in-flight positron annihilation, previously not applied in NIST calculations for energies above 10 MeV.

Higgins, P.D.; Attix, F.H.; Hubbell, J.H.; Seltzer, S.M.; Berger, M.J.

1991-11-01

293

A Monte-Carlo simulation of positron diffusion in solids  

NASA Astrophysics Data System (ADS)

A Monte-Carlo simulation of positron diffusion in layered structures is described. This simulation is a useful tool to extract physical properties of slow positron data. The basis of the simulation is the random-walk model of the positron diffusion (C. Hübner, T. Staab and R. Krause-Rehberg, Appl. Phys. A 61 (1995) 203) [1]. The input parameters are the temperature, the positron diffusion constants, the positron lifetimes, the positron trapping rates, and the S parameters of bulk and defect annihilation. Furthermore, assumptions on the sample layer structure, the properties of surface and interfaces, the structure of the electric field, and the positron implantation profile must be included. The results of the simulation are the S parameter, F parameter, or positron lifetimes as a function of positron implantation energy, respectively. This means that a real positron beam experiment is simulated. The resulted data can directly be compared to measured curves. A typical simulation of layered structures, such as a Schottky contact (Au-film on Si), was performed, and the data were compared with experimental data of the literature. Furthermore, we studied the effect of internal and external electric fields on defect profiling in semiconductors by slow positron beam technique.

Eichler, S.; Hübner, C.; Krause-Rehberg, R.

1997-05-01

294

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

1998-01-01

295

Beam-based age-momentum correlation studies of positronium spin conversion in paramagnetic solutions and of positron trapping at defects in diamonds  

Microsoft Academic Search

Correlated measurements of the lifetime and of the Doppler broadening of the 511 keV annihilation radiation of positrons (Age-Momentum Correlation, AMOC) using an MeV positron beam have become a powerful tool for investigating reactions of positrons or positronium as a function of time. The room-temperature reaction rate of the spin conversion of positronium in methanol induced by the presence of

H. Stoll; M. Koch; U. Lauff; K. Maier; J. Major; A. Seeger; P. Wesolowski; I. Billard; J. Ch. Abbe´; S. H. Connell; J. P. F. Sellschop; E. Sideras-Haddad; K. Bharuth-Ram; H. Haricharun

1994-01-01

296

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

297

Direct Observation of the Surface Segregation of Cu in Pd by Time-Resolved Positron-Annihilation-Induced Auger Electron Spectroscopy  

SciTech Connect

Density functional theory calculations predict the surface segregation of Cu in the second atomic layer of Pd which has not been unambiguously confirmed by experiment so far. We report measurements on Pd surfaces covered with three and six monolayers of Cu using element selective positron-annihilation-induced Auger electron spectroscopy (PAES) which is sensitive to the topmost atomic layer. Moreover, time-resolved PAES, which was applied for the first time, enables the investigation of the dynamics of surface atoms and hence the observation of the segregation process. The time constant for segregation was experimentally determined to {tau}=1.38(0.21) h, and the final segregated configuration was found to be consistent with calculations. Time-dependent PAES is demonstrated to be a novel element selective technique applicable for the investigation of, e.g., heterogeneous catalysis, corrosion, or surface alloying.

Mayer, J.; Hugenschmidt, C.; Schreckenbach, K. [ZWE FRM II, Lichtenbergstrasse 1, 85747 Garching (Germany); Physik Department E21, Technische Universitaet Muenchen, James Franck Strasse, 85748 Garching (Germany)

2010-11-12

298

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

299

Positron annihilation study of the influence of doping on the 3 d electron states in the Ni3Al intermetallic compound  

NASA Astrophysics Data System (ADS)

The 3 d electron states in Ni3Al single crystals doped with Fe, Co, and Nb have been investigated using angular correlation of annihilation radiation (ACAR). The ACAR spectra contain information on the momentum distribution of valence electrons and strongly bound 3 d electrons of the intermetallic compound. It has been established that the positrons in the Ni3Al crystals predominantly annihilate in the nickel sublattice from delocalized states. The doping of the compound by the third element leads to a variation in the momentum distribution of Ni 3 d electrons due to the change in the character of interatomic bonds. An analysis of the momentum distribution has demonstrated that the niobium atoms increase the covalent component of the chemical bond as compared to the binary compound due to the d Nb- d Ni hybridization. The doping with cobalt atoms also enhances the tendency toward the formation of the covalent bond. At the same time, iron atoms have a weak effect on the electronic structure of the intermetallic compound.

Druzhkov, A. P.; Perminov, D. A.; Stepanova, N. N.

2010-10-01

300

Present status of the low energy linac-based slow positron beam and positronium spectrometer in Saclay  

NASA Astrophysics Data System (ADS)

A new slow positron beamline featuring a large acceptance positronium lifetime spectrometer has been constructed and tested at the linac-based slow positron source at IRFU CEA Saclay, France. The new instrument will be used in the development of a dense positronium target cloud for the GBAR experiment. The GBAR project aims at precise measurement of the gravitational acceleration of antihydrogen in the gravitational field of the Earth. Beyond application in fundamental science, the positron spectrometer will be used in materials research, for testing thin porous films and layers by means of positronium annihilation. The slow positron beamline is being used as a test bench to develop further instrumentation for positron annihilation spectroscopy (Ps time-of-flight, pulsed positron beam). The positron source is built on a low energy linear electron accelerator (linac). The 4.3 MeV electron energy used is well below the photoneutron threshold, making the source a genuine on-off device, without remaining radioactivity. The spectrometer features large BGO (Bismuth Germanate) scintillator detectors, with sufficiently large acceptance to detect all ortho-positronium annihilation lifetime components (annihilation in vacuum and in nanopores).

Liszkay, L.; Comini, P.; Corbel, C.; Debu, P.; Grandemange, P.; Pérez, P.; Rey, J.-M.; Reymond, J.-M.; Ruiz, N.; Sacquin, Y.; Vallage, B.

2014-04-01

301

Structure and sublimation of water ice films grown in vacuo at 120-190 K studied by positron and positronium annihilation.  

PubMed

The crystalline structure of ? 5-20 ?m water ice films grown at 165 and 172 K has been probed by measuring the fraction of positrons forming ortho-positronium (ortho-Ps) and decaying into three gamma photons. It has been established that films grown at slower rates (water vapour pressure ? 1 mPa) have lower concentrations of lattice defects and closed pores, which act as Ps traps, than those grown at higher rates (vapour pressure ? 100 mPa), evidenced by ortho-Ps diffusion lengths being approximately four times greater in the former. By varying the growth temperature between 162 and 182 K it was found that films become less disordered at temperatures above ? 172 K, with the ortho-Ps diffusion length rising by ? 60%, in this range. The sublimation energy for water ice films grown on copper has been measured to be 0.462(5) eV using the time dependence of positron annihilation parameters from 165 to 195 K, in agreement with earlier studies and with no measurable dependence on growth rate and thermal history. PMID:24599176

Townrow, S; Coleman, P G

2014-03-26

302

Detection of a feature at 0.44 MeV in the Crab pulsar spectrum with FIGARO II - A redshifted positron annihilation line?  

NASA Astrophysics Data System (ADS)

The balloon-borne experiment FIGARO II was launched from the base of Trapani-Milo on July 9, 1990 at 0433 UT and observed the Crab pulsar for about 5 hr. The light curve of the signal from PSR 0531 + 21 folded with the radio period shows clearly the known double-peak structure. The spectrum of the second peak, the dominant structure in the hard X-rays and low-energy gamma rays, is characterized by a feature which, if interpreted as an emission line, gives the intensity of (0.86 + or - 0.33) x 10 to the -4th photons/sq cm per sec at the energy of 0.44 + or - 0.01 MeV at a confidence level of 99.6 percent. If this feature is the signature of positron annihilation close to the neutron star surface, redshifted by the intense gravitational field, it is possible to estimate the mass-to-radius ratio of the Crab neutron star and the positron production rate from one of the polar caps. The former is equal to 0.087 solar masses/km and the latter 8.2 x 10 to the 39th e(+)/s for a pencil-beam pattern, in reasonable agreement with some heuristic estimates based on the number of energetic particles radiating the optical pulses.

Massaro, E.; Matt, G.; Salvati, M.; Costa, E.; Mandrou, P.; Niel, M.; Olive, J. F.; Mineo, T.; Sacco, B.; Scarsi, L.; Gerardi, G.; Agrinier, B.; Barouch, E.; Comte, R.; Parlier, B.; Masnou, J. L.

1991-07-01

303

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

304

Study of the optimal conditions for NV- center formation in type 1b diamond, using photoluminescence and positron annihilation spectroscopies  

E-print Network

We studied the parameters to optimize the production of negatively-charged nitrogen-vacancy color centers (NV-) in type~1b single crystal diamond using proton irradiation followed by thermal annealing under vacuum. Several samples were treated under different irradiation and annealing conditions and characterized by slow positron beam Doppler-broadening and photoluminescence (PL) spectroscopies. At high proton fluences another complex vacancy defect appears limiting the formation of NV-. Concentrations as high as 2.3 x 10^18 cm^-3 of NV- have been estimated from PL measurements. Furthermore, we inferred the trapping coefficient of positrons by NV-. This study brings insight into the production of a high concentration of NV- in diamond, which is of utmost importance in ultra-sensitive magnetometry and quantum hybrid systems applications.

Botsoa, J; Adam, M -P; Desgardin, P; Leoni, E; Courtois, B; Treussart, F; Barthe, M -F

2011-01-01

305

Study of the optimal conditions for NV- center formation in type 1b diamond, using photoluminescence and positron annihilation spectroscopies  

E-print Network

We studied the parameters to optimize the production of negatively-charged nitrogen-vacancy color centers (NV-) in type~1b single crystal diamond using proton irradiation followed by thermal annealing under vacuum. Several samples were treated under different irradiation and annealing conditions and characterized by slow positron beam Doppler-broadening and photoluminescence (PL) spectroscopies. At high proton fluences another complex vacancy defect appears limiting the formation of NV-. Concentrations as high as 2.3 x 10^18 cm^-3 of NV- have been estimated from PL measurements. Furthermore, we inferred the trapping coefficient of positrons by NV-. This study brings insight into the production of a high concentration of NV- in diamond, which is of utmost importance in ultra-sensitive magnetometry and quantum hybrid systems applications.

J. Botsoa; T. Sauvage; M. -P. Adam; P. Desgardin; E. Leoni; B. Courtois; F. Treussart; M. -F. Barthe

2011-08-24

306

Constraints on Resonant Dark Matter Annihilation  

E-print Network

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

Backovic, Mihailo

2011-07-04

307

Positron accumulation effect in particles embedded in a low-density matrix  

NASA Astrophysics Data System (ADS)

Systematic studies of the so-called positron accumulation effect for samples with particles embedded in a matrix are reported. This effect is related to energetic positrons which penetrate inhomogeneous medium. Due to differences in the linear absorption coefficient, different amounts of positrons are accumulated and annihilate in the identical volume of both materials. Positron lifetime spectroscopy and Doppler broadening of the annihilation line using Na-22 positrons were applied to the studies of the epoxy resin samples with embedded micro-sized particles of transition metals, i.e., Ni, Sn, Mo, W, and nonmetal particles, i.e., Si and NaF. The significant difference between the determined fraction of positrons annihilating in the particles and the particle volume fraction indicates the positron accumulation effect. The simple phenomenological model and Monte Carlo simulations are able to describe the main features of the obtained dependencies. The aluminum alloy with embedded Sn nanoparticles is also considered for demonstration differences between the accumulation and another related effect, i.e., the positron affinity.

Dryzek, Jerzy; Siemek, Krzysztof

2015-02-01

308

Inclusive Electron Production from Heavy Quarks with the Tasso Lead-Liquid Argon Calorimeters in Electron Positron Annihilation at PETRA  

NASA Astrophysics Data System (ADS)

The production of electrons from the decay of bottom and charm hadrons has been studied in e('+)e('-) annihilation at 34.6 GeV center of mass energy using lead -liquid argon calorimeters. It is observed that the b quark fragmentation function is peaked at large values of the scaling variable z with. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). For c quarks. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). is observed. A forward-backward charge asymmetry of A = -0.25 (+OR-) 0.22 was measured in b production. As the calorimeters and the methods of pattern recognition are well suited to (pi)('0) identification, the inclusive (pi)('0) cross section has also been measured.

Wicklund, Eric John

309

Correlation of polycrystalline Cu(In,Ga)Se2 device efficiency with homojunction depthand interfacial structure: X-ray photoemission and positron annihilation spectroscopic characterization  

NASA Astrophysics Data System (ADS)

Angled-resolved high resolution photoemission measurements on valence band electronic structure and Cu 2p, In 3d, Ga 2p, and Se 3d core lines were used to evaluate surface and near-surface chemistry of CuInSe2 and Cu(In,Ga)Se2 device grade thin films. XPS compositional depth profiles were also acquired from the near-surface region, and bonding of the Cu, In, Ga, and Se was determined as a function of depth. A Cu-poor region was found, indicating CuIn5Se8 or a CuIn3Se5-In2Se3 mixture. Correlation between the depth of the Cu-poor region/bulk interface and device efficiency showed that the depth was 115 angstroms for a 16.4% CIGS device, 240 angstroms for a 15.0% CIGS, and 300 angstroms for 14.0% CIGS, with similar trends for CIS films. The surface region is n-type, the bulk is p-type, with a 0.5 eV valence band offset. Depth of homojunction may be the determining factor in device performance. Positron annihilation spectroscopy gave similarly illuminating results.

Nelson, A. J.; Sobol, P. E.; Gabor, A. M.; Contreras, M. A.; Asoka-Kumar, P.; Lynn, K. G.

1994-12-01

310

Calculation of the expectation values of the spin and the magnetic moment of the gamma photons created as a result of the electron-positron annihilation  

E-print Network

We have calculated the expectation values of the spin and the intrinsic magnetic moment of the gamma photons created as a result of the electron-positron annihilation. We show that, depending on its helicity a gamma photon propagating in z direction with an angular frequency carries a magnetic moment of along the propagation direction. Here the (+) and (-) signs stand for the right hand and left circular helicity respectively. We also show that whatever the helicity is, the spin of each gamma photon is equal to zero (but not !). We argue that in a Stern-Gerlach experiment (SGE) the magnetic moment is an important property but not the spin of the particles. Because of these two symmetric values of the magnetic moment, we expect a splitting of the gamma photon beam into two symmetric subbeams in a (SGE). We believe that the present result will be helpful for understanding the recent attempts on the (SGE) with slow light and the behavior of the dark polaritons and also the atomic spinor polaritons.

Mesude Saglam; Ziya Saglam

2012-01-20

311

Oxygen-Atom Defects In 6H Silicon Carbide Implanted Using 24- MeV O{sup 3+} Ions Measured Using Three-Dimensional Positron Annihilation Spectroscopy System (3DPASS)  

SciTech Connect

Three dimensional electron-positron (e{sup -}-e{sup +}) momentum distributions were measured for single crystal 6H silicon carbide (SiC); both virgin and having implanted oxygen-atom defects. 6H SiC samples were irradiated by 24- MeV O{sup 3+} ions at 20 particle-nanoamps at the Sandia National Laboratory's Ion Beam Facility. O{sup 3+} ions were implanted 10.8 {mu}m deep normal to the (0001) face of one side of the SiC samples. During positron annihilation measurements, the opposite face of the 254.0-{mu}m thick SiC samples was exposed to positrons from a {sup 22}Na source. This technique reduced the influence on the momentum measurements of vacancy-type defects resulting from knock-on damage by the O{sup 3+} ions. A three-dimensional positron annihilation spectroscopy system (3DPASS) was used to measure e{sup -}-e{sup +} momentum distributions for virgin and irradiated 6H SiC crystal both before and following annealing. 3DPASS simultaneously measures coincident Doppler-broadening (DBAR) and angular correlation of annihilation radiation (ACAR) spectra. DBAR ratio plots and 2D ACAR spectra are presented. Changes in the momentum anisotropies relative to crystal orientation observed in 2D ACAR spectra for annealed O-implanted SiC agree with the local structure of defect distortion predicted using Surface Integrated Molecular Orbital/Molecular Mechanics (SIMOMM). Oxygen atoms insert between Si and C atoms increasing their separation by 0.9 A forming a Si-O-C bond angle of {approx}150 deg.

Williams, Christopher S.; Petrosky, James C.; Burggraf, Larry W. [Air Force Institute of Technology, AFIT/ENP, 2950 Hobson Way, Wright-Patterson Air Force Base, OH 45433 (United States); Duan, Xiaofeng F. [Air Force Research Laboratory's (AFRL) Department of Defense Supercomputing Resource Center (DSRC), Wright-Patterson Air Force Base, OH 45433 (United States)

2011-06-01

312

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

313

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

314

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

315

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

316

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

317

Influence of fillers on free volume and gas barrier properties in styrene-butadiene rubber studied by positrons  

Microsoft Academic Search

The measurements of free-volume hole property by positron annihilation lifetime spectroscopy (PALS) have been carried out for polymer–clay nanocomposite materials that consist of styrene–butadiene rubber (SBR) and layered silicate clay of rectorite and conventional composite materials N326 (carbon black) \\/SBR. The PALS and differential scanning calorimeter (DSC) results show layered rectorite has a stronger effect on restraining polymer chain mobility

Z. F. Wang; B. Wang; N. Qi; H. F. Zhang; L. Q. Zhang

2005-01-01

318

Annihilation in Gases and Galaxies  

NASA Technical Reports Server (NTRS)

This publication contains most of the papers, both invited and contributed, that were presented at the Workshop of Annihilation in Gases and Galaxies. This was the fifth in a biennial series associated with the International Conference on the Physics of Electronic and Atomic Collisions. Subjects covered included the scattering and annihilation of positrons and positronium atoms in various media, including those of astrophysical interest. In addition, the topics of antimatter and dark matter were covered.

Drachman, Richard J. (editor)

1990-01-01

319

Mass energy-transfer and mass energy-absorption coefficients, including in-flight positron annihilation for photon energies 1 keV to 100 MeV, 1992  

SciTech Connect

Mass energy-transfer and mass energy-absorption coefficients are tabulated in units of sq cm/g for photon energies between 1 keV and 100 MeV for 29 elements (Z = 1-92), and 14 mixtures and compounds of general dosimetric interest. Cross sections for photo-effect, incoherent scattering, pair and triplet production are those compiled or generated by the National Institute of Standards and Technology (NIST) (formerly the National Bureau of Standards). Corrections are included for in-flight positron annihilation, previously not applied in NIST calculations for energies above 10 MeV. Agreement with recently published data is good for energies above 1 MeV, but the authors find differences in mass energy-absorption coefficients in the low energy region of as much as 4% compared with the last NIST compilation, and as much as 9% when compared with other recent compilations.

Higgins, P.D.; Attix, F.H.; Hubbell, J.H.; Seltzer, S.M.; Berger, M.J.

1992-03-01

320

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

E-print Network

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

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

2015-01-01

321

Portable Positron Measurement System (PPMS)  

ScienceCinema

Portable Positron Measurement System (PPMS) is an automated, non-destructive inspection system based on positron annihilation, which characterizes a material's in situatomic-level properties during the manufacturing processes of formation, solidification, and heat treatment. Simultaneous manufacturing and quality monitoring now are possible. Learn more about the lab's project on our facebook site http://www.facebook.com/idahonationallaboratory.

None

2013-05-28

322

Study on the microstructure and mechanical properties for epoxy resin/montmorillonite nanocomposites by positron  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetimes have been measured for epoxy resin/organic montmorillonite (OMMT) nanocomposites. Effects of different dispersion states of nano-layered OMMT on the positron annihilation parameters and the mechanical properties were studied. We found that the ortho-positronium ( o-Ps) intensity decreased with increasing OMMT content, which indicated that the interaction between the host and nanofillers restrained the segmental motion, resulting in a decrease of the free volume. On the other hand, it is very interesting to observe a good correlation between the interfacial interaction and mechanical properties, suggesting that the dispersion states of OMMT and interfacial property between clay layers and matrix played an important role in determining the mechanical properties.

Wang, B.; Qi, N.; Gong, W.; Li, X. W.; Zhen, Y. P.

2007-02-01

323

Vacancy-type defects introduced by plastic deformation of GaN studied using monoenergetic positron beams  

NASA Astrophysics Data System (ADS)

Vacancy-type defects in plastically deformed GaN were studied using monoenergetic positron beams. Dislocations with a Burgers vector of (1/3)[12¯10] were introduced by applying compressive stress at 950 °C. Measurements of Doppler broadening spectra of the annihilation radiation and positron lifetime spectra showed that both microvoids and Ga-vacancy-type defects were introduced into the deformed sample. The former defects are considered to be introduced through an agglomeration of vacancies introduced by dislocation motions. We observed a distribution of the mean positron lifetime along a long side of the deformed sample, which corresponds to the stress distribution during the deformation. In photoluminescence studies, yellow-band luminescence (2.2 eV) decreased due to the deformation. The suppression of this band was attributed to the vacancy-type defects and/or dislocations introduced by the deformation.

Uedono, Akira; Yonenaga, Ichiro; Watanabe, Tomohito; Kimura, Shogo; Oshima, Nagayasu; Suzuki, Ryoichi; Ishibashi, Shoji; Ohno, Yutaka

2013-08-01

324

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

325

Studies of positron induced luminescence from polymers  

SciTech Connect

Light emission from polymers (anthracene dissolved in polystryrene) induced by low-energy positrons and electrons has been studied. Results indicate a clear difference between optical emissions under positron and electron bombardment. The positron-induced luminescence spectrum is believed to be generated by both collisional and annihilation processes.

Xu, J.; Hulett, L.D. Jr.; Lewis, T.A. [Oak Ridge National Lab., TN (United States); Tolk, N.H. [Vanderbilt Univ., Nashville, TN (United States). Dept. of Physics and Astronomy

1994-06-01

326

The correlation between lifetime and momentum of e+-e- pair  

NASA Astrophysics Data System (ADS)

The triple coincident measurements of the Doppler broadening of annihilation line and the positron lifetime have been performed on the polymer (polypropylene PP/polyethylene PE) and the high-temperature superconductor (YBa2Cu3O7) samples. Two time spectrometers have simultaneously been used with the time resolution of 320 and 620 ps, respectively. The energy resolution of common HP Ge detector was 1.5 keV at 570 keV. Preliminary results show that correlation between the centroid of annihilation line and the time channel has been observed in both types of materials. The energy shifts up to +30 eV (for polymer) and up to ˜-200 eV for YBa2Cu3O7 have been found. The existence of correlation between the intensity of long-lived component of ˜1.9 ns and a momentum interval of annihilating e+-e- pair can be concluded in the case of polymer sample.

Krištiaková, K.; Krištiak, J.; Šauša, O.; Morhá?, M.; BandzÌ?uch, P.

1994-06-01

327

Measurement of the Total Charm Cross Section by Electron-Positron Annihilation at Energies Between 3.97-4.26 GeV  

E-print Network

Using the CLEO-c detector, we have measured the charm hadronic cross sections for $e^{+}e^{-}$ annihilations at a total of thirteen center-of-mass energies between 3.97 and 4.26 GeV. Observed cross sections for the production of $D\\bar{D}$, $D^{*}\\bar{D}$, $D^{*}\\bar{D}^{*}$, $D_{s}\\bar{D}_{s}$, $D_{s}^{*}\\bar{D}_{s}$, and $D_{s}^{*}\\bar{D}_{s}^{*}$, in addition to the total charm cross section are presented. Observed cross sections were radiatively corrected to obtain tree-level cross sections and R.

Brian W. Lang

2008-01-28

328

Slow-Positron Generator For Studying Polymer Films  

NASA Technical Reports Server (NTRS)

Aspects of molecular structures probed by positron-annihilation spectroscopy (PAS). Slow-positron-beam generator suitable for PAS measurements in thin polymer films. Includes Na22 source of positrons and two moderators made of well-annealed tungsten foil. With proper choice of voltage, positrons emitted by inward-facing surfaces of moderators made to stop in polymer films tested.

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

1992-01-01

329

Cosmic rays from dark matter annihilation and big-bang nucleosynthesis  

SciTech Connect

Recent measurements of cosmic-ray electron and positron fluxes by PAMELA and ATIC experiments may indicate the existence of annihilating dark matter with large annihilation cross section. We show that the dark matter annihilation in the big-bang nucleosynthesis epoch affects the light element abundances, and it gives stringent constraints on such annihilating dark matter scenarios.

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); Moroi, Takeo [Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa 277-8568 (Japan); Department of Physics, Tohoku University, Sendai 980-8578 (Japan); Nakayama, Kazunori [Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582 (Japan)

2009-04-15

330

Positron program at the Idaho Accelerator Center  

SciTech Connect

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

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

2009-09-02

331

Stringent Constraint on Galactic Positron Production  

E-print Network

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

John F. Beacom; Hasan Yuksel

2006-08-15

332

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

333

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

334

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

335

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

336

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

337

Cosmic positron signature from dark matter in the littlest Higgs model with T parity  

Microsoft Academic Search

We calculate the flux of cosmic positrons from the dark matter annihilation in the littlest Higgs model with T parity. The dark matter annihilates mainly into weak gauge bosons in the halo, and high-energy positrons are produced through leptonic and hadronic decays of the bosons. We investigate a possibility to detect the positron signal in upcoming experiments such as PAMELA

Masaki Asano; Shigeki Matsumoto; Nobuchika Okada; Yasuhiro Okada

2007-01-01

338

Gamma ray bursts and the origin of galactic positrons  

E-print Network

A recent observation of the 511 keV electron-positron annihilation line from the Galactic bulge has prompted a debate on the origin of the galactic positrons responsible for this emission. Assuming equilibrium between annihilation and injection of positrons in the Galaxy, we investigate the possibility that positrons were produced by past gamma ray bursts (GRBs). We compare the positron annihilation rate inferred from the observed 511 keV line with the predicted injection rate due to electron-positron pairs leaking out of individual GRBs and those pairs produced by GRB photons in the dense molecular clouds at the Galactic center. We conclude that the proposed scenario is marginally consistent with observations, and can reproduce the observed normalization of the annihilation line only for higher-than-average values of the GRB rate in the Galaxy and/or the molecular clouds optical depth for pair production.

Gianfranco Bertone; Alexander Kusenko; Sergio Palomares-Ruiz; Silvia Pascoli; Dmitry Semikoz

2006-03-10

339

Positron emission tomography wrist detector  

DOEpatents

A method of serially transferring annihilation information in a compact positron emission tomography (PET) scanner includes generating a time signal representing a time-of-occurrence of an annihilation event, generating an address signal representing a channel detecting the annihilation event, and generating a channel signal including the time and address signals. The method also includes generating a composite signal including the channel signal and another similarly generated channel signal concerning another annihilation event. An apparatus that serially transfers annihilation information includes a time signal generator, address signal generator, channel signal generator, and composite signal generator. The time signal is asynchronous and the address signal is synchronous to a clock signal. A PET scanner includes a scintillation array, detection array, front-end array, and a serial encoder. The serial encoders include the time signal generator, address signal generator, channel signal generator, and composite signal generator.

Schlyer, David J. (Bellport, NY); O'Connor, Paul (Bellport, NY); Woody, Craig (Setauket, NY); Junnarkar, Sachin Shrirang (Sound Beach, NY); Radeka, Veljko (Bellport, NY); Vaska, Paul (Sound Beach, NY); Pratte, Jean-Francois (Stony Brook, NY)

2006-08-15

340

POSITRON DEPTH PROFILING IN SOLID SURFACE LAYERS  

Microsoft Academic Search

We briefly review the principles of the Doppler Broadening of the positron annihilation radiation line, the most common technique used in defect depth profiling of solids relevant to dc- beams. We focus on some specific examples of Slow Positron Implantation Spectroscopy (SPIS) investigations related to technological issues such as, for instance, i) phase transitions in metal coatings possibly induced by

Robert I. GRYNSZPAN; Paul G. COLEMANc; Bath BA

341

Electron-Positron Plasma Drop Formed by Ultra-Intense Laser Pulses  

E-print Network

We study the initial properties and positron annihilation within a small electron-positron plasma drop formed by intense laser pulse. Such QED cascade initiated plasma is, in general, far below the chemical (particle yield) equilibrium. We find that the available electrons and positrons equilibrate kinetically, yet despite relatively high particle density, the electron-positron annihilation is very slow, suggesting a rather long lifespan of the plasma drop.

Inga Kuznetsova; Johann Rafelski

2012-04-25

342

Electron-Positron Plasma Drop Formed by Ultra-Intense Laser Pulses  

E-print Network

We study the initial properties, and positron annihilation within a small electron-positron plasma drop formed by intense laser pulse energy initiated QED cascade in general far below the chemical (particle yield) equilibrium. We find that the available electrons and positrons equilibrate kinetically, yet despite relatively high particle density the electron-positron annihilation is very slow suggesting a rather long lifespan of the plasma drop.

Kuznetsova, Inga

2011-01-01

343

Mesopore quality determines the lifetime of hierarchically structured zeolite catalysts  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

344

Positron deposition in plasmas by positronium beam ionization and transport of positrons in tokamak plasmas  

SciTech Connect

In a recently proposed positron transport experiment, positrons would be deposited in a fusion plasma by forming a positronium (Ps) beam and passing it through the plasma. Positrons would be deposited as the beam is ionized by plasma ions and electrons. Radial transport of the positrons to the limiter could then be measured by detecting the gamma radiation produced by annihilation of positrons with electrons in the limiter. This would allow measurements of the transport of electron-mass particles and might shed some light on the mechanisms of electron transport in fusion plasmas. In this paper, the deposition and transport of positrons in a tokamak are simulated and the annihilation signal determined for several transport models. Calculations of the expected signals are necessary for the optimal design of a positron transport experiment. There are several mechanisms for the loss of positrons besides transport to the limiter. Annihilation with plasma electrons and reformation of positronium in positron-hydrogen collisions are two such processes. These processes can alter the signal and place restrictions ons on the plasma conditions in which positron transport experiments can be effectively performed.

Murphy, T.J.

1986-11-01

345

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

E-print Network

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

Gribakin, Gleb

346

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

NASA Astrophysics Data System (ADS)

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

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

2015-02-01

347

A continuous time-tagged positron beam and its application to materials research  

Microsoft Academic Search

A time-tagged relativistic positron beam with very low time jitter (? 80 ps with the present setup) has been installed at the Stuttgart pelletron accelerator. A position-sensitive ß+ detector with good angular resolution and small e+ directional straggling supplies the start signal for positron lifetime measurements. The stop signal of the positron lifetime spectrometer based on this ``positron clock'' is

P. Wesolowski; K. Maier; J. Major; A. Seeger; H. Stoll; T. Grund; M. Koch

1992-01-01

348

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

2014-06-01

349

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

350

Multiwavelength Observation of WIMP Annihilation  

E-print Network

The annihilation of neutralino dark matter may result in observable signals in different wavelength. In the present paper we will discuss the effect of neutralino annihilation in the halo of our Galaxy and in its center. According to high resolution cold dark matter simulations, large virialized halos are formed through the constant merging of smaller halos appeared at previous times. At each epoch, dark matter halos have then a clumpy component which is made of these merging subhalos. The annihilation of dark matter in these clumps, always present in the halo of our Galaxy, may be responsible for appreciable fluxes of $\\gamma$-rays, potentially detectable. We find that, depending on the fundamental parameters of the clump density profile and on the distribution of clumps in the Galactic halo, the contribution to the diffuse $\\gamma$-ray background from clumps could be used to obtain constraints on the neutralino properties such as mass and annihilation cross section. On the other hand the annihilation of neutralino dark matter in the galactic center may result in radio signals. At the galactic center, infact, the accretion flow onto the central black hole sustains strong magnetic fields that can induce synchrotron emission, in the radio wavelength, by electrons and positrons generated in neutralino annihilations during advection onto the black hole. We find that the observed emission from the galactic center is consistent with neutralinos following a Navarro Frenk and White density profile at the galactic center while it is inconsistent with the presence of a spike density profile, supposed to be generated by the formation history of the central black hole.

Roberto Aloisio

2004-05-06

351

Unthermalized positrons in gamma ray burst sources  

NASA Technical Reports Server (NTRS)

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

Tkaczyk, W.; Karakula, S.

1992-01-01

352

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

353

On the positron in a metal  

NASA Astrophysics Data System (ADS)

The self-consistent calculations of the spatial distributions of electrons and potentials in vacancies of metals with a localized positron, the spatial distributions of positrons localized in surface states, and the binding energies of positrons and their lifetimes have been performed in terms of the Kohn-Sham method and the stabilized jellium model. The presence of a localized positron in a vacancy leads to the effect that the vacancy is weakly distinguishable for electron waves: the positron weakens the potential field in the vicinity of the vacancy and leads to a phase shift of the scattering electron wave functions. The calculation of the phase shifts of the wave functions for quasi-free positrons scattered by unperturbed vacancies and the representation of a system of vacancies as a "superlattice" in a metal have made it possible to find the shift of the positron work function and the vacancy contribution to the positron effective mass.

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

2014-09-01

354

Positron scattering with molecules of biological relevance  

NASA Astrophysics Data System (ADS)

Positron annihilation provides the basis of Positron Emission Tomography (PET), a medical imaging technology now in widespread use throughout the world. Understanding radiation dose rates is crucial in applying this technique, and current methods are based on models assuming the positron behaves identically to an electron, as well as lacking the real physics in the scattering processes as represented by measured scattering cross sections. At the Australian Positron Beamline Facility we have undertaken a program of measurements investigating positron scattering from biologically relevant molecules, to form the basis of a new model of dosimetry in PET scans. Measurements of positron scattering from water and formic acid will be presented, including the first measurements of positronium formation cross sections for these targets.

Sullivan, James; Makochekanwa, Casten; Slaughter, Dan; Caradonna, Peter; Jones, Adric; Tattersall, Wade; Buckman, Stephen; Bankovic, Ana; Petrovic, Zoran; Nixon, Kate; Brunger, Michael

2010-03-01

355

Positron Spectroscopy Investigation of Normal Brain Section and Brain Section with Glioma Derived from a Rat Glioma Model  

SciTech Connect

The application of positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS) to the study of animal or human tissue has only recently been reported [G. Liu, et al. phys. stat. sol. (C) 4, Nos. 10, 3912-3915 (2007)]. We have initiated a study of normal brain section and brain section with glioma derived from a rat glioma model. For the rat glioma model, 200,000 C6 cells were implanted in the basal ganglion of adult Sprague Dawley rats. The rats were sacrificed at 21 days after implantation. The brains were harvested, sliced into 2 mm thick coronal sections, and fixed in 4% formalin. PALS lifetime runs were made with the samples soaked in formalin, and there was not significant evaporation of formalin during the runs. The lifetime spectra were analyzed into two lifetime components. While early results suggested a small decrease in ortho-Positronium (o-Ps) pickoff lifetime between the normal brain section and brain section with glioma, further runs with additional samples have showed no statistically significant difference between the normal and tumor tissue for this type of tumor. The o-Ps lifetime in formalin alone was lower than either the normal tissue or glioma sample. So annihilation in the formalin absorbed in the samples would lower the o-Ps lifetime and this may have masked any difference due to the glioma itself. DBS was also used to investigate the difference in positronium formation between tumor and normal tissue. Tissue samples are heterogeneous and this needs to be carefully considered if PALS and DBS are to become useful tools in distinguishing tissue samples.

Yang, SH. [Department of Pharmacology and Neuroscience, University of North Texas Health Science Center, Fort Worth TX 76107 (United States); Ballmann, C.; Quarles, C. A. [Department of Astronomy and Physics, Texas Christian University, TCU Box 298840, Fort Worth TX 76129 (United States)

2009-03-10

356

Positron astrophysics and areas of relation to low-energy positron physics  

NASA Astrophysics Data System (ADS)

I briefly review our general knowledge of positron astrophysics, focusing mostly on the theoretical and modelling aspects. The experimental/observational aspects of the topic have recently been reviewed elsewhere [E. Churazov et al., Mon. Nat. R. Astron. Soc. 411, 1727 (2011); N. Prantazos et al., Rev. Mod. Phys. 83, 1001 (2011)]. In particular, I highlight the interactions and cross sections of the reactions that the positrons undergo in various cosmic media. Indeed, these must be of high interest to both the positron astrophysics community and the low-energy positron physics community in trying to find common areas of potential collaboration for the future or areas of research that will help the astrophysics community make further progress on the problem. The processes undergone by positrons from the moments of their birth to their annihilation (in the interstellar medium or other locations) are thus examined. The physics of the positron interactions with gases and solids (dust grains) and the physical conditions and characteristics of the environments where the processes of energy loss, positronium formation, and annihilation take place, are briefly reviewed. An explanation is given about how all the relevant physical information is taken into account in order to calculate annihilation rates and spectra of the 511 keV emission in the ISM; special attention is paid to positron interactions with dust and with polycyclic aromatic hydrocarbons. In particular, an attempt is made to show to what extent the interactions between positrons and interstellar dust grains are similar to laboratory experiments in which beams of low-energy positrons impinge upon solids and surfaces. Sample results are shown for the effect of dust grains on positron annihilation spectra in some phases of the ISM which, together with high resolution spectra measured by satellites, can be used to infer useful knowledge about the environment where the annihilation is predominantly taking place, and ultimately about the birth place and history of positrons in the Galaxy. The important complementarity between work done by the astrophysical and the positron physics communities is emphasised, and attempts are made to suggest avenues of future research for progress in the two fields. Contribution to the Topical Issue "Electron and Positron Induced Processes", edited by Michael Brunger, Radu Campeanu, Masamitsu Hoshino, Oddur Ingólfsson, Paulo Limão-Vieira, Nigel Mason, Yasuyuki Nagashima and Hajime Tanuma.

Guessoum, Nidhal

2014-05-01

357

Improved source and transport of monoenergetic MeV positrons  

NASA Astrophysics Data System (ADS)

A new monoenergetic MeV positron beam source was designed and constructed at the Lawrence Livermore National Laboratory (LLNL) positron facility. The positron source provides a low emittance beam with high energy resolution for channeling, scattering, and in-flight annihilation experiments. New beam optics for extracting slow positrons from a moderator mounted in a Pelletron electrostatic accelerator is presented. A beam line was designed and constructed to energy analyze the output of the accelerator and to provide adjustable size and angular divergence beams to a target station. A post target large angle acceptance magnet separates positrons transmitted through the target from forward gamma ray emission.

Selim, F. A.; Hunt, A. W.; Golovchenko, J. A.; Howell, R. H.; Haakenaasen, R.; Lynn, K. G.

2000-08-01

358

Positron production using a 1.7 MV pelletron accelerator  

NASA Astrophysics Data System (ADS)

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 19F(p,?e+e-)16O reaction, where the fluorine target is in the form of a CaF2 pellet. Positron production has been observed by detecting 511 keV annihilation gamma rays emerging from the irradiated CaF2 target.

Alcantara, K. F.; Crivelli, P.; Santos, A. C. F.

2013-04-01

359

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

360

The lives and deaths of positrons in the interstellar medium  

E-print Network

We reexamine in detail the various processes undergone by positrons in the ISM from their birth to their annihilation using the most recent results of positron interaction cross sections with H, H2 and He. The positrons' lives are divided into two phases: the 'in-flight' phase and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.

N. Guessoum; P. Jean; W. Gillard

2005-04-07

361

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

362

Positron binding to molecules  

NASA Astrophysics Data System (ADS)

˜ Positron annihilation on molecules as a function of incident positron energy exhibit vibrational Feshbach resonances (VFR).ootnotetextJ. A. Young and C. M. Surko, Phys. Rev. A 77, 052704 and 78, 032702 (2008). The energy differences between the vibrational modes and the VFR provide a measure of the positron-molecule binding energy, ?b. Measurements of ?b for 30 molecules are now available from this procedure and from comparison with theoretical spectra in the case of very small binding energies.^2 The dependence of ?b on molecular parameters can be expressed as ?b= 12.4(?+ 1.6?+ 2.4N?- 5.6) [meV], where ? is the molecular dipole polarizability, ? is the permanent dipole moment, and N? is the number of ? bonds in aromatic molecules.ootnotetextJ. R. Danielson, J. A. Young, and C. M. Surko, J. Phys. B 42, 235203 (2009). Insights into chemical trends from this analysis will be discussed. A key outstanding goal is to measure ?b for molecules that are sufficiently small and simple that ?b can be calculated theoretically. The empirical formula is used to identify candidate molecules. New measurements of ?b for CS2 (predicted ?b40 meV) will be discussed.

Surko, C. M.; Danielson, J. R.; Gosselin, J. J.

2010-03-01

363

The Beaming Pattern of Doppler-boosted Thermal Annihilation Radiation: Application to MeV Blazars  

Microsoft Academic Search

The beaming pattern of thermal annihilation radiation is broader than the beaming pattern produced by isotropic nonthermal electrons and positrons in the jets of radio-emitting active galactic nuclei that Compton scatter photons from an external isotropic radiation field. Thus, blueshifted thermal annihilation radiation can provide the dominant contribution to the high-energy radiation spectrum at observing angles theta >~ 1\\/ Gamma

Jeffrey G. Skibo; Charles D. Dermer; Reinhard Schlickeiser

1997-01-01

364

J. Phys. B: At. Mol. Opt. Phys. 29 (1996) 31513175. Printed in the UK Many-body calculations of positron scattering and  

E-print Network

of positron scattering and annihilation from noble-gas atoms V A Dzuba, V V Flambaum, G F Gribakin and W of low-energy positrons with noble-gas atoms. The positron­atom correlation potential includes explicitly the contribution from the target polarization by the positron and that from the virtual positronium (Ps) formation

Gribakin, Gleb

365

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

366

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

367

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

E-print Network

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

Stillinger, Frank

368

Neutralino Annihilation at the Galactic Center Revisited  

E-print Network

The annihilation of neutralino dark matter in the Galactic Center (GC) may result in radio signals that can be used to detect or constrain the dark matter halo density profile or dark matter particle properties. At the Galactic Center, the accretion flow onto the central Black Hole (BH) sustains strong magnetic fields that can induce synchrotron emission by electrons and positrons generated in neutralino annihilations during advection onto the BH. Here we reanalyze the radiative processes relevant for the neutralino annihilation signal at the GC, with realistic assumptions about the accretion flow and its magnetic properties. We find that neglecting these effects, as done in previous papers, leads to the incorrent electron and photon spectra. We find that the magnetic fields associated with the flow are significantly stronger than previously estimated. We derive the appropriate equilibrium distribution of electrons and positron and the resulting radiation, considering adiabatic compression in the accretion flow, inverse Compton scattering off synchrotron photons (synchrotron self-Compton scattering), and synchrotron self-absorption of the emitted radiation. We derive the signal for a Navarro-Frenk-White (NFW) dark matter halo profile and a NFW profile with a dark matter spike due to the central BH. We find that the observed radio emission from the GC is inconsistent with the scenario in which a spiky distribution of neutralinos is present. We discuss several important differences between our calculations and those previously presented in the literature.

Roberto Aloisio; Pasquale Blasi; Angela V. Olinto

2004-06-10

369

Positron production during relativistic runaway processes associated with thunderstorms  

NASA Astrophysics Data System (ADS)

Recent spacecraft observations of terrestrial gamma-ray flashes (TGFs) by Fermi/GBM and aircraft observations of the gamma-ray emissions from thunderclouds by ADELE have shown prominent 511 keV positron annihilation lines, demonstrating large enhancements in the positron populations. These observations show that significant pair-production must be taking place, most likely in association with the production of relativistic runaway electron avalanches. Using detailed Monte Carlo simulations, we investigate the production and subsequent transport of positrons by strong electric fields associated with thunderstorms. It will be shown that intense high-energy beams of positrons can be produced with energies reaching 100 MeV, well exceeding the average energy of the runaway electron population. These positrons, which may travel many kilometers before annihilating, generate a substantial amount of bremsstrahlung x-rays and annihilation gamma-rays. In this presentation, we shall discuss the theory of positron production by runaway electron avalanches and the feedback effects produced by these positrons. In addition, we shall use the Monte Carlo simulations to model the recent Fermi/GBM TGF and ADELE gamma-ray data.

Dwyer, J. R.; Smith, D. M.; Rassoul, H.; Cramer, E. S.; Schaal, M.; Saleh, Z. H.; Grefenstette, B.; Hazelton, B. J.; Splitt, M. E.; Lazarus, S. M.; Fishman, G. J.; Briggs, M. S.; Connaughton, V.

2009-12-01

370

Hypernovae and light dark matter as possible Galactic positron sources  

E-print Network

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

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

2005-07-25

371

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

372

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

373

Characterization of defects in Si and SiO{sub 2}-Si using positrons  

SciTech Connect

Positron annihilation spectroscopy of overlayers, interfaces, and buried regions of semiconductors has seen a rapid growth in recent years. The characteristics of the annihilation gamma rays depend strongly on the local environment of the annihilation sites, and can be used to probe defect concentrations in a range inaccessible to conventional defect probes. Some of the recent success of the technique in examining low concentrations of point defects in technologically important Si-based structures is discussed.

Asoka-Kumar, P.; Lynn, K.G.

1993-12-31

374

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

375

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

376

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

2009-01-25

377

A method to detect positron anisotropies with Pamela data  

NASA Astrophysics Data System (ADS)

The PAMELA experiment is collecting data since 2006; its results indicate the presence of a large flux of positron with respect to electrons in the CR spectrum above 10 GeV. This excess might also be originated in objects such as pulsars and microquasars or through dark matter annihilation. Here the electrons and positrons events collected by PAMELA have been analized searching for anisotropies. The analysis is performed at different angular scales and results will be presented at the conference.

Panico, B.; Adriani, O.; Barbarino, G. C.; Bazilevskaya, G. A.; Bellotti, R.; Boezio, M.; Bogomolov, E. A.; Bongi, M.; Bonvicini, V.; Bottai, S.; Bruno, A.; Cafagna, F.; Campana, D.; Carbone, R.; Carlson, P.; Casolino, M.; Castellini, G.; De Donato, C.; De Santis, C.; De Simone, N.; Di Felice, V.; Formato, V.; Galper, A. M.; Giaccari, U.; Karelin, A. V.; Koldashov, S. V.; Koldobskiy, S.; Krutkov, S. Y.; Kvashnin, A. N.; Leonov, A.; Malakhov, V.; Marcelli, L.; Martucci, M.; Mayorov, A. G.; Menn, W.; Merge, M.; Mikhailov, V. V.; Mocchiutti, E.; Monaco, A.; Mori, N.; Munini, R.; Osteria, G.; Palma, F.; Papini, P.; Pearce, M.; Picozza, P.; Pizzolotto, C.; Ricci, M.; Ricciarini, S. B.; Sarkar, R.; Scotti, V.; Simon, M.; Sparvoli, R.; Spillantini, P.; Stozhkov, Y. I.; Vacchi, A.; Vannuccini, E.; Vasilyev, G. I.; Voronov, S. A.; Yurkin, Y. T.; Zampa, G.; Zampa, N.; Zverev, V. G.

2014-11-01

378

Local dark matter clumps and the cosmic ray positron excess  

E-print Network

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

Daniel T. Cumberbatch; Joseph Silk

2006-07-24

379

Point defects in As-grown and ion implanted GaAs probed by a monoenergetic positron beam  

NASA Astrophysics Data System (ADS)

Defects in ion implanted GaAs were studied by monoenergetic positron beams. From measurements of Doppler broadening profiles of the positron annihilation as a function of incident positron energy, it was found that a species of defects strongly depends on a species of implanted ions. Native defects in GaAs wafers were also studied. The present investigation shows that positrons provide a sensitive and nondestructive probe for the detection of both vacancies and interstitials.

Uedono, A.; Fujii, S.; Wei, L.; Tanigawa, S.

1994-06-01

380

PhytoBeta imager: a positron imager for plant biology  

SciTech Connect

Several positron emitting radioisotopes such as 11C and 13N can be used in plant biology research. The 11CO2 tracer is used to facilitate plant biology research toward optimization of plant productivity, biofuel development and carbon sequestration in biomass. Positron emission tomography (PET) imaging has been used to study carbon transport in live plants using 11CO2. Because plants typically have very thin leaves, little medium is present for the emitted positrons to undergo an annihilation event. The emitted positrons from 11C (maximum energy 960 keV) could require up to approximately 4 mm of water equivalent material for positron annihilation. Thus many of the positrons do not annihilate inside the leaf, resulting in limited sensitivity for PET imaging. To address this problem we have developed a compact beta-positive, beta-minus particle imager (PhytoBeta imager) for 11CO2 leaf imaging. The detector is based on a Hamamatsu H8500 position sensitive photomultiplier tube optically coupled via optical grease to a 0.5 mm thick Eljen EJ-212 plastic scintillator. The detector is equipped with a flexible arm to allow its placement and orientation over or under the leaf to be studied while maintaining the leaf's original orientation. To test the utility of the system the detector was used to measure carbon translocation in a leaf of the spicebush (Lindera benzoin) under two transient light conditions.

Weisenberger, Andrew G; Lee, Seungjoon; McKisson, John; McKisson, J E; Xi, Wenze; Zorn, Carl; Reid, Chantal D; Howell, Calvin R; Crowell, Alexander S; Cumberbatch, Laurie; Fallin, Brent; Stolin, Alexander

2012-06-01

381

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

382

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

SciTech Connect

If dark matter annihilation accounts for the tantalizing excess of cosmic ray electron/positrons, as reported by the PAMELA, ATIC, HESS, and FERMI observatories, then the implied annihilation cross section must be relatively large. This results, in the context of standard cosmological models, in very small relic dark matter abundances that are incompatible with astrophysical observations. We explore possible resolutions to this apparent conflict in terms of nonstandard cosmological scenarios; plausibly allowing for large cross sections, while maintaining relic abundances in accord with current observations.

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

2010-06-15

383

Development of a method to study positron diffusion in metals by the observation of positronium negative ions  

NASA Astrophysics Data System (ADS)

We have developed a new method to study positron diffusion in metals. In this method, we observe positronium negative ions emitted from the sample surfaces after coating with alkali-metals to evaluate the yields of the positrons which return to the surfaces. ?-rays from the ions accelerated using an electric field are clearly distinguished from those emitted from pair-annihilation of positrons in the bulk or on the surface and self-annihilation of emitted positronium atoms. Reliable studies on positron diffusion in metals have been enabled by this method.

Suzuki, Takuji; Terabe, Hiroki; Iida, Shimpei; Yamashita, Takashi; Nagashima, Yasuyuki

2014-09-01

384

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

385

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

386

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

387

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

388

MEDICAL APPLICATION OF THE POSITRON EMITTER BEAM AT HIMAC  

Microsoft Academic Search

A treatment planning of heavy ion cancer therapy is performed with an aid of X-ray CT, which has uncertainties in an estimation of ion range. In order to obtain a dose distribution with high accuracy, irradiation and range measurement systems are developed at HIMAC using positron emitter beams. Exact stopping points of the ions can be measured by detecting annihilation

M. Suda; Y. Futami; T. Kanai; M. Kanazawa; A. Kitagawa; T. Murakami; K. Noda; M. Shimbo; T. Tomitani; M. Torikoshi; E. Urakabe; K. Hanawa; Y. Iseki; K. Sato

389

WIMP annihilation in caustics  

E-print Network

The continuous infall of dark matter with low velocity dispersion from all directions in a galactic halo leads to the formation of caustics which are very small scale ($\\sim$parsec) high density structures. If the dark matter is made up of SUSY neutralinos, the annihilation of these particles produces a characteristic spectrum of gamma rays which in principle, could be detected. The annihilation signal at different energy bands is computed and compared with the expected gamma ray background.

Aravind Natarajan

2007-05-22

390

The detection of a cosmic-ray electron-positron anisotropy is a sufficient (but not necessary) condition to discard a Dark Matter origin for the anomalous positron fraction  

E-print Network

I demonstrate that if an anisotropy in the arrival direction of high-energy cosmic-ray electrons and positrons is observed, then dark matter annihilation is ruled out as an explanation to the positron excess. For an observable anisotropy to originate from dark matter annihilation, the high-energy electrons and positrons must be produced in a nearby clump. I consider the annihilation pathway producing the smallest flux of gamma rays versus electrons and positrons, and the combination of clump distance and luminosity that minimizes the gamma-ray flux. I show that if an anisotropy from such a clump were detected, then the clump would be clearly detectable as an anomalous, bright gamma-ray source with the Fermi Large Area Telescope. I also point out that the non-detection of an anisotropy is perfectly compatible with an astrophysical origin for the excess positrons that has nothing to do with dark matter.

Profumo, Stefano

2014-01-01

391

Calculation of positron characteristics for elements of the periodic table  

NASA Astrophysics Data System (ADS)

Positron characteristics have been calculated in bulk and monovacancies 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, and different parametrizations for the positron enhancement factor and correlation energy. As it is known, positron lifetimes in bulk show a periodic behaviour with atomic number. These calculations also confirm that monovacancy lifetimes follow the same behaviour. The results obtained have been compared with selected experimental lifetime data, which confirms the calculated theoretical trends. Positron binding energies to a monovacancy have been calculated also for most of the elements of the periodic table. The binding energy shows a periodic behaviour with atomic number too.

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

2011-01-01

392

Compact Beta Particle/Positron Imager for Plant Biology  

SciTech Connect

The 11CO2 tracer is used to facilitate plant biology research towards optimization of plant productivity, biofuel development and carbon sequestration in biomass. Positron emission tomography (PET) imaging has been used to study carbon transport in live plants using 11CO2. Plants typically have very thin leaves resulting in little medium for the emitted positrons to undergo an annihilation event. For the emitted positron from 11C decay approximately 1mm of water equivalent material is needed for positron annihilation. Thus most of the positrons do not annihilate inside the leaf, resulting in limited sensitivity for PET imaging. To address this problem we have developed a compact beta-positive beta-minus particle (BPBM) imager for 11CO2 leaf imaging. The detector is based on a Hamamatsu H8500 position sensitive photomultiplier tube optically coupled via optical grease and a 3mm thick glass plate to a 0.5mm thick Eljin EJ-212 plastic scintillator. The detector is equipped with a flexible arm to allow its placement and orientation on the leaf of the plant of interest while maintaining the leaf's original orientation. We are planning to utilize the imaging device at the Duke University Phytotron to investigate dynamic carbon transport differences between invasive and native species.

Weisenberger, Andrew; Lee, Seung Joon; McKisson, John; Xi, Wenze; Zorn, Carl; Stolin, Alexander; Majewski, Stan; Majewski, Stanislaw; Howell, Calvin; Crowell, Alec

2011-06-01

393

Enhanced lifetime of positronium atoms via collective radiative effects  

E-print Network

A method is proposed to manipulate the annihilation dynamics of a dense gas of positronium atoms employing superradiance and subradiance regimes of the cooperative spontaneous emission of the system. The annihilation dynamics is controlled by the gas density and by the intensity of the driving strong resonant laser field. In particular, the method allows to increase the annihilation lifetime of an ensemble of positronium atoms more than hundred times by trapping the atoms in the excited state via collective radiative effects in the resonant laser and cavity fields.

Ni Cui; Mihai Macovei; Karen Z. Hatsagortsyan; Christoph H. Keitel

2011-12-07

394

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

SciTech Connect

The fluxes and spectra of quasimonochromatic photons originating from the in-flight annihilation of positrons interacting with electrons of targets are analyzed in the energy region characteristic of the excitation of E1 giant resonances in nuclei. Targets of small thickness and low atomic number are used. The dependences of the spectra on the energy and angle (and their scatter) for positrons incident to the target, on the collimation angle for photons, and on the target thickness are studied.

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

2014-12-15

395

Measurement of the F Meson Lifetime.  

NASA Astrophysics Data System (ADS)

A result on the first measurement of the lifetime of the F('(+OR-)) mesons produced in e('+)e('-) annihilations at 29 GeV is presented. These measurements have been made by using a high precision straw-type drift chamber along with the HRS detector at PEP. The data which. were analysed for these measurements corresponds to an integrated luminosity of 145 Pb('-1). The lifetime is measured to be. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). based on 17 F('(+OR-)) Decays.

Jung, Chang Kee

396

The Isotropic Radio Background and Annihilating Dark Matter  

SciTech Connect

Observations by ARCADE-2 and other telescopes sensitive to low frequency radiation have revealed the presence of an isotropic radio background with a hard spectral index. The intensity of this observed background is found to exceed the flux predicted from astrophysical sources by a factor of approximately 5-6. In this article, we consider the possibility that annihilating dark matter particles provide the primary contribution to the observed isotropic radio background through the emission of synchrotron radiation from electron and positron annihilation products. For reasonable estimates of the magnetic fields present in clusters and galaxies, we find that dark matter could potentially account for the observed radio excess, but only if it annihilates mostly to electrons and/or muons, and only if it possesses a mass in the range of approximately 5-50 GeV. For such models, the annihilation cross section required to normalize the synchrotron signal to the observed excess is sigma v ~ (0.4-30) x 10^-26 cm^3/s, similar to the value predicted for a simple thermal relic (sigma v ~ 3 x 10^-26 cm^3/s). We find that in any scenario in which dark matter annihilations are responsible for the observed excess radio emission, a significant fraction of the isotropic gamma ray background observed by Fermi must result from dark matter as well.

Hooper, Dan; Belikov, Alexander V.; Jeltema, Tesla E.; Linden, Tim; Profumo, Stefano; Slatyer, Tracy R.

2012-11-01

397

An intra-operative positron probe with background rejection capability for FDG-guided surgery.  

PubMed

For radio-guided surgery on tumors using F-18-FDG, detection of annihilation gamma photons emanating from other parts of the body produces background radiation counts and limits its use in clinical situations. To overcome this limitation, we have developed an intra-operative positron probe with background-rejection capability. The positron probe uses a phoswich detector composed of a plastic scintillator and a bismuth germinate (BGO). A positron from a positron emitter such as F-18 is detected by the plastic scintillator and emits annihilation photons. The BGO detects one of the annihilation photons while a photo-multiplier tube (PMT) detects scintillation photons from both scintillators. The decay time differences of these two scintillators are used to distinguish whether the event is a true event where a positron and a following annihilation photon are detected simultaneously, or a background event. In this configuration, only positrons can be selectively detected, even in an environment of high background gamma photon flux. Spatial resolution was 11-mm full width at half maximum (FWHM) 5 mm from the detector surface. Measured sensitivity for the F-18 point source was 2.6 cps/kBq 5 mm from the detector surface. The background count rate was less than 0.5 cps for a 20-cm diameter cylindrical phantom containing 37 MBq of F-18 solution measured on the phantom surface, while the positron count rate was almost linear over a range of approximately 6 kcps. These results indicate that our developed intra-operative positron probe is valuable for radio-guided surgery on tumors using F-18-FDG in a high flux of background annihilation gamma photons. PMID:15770969

Yamamoto, Seiichi; Matsumoto, Keiichi; Sakamoto, Setsu; Tarutani, Kazumasa; Minato, Kotaro; Senda, Michio

2005-02-01

398

Intensity dependence of the fluorescence lifetime of in vivo chlorophyll excited by a picosecond light pulse. [Chlorella pyrenoidosa  

Microsoft Academic Search

New data on intensity-dependent lifetimes indicate that all previous in vivo fluorescence studies of chlorophyll by picosecond techniques must be reinterpreted. Anomalously short lifetimes result from high-intensity effects due to exciton-exciton annihilation processes. Measurements in Chlorella pyrenoidosa with single-pulse, low-intensity excitation indicate a longer ''true'' lifetime of 650 +- 150 picoseconds.

A. J. Campillo; V. H. Kollman; S. L. Shapiro

1976-01-01

399

Modular Annihilator Jordan Pairs  

Microsoft Academic Search

Barnes proved, in [2] and [3], that a complex semiprimitive associative Banach algebra A is modular annihilator if and only if 0 is the only possible accumulation point of the spectrum of x for each x 2 A. A complex Jordan Banach algebra J which satisfies the above spectral property is called inessen-tial. Fern'andez proved in [9] that inessential complex

M. Benslimane; H. Marhnine; C. Zarhouti

2001-01-01

400

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

401

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

402

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

403

Compact conscious animal positron emission tomography scanner  

DOEpatents

A method of serially transferring annihilation information in a compact positron emission tomography (PET) scanner includes generating a time signal for an event, generating an address signal representing a detecting channel, generating a detector channel signal including the time and address signals, and generating a composite signal including the channel signal and similarly generated signals. The composite signal includes events from detectors in a block and is serially output. An apparatus that serially transfers annihilation information from a block includes time signal generators for detectors in a block and an address and channel signal generator. The PET scanner includes a ring tomograph that mounts onto a portion of an animal, which includes opposing block pairs. Each of the blocks in a block pair includes a scintillator layer, detection array, front-end array, and a serial encoder. The serial encoder includes time signal generators and an address signal and channel signal generator.

Schyler, David J. (Bellport, NY); O'Connor, Paul (Bellport, NY); Woody, Craig (Setauket, NY); Junnarkar, Sachin Shrirang (Sound Beach, NY); Radeka, Veljko (Bellport, NY); Vaska, Paul (Sound Beach, NY); Pratte, Jean-Francois (Stony Brook, NY); Volkow, Nora (Chevy Chase, MD)

2006-10-24

404

The NEPOMUC upgrade and advanced positron beam experiments  

NASA Astrophysics Data System (ADS)

The neutron-induced positron source NEPOMUC at the FRM II provides a mono-energetic positron beam of high intensity of the order of 109 moderated positrons per second. The new layout of NEPOMUC upgrade is presented and the constraints for operating an in-pile positron source at a research reactor are discussed. Inside the tip of the new beam tube, 80% 113Cd-enriched Cd is used as a neutron-?-converter that has a projected lifetime of 25 years of reactor operation and thus ensures positron beam experiments in the long term. The source consists of Pt foils that both generate positrons, by pair production, and moderate them. The layout of these foils, the electric lenses and the magnetic fields for positron extraction and beam formation have been improved. In addition to a higher beam intensity, it is expected that the beam brightness will improve by at least one order of magnitude. The present and planned experiments range from fundamental studies in nuclear, atomic and plasma physics to high-sensitivity and element-selective investigations in surface and solid state physics to applications in materials science. The upgrade of several positron spectrometers as well as new positron beam experiments are presented. In addition, a new switching and remoderation unit will allow us to toggle from the high-intensity primary beam to a brightness enhanced remoderated positron beam.

Hugenschmidt, Christoph; Piochacz, Christian; Reiner, Markus; Schreckenbach, Klaus

2012-05-01

405

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

406

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

E-print Network

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

Masaki Asano; Shigeki Matsumoto; Nobuchika Okada; Yasuhiro Okada

2006-11-22

407

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

408

Performance of the Beijing pulsed variable-energy positron beam  

NASA Astrophysics Data System (ADS)

In order to study the depth-dependent characteristics of open-volume defects in thin surface layers, the variable-energy positron lifetime spectroscopy (VEPLS) has been enabled by pulsing a continuous positron beam. The buncher is a quarter-wave coaxial resonator and the RF-signal is fed in by a coupling loop with a frequency of 149.89 MHz and the reflection factor of 0.05 measured by a Network Analyzer. Three synchronic signals with their phases and amplitudes adjusted independently are supplied for start signal of the positron lifetime measurement and the power signal by an electronic system. The stop signal is derived from a detector, a BaF 2 scintillator coupled to a photomultiplier-tube (Hamamatsu). The time resolution of 295 ps (FWHM) was achieved for a Kapton film and a Ti sample at positron energies in the range between 1 keV and 30 keV.

Wang, B. Y.; Ma, Y. Y.; Zhang, Z.; Yu, R. S.; Wang, P.

2008-10-01

409

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

410

Intensity Dependence of the Fluorescence Lifetime of in vivo Chlorophyll Excited by a Picosecond Light Pulse  

Microsoft Academic Search

New data on intensity-dependent lifetimes indicate that all previous in vivo fluorescence studies of chlorophyll by picosecond techniques must be reinterpreted. Anomalously short lifetimes result from high-intensity effects due to exciton-exciton annihilation processes. Measurements in Chlorella pyrenoidosa with single-pulse, low-intensity excitation indicate a longer \\

A. J. Campillo; V. H. Kollman; S. L. Shapiro

1976-01-01

411

Indirect and direct signatures of Higgs portal decaying vector dark matter for positron excess in cosmic rays  

E-print Network

We investigate the indirect signatures of the Higgs portal $U(1)_X$ vector dark matter (VDM) $X_\\mu$ from both its pair annihilation and decay. The VDM is stable at renormalizable level by $Z_2$ symmetry, and thermalized by Higgs-portal interactions. It can also decay by some nonrenormalizable operators with very long lifetime at cosmological time scale. If dim-6 operators for VDM decays are suppressed by $10^{16}$ GeV scale, the lifetime of VDM with mass $\\sim$ 2 TeV is just right for explaining the positron excess in cosmic ray recent observed by PAMELA and AMS02 Collaborations. The VDM decaying into $\\mu^+ \\mu^-$ can fit the data, evading various constraints on cosmic rays. We give one UV-complete model as an example. This scenario for Higgs portal decaying VDM with mass around $\\sim2$ TeV can be tested by DM direct search at XENON1T and at the future colliders by measuring the Higgs self-couplings.

Seungwon Baek; P. Ko; Wan-Il Park; Yong Tang

2014-06-19

412

A new look at the cosmic ray positron fraction  

NASA Astrophysics Data System (ADS)

Context. The positron fraction in cosmic rays has recently been measured with improved accuracy up to 500 GeV, and it was found to be a steadily increasing function of energy above ~10 GeV. This behaviour contrasts with standard astrophysical mechanisms, in which positrons are secondary particles, produced in the interactions of primary cosmic rays during their propagation in the interstellar medium. The observed anomaly in the positron fraction triggered a lot of excitement, as it could be interpreted as an indirect signature of the presence of dark matter species in the Galaxy, the so-called weakly interacting massive particles (WIMPs). Alternatively, it could be produced by nearby sources, such as pulsars. Aims: These hypotheses are probed in light of the latest AMS-02 positron fraction measurements. As regards dark matter candidates, regions in the annihilation cross section to mass plane, which best fit the most recent data, are delineated and compared to previous measurements. The explanation of the anomaly in terms of a single nearby pulsar is also explored. Methods: The cosmic ray positron transport in the Galaxy is described using a semi-analytic two-zone model. Propagation is described with Green functions as well as with Bessel expansions. For consistency, the secondary and primary components of the positron flux are calculated together with the same propagation model. The above mentioned explanations of the positron anomaly are tested using ?2 fits. The numerical package MicrOMEGAs is used to model the positron flux generated by dark matter species. The description of the positron fraction from conventional astrophysical sources is based on the pulsar observations included in the Australia Telescope National Facility (ATNF) catalogue. Results: The masses of the favoured dark matter candidates are always larger than 500 GeV, even though the results are very sensitive to the lepton flux. The Fermi measurements point systematically to much heavier candidates than the recently released AMS-02 observations. Since the latter are more precise, they are much more constraining. A scan through the various individual annihilation channels disfavours leptons as the final state. On the contrary, the agreement is excellent for quark, gauge boson, or Higgs boson pairs, with best-fit masses in the 10 to 40 TeV range. The combination of annihilation channels that best matches the positron fraction is then determined at fixed WIMP mass. A mixture of electron and tau lepton pairs is only acceptable around 500 GeV. Adding b-quark pairs significantly improves the fit up to a mass of 40 TeV. Alternatively, a combination of the four-lepton channels provides a good fit between 0.5 and 1 TeV, with no muons in the final state. Concerning the pulsar hypothesis, the region of the distance-to-age plane that best fits the positron fraction for a single source is determined. Conclusions: The only dark matter species that fulfils the stringent gamma ray and cosmic microwave background bounds is a particle annihilating into four leptons through a light scalar or vector mediator, with a mixture of tau (75%) and electron (25%) channels, and a mass between 0.5 and 1 TeV. The positron anomaly can also be explained by a single pulsar, and a list of five pulsars from the ATNF catalogue is given. We investigate how this list could evolve when more statistics are accumulated. Those results are obtained with the cosmic ray transport parameters that best fit the B/C ratio. Uncertainties in the propagation parameters turn out to be very significant. In the WIMP annihilation cross section to mass plane for instance, they overshadow the error contours derived from the positron data.

Boudaud, M.; Aupetit, S.; Caroff, S.; Putze, A.; Belanger, G.; Genolini, Y.; Goy, C.; Poireau, V.; Poulin, V.; Rosier, S.; Salati, P.; Tao, L.; Vecchi, M.

2015-03-01

413

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

E-print Network

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

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

2014-09-25

414

Microstructure of polymer clay nanocomposites studied by positrons  

NASA Astrophysics Data System (ADS)

The epoxy-rectorite nanocomposites with different rectorite contents, epoxide equivalent were prepared and its microstructure was studied by positron annihilation and X-ray diffraction (XRD). At low rectorite content (0-2.0%), the free volume size in nanocomposites is nearly the same, but its concentration decreases with increasing content; the exfoliated structure was observed by XRD and interfacial layer formation between rectorite platelets and epoxy matrix was probed by positrons. Comparing with epoxy-montmorillonite, the exfoliated structure and interfacial layers are easier formed in epoxy-rectorite nanocomposites.

Wang, S. J.; Liu, L. M.; Fang, P. F.; Chen, Z.; Wang, H. M.; Zhang, S. P.

2007-02-01

415

Measurements of heavy quark and lepton lifetimes  

SciTech Connect

The PEP/PETRA energy range has proved to be well-suited for the study of the lifetimes of hadrons containing the b and c quarks and the tau lepton for several reasons. First, these states comprise a large fraction of the total interaction rate in e/sup +/e/sup -/ annihilation and can be cleanly identified. Second, the storage rings have operated at high luminosity and so produced these exotic states copiously. And finally, thanks to the interplay of the Fermi coupling strength, the quark and lepton masses, and the beam energy, the expected decay lengths are in the 1/2 mm range and so are comparatively easy to measure. This pleasant coincidence of cleanly identified and abundant signal with potentially large effects has made possible the first measurements of two fundamental weak couplings, tau ..-->.. nu/sub tau/W and b ..-->.. cW. These measurements have provided a sharp test of the standard model and allowed, for the first time, the full determination of the magnitudes of the quark mixing matrix. This paper reviews the lifetime studies made at PEP during the past year. It begins with a brief review of the three detectors, DELCO, MAC and MARK II, which have reported lifetime measurements. Next it discusses two new measurements of the tau lifetime, and briefly reviews a measurement of the D/sup 0/ lifetime. Finally, it turns to measurements of the B lifetime, which are discussed in some detail. 18 references, 14 figures, 1 table.

Jaros, J.A.

1985-02-01

416

Novel Positron-beam Systems for Atomic Physics  

NASA Astrophysics Data System (ADS)

Trapped positron plasmas are now routinely used to generate high resolution positron beams for a range of atomic physics experiments. Described here are the designs of two new positron beam systems intended for study of the nature and mechanisms of positron attachment to ordinary matter. Positrons attach to molecules in two-body collisions via the excitation of vibrational modes, resulting in huge enhancements in the annihilation rate. While this technique provides a way to measure binding energies,\\footnotetextGribakin, Young, and Surko, Rev. Mod. Phys. 82, 2557 (2010). improved energy resolution is critical to further progress. The design of a new system using a cryogenically cooled buffer gas is described that is intended to meet this need (i.e., resolution <=10 meV, FWHM, a factor of 4 improvement). The analogous process of positron attachment to atoms has not yet been studied experimentally. Also described here is the design of a tailored beam system, combined with a pulsed laser, intended to do this via photo-induced recombination.\\footnotetextSurko, Danielson, Gribakin, and Continetti, NJP 14, 065004 (2012). The example of positron binding to zinc (predicted binding energy ˜ 0.1 eV) is discussed.

Natisin, M. R.; Danielson, J. R.; Surko, C. M.

2012-10-01

417

Remarks on calculation of positron flux from galactic dark matter  

SciTech Connect

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

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

2010-08-15

418

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

419

A Doppler-broadening facility for positron spin relaxation (e +SR) experiments  

Microsoft Academic Search

A set-up is described for the determination of the spin polarization of positrons emitted from radioactive sources that makes use of the dependence of the Doppler broadening of the 511 keV annihilation photon line on the strength and direction (with regard to the spin polarization) of an applied magnetic field. In the so-called e+SR (positron spin relaxation) technique the sample

Th. Gessmann; P. Harmat; J. Major; A. Seeger

1997-01-01

420

High resolution positron Q-value measurements and nuclear structure studies far from the stability line. Progress report, July 1, 1979-June 30, 1980. [Dept. of Physics and Astronomy, Univ. of South Carolina, Columbia  

SciTech Connect

This document represents a progress report and renewal proposal for the contract DEAS 09 79 ER10434 between the USDOE and USC. During the time from 1 July 1979 to 1 March 1980, the large 35% intrinsic Ge detector was purchased and tested, and the new hyperpure Ge detector for positron end-point-energy measurements was designed and fabricated by ORTEC. It was delivered just prior to January 1, 1980. Measurements using this special equipment began in January 1980. During this period, a new effort in the measurement of short nuclear lifetimes was completed, and nuclear structure measurements of /sup 206/Rn nd /sup 208/Rn were completed. The results of these efforts are described in the text. A search for the Post Doctoral Research Associate was started last summer and the position will be filled on or about 1 March 1980. The first experiments to measure positron end-point-energies were proposed to the ORIC scheduling committee, and measurements will begin in March 1980. Theoretical efforts describe accurately the interference of annihilation radiation with positron end-point-energy measurements were begun.

Avignone, F.T. III

1980-02-22

421

Positron Reaction Microscope  

NASA Astrophysics Data System (ADS)

We are developing a positron reaction microscope to measure kinematically complete ionization reactions of atoms and dissociative ionization of simple molecules by positron impact. The experiment is designed to use the slow positron beamline at the ARC Centre for Antimatter Matter Studies (CAMS) node at the Australian National University (ANU). This project is a collaboration among the University of North Texas, CAMS, and the Max Planck Insitute for Kern Phyzik in Heidelberg. Initial measurements and apparatus calibration will be performed using electrons. For positron measurements, the apparatus will be rolled into position on the slow positron beamline at the CAMS site at ANU.

Mueller, D. W.; Lee, C.; Vermet, C.; Armitage, S.; Slaughter, D.; Hargrave, L.; Dorn, A.; Brunton, J.; Buckman, S. J.; Sullivan, J. P.

2012-06-01

422

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

423

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

424

Induced Positron Annihiliation Investigation of Cadmium Zinc Telluride Crystal Microstructures  

SciTech Connect

Cadmium-Zinc-Telluride (CZT) crystals are used in semiconductor radiation detectors for the detection of x-ray and gamma radiation. However, production of detector grade crystals is difficult as small variations in compositional uniformity and primarily the zinc content can significantly affect the ability of the CZT crystal to function as a radiation detector. Currently there are no known nondestructive methods that can be used to identify detector grade crystals. The current test method is to fabricate and test the detector to determine if the crystal is sufficiently uniform and of the correct composition to be considered a detector grade crystal. Consequently, nondestructive detection methods are needed to identify detector grade crystals prior to the fabrication process. The purpose of this feasibility study was to perform a preliminary assessment of the ability of several new, nondestructive technologies based on Induced Positron Annihilation (IPA) to determine if detector grade CZT crystals can be identified. Results of measurements performed on specimens from Fisk University and EV Products, Inc. indicate that both the near surface Distributed Source Positron Annihilation (up to 3 mm penetration) and the volumetric Photon Induced Positron Annihilation methods may be suitable for determining CZT crystal quality. Further work on CZT crystals with a broader range of compositions and detector characteristics is needed to provide a well defined, calibrated, method for assessing CZT crystal quality.

D. W. Akers

2005-06-01

425

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

426

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

427

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

428

Positron ionization mass spectrometry: An organic mass spectrometrist's view  

SciTech Connect

We are currently engaged in a research program to study the ionization of polyatomic molecules by positrons. We refer to the technique herein as positron ionization mass spectrometry which includes all of the possible ionization mechanisms. In the course of this work we will attempt to characterize each of the important ionization mechanisms. Our ultimate objective is to explore the use of positron ionization mass spectrometry for chemical analysis. Several other groups have also begun to pursue aspects of positron ionization in parallel with our efforts although with somewhat different approaches and, perhaps with slightly different emphases. Recently, for example, Passner et al. have acquired mass spectra in a Penning trap resulting from the ionization of several different polyatomic molecules by near thermal kinetics energy positrons. Our research involves studying the different types of ionizing interactions of positrons with organic molecules, as a function of positron kinetic energy. For ionization of polyatomic molecules by positrons, several possible mechanisms are apparent from lifetime and scattering cross-section data. These mechanisms are discussed.

Glish, G.L.; Donohue, D.L.; McLuckey, S.A.; Eckenrode, B.A.; Hulett, L.D. Jr.

1990-01-01

429

Positron-rubidium scattering  

NASA Technical Reports Server (NTRS)

A 5-state close-coupling calculation (5s-5p-4d-6s-6p) was carried out for positron-Rb scattering in the energy range 3.7 to 28.0 eV. In contrast to the results of similar close-coupling calculations for positron-Na and positron-K scattering the (effective) total integrated cross section has an energy dependence which is contrary to recent experimental measurements.

Mceachran, R. P.; Horbatsch, M.; Stauffer, A. D.

1990-01-01

430

Positrons from quantum evaporation of primordial black-holes  

NASA Technical Reports Server (NTRS)

The unconfirmed prediction of quantum evaporation of primordial black holes (PBHs) is considered together with the related unanswered questions of whether PBHs ever existed and whether any could still exist. The behavior of the positrons from PHBs is modeled in relation to three facts. Firstly, the integrated emitted number spectrum of positrons is six to eight times larger than that of photons. Secondly, positrons emitted from PBHs lose energy and annihilate, producing a prominent line at 511 keV which is redshifted by the expansion of the universe. Thirdly, these photons may be detectable in the X-ray and low gamma ray energy ranges. The model predicts a flux which is significantly inferior to the instrument sensitivities of the foreseeable future.

Durouchoux, P.; Wallyn, P.; Dubus, G.

1997-01-01

431

PAMELA Positron Excess as a Signal from the Hidden Sector  

E-print Network

The recent positron excess observed in the PAMELA satellite experiment strengthens previous experimental findings. We give here an analysis of this excess in the framework of the Stueckelberg extension of the standard model which includes an extra $U(1)_X$ gauge field and matter in the hidden sector. Such matter can produce the right amount of dark matter consistent with the WMAP constraints. Assuming the hidden sector matter to be Dirac fermions it is shown that their annihilation can produce the positron excess with the right positron energy dependence seen in the HEAT, AMS and the PAMELA experiments. Further test of the proposed model can come at the Large Hadron Collider. The predictions of the $\\bar p/p$ flux ratio also fit the data.

Daniel Feldman; Zuowei Liu; Pran Nath

2009-03-14

432

AMS02 positron excess from decaying fermion DM with local dark gauge symmetry  

E-print Network

Positron excess observed by PAMELA, Fermi and AMS02 may be due to dark matter (DM) pair annihilation or decay dominantly into muons. In this paper, we consider a scenario with thermal fermionic DM ($\\chi$) with mass $\\sim O(1-2)$ TeV decaying into a dark Higgs ($\\phi$) and an active neutrino ($\

P. Ko; Yong Tang

2015-01-11

433

A Multicell Trap for Positron Storage  

NASA Astrophysics Data System (ADS)

There are many potential applications of high-capacity and/or portable antimatter traps, including multiplexing the output of high-flux positron beams, study of electron-positron plasmas, and eventually the construction of an annihilation gamma-ray laser at 0.51 MeV. We describe recent progress in the design and construction of a novel multicell Penning-Malmberg (PM) trap to store in excess of 5 x10^11 positrons.ootnotetextJ. R. Danielson, T. R. Weber, and C. M. Surko, Phys. Plasmas 13, 123502 (2006). The construction and planned objectives of a test electrode structure will be discussed. A key issue to be determined is the quality of confinement in off-axis cells in the less uniform portions of the magnetic field near the ends of the magnet. Protocols for filling off-axis cells and handling plasmas with kiloelectron volt levels of space charge will be described. A new scenario to rapidly dump the contents of the multicell trap will be discussed.

Danielson, J. R.; Surko, C. M.

2011-11-01

434

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

435

An observable electron-positron anisotropy cannot be generated by dark matter  

NASA Astrophysics Data System (ADS)

I demonstrate that if an anisotropy in the arrival direction of high-energy cosmic-ray electrons and positrons is observed then, barring local anisotropic diffusion, dark matter annihilation is ruled out as an explanation to the positron excess. For an observable anisotropy to originate from dark matter annihilation, the high-energy electrons and positrons must be produced in a nearby clump. I consider the annihilation pathway producing the smallest flux of gamma rays versus electrons and positrons, and the combination of clump distance and luminosity that minimizes the gamma-ray flux. I show that if an anisotropy from such a clump were detected, and if such anisotropy did not generate from anisotropic diffusion effects, then the clump would be clearly detectable as an anomalous, bright gamma-ray source with the Fermi Large Area Telescope. I also point out that the non-detection of an anisotropy is perfectly compatible with an astrophysical origin for the excess positrons that has nothing to do with dark matter.

Profumo, Stefano

2015-02-01

436

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.

Asoka kumar, Palakkal P. V. (Coram, NY); Lynn, Kelvin G. (Center Moriches, NY)

1993-01-01

437

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

438

Positrons for linear colliders  

SciTech Connect

The requirements of a positron source for a linear collider are briefly reviewed, followed by methods of positron production and production of photons by electromagnetic cascade showers. Cross sections for the electromagnetic cascade shower processes of positron-electron pair production and Compton scattering are compared. A program used for Monte Carlo analysis of electromagnetic cascades is briefly discussed, and positron distributions obtained from several runs of the program are discussed. Photons from synchrotron radiation and from channeling are also mentioned briefly, as well as positron collection, transverse focusing techniques, and longitudinal capture. Computer ray tracing is then briefly discussed, followed by space-charge effects and thermal heating and stress due to showers. (LEW)

Ecklund, S.

1987-11-01

439

Compton-backscattered annihilation radiation from the Galactic Center region  

NASA Technical Reports Server (NTRS)

On 1989 May 22, the High Energy X-ray and Gamma-ray Observatory for Nuclear Emissions, a balloon-borne high-resolution germanium spectrometer with an 18-deg FOV, observed the Galactic Center (GC) from 25 to 2500 keV. The GC photon spectrum is obtained from the count spectrum by a model-independent method which accounts for the effects of passive material in the instrument and scattering in the atmosphere. Besides a positron annihilation line with a flux of (10.0 +/- 2.4) x 10 exp -4 photons/sq cm s and a full width at half-maximum (FWHM) of (2.9 + 1.0, -1.1) keV, the spectrum shows a peak centered at (163.7 +/- 3.4) keV with a flux of (1.55 +/- 0.47) x 10 exp -3 photons/sq cm s and a FWHM of (24.4 +/- 9.2) keV. The energy range 450-507 keV shows no positronium continuum associated with the annihilation line, with a 2-sigma upper limit of 0.90 on the positronium fraction. The 164 keV feature is interpreted as Compton backscatter of broadened and redshifted annihilation radiation, possibly from the source 1E 1740.7-2942.

Smith, D. M.; Lin, R. P.; Feffer, P.; Slassi, S.; Hurley, K.; Matteson, J.; Bowman, H. B.; Pelling, R. M.; Briggs, M.; Gruber, D.

1993-01-01

440

The Beaming Pattern of Doppler Boosted Thermal Annihilation Radiation: Application to MeV Blazars  

Microsoft Academic Search

The beaming pattern of thermal annihilation radiation is broader than the\\u000abeaming pattern produced by isotropic nonthermal electrons and positrons in the\\u000ajets of radio-emitting active galactic nuclei which Compton scatter photons\\u000afrom an external isotropic radiation field. Thus blueshifted thermal\\u000aannihilation radiation can provide the dominant contribution to the high-energy\\u000aradiation spectrum at observing angles theta > 1\\/Gamma, where

Jeffrey G. Skibo; Charles D. Dermer; Reinhard Schlickeiser

1997-01-01