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1

Position-resolved Positron Annihilation Lifetime Spectroscopy  

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

2

A positron annihilation lifetime study of shock loading in tantalum  

Microsoft Academic Search

Shock loading and quasistatic loading were studied in annealed high purity (99.95%) tantalum in order to characterize the response and identify the physical mechanisms which are responsible. Samples were investigated with positron annihilation lifetime spectroscopy, microhardness measurements, and optical microscopy. The results allow increased damage in the form of dislocation density with increased true strain, but no significant difference in

David Michael Stepp

1998-01-01

3

Positron annihilation lifetime measurements of He-ion-irradiated Fe using pulsed positron beam  

NASA Astrophysics Data System (ADS)

The positron annihilation lifetimes of He-ion-irradiated Fe samples were measured by a pulsed positron beam technique. The relative thickness of the surface oxidation layer was measured by Auger microprobe measurements. Unirradiated samples with different surface oxidation layer thicknesses had very similar second lifetime component intensities, indicating that the surface oxidation layer thickness does not affect positron lifetime spectra. In lifetime measurements using 10-keV positrons, the second lifetime component decreased monotonically with increasing irradiation dose. The mean lifetime in the 10-keV measurements was longer than that in 15-keV measurements (which correspond to the peak region of vacancy production); this is thought to be due to vacancy clusters absorbing He atoms.

Sato, K.; Kinomura, A.; Omura, T.; Xu, Q.; Yoshiie, T.; Kasada, R.; Kimura, A.; Morishita, K.

2011-01-01

4

Measurement and Analysis of the Positron Annihilation Lifetime Spectra for Mesoporous Silica  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy as a technique of porous materials investigation is still under development. Both measurement conditions and lifetime spectra analysis methods have to be adapted to the requirements of positron porosimetry. If complex positron annihilation lifetime spectra of porous materials are processed, numerical methods of lifetime spectra analysis have to be used with a particular care. Among various methods of positron annihilation lifetime spectra analysis available, the most popular ones were tested implemented in programs MELT and LT. It was found, from the simulated spectra study, that a large number of counts in a spectrum is needed to obtain reliable results of analysis. Even then only an approximate solution is available. Determination of the approximation nature helps in a further interpretation of the results and in positron annihilation lifetime spectrometer setting adequate to the problem. For this purpose, a few complex spectra obtained from positron beam facility were tested. The results show a negligible influence of resolution function on long lifetime components, while the precise estimation of the background level is crucial. Also some distortions caused by imperfection of numerical analysis methods are pointed out.

Zaleski, R.

2006-11-01

5

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

6

Positron annihilation lifetime in mesoporous silica MCM-41 at different vacuum levels  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectra of MCM-41 and zeolite Y were measured at different vacuum levels. When the experiments were carried out in air, a very long lifetime component (?4 = 35-45 ns, I4 = 15-20%) was observed for MCM-41, while the longest lifetime for zeolite Y was only 2-4 ns with an intensity of 15-25%. However, when the experiments were carried out in vacuum, the very long lifetime components could be observed for both samples, although with different intensities, ~30% for MCM-41 and ~10% for zeolite Y. For MCM-41 in air, the longest lifetime (?4) is ~42 ns, corresponding to the ortho-positronium (o-Ps) annihilation lifetime in MCM-41 cavities. This value is slightly longer but very close to the value of 39 ns, which was estimated by using a bouncing quantum particle model. These peculiar positron annihilation characteristics were explained by air quenching mechanism of o-Ps annihilation in MCM-41. It was suggested that because of the existence of the very long lifetime component even in air, positron lifetime spectroscopy could be a very useful tool for nondestructive measurement of the cavity size of mesoporous solids such as MCM-41.

He, Y. J.; Zhang, H. Y.; Chen, Y. B.; Wang, H. Y.; Horiuchi, T.

2001-03-01

7

Investigation on the porosity of zeolite NU-88 by means of positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

Seven well characterized zeolites were investigated by positron annihilation lifetime spectroscopy. The lifetime spectra were analysed in four discrete components. The third one was associated with ortho-positronium annihilation in the channels, framed in terms of infinite cylinders. Differences between the radii determined from the positron annihilation technique and X-ray diffraction data were found and explained in terms of the physical structure of the channel. An analogous study on a high-silica NU-88 zeolite gave a value of 0.33 nm for the corresponding radius, in agreement with Ar and N2 adsorption data as well as with the catalytic behaviour of this zeolite in several acid catalyzed reactions. The longest lifetime component in NU-88 reveals the existence of mesopores, with average radius of about 1.8 nm, which could explain the importance of hydrogen transfer reactions in this zeolite.

Consolati, G.; Mariani, M.; Millini, R.; Quasso, F.

2009-08-01

8

Dynamics of polycarbonate far below the glass transition temperature: A positron annihilation lifetime study  

Microsoft Academic Search

In this study, we have extended the knowledge of the dynamics of polycarbonate (PC) far below Tg by means of positron annihilation lifetime spectroscopy (PALS), which is able to detect the isothermal relaxation of the free volume after rejuvenation above Tg. The free volume relaxation patterns at different temperatures below Tg were modeled according to a phenomenological model presented by

D. Cangialosi; M. Wübbenhorst; H. Schut; A. van Veen; S. J. Picken

2004-01-01

9

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

10

Positron annihilation lifetime study of pure and Na(acac) treated polyvinylalcohol  

Microsoft Academic Search

Positron annihilation lifetime studies of pxre and treated polyvinyl alcohol (PYA) reflect the effect of concentration as well ae temperature on free volume. Therefore, variations of the lifetime of the o-PS longlived component r3 and the probability of its formation I3 have been studied and correlated with changes in the electrical conductivity CY as well as with X-ray diffraction data.

A. M. A. EL-SAYED; E. E. ABDEL-HADY; M. MOHSEN; A. SAWABY

1993-01-01

11

Design, simulation and performance of a slow positron beam with secondary electron tagging for positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

A positron annihilation lifetime spectrometer using a magnetically guided continuous positron beam and the signal from secondary electrons (SE) emitted from the sample is presented. The positron and secondary electrons trajectories as well as the physics of SE were simulated using GEANT4 program. A special attention was paid to the consequences of positron backscattering, in particular on the shape of the lifetime spectrum. A very good agreement was obtained between simulation and measurements concerning the time resolution (~300–400 ps FWHM) for the studied range of incident positron energies. Due to the complex shape of the resolution function, the present design of the spectrometer is not suited to study of short lifetimes, but can be used for depth profiling porosimetry of materials with Ps formation with the an excellent peak to background ratio of ~105.

Marinov, H.; Djourelov, N.; Nédélec, P.; Petrov, L.

2013-11-01

12

Evolution of porosity in a Portland cement paste studied through positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy experiments were carried out in an ordinary Portland cement paste characterized by a water-to-cement ratio /w/c=0.8, in order to monitor the porosity of the paste. It was found that ortho-positronium intensity is a suitable quantity to this purpose, being sensitive to the amount of water contained in the pores. The experimental data show good agreement with the porosity calculated according to the Powers' model.

Consolati, G.; Quasso, F.

2003-10-01

13

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

14

Positron annihilation lifetime spectroscopy based on secondary electron emission from carbon foil  

NASA Astrophysics Data System (ADS)

Using the emission of secondary electron (SE) from a thin carbon foil in transmission geometry guided by a strong radial electric field in the region between the foil and the sample under investigation, a new variable energy positron annihilation lifetime spectroscopy (PALS) system is being implemented. To this end, the SE emission from a carbon foil of 30nm thick has been investigated in transmission geometry. Considerable emission of SE from the foil with peak energy of about 5eV is observed. We have found that both the energy loss and dispersion of the positrons after transmission through the carbon foil are small enough for the proposed positron lifetime-depth profiling PALS system to be realised. These experimental results and the timing simulations of Cai et. al. [1] indicate that the proposed variable PALS based on secondary electrons generated by carbon foil have a high time resolution.

Ng, C. K.; Yang, B.; Fung, S.

2013-06-01

15

A simple shape-free model for pore-size estimation with positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy is one of the methods for estimating pore size in insulating materials. We present a shape-free model to be used conveniently for such analysis. A basic model in classical picture is modified by introducing a parameter corresponding to an effective size of the positronium (Ps). This parameter is adjusted so that its Ps-lifetime to pore-size relation merges smoothly with that of the well-established Tao-Eldrup model (with modification involving the intrinsic Ps annihilation rate) applicable to very small pores. The combined model, i.e., modified Tao-Eldrup model for smaller pores and the modified classical model for larger pores, agrees surprisingly well with the quantum-mechanics based extended Tao-Eldrup model, which deals with Ps trapped in and thermally equilibrium with a rectangular pore.

Wada, Ken; Hyodo, Toshio

2013-06-01

16

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

Microsoft Academic Search

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

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

1995-01-01

17

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

18

Positron Annihilation Energy and Lifetime Spectroscopy Studies for Radiation Defects in Stainless Steel  

SciTech Connect

High Energy proton (up to 800 MeV) and spallation neutron irradiated samples of stainless steel 316 L and Mod 9 CrlMo were studied using positron annihilation energy and lifetime spectroscopy. Irradiation damage caused to 316 L were up to 10 displacements per atom (dpa) and those for 9 CrlMo were up to about 2.5 dpa. Studies of T-parameter, which is calculated as the ratio of the number of counts in the wings of the Doppler-broadened 511 keV peak to the number of counts in the center of the peak, showed that it dropped sharply from 0 to 3.5 dpa, and continued dropping up to 10 dpa in 316 L. In 9 CrlMo, similarly, T-parameter dropped sharply from 0 dpa to 1 dpa, but from 1 dpa to 2.5 dpa it remained constant, indicating that the ''effective'' density of defects, as indicated by T-parameter saturated with the irradiation damage above 1 dpa in 9 CrlMo. We saw no apparent change in the positron lifetime for radiation damages above 3.5 dpa in 316 L. However, for steel 9 CrlMo the positron lifetime in defects increased with irradiation damage from 1.02 dpa to 2.46 dpa. This implies that the electron density at the annihilation sites, defects in this case, decreased. These results led us in both cases to investigate lower radiation damaged samples. We measured energy and lifetime spectra in 316 L and 9 CrlMo samples that were irradiated under the similar conditions as the above samples, but with radiation damage less than 0.1 dpa. These results fill in some of the gap between 0 and 1 dpa and suggest that most of the change in T-parameter occurs at irradiation damages below 0.054 dpa.

Tchelidze, L. [Department of Physics, Idaho State University, 785 S. 8th Ave., Campus Box 8106, Pocatello, Idaho 83209 (United States); Wells, D. P. [Department of Physics, Idaho State University, 785 S. 8th Ave., Campus Box 8106, Pocatello, Idaho 83209 (United States); Idaho Accelerator Center, 1500 Alvin Ricken Drive, Pocatello, Idaho 83201 (United States); Maloy, S. A. [Los Alamos National Lab, P.O. Box 1663, Los Alamos, NM 87545 (United States)

2009-03-10

19

Positron annihilation in small molecules  

NASA Astrophysics Data System (ADS)

Positron annihilation in room temperature samples of the molecular gases N2, O2, CO, N2O and CH4 has been studied in the density range below 10 amagat using the positron lifetime technique. Careful analyses of the density dependence of the free positron annihilation rates have been performed that have allowed the annihilation parameter, , to be extracted. We compare our results to those in the literature, and give recommended values from experiment. We have also synthesized the existing data for H2 and derived a recommended value for the annihilation parameter for this molecule.

Charlton, M.; Giles, T.; Lewis, H.; van der Werf, D. P.

2013-10-01

20

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

21

Chemical environment in halogenated styrene polymers studied by using positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

Polystyrene samples, incorporated with halogen elements (F, Cl, Br, I) on the para-position of the benzene ring, were studied using positron annihilation lifetime spectroscopy. It was found that the free-volume hole size is significantly affected by the internal Coulombic interaction of the halogen group, and is mainly related to the electronegativity of halogen-carbon bonds. In addition, it is found that the free-volume is secondarily modified by the steric effect of the side groups. The intensity of o-Ps has a linear relationship with the strength of the C-X bond and is strongly affected by the chemical environment in a halogenated styrene polymer system.

Zhang, R.; Wu, Y. C.; Chen, H.; Zhang, J.; Li, Y.; Sandreczki, T. C.; Jean, Y. C.

2003-10-01

22

Development of a pico-second life-time spectrometer for positron annihilation studies.  

National Technical Information Service (NTIS)

Positron annihilation technique is a sensitive probe to investigate various physico-chemical phenomena due to the ability to provide information about the electron momentum and density in any medium. While measurements on the Doppler broadening and angula...

P. K. Pujari T. Datta B. S. Tomar S. K. Das

1992-01-01

23

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

24

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

25

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

PubMed

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

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

2013-05-01

26

Positron Lifetime Spectroscopy  

NASA Astrophysics Data System (ADS)

We have assembled a positron lifetime spectrometer using two scintillation detectors in a slow-fast coincidence configuration. The decay of ^22Na serves as a convenient source of positrons. Positron energies are sufficiently high that positrons penetrate into the bulk of the surrounding material under study. A 1275 keV gamma ray emitted less than 10 ps following the positron decay serves as the lifetime start signal, while the detection of a 511 keV annihilation gamma signals the end of life of the positron. The first version of our spectrometer employs NaI scintillators that have good detection efficiencies but with time resolution of several nanoseconds are not particularly suitable for measuring sub-nanosecond lifetimes of positrons in metals. Recently we have replaced the NaI detectors with ones employing plastic scintillators, which offer a time resolution of better than 1 ns. First results of measurements performed with this apparatus will be discussed.

Martin, Jacob; Jaeger, Herbert

2010-04-01

27

Development of an Electron-Positron Source for Positron Annihilation Lifetime Spectroscopy.  

National Technical Information Service (NTIS)

The extremely strong fields produced by focusing powerful laser pulses into minute volumes of material enable relativistic effects to be used to generate radiation, to accelerate particles, and to produce electrons and positrons from vacuum. From applicat...

A. Modovanakis B. Hou I. Sokolov J. Easter J. Nees

2009-01-01

28

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

29

Structural study of polymer hydrogel contact lenses by means of positron annihilation lifetime spectroscopy and UV-vis-NIR methods.  

PubMed

A study has been conducted in order to determine presence of free volume gaps in the structure of structure of polymer hydrogel contact lenses made in phosphoryl choline technology and of the degree of defect of its structure. The study was made by means of positron annihilation lifetime spectroscopy. As a result of the conducted measurements, curves were obtained, which described numbers of counts of the acts of annihilation in the time function. The conducted studies revealed existence of three components ?(1), ?(2) and ?(3). The ?(3) component is attributed to the pick-off annihilation of o-Ps orthopositronium trapping by free volume gaps and provides information about geometrical parameters of the volumes. At the same time, the UV-vis-NIR spectrometry examination was conducted on the same samples in the spectral range 200-1,000 nm. PMID:23695358

Filipecki, J; Kocela, A; Korzekwa, P; Miedzinski, R; Filipecka, K; Golis, E; Korzekwa, W

2013-05-22

30

Positron annihilation studies in pyrophyllite  

Microsoft Academic Search

Results of positron annihilation studies in thermally treated natural pyrophyllite (PP) are presented. In this material we have found the positron lifetime component to range between 0.5 and 1 ns. This can be associated with voids whose evolution follows the dehydroxylation process induced by thermal treatment. The annealing of the PP at 1100circC caused the transition of the voids to

Jerzy Dryzek; Ewa Dryzek; Piotr Golonka

2002-01-01

31

Galactic Positron Annihilation Radiation  

NASA Astrophysics Data System (ADS)

Nine years of observations with the CGRO/OSSE instrument have allowed the first maps of positron annihilation radiation. The extraction of annihilation radiation from a complex spectrum composed of both compact and diffuse sources depends upon the unique spectral signatures of 511 keV line and positronium continuum emissions. We report here results of a re-analysis of OSSE obtained spectra of the galactic center region. We show updated maps of annihilation radiation, and discuss how these maps influence the interpretation of suggested sources of galactic positrons.

Milne, P. A.; Kurfess, J. D.; Strickman, M. S.; Leising, M. D.

2002-05-01

32

Positron lifetime in neutron-irradiated germanium  

SciTech Connect

A model is proposed for calculating the average positron lifetime (before annihilation) in the case when positrons are captured by traps of two types. The average positron lifetime calculated from the experimentally determined angular correlation of the annihilation ..gamma.. rays increases during stage I of isochronous annealing of neutron-irradiated germanium and decreases during stage II to the value before irradiation. The experimental and calculated dependences of the average positron lifetime on the annealing temperature are in qualitative agreement. The observed narrowing of the angular correlation curves and increase in the positron lifetime in neutron-irradiated germanium are inversely proportional to one another. A combined analysis of the annihilation, electrical, and x-ray experiments shows that during annealing of neutron-irradiated germanium the process of positron annihilation is influenced by two types of positron-sensitive defects, which are formed by irradiation (mainly disordered regions) and during stage I of the annealing process (probably divacancies).

Arifov, P.U.; Arutyunov, N.Y.

1980-02-01

33

Positron annihilation lifetime and Doppler broadening study in 50 MeV Li3+ ion irradiated polystyrene films  

NASA Astrophysics Data System (ADS)

Swift heavy ion (SHI) irradiation of polymeric materials results in the change of their free volume properties which have strong correlation with their macroscopic properties. The modification depends on the polymer and ion beam parameters, namely ion energy, fluence and ion species. Polystyrene films were irradiated with Li3+ ions of energy 50 MeV from 15 UD Pelletron accelerators at Inter University Accelerator Centre (IUAC), New Delhi, India to the fluences of 1011, 1012 and 1013 ions/cm2. Nanosized free volume parameters in the polymer have been studied by positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS). From o-Ps lifetime ?3, free volume hole radius, mean free volume of microvoids and fractional free volume are computed and modification in free volume with the fluence is studied. Free volume parameters change slowly with ion fluence with a decrease at the highest fluence of 1013 ions/cm2. The decrease in ?3 and I3 (reflecting the number of free volume holes) may be interpreted on the process of cross-linking. S parameter obtained from DBS measurements showed a minor decrease with increasing fluence.

Asad Ali, S.; Kumar, Rajesh; Nambissan, P. M. G.; Singh, F.; Prasad, Rajendra

2010-06-01

34

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

SciTech Connect

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

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

1995-09-01

35

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

SciTech Connect

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

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

2006-03-01

36

Positron Annihilation in Insulating Materials  

SciTech Connect

We describe positron results from a wide range of insulating materials. We have completed positron experiments on a range of zeolite-y samples, KDP crystals, alkali halides and laser damaged SiO{sub 2}. Present theoretical understanding of positron behavior in insulators is incomplete and our combined theoretical and experimental approach is aimed at developing a predictive understanding of positrons and positronium annihilation characteristics in insulators. Results from alkali halides and alkaline-earth halides show that positrons annihilate with only the halide ions, with no apparent contribution from the alkali or alkaline-earth cations. This contradicts the results of our existing theory for metals, which predicts roughly equal annihilation contributions from cation and anion. We also present result obtained using Munich positron microprobe on laser damaged SiO{sub 2} samples.

Asoka-Kumar, P; Sterne, PA

2002-10-18

37

Free-Radical Quenching of Positron Lifetimes  

Microsoft Academic Search

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

Stephan Berko; A. Joseph Zuchelli

1956-01-01

38

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

SciTech Connect

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

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

2010-12-01

39

Ligament coarsening in nanoporous gold: Insights from positron annihilation study  

NASA Astrophysics Data System (ADS)

Positron lifetime experiments, coupled with scanning electron microscopy studies, have been used to follow the coarsening of the ligaments of nanoporous Au (np-Au), prepared by electrochemical dealloying of Ag75Au25. Positron lifetime measurements in the as-prepared np-Au indicate two lifetime components, identified with annihilation at vacancies within the ligaments and at the ligament-pore interface. The variation of these lifetime parameters with annealing temperature indicates distinct changes that correlate the migration of vacancies to the growth of ligaments. The lifetime component, corresponding to the annihilation at the ligament-pore interface, shows signature of ligament surface instability.

Viswanath, R. N.; Chirayath, V. A.; Rajaraman, R.; Amarendra, G.; Sundar, C. S.

2013-06-01

40

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

NASA Astrophysics Data System (ADS)

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

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

2006-02-01

41

Photon correlations in positron annihilation  

SciTech Connect

The two-photon positron annihilation density matrix is found to separate into a diagonal center-of-energy factor implying maximally entangled momenta, and a relative factor describing decay. For unknown positron injection time, the distribution of the difference in photon arrival times is a double exponential at the para-Ps decay rate, consistent with experiment [V. D. Irby, Meas. Sci. Technol. 15, 1799 (2004)].

Gauthier, Isabelle; Hawton, Margaret [Department of Physics, Lakehead University, Thunder Bay, Ontario, P7B 5E1 (Canada)

2010-06-15

42

Positron lifetime measurements by proton capture  

NASA Astrophysics Data System (ADS)

A positron lifetime spectroscopy (PLS) technique was developed using coincident ? rays induced by proton capture. Proton capture in some light elements induce coincident MeV ? rays, allowing positron lifetime to be measured. One ? quantum provides a start signal for the positron lifetime spectrometer, whereas the other ? quantum bombards the sample under investigation, generating a positron inside it through pair production. The stop signal is obtained from the detection of one of the two 511 keV photons emitted from positron annihilation with the sample electrons. This new technique can extend PLS, which is a powerful tool to identify the size and concentration of defects, to thick materials and a broad range of applications. It also eliminates the source contribution from the measured spectra, which may lead to the identification of more defect types in a sample.

Selim, F. A.; Wells, D. P.; Harmon, J. F.

2005-03-01

43

Free volume properties in liquid crystals studied by positron annihilation  

Microsoft Academic Search

Positron annihilation lifetime technique was applied to study the electric field dependence of size, intensity and size distribution of free volumes in various liquid crystals negative nematic MBBA(N-(4-methoxybenzylidene)-4-n-butylaniline), positive nematic 5CB (4-cyano-4-n-pentylbiphenyl) and cholesteric mixture of MBBA and cholesteryl oleate. Positron annihilation decay curves were obtained in the direct and alternate electric field range from 0 to 120 V\\/mm, and

Ch. Zhao; Y. Ujihira

1996-01-01

44

Positron annihilation and cluster formation in nitrogen fluid  

Microsoft Academic Search

The annihilation of positrons in nitrogen has been studied both experimentally and theoretically. Lifetime measurements over the temperature range 60 to 400 K and at pressures up to 80 bar show that below 300 K, clustering of nitrogen molecules occurs around the positron. Experimental data are compared to those in other gases. Results of density functional calculations are in very

K. Rytsola; K. Rantapuska; P. Hautojarvi

1984-01-01

45

Positron annihilation spectroscopy in carbon black–silica–styrene butadiene rubber (SBR) composites under deformation  

Microsoft Academic Search

Positron annihilation lifetime (PAL) spectroscopy has been used to investigate microstructural properties of carbon black–silica–styrene butadiene rubber (SBR) composites. The positron annihilation lifetime shows a direct dependence on the carbon black type as well as the silica content added to the rubber compound. The effect of the tensile strain (?=0–150%) on the positron annihilation parameters has been investigated by constructing

M. Mohsen; M. H. Abd-El Salam; A. Ashry; A. Ismail; H. Ismail

2005-01-01

46

Positron annihilation measurements of Dy-doped YBCO superconductors  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime and Doppler broadening energy spectra have been measured in Dy-doped high-temperature YBCO superconductors as a function of temperature between 40 and 295 K. The defect-related lifetime component ?2 was approximately constant from room temperature to above Tc at about 90 K and then showed a step-like decrease. These results indicated that the electronic structure changed below Tc. It was also found that a transition in the positron Doppler line-shape parameter S occurred at the superconducting transition temperature. These results show that the average electron momentum at the annihilation sites increases as temperature is lowered across the superconducting transition range.

Jung, K.; Byrne, J. G.; de Andrade, M. C.; Maple, M. B.

1996-02-01

47

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

PubMed

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

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

2002-01-15

48

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

NASA Astrophysics Data System (ADS)

An experimental investigation was performed to study the effect on the free volume of the advance of the cross-linking reaction in a copolymer of styrene butadiene rubber by sulfur vulcanization. The dynamic modulus and loss tangent were evaluated over samples cured for different times at 433 K by dynamic mechanical tests over a range of frequencies between 5 and 80 Hz at temperatures between 200 and 300 K. Using the William-Landel-Ferry relationship, master curves were obtained at a reference temperature of 298 K and the coefficients c01 and c02 were evaluated. From these parameters the dependence of the free volume on the cure time is obtained. Positron annihilation lifetime spectroscopy was also used to estimate the size and number density of free volume sites in the material. The spectra were analyzed in terms of continuous distributions of free volume size. The results suggest an increase of the lower free volume size when cross linking takes place. Both techniques give similar results for the dependence of free volume on the time of cure of the polymer.

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

2002-02-01

49

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

NASA Astrophysics Data System (ADS)

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

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

2012-02-01

50

Positron Annihilation Induced Auger Electron Spectroscopy.  

National Technical Information Service (NTIS)

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...

A. Weiss A. R. Koymen D. Mehl K. O. Jensen C. Lei

1990-01-01

51

Positron annihilation induced Auger electron emission  

SciTech Connect

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

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

1988-01-01

52

Distribution of positron annihilation radiation  

NASA Astrophysics Data System (ADS)

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.

Milne, Peter A.

2006-10-01

53

Positron scattering and annihilation in hydrogenlike ions  

NASA Astrophysics Data System (ADS)

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

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

2013-09-01

54

Low energy positron annihilation study of composite reverse osmosis membranes  

NASA Astrophysics Data System (ADS)

Positron annihilation with a slow positron beam was applied to the characterization of composite reverse osmosis membranes. The results, obtained at different positron incident energies, indicated that the membranes are asymmetric with respect to the pore structure, consisting of a thin top layer with little porosity and an underlying thick porous layer. A relationship between the longest positron lifetime near the membrane surface and the salt rejection rate was discussed in terms of the free-volume hole size for the thin top layer.

Chen, Z.; Ito, K.; Yanagishita, H.; Oshima, N.; Suzuki, R.; Kobayashi, Y.

2011-01-01

55

Positron annihilation study on ZnO-based scintillating glasses  

NASA Astrophysics Data System (ADS)

Positron lifetime of ZnO-based scintillating glasses (55 - x)SiO 2-45ZnO- xBaF 2 ( x = 5, 10, 15 mol%) were measured with a conventional fast-fast spectrometer. Three positron lifetime components ?1, ? 2, and ?3 are ˜0.23 ns, ˜0.45 ns, and ˜1.6 ns, respectively. All the three positron lifetime components first increase with increasing BaF 2 concentration from 5 mol% to 10 mol%, then decreases as BaF 2 further increases to 15 mol%. The result suggests that the glass sample with 10 mol% BaF 2 contains the highest defect density, and is in excellent agreement with glass chemistry, glass density, thermal properties, and calculated crystallinity. Therefore, positron annihilation lifetime measurement is an effective tool for analyzing defects in ZnO-based scintillating glasses.

Nie, Jiaxiang; Yu, Runsheng; Wang, Baoyi; Ou, Yuwen; Zhong, Yurong; Xia, Fang; Chen, Guorong

2009-04-01

56

Positron Annihilation in the Bipositronium Ps2  

SciTech Connect

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

Bailey, David H.; Frolov, Alexei M.

2005-07-01

57

Positron annihilation in solid and liquid Ni  

SciTech Connect

New techniques have been developed for the study of metals via positron annihilation which provide for the in-situ melting of the samples and subsequent measurements via Doppler broadening of positron-annihilation radiation. Here we report these metods currently in use at our laboratory; ion implantation of /sup 58/Co and the use of Al/sub 2/O/sub 3/ crucibles for in-situ melting followed by the decomposition of the Doppler-broadened spectrum into a parabolic and a Gaussian component. Our earliest results obtained for pure Ni in the polycrystalline solid and in the liquid state are compared. An interesting similarity is reported for the distributions of the high-momentum (Gaussian) component for positrons annihilating in vacancies at high temperatures and those annihilating in liquid Ni.

Fluss, M.J.; Smedskjaer, L.C.; Chakraborty, B.; Chason, M.K.

1982-03-01

58

Defect characterization with positron annihilation. [Review  

SciTech Connect

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

Granatelli, L; Lynn, K G

1980-01-01

59

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

60

Positron annihilation in low-temperature rare gases. II. Argon and neon  

Microsoft Academic Search

Lifetime measurements of slow-positron and ortho-positronium (o-Ps) annihilation have been made in argon and neon gases at room temperature and below. The argon experiments cover the temperature range 115-300 K and the density range 0.0356-0.0726 g\\/cm3 (~= 20-40 amagat). The slow-positron spectra in argon exhibit a departure from free-positron annihilation below 200 K. The departure becomes more marked as the

K. F. Canter; L. O. Roellig

1975-01-01

61

Molecular motions at low temperature observed by positron annihilation  

Microsoft Academic Search

Positron annihilation lifetime spectroscopy (PALS) was applied to low-density polyethylene (LDPE) and ethylene-methylmethacrylate (EMMA) copolymers with various MMA contents. The experiments were conducted in the dark as well as under visible light condition. In the dark at a low temperature of about 30 K, the o-Ps intensity was observed to increase with the elapsed time, and the increasing rate depended on

Chunqing He; Takenori Suzuki; Lin Ma; Masaru Matsuo; V. P Shantarovich; Kenjiro Kondo; Yasuo Ito

2002-01-01

62

Analysis of positron lifetime spectra via a fast Prony algorithm  

NASA Astrophysics Data System (ADS)

A new fast method is used to analyze positron annihilation lifetime spectra. The regularization procedure to solve the inverse problem of finding parameters of useful signals present in the data is formulated. A new approach to determine the number of components in a spectrum is proposed. Results on simulated spectra demonstrate the high resolution capability of the method.

Kul'Ment'ev, A. I.

2004-03-01

63

The Influence of Magnetic Field on Annihilation of Positrons in Corroded Steel St-20  

NASA Astrophysics Data System (ADS)

The positron lifetime in steel St-20 were determined before and following corrosion in HCl vapors. The influence of external magnetic field and remanent magnetization state on the parameters of the positron lifetime spectrum in the steel were investigated. It was found that the presence of magnetic field causes a decrease in the intensity of the component connected with annihilation of positrons in the close-to-the-surface lattice defects.

Pietrzak, R.; Szatanik, R.; Jaworska, A.

2006-11-01

64

Symmetry based study of positron annihilation spectra.  

National Technical Information Service (NTIS)

We describe a method for off-line analysis of spectra measured by two-dimensional angular correlation of annihilation radiation (2D-ACAR) positron spectroscopy. The method takes into account, at all its stages, two salient data features: the piecewise con...

G. Adam S. Adam

1995-01-01

65

Positron Annihilation in gamma-Ray Bursts.  

National Technical Information Service (NTIS)

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...

A. K. Harding

1990-01-01

66

Positron Annihilation in Rare-Earth Hydrides  

Microsoft Academic Search

Analysis of the angular correlations of positron-annihilation radiation from erbium, gadolinium, holmium, and ytterbium hydrides indicates that the electronic structure of these materials is basically metallic in nature. As was previously found in the case of cerium hydride, the angular correlations were found to be broader than those from their parent metals by amounts which were consistent with the assumption

M. P. CHOUINARDt; D. R. Gustafson

1971-01-01

67

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

Microsoft Academic Search

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

Chakraborty

1982-01-01

68

Defects in metals. [Positron annihilation spectroscopy  

SciTech Connect

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

Siegel, R.W.

1982-06-01

69

In-situ positron lifetime spectroscopy of radiation damage by simultaneous irradiation of slow-positron and ion beams  

NASA Astrophysics Data System (ADS)

An in-situ positron analysis system has been developed for positron annihilation lifetime spectroscopy during ion beam irradiation. An electron linear accelerator was used to produce a slow positron beam for this system. Positron lifetime spectroscopy of SiO2/Si and annealed pure Fe samples was successfully demonstrated during simultaneous irradiation with a 150 keV Ar+ beam. The lifetime spectra changed with increasing ion dose, indicating a decrease in ortho-positronium intensity (SiO2) and the decrease in positron diffusion length (Fe).

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

2013-06-01

70

Study of the Mg-Cd system by positron annihilation methods  

NASA Astrophysics Data System (ADS)

The problem of preferential positron annihilation in binary alloys has not been satisfactorily solved in the past. We examine this effect experimentally in Mg-Cd alloys using the new technique of Doppler broadening measurements with background reduction which allows us to observe positron annihilation with core electrons. Conventional positron lifetime spectroscopy is applied as well. We measure the Doppler spectra and positron lifetimes for selected alloys in the whole concentration range of the Mg-Cd system. The analysis of experimental results is supported by theoretical calculations. The conclusion is given that a small positron preferential annihilation occurs at Mg sites. Besides, the temperature dependencies of the peak counting rate of the angular correlation curve are measured for several samples and the vacancy formation energies are established. In the case of Mg-20 at.% Cd alloy, the increase of the peak counting rate characteristic for generation of thermal vacancies shows a discontinuity close to the order-disorder transition temperature.

Dryzek, E.; Kuriplach, J.; Dryzek, J.

1998-07-01

71

High energy positrons from annihilating dark matter  

SciTech Connect

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

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

2009-12-15

72

Positron Annihilation Technique Applied to the Study of Inhomogeneous Solids: Aluminium Alloys, Layered Compound 1T-TaS sub 2.  

National Technical Information Service (NTIS)

This thesis is an experimental investigation of inhomogeneous solids using the standard positron annihilation techniques (angular correlation of annihilation gamma -rays, Doppler broadening, lifetime). The investigations are concerned with different types...

F. Boileau

1983-01-01

73

Diffusion of permanent liquid dye molecules in human hair investigated by positron lifetime spectroscopy  

Microsoft Academic Search

The diffusion behavior of a commercial permanent liquid hair dye in human hair has been investigated using positron annihilation lifetime spectroscopy (PALS) and gravimetric sorption method. The positron technique makes it possible to non-invasively characterize the angstrom sized free volume holes in hair, which are supposed to be express pathways for diffusion of small molecules. The o-Ps lifetime parameters ?3

M. N. Chandrashekara; C. Ranganathaiah

2009-01-01

74

Apparatus for photon activation positron annihilation analysis  

DOEpatents

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

Akers, Douglas W. (Idaho Falls, ID)

2007-06-12

75

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

76

Positron scattering and annihilation from hydrogenlike ions  

Microsoft Academic Search

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

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

2004-01-01

77

Positron annihilation in low-temperature rare gases. II. Argon and neon  

Microsoft Academic Search

Lifetime measurements of slow-positron and ortho-positronium (o-Ps) ; annihilation were made in argon and neon gases at room temperature and below. ; The argon experiments cover the temperature range 115 to 300°K and the ; density range 0.0356 to 0.0726 g\\/cm³ (approximately equal to 20 to 40 ; amagat). The slow-positron spectra in argon exhibit a departure from free-; positron

K. F. Canter; L. O. Roellig

1975-01-01

78

Positron annihilation in carbon black-polymer composites  

NASA Astrophysics Data System (ADS)

Positron lifetime spectra and Doppler broadening of the annihilation line were measured for samples of carbon black/polyethylene and polypropylene composites with varying amount of the filler. Tensile strength, resistivity, EPR resonance were studied in addition to have the samples better characterized. The decrease in resistivity of samples, accompanied by the worsening of mechanical properties, the drop both in the intensities of Ps lifetime components in the lifetime spectra and in the line-shape parameter values, were observed with increase in the carbon black content. The presence of radicals associated with aromatic structure of the carbon sheets and others associated with the surface oxygen functional groups was established by EPR measurements for the carbon blacks being used as fillers. The carbon black of the highest specific surface area influenced the measured characteristics the most.

D?bowska, M.; Rudzi?ska-Girulska, J.; Jezierski, A.; Pasternak, A.; Po?niak, R.

2000-06-01

79

Optimization of the Scintillator Size for Positron Lifetime Measurements  

NASA Astrophysics Data System (ADS)

Optimization of the measurement condition for positron annihilation lifetime spectroscopy (PALS) is discussed from the viewpoint of non-destructive "on-site" material inspection. Numerical analysis based on a least-squares estimation and experiments with various sizes of BaF2 scintillators by conventional PALS suggested that the use of relatively large BaF2 scintillators enables on-site material inspection by PALS with reasonable accumulation time and time resolution.

Yamawaki, M.; Kobayashi, Y.; Hattori, K.; Watanabe, Y.

2013-06-01

80

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

81

Positron annihilation measurements of Pr-containing superconductor compounds  

SciTech Connect

Positron annihilation lifetime and Doppler broadening energy spectra have been measured in Pr-containing YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} as a function of temperature between 18 and 295 K. The defect-related positron lifetime component {tau}{sub 2} was independent of temperature for the PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} compound and was very close to the value {tau}{sub 1} associated with bulk or defect-free material. These results indicate that the structure of this compound is nearly free of defects which trap positrons and that there is no electronic structure change during cooling. There was a slight temperature dependence below {ital T}{sub {ital c}} for the Y{sub 0.5}Pr{sub 0.5}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} compound. It was also found that a change in the positron Doppler line-shape parameter {ital S} occurred at the superconducting transition temperature for 0.5 Pr-containing compound but not for the 1.0 Pr-containing compound. These results show that the average electron momentum at the annihilation sites increases as temperature is lowered across the superconducting transition range for the 0.5 Pr-containing compound but not for the PrBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} compound. {copyright} {ital 1995} {ital American} {ital Institute} {ital of} {ital Physics}.

Jung, K.; Byrne, J.G. [Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84112 (United States); de Andrade, M.C.; Maple, M.B. [Department of Physics and Institute for Pure and Applied Physical Sciences, University of California at San Diego, La Jolla, California 92093-0360 (United States)

1995-11-01

82

Positron-annihilation study of radiation defects in sodium azide  

SciTech Connect

Annihilation-photon angular correlation has been used to examine radiation defects in sodium azide capable of trapping positrons. The calculated and measured characteristics have been determined for various defects, including micropores filled by radiolytic nitrogen. The positron annihilation rates have been determined for the regions around radiation defects.

Etin, G.I.; Ryabykh, S.M.

1987-07-01

83

Positron Annihilation Techniques (PAT) in Polymer Science and Engineering  

Microsoft Academic Search

For more than 20 years positron annihilation research has been a branch of basic physics and chemistry. Recently, however, the positron annihilation technique (PAT) has emerged into the field of applied science, thus following the well-known trend that fundamental techniques of basic physics turn out to be of great importance in applied science and technology. Examples of this are x-ray

A. E. Hamielec; M. Eldrup; O. Mogensen

1973-01-01

84

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

85

RHESSI Positron-Electron Annihilation Line Observations  

NASA Astrophysics Data System (ADS)

We discuss our analysis of the electron-positron annihilation line and continuum from flares observed by RHESSI. RHESSI observed what appears to be a thermally broadened line (~6.5 keV FWHM) at 511 keV over the first 10 min of the 2003 October 28 flare that was consistent with emission from a plasma >105 K. The line narrowed to a width of ~1 keV over the next 2 min. The narrow width suggests emission from a <104 K region that is at least 20% ionized. This suggests a rapid variation in the solar atmosphere at densities <1014 H cm-3. In contrast the shape of the annihilation line from the first ten minutes of the 2003 November 2 flare is consistent with emission from a quiet solar atmosphere at ~5200 K; however, the flux in the 3 ? continuum may be inconsistent with this location. RHESSI has recently observed gamma-ray emission from flares erupting from AR0720 on 2005 January 15, 17, 19, and 20. Our preliminary analysis of the January 20 flare reveals a gamma spectrum with a hard continuum, intense lines at 511 keV and 2223 keV (neutron capture), and relatively weak nuclear de-excitation lines. This gamma-ray spectrum is consistent with a very-hard accelerated particle spectrum that produces most of the positrons from decay of positive pi mesons produced near the photosphere. The annihilation line has a width of ~6.5 keV that is again suggestive of thermal broadening. This work has been supported by NASA DPR NNG04ED181 and by ONR.

Share, G. H.; Murphy, R. J.; Smith, D. M.; Lin, R. P.; Schwartz, R. A.

2005-05-01

86

Positron transport and annihilation in expanding flows: A model for the high-latitude annihilation feature  

SciTech Connect

Positron sources in the vicinity of the Galactic Center can account for the recently discovered high-latitude 0.511 MeV annihilation radiation if the positrons are convected away from the disk of the Galaxy and annihilate during transport. Here we describe our treatment for positron energy loss and thermalization, transport, and annihilation. The concept of the Maxwell-Boltzmann length (a generalized stopping distance) is outlined, and a model map of the distribution of annihilation radiation is presented for idealized flows.

Dermer, Charles D.; Skibo, Jeffrey G. [E. O. Hulburt Center for Space Research, Code 7653, Naval Research Laboratory, Washington, District of Columbia 20375 (United States)

1997-05-10

87

Yield and internal stresses in aluminum filled epoxy resin. A compression test and positron annihilation analysis  

Microsoft Academic Search

The influence of the filler content on the mechanical properties of an epoxy resin composite filled with aluminum powder was investigated. Compressive tests were performed at room temperature and at different strain rates. The response of the composites was also studied by positron annihilation lifetime spectroscopy. The dependence of the yield stress on the filler content is shown. The results

S. Goyanes; G. Rubiolo; A. Marzocca; W. Salgueiro; A. Somoza; G. Consolati; I. Mondragon

2003-01-01

88

On the free volume evolution in a deformed epoxy composite. A positron annihilation study  

Microsoft Academic Search

After fabrication of an epoxy system filled with aluminum powder, followed by inelastic deformations under compression of the specimens, Positron Annihilation Lifetime Spectroscopy (PALS) was used to follow the evolution of the free-volume holes in the matrix. In order to describe the micromechanical deformation mechanism that takes place in the matrix around the inclusions, the experimental free-volume holes data were

S. Goyanes; G. Rubiolo; W. Salgueiro; A. Somoza

2005-01-01

89

Positron annihilation study of the high-Tc (Bi, Pb)2Sr2Ca2Cu3Ox superconductor  

NASA Astrophysics Data System (ADS)

Positron lifetime spectroscopy (PLS) and positron Doppler-broadening spectroscopy (PDBS) were applied to the high-Tc lead-doped Bi2Sr2Ca2Cu3Ox (BPSCCO 2223) superconductor as a function of temperature. Neither positron lifetimes nor Doppler parameters (S, W, and S/W) showed significant change through Tc. This may result from having the highest positron density in the open BiO2 double layers and no significant positron density in the superconducting CuO2 layers where positrons, if mainly present, are known to be sensitive to the transition in other high-Tc superconductors. Doppler parameters showed that the probability of positron annihilations with core electrons in the lattice slightly increased and that the probability of positron annihilations with conduction electrons slightly decreased as temperature decreased from ambient temperature to 20 K. The lifetime associated with positron annihilations in the perfect lattice of the sample (?1) was 209 ps and, due to the annihilations at internal surfaces or voids in the sample (?2) was about 540 ps, independent of temperature. Finally, the mean lifetime for BSCCO 2223 was about 307 ps.

Lim, H. J.; Byrne, J. G.

1997-03-01

90

Positron annihilation and ESR study of irradiation-induced defects in silica glass  

Microsoft Academic Search

The angular correlation of positron annihilation radiation (ACAR), positron lifetime and electron spin resonance (ESR) have been measured on fused and synthetic silica glass samples before and after irradiation with fast neutrons up to a dose of 8.6 × 1018 n\\/cm2 at about 150°C or with 3 MeV electrons up to a dose of 1 × 1018 e\\/cm2 below 50°C.

M. Hasegawa; M. Tabata; M. Fujinami; Y. Ito; H. Sunaga; S. Okada; S. Yamaguchi

1996-01-01

91

Study of defects in implanted silica glass by depth profiling Positron Annihilation Spectroscopy  

NASA Astrophysics Data System (ADS)

Positron Annihilation Spectroscopy (PAS) performed with continuous and pulsed positron beams allows to characterize the size of the intrinsic nano-voids in silica glass, their in depth modification after ion implantation and their decoration by implanted ions. Three complementary PAS techniques, lifetime spectroscopy (LS), Doppler broadening spectroscopy (DBS) and coincidence Doppler broadening spectroscopy (CDBS) will be illustrated by presenting, as a case study, measurements obtained on virgin and gold implanted silica glass.

Brusa, R. S.; Mariazzi, S.; Ravelli, L.; Mazzoldi, P.; Mattei, G.; Egger, W.; Hugenschmidt, C.; Löwe, B.; Pikart, P.; Macchi, C.; Somoza, A.

2010-10-01

92

Resolvability of defect ensembles with positron annihilation studies  

SciTech Connect

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

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

1984-11-12

93

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

NASA Astrophysics Data System (ADS)

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

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

2013-02-01

94

Observations of diffuse galactic positron annihilation radiation  

NASA Astrophysics Data System (ADS)

The Galactic plane was scanned nearly three times by the University of California, San Diego/Massachusetts Inst. of Tech. (UCSD/MIT) Hard X-ray and Low Energy Gamma-Ray Experiment on HEAO-1 from August 1977 through September 1978. Its Medium Energy Detectors were of the NaI/CsI phoswich type and operated over the 100 keV to 2 MeV range, with a 17 deg Full Width at Half Maximum (FWHM) field of view and a 9% energy resolution at 511 keV. Sky maps for each epoch of observation were constructed in several energy bands. After subtraction of known point sources, a component associated with the galactic plane remains, whose spectrum is consistent with a power law and a positron annihilation spectrum. In the 333 to 583 keV energy band the flux is concentrated within +/- 35 deg of the galactic center, and the ratio of lux/radian (anticenter) is high, with a 2 sigma lower limit of 13. The parameters of the galactic center region's annihilation spectrum are positronium fraction of 0.9 +/- 0.1 and 511 keV flux of (2.0 +/- 0.7) x 10-3 photons/((sq cm(s)(rad)).

Briggs, M. S.; Gruber, D. E.; Matteson, J. L.; Peterson, L. E.

1993-12-01

95

Resonances and Bound States in Positron Annihilation on Molecules  

NASA Astrophysics Data System (ADS)

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

Surko, C. M.

2007-10-01

96

Free-volume changes at nanoscale in doped polyacrylic acid studied by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Polyacrylic acid (PAA) doped with carbon black (CB), chromium oxide (Cr2O3) and cupferron with different wt% (0.25%, 0.50%, 0.75%, and 1%) was studied using positron annihilation lifetime (PAL) technique and Doppler broadening of annihilation radiation (DBAR). Ortho-positronium lifetime components (?3 and I3) were used to estimate the nanoscale free-volume hole sizes (Vf) and its fractions (f). It was found that the hole size Vf and its fractions f as well as S-parameters decreased at high value of doping concentration due to dopants-polymer formation. These results are supported by a significant narrowing in the nanoscale free-volume hole size distributions. The correlation between positron annihilation parameters and electric conductivity are discussed.

Gomaa, Ehsan

2007-03-01

97

Enhancement of positron-atom annihilation near the positronium formation threshold.  

PubMed

The behavior of the positron- 2 gamma annihilation rate on an atomic target near the positronium (Ps) formation threshold is determined. When the positron energy epsilon approaches the threshold epsilon(thr) from below, the effective number of electrons contributing to the annihilation, Z(eff), grows as Z(eff) approximately A/square root of [epsilon(thr)-epsilon], where A is related to the size of the Ps formation cross section, sigma(Ps) approximately B square root of [epsilon-epsilon(thr)], by A = B square root of [2 epsilon(thr)]/32 pi (in atomic units). Taking account of the finite Ps lifetime eliminates the singularity in Z(eff) and shows that close to threshold the positron annihilation cross section is identical to the para-Ps formation cross section. PMID:11955233

Gribakin, G F; Ludlow, J

2002-04-05

98

Positron annihilation studies of zirconia doped with metal cations of different valence  

NASA Astrophysics Data System (ADS)

New results obtained by applying positron annihilation spectroscopy to the investigation of zirconia-based nanomaterials doped with metal cations of different valence are reported. The slow-positron implantation spectroscopy combined with Doppler broadening measurements was employed to study the sintering of pressure-compacted nanopowders of tetragonal yttria-stabilised zirconia (t-YSZ) and t-YSZ with chromia additive. Positronium (Ps) formation in t-YSZ was proven by detecting 3?-annihilations of ortho-Ps and was found to gradually decrease with increasing sintering temperature. A subsurface layer with enhanced 3?-annihilations, compared to the deeper regions, could be identified. Addition of chromia was found to inhibit Ps formation. In addition, first results of positron lifetime measurements on nanopowders of zirconia phase-stabilised with MgO and CeO2 are presented.

Prochazka, I.; Cizek, J.; Melikhova, O.; Konstantinova, T. E.; Danilenko, I. A.; Yashchishyn, I. A.; Anwand, W.; Brauer, G.

2013-06-01

99

Positron annihilation and ESR study of irradiation-induced defects in silica glass  

NASA Astrophysics Data System (ADS)

The angular correlation of positron annihilation radiation (ACAR), positron lifetime and electron spin resonance (ESR) have been measured on fused and synthetic silica glass samples before and after irradiation with fast neutrons up to a dose of 8.6 × 1018 n/cm2 at about 150°C or with 3 MeV electrons up to a dose of 1 × 1018 e/cm2 below 50°C. The ACAR curves are deconvoluted into two Gaussian components: a narrow and a broad component. The narrow component is due to self-annihilation of ortho-state of positronium (p-Ps) formed in "intrinsic" structural voids with radius of about 0.3 nm, while the broad component comes from pick-off annihilation of ortho-Ps and annihilation of positrons with valence electrons presumably associated with oxygen. Detailed post-irradiation experiments reveal that two kinds of positron trapping centers (defects) are introduced. Both kinds of the defects give almost the same broad component and markedly suppress Ps formation. This suggests that they are oxygen related centers. The first type defects with the positron lifetime of about 0.25 ns anneal out at 400-500°C, while the second type defects with lifetime of 0.47 ns recover after annealing at about 600°C. Electron spin resonance (ESR) measurements were also made on the same samples to detect irradiation-induced paramagnetic center (defects): E' centers, peroxyradicals (POR) and non-bridging oxygen hole centers (NBOHC). The correspondence between the positron trapping centers and the ESR-active paramagnetic centers is not straightforward, partly because positrons can be trapped not only at the paramagnetic defects but also at diamagnetic defects. However, possible relations between these positron trapping centers and the paramagnetic defects are discussed.

Hasegawa, M.; Tabata, M.; Yamaguchi, S.; Fujinami, M.; Ito, Y.; Sunaga, H.; Okada, S.

1996-08-01

100

Positron annihilation studies of moisture in graphite-reinforced composites  

SciTech Connect

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

Singh, J.J. (NASA, Langley Research Center, Hampton, Va.); Holt, W.H. (U.S. Navy, Naval Surface Weapons Center, Dahlgren, Va.); Mock, W., Jr. (Vought Corp., Dallas, Tex.)

1980-07-01

101

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

NASA Astrophysics Data System (ADS)

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 rates resolved as a function of positron energy reveal the presence of vibrational Feshbach resonances (VFRs) for many polyatomic molecules. These resonances lead to strong enhancement of the annihilation rates. They also provide evidence that positrons bind to many molecular species. A quantitative theory of VFR-mediated attachment to small molecules is presented. It is tested successfully for selected molecules (e.g., methyl halides and methanol) where all modes couple to the positron continuum. Combination and overtone resonances are observed and their role is elucidated. Molecules that do not bind positrons and hence do not exhibit such resonances are discussed. In larger molecules, annihilation rates from VFR far exceed those explicable on the basis of single-mode resonances. These enhancements increase rapidly with the number of vibrational degrees of freedom, approximately as the fourth power of the number of atoms in the molecule. While the details are as yet unclear, intramolecular vibrational energy redistribution (IVR) to states that do not couple directly to the positron continuum appears to be responsible for these enhanced annihilation rates. In connection with IVR, experimental evidence indicates that inelastic positron escape channels are relatively rare. Downshifts of the VFR from the vibrational mode energies, obtained by measuring annihilate rates as a function of incident positron energy, have provided binding energies for 30 species. Their dependence upon molecular parameters and their relationship to positron-atom and positron-molecule binding-energy calculations are discussed. Feshbach resonances and positron binding to molecules are compared with the analogous electron-molecule (negative-ion) cases. The relationship of VFR-mediated annihilation to other phenomena such as Doppler broadening of the gamma-ray annihilation spectra, annihilation of thermalized positrons in gases, and annihilation-induced fragmentation of molecules is discussed. Possible areas for future theoretical and experimental investigation are also discussed.

Gribakin, G. F.; Young, J. A.; Surko, C. M.

2010-07-01

102

Photoluminescence and positron annihilation spectroscopic investigation on a H(+) irradiated ZnO single crystal.  

PubMed

Low temperature photoluminescence and room temperature positron annihilation spectroscopy have been employed to investigate the defects incorporated by 6 MeV H(+) ions in a hydrothermally grown ZnO single crystal. Prior to irradiation, the emission from donor bound excitons is at 3.378 eV (10 K). The irradiation creates an intense and narrow emission at 3.368 eV (10 K). The intensity of this peak is nearly four times that of the dominant near band edge peak of the pristine crystal. The characteristic features of the 3.368 eV emission indicate its origin as a 'hydrogen at oxygen vacancy' type defect. The positron annihilation lifetime measurement reveals a single component lifetime spectrum for both the unirradiated (164 ± 1 ps) and irradiated crystal (175 ± 1 ps). It reflects the fact that the positron lifetime and intensity of the new irradiation driven defect species are a little higher compared to those in the unirradiated crystal. However, the estimated defect concentration, even considering the high dynamic defect annihilation rate in ZnO, comes out to be ?4 × 10(17) cm(-3) (using SRIM software). This is a very high defect concentration compared to the defect sensitivity of positron annihilation spectroscopy. A probable reason is the partial filling of the incorporated vacancies (positron traps), which in ZnO are zinc vacancies. The positron lifetime of ?175 ps (in irradiated ZnO) is consistent with recent theoretical calculations for partially hydrogen-filled zinc vacancies in ZnO. Passivation of oxygen vacancies by hydrogen is also reflected in the photoluminescence results. A possible reason for such vacancy filling (at both Zn and O sites) due to irradiation has also been discussed. PMID:22790024

Sarkar, A; Chakrabarti, Mahuya; Sanyal, D; Bhowmick, D; Dechoudhury, S; Chakrabarti, A; Rakshit, Tamita; Ray, S K

2012-07-12

103

High-resolution of positron annihilation on molecules  

Microsoft Academic Search

The use of positrons as atomic and molecular probes opens new avenues to understanding of atomic and molecular structure. The mechanism responsible for unexpectedly large positron annihilation rates exhibited by certain large molecules has remained an unsolved problem in this field for decades. To date, experimental attempts to gain insight into the cause of these large rates used thermal distributions

L. D. Barnes; S. J. Gilbert; C. M. Surko

2002-01-01

104

Positron scattering and annihilation from hydrogen-like ions  

Microsoft Academic Search

The past few years has seen the Configuration Interaction (CI) method applied to study the interactions of positrons with various one- and two-electron atoms. Here the Kohn-variational method is used with a CI type trial wave function and applied to the scattering and annihilation of positrons and hydrogenic ions. The resultant J = 0 and J = 1 phase shifts

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

2004-01-01

105

Defects in nitrides, positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

In-grown group III (cation) vacancies (VGa, VAl, VIn) in GaN, AlN and InN tend to be complexed with donor-type defects These donor defects may in principle be residual impurities such as O or H, n-type dopants such as Si, or intrinsic defects such as the N vacancy (VN). The cation vacancies and their complexes are generally deep acceptors, and hence they compensate for the n-type conductivity and add to the scattering centers limiting the carrier mobility in these materials. Mg doping reduces the group III vacancy concentrations, but other kinds of vacancy defects emerge. This work presents results obtained with positron annihilation spectroscopy in GaN, AlN, and InN. The vacancy-donor complexes are different in these three materials, and their importance in determining the opto-electronic properties of the material varies as well. The formation of these defects is discussed in the light of the differences in the growth methods.

Tuomisto, Filip

2013-02-01

106

Coincidence Doppler Broadening of Positron Annihilation Radiation in Fe  

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

107

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

108

Positron annihilation spectroscopic studies of the influence of heat treatment on defect evolution in hybrid MWCNT-polyacrylonitrile-based carbon fibers  

Microsoft Academic Search

Polyacrylonitrile-based carbon fibers, embedded with multi-wall carbon nanotubes (MWCNTs) in different concentrations, have been prepared by an electrospinning technique. The samples were subjected to oxidative stabilization followed by carbonization and graphitization at temperatures from 1000 to 3000°C. Scanning electron microscopy, X-ray diffraction, and positron annihilation spectroscopy were used to investigate the samples. Positron annihilation studies, viz. analysis of positron lifetime

K. Chakrabarti; P. M. G. Nambissan; C. D. Mukherjee; K. K. Bardhan; C. Kim; K. S. Yang

2007-01-01

109

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

NASA Astrophysics Data System (ADS)

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

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

2008-08-01

110

Analysis of semi-insulating GaAs and the role of positron annihilation  

NASA Astrophysics Data System (ADS)

The uses of Positron Annihilation (PA) techniques for the analysis of native defects in bulk GaAs are reported. PA has allowed the structure of the metastable state of the important EL2 defect to be examined and has demonstrated that a vacancy is present in the complex. PA has also provided strong evidence that the centre responsible for minority-carrier lifetime degradation in this material is the arsenic vacancy.

Brozel, M. R.

1995-06-01

111

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

Microsoft Academic Search

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 tau3 and I3. The experimental results showed that before implantation

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

2001-01-01

112

Characterizing microstructural changes in ferritic steels by positron annihilation spectroscopy: Studies on modified 9Cr-1Mo steel  

NASA Astrophysics Data System (ADS)

Applicability of positron annihilation spectroscopy in probing the microstructural changes in ferritic steels has been investigated with thermal treatment studies on modified 9Cr-1Mo steel, during 300-1273 K. Positron lifetime results are compared with those of ultrasonic velocity and hardness techniques with two initial microstructural conditions i.e., normalized and tempered condition as well as only normalized condition. In first case, positron lifetime is found to be sensitive to small changes in metal carbide precipitation which could not be probed by other two techniques. In later case, positron lifetime is found to be sensitive to defect annealing until 673 K and in distinguishing the growth and coarsening of metal carbide precipitation stages during 773-1073 K. The present study suggests that by combining positron lifetime, ultrasonic velocity and hardness measurements, it is possible to distinguish distinct microstructures occurring at different stages.

Hari Babu, S.; Rajkumar, K. V.; Hussain, S.; Amarendra, G.; Sundar, C. S.; Jayakumar, T.

2013-01-01

113

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

114

Neutrino annihilation of an electron-positron pair  

SciTech Connect

In this study the authors carry out the analysis of the differential cross sections of the electron-positron pair annihilation process by simultaneously taking into account the spin effects, the interference of the charged and neutral currents, and also the possible nonzero neutrino rest mass. The differential cross sections are calculated in the COM system to first order in the weak interaction coupling constant with arbitrary electron and positron spin orientations and arbitrary neutrino and antineutrino polarizations.

Samsonenko, N.V.; Lal, K.C.

1987-01-01

115

Temperature Dependence of Positron Annihilation Rates on Atoms and Molecules%  

Microsoft Academic Search

We previously reported experimental data on positron annihilation rates, Z_eff, measured in a Penning trap geometry at room temperature for a large variety of organic and inorganic substances.(K. Iwata, et al.,) Phys. Rev. A 51, 473 (1995) We have now been successful in heating the stored positrons to temperatures up to 1 eV in the presence of a test gas.

C. Kurz; R. G. Greaves; C. M. Surko

1996-01-01

116

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

117

The application of artificial neural networks to the inversion of the positron lifetime spectrum  

NASA Astrophysics Data System (ADS)

A new method of processing positron annihilation lifetime spectra is proposed. It is based on an artificial neural network (ANN)-back propagation network (BPN). By using data from simulated positron lifetime spectra which are generated by a simulation program and tested by other analysis programs, the BPN can be trained to extract lifetime and intensity from a positron annihilation lifetime spectrum as an input. In principle, the method has the potential to unfold an unknown number of lifetimes and their intensities from a measured spectrum. So far, only a proof-of-principle type preliminary investigation was made by unfolding three or four discrete lifetimes. The present study aims to design the network. Besides, the performance of this method requires both the accurate design of the BPN structure and a long training time. In addition, the performance of the method in practical applications is dependent on the quality of the simulation model. However, the chances of satisfying the above criteria appear to be high. When appropriately developed, a trained network could be a very efficient alternative to the existing methods, with a very short identification time. We have used the artificial neural network codes to analyze data such as the positron lifetime spectra for single crystal materials and monocrystalline silicon. Some meaningful results are obtained.

An, Ran; Zhang, Jie; Kong, Wei; Ye, Bang-Jiao

2012-11-01

118

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

119

Signature of crystal symmetry in positron annihilation data.  

National Technical Information Service (NTIS)

While the technique of two-dimensional angular correlation of the electron-positron annihilation radiation (2D-ACAR) yields data in a reference frame related to detectors, data interpretation is made in a reference frame related to the crystal. The paper ...

G. Adam S. Adam

1994-01-01

120

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

SciTech Connect

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

Colgate, S.A.

1983-03-17

121

Positron-Annihilation Characteristics in Real Solids Including Many-Body Enhancement Effects.  

National Technical Information Service (NTIS)

A density functional scheme has been developed for incorporating electron-positron correlation effects into band-structure calculations of positron-annihilation characteristics. The electron and positron densities are determined from a self-consistent set...

B. Chakraborty

1982-01-01

122

Senile scaling in electron-positron annihilation  

Microsoft Academic Search

An important difference is pointed out between the approach to scaling behaviour in deep inelastic scattering and annihilation. In the framework of the massive quark model a quantitative analysis of the breaking terms is carried out for the total and the one-particle inclusive cross-sections. Good agreement with available data is found.

R. Gatto; Giuliano Preparata

1974-01-01

123

Detector shape and arrangement for positron annihilation imaging device  

SciTech Connect

A circular array of detectors is used for detecting coincident events and the point of annihilation in a positron annihilation process in a selected plane wherein the output from the detectors is processed in a computer to provide image reconstruction for the selected plane. Bismuth germanate is shown to be more efficient than previously used detectors and the adaption of trapezoidal shape for the detectors enables more efficient utilization of the photons produced in the process. The use of absorbing metal reducing plugs between adjacent detectors may be used to decrease the aperture function of the detectors for low angles of incident radiation.

Thompson, C.J.

1981-09-22

124

Single crystal growth of Ga2(SexTe)3 semiconductors and defect studies via positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Small single crystals of Ga2(SexTe)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 Ga2Te3. Defect studies via positron annihilation spectroscopy show an average positron lifetime of ?400ps 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.

2012-08-01

125

Positron annihilation Doppler broadening study of Xe-implanted aluminum  

NASA Astrophysics Data System (ADS)

Positron annihilation Doppler broadening measurements were conducted to characterize information of defects in 380 keV Xe+-implanted aluminum upon thermal annealing at temperatures ranging from 100 to 600 °C. The results suggest a broad distribution in the depth of vacancy-type defects in all the as-implanted samples. Meanwhile, with an increase in implantation dose the defect-rich region shifts toward the sample surface. It was found that increasing the annealing temperature triggers surface-directed migration and coalescence of vacancy and XenVm clusters in samples with implantation doses of 1E15 and 1E16 Xe+cm-2. In the sample implanted with a high dose of 1E17 Xe+cm-2, positron annihilation revealed a decomposition and even elimination of such defects under post-implantation annealing treatment.

Yu, R. S.; Maekawa, M.; Kawasuso, A.; Wang, B. Y.; Wei, L.

2013-10-01

126

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

NASA Astrophysics Data System (ADS)

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

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

2011-04-01

127

Angular Correlation of Positron-Annihilation Radiation from Cerium Hydride  

Microsoft Academic Search

The angular correlation of positron-annihilation radiation from cerium hydride was measured for hydrogen concentrations in the range H\\/Ce = 1.8–2.8. The angular correlations were found to be broader than that from cerium metal and to increase in width with increasing hydrogen concentration in a manner consistent with the screened protonic model of the electronic structure. The anionic model does not

M. P. Chouinard; D. R. Gustafson; R. C. Heckman

1969-01-01

128

Galactic Positron Annihilation Observations with OSSE  

NASA Astrophysics Data System (ADS)

SMM/GRS, CGRO/OSSE and WIND/TGRS results indicate that the dominant 511 keV emission from the galactic center region is steady and of diffuse origin. The OSSE instrument on CGRO has provided the first maps of the 511 keV emission, and the discovery of a surprising excess at positive galactic latitudes above the center of the Galaxy (Purcell et al. 1997b). Most of the annihilation emission is in a 3-photon continuum below 500 keV. Although the continuum signal is stronger, with a positronium fraction near unity, the analysis is more complicated because of continuum contributions to the spectral fits from discrete sources and cosmic ray interactions with the. Kinzer et al. (1999b) report on 1-dimensional latitude and longitude distributions of the positronium continuum emission. Mapping the continuum emission is of importance in order to confirm the features in the 511 keV maps and to study the annihilation sites. We report on OSSE results with an expanded data set, which includes observations from CGRO cycles 7 and 8, inclusion of the positronium continuum emission, and additional pointed observations along the galactic plane not included in earlier analyses.

Kurfess, J. D.

1999-08-01

129

Resonant positron annihilation in rings and substituted alkanes  

NASA Astrophysics Data System (ADS)

Energy-resolved positron-on-molecule annihilation rates have been measured for a variety of molecules by passing a cold positron beam through test gases [1,2]. In many cases, the annihilation rates exceed the free electron annihilation rate by orders of magnitude. In particular, when the positron energy equals a vibrational mode energy minus the binding energy, large vibrational Feshbach resonances (VFR) are observed. In alkane molecules, the height of the C-H stretch resonances grow exponentially and the binding energy grows linearly with the number of carbons [1]. In this paper, we report new results for benzene, d-benzene, and 1-chlorohexane. Specifically, we examine the relationship between binding, C-H peak height, and molecular size. All these molecules have deeper binding than alkanes with similar numbers of carbons or atoms. An empirical trend will be discussed such that molecules with the same number of atoms have similar C-H peak heights when the simple kinematic effects of the binding energy on VFR are normalized out. [1] L. D. Barnes, et al., Phys. Rev. A 67, 032706 (2003). [2] L. D. Barnes, et al., Phys. Rev. A 74, 012706 (2006).

Young, J. A.; Surko, C. M.

2007-06-01

130

Characterization of point defects in UO2 by positron annihilation spectroscopy: a first-principles study  

NASA Astrophysics Data System (ADS)

abstract- Positron Annihilation Spectroscopy is a powerful experimental tool for probing defects in crystalline materials. The correct identification of defects with PAS requires the knowledge of accurate positron lifetimes for the various kinds of defects. That can be provided by numerical calculations in the framework of the Two-Component Density-Functional Theory. This method accurately treat on the same footing, the electrons and positron densities as well as the atomic structure. We have implemented this formalism within the Projector Augmented-Wave method in the ABINIT code, optimizing the electrons and positron densities self-consistently and calculating positron-induced forces accurately. That allows to properly determining the relaxed geometries of defects that trapped positron. We have applied the TC-DFT to various point defects in UO2. The use of the PAW method allows considering large super cells to simulate point defects, we have typically used cells that contain 32 UO2 unit formulas. We use the LDA+U framework in order to treat strong electronic correlations. This work is a first attempt to help for the interpretation of PAS experiments on UO2 but it seems really promising.-

Torrent, Marc; Jomard, Gerald

2011-03-01

131

Statistical Multimode Resonant Annihilation of Positrons on Molecules  

NASA Astrophysics Data System (ADS)

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

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

2012-06-01

132

Positron lifetime and Doppler broadening techniques applied to irradiation-damaged silver  

SciTech Connect

Positron lifetime and Doppler broadening measurements have been used to study defect production resulting from room temperature irradiation of pure silver by D-T fusion neutrons or energetic protons. Use of the positron annihilation analysis has established that the surviving defects from both irradiations have the same dose dependence and that defect concentration can be quantitatively measured and compared to damage models. The relative merit of the lifetime and Doppler broadening measurements in arriving at these conclusions is discussed in this report along with some practical aspects of the measurements. In the proton damaged samples the trapping rate approaches saturation, a circumstance which could be misinterpreted if a less extensive data set containing only Doppler broadening data were available. Some remarks about the analysis of positron data and general conclusions about the defect structure are given. 3 figures.

Howell, R.H.

1981-07-27

133

Positron annihilation in latex-templated macroporous silica films: pore size and ortho-positronium escape  

NASA Astrophysics Data System (ADS)

Depth profiling of positron annihilation characteristics has been used to investigate the pore size distribution in macroporous PMMA latex-templated SiO2 films deposited on glass or Si and prepared with 11-70% porosity. The correlation between the annihilation characteristics shows that ortho-positronium (o-Ps) escape (re-emission) into vacuum occurs in all films with a porosity threshold that is pore size dependent. For 60 ± 2% porosity, the o-Ps re-emission yield decreases from ˜0.25 to ˜0.11 as the pore size increases from 32 to 75 nm. The o-Ps re-emission yield is shown to vary linearly with the specific surface area per mass unit and the slope is independent of pore size, 9.1 ± 0.4 g cm-2. For 32 nm pores, the o-Ps annihilation lifetimes in the films, 17(2) ns and 106(5) ns, show that o-Ps annihilates from micropores with small effective size (1.4 ± 4 nm) and from macropores with large effective size (˜32 nm). Above the porosity threshold, the o-Ps-escape model predicts the annihilation lifetime in the films to be 19 ± 2 ns. Our results imply that o-Ps efficiently detects the microporosity present in the silica walls. At low porosity, its capture into the micropores competes with its capture into the macropores. At higher porosity (when the distance between micropores and macropores becomes small), this capture into the micropores assists the capture into the macropores.

Liszkay, L.; Guillemot, F.; Corbel, C.; Boilot, J.-P.; Gacoin, T.; Barthel, E.; Pérez, P.; Barthe, M.-F.; Desgardin, P.; Crivelli, P.; Gendotti, U.; Rubbia, A.

2012-06-01

134

A positron annihilation study of the formation and dissolution of L12 precipitates in Al - Li alloys  

Microsoft Academic Search

The formation and dissolution of ?? precipitates has been followed by positron annihilation spectroscopy in two Al-Li alloys (Al–9·9 at.% Li and Al-8·6 at.% Li-0·04 at.% Zr). Lifetime measurements have been performed by annealing the samples from the as-quenched state to 750 K, which is well above the dissolution temperature for the ??-phase Al3Li. The results suggest that small ??-like

J. Del Río; F. Plazaola; N. de Diego

1994-01-01

135

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.

136

The effects of positron binding and annihilation mechanisms in biomolecules on PET resolution  

Microsoft Academic Search

The angular correlation distribution of electron-positron annihilation photons is presented for polar biological molecules, and the associated noncollinearity effect on resolution in positron emission tomography (PET) is discussed. The image resolution in PET is known to be limited by a number of factors such as radioactive decay statistics, attenuation, scatter, or random coincidence of annihilation photons. This paper deals with

L. Pichl; M. Tachikawa; R. J. Buenker; M. Kimura; J.-M. Rost

2005-01-01

137

Feshbach-resonance-mediated annihilation in positron interactions with large molecules  

NASA Astrophysics Data System (ADS)

Measurements of positron-on-molecule annihilation have established that positrons attach to a variety of molecules via vibrational Feshbach resonances (VFRs). These resonances, in turn, result in greatly enhanced positron annihilation rates beyond those expected in simple collisions. A recent paper [J. A. Young and C. M. Surko, Phys. Rev. Lett. 99, 133201 (2007)] described experimental studies and analysis of positron annihilation in large molecules. Presented here is a more complete description of those results as well as data and analysis. Positron-molecule binding energy data are presented for twenty-five molecules, including the relatively deeply bound species 1-chlorohexane, benzene, and naphthelene. When a relatively weak dependence of resonant annihilation rate on incident positron energy and binding energy is removed, the resulting rates for hydrocarbons are shown to obey a universal scaling as a function of the number of vibrational degrees of freedom. Previously measured annihilation rates for alkanes using thermal positrons at 300 K are shown to be in quantitative agreement with measurements at higher energies. Results are presented for partially fluorinated hydrocarbons, providing evidence that the unusual suppression of resonant annihilation observed in these molecules occurs via a vibrational deexcitation process. The role of molecular temperature and inelastic escape channels in VFR-mediated positron annihilation in molecules is discussed. The implications of these results for theories of the annihilation process are also discussed.

Young, J. A.; Surko, C. M.

2008-05-01

138

Positron annihilation on polycyclic aromatic hydrocarbon molecules in the interstellar medium  

NASA Astrophysics Data System (ADS)

We examine the annihilation of positrons on polycyclic aromatic hydrocarbon (PAH) molecules in interstellar medium (ISM) conditions. We estimate the annihilation rates of positrons on PAHs by a semi-empirical approach. We show that PAHs can play a significant role in the overall Galactic positron annihilation picture and use the annihilation rates and International Gamma-Ray Astrophysics Laboratory Galactic emission measurements to constrain the number of PAHs present in the ISM. We find an upper limit of 4.6 × 10-7 for the PAH abundance (by number, relative to hydrogen).

Guessoum, N.; Jean, P.; Gillard, W.

2010-02-01

139

Positron lifetimes in solids from first principles calculations  

SciTech Connect

We present a first principles method for calculating positron lifetimes in solids, based on self-consistent calculations using the Linear Muffin-Tin Orbital method. Local density approximations are used for both electron-electron and electron-positron interactions. Results are presented for a variety of elemental metals and vacancies to demonstrate the reliability of this approach. Theoretical calculations of positron lifetimes can be used to interpret experimental data. As an examples of this, we interpret our experimental lifetime data for the oxide superconductor Ba{sub 1-x}K{sub x}BiO{sub 3} using calculations based on this method. 12 refs., 3 figs.

Sterne, P.A.; O'Brien, J.C.; Howell, R.H. (Lawrence Livermore National Lab., CA (United States)); Kaiser, J.H. (Texas Univ., Arlington, TX (United States). Dept. of Physics)

1991-08-07

140

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

141

Role of Binding Energy in Feshbach-Resonant Positron-Molecule Annihilation  

NASA Astrophysics Data System (ADS)

Measurements of positron-on-molecule annihilation have established that positrons bind to a variety of molecules via vibrational Feshbach resonances. Data for deeply bound states in benzene and 1-chlorohexane and for positronically excited (i.e., second) bound states in alkanes are used to establish the dependence of annihilation rates on the binding energy and incident positron energy. With this dependence removed, annihilation rates for a broad class of molecules lie on a universal curve as a function of the number of molecular vibrational degrees of freedom. The implications of these results for theoretical models are discussed.

Young, J. A.; Surko, C. M.

2007-09-01

142

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

NASA Astrophysics Data System (ADS)

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

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

2013-05-01

143

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

SciTech Connect

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

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

2000-04-15

144

Positron annihilation in AlN and GaN  

NASA Astrophysics Data System (ADS)

The measurements of one-dimensional angular correlation of the annihilation radiation (1D-ACAR) have been carried out for AlN and GaN as well as for some related materials (Al, Ga, GaP, GaAs, GaSb) which have been used as samples of references the analysis of results. The numeral values of characteristic length of radius of spherical volume to be occupied by annihilating electron (rs') have differed significantly from the corresponding values (rs) calculated by the conventional independent-particle-model (IPM) for ideal Fermi-gas: rs' (AlN)~=1.28 rs, where rs (AlN)~=1.61a.u., and rs' (GaN)~=1.66 rs, where rs (GaN)~=1.64a.u. The electron-positron ``ion radii'' reconstructed by the high-momentum components (HMC) of 1D-ACAR for Al3+, Ga3+ cores as well as numeral rs' values provide some reasons to believe that Ga- and Al-vacancies and their impurity complexes are effective centers of the positron localization in AlN and GaN; it is assumed that these complexes include VGa, VAl, and /N atom (VGa-NGa in GaN and VAl-NAl in AlN) where the nitrogen atom is likely to be in the configuration of substitution (anti-site), N+Ga and N+Al, respectively.

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

2001-12-01

145

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

2013-08-19

146

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

147

INTEGRAL Observations of Galactic Electron-Positron Annihilation Radiation  

NASA Astrophysics Data System (ADS)

One of the primary goals of the spectrometer (SPI) on the INTEGRAL mission is detailed mapping and spectroscopy of the Galactic electron-positron annihilation radiation. The INTEGRAL coded-aperture array and high-resolution germanium detectors are optimized to resolve the spectral components and map the Galactic distribution of this emission. INTEGRAL has found that the bulk of the radiation is concentrated towards the center of our Galaxy. It can be well-described by a symmetric gaussian with a FWHM of 8 degrees. Evidence for a more extended Galactic Plane component is emerging from the data (currently at the 3-4 sigma level). The data are generally consistent with the earlier results from the CGRO/OSSE mission However, the so-called ``Galactic fountain" (an enhanced region of emission above the Galactic Plane reported in early analyses of the CGRO/OSSE data) is not seen. INTEGRAL detects the 3-photon positronium annihilation continuum and finds that its morphology is similar to the 511-keV narrow-line distribution. Recently, evidence has been found for a 2-component structure for the 511 keV line, a narrow component (FWHM ˜ 1 keV) and a broadened component (FWFM ˜ 5 keV). The latter expected from the 2-photon decay of positronium in flight. The spectroscopic results are consistent with the annihilation taking place in the warm, partially-ionized phase of the interstellar medium.

Teegarden, B. J.

2005-12-01

148

Nondestructive positron-lifetime measurements during fatigue of austenitic stainless steel using a mobile positron beam  

NASA Astrophysics Data System (ADS)

Nondestructive positron-lifetime measurements have been performed during stress-controlled symmetric fatigue of austenitic stainless steel up to failure for different applied stress amplitudes between 230 and 300 MPa. For this purpose a fatigue machine has been equipped with a mobile positron beam emitted from a 22 ?Ci (0.8 MBq) 72Se/72As generator. A pronounced increase of the mean positron lifetime has been found within the first 10% of fatigue life. Saturation has been reached after 40% of fatigue life. The mean positron lifetime at failure depends strongly on the applied stress amplitude.

Barbieri, Andrea; Hansen-Ilzhöfer, Sabine; Ilzhöfer, Achim; Holzwarth, Uwe

2000-09-01

149

Positron annihilation in diamond, silicon and silicon carbide  

Microsoft Academic Search

Recent positron lifetime and doppler broadening results on silicon, diamond and silicon carbide are presented in this contribution. In as-grown Czochralski Si ingols vacancies are found to be retained after growth at concentrations typically around 3×1016\\/cm3. 10 MeV eleciron irradiation of variously doped Si wafers shows that only high doping concentrations well in excess of the interstitial oxygen concentration causes

S. Dannefaer

1995-01-01

150

Auger-electron emission resulting from the annihilation of core electrons with low-energy positrons  

Microsoft Academic Search

We report the first demonstration of positron-induced Auger-electron spectroscopy. A beam of low-energy (101 eV) positrons was used to create core holes at the surface of Ni and Cu by matter-antimatter annihilation. Estimates are developed for the probability of positrons annihilating with a 3p electron and found to be as high as 3.7(7)×10-2 in Ni. The implications of the extremely

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

1988-01-01

151

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

Microsoft Academic Search

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

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

1997-01-01

152

Annihilation Characteristics of Positrons Trapped at the Surfaces of Ge, Si, GaAs, Cu, and Ag using Positron annihilation induced Auger Electron Spectroscopy (PAES)  

Microsoft Academic Search

In PAES, a low energy beam of positrons is used to induce the Auger transition. PAES is more surface sensitive than Electron induced Auger Electron Spectroscopy (EAES) due to the fact that positrons implanted into the sample diffuse back to the surface and are trapped into a surface state where they annihilate. PAES Auger intensities were measured to obtain estimates

Shannon Starnes; Nial Fazleev; Eunseung Jung; Amy Zhou; John Fry; Alex Weiss

2001-01-01

153

Low-temperature positron lifetime and Doppler-broadening measurements for single-crystal nickel oxide containing cation vacancies  

SciTech Connect

Lifetime and Doppler-broadening measurements for positron annihilation in substoichiometric nickelous oxide have been made concomitantly from liquid-helium to room temperature. The concentration of cation vacancies is readily controlled by altering the ambient oxygen pressure while annealing the crystals at 1673/sup 0/K. It was found that neither of the three lifetimes observed or their relative intensities varied significantly with the oxygen pressure, and the bulk rate only increased slightly when the specimen was cooled from room to liquid-helium temperatures. These results are interpreted as indicating that some of the positrons are trapped by the existing cation vacancies and a smaller fraction by vacancy clusters.

Waber, J.T.; Snead, C.L. Jr.; Lynn, K.G.

1985-01-01

154

Full potential LAPW calculations of positron lifetimes in materials  

NASA Astrophysics Data System (ADS)

We report positron lifetime calculations for a large number of semiconductors and insulators, including materials of interest for radiation detection. These include CdTe, ZnTe, lanthanide trihalides, orthophosphates, ZnO. Trends in lifetimes with structural features are discussed.

Takenaka, Hiroyuki; Singh, David

2007-03-01

155

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

156

Cosmic ray positrons from annihilations into a new, heavy lepton  

SciTech Connect

Recent results from the PAMELA experiment indicate an excess in the positron spectrum above 10 GeV, but antiproton data are consistent with the expected astrophysical backgrounds. We propose a scenario that reproduces these features. Dark matter annihilates through channels involving a new heavy vectorlike lepton which then decays by mixing with standard model leptons. If charged, this heavy lepton might be produced at the LHC, and could lead to multilepton final states or to long-lived charged tracks. Large neutrino detectors such as ANTARES or IceCube might be sensitive to a monochromatic neutrino line. This scenario may be simply embedded in various models, including an extension to the next-to-minimal supersymmetric standard model.

Phalen, Daniel J.; Pierce, Aaron [Michigan Center for Theoretical Physics (MCTP), Department of Physics, University of Michigan, Ann Arbor, Michigan 48109 (United States); Weiner, Neal [Center for Cosmology and Particle Physics, Department of Physics, New York University, New York, New York 10003 (United States)

2009-09-15

157

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

158

Defect studies of stainless steel via positron annihilation energy spectroscopy  

NASA Astrophysics Data System (ADS)

High Energy proton (up to 800 MeV) and spallation neutron irradiated samples of stainless steel 316L and Mod 9Cr1Mo were studied using positron annihilation energy spectroscopy. Doses delivered to 316L were up to 10 displacements per atom (dpa) and doses to 9Cr1Mo were up to 2.5dpa. We studied the change of T-parameter, which is calculated as the ratio of the number of counts in the wings of the Doppler-broadened 511 keV peak to the number of counts in the center of the peak. T-parameter is related to the density of defects in the sample of interest. Higher defect densities induce, generally, smaller T-parameter, although this is complicated by additional effects that include the size, nature and other properties of defects that may lead to saturation of T-parameter. For the large doses studied, positron annihilation energy spectroscopy showed that the T-parameter dropped sharply from 0 to 3 dpa, and continued dropping up to 10 dpa. In 9Cr1Mo, similarly, T-parameter dropped sharply from 0 dpa to 1dpa, but from 1 dpa to 2.5 dpa it remained constant, indicating that the density of defects or T-parameter saturated with dose above 1 dpa in 9Cr1Mo. These results, where the change in T-parameter from zero dose to 1 or more dpa, is much larger than the effect that we see from one irradiated specimen to another, led us in both cases to investigate lower doses. We measured energy spectra in 316L and 9Cr1Mo that were irradiated under the similar conditions as the above samples, but with doses less than 0.1dpa. These results fill in the gap between 0 and 1 dpa and suggest that most of the change in T-parameter occurs below 0.05 dpa.

Tchelidze, L.; Wells, D. P.; Maloy, S. A.

2011-01-01

159

Defect dependence of positron lifetimes in oxide superconductors  

SciTech Connect

First principles calculations of the positron lifetimes are used to interpret experimental lifetime data for Ba{sub 1-x}K{sub x}BiO{sub 3}. The observed lifetimes are found to be very sensitive to the presence of defects in the sample. The temperature dependence of the lifetime and the change in lifetime with sample aging are both attributed to defects. The qualitative differences between metal vacancy defects and oxygen vacancy defects in metal oxides are also discussed. 6 refs., 2 figs.

Sterne, P.A.; O'Brien, J.C.; Howell, R.H. (Lawrence Livermore National Lab., CA (USA)); Kaiser, J.H. (Texas Univ., Arlington, TX (USA). Dept. of Physics)

1990-11-01

160

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

161

Positron Annilation Lifetime and Glass Transition Temperatures in CO2 Sorption Polystyrene  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy was used to measure the free-volume size and distributions as a function of temperature in polystyrene with and without 400 psi CO2 sorption. The transition temperatures in the polystyrene with CO2 sorption obtained from ortho-positronium lifetimes were found to depend on the thermal cycles and a meta-stable state showing a negative thermal expansion coefficient was observed between 53°C and 82°C during the first heating up experiment. The observed Tg in polystyrene with, and without CO2 sorption after annealing from ortho-positronium lifetimes were found to be 86°C and 91°C, which are 5°C higher, and 10°C lower than from the differential scanning calorimetry data, respectively. The observed free-volume variations are discussed in terms of hole expansion, creation, free-volume relaxation, plasticization, and hole filling in amorphous polymers.

Jean, Y. C.; Chen, H.; Lee, L. J.; Yang, J.

2008-05-01

162

Hydrogen implantation effect in copper alloys selected for ITER investigated by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Defects in the form of vacancies (loops, voids, etc) created by hydrogen implantation into selected Cu alloys foreseen for the International Thermonuclear Experimental Reactor (ITER) first wall cladding were studied using positron annihilation spectroscopy. The pulsed low-energy positron system, which enables depth profiling of the positron lifetime spectra in the near-surface region (20-460 nm) of hydrogen-implanted copper alloys, was applied, and its results were compared with TRIM calculations and transmission electron microscopy studies. The selected specimens were implanted in the Ion Beam Laboratory of FEI STU Bratislava. The energy of implantation was EH = 2 × 95 keV for the molecular H_{2}^{+} ion beam. The temperature during this process was lower than 90°C. Two implantation doses were chosen for both the alloys: 1.3 × 1019 ions cm-2 (1.1 C cm-2) and 5 × 1018 ions cm-2 (0.4 C cm-2). Although the influences of neutrons with energy 14 MeV and protons with energy 95 keV are not the same (differences in energy and existence of proton charge), experimental simulation of radiation damage of ITER construction materials was successfully performed. The results are discussed in terms of microstructural changes of the studied materials upon irradiation. The CuAl25 alloy seems to be more resistant to proton bombardment than CuCrZr.

Slugen, V.; Kuriplach, J.; Ballo, P.; Domonkoš, P.

2004-01-01

163

Detection of Hidden Explosives Using Resonant Gamma Rays From In-Flight Annihilation of Fast Positrons  

SciTech Connect

Gamma rays with tunable energies can be produced by the in-flight annihilation of fast positrons. The kinetic energy of the positron beam determines the gamma-ray energy of annihilation photons emitted in a narrow cone in the forward direction. These photons can be used for nuclear resonant fluorescence determination of explosive materials rich in 14N selecting gamma rays with energies that match excited states in 14N and then observing the emitted nuclear fluorescence.

Guardala, Noel A. [Naval Surface Warfare Center/Carderock Division, Code 644, 9500 MacArthur Blvd, W. Bethesda MD 20817 (United States); Farrell, J. Paul; Dudnikov, Vadim [Brookhaven Technology Group, 12-12 Tech Drive, Nesconsett, NY 11793 (United States); Merkel, George [Army Research Lab, 2800 Powder Mill Road, Adelphi, MD (United States)

2003-08-26

164

Positron-annihilation studies on Al-Cu-Fe: Their significance for the structural models of icosahedral quasicrystals  

NASA Astrophysics Data System (ADS)

We have investigated the vacancy defects in samples of quasicrystalline Al-Cu-Fe using positron-lifetime measurements as a function of various heat treatments and alloy compositions. In general, a two-component positron-annihilation-lifetime spectrum seems characteristic of quasicrystalline Al-Cu-Fe and other icosahedral quasicrystals. This can be interpreted in terms of vacancy defects, whose concentrations are about three orders of magnitude more than what one usually observes in crystalline metals at room temperature. We discuss the significance of the results of our studies for the structure of icosahedral quasicrystals, for which several models have been postulated in the literature. The concentration of the vacancy defects observed by us seems incompatible with the icosahedral-twin model or, at first sight, with the space-filling Penrose tiling based models or random-tiling models.

Chidambaram, R.; Sanyal, M. K.; Raghunathan, V. S.; Nambissan, P. M. G.; Sen, P.

1993-08-01

165

Auger Line Shape Measurements Using High Resolution Positron Annihilation Induced Auger Electron Spectroscopy  

Microsoft Academic Search

Conventional methods of Auger electron spectroscopy (AES) make use of energetic electron or photon beams to create the core-hole excitations that lead to the Auger transition. The energetic beams result in a large secondary electron background in the Auger peak region. In Positron annihilation induced Auger Electron Spectroscopy (PAES), the core holes are created by matter-antimatter annihilation and not through

Haiqing Zhou

1996-01-01

166

Auger line shape measurements using positron annihilation induced Auger electron spectroscopy  

Microsoft Academic Search

Conventional methods of Auger electron spectroscopy (AES) make use of energetic electron or photon beams to create the core-hole excitations that lead to the Auger transition. The energetic beams result in a large secondary electron background in the Auger peak region. In positron annihilation induced Auger electron spectroscopy (PAES), the core holes are created by matter-antimatter annihilation and not through

A. H. Weiss; S. Yang; H. Q. Zhou; E. Jung; S. Wheeler

1995-01-01

167

A gamma ray laser based on induced annihilation of electron-positron pairs  

Microsoft Academic Search

In this paper, the coherent amplification of gamma radiation of a system of parapositronium atoms is proposed. The nonlinear optics of positronium media is suggested. The induced annihilation transitions for the electron-positron plasma are compared with those of the positronium medium. It is suggested that the Bose-Einstein condensation could play a crucial role in the estimation of the induced annihilation

A. Loeb; S. Eliezer

1986-01-01

168

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

169

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

Microsoft Academic Search

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

M. V. Deepa Urs; C. Ranganathaiah

2007-01-01

170

Temperature dependence of positron annihilation characteristics on the surfaces of graphite powders  

Microsoft Academic Search

Positron lifetime measurements have been made on graphite powders, grafoils, and pyrolytic graphite crystals with different surface areas in the temperature range between 25° and 600 °C. Three positron lifetimes were found in these systems: a short-lived component (~0.2 ns) due to positrons in the bulk; a component (~0.45 ns) due to surface-trapped positrons; and a long-lived component (~2 ns)

Y. C. Jean; K. Venkateswaran; E. Parsai; K. L. Cheng

1984-01-01

171

Temperature dependence of positron annihilation characteristics on the surfaces of graphite powders  

Microsoft Academic Search

Positron lifetime measurements have been made on graphite powders, grafoils, and pyrolytic graphite crystals with different surface areas in the temperature range between 25° and 600 °C. Three positron lifetimes were found in these systems: a short-lived component (˜0.2 ns) due to positrons in the bulk; a component (˜0.45 ns) due to surface-trapped positrons; and a long-lived component (˜2 ns)

Y. C. Jean; K. Venkateswaran; E. Parsai; K. L. Cheng

1984-01-01

172

Investigations of Positron Annihilation with Atoms and Molecules using PsARS  

NASA Astrophysics Data System (ADS)

Positrons, being the antiparticles of electrons, ultimately annihilate either directly with electrons (that are free or attached to atoms or molecules), or via the formation of positronium (Ps, a short-lived atom composed of a positron and an electron) with subsequent annihilation. In this work we have developed positronium annihilation ratio spectroscopy (PsARS), and are using PsARS to investigate the formation and destruction of Ps [1], as well as positron attachment to molecules. For this experiment a 3 to 100 eV positron beam obtained from a sodium-22 radioactive source is passed through a gas scattering cell and resulting annihilation gamma rays of different energies are detected in coincidence. Annihilation measurements, such as these, have astrophysical relevance since characteristic positron annihilation gamma rays have been observed from various extraterrestrial sources (e.g., solar flares and the direction towards the center of our galaxy). [1] W.E. Kauppila, E.G. Miller, H.F.M. Mohamed, K. Pipinos, T.S. Stein and E. Surdutovich, Phys. Rev. Lett. 93, 113401 (2004).

Kauppila, W. E.; Edwards, J. J.; Miller, E. G.; Stein, T. S.; Surdutovich, E.

2006-03-01

173

Positron Annihilation Studies of the Subsurface Zone in Aluminium Alloys  

NASA Astrophysics Data System (ADS)

The paper presents positron lifetime studies of the subsurface region of the aluminium cast AK64 alloy containing silicon as a main alloying element after sliding against stainless steel in the pin-on-disc tester. The subsurface zone of the depth exceeding 150 ?m has been detected. The scanning electron microscopy revealed only the plastically deformed region occurring at the depth up to ca. 50 ?m and cracks at the depth less than 30 ?m. The comparison with the results obtained for the AK12 alloy containing almost a twice higher amount of silicon demonstrates the influence of the hard silicon rich particles on the total depth of the subsurface zone. A higher amount of hard particle prevents to some extend the propagation of defects inside the material.

Dryzek, E.; Dryzek, J.

2006-11-01

174

Investigation of Positron-CO scattering using Positronium Annihilation Ratio Spectroscopy  

NASA Astrophysics Data System (ADS)

We are using the technique [1] of positronium annihilation ratio spectroscopy (PsARS) to study the annihilation of 6 - 13 eV positrons colliding with CO. In this method we detect in coincidence (a) two 511 keV annihilation gamma rays, and simultaneously (b) two 300 - 460 keV gamma rays from the three gamma decay of ortho-Ps. The ratio of these signals R3?/2? versus positron impact energy reveals anomalous behavior for CO when compared with other gases [1] suggesting that some other mechanism than Ps formation is contributing to positron annihilation at an energy about 1 eV above the Ps formation threshold of 7.2 eV. Recognizing that the threshold for electronic excitation by positron impact is located where the anomalous R3?/2? behavior occurs suggests that we may be observing an effect where the positron is electronically exciting CO and temporarily binding to the molecule in a resonance-like state from which the positron can annihilate with an electron producing an enhanced 511 keV coincidence signal consistent with our measurements. [1] W.E. Kauppila, E.G. Miller, H.F.M. Mohamed, K. Pipinos, T.S. Stein and E. Surdutovich, Phys. Rev. Lett. 93, 113401 (2004).

Kauppila, W. E.; Edwards, J. J.; Miller, E. G.; Stein, T. S.; Surdutovich, E.

2006-05-01

175

Positron lifetime study of poly(dimethylsiloxane) based polymer-nanoparticle composites  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy was employed to study the changes in the size of the local free volume elements (holes) of poly(dimethylsiloxane) (PDMS) based nanocomposites incorporating two different types of nanoparticles, meta-carborane (m-CB) and iso-octyl polyhedral oligomeric silsesquioxane (POSS). Materials were prepared with various loading levels of each nanoparticle and the temperature dependence of the o-Ps lifetime, and thus the free volume, and its distribution were measured from 100–300 K. It was observed that m-CB reduces the size of the local free volume elements while POSS has the opposite effect, in each case the change is only seen in the rubbery state below the so-called "knee temperature" (Tk ~ 210 K).

Hughes, D. J.; Roussenova, M. V.; Beavis, P.; Swain, A. C.; Alam, M. A.

2013-06-01

176

Effect of positron-atom interactions on the annihilation gamma spectra of molecules  

NASA Astrophysics Data System (ADS)

Calculations of ?-spectra for positron annihilation on a selection of molecules, including methane and its fluoro-substitutes, ethane, propane, butane and benzene are presented. The annihilation ?-spectra characterise the momentum distribution of the electron-positron pair at the instant of annihilation. The contribution to the ?-spectra from individual molecular orbitals is obtained from electron momentum densities calculated using modern computational quantum chemistry density functional theory tools. The calculation, in its simplest form, effectively treats the low-energy (thermalised, room-temperature) positron as a plane wave and gives annihilation ?-spectra that are about 40% broader than experiment, although the main chemical trends are reproduced. We show that this effective ‘narrowing’ of the experimental spectra is due to the action of the molecular potential on the positron, chiefly, due to the positron repulsion from the nuclei. It leads to a suppression of the contribution of small positron-nuclear separations where the electron momentum is large. To investigate the effect of the nuclear repulsion, as well as that of short-range electron-positron and positron-molecule correlations, a linear combination of atomic orbital description of the molecular orbitals is employed. It facilitates the incorporation of correction factors which can be calculated from atomic many-body theory and account for the repulsion and correlations. Their inclusion in the calculation gives ?-spectrum linewidths that are in much better agreement with experiment. Furthermore, it is shown that the effective distortion of the electron momentum density, when it is observed through positron annihilation ?-spectra, can be approximated by a relatively simple scaling factor.

Green, D. G.; Saha, S.; Wang, F.; Gribakin, G. F.; Surko, C. M.

2012-03-01

177

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

PubMed

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

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

2012-12-13

178

Positron lifetime studies of 100MeV oxygen irradiated Pb-doped Bi2223 superconductors  

Microsoft Academic Search

Positron lifetime studies have been carried out for unirradiated and 100-MeV oxygen ion irradiated Pb-doped Bi-2223 superconductors. The analysis of positron lifetime spectra revealed three lifetime components: a short lifetime, ?1=153–196ps; an intermediate lifetime, ?2=269–339ps; and a long lifetime, ?3=616–812ps. A decrease in all the lifetime components, ?1, ?2 and ?3 with an increase of the relative intensities of I2

T. Banerjee; R. N. Viswanath; D. Kanjilal; R. Kumar; S. Ramasamy

2000-01-01

179

Influence of defects on positron transmission and annihilation in the lithium fluoride crystal  

SciTech Connect

The positron implantation profile and the angular distribution of annihilation ..gamma.. quanta were determined for a lithium fluoride crystal under ..beta../sup +/ and ..gamma.. irradiation (/sup 22/Na source). The positron absorption coefficient of the irradiated crystal was ..cap alpha.. = 76.2 +- 1.5 cm/sup -1/. The angular distribution had a strong narrow component. After thermal bleaching of the crystal, ..cap alpha.. = 91.9 +- 1.5 cm/sup -1/, the narrow component made a smaller contribution to the angular distribution, and its half-width increased. The positron mobility was found to be ..mu.. = 18 +- 8 cm/sup 2/ x V/sup -1/ x sec/sup -1/. It is suggested that defects influence in two ways the fate of positrons in the lithium fluoride crystal: free positrons may be trapped by some defects (cationic vacancies) or annihilated in collisions with others (F centers). The defect concentration is estimated.

Varisov, A.Z.; Kozlov, V.G.

1984-05-01

180

Positron Annihilation in Corroded Steels St20 and St3S  

NASA Astrophysics Data System (ADS)

The paper deals with examining the lifetimes of positrons emitted from 22Na source in corroded St20 and St3S steels. Influence of the method of defecting the sample surfaces on the spectrum of positron lifetimes was investigated. Influence of temperature of initial annealing of samples, plastic deformation, electrolytic hydrogenation and corrosion in HCl vapors on trapping positrons in the examined steels was also determined.

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

2006-11-01

181

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

182

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

183

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

NASA Astrophysics Data System (ADS)

This paper presents a combined description of the ortho-positronium (o-Ps) response from positron annihilation lifetime spectroscopy (PALS) and viscosity data on bis[m-(m-phenoxy) phenoxyphenyl] ether within the free volume version of the two-order parameter (TOP) model of disordered phase. The quasi-sigmoidal form of the temperature dependence of the o-Ps lifetime, ?3, over a wide temperature interval, can be described by an expression with the distribution function for liquid-like domains 1-F(T) which follows from a phenomenological analysis of the viscosity using the modified Vogel-Fulcher-Tamman-Hesse (MVFTH) equation. This simultaneous description indicates a close connection between the PALS response and the viscosity behavior for the matrix and gives support for the liquid-like and solid-like domain physical picture of all the physical states of the disordered phase.

Bartoš, J.

2008-07-01

184

Mössbauer and positron annihilation studies of pharmaceutically important iron-dextran complexes  

NASA Astrophysics Data System (ADS)

Iron-dextran complexes are pharmaceutically important models of iron-storage protein ferritin. These complexes are used for treatment of iron-deficiency anemias. In this work we present the results of the study of various iron-dextran complexes by Mössbauer spectroscopy and the positron annihilation technique. Mössbauer spectroscopy indicated the differences between the electronic and magnetic structures of iron cores in iron-dextran complexes while positron annihilation showed variations of dextran shells in those complexes. Both techniques appeared to be useful to study microstructural variations in iron-dextran complexes.

Oshtrakh, M. I.; Kopelyan, E. A.; Semionkin, V. A.; Livshits, A. B.; Krylova, V. E.; Kozlov, A. A.

1993-04-01

185

Is There a Dark Matter Signal in the Galactic Positron Annihilation Radiation?  

SciTech Connect

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

Lingenfelter, R. E.; Rothschild, R. E. [Center for Astrophysics and Space Sciences, University of California San Diego, La Jolla, California 92093 (United States); Higdon, J. C. [Keck Science Center, Claremont Colleges, Claremont, California 91711-5916, USA and California Institute of Technology, Pasadena, California 91125 (United States)

2009-07-17

186

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

187

Spatially resolved deformation studies on carbon steel employing X-rays and positron annihilation  

NASA Astrophysics Data System (ADS)

Spatially resolved studies on plasticity in a polycrystalline sample of the ferritic carbon steel C45E (AISI 1045) were performed after deformation in a three-point bending test. The local effects of deformation were investigated by scanning the sample with two different methods: Positron Annihilation Spectroscopy (S-parameter) and Debye-Scherrer diffraction (reflex broadening). A simple relation between the results of both experiments and the true strain over the cross-section of the bent sample is presented in this letter. Comparing the methods a linear correlation between the lattice distortion of -iron and the defect sensitive positron annihilation parameter is found.

Haaks, Matz; Müller, Ingo; Schoeps, Andreas; Franz, Hermann

2006-03-01

188

Effects of quantum chemistry models for bound electrons on positron annihilation spectra for atoms and small molecules  

Microsoft Academic Search

The Doppler-shift spectra of the ?-rays from positron annihilation in molecules were determined by using the momentum distribution of the annihilation electron–positron pair. The effect of the positron wavefunction on spectra was analysed in a recent paper (Green et al 2012 New J. Phys.14 035021). In this companion paper, we focus on the dominant contribution to the spectra, which arises

Feng Wang; Xiaoguang Ma; Lalitha Selvam; Gleb Gribakin; Clifford M Surko

2012-01-01

189

A simulation study of a method to reduce positron annihilation spread distributions using a strong magnetic field in positron emission tomography  

SciTech Connect

The positron trajectories have been three-dimensionally simulated using a Monte-Carlo method under various strength of the magnetic field. More than 5 tesla of the field confined the positrons effectively, resulting in increase of the probability of the annihilation within a limited small region, hence the higher spatial resolution in positron emission tomography.

Iida, H.; Kanno, I.; Miura, S.; Murakami, M.; Takahashi, V.; Kemura, K.

1986-02-01

190

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

191

Characterization of defect accumulation in neutron-irradiated Mo by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy measurements were performed on neutron-irradiated low carbon arc cast Mo. Irradiation took place in the high flux isotope reactor, Oak Ridge National Laboratory, at a temperature of 80 ± 10 °C. Neutron fluences ranged from 2 × 1021 to 8 × 1024 n/m2 (E > 0.1 MeV), corresponding to displacement damage levels in the range from 7.2 × 10-5 to 2.8 × 10-1 displacements per atom (dpa). A high density of submicroscopic cavities was observed in the neutron-irradiated Mo and their size distributions were estimated. Cavities were detected even at a very low-dose of ˜10-4 dpa. The average size of the cavities did not change significantly with dose, in contrast to neutron-irradiated bcc Fe where cavity sizes increased with increasing dose. It is suggested that the in-cascade vacancy clustering may be significant in neutron-irradiated Mo, as predicted by molecular dynamics simulations.

Eldrup, M.; Li, Meimei; Snead, L. L.; Zinkle, S. J.

2008-08-01

192

Point defects in YBa2Cu3O7-x studied using positron annihilation  

NASA Astrophysics Data System (ADS)

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

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

2012-07-01

193

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

194

Anomalous Electron--Muon and Electron--Hadron Production in Electron--Positron Annihilation.  

National Technical Information Service (NTIS)

Results of studies of anomalous electron-muon and electron-hadron events produced in electron-positron annihilation are presented. The data for this work were obtained with a lead-glass counter system, which was added to one octant of the Stanford Linear ...

B. P. Kwan

1978-01-01

195

Window least squares method applied to statistical noise smoothing of positron annihilation data.  

National Technical Information Service (NTIS)

The paper deals with the off-line processing of experimental data obtained by two-dimensional angular correlation of the electron-positron annihilation radiation (2D-ACAR) technique on high-temperature superconductors. A piecewise continuous window least ...

G. Adam S. Adam B. Barbiellini L. Hoffmann A. A. Manuel

1993-01-01

196

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

SciTech Connect

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

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

1982-01-01

197

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

Microsoft Academic Search

We calculate the extragalactic diffuse emission originating from the up-scattering of cosmic microwave photons by energetic electrons and positrons produced in particle dark matter annihilation events at all redshifts and in all halos. We outline the observational constraints on this emission and we study its dependence on both the particle dark matter model (including the particle mass and its dominant

Stefano Profumo; Tesla E. Jeltema

2009-01-01

198

Search for positron annihilation line and continuum radiation from the Galactic Center  

NASA Astrophysics Data System (ADS)

A balloon-borne germanium ?-ray telescope was flown over Alice Springs, Australia, on 1984 November 20 to search for the 511 keV positron annihilation line from the Galactic Center. The measured line flux at Earth was (0.6±4.4)×10-4ph cm-2s-1 indicating that the source was still in a "low" or "off" state.

MacCallum, C. J.; Leventhal, M.

1985-08-01

199

Second-Order QCD Corrections to the Thrust Distribution in Electron-Positron Annihilation  

SciTech Connect

We compute the next-to-next-to-leading-order (NNLO) QCD corrections to the thrust distribution in electron-positron annihilation. The corrections turn out to be sizable, enhancing the previously known next-to-leading-order prediction by about 15%. Inclusion of the NNLO corrections significantly reduces the theoretical renormalization scale uncertainty on the prediction of the thrust distribution.

Gehrmann-De Ridder, A. [Institute for Theoretical Physics, ETH, CH-8093 Zurich (Switzerland); Gehrmann, T. [Institut fuer Theoretische Physik, Universitaet Zuerich, CH-8057 Zurich (Switzerland); Glover, E. W. N. [Institute of Particle Physics Phenomenology, Department of Physics, University of Durham, Durham, DH1 3LE (United Kingdom); Heinrich, G. [School of Physics, University of Edinburgh, Edinburgh EH9 3JZ (United Kingdom)

2007-09-28

200

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

NASA Astrophysics Data System (ADS)

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

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

2012-08-01

201

Measuring electron-positron annihilation radiation from laser plasma interactions  

SciTech Connect

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

Chen, Hui; Tommasini, R. [Lawrence Livermore National Laboratory, Livermore, California 94551 (United States); Seely, J.; Szabo, C. I.; Feldman, U.; Pereira, N. [Artep Inc., Ellicott City, Maryland 21042 (United States); Gregori, G.; Falk, K.; Mithen, J.; Murphy, C. D. [Clarendon Laboratory, University of Oxford, Oxford OX1 3PU (United Kingdom)

2012-10-15

202

OSSE/SMM/TGRS observations of positron annihilation radiation  

NASA Astrophysics Data System (ADS)

Extensive observations using the CGRO/OSSE instrument have permitted the first investigations of the distribution of both line and continuum positronium emission in the Galaxy. Maps have been generated of both components of the total annihilation radiation. In addition, potential bulge and disk distributions have been tested against the measurements of each component. At the level of this study, the two emissions appear to follow a similar distribution for both bulge and disk components. Differences between these maps would be suggestive of a variation of the positronium fraction, which is a diagnostic of the characteristics of the annihilation media. The previously reported enhancement of 511 keV emission from positive latitudes is not present at a significant level in the positronium continuum maps. We conclude this discussion with an assessment of the ability of the SPI and IBIS instruments to resolve ambiguities from the OSSE/SMM/TGRS studies.

Milne, P. A.; Kurfess, J. D.; Kinzer, R. L.; Leising, M. D.; Dixon, D. D.

2001-09-01

203

Helium bubbles in neutron-irradiated copper-boron studied by positron annihilation  

Microsoft Academic Search

The influence of helium, introduced by the B(n, ?)Li reaction, on the evolution of defect structure in copper containing a few hundred ppm boron has been studied by detailed positron lifetime and two-photon angular correlation measurements, supplemented by TEM studies. In the as-irradiated state of Cu-B, two lifetime components have been resolved. The shorter lifetime, ?1, = 167 ps of

B. Viswanathan; G. Amarendra; K. P. Gopinathan

1989-01-01

204

Binding energy, structure, and annihilation properties of the positron-LiH molecule complex, studied with explicitly correlated Gaussian functions  

NASA Astrophysics Data System (ADS)

The energy and two-photon annihilation rate of the complex of the LiH molecule with a positron have been calculated for the equilibrium geometry of LiH in the gas phase, using the explicitly correlated Gaussian basis set. The resulting energy of e+LiH (-8.104 850 hartree) is the lowest obtained to date. The binding energy of the positron in this system has been predicted to be at least 34.401 mhartree, and the annihilation rate has been estimated to be 1.26 ns-1. Maps of the electron, positron, and contact (electron-positron) densities have been prepared.

Strasburger, Krzysztof

1999-12-01

205

Effect of free volume and temperature on the structural relaxation in polymethylphenylsiloxane: A positron lifetime and pressure-volume-temperature study  

Microsoft Academic Search

The microstructure of the free volume and its temperature dependence in polymethylphenylsiloxane (PMPS) have been examined using positron annihilation lifetime spectroscopy (PALS) and pressure-volume-temperature experiments. The hole-free volume fraction h and the specific hole-free and occupied volumes, Vf=hV and Vocc=(1-h)V, were estimated employing the Simha-Somcynsky (SS) lattice-hole theory. From the PALS spectra analyzed with the new routine LT9.0 the hole

G. Dlubek; M. Q. Shaikh; R. Krause-Rehberg; M. Paluch

2007-01-01

206

Prevacancy effects in metals observed by positron annihilation  

SciTech Connect

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

Smedskjaer, L.C.

1982-03-01

207

On positron annihilation in concentrated random alloys and superconducting cuprates  

SciTech Connect

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

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

1988-01-01

208

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

SciTech Connect

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

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

1992-09-01

209

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

SciTech Connect

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

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

1992-09-01

210

Study on Cd vacancy in CdZnTe Crystal by Positron Annihilation Technology  

NASA Astrophysics Data System (ADS)

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

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

2013-03-01

211

Gallium Vacancy in GaSb Studied by Positron Lifetime Spectroscopy and p Photoluminescence.  

National Technical Information Service (NTIS)

Positron lifetime technique and photoluminescence (PL) were employed to study the vacancy type defects in p-type Zn-doped and undoped GaSb samples. In the positron lifetime study, Ga vacancy related defect was identified in these materials and it was foun...

C. C. Ling C. D. Beling M. K. Lui S. Fung W. K. Mui

2002-01-01

212

3DAP and positron annihilation study of precipitation behavior in Cu–Cr–Zr alloy  

Microsoft Academic Search

Precipitation behavior in a Cu–0.78%Cr–0.13%Zr alloy during aging and reaging has been studied by laser-assisted local electrode three-dimensional atom probe (Laser-LEAP) and positron annihilation spectroscopy (PAS). After the prime aging at 460°C, Cr clusters enriched with Zr were observed. Further reaging at 600°C caused the clusters to grow to almost spherical (1h) and thick platelets (4h) Cr precipitates. Zr and

M. Hatakeyama; T. Toyama; J. Yang; Y. Nagai; M. Hasegawa; T. Ohkubo; M. Eldrup; B. N. Singh

2009-01-01

213

Positron Annihilation in Carbon Nanotubes Studied by Coincidence Doppler Broadening Spectroscopy  

Microsoft Academic Search

In order to assign the sites of positron annihilation, coincidence Doppler broadening spectra were measured for a highly oriented pyrolytic graphite crystal, graphite powder, multi-walled carbon nanotubes (MNTs) and cup-stacked carbon nanotubes (CNTs). The spectrum for graphite powder normalized to that for highly oriented pyrolytic graphite (HOPG) is almost flat in the momentum region from 7×10-3 to 13×10-3 mec, having

H. Murakami; K. Sato; I. Kanazawa; M. Sano

2008-01-01

214

Development of Transition Edge Sensor Gamma-Ray Detectors for Positron-Annihilation-Spectroscopy  

NASA Astrophysics Data System (ADS)

We are beginning development of a Transition Edge Sensor (TES) based gamma-ray detector for use in Coincidence-Doppler-Broadening (CDB) Positron-Annihilation-Spectroscopy (PAS) studies. We are specifically motivated to asses and study damage to nuclear Reactor Pressure Vessels (RPVs) which results from neutron irradiation during reactor operation. In this paper we provide a brief introduction to CDB-PAS and discuss our detector design.

Leman, Steven W.; Takahashi, Hiroyuki

2008-05-01

215

Vacancy-impurity interaction in dilute iron alloys by positron annihilation  

Microsoft Academic Search

Positron annihilation measurements of the coincidence count rate at the peak of the angular correlation curve (CCR) have been performed as a function of temperature for the alloy systemFeTi in a concentration range up to about 1.3 at. %. The concentration dependence of the effective vacancy formation enthalpy (H1VF)eff suggests the existence of an attractive interaction between vacancies and the

W. Triftshäuser; H. Matter; J. Winter

1982-01-01

216

Production of positron annihilation radiation by cosmic-rays near sea level  

NASA Astrophysics Data System (ADS)

Production of positron annihilation radiation by cosmic-rays in Al, Fe, Sn and Pb is measured by means of a triggered HPGe detector. The equipment is located in Belgrade, at an absolute height of 125m a.s.l. The production rate per unit mass is found to be proportional to the square of the atomic number of the material divided by its mass number, with the proportionality constant equal to 8.1(3)×10-6s-1g-1.

Puzovic, J. M.; Anicin, I. V.

2002-03-01

217

Search for positron annihilation line and continuum radiation from the Galactic Center  

SciTech Connect

Our balloon-borne germanium ..gamma..-ray telescope was flown over Alice Springs, Australia, on 1984 November 20 to search for the 511 keV positron annihilation line from the Galactic Center. The measured line flux at Earth was (0.6 +- 4.4) x 10/sup -4/ photons per square centimeter per second indicating that the source was still in a ''low'' or ''off'' state. 18 refs., 2 figs.

MacCallum, C.J.; Leventhal, M.

1985-01-01

218

Search for positron annihilation line and continuum radiation from the Galactic Center  

NASA Astrophysics Data System (ADS)

Our balloon-borne germanium (GAMMA)-ray telescope was flown over Alice Springs, Australia, on 1984 November 20 search for the 511 keV positron annihilation line from the Galactic Center. The measured line flux at Earth was (0.6 +- 4.4) x 10 to the minus 4 power photons per square centimeter per second indicating that the source was still in a low or off state.

MacCallum, C. J.; Leventhal, M.

219

Effects of transition metal ions on positron annihilation characteristics in epoxies  

Microsoft Academic Search

Effects of several 3d-transition metal ions on positron annihilation characteristics in MY-720 epoxy resin have been investigated. All metal complexes were added at the mole ratio of 1 metal complex for every 10 MY-720 repeat units. In all cases, it appears that the 3d magnetic electrons are localized on their respective ions. Cr3+-ions make the strongest impact - both on

Jag J. Singh; Diane M. Stoakley; William H. Holt; Willis M. Mock; Joseph P. Teter

1987-01-01

220

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

Microsoft Academic Search

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

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

1996-01-01

221

OSSE, SMM & TGRS Observations of Positron Annihilation Radiation  

NASA Astrophysics Data System (ADS)

Extensive observations using the CGRO/OSSE instrument have permitted the first investigations of the distribution of positronium continuum emission in the Galaxy. Maps have been generated of both the 511 keV line and the positronium continuum portions of the total annihilation radiation. Additionally, potential bulge and disk distributions have been tested against the measurements of each component. The two emissions appear to follow a similar distribution for both bulge and disk components. Differences between these maps would be suggestive of a variation of the positronium faction, which is a diagnostic of the characteristics of the annihilation media. The previously reported enhancement of 511 keV emission from positive latitudes is not present at a significant level in the positronium continuum maps. Potential interpretations of this finding are given. The ramifications of the overall picture emerging from the OSSE analysis in the context of supernova science concludes this discussion. This research was funded by a Research Assistantship from the National Research Council.

Milne, P. A.; Kurfess, J. D.; Leising, M. D.; Dixon, D. D.

2000-10-01

222

Unstable GRB photospheres and electron-positron annihilation lines  

Microsoft Academic Search

We propose an emission mechanism of prompt gamma-ray bursts (GRBs) that can\\u000areproduce the observed non-thermal spectra with high radiative efficiencies,\\u000a>50%. Internal dissipation below a photosphere can create a radiation-dominated\\u000athermal fireball. If electron-positron pairs outnumber protons, radiative\\u000aacceleration of pairs drives the two-stream instabilities between pairs and\\u000aprotons, leading to the ``proton sedimentation'' in the accelerating pair\\u000aframe.

Kunihito Ioka; Kohta Murase; Kenji Toma; Shigehiro Nagataki; Takashi Nakamura

2007-01-01

223

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

224

Recent progress in annihilation related studies by slow positrons  

SciTech Connect

The field of slow-positron physics has expanded significantly in the last few years to include particles and atomic physics but has been most extensive in those associated with condensed matter or material science. This can primarily be attributed to the development of more efficient moderators. These moderators have been associated with both laboratory- and facility-based beams. In this paper I will focus only on the material-science aspects however. Positron can and are being used to examine all of the various fields. I feel the contribution in all these areas will be significant. I will primarily discuss those developments that have been developed in the area of interface science; a field that has both scientific and technological importance and has a limited number of nondestructive probes used in studying a buried interface. Interfaces are technologically important for applications such as electrical properties and mechanical properties. Such applications help to motivate the fundamental research of interface properties and dynamics, which is necessary to develop the basic understanding of new types of interfaces. The role of the interface is also important since it limited length of this paper. Results will be presented in interface studies that have occurred in the last year, including some unpublished results obtained at Brookhaven over the past few months. This field is in the early stages and I expect that the full utilization of this relatively new probe can be anticipated in the next few years. 17 refs., 7 figs.

Lynn, K.G.

1989-01-01

225

Donor-doping effect in BaTiO3 ceramics using positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Temperature-dependent measurements using positron lifetime spectroscopy (PLS) were carried out to study various defects in polycrystalline donor doped BaTiO3 (BT) samples. Annealing under different donor-doping level of La and Y were performed. At high doping level, it was found that with increasing doping level the average lifetime increases. This is attributed to the interplay of complexes containing oxygen vacancies VO and other vacancies, such as VBa, VTi as well as grain boundaries.

Mohsen, M.; Krause-Rehberg, R.; Massoud, A. M.; Langhammer, H. T.

2003-10-01

226

The 511 keV emission from positron annihilation in the Galaxy  

NASA Astrophysics Data System (ADS)

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

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

2011-07-01

227

Hunting for glueballs in electron-positron annihilation  

SciTech Connect

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

Stanley Brodsky; Alfred Scharff Goldhaber; Jungil Lee

2003-05-01

228

Hunting for Glueballs in Electron-Positron Annihilation  

SciTech Connect

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

Brodsky, Stanley J.

2003-05-28

229

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

NASA Astrophysics Data System (ADS)

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

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

2012-07-01

230

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

231

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

232

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

233

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

NASA Astrophysics Data System (ADS)

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

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

2009-02-01

234

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

SciTech Connect

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

Rudaz, S.; Stecker, F.W.

1988-02-01

235

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

SciTech Connect

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

Arbuzov, A. B.; Kuraev, E. A.; Volkov, M. K. [Bogoliubov Laboratory of Theoretical Physics, JINR, Dubna, 141980 (Russian Federation); Department of Higher Mathematics, University Dubna, Dubna, 141980 (Russian Federation); Bogoliubov Laboratory of Theoretical Physics, JINR, Dubna, 141980 (Russian Federation)

2011-04-15

236

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

237

Positron annihilation and thermoluminescence studies of thermally induced defects in ?-Al2O3 single crystals  

NASA Astrophysics Data System (ADS)

?-Al2O3 crystals were subjected to different thermal treatments at a temperature of 1500 °C in a strongly reducing ambience of carbon and vacuum. Positron annihilation spectroscopy (PAS) and thermally stimulated luminescence (TL) studies were carried out to understand the nature of defects generated. Results show the presence of aluminium vacancies in crystals annealed in vacuum. On annealing in the presence of graphite, ingress of carbon in these vacancies is indicated by different PAS measurements. A simultaneous enhancement of dosimetry properties has been observed. The study provides evidence that association of carbon with aluminium vacancies helps in creation of effective dosimetry traps.

Muthe, K. P.; Sudarshan, K.; Pujari, P. K.; Kulkarni, M. S.; Rawat, N. S.; Bhatt, B. C.; Gupta, S. K.

2009-05-01

238

Investigation of Microstructural Changes in Polyetherether-Ketone Films at Cryogenic Temperatures by Positron Lifetime Spectroscopy.  

National Technical Information Service (NTIS)

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 ...

A. Eftekhari D. R. Sprinkle J. J. Singh T. L. St.Clair

1991-01-01

239

Defect Density Mapping of Shot Peened Materials Using Positron Annihilation Spectroscopy  

NASA Astrophysics Data System (ADS)

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

Gagliardi, Marcus A.; Hunt, Alan W.

2009-03-01

240

Defect Density Mapping of Shot Peened Materials Using Positron Annihilation Spectroscopy  

SciTech Connect

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

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

2009-03-10

241

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

242

Microstructural Characterization of Ferritic/Martensitic Steels by Positron Annihilation Spectroscopy  

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

243

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

244

Positron Annihilation Spectroscopy as a Novel Interfacial Probe for Thin Polymeric Films and Nano-Composites  

NASA Astrophysics Data System (ADS)

Positron annihilation spectroscopy (PAS) has been developed as a novel probe to characterize the sub-nanometer defect, free volume, profile from the surface, interfaces, and to the bulk in polymeric materials when a variable mono-energy slow positron beam is used. Free-volume hole sizes, fractions, and distributions are measurable as a function of depth at the high precision. PAS has been successfully used to study the interfacial properties of polymeric nanocomposites at different chemical bonding. In nano-scale thin polymeric films, such as in PS/SiO2, and PU/ZnO, significant variations of Tg as a function of depth and of wt% oxide are observed. Variations of Tg are dependent on strong or weak interactions between polymers and nano-scale oxides surfaces.

Awad, Somia; Chen, Hongmin; Maina, Grace; Lee, L. James; Gu, Xiaohong; Jean, Y. C.

2010-03-01

245

Modeling and simulation of charged particle beam transport in the UTA 2 meter Time of Flight Positron Annihilation Induced Auger Spectrometer  

Microsoft Academic Search

Time of Flight Positron Annihilation Induced Auger Electron Spectroscopy (TOF PAES) is a surface analytical technique with high surface selectivity. Almost 95% of the PAES signal originates from the sample's topmost layer due to the trapping of positrons just above the surface in an image-potential well before annihilation. This talk presents a description of the TOF technique as the results

Prasad Joglekar; Lawrence Lim; Sushant Kalaskar; Karthik Shastry; Suman Satyal; Alexander Weiss

2010-01-01

246

Tuning porosity of silica films by using various surfactants and changing their loading: A study of positron annihilation Doppler broadening based on a slow positron beam  

NASA Astrophysics Data System (ADS)

Porous silica films were synthesized via a sol-gel method using a nonionic amphiphilic triblock copolymer (F127) and a cationic surfactant (CTAB) as the structural templates with varying weight ratio. Positron annihilation Doppler broadening spectroscopy based on a slow positron beam was used to study the prepared silica films. For the porous silica films, the S parameter increased gradually with increasing the surfactant loading, which showed that higher porosity was introduced in the silica films with more porogen amount.

Xiong, Bangyun; Mao, Wenfeng; He, Chunqing

2013-06-01

247

Evidence of Positron Annihilation at Electronic Excitation Threshold for N2 ^*  

NASA Astrophysics Data System (ADS)

We are investigating Positronium (Ps) formation for < 20 eV positrons interacting with N2 in a gas scattering cell. The technique [1] of Ps annihilation ratio spectroscopy (PsARS) is used to obtain the ratios of coincidence signals for two of the three gamma rays (S3?) in the photon energy window 300 to 460 keV resulting from ortho-Ps decay to that for two 511 keV gamma rays (S2?) arising from para-Ps decay and other processes. By comparing these ratios of S3 ?/S2? for N2 to those for Ar it is found that N2 exhibits strikingly anomalous behavior near and below the Ps formation threshold. Typically, this ratio remains constant within 2 eV above the Ps threshold. For N2, this ratio decreases to zero at the threshold and an S2 ? signal remains for an energy of ˜0.3 eV below. Since N2 has an electronic excitation threshold for positron impact that opens up at ˜0.3 eV below the Ps threshold, the present results strongly suggest that the incident positron is electronically exciting N2 and then binding to the excited N2 in a temporary resonance-like state from which the bound positron annihilates with a molecular electron. ^*Research supported by NSF Grant PHY 99-88093.[1] W.E. Kauppila, E.G. Miller, H. F.M. Mohamed, K. Pipinos, T. S. Stein, and E. Surdutovich, Phys. Rev. Lett. 93, 113401 (2004).

Miller, E. G.; Edwards, J. J.; Kauppila, W. E.; Stein, T. S.; Surdutovich, E.

2006-05-01

248

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

249

Positron annihilation doppler broadening spectroscopy of carbon black and carbon black-filled rubber polymers  

NASA Astrophysics Data System (ADS)

We have used positron annihilation Doppler Broadening Spectroscopy to investigate the effects of filling various rubber polymers with different amounts and types of carbon blacks. The S parameter was measured for a wide variety of samples. The results show direct correlation between the S parameter and the various physical characteristics of the carbon black. The S parameter is found to decrease with carbon black particle and aggregate diameter, to increase with carbon black surface area, and to decrease as more carbon black is added to the polymer samples. This behavior can be understood as due to a combination of reduction in positronium formation as carbon black is added to the polymer, and the decrease in positron annihilation with valence electrons in the sample as the amount of carbon black-polymer links are increased. No effect on the S parameter is observed for sample mixing times longer than 45 seconds. The authors express their thanks to Dr. Michel Gerspacher and Dr.Leszek Nikiel, Sid Richardson Carbon and Gasoline Co., Ft. Worth Tx, for providing the samples and for many helpful discussions.

Semaan, Mars E.; Quarles, C. A.

1999-10-01

250

Chemical structural effects on ?-ray spectra of positron annihilation in fluorobenzenes  

NASA Astrophysics Data System (ADS)

Spectra of ?-ray Doppler shifts for positron annihilation in benzene and its fluoro-derivatives are simulated using low energy plane wave positron (LEPWP) approximation. The results are compared with available measurements. It is found that the Doppler shifts in these larger aromatic compounds are dominated by the contributions of the valence electrons and that the LEPWP model overestimates the measurements by approximately 30%, in agreement with previous findings in noble gases and small molecules. It is further revealed that the halogen atoms not only switch the sign of the charges on carbon atoms that they bond to, but that they also polarize other C-H bonds in the molecule leading to a redistribution of the molecular electrostatic potentials. As a result, it is likely that the halogen atoms contribute more significantly to the annihilation process. The present study also suggests that, while the Doppler shifts are sensitive to the number of valence electrons in the molecules, they are less sensitive to the chemical structures of isomers that have the same numbers and type of atoms and, hence, the same numbers of electrons. Further investigation of this effect is warranted.

Wang, F.; Ma, X. G.; Selvam, L.; Gribakin, G. F.; Surko, C. M.

2012-04-01

251

Creation and uses of positron plasmas  

SciTech Connect

Advances in positron trapping techniques have led to room-temperature plasmas of 10[sup 7] positrons with lifetimes of 10[sup 3] s. Improvements in plasma manipulation and diagnostic methods make possible a variety of new experiments, including studies just being initiated of electron--positron plasmas. The large numbers of confined positrons have also opened up a new area of positron annihilation research, in which the annihilation cross sections for positrons with a variety of molecules have been measured, as well as the energy spread of the resulting gamma rays. Such measurements are of interest for fundamental physics and for the modeling of astrophysical plasmas.

Greaves, R.G.; Tinkle, M.D.; Surko, C.M. (Physics Department, University of California, San Diego, La Jolla, California 92093-0319 (United States))

1994-05-01

252

Positron annihilation and differential scanning calorimetric study of poly(trimethylene terephthalate)\\/EPDM blends  

Microsoft Academic Search

The blends of poly(trimethylene terephthalate) (PTT) and ethylene propylene diene monomer (EPDM) with different composition have been studied by positron lifetime technique (PLT) and differential scanning calorimetric (DSC) measurements. The DSC results for the blends of 50\\/50 and 40\\/60 show clear two glass transition temperatures indicating two-phase system. No melting point depression was observed for the blend system, which strongly

H. B. Ravikumar; C. Ranganathaiah; G. N. Kumaraswamy; S. Thomas

2005-01-01

253

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

254

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

NASA Astrophysics Data System (ADS)

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

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

2012-08-01

255

Fermi surface ridge at second and third Umklapp positron annihilations in Y Ba2Cu3O(7-delta).  

National Technical Information Service (NTIS)

Results of statistical noise smoothing of the electron momentum distribution found by two-dimensional angular correlation of the electron-positron annihilation radiation technique on untwinned Y Ba2Cu3O( 7-delta) single crystals are reported. Two distinct...

G. Adam S. Adam B. Barbiellini L. Hoffmann A. A. Manuel

1993-01-01

256

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

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

257

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

NASA Astrophysics Data System (ADS)

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

Amrane, N.

258

Defects in virgin and N+ -implanted ZnO single crystals studied by positron annihilation, Hall effect, and deep-level transient spectroscopy  

Microsoft Academic Search

High-quality single crystals of ZnO in the as-grown and N+ ion-implanted states have been investigated using a combination of three experimental techniques---namely, positron lifetime\\/slow positron implantation spectroscopy accompanied by theoretical calculations of the positron lifetime for selected defects, temperature-dependent Hall (TDH) measurements, and deep level transient spectroscopy (DLTS). The positron lifetime in bulk ZnO is measured to be (151±2)ps and

G. Brauer; W. Anwand; W. Skorupa; J. Kuriplach; O. Melikhova; C. Moisson; H. von Wenckstern; H. Schmidt; M. Lorenz; M. Grundmann

2006-01-01

259

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

SciTech Connect

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

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

2009-01-01

260

Annihilation of an electron-positron pair into five photons at high energies  

SciTech Connect

The cross section for electron-positron annihilation into five photons at high energies is calculated using the method of helicity amplitudes. For {var_phi}-meson factories with luminosity L {approximately}10{sup 33} cm{sup -2} s{sup -1} and large-angle photon detection with a threshold {omega}{sub min}/E > 0.1, the number of such events can be as large as several tens per day. These events must be considered as background in studying the process e{sup -}e{sup +} {yields} {pi}{sup 0}{pi}{sup 0}{gamma}. Explicit expressions for the helicity amplitudes are presented. The above process is also interesting as a higher-order QED transition. 6 refs.

Bratkovskaya, E.L.; Kuraev, E.A. [Joint Institute for Nuclear Research, Dubna (Russian Federation); Peryshkin, A.N.; Silagadze, Z.K. [State Research Center Siberian Division, Novosibirsk (Russian Federation)

1995-09-01

261

Spatially resolved positron annihilation spectroscopy on friction stir weld induced defects  

NASA Astrophysics Data System (ADS)

A friction stir welded (FSW) Al alloy sample was investigated by Doppler broadening spectroscopy (DBS) of the positron annihilation line. The spatially resolved defect distribution showed that the material in the joint zone becomes completely annealed during the welding process at the shoulder of the FSW tool, whereas at the tip, annealing is prevailed by the deterioration of the material due to the tool movement. This might be responsible for the increased probability of cracking in the heat affected zone of friction stir welds. Examination of a material pairing of steel S235 and the Al alloy Silafont36 by coincident Doppler broadening spectroscopy (CDBS) indicates the formation of annealed steel clusters in the Al alloy component of the sample. The clear visibility of Fe in the CDB spectra is explained by the very efficient trapping at the interface between steel cluster and bulk.

Hain, Karin; Hugenschmidt, Christoph; Pikart, Philip; Böni, Peter

2010-04-01

262

Radiation damage in nanocrystalline Ni under irradiation studied using positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

We studied the grain size dependence of the defect evolution that occurred in a nanocrystalline (NC) Ni and its coarse-grained counterpart under irradiation. The vacancy defects produced during irradiation were investigated via in situ observations of the positron annihilation Doppler broadening spectra. The annihilation line-shape parameter S was measured under beam-on (during irradiation) and beam-off (after irradiation) conditions. We found that different variations in S were observed depending on the grain size; for the coarse-grained specimen, the S measured under beam-on conditions was larger than that observed under beam-off conditions, while for the NC specimen, the S value remained unchanged under beam-on and beam-off conditions. The former result indicated that the vacancy concentration was enhanced during irradiation, due to transient vacancy production. The latter result may imply that defect accumulation was strongly suppressed under irradiation. We also studied the flux dependence of the radiation damage effects for the NC specimen, using high-flux He ions in the flux range of 1.2 × 1013-6 × 1014 ions/(cm2 s). A slight flux dependence was observed, which was due to microstructural changes at the intersections of the crystallite interfaces under the highest flux irradiation studied.

Tsuchida, H.; Iwai, T.; Awano, M.; Oshima, N.; Suzuki, R.; Yasuda, K.; Batchuluun, C.; Itoh, A.

2013-11-01

263

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

SciTech Connect

We present laboratory measurements obtained with a ground-based prototype of a focusing positron-annihilation-radiation telescope developed by the Toulouse-Argonne collaboration. This balloon-borne telescope has been designed to collect 511-keV photons with an extremely low instrumental background. The telescope features a Laue diffraction lens and a detector module containing a small array of germanium detectors. It will provide a combination of high spatial and energy resolution (15 arc sec and 2 keV, respectively) with a sensitivity of {approximately}3{times}10{sup {minus}5} photons cm{sup {minus}2}s{sup {minus}1}. These features will allow us to resolve a possible narrow 511-keV line both energetically and spatially within a Galactic center ``microquasar`` or in other broad-class annihilators. The ground-based prototype consists of a crystal lens holding small cubes of diffracting germanium crystals and a 3{times}3 germanium array that detects the concentrated beam in the focal plane. Measured performances of the instrument at different line energies (511 keV and 662 keV) are presented and compared with Monte-Carlo simulations. The advantages of a 3{times}3 Ge-detector array with respect to a standard-monoblock detector have been confirmed. The results obtained in the laboratory have strengthened interest in a crystal-diffraction telescope, offering new perspectives for die future of experimental gamma-ray astronomy.

Naya, J.E.; von Ballmoos, P.; Albernhe, F.; Vedrenne, G. [Centre d`Etude Spatial des Rayonnements, Toulouse (France); Smither, R.K.; Faiz, M.; Fernandez, P.B.; Graber, T. [Argonne National Lab., IL (United States)

1995-10-01

264

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

265

Effect of microstructure on positron-annihilation parameters in YBa2Cu3O7  

NASA Astrophysics Data System (ADS)

The temperature dependence of positron Doppler line shape has been measured for several samples of YBa2Cu3O7-x (x<=0.1). Two distinct temperature dependences have been observed. In one case the line-shape parameter S increases by ~=0.5% between 15 and 100 K, remains constant between 100 and 230 K, and then decreases by ~=0.25% between 230 and 290 K. In the second case, S remains constant between 20 and 100 K, decreases by ~=1% between 100 and 200 K, and then remains constant up to 290 K. Room-temperature positron-lifetime measurements also reveal distinct differences between samples. An explanation of these differences based on micro- structural differences observed by transmission electron microscopy is suggested.

Usmar, S. G.; Lynn, K. G.; Moodenbaugh, A. R.; Suenaga, M.; Sabatini, R. L.

1988-09-01

266

Free-volume studies in polyurethane membranes by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

The free-volume parameters in various urethane/urea membranes obtained by varying the ratio of the structural constituents, polypropylene oxide and polybutadiene, were studied by positron lifetime and Doppler broadening measurements. On bi-soft segment membranes, a correlation was found between the composition of membranes, the normalized free volume, the radii of the holes and gas permeability. However, the correlation is not clear when PU data are also considered, indicating that other features must also play an important role in the permeation mechanism.

Ferreira Marques, M. F.; Lopes Gil, C.; Gordo, P. M.; Kajcsos, Zs.; de Lima, A. P.; Queiroz, D. P.; de Pinho, M. N.

2003-10-01

267

Defect Characterization in SiGe/SOI Epitaxial Semiconductors by Positron Annihilation  

PubMed Central

The potential of positron annihilation spectroscopy (PAS) for defect characterization at the atomic scale in semiconductors has been demonstrated in thin multilayer structures of SiGe (50 nm) grown on UTB (ultra-thin body) SOI (silicon-on-insulator). A slow positron beam was used to probe the defect profile. The SiO2/Si interface in the UTB-SOI was well characterized, and a good estimation of its depth has been obtained. The chemical analysis indicates that the interface does not contain defects, but only strongly localized charged centers. In order to promote the relaxation, the samples have been submitted to a post-growth annealing treatment in vacuum. After this treatment, it was possible to observe the modifications of the defect structure of the relaxed film. Chemical analysis of the SiGe layers suggests a prevalent trapping site surrounded by germanium atoms, presumably Si vacancies associated with misfit dislocations and threading dislocations in the SiGe films.

2010-01-01

268

Effects of quantum chemistry models for bound electrons on positron annihilation spectra for atoms and small molecules  

NASA Astrophysics Data System (ADS)

The Doppler-shift spectra of the ?-rays from positron annihilation in molecules were determined by using the momentum distribution of the annihilation electron-positron pair. The effect of the positron wavefunction on spectra was analysed in a recent paper (Green et al 2012 New J. Phys. 14 035021). In this companion paper, we focus on the dominant contribution to the spectra, which arises from the momenta of the bound electrons. In particular, we use computational quantum chemistry models (Hartree-Fock with two basis sets and density functional theory (DFT)) to calculate the wavefunctions of the bound electrons. Numerical results are presented for noble gases and small molecules such as H2, N2, O2, CH4 and CF4. The calculations reveal relatively small effects on the Doppler-shift spectra from the level of inclusion of electron correlation energy in the models. For atoms, the difference in the full-width at half-maximum of the spectra obtained using the Hartree-Fock and DFT models does not exceed 2%. For molecules the difference can be much larger, reaching 8% for some molecular orbitals. These results indicate that the predicted positron annihilation spectra for molecules are generally more sensitive to inclusion of electron correlation energies in the quantum chemistry model than the spectra for atoms are.

Wang, Feng; Ma, Xiaoguang; Selvam, Lalitha; Gribakin, Gleb; Surko, Clifford M.

2012-08-01

269

The Measurement of the Hadronic Cross Section 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 multi-hadron events at a center-of-mass energy of 20 GeV. The events are used in the measurement of the ratio R, the hadronic cross section in electron-positron annihilation to the cross section for muon pair production in electron -positron annihilation. The experiment is important because R is a fundamental parameter that tests in a model independent way the basic assumptions of strong interaction theories. The events are selected by cutting on three quantities; momentum thrust angle, charged multiplicity, and total energy. The cuts are necessary to reduce backgrounds from two photon processes and tau-pair production. The final event sample corresponds about 18000 events in an integrated luminosity of 80.9pb('-1). The accepted event sample is corrected for backgrounds and detection efficiencies in order to obtain the actual number of events produced. The cuts are defined to minimize the errors associated with extrapolation of the observed numbers to the actual number of events. Monte Carlo techniques are used to model the physics processes and detector acceptance. The sensitivity of the acceptance to the cuts was checked by loosening and tightening the cuts. The acceptance was also checked for sensitivity to variation in Monte Carlo parameters. The integrated luminosity was measured with large angle Bhabha scattering. Good agreement was found between the luminosity measured with large angle Bhabha scattering and measured with smaller angle Bhabha scattering and collinear mu-pairs. The radiative corrections were done by summing leading logs to obtain higher order corrections. The major source of error with radiative effects is due to the poor precision of the lower energy R measurements. The value obtained is: R = 3.81 (+OR-) 0.04 (+OR -) 0.10 the most precise to date. The value of (alpha)(,s) extracted from the R-value is (alpha)(,s) = 0.12 (+OR-) 0.09. Three sources of error dominate to systematic error; the acceptance modelling, integrated luminosity, and radiative corrections. Without simultaneous reduction in all these sources the systematic error has little chance of being reduced in the near future.

Clearwater, Scott Harvey

270

Inclusive hadron production in upsilon decays and in nonresonant electron-positron annihilation at 10.49 GeV  

Microsoft Academic Search

We report measurements of single-particle inclusive spectra and two-particle correlations in decays of the Upsilon(1S) resonance and in nonresonant annihilations of electrons and positrons at center-of-mass energy 10.49 GeV, just below BB¯ threshold. These data were obtained using the CLEO detector at the Cornell Electron Storage Ring (CESR) and provide information on the production of pi, K, rho, K*, phi,

S. Behrends; K. Chadwick; T. Gentile; Jan M. Guida; Joan A. Guida; A. C. Melissinos; S. L. Olsen; G. Parkhurst; R. Poling; C. Rosenfeld; E. H. Thorndike; P. Tipton; D. Besson; J. Green; R. Namjoshi; F. Sannes; P. Skubic; A. Snyder; R. Stone; A. Chen; M. Goldberg; N. Horwitz; A. Jawahery; P. Lipari; G. C. Moneti; C. G. Trahern; H. van Hecke; S. E. Csorna; L. Garren; M. D. Mestayer; R. S. Panvini; Xia Yi; M. S. Alam; P. Avery; C. Bebek; K. Berkelman; D. G. Cassel; J. W. Dewire; R. Ehrlich; T. Ferguson; R. Galik; M. G. D. Gilchriese; B. Gittelman; M. Halling; D. L. Hartill; S. Holzner; M. Ito; J. Kandaswamy; D. L. Kreinick; Y. Kubota; N. B. Mistry; F. Morrow; E. Nordberg; M. Ogg; D. Peterson; K. Read; A. Silverman; P. C. Stein; S. Stone; Xu Kezun; A. J. Sadoff; R. T. Giles; J. Hassard; M. Hempstead; K. Kinoshita; W. W. Mackay; F. M. Pipkin; Richard Wilson; P. Haas; T. Jensen; H. Kagan; R. Kass

1985-01-01

271

A positron annihilation radiation telescope using Laue diffraction in a crystal lens  

SciTech Connect

We present a new type of gamma-ray telescope featuring a Laue diffraction lens, a detector module with a 3-by-3 germanium array, and a balloon gondola stabilized to 5 arc sec pointing accuracy. The instrument`s lens is designed to collect 511 keV photons on its 150 CM{sup 2} effective area and focus them onto a small detector having only {approx}14 CM{sup 3} of equivalent volume for background noise. As a result, this telescope overcomes the mass-sensitivity impasse of present detectors in which the collection areas are identical to the detection area. The sensitivity of our instrument is anticipated to be 3 {times} 10{sup {minus}5} ph cm{sup {minus}2} S{sup {minus}1} at 511 key with an angular resolution of 15 arc sec and an energy resolution of 2 keV. These features will allow the resolve of a possible energetically narrow 511 keV positron annihilation line both energy-wise and spatially within a Galactic Center ``microquasar`` as 1El740.7-2942 or GRS1758-258. In addition to the galactic ``microquasars,`` other prime objectives include Cyg X-1, X-ray binaries, pulsars, and AGNS.

Smither, R.K. [Argonne National Lab., IL (United States); von Ballmoos, P. [Toulouse-3 Univ., 31 (France). Centre d`Etude Spatiale des Rayonnements

1993-03-01

272

A positron annihilation radiation telescope using Laue diffraction in a crystal lens  

SciTech Connect

We present a new type of gamma-ray telescope featuring a Laue diffraction lens, a detector module with a 3-by-3 germanium array, and a balloon gondola stabilized to 5 arc sec pointing accuracy. The instrument's lens is designed to collect 511 keV photons on its 150 CM[sup 2] effective area and focus them onto a small detector having only [approx]14 CM[sup 3] of equivalent volume for background noise. As a result, this telescope overcomes the mass-sensitivity impasse of present detectors in which the collection areas are identical to the detection area. The sensitivity of our instrument is anticipated to be 3 [times] 10[sup [minus]5] ph cm[sup [minus]2] S[sup [minus]1] at 511 key with an angular resolution of 15 arc sec and an energy resolution of 2 keV. These features will allow the resolve of a possible energetically narrow 511 keV positron annihilation line both energy-wise and spatially within a Galactic Center microquasar'' as 1El740.7-2942 or GRS1758-258. In addition to the galactic microquasars,'' other prime objectives include Cyg X-1, X-ray binaries, pulsars, and AGNS.

Smither, R.K. (Argonne National Lab., IL (United States)); von Ballmoos, P. (Toulouse-3 Univ., 31 (France). Centre d'Etude Spatiale des Rayonnements)

1993-03-01

273

Hydrogen-embrittlement study and positron annihilation response of hydrogen in AISI 4340 steel  

SciTech Connect

Hydrogen degradation of mechanical properties and the positron annihilation response of hydrogen in ultra-high strength 4340 steel were studies. Hydrogen dissolves in the steel up to about 8 wt. ppm with very high diffusivity of the order of 10{sup {minus}7} cm{sup 2}/sec at room temperature during cathodic charging. The strength was greatly lowered by implanted hydrogen, and as the hydrogen was removed by aging at room and elevated temperature, the strength was recovered fully or partially depending on the hydrogen-induced damage. Intergranular fracture along prior austenite grain boundaries was produced by initial hydrogen and with further implantation of high fugacity hydrogen the fracture mode changed to transgranular fracture. Severe charging increased dislocation density and formed dislocation substructures. The fractographs indicated that hydrogen-induced cracks were localized in the surface region, presumable nucleating at trapping sites such as non-metallic inclusions, prior austenite grain boundaries, martensite lath boundaries, and dislocation tangles. Hydrogen-induced damage could be nondestructively detected by the Dopper-broadening technique by virtue of a higher value of the parameter, P/W.

Shim, In-Ok.

1988-01-01

274

Effect of barium chloride doping on PVA microstructure: positron annihilation study  

NASA Astrophysics Data System (ADS)

Microstructural studies on BaCl2 doped polyvinyl alcohol (PVA) polymer films were carried out using density, PAL and dielectric measurements at room temperature. The positron annihilation studies on these samples shows considerable effect on the PVA microstructure due to doping and is understood by invoking the chemical reaction between Ba2+ ions of BaCl2 with OH groups of PVA via intra/inter molecular hydrogen bonding, which forms the complex. This complex formation modifies the free volume content in the amorphous fraction, and results in an enhancement of the polymer crystallinity. At higher dopant concentrations, the number of such complexes increases, and ends up with the formation of dopant aggregates or agglomerates leading to certain phase separation into a polymer-rich phase and a dopant-rich phase. These phase separations are thought to be due to the existence of two or more crystalline phases within the polymer matrix. The XRD study also supports this enhancement of PVA crystallinity due to doping. The electrical studies on the doped PVA reflects that the complex formation due to doping affects the microstructure and hence the dielectric properties including the dc and ac conductivities of the polymer. All of these observed results were analyzed and understood based on the microstructural modification of PVA as a function of dopant concentrations.

Bhajantri, R. F.; Ravindrachary, V.; Harisha, A.; Ranganathaiah, C.; Kumaraswamy, G. N.

2007-06-01

275

Positron annihilation spectroscopy of AlGaAs/GaAs interfaces in metalorganic chemical vapor deposition grown GaAs heterojunction solar cells  

SciTech Connect

The defect density profile of high-efficiency epitaxial metalorganic chemical vapor deposition (MOCVD) grown GaAs heterojunction solar cell structures has been characterized using a variable-energy positron beam. Spatial defect changes, film thickness variations, and possibly interfacial space charge and disorder may be resolved from annihilation characteristics by control of the implantation depth of positrons. Correlations were made relating positron annihilation spectroscopy (PAS) measurements to surface photovoltage data, band bending, and known MOCVD growth parameter variations. Based upon these correlations, it is expected that PAS may provide a valuable means for probing defect profiles that may affect the electrical and optical response of MOCVD-grown semiconductor materials.

DeWald, A.B.; Frost, R.L.; Ringel, S.A.; Schaffer, J.P.; Rohatgi, A.; Nielsen, B.; Lynn, K.G.

1988-07-01

276

Positron line-shape parameters and lifetimes for semiconductors: Systematics and temperature effects  

NASA Astrophysics Data System (ADS)

Positron Doppler broadening and lifetime experiments have been performed on C (diamond), SiC, Si, Ge, GaN, GaP, GaAs, GaSb, InP, InAs, and InSb. It was found that the Doppler-broadening parameter arising from valence electrons depends linearly on the free-electron gas rs value when calculated from the valence-electron density. Positron lifetimes due to valence electrons are found to be proportional to r3>s. Finer details, which result from the bonded character of the valence electrons, are revealed by slight anisotropies of the S parameter. Temperature dependencies in the 100-600-K temperature range studied by means of Doppler broadening show a complex behavior which in part may be defect influenced.

Dannefaer, S.; Puff, W.; Kerr, D.

1997-01-01

277

Identification of Zn-vacancy-hydrogen complexes in ZnO single crystals: A challenge to positron annihilation spectroscopy  

Microsoft Academic Search

A systematic study of various, nominally undoped ZnO single crystals, either hydrothermally grown (HTG) or melt grown (MG), has been performed. The crystal quality has been assessed by x-ray diffraction, and a comprehensive estimation of the detailed impurity and hydrogen contents by inductively coupled plasma mass spectrometry and nuclear reaction analysis, respectively, has been made also. High precision positron lifetime

G. Brauer; W. Anwand; D. Grambole; J. Grenzer; W. Skorupa; J. Cízek; J. Kuriplach; I. Procházka; C. C. Ling; C. K. So; D. Schulz; D. Klimm

2009-01-01

278

Positron lifetime studies of defect structures in Ba sub 1-x K sub x BiO sub 3  

SciTech Connect

Temperature-dependent positron lifetime experiments have been performed from room temperature to cryogenic temperatures on Ba{sub 1-x}K{sub x}BiO{sub 3}, for x = 0.4 and 0.5. From the temperature dependence of the positron lifetime in the normal state, we observe a clear signature of competition between separate defect populations to trap the positron. Theoretical calculations of lifetimes of free or trapped positrons have been performed on Ba{sub 1-x}K{sub x}BiO{sub 3}, to help identify these defects. Lifetime measurements separated by long times have been performed and evidence of aging effects in the sample defect populations is seen in these materials. 5 refs., 3 figs.

O'Brien, J.C.; Howell, R.H.; Radousky, H.B.; Sterne, P.A. (Lawrence Livermore National Lab., CA (USA)); Hinks, D.G. (Argonne National Lab., IL (USA)); Folkerts, T.J.; Shelton, R.N. (California Univ., Davis, CA (USA))

1990-12-20

279

3-D defect density plots of large scale structural materials with positron annihilation spectroscopy: feasibility and optimization  

NASA Astrophysics Data System (ADS)

We investigated the feasibility of the pair-production based positron annihilation spectroscopy (PAS) technique to generate 2-D and possibly 3-D defect density images (defect density plots) of structural materials widely used in industry. After assessing the possible PAS techniques and performing MCNPX simulations on different positron production mechanisms, we chose to employ Doppler broadening analysis of the annihilation peak coupled with the pair-production method of positron creation using bremsstrahlung photon beams from a linear accelerator. First, to optimize the important parameters of the proposed experimental setup, MCNPX simulations were performed. These parameters are: thickness of the bremsstrahlung beam hardener, detector orientation, thickness of the bremsstrahlung converter, beam energy and current, etc. These parameters were studied to maximize the number of 511 keV counts in the detector and minimize the low energy background photons that Compton scatter into the detector. A Figure of Merit (FOM) was used to quantitatively track effectiveness (or ineffectiveness) of each value for all the parameters. We conducted experiments with the simulated setup and these results were compared to MCNPX calculations.

Makarashvili, V.; Wells, D. P.; Roy, A. K.

2011-01-01

280

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

Microsoft Academic Search

In the current paper, the decomposition behavior of the engineering alloys 2024 (Al-Cu-Mg) and 7010 (Al-Zn-Cu-Mg) is studied using positron lifetime measurements. Positrons probe open volume defects such as vacancies and dislocations. However, they may also be used to investigate coherent zones and incoherent precipitates. In order to understand the rather complicated precipitation sequences and the response of positrons to

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

1998-01-01

281

Positron spectroscopy of an ion-modified lyomeso structure  

NASA Astrophysics Data System (ADS)

The effect of magnesium sulfate on the structural parameters of aqueous lecithin solutions has been studied using electron-positron annihilation spectroscopy. Changes in the lifetime and annihilation of ortho-positronium in the solutions arise from the introduction of low concentrations of dopant ions. The experimental results are interpreted in terms of the modifying role of magnesium cations and sulfate ions.

Popov, E. V.; Lyashenko, K. A.

1993-05-01

282

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

SciTech Connect

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

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

2007-12-24

283

Observations of the growth and sublimation of water ice films by variable-energy positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

The near-surface structure of thin vapor-deposited water ice films during formation and sublimation in vacuum has been probed using in-situ positron annihilation spectroscopy. While amorphous solid water films prepared at 120K are highly dense with low porosity micro- and interconnected meso-pores have been observed in the top few tens of nm of ice films prepared at temperature of 170 – 180K as the films are forming and during sublimation. Responses to these near-surface pores are much more pronounced in films grown at 170 – 180K than in those grown at lower temperatures.

Townrow, S.; Coleman, P. G.; Wu, Y. C.; Jiang, J.; Wang, S. J.

2013-06-01

284

Modification of the mesoscopic structure in neutron irradiated EPDM viewed through positron annihilation spectroscopy and dynamic mechanical analysis  

NASA Astrophysics Data System (ADS)

This article focuses on the study of the mesoscopic structure in neutron irradiated EPDM both from experimental and theoretical points of view. In this work we reveal completely the modification of the mesostructure of the EPDM due to neutron irradiation, resolving volume fraction, size and distribution of the crystalline zones as a function of the irradiation dose. Positron annihilation spectroscopy and dynamic mechanical analysis techniques are applied and the results are discussed by means of new theoretical results for describing the interaction process between the crystals and amorphous zones in EPDM.

Lambri, O. A.; Plazaola, F.; Axpe, E.; Mocellini, R. R.; Zelada-Lambri, G. I.; García, J. A.; Matteo, C. L.; Sorichetti, P. A.

2011-02-01

285

Radiative muon (pion) pair production in high energy electron-positron annihilation (the case of small invariant pair mass)  

Microsoft Academic Search

The process of the muon (pion) pair production with small invariant mass in\\u000athe electron--positron high--energy annihilation, accompanied by emission of\\u000ahard photon at large angles, is considered. We find that the Drell--Yan picture\\u000afor differential cross section is valid in the charge--even experimental\\u000aset--up. Radiative corrections both for electron block and for final state\\u000ablock are taken into account.

A. B. Arbuzov; E. Bartoÿs; V. V. Bytev; E. A. Kuraev; Z. K. Silagadze

2003-01-01

286

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

NASA Astrophysics Data System (ADS)

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

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

2011-12-01

287

Electrical properties and positron annihilation study of (Ba 1? x Ho x )TiO 3 ceramics  

Microsoft Academic Search

DC resistivity, dielectric constant, dielectric loss and positron annihilation spectra of (Ba1?x\\u000a Ho\\u000a x\\u000a )TiO3 ceramics have been measured as a function of holmium concentration x. It has been found that the DC resistivity of (Ba1?x\\u000a Ho\\u000a x\\u000a )TiO3 is strongly dependent on the Ho content: it decreases three orders of magnitude and reaches a minimum at x = 0.4%. Doping with

Fanggao Chang; Tao Li; Yongxia Ge; Zhenping Chen; Zhongshi Liu; Xiping Jing

2007-01-01

288

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

289

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

PubMed

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

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

2010-09-28

290

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

NASA Astrophysics Data System (ADS)

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

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

2009-12-01

291

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

SciTech Connect

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

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

1998-09-04

292

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

NASA Astrophysics Data System (ADS)

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 disc 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 colour-flavour-locked phase, respectively. The electron-positron energy deposition rate on the rotation axis of rotating neutron and quark stars is studied for two accretion disc models (isothermal disc and accretion disc 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 general relativistic objects, 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.

Kovács, Z.; Cheng, K. S.; Harko, T.

2011-03-01

293

Gamma-ray and radio constraints of high positron rate dark matter models annihilating into new light particles  

SciTech Connect

The possibility of explaining the positron and electron excess recently found by the PAMELA and ATIC Collaborations in terms of dark matter (DM) annihilation has attracted considerable attention. Models surviving bounds from, e.g., antiproton production generally fall into two classes, where either DM annihilates directly with a large branching fraction into light leptons, or, as in the recent models of Arkani-Hamed et al., and of Nomura and Thaler, the annihilation gives low-mass (pseudo)scalars or vectors {phi} which then decay into {mu}{sup +}{mu}{sup -} or e{sup +}e{sup -}. While the constraints on the first kind of models have recently been treated by several authors, we study here specifically models of the second type which rely on an efficient Sommerfeld enhancement in order to obtain the necessary boost in the annihilation cross section. We compute the photon flux generated by QED radiative corrections to the decay of {phi} and show that this indeed gives a rather spectacular broad peak in E{sup 2}d{sigma}/dE, which for these extreme values of the cross section violates gamma-ray observations of the Galactic center for DM density profiles steeper than that of Navarro, Frenk and White. The most stringent constraint comes from the comparison of the predicted synchrotron radiation in the central part of the Galaxy with radio observations of Sgr A*. For the most commonly adopted DM profiles, the models that provide a good fit to the PAMELA and ATIC data are ruled out, unless there are physical processes that boost the local antimatter fluxes more than 1 order of magnitude, while not affecting the gamma-ray or radio fluxes.

Bergstroem, Lars; Bringmann, Torsten; Edsjoe, Joakim [Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, AlbaNova, SE--106 91 Stockholm (Sweden); Bertone, Gianfranco [Institut d'Astrophysique de Paris, UMR7095-CNRS Universite Pierre et Marie Curie, 98bis Boulevard Arago, 75014 Paris (France); Taoso, Marco [Institut d'Astrophysique de Paris, UMR7095-CNRS Universite Pierre et Marie Curie, 98bis Boulevard Arago, 75014 Paris (France); INFN, Sezione di Padova, via Marzolo 8, Padova, 35131 (Italy)

2009-04-15

294

Anti-proton and positron cosmic rays from dark matter annihilation around intermediate mass black holes  

NASA Astrophysics Data System (ADS)

Intermediate Mass Black Holes (IMBHs) are candidates to seed the Supermassive Black Holes (SMBHs), and some could still wander in the Galaxy. In the context of annihilating DM, they are expected to drive huge annihilation rates, and could therefore significantly enhance the primary cosmic rays (CRs) expected from annihilation of the DM of the Galactic halo. In this proceeding (the original paper is Brun et al. [Phys. Rev. D 76 (8) (2007) 083506]), we briefly explain the method to derive estimates of such exotic contributions to the p¯ and e CR spectra, and the associated statistical uncertainties connected to the properties of IMBHs. We find boost factors of order 10 to the exotic fluxes, but associated with very large statistical uncertainties.

Lavalle, Julien

2008-04-01

295

Positrons in cosmic rays from dark matter annihilations for uplifted Higgs regions in the MSSM  

SciTech Connect

We point out that there are regions in the minimal supersymmetric standard model (MSSM) parameter space which successfully provide a dark matter (DM) annihilation explanation for observed e{sup +} excess (e.g., PAMELA), while still remaining in agreement with all other data sets. Such regions (e.g., the uplifted Higgs region) can realize an enhanced neutralino DM annihilation dominantly into leptons via a Breit-Wigner resonance through the CP-odd Higgs channel. Such regions can give the proper thermal relic DM abundance, and the DM annihilation products are compatible with current antiproton and gamma ray observations. This scenario can succeed without introducing any additional degrees of freedom beyond those already in the MSSM.

Kadota, Kenji; Freese, Katherine [Michigan Center for Theoretical Physics, University of Michigan, Ann Arbor, Michigan 48109 (United States); Gondolo, Paolo [Department of Physics, University of Utah, Salt Lake City, Utah 84112 (United States)

2010-06-01

296

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

297

Effect of annealing on microstructure evolution in CoFeB/MgO/CoFeB heterostructures by positron annihilation  

NASA Astrophysics Data System (ADS)

As one of the most powerful tools for investigation of defects of materials, positron annihilation spectroscopy was employed to explore the thermal effects on the film microstructure evolution in CoFeB/MgO/CoFeB heterostructures. It is found that high annealing temperature can drive vacancy defects agglomeration and ordering acceleration in the MgO barrier. Meanwhile, another important type of defects, vacancy clusters, which are formed via the agglomeration of vacancy defects in the MgO barrier after annealing, still exists inside the MgO barrier. All these behaviors in the MgO barrier could potentially impact the overall performance in MgO based magnetic tunnel junctions.

Zhao, Chong-Jun; Lu, Xiang-An; Zhao, Zhi-Duo; Li, Ming-Hua; Zhang, Peng; Wang, Bao-Yi; Cao, Xing-Zhong; Zhang, Jing-Yan; Yu, Guang-Hua

2013-09-01

298

Positron annihilation spectroscopy and transport properties of double perovskite compounds Sr2-xGdxFeMoO6  

NASA Astrophysics Data System (ADS)

Single phases of double layered perovskitelike Sr2-xGdxFeMoO6 samples with x varying from 0.00 to 0.25 have been synthesized. Distortion in FeO6 and MoO6 octahedrons is characterized by Rietveld refinements using the powder x-ray diffraction data. Antisite defects are considered during the refinements. Positron annihilation spectroscopy shows us that the density of antisite defects increases with increasing x. The size of defects reaches the largest values, and conducting electrons are most extensive in sample with x=0.20. The insulating behavior is observed. We argue that the valence states between Fe and Mo play the critical role in magnetism and transition properties in Gd doped Sr2FeMoO6.

Hu, Y. C.; Wang, P. F.; Lv, Bin; Ji, Qing; Wu, X. S.; Lu, Qingfeng

2009-04-01

299

Hydrogen behaviour in electron-irradiated titanium alloys studied by positron annihilation method  

Microsoft Academic Search

Measurements of angular distribution of annihilation photons are applied to investigations of hydrogen behavior in annealed, plastically deformed, and irradiated titanium alloys. It is shown that hydrogen interaction with materials becomes more intensive in presence of structural imperfections of deformational and radiation origin. Probable mechanisms of hydrogen interaction with deformational and radiation defects are discussed.

K. M. Mukashev; F. F. Umarov

2007-01-01

300

Many-body enhancement of positron annihilation in metals: the choice of electron density parameter rs  

Microsoft Academic Search

It is shown that the choice of electron density parameter rs, based on the electron density in the interstitial region, leads to a universal correlation between annihilation rate ( lambda ) and rs for all metals, known previously only for simple (sp) metals. This new choice of rs also gives a lower momentum dependence of the enhancement factor in Cu,

S. M. Sharma; S. K. Sikka

1984-01-01

301

The role of positron annihilation lifetime studies and nuclear sensors for characterising porous materials  

NASA Astrophysics Data System (ADS)

A series of nuclear sensors were designed to assess the chemistry within the nanopores of a porous material. The nuclear sensors of varying size, charge, and hydrophobicity were exposed to hollow silica shells (HSS) at varying pH. Uptake and release kinetics were studied over a 24 h period at room temperature. Preliminary study indicate positively charged nuclear sensors were selectively and rapidly (within 10 min) absorbed by the HSS at pH 7 to 9. PALS showed there were two types of pores (1.7 and 0.7 nm) present. The data suggest the nuclear sensors sit within the larger pore of the HSS. Both PALS and nuclear sensors are required to obtain an accurate insight into the nanoporosity of the hollow silica shells.

Mume, E.; Uedono, A.; Mizunaga, G.; Lynch, D. E.; Smith, S. V.

2011-01-01

302

Defect properties of as-grown and electron-irradiated Te-doped GaSb studied by positron annihilation  

NASA Astrophysics Data System (ADS)

Defects in as-grown undoped and tellurium-doped gallium antimonide were studied using positron lifetime and coincidence Doppler broadening measurements. The grown-in defects in these samples were supposed to be Ga vacancy (VGa)-related defects. More VGa-related defects were introduced into undoped and lightly Te-doped GaSb after electron irradiation at the doses of 1.0 × 1017 cm-2 and 1.0 × 1018 cm-2 however, in the heavily Te-doped GaSb, electron irradiation led to partial recovery of VGa. The role of Te content in the defect evolution is also discussed.

Hui, Li; Kai, Zhou; Jingbiao, Pang; Yundong, Shao; Zhu, Wang; Youwen, Zhao

2011-07-01

303

Positron annihilation spectroscopy as a diagnostic tool for process monitoring of buried oxide layer formation in Si  

NASA Astrophysics Data System (ADS)

The sensitivity of positron annihilation spectroscopy (PAS) to process parameters associated with the formation of buried oxide layers in silicon (SIMOX, Separation by IMplantation of OXygen, structures) has been investigated and evaluated. Ion beam current density, implantation temperature, and ion dose and energy were varied over ranges used in standard and low-dose SIMOX formation. Whereas the positron (Doppler broadening) parameter for as-implanted samples was found to have no measurable sensitivity to low-energy ion current density, the results suggest that there is a small dependence on implantation temperature. More significantly, PAS is highly sensitive to ion dose (i.e., to changes in dose of a few percent), and to ion energy. An empirical parameter is proposed as a measure of the former. The results for as-implanted samples suggest that beam-based PAS is an ideal method for improved dose control, particularly during the formation of low-dose SIMOX. Measurements on fully-formed SIMOX samples are also rich in information related to the structure of the samples.

Coleman, P. G.; Knights, A. P.; Anc, M. J.

2003-01-01

304

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

305

Vacancy-type defects in 6H silicon carbide induced by He-implantation: a positron annihilation spectroscopy approach  

NASA Astrophysics Data System (ADS)

Six-fold helium ion implantation was carried out on nitrogen doped n-type 6H-SiC epi samples. A box-shaped He-implantation profile and damage region was thus introduced. Vacancy-type defects in the implanted region were studied by positron annihilation spectroscopy using a monoenergetic positron beam. The average size of the vacancy-type defect detected in the as-He-implanted sample was the divacancy (V2). Thermal annealing had the effect of shrinking the defective region. Annealing at temperatures lower than 900 °C had the effect of removing vacancy-type defects in the defective region. While the annealing temperature is above 900 °C, the size of the vacancy-type defects in the defective region increased with annealing temperature. At the annealing temperature of 1600 °C, the defective region reduced to ~100 nm and the vacancy-type defects within the region agglomerated to clusters having an average size of 14 V2.

Zhu, C. Y.; Ling, C. C.; Brauer, G.; Anwand, W.; Skorupa, W.

2008-10-01

306

SYNCHROTRON RADIATION, FREE ELECTRON LASER, APPLICATION OF NUCLEAR TECHNOLOGY, ETC.: Positronium annihilation in silica aerogel studied by a positron age-momentum correlation technique  

NASA Astrophysics Data System (ADS)

A high-performance positron age-momentum correlation (AMOC) spectrometer was newly developed. The counting rate is increased up to 200 cps much larger than the value 20 cps reported by other international groups. And at the same time, the time resolution still keeps at the international level of 220 ps. Furthermore, positronium (Ps) annihilation in silica aerogel was investigated by AMOC, which indicates: (1) Ps annihilation between the grains dominantly undergoes pick-off process and spin conversion from o-Ps to p-Ps; (2) Annealing below 400 °C changes the grain surface conditions, i. e. the desorption of hydrogen and the decrease of the defect centers concentration.

Wang, Dan-Ni; Zhang, Lan-Zhi; Wang, Bao-Yi; Yu, Run-Sheng; Zhang, Zhi-Ming; Li, Dao-Wu; Wei, Long

2009-01-01

307

Annihilation of electron-positron pairs in the positronium ion Ps{sup -} and bipositronium Ps{sub 2}  

SciTech Connect

Rates of the two-, three-, four-, and five-photon annihilations of the electron-positron pairs are determined numerically for the three-body positronium ion Ps{sup -}(e{sup -}e{sup +}e{sup -}) and four-body bipositronium ''molecule''Ps{sub 2}(e{sup -}e{sup +}e{sup -}e{sup +}). The values obtained in our computations are {gamma}{sub 2{gamma}}(Ps{sup -}){approx_equal}2.080 485 305 25x10{sup 9} s{sup -1}, {gamma}{sub 3{gamma}}(Ps{sup -}){approx_equal}5.636 415 155 0x10{sup 6} s{sup -1}, {gamma}{sub 4{gamma}}(Ps{sup -}){approx_equal}3.075x10{sup 3} s{sup -1}, {gamma}{sub 5{gamma}}(Ps{sup -}){approx_equal}5.383 s{sup -1}, and {gamma}{sub 2{gamma}}(Ps{sub 2}){approx_equal}4.438 595 2x10{sup 9} s{sup -1}, {gamma}{sub 3{gamma}}(Ps{sub 2}){approx_equal}1.202 497x10{sup 7} s{sup -1}, {gamma}{sub 4{gamma}}(Ps{sub 2}){approx_equal}6.562x10{sup 3} s{sup -1}, {gamma}{sub 5{gamma}}(Ps{sub 2}){approx_equal}11.484 s{sup -1}. The four- and five-photon annihilation rates are significantly smaller than the corresponding two- and three-photon annihilation rates known for these systems. We also determine the rates of one- and zero-photon annihilation for the Ps{sup -} ion and Ps{sub 2} system. The corresponding numerical values are {gamma}{sub 1{gamma}}(Ps{sup -}){approx_equal}3.824 91x10{sup -2} s{sup -1}, {gamma}{sub 1{gamma}}(Ps{sub 2}){approx_equal}1.941 88x10{sup -1} s{sup -1}, and {gamma}{sub 0{gamma}}(Ps{sub 2}){approx_equal}2.321 97x10{sup -9} s{sup -1}.

Frolov, Alexei M. [Department of Chemistry, University of Western Ontario, London, Ontario, N6A 5B7 (Canada)

2009-07-15

308

Identification of Zn-vacancy-hydrogen complexes in ZnO single crystals: A challenge to positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

A systematic study of various, nominally undoped ZnO single crystals, either hydrothermally grown (HTG) or melt grown (MG), has been performed. The crystal quality has been assessed by x-ray diffraction, and a comprehensive estimation of the detailed impurity and hydrogen contents by inductively coupled plasma mass spectrometry and nuclear reaction analysis, respectively, has been made also. High precision positron lifetime experiments show that a single positron lifetime is observed in all crystals investigated, which clusters at 180-182 ps and 165-167 ps for HTG and MG crystals, respectively. Furthermore, hydrogen is detected in all crystals in a bound state with a high concentration (at least 0.3at.% ), whereas the concentrations of other impurities are very small. From ab initio calculations it is suggested that the existence of Zn-vacancy-hydrogen complexes is the most natural explanation for the given experimental facts at present. Furthermore, the distribution of H at a metal/ZnO interface of a MG crystal, and the H content of a HTG crystal upon annealing and time afterward has been monitored, as this is most probably related to the properties of electrical contacts made at ZnO and the instability in p -type conductivity observed at ZnO nanorods in literature. All experimental findings and presented theoretical considerations support the conclusion that various types of Zn-vacancy-hydrogen complexes exist in ZnO and need to be taken into account in future studies, especially for HTG materials.

Brauer, G.; Anwand, W.; Grambole, D.; Grenzer, J.; Skorupa, W.; ?ížek, J.; Kuriplach, J.; Procházka, I.; Ling, C. C.; So, C. K.; Schulz, D.; Klimm, D.

2009-03-01

309

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

310

Higgs-boson production at or near the Z0 peak in electron-positron annihilation  

Microsoft Academic Search

In the minimal standard model, Higgs-boson production in e--e+ annihilation at or near the Z0 peak is studied in detail. We focus on e-e+-->Z0-->H(-->qq¯)+nunu¯, i.e., Z0-->2 jets+missing momentum. Another type of standard-model background (Z0-->qq¯nunu¯) is calculated in detail; and it is shown that, by introducing a proper acceptance cut, it can be practically eliminated, so that detection of a Higgs

D. Atwood; A. P. Contogouris; N. Mebarki; H. Tanaka

1986-01-01

311

Positron annihilation induced Auger electron spectroscopic study of the changes in the top layers of electrodeposited Cu2O surfaces resulting from vacuum annealing  

NASA Astrophysics Data System (ADS)

Positron Annihilation induced Auger electron Spectroscopy (PAES) and electron stimulated Auger Spectroscopy (EAES) were used to measure changes in the surface of electrodeposited Cu2O resulting from vacuum annealing. The PAES measurements show a very large increase in the intensity of the Cu MVV Auger peak after annealing at 535 K. Similar but significantly smaller changes were observed in the EAES spectra consistent with the fact that PAES is primarily sensitive to the top-most atomic layer due to the fact that the positrons are trapped just outside the surface prior to annihilation while EAES samples several atomic layers. The PAES and EAES data indicate that vacuum annealing of Cu2O results in a surface with a high concentration of Cu in the top-most atomic layer. Research supported by the Welch Foundation Y-1100 and NSF DMR 98-12628.

Nadesalingam, M.; Zhu, J.; Fazleev, N.; Mukherjee, S.; de Tacconi, N. R.; Somasundaram, S.; Chenthamarakshan, C. R.; Rajeshwar, K.; Weiss, A. H.

2006-03-01

312

A new ab initio method of calculating $Z_{eff}$ and positron annihilation rates using coupled-channel T-matrix amplitudes  

Microsoft Academic Search

A new {\\\\it ab initio} theoretical formulation to calculate $Z_{eff}$ and hence the positron annihilation rates is presented using the on-shell and half-offshell T-matrix scattering amplitudes without any explicit use of the scattering wave function. The formulation is exact and universal, as long as the dynamics of the system can be described by a Lippmann-Schwinger type equation. It could serve

P K Biswas

2002-01-01

313

Radiative corrections to the cross section for the three-photon annihilation of an electron-positron pair at high energies  

SciTech Connect

Radiative corrections to the differential cross section for three-photon annihilation in collisions of high-energy electron-positron beams are calculated in the logarithmic approximation for the case of photons emitted at large c.m.s. angles with respect to the beams. Structure functions are used to represent this cross section in the form of the cross section for the Drell-Yan process. The accuracy of the result is estimated. 4 refs., 1 fig.

Kuraev, E.A. [Joint Institute for Nuclear Research, Dubna (Russian Federation); Silagadze, Z.K. [Budker Institute for Nuclear Physics, Novosibirsk (Russian Federation)

1995-10-01

314

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

NASA Astrophysics Data System (ADS)

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 obtained in the conventional way from one of the annihilation ? quanta. Compared with the conventional ?? lifetime-measurement technique, this ß+? lifetime spectrometer does not only give a substantially better time resolution but also, owing to the ß+ detector efficiency unity, a much higher coincidence count rate. This will permit routine age-momentum-correlation (AMOC) measurements, using the second annihilation photon. The positron clock can handle very high positron beam fluxes such as they will become available in e+ factories. In the Stuttgart setup we hope to reach, by using solid rare-gas moderators in the accelerator terminal, positron fluxes of about 106e+/s. A comparison between the moderation efficiencies of tungsten and rare-gas moderators under the vacuum conditions of the accelerator terminal is presented. AMOC measurements allow us to obtain time-resolved information on the evolution of the positron states (e.g., trapping of positrons at defects or positronium formation). As a surprising effect, “giant” Doppler broadening of the annihilation quanta of “young” positrons in diamond has been observed by means of a beam-based AMOC technique.

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

1992-05-01

315

Search for thermally generated monovacancies in silicon using monoenergetic positrons  

NASA Astrophysics Data System (ADS)

Using a monoenergetic, variable-energy positron beam, measurements of Doppler broadening and positronium fraction for positron-annihilation radiation from high-resistivity silicon have been made up to the melting point. The measurements show no clear evidence of positron trapping into thermally generated monovacancies which Dannefaer, Mascher, and Kerr observed by measuring the lifetimes of positrons emitted by an in situ source. A lower limit of about 3.6 eV is placed on the formation enthalpy of a neutral monovacancy, subject to assumptions about positron-trapping rate, binding energy, and monovacancy formation entropy.

Throwe, J.; Leung, T. C.; Nielsen, B.; Huomo, H.; Lynn, K. G.

1989-12-01

316

Time-variable positron annihilation radiation from the galactic center direction  

SciTech Connect

Our balloon-borne germanium ..gamma..-ray telescope has been flown for a third time over Alice Springs, Australia to further study the galactic center source 511 keV annihilation-line radiation. The source was found to be in an ''off'' or ''low'' state. Our best fit experimental value for the line flux at Earth is zero, with a ''1 sigma'' upper bound of 3.8 x 10/sup -4/ photons cm/sup -2/ s/sup -1/. This null result is discussed in the context of other measurements and available theoretical models, the most promising of which involve a galactic supernova or central compact object like a massive black hole.

Leventhal, M.; MacCallum, C.J.; Huters, A.F.; Stang, P.D.

1982-09-01

317

Positron trapping and possible presence of SO3H clusters in dry fluorinated polymer electrolyte membranes  

NASA Astrophysics Data System (ADS)

The behavior of positrons that do not form positronium in dry fluorinated polymer electrolyte membranes (Nafion®, Fumapem® and Aquivion®) with various ion exchange capacities (IECs) was studied by the combined use of Doppler broadening of annihilation radiation (DBAR) and the positron lifetime technique. The drastic increase of the S parameter, measured by DBAR, with increasing IEC above 0.91 meq/g indicates that increasing numbers of positrons are trapped by oxygen atoms and annihilate with the electrons bound in them. Reversed micelle like SO3H nanoclusters to trap positrons possibly appear at IEC = 0.91 meq/g and their concentration increases with increasing IEC.

Mohamed, Hamdy F. M.; Kobayashi, Y.; Kuroda, S.; Ohira, A.

2012-08-01

318

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

NASA Astrophysics Data System (ADS)

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 analyse the influence of general relativistic effects, and 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 Massachusetts Institute of Technology bag model equation of state and in the colour-flavour-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.

Kovács, Z.; Cheng, K. S.; Harko, T.

2010-03-01

319

Positron scattering and annihilation from the hydrogen molecule at zero energy.  

PubMed

The confined variational method is used to generate a basis of correlated Gaussians to describe the interaction region wave function for positron scattering from the H2 molecule. The scattering length was approximately = -2.7a(0) while the zero energy Z(eff) of 15.7 is compatible with experimental values. The variation of the scattering length and Z(eff) with internuclear distance was surprisingly rapid due to virtual state formation at R approximately = 3.4a(0). PMID:20366093

Zhang, J-Y; Mitroy, J; Varga, K

2009-11-24

320

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

321

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

NASA Astrophysics Data System (ADS)

Induced Positron Analysis (IPA) has demonstrated the ability to nondestructively quantify shot peening/surface treatments and relaxation effects in single crystal superalloys, steels, titanium and aluminum with a single measurement as part of a National Science Foundation SBIR program and in projects with commercial companies. IPA measurement of surface treatment effects provides a demonstrated ability to quantitatively measure initial treatment effectiveness along with the effect of operationally induced changes over the life of the treated component. Use of IPA to nondestructively quantify surface and subsurface residual stresses in turbine engine materials and components will lead to improvements in current engineering designs and maintenance procedures.

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

2007-03-01

322

Positron annihilation study of the hardening behavior in Al-Cu based alloy by electron and heavy ion irradiation  

NASA Astrophysics Data System (ADS)

Al-Cu based alloy, which is generally called duralumin (JIS2017), was irradiated with 10 MeV Iodine ions, 200 MeV Xenon ions and 3 MeV electrons at room temperature respectively. The micro Vicker's hardness and positron annihilation coincidence Doppler broadening (CDB) measurements have been performed before and after irradiation. Only in the case of ion irradiation, the Vicker's hardness increases with increasing ion dose. Nevertheless, there was no difference in the profile CDB spectrum for before and after irradiation. On the other hand, we found that the micro hardness of this alloy, which was Xe ion irradiated and subsequently annealed at 423 K, is greater than that of age hardened alloy without irradiation. CDB ratio curve of the age hardened Duralumin is clearly different in the electron momentum range around 0.015-0.025 mc from that of the ion irradiated alloy. The results of three-dimensional atom probe (3DAP) also show that a lot of small clusters were found after ion irradiation but large precipitations have found in annealed Duralumin. These results reveal that a number of small clusters formed in this alloy after ion irradiation, and they should strongly affects the micro hardness.

Hori, Fuminobu; Kobayashi, Ippei; Saito, Yuichi; Ishikawa, Norito; Oshima, Takeshi; Iwase, Akihiro

2010-04-01

323

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

NASA Astrophysics Data System (ADS)

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 requirements. The unique and optimal features of MAC are fully exploited to reduce the systematics involved in both measurements, resulting in significant quantitative tests of the theory of quantum chromodynamics. Special attention is focussed on radiative corrections to the total cross section, which constitute a critical component of the acceptance determination, and for the first time the effects of higher order than (alpha)('3) QED processes are included. The total cross section measurement yields R = 3.91 with a total error of (+OR-)2.7%, an accuracy not previously attained by other experiments. For the energy-energy correlation cross section, the consequences of combining pure quantum chromodynamics with contrasting fragmentation models are explored and compared with the data, and result in different values for the strong coupling constant, (alpha)(,s) (TURNEQ) 0.13 (+OR-) 0.02 for incoherent jet formation and 0.24 (+OR-) 0.04 in the string model.

Heltsley, Brian Keith

324

Characterization of helium ion implanted reduced activation ferritic/martensitic steel with positron annihilation and helium thermal desorption methods  

NASA Astrophysics Data System (ADS)

Low energy 3 keV He implantations were carried out on Eurofer97 steel to reproduce the radiation damage induced by the creation and thermal evolution of the defects and the behavior of the implanted He is studied with the positron annihilation Doppler broadening technique (PADB) and thermal desorption spectroscopy (TDS). PADB results show that annealing at 1200 K for 1 h removes the majority of pre existing defects. The 3 keV He+ implantations were carried out in two sets of samples - 'surface polished' and 'annealed' (1200 K, 1 h), up to doses of 1018-1019 He/cm2. The samples were implanted at temperatures of 375 K and 525 K. The preliminary TDS studies show that when implanting at 375 K, the majority of He is released at desorption temperatures below 1000 K and is likely to be related to trapping at vacancies and small He clusters. At 575 K, the larger part of the desorption takes place at higher (>1000 K) temperatures and is associated with the formation of larger He-vacancy clusters, such as bubbles or voids. The migration of He during implantation towards the bulk is observed as a decrease in Doppler broadening parameter at depths much deeper the He implantation range.

Carvalho, I.; Schut, H.; Fedorov, A.; Luzginova, N.; Sietsma, J.

2013-11-01

325

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

Microsoft Academic Search

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

Raju Gupta; R. W. Siegel

1977-01-01

326

Positron annihilation study in binary molecular solid solutions of metal acetylacetonate complexes using positron annihilation lifetime (PAL) and Doppler broadening (DBS) spectroscopies  

Microsoft Academic Search

PAL and DBS techniques have been used to study inhibition of positronium (Ps) formation in binary molecular solid solutions of Al(III) (tris)-acetylacetonate (acac), as matrix, and Co(III), Fe(III), Ru(III), Rh(III), and Ir(III) (tris)-acetylacetonates, as guest molecules. For all the studied systems, no Ps quenching reactions have been observed. For Al(1?x)Co(x)(acac)3, Al(1?x)Fe(x)(acac)3, and Al(1?x)Ru(x)(acac)3 systems, a strong inhibition of Ps formation

Arilza O. Porto; Welington F. Magalhães; Nelson G. Fernandes; José C. Machado

1997-01-01

327

High-temperature vacant lattice site formation in solids and free volumes in melts studied by positron lifetime measurements  

Microsoft Academic Search

In the present paper a concise review is given of the application of positron lifetime measurements to the study of high-temperature vacancies in intermetallic compounds (F76.3Al23.7), in metal oxides (NiO), in elemental semiconductors (Si, Ge), and of the oxygen loss or uptake in YBa2Cu3O7-delta. Investigations of free volumes in elemental melts (Al, In, Ge) are included.

H.-E. Schaefer

1991-01-01

328

Chromium–manganese iron alloy system design cast in metal and sand moulds for erosion resistance: a positron lifetime study  

Microsoft Academic Search

Erosion characteristics of high chromium (Cr, 16–19%) alloy cast iron with 5% and 10% manganese (Mn) prepared in metal and\\u000a sand moulds through induction melting are investigated using jet erosion test setup in both as-cast and heat-treated conditions.\\u000a The samples were characterised for hardness and microstructural properties. A new and novel non-destructive evaluation technique\\u000a namely positron lifetime spectroscopy has also

P. Sampathkumaran; C. Ranganathaiah; S. Seetharamu; Kishore

2011-01-01

329

Hard photon processes in electron-positron annihilation at 29 GeV  

SciTech Connect

The hard photon processes ..mu mu gamma.. and hadrons + ..gamma.. in e/sup +/e/sup -/ annihilation at 29 GeV have been studied. The study is based on an integrated luminosity of 226 pb/sup -1/ taken at PEP with the Mark II detector. For the ..mu mu gamma.. process, a small fraction of non-planar events are observed with missing momentum along the beam direction. The resulting missing energy spectrum is consistent with that expected from higher order effects. The observed cross section is consistent with the predicted cross section for this process, sigma/sup exp/sigma/sup th/ = .90 +- .05 +- .06. The observed hard photon energy spectrum and mass distributions are found to be in agreement with O(..cap alpha../sup 3/) QED. The measured charge asymmetry is in good agreement with the predicted value, A/sub exp/A/sub th/ = .83 +- .25 +- .12. The ..mu gamma.. invariant mass distribution is used to place a limit on a possible excited muon coupling G..gamma../M* for excited muon masses in the range 1 < M* < 21 GeV of (G..gamma../M*)/sup 2/ < 10/sup -5/ GeV/sup -2/ at a 95% confidence level. In the hadrons + ..gamma.. process, evidence for final state radiation is found in an excess of events over that predicted from initial state radiation alone of 253 +- 54 +- 60 events. Further evidence for final state radiation is found in a large hadronic charge asymmetry A/sub Had+..gamma../= (-24.6 +- 5.5)%.

Gold, M.S.

1986-11-01

330

A Comparative Study of Vacancy Related Defects in Bulk and Thin Film La_1-xSrxCoO3 using Positron Annihilation  

NASA Astrophysics Data System (ADS)

The defect characteristics in single crystal and polycrystalline La_1-xSr_xCoO3 have been studied by positrons. Both, Doppler-broadening defect profiling and positron lifetime spectroscopy have been performed on film and bulk samples, respectively. The Doppler-broadening S parameter was found to increase with increasing oxygen non-stoichiometry in the film indicating an increase in the concentration and size of vacancy type defects. The stability of these defects after various temperature treatments is investigated. In order to examine changes in the defect structure as a function of Sr content we conducted positron lifetime measurements on single- and polycrystalline bulk material. These results will be discussed together with Doppler-broadening data obtained for films with varying Sr content.

Friessnegg, T.; Nielsen, B.; Keeble, D. J.; Madhukar, S.; Aggarwal, S.; Ramesh, R.; Poindexter, E. H.; Moodenbaugh, A. R.; Asai, K.; Kobayashi, Y.

1998-03-01

331

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

PubMed Central

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

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

2012-01-01

332

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

333

A new ab initio method of calculating $\\\\mathsf{Z_{eff}}$ and positron annihilation rates using coupled-channel T-matrix amplitudes  

Microsoft Academic Search

A new {\\\\it ab initio} theoretical formulation to calculate $Z_{eff}$ and\\u000ahence the positron annihilation rates is presented using the on-shell and\\u000ahalf-offshell T-matrix scattering amplitudes without any explicit use of the\\u000ascattering wave function. The formulation is exact and universal, as long as\\u000athe dynamics of the system can be described by a Lippmann-Schwinger type\\u000aequation. It could serve

P. K. Biswas

2004-01-01

334

Oxygen-vacancy behavior in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ by positron annihilation and oxygen diffusion  

SciTech Connect

Oxygen-diffusion and positron-annihilation results for La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ compounds are reported. A qualitative explanation of the observed results is given on the basis of a model in which the oxygen-vacancy concentration in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/ is determined by Sr/sup 2+/ ion clustering on the La sublattice. This model also leads to a maximum in the Cu/sup 3+/ ion concentration as a function of the Sr/sup 2+/ ion concentration.

Smedskjaer, L.C.; Routbort, J.L.; Flandermeyer, B.K.; Rothman, S.J.; Legnini, D.G.; Baker, J.E.

1987-09-01

335

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

336

Material Science with Positrons: From Doppler Spectroscopy to Failure Prediction  

NASA Astrophysics Data System (ADS)

We describe an alternative approach for a reliable lifetime prediction employing the local concentration of lattice defects as a precursor for fatigue failure. We present positron annihilation spectroscopy (PAS) as a non-destructive technique sensitive for defect concentrations in the range relevant to plasticity in metals.

Haaks, Matz; Eich, Patrik; Fingerhuth, Judith; Müller, Ingo

337

A Study of Double-Charm and Charm-Strange Baryons inElectron-Positron Annihilations  

SciTech Connect

In this dissertation I describe a study of double-charm and charm-strange baryons based on data collected with the BABAR Detector at the Stanford Linear Accelerator Center. In this study I search for new baryons and make precise measurements of their properties and decay modes. I seek to verify and expand upon double-charm and charm-strange baryon observations made by other experiments. The BABAR Detector is used to measure subatomic particles that are produced at the PEP-II storage rings. I analyze approximately 300 million e+e- {yields} c{bar c} events in a search for the production of double-charm baryons. I search for the double-charm baryons {Xi}{sup +}{sub cc} (containing the quarks ccd) and {Xi}{sup ++}{sub cc} (ccu) in decays to {Lambda}{sup +}{sub c}K{sup -}{pi}{sup +} and {Lambda}{sup +}{sub c}K{sup -}{pi}{sup +}{pi}{sup +}, respectively. No statistically significant signals for their production are found, and upper limits on their production are determined. Statistically significant signals for excited charm-strange baryons are observed with my analysis of approximately 500 million e+e- {yields} c{bar c} events. The charged charm-strange baryons {Xi}{sub c}(2970){sup +}, {Xi}{sub c}(3055){sup +}, {Xi}{sub c}(3123){sup +} are found in decays to {Lambda}{sup +}{sub c}K{sup -}{pi}{sup +}, the same decay mode used in the {Xi}{sup +}{sub cc} search. The neutral charm-strange baryon {Xi}{sub c}(3077){sup 0} is observed in decays to {Lambda}{sup +}{sub c}K{sub 8}{pi}{sup -}. I also search for excited charm-strange baryon decays to {Lambda}{sup +}{sub c}K{sub 8}, {Lambda}{sup +}{sub c}K{sup -}, {Lambda}{sup +}{sub c}K{sub 8}{pi}{sup -}{pi}{sup +}, and {Lambda}{sup +}{sub c}K{sup -}{pi}{sup -}{pi}{sup +}. No significant charm-strange baryon signals a f h these decay modes. For each excited charm-strange baryon state that I observe, I measure its mass, natural width (lifetime), and production rate. The properties of these excited charm-strange baryons and their decay modes provide constraints for phenomenological models of quark interactions through quantum chromodynamics. My discovery of the two new charm-strange baryons {Xi}{sub c}(3055){sup +} and {Xi}{sub c}(3123){sup +} influences our theoretical understanding of charm-strange baryon states.

Edwards, Adam J.; /SLAC

2007-10-15

338

Bounds on Cross-sections and Lifetimes for Dark Matter Annihilation and Decay into Charged Leptons from Gamma-ray Observations of Dwarf Galaxies  

SciTech Connect

We provide conservative bounds on the dark matter cross-section and lifetime from final state radiation produced by annihilation or decay into charged leptons, either directly or via an intermediate particle {phi}. Our analysis utilizes the experimental gamma-ray flux upper limits from four Milky Way dwarf satellites: HESS observations of Sagittarius and VERITAS observations of Draco, Ursa Minor, and Willman 1. Using 90% confidence level lower limits on the integrals over the dark matter distributions, we find that these constraints are largely unable to rule out dark matter annihilations or decays as an explanation of the PAMELA and ATIC/PPB-BETS excesses. However, if there is an additional Sommerfeld enhancement in dwarfs, which have a velocity dispersion {approx} 10 to 20 times lower than that of the local Galactic halo, then the cross-sections for dark matter annihilating through {phi}'s required to explain the excesses are very close to the cross-section upper bounds from Willman 1. Dark matter annihilation directly into {tau}'s is also marginally ruled out by Willman 1 as an explanation of the excesses, and the required cross-section is only a factor of a few below the upper bound from Draco. Finally, we make predictions for the gamma-ray flux expected from the dwarf galaxy Segue 1 for the Fermi Gamma-ray Space Telescope. We find that for a sizeable fraction of the parameter space in which dark matter annihilation into charged leptons explains the PAMELA excess, Fermi has good prospects for detecting a gamma-ray signal from Segue 1 after one year of observation.

Essig, Rouven; /SLAC; Sehgal, Neelima; Strigari, Louis E.; /KIPAC, Menlo Park

2009-06-19

339

Free volume of an epoxy resin and its relation to structural relaxation: Evidence from positron lifetime and pressure-volume-temperature experiments  

NASA Astrophysics Data System (ADS)

The microstructure of the free volume and its temperature dependence in the epoxy resin diglycidyl ether of bisphenol-A (DGEBA) have been examined using positron annihilation lifetime spectroscopy (PALS, 80-350K , 10-5Pa ) and pressure-volume-temperature (PVT, 293-470K , 0.1-200MPa ) experiments. Employing the Simha-Somcynsky lattice-hole theory (S-S eos), the excess (hole) free volume fraction h and the specific free and occupied volumes, Vf=hV and Vocc=(1-h)V , were estimated. From the PALS spectra analyzed with the new routine LT9.0 the hole size distribution, its mean, ?vh? , and mean dispersion, ?h , were calculated. ?vh? varies from 35to130Å3 . From a comparison of ?vh? with V and Vf , the specific hole number Nh' was estimated to be independent of the temperature [Nh(300K)=Nh'/V=0.65nm-3] . From comparison with reported dielectric and viscosity measurements, we found that the structural relaxation slows down faster than the shrinkage of the hole free volume Vf would predict on the basis of the free volume theory. Our results indicate that the structural relaxation in DGEBA operates via the free-volume mechanism only when liquidlike clusters of cells of the S-S lattice appear which contain a local free volume of ˜1.5 or more empty S-S cells. The same conclusion follows from the pressure dependency of the structural relaxation and Vf . It is shown that PALS mirrors thermal volume fluctuations on a subnanometer scale via the dispersion in the ortho-positronium lifetimes. Using a fluctuation approach, the temperature dependency of the characteristic length of dynamic heterogeneity, ? , is estimated to vary from ?=1.9nm at Tg to 1.0nm at T/Tg>1.2 . A model was proposed which relates the spatial structure of the free volume as concluded from PALS to the known mobility pattern of the dynamic glass transition at low (cooperative ? -relaxation) and high ( a -relaxation) temperatures. We discuss possible reasons for the differences between the results of our method and the conclusion from dynamic heat capacity.

Dlubek, Günter; Hassan, E. M.; Krause-Rehberg, Reinhard; Pionteck, Jürgen

2006-03-01

340

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

341

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

342

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

NASA Astrophysics Data System (ADS)

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

Abo-Elsoud, M.

2013-04-01

343

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

344

Bose-Einstein Correlations of Pions in Positron Electron Annihilation at 29 GEV Center-Of Energy  

Microsoft Academic Search

Measurements of two and three particle correlations between like-sign pions produced in e^+e ^- annihilation at 29 GeV center-of-mass energy are presented. The analysis is based on data taken during the period 1982-1986 using the TPC\\/2gamma detector at P scEP. Two particle correlations are studied as a function of Q, the momentum difference as measured in the rest frame of

Robert Ellsworth Avery

1989-01-01

345

Effect of free volume and temperature on the structural relaxation in polymethylphenylsiloxane: a positron lifetime and pressure-volume-temperature study.  

PubMed

The microstructure of the free volume and its temperature dependence in polymethylphenylsiloxane (PMPS) have been examined using positron annihilation lifetime spectroscopy (PALS) and pressure-volume-temperature experiments. The hole-free volume fraction h and the specific hole-free and occupied volumes, Vf=hV and Vocc=(1-h)V, were estimated employing the Simha-Somcynsky (SS) lattice-hole theory. From the PALS spectra analyzed with the new routine LT9.0 the hole size distribution, its mean, , and mean dispersion, sigmah, were calculated. A comparison of with V and Vf delivered a constant specific hole number Nh'. Using a fluctuation approach the temperature dependency of the volume of the smallest representative freely fluctuating subsystem, , is estimated to vary from approximately 8.5 nm3 at Tg to approximately 3 nm3 at T/Tg>or=1.15. Unlike other polymers, the segmental relaxation from dielectric spectroscopy of PMPS follows the Cohen-Turnbull free volume theory almost perfectly in the temperature and pressure ranges between 243 and 279 K and 0 and approximately 100 MPa. This behavior correlates with the small mass of the SS lattice mer which indicates the high flexibility of the PMPS chain. Above 293 K and approximately 150 MPa, the free volume prediction gives relaxation times that are too small, which indicates that effects of thermal energy must be included in the analysis. To quantify the degree to which volume and thermal energy govern the structural dynamics the ratio of the activation enthalpies, Ei=R[(d ln taudT1)]i (tau-relaxation time of alpha relaxation), at constant volume V and constant pressure P, EV/EP, is frequently determined. The authors present arguments for necessity to substitute EV with EVf, the activation enthalpy at constant (hole) free volume, and show that EVf/EP changes as expected: increasing with increasing free volume, i.e., with increasing temperature and decreasing pressure. EVf/EP (=0.04-0.1) exhibits remarkably smaller values than EV/EP (=0.44-0.53), which leads to the inference that the free volume plays a distinctly larger role in dynamics than traditionally concluded from EV/EP. This conclusion is in agreement with the results of our more direct Cohen-Turnbull free volume analysis. PMID:17228972

Dlubek, G; Shaikh, M Q; Krause-Rehberg, R; Paluch, M

2007-01-14

346

Positronium annihilation lifetimes and dielectric spectroscopy studies on diethyl phthalate: Phenomenological correlations and microscopic analyses in terms of the extended free volume model by Cohen-Grest  

NASA Astrophysics Data System (ADS)

A combined positronium annihilation lifetime spectroscopy (PALS) and dielectric spectroscopy (DS) study on a typical van der Waals glass-former diethyl phthalate (DEP) was performed and the results were compared. From phenomenological point of view, the mutual relationships between the characteristic PALS temperatures, the glass temperature TgPALS, and the crossover temperatures Tb1L and Tb2L on the ortho-positronium (o-Ps) lifetime versus the temperature plot, have been discussed with respect to the characteristic DS temperatures, the glass temperature TgDS and the dynamic crossover temperature TBST, concerning the crossover behavior of primary ?-relaxation times. Next, simultaneous application of the extended free volume (EFV) model by Cohen-Grest on the temperature dependence of both the mean free volume hole size data as extracted from PALS and the dielectric ?-relaxation time revealed a good agreement between the experimental Tb1L and the characteristic EFV temperatures T0DS and T0PALS at which a free volume percolation should occur. These results indicate the important role of free volume in control of the primary (?) dynamics of supercooled DEP.

Pawlus, S.; Bartoš, J.; Šauša, O.; Krištiak, J.; Paluch, M.

2006-03-01

347

Correlation of polycrystalline Cu(In,Ga)Se{sub 2} device efficiency with homojunction depth and interfacial structure: X-ray photoemission and positron annihilation spectroscopic characterization  

SciTech Connect

Angled-resolved high resolution photoemission measurements on valence band electronic structure and Cu 2p, In 3d, Ga 2p, and Se 3d core lines were used to evaluate surface and near-surface chemistry of CuInSe{sub 2} and Cu(In,Ga)Se{sub 2} device grade thin films. XPS compositional depth profiles were also acquired from the near-surface region, and bonding of the Cu, In, Ga, and Se was determined as a function of depth. A Cu-poor region was found, indicating CuIn{sub 5}Se{sub 8} or a CuIn{sub 3}Se{sub 5}-In{sub 2}Se{sub 3} mixture. Correlation between the depth of the Cu-poor region/bulk interface and device efficiency showed that the depth was 115 {angstrom} for a 16.4% CIGS device, 240 {angstrom} for a 15.0% CIGS, and 300 {angstrom} for 14.0% CIGS, with similar trends for CIS films. The surface region is n-type, the bulk is p-type, with a 0.5 eV valence band offset. Depth of homojunction may be the determining factor in device performance. Positron annihilation spectroscopy gave similarly illuminating results.

Nelson, A.J.; Sobol, P.E.; Gabor, A.M.; Contreras, M.A. [National Renewable Energy Lab., Golden, CO (United States); Asoka-Kumar, P.; Lynn, K.G. [Brookhaven National Lab., Upton, NY (United States)

1994-06-01

348

Development of a Simple Positron Age-Momentum Setup  

NASA Astrophysics Data System (ADS)

A positron age-momentum setup that uses NIM Bin electronic modules and a conventional multichannel analyzer (MCA) is described. The essential idea is to accumulate a Doppler broadened spectrum (sensitive to the annihilation electron momentum) using a high purity Germanium detector in coincidence with a BaF2 scintillation counter, which also serves as the stop signal in a conventional positron lifetime setup. The MCA that collects the Doppler spectrum is gated by a selected region of the lifetime spectrum. Thus we can obtain Doppler broadening spectra as a function of positron lifetime: an age-momentum spectrum. The apparatus has been used so far to investigate a ZnO sample where the size of different vacancy trapping sites may affect the positron lifetime and the Doppler broadening spectrum. We are also looking at polymer and rubber carbon-black composite samples where differences in the Doppler spectrum may arise from positron trapping or positronium formation in the samples. Correction for background and contribution from the positron source itself to the Doppler spectrum will be discussed.

Sheffield, Thomas; Quarles, C. A.

2009-04-01

349

Oxygen-Atom Defects In 6H Silicon Carbide Implanted Using 24- MeV O3+ Ions Measured Using Three-Dimensional Positron Annihilation Spectroscopy System (3DPASS)  

NASA Astrophysics Data System (ADS)

Three dimensional electron-positron (e--e+) momentum distributions were measured for single crystal 6H silicon carbide (SiC) both virgin and having implanted oxygen-atom defects. 6H SiC samples were irradiated by 24- MeV O3+ ions at 20 particle-nanoamps at the Sandia National Laboratory's Ion Beam Facility. O3+ ions were implanted 10.8 ?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-?m thick SiC samples was exposed to positrons from a 22Na source. This technique reduced the influence on the momentum measurements of vacancy-type defects resulting from knock-on damage by the O3+ ions. A three-dimensional positron annihilation spectroscopy system (3DPASS) was used to measure e--e+ momentum distributions for virgin and irradiated 6H SiC crystal both before and following annealing. 3DPASS simultaneously measures coincident Doppler-broadening (DBAR) and angular correlation of annihilation radiation (ACAR) spectra. DBAR ratio plots and 2D ACAR spectra are presented. Changes in the momentum anisotropies relative to crystal orientation observed in 2D ACAR spectra for annealed O-implanted SiC agree with the local structure of defect distortion predicted using Surface Integrated Molecular Orbital/Molecular Mechanics (SIMOMM). Oxygen atoms insert between Si and C atoms increasing their separation by 0.9 A? forming a Si-O-C bond angle of ~150°.

Williams, Christopher S.; Duan, Xiaofeng F.; Petrosky, James C.; Burggraf, Larry W.

2011-06-01

350

Positron and positronium studies of irradiation-induced defects and microvoids in vitreous metamict silica  

Microsoft Academic Search

To study irradiation-induced defects and structural microvoids in vitreous silica (v-SiO2), positron lifetime, angular correlation of positron annihilation radiation (ACAR), and electron spin resonance (ESR) were measured on v-SiO2 and quartz (c-SiO2) samples irradiated with fast neutrons up to a dose of 4.1×1020 n\\/cm2. Two kinds of positron-trapping defects have been found to form in v-SiO2 by fast neutron irradiation:

M. Hasegawa; M. Saneyasu; M. Tabata; Z. Tang; Y. Nagai; T. Chiba; Y. Ito

2000-01-01

351

Comparison of three-jet and radiative two-jet events in electron-positron annihilation at 29 GeV  

SciTech Connect

By comparing 3-jet (e/sup +/e/sup -/ ..-->.. q anti q g) and radiative 2-jet (e/sup +/e/sup -/ ..-->.. q anti q ..gamma..) events from electron-positron annihilation, we have studied the local and global effects of the presence of a hard bremsstrahlung gluon in hadronic events. Detector and event selection efficiencies and biases affect these two kinds of events almost equally because they have very similar kinematics and topologies. Accurate comparisons of q anti q g and q anti q ..gamma.. events can therefore be made. Globally, we observe a depletion of hadrons in q anti q g events relative to q anti q ..gamma.. events on the opposite side of the event plane from the gluon, in the angular region between the q and anti q jets. This depletion is shown to be in agreement with the predictions of Quantum Chromodynamics (QCD). The existence of this effect demonstrates that the presence of a gluon significantly alters the color forces and hence the fragmentation process in hadronic events. We also use these q anti q ..gamma.. and q anti q g events to compare low energy (4.5 GeV) gluon and quark jets. Our data indicate that gluon jets have softer x/sub p/ distributions than quark jets, while the transverse momentum distributions of these two types of jets are identical within our errors. Although we are unable to determine if the multiplicities of gluon (n/sub /) and quark (n/sub q/) jets are different, the ratio n/sub g//n/sub q/ = 9/4 predicted asymptotically in QCD would not be consistent with our data.

Sheldon, P.D.

1986-11-01

352

Pulsed low-energy positron beams: A useful tool to investigate defect structures in deformed metals and alloys  

NASA Astrophysics Data System (ADS)

To understand in more detail the behaviour of deformed metallic materials, the knowledge of defects and defect distributions at an atomistic level is important. To investigate the plastic zone in front of a crack tip, the positron annihilation lifetime spectroscopy with a pulsed monoenergetic positron beam of variable energy allows the detection of vacancies, dislocations, vacancy clusters and micro voids in the crack surface near region. Moreover, for a given defect type it is possible to determine its concentration. The positron lifetime measurements in samples of different materials (aluminium, copper) showed different defect profiles for crack surfaces produced by monotonic and cyclic deformation. In addition, this technique was able to characterize the kind of damage (monotonic or cyclic) by analysing the different positron lifetimes.

Egger, W.; Sperr, P.; Kögel, G.; Gudladt, H.-J.

2010-07-01

353

Annihilation of antimatter meteors  

Microsoft Academic Search

Antimatter meteors probably enter the Earth's atmosphere. If they have the ability to escape complete vaporization during their infall flight, it may be possible, that a fraction of their original mass could survive for short or long time, depending on the mechanisms of ablation. In case of ablation through the annihilation process only, the lifetime of such an object follows

P. M. Papaelias

1993-01-01

354

Lifetime of antimatter meteors  

Microsoft Academic Search

The lifetime of antimatter fragments which may enter the Earth's atmosphere in the form of meteors is determined in this paper, for cases in which the annihilation may be accompanied by the evaporation process. The antimatter object can be penetrated by the nucleon -- antinucleon annihilation products, which can be generated by interactions of atoms of antimatter fragments with the

P. M. Papaelias

1994-01-01

355

Lifetime of antimatter meteors  

Microsoft Academic Search

The lifetime of antimatter fragments which may enter the Earth's atmosphere in the form of meteors is determined in this paper, for cases in which the annihilation may be accompanied by the evaporation process. The antimatter object can be penetrated by the nucleon - antinucleon annihilation products, which can be generated by interactions of atoms of antimatter fragments with the

P. M. Papaelias

1994-01-01

356

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

357

Positrons and positronium  

SciTech Connect

Positrons and positronium are finding increasing applications in solid state physics, chemical dynamics, radiation chemistry, the material properties of defects, voids and vacancies etc., surface phenomena, chemical analysis, biological functions, medical imaging techniques, and radiopharmaceuticals labeled with positron-emitting nuclides. The availability of modern electronic modules and computers has encouraged researchers to apply positron and positronium methods to almost every scientific discipline. This compilation provides researchers with a literature search. It includes articles, proceedings, abstracts, reports and patents published between 1930 and 1984 on the subject of positrons, positron annihilation and positronium.

Jean, Y.C. (Univ. of Missouri-Kansas City, KS, MO (US)); (Lambrecht, R.M. (Radionuclide and Cyclotron Operations, King Feisal Specialist Hospital and Research Center, Kingdom of Saudi Arabia (SA)); Horrath, D. (Central Research Inst. for Physics, Budapest (HU)))

1988-01-01

358

Annihilation emission from young supernova remnants  

NASA Astrophysics Data System (ADS)

Context. A promising source of the positrons that contribute through annihilation to the diffuse Galactic 511 keV emission is the ?+-decay of unstable nuclei like 56Ni and 44Ti synthesised by massive stars and supernovae. Although a large fraction of these positrons annihilate in the ejecta of SNe/SNRs, no point-source of annihilation radiation appears in the INTEGRAL/SPI map of the 511 keV emission. Aims: We exploit the absence of detectable annihilation emission from young local SNe/SNRs to derive constraints on the transport of MeV positrons inside SN/SNR ejecta and their escape into the CSM/ISM, both aspects being crucial to the understanding of the observed Galactic 511 keV emission. Methods: We simulated 511 keV lightcurves resulting from the annihilation of the decay positrons of 56Ni and 44Ti in SNe/SNRs and their surroundings using a simple model. We computed specific 511 keV lightcurves for Cas A, Tycho, Kepler, SN1006, G1.9+0.3 and SN1987A, and compared these to the upper-limits derived from INTEGRAL/SPI observations. Results: The predicted 511 keV signals from positrons annihilating in the ejecta are below the sensitivity of the SPI instrument by several orders of magnitude, but the predicted 511 keV signals for positrons escaping the ejecta and annihilating in the surrounding medium allowed to derive upper-limits on the positron escape fraction of ~13% for Cas A, ~12% for Tycho, ~30% for Kepler and ~33% for SN1006. Conclusions: The transport of ~MeV positrons inside SNe/SNRs cannot be constrained from current observations of the 511 keV emission from these objects, but the limits obtained on their escape fraction are consistent with a nucleosynthesis origin of the positrons that give rise to the diffuse Galactic 511 keV emission.

Martin, P.; Vink, J.; Jiraskova, S.; Jean, P.; Diehl, R.

2010-09-01

359

Experimental Evidence for the Constancy of the Velocity of Gamma Rays, Using Annihilation in Flight  

Microsoft Academic Search

The annihilation in flight of positrons was used to determine the ; constancy of gamma ray velocity as a test for the second postulate of special ; relativity. In the annihilation the center-of-mass system of the positron and ; electron moves with a velocity close to 1\\/2 C, and two gamma rays are emitted. ; In the case of annihilation

D. Sadeh

1963-01-01

360

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

361

Positronium Annihilation Gamma Ray Laser.  

National Technical Information Service (NTIS)

The goal of the work reported here is the perfection of a powerful laser based on the coherent annihilation of a Bose-Einstein condensate of positronium, the hydrogen-like atoms formed from bound electron-positron pairs. We have made progress in several a...

J. A. Mills

2009-01-01

362

Positron studies of metal-oxide-semiconductor structures  

SciTech Connect

Positron annihilation spectroscopy provides a new probe to study the properties of interface traps in metal-oxide semiconductors (MOS). Using positrons, we have examined the behavior of the interface traps as a function of gate bias. We propose a simple model to explain the positron annihilation spectra from the interface region of a MOS capacitor.

Au, H.L.; Asoka-Kumar, P.; Nielsen, B.; Lynn, K.G. (Brookhaven National Laboratory, Upton, New York 11973 (United States))

1993-03-15

363

Positron Annihilation Study of Dislocations Produced by Polishing in the Surface of Iron Single Crystals: Part 1. Density Profile and Removal by Annealing. (Reannouncement with New Availability Information).  

National Technical Information Service (NTIS)

The trap concentration as a function of the depth from the abraded surface of iron single crystals was investigated by means of positron Doppler broadening. The deformation depth was about 40 micrometers from the surface when the specimens was mechanicall...

J. L. Lee J. T. Waber

1990-01-01

364

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

365

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

366

Positron and positronium studies of silica aerogel  

NASA Astrophysics Data System (ADS)

Variable-energy positron annihilation spectroscopy (VEPAS) has been used to probe the structure of gold nanoparticles suspended in a silica aerogel matrix by measuring the annihilation parameters S and W, and the ortho-positronium (o-Ps) fraction parameter F, as a function of incident positron energy. Correction methods have been developed to improve the sensitivity of F by removing background contributions to the number of recorded o-Ps annihilation events. Charging effects have also been observed and investigated.

Edwardson, C. J.; Grogan, M. D. W.; Birks, T. A.; Coleman, P. G.

2011-01-01

367

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

368

Positronium molecule formation, Bose-Einstein condensation and stimulated annihilation  

Microsoft Academic Search

Low energy positron production and storage may be used to make and observe some interesting many-positron, many-electron systems both in vacuum and in the presence of ordinary matter. One of the most interesting possibilities is to make an annihilation photon laser. One might start on the path to this laser by creating a high density positron burst and forming a

Allen Paine Mills

2002-01-01

369

Adiabatic method for positronic atoms and molecules  

NASA Astrophysics Data System (ADS)

The adiabatic separation of electronic and positronic degrees of freedom is proposed. The positronic charge in expressions for the potential energy is partitioned between the electronic and positronic Hamiltonians. This partitioning assures an acceptable electron density distribution around the positron. Our adiabatic method gives good values of binding energies and annihilation rates for model systems, in which the positron is weakly bound. On the other hand, it does not work well if the wave function contains a significant contribution of the positronium cluster.

Strasburger, Krzysztof; Wo?cyrz, Ma?gorzata

2007-03-01

370

Calculation of the Detection Efficiency in Liquid Scintillators. II. Single Positron Emitters.  

National Technical Information Service (NTIS)

Counting efficiency as a function of the figure of merit for 30 positron emitters has been computed from the positron energy spectrum. Only the efficiency contribution of positrons has been taken into consideration. The contribution of the annihilation fo...

A. Grau E. Garcia-Torano

1982-01-01

371

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

NASA Astrophysics Data System (ADS)

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

Dinesh, Meghala; Chikkakuntappa, Ranganathaiah

2013-09-01

372

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

PubMed

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

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

2011-03-15

373

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

Microsoft Academic Search

An experimental investigation was performed to study the effect on the free volume of the advance of the cross-linking reaction in a copolymer of styrene butadiene rubber by sulfur vulcanization. The dynamic modulus and loss tangent were evaluated over samples cured for different times at 433 K by dynamic mechanical tests over a range of frequencies between 5 and 80

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

2002-01-01

374

Positron confinement in embedded lithium nanoclusters  

Microsoft Academic Search

Quantum confinement of positrons in nanoclusters offers the opportunity to obtain detailed information on the electronic structure of nanoclusters by application of positron annihilation spectroscopy techniques. In this work, positron confinement is investigated in lithium nanoclusters embedded in monocrystalline MgO. These nanoclusters were created by means of ion implantation and subsequent annealing. It was found from the results of Doppler

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

2002-01-01

375

Galactic secondary positron flux at the Earth  

Microsoft Academic Search

Context: Secondary positrons are produced by spallation of cosmic rays within the interstellar gas. Measurements have been typically expressed in terms of the positron fraction, which exhibits an increase above 10 GeV. Many scenarios have been proposed to explain this feature, among them some additional primary positrons originating from dark matter annihilation in the Galaxy. Aims: The PAMELA satellite has

T. Delahaye; R. Lineros; F. Donato; N. Fornengo; J. Lavalle; P. Salati; R. Taillet

2009-01-01

376

Positron states in vacancies and voids  

Microsoft Academic Search

The authors have calculated the positron wavefunctions and annihilation rates in voids in aluminium as functions of void radius using density functional theory with the non-local approximation to the positron-electron correlation potential developed by Jensen and Walker (1988). Their calculations show the gradual evolution of the positron state, as the radius increases, from a state extending over the whole of

G. M. Dunn; K. O. Jensen; A. B. Walker

1991-01-01

377

Positron annihilation study of Sr Doping in La{sub 2-x}Sr{sub x}CuO{sub 4}  

SciTech Connect

We present a combined experimental and threshold study of effects of Sr doping on electronic structure of La{sub 2-x}Sr{sub x}CuO{sub 4}. Electron-positron momentum distributions have been measured to high statistical precision (> 4 {times} 10{sup 8} counts) at room temperature for samples with Sr concentrations of x = 0.0, 0.1, 0.13 and 0.2. Analysis of all four spectra reveal strong features due to electron-positron wavefunction overlap, in quantitative agreement with theoretical calculations. The Sr doped samples show discontinuities consistent with presence of a Fermi surface. The form and position of these features are in general agreement with the predictions of band theory. Correspondence between theory and experiment, as well as some differences, are revealed by a detailed study of the changes in electron-position momentum density with increasing Sr doping.

Sterne, P.A. [Lawrence Livermore National Lab., CA (United States)]|[California Univ., Davis, CA (United States); Howell, R.H.; Fluss, M.J. [Lawrence Livermore National Lab., CA (United States); Kaiser, J.H. [Texas Univ., Arlington, TX (United States); Kitazawa, K. [Tokyo Univ. (Japan); Kojima, H. [Yamanashi Univ., Kofu (Japan)

1993-04-22

378

Measurement of the (27)P lifetime  

NASA Astrophysics Data System (ADS)

The lifetime of 27P has been measured using the Recoil Mass Spectrometer (RMS) at the Nuclear Structure Research Laboratory (NSRL) at the University of Rochester. 27P was produced by bombarding a BeO target with a 24Mg beam at a lab energy of 118 MeV. A focal-plane detector system, consisting of a parallel-grid avalanche counter (PGAC) backed by an ionization counter (IC) and a silicon detector, was used to provide particle identification. A sodium iodide detector array was used to detect the 511 keV positron annihilation radiation produced by the decay of 27P. The result obtained for the half-life of 27P is 0.32 -0.15+0.22 s.

Freeman, Charles George

379

Positron studies of the temperature-dependence of free volumes in Polydimethylsiloxane/poly(propylene oxide) urethane/urea membranes  

NASA Astrophysics Data System (ADS)

Free-volume parameters in polyurethane/urea membranes have been studied by positron annihilation lifetime and Doppler broadening measurements. The bi-soft segment membranes were obtained by varying the ratio of the structural constituents, polypropylene oxide and polydimethylsiloxane (PU/PDMS), with PDMS content from 25 to 75 wt%. The free-volume holes determined by PALS are correlated with gas permeation features. The phase separation of the various soft and hard segments in the membranes is mirrored in both lifetime and Doppler results.

Ferreira Marques, M. F.; Gordo, P. M.; Kajcsos, Zs.; Lopes Gil, C.; de Lima, A. P.; Queiroz, D. P.; de Pinho, M. N.

2007-02-01

380

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

SciTech Connect

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

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

2010-02-19

381

100 Picosecond Timing Resolution Detection of Annihilation Photons  

NASA Astrophysics Data System (ADS)

In this work, we have developed electronic techniques which have dramatically improved the timing resolution in microchannel-plate detection of high energy gamma photons. The developed methods involve over-range ``tagging" of large amplitude pulses and ``electronic selection" of detector transit times. Currently, a full width half maximum (FWHM) timing resolution of 138 ps has been achieved. The high resolution timing measurements were obtained by utilizing a ^22Na positron source positioned between two opposing MCP detectors situated 180^rc from each other. Subsequent annihilation photons generate start and stop signals on the opposing MCP detectors. It is important to note that the timing resolution reported here, for 0.511 MeV photons, is only 13 ps greater than the lifetime of positronium atoms in vacuum. The electronic tagging and selection techniques will be discussed.

Irby, Victor

2002-05-01

382

Photoluminescence and positron annihilation spectroscopy of MeV Si{sup +} ion-irradiated Si{sub y}O{sub 1-y}:Er (y{approx_equal}1/3) thin films  

SciTech Connect

Amorphous erbium-doped silicon oxide (Si{sub y}O{sub 1-y}:Er, y{>=}1/3) thin films are currently under investigation as a luminescent material system for complementary metal-oxide semiconductor compatible light emitters. We have grown films with y{approx_equal}1/3 and investigated their properties using both positron annihilation and photoluminescence (PL) spectroscopies. Films were characterized ''as deposited,'' following irradiation with 1 MeV Si{sup +} ions and after isochronal annealing. The PL yield from both Er{sup 3+} ions and sensitizing defects is reduced by irradiation, depending strongly on the irradiation fluence and reaching saturation at {approx}4x10{sup 13} Si{sup +}/cm{sup 2}. Higher implantation fluences result in an open-volume defect structure in the film that persists after annealing. This annealing behavior is similar to that of an unrecoverable quenching effect on Er{sup 3+}-related PL near 1540 nm, and we suggest that these open-volume defects may cause a decoupling of the Er{sup 3+} ions from sensitizing oxide point defects that form as a result of the film deposition process.

Blakie, D. E.; Zalloum, O. H. Y.; Wojcik, J.; Irving, E. A.; Knights, A. P.; Mascher, P. [Department of Engineering Physics, Centre for Emerging Device Technologies, McMaster University, Hamilton, Ontario L8S 4L7 (Canada); Simpson, P. J. [Department of Physics and Astronomy, University of Western Ontario, London N6A 3K7 (Canada)

2009-03-01

383

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

384

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

385

Sputter Depth Profiling of the Ge Wetting Layer on Si(001) Using Time-of-Flight Positron Annihilation Induced Auger Electron Spectroscopy  

NASA Astrophysics Data System (ADS)

Ge and Si inter-diffusion is important for application of GeSi nano-structures and other devices. A sample consisting of 4 layers of Ge deposited on Si(001) substrate at 650ºC by chemical vapor deposition was investigated using Time-Of-Flight Positron induced Auger Electron Spectroscopy (TOF-PAES). Previous work has demonstrated that the combination of PAES and sputter depth profiling is able to measure concentration profiles with single atomic layer resolution. Depth profiling of the Ge on Si sample shows clear evidence of Si diffusion into Ge. Si was found throughout the Ge overlayer with highest Ge concentration on the surface and lowest at the Ge/Si interface where a sharp change of composition was found. Research supported by the Welch Foundation and NSF DMR 98-12628.

Zhu, J.; Yan, J.; Xie, S.; Eshed, A.; Weiss, A. H.

2004-03-01

386

High current pulsed positron microprobe  

SciTech Connect

We are developing a low energy, microscopically focused, pulsed positron beam for defect analysis by positron lifetime spectroscopy to provide a new defect analysis capability at the 10{sup 10} e{sup +}s{sup -l} beam at the Lawrence Livermore National Laboratory electron linac. When completed, the pulsed positron microprobe will enable defect specific, 3-dimensional maps of defect concentrations with sub-micron resolution of defect location. By coupling these data with first principles calculations of defect specific positron lifetimes and positron implantation profiles we will both map the identity and concentration of defect distributions.

Howell, R.H.; Stoeffl, W.; Kumar, A.; Sterne, P.A.; Cowan, T.E.; Hartley, J.

1997-05-01

387

a Study of Hadronization Using Energy Flow from Electron-Positron Annihilation Into Quarks and Gluons at Square Root S of 29 GEV  

NASA Astrophysics Data System (ADS)

We have made a high statistics study of QCD jets produced in e('+)e('-) annihilations at SQRT.(s) of 29 GeV and observed in the MAC detector located at the PEP storage ring at SLAC. The MAC detector uses calorimetry and provides a homogeneous response over much of its 98%(.)4(pi) sr instrumented solid angle. A data sample of well reconstructed hadronic events was selected by requiring that E(,vis) in the calorimeters be near SQRT.(s), and almost all the energy be deposited in the central calorimeters. Fits of the jet transverse energy flow are made to the data using String (STR) model and several types of Independent Jet (IJM) model hypotheses, where (alpha)(,s), the strong coupling constant, and (sigma)(,q), the width of the secondary quark P(,(PERP)) distribution, are free parameters. The fits to O((alpha)(,s)('2)) using MS renormalization yield (alpha)(,s) (TURN) 0.17 with the STR hypothesis, and (alpha)(,s) (TURN) 0.12 with the various IJM hypotheses. The correlations between (alpha)(,s) and (sigma)(,q) are examined. Detailed comparisons were made with other experimental results. The energy flow projected onto the event plane of 3-jet events selected from the above data sample was studied. The data shows an asymmetric energy flow around the thin jet. Such an asymmetry was predicted by the STR model, and a cluster model (Webber) incorporating soft gluon interference. The various IJM models show no such asymmetry. We associate this asymmetry with coherence effects during hadronization.

Rosenberg, Leslie J.

388

Positron annihilation study of dislocations produced by polishing in the surface of iron single crystals: Part I. Density profile and removal by annealing  

NASA Astrophysics Data System (ADS)

The trap concentration as a function of the depth from the abraded surface of iron single crystals was investigated by means of positron Doppler broadening. The deformation depth was about 40 ?m from the surface when the specimen was mechanically polished on 240-grit SiC paper. The depth of the damaged region is less than 15 ?m from the surface when a slurry of 0.3 /xm A1"2O3 particles is used. A mixture of edge and screw dislocations was found in the deformed layer. The two stages in recovery observed upon annealing mechanically polished iron are interpreted as being due to the disappearance of screw components during lower temperature anneals (273 to 473 K). When the number of screw dislocations has fallen considerably, the edge-dislocation dipoles begin to disappear. The activation energies for recovery of screw and edge dislocations are estimated at 30.5 and 103.2 kJ/mol, respectively, using the second-order Li model. Based on these small energies, dislocation dipoles disappear by glide rather than by climb in pure iron. Migration of monovacancies and climb are apparently important with impure iron, since the activation energy is similar to that for vacancy migration.

Lee, Jong-Lam; Waber, James T.

1990-07-01

389

Electron-Positron Colliding Beam Experiments  

Microsoft Academic Search

Possible experiments with high-energy colliding beams of electrons and positrons are discussed. The role of the proposed two-pion resonance and of the three-pion resonance or bound state is investigated in connection with electron-positron annihilation into pions. The existence of a three-pion bound state would give rise to a very large cross section for annihilation into pi0+gamma. A discussion of the

N. Cabibbo; R. Gatto

1961-01-01

390

Microstructure of thermally grown and deposited alumina films probed with positrons  

NASA Astrophysics Data System (ADS)

Aluminum oxide films used for corrosion protection of iron and nickel aluminides were generated by substrate oxidation as well as plasma and physical vapor depositions. The films grown by oxidation were crystalline. The others were amorphous. Defect structures of the films were studied by positron spectroscopy techniques. Lifetimes of the positrons, and Doppler broadening of the ? photons generated by their annihilation, were measured as functions of the energies with which they were injected. In this manner, densities and sizes of the defects were determined as functions of depths from the outer surfaces of the films. Alumina films generated by oxidation had high densities of open volume defects, mainly consisting of a few aggregated vacancies. In the outer regions of the films the structures of the defects did not depend on substrate compositions. Positron lifetime measurements, and the S and W parameters extracted from Doppler broadening spectra, showed uniform distributions of defects in the crystalline Al2O3 films grown on nickel aluminide substrates, but these data indicated intermediate layers of higher defect contents at the film/substrate interfaces of oxides grown on iron aluminide substrates. Amorphous films generated by plasma and physical vapor deposition had much larger open volume defects, which caused the average lifetimes of the injected positrons to be significantly longer. The plasma deposited film exhibited a high density of large cavities.

Somieski, Bertram; Hulett, Lester D.; Xu, Jun; Pint, Bruce A.; Tortorelli, Peter F.; Nielsen, Bent; Asoka-Kumar, Palakkal; Suzuki, Ryoichi; Ohdaira, Toshiyuki

1999-03-01

391

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

392

Applicability of the MCPNX particle transport code for determination of the source correction effect in positron lifetime measurements on thin polymer films  

SciTech Connect

The method presented herein uses the MCNPX Monte Carlo particle transport code to track individual positrons and other particles through geometry that accounts for the detectors, backing foils, samples and sources with their actual sizes, positions and material characteristics. Polymer material, polydimethylsiloxane (PDMS), with different thickness of films served as samples. The excellent agreement between the experimental results and the MCNPX simulation of source correction effects for varied positron sources and different film thicknesses validates the applicability of the MCNPX code.

J.M. Urban-Klaehn

2007-09-01

393

Effects of annealing on carrier lifetime in 4H-SiC  

NASA Astrophysics Data System (ADS)

We present results of a thermal anneal process that increases the minority carrier lifetime in SiC substrates to in excess of 3 ?s, compared to the starting as-grown substrates with lifetimes typically in the <10 ns range. Measurement of lifetimes was conducted using microwave-photoconductive decay. Electron beam induced current measurements exhibited minority carrier diffusion lengths of up to 65 ?m, confirming the enhanced carrier lifetime of the annealed substrate material. Additionally, positron annihilation spectroscopy and deep level transient spectroscopic (DLTS) analysis of samples subjected to this anneal process indicated that a significant reduction of deep level defects, particularly Z1/Z2, may account for the significantly enhanced lifetimes. The enhanced lifetime is coincident with a transformation of the original as-grown crystal into a strained or disordered lattice configuration as a result of the high temperature anneal process. The operational performance of p-i-n diodes employing drift layers fabricated from the annealed high-lifetime substrates confirmed conductivity modulation in the diodes consistent with ambipolar carrier lifetimes in the microsecond range.

Jenny, J. R.; Malta, D. P.; Tsvetkov, V. F.; Das, M. K.; Hobgood, H. Mcd.; Carter, C. H.; Kumar, R. J.; Borrego, J. M.; Gutmann, R. J.; Aavikko, R.

2006-12-01

394

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

395

Profiling Multilayer Structures with Monoenergetic Positrons.  

National Technical Information Service (NTIS)

Variable-energy (0-25keV) positron stopping and annihilation behavior is studied in a multilayer structure, which has subsequent (about 3000 thick) ZnS and Al2O3 layers on a glass substrate. Direct information on positron slowing down properties is obtain...

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

1986-01-01

396

Damage-depth profiling of an ion-irradiated polymer by monoenergetic positron beams  

Microsoft Academic Search

Poly(aryl-ether-ether ketone) (PEEK) films irradiated with 1-MeV and 2-MeV O+ ions were exposed to positron beams to measure the positron annihilation Doppler broadening as a function of the positron energy. The annihilation lines recorded at relatively low positron energies were found to become broader with increasing irradiation dose, suggesting that positronium (Ps) formation is inhibited in the damaged regions. The

Yoshinori Kobayashi; Isao Kojima; Shunichi Hishita; Takenori Suzuki; Eiji Asari; Masahiro Kitajima

1995-01-01

397

A positron-decay triggered transmission source for positron emission tomography  

Microsoft Academic Search

Attenuation of gamma rays is the most important correction required to provide artefact-free quantitative images in positron emission tomography (PET). We have tested a new technique, which allows multiple sources to be used for transmission scans. Each point source is surrounded by plastic scintillator, and the energy the positron must lose before annihilation is collected in the plastic scintillator coupled

Marie-Laure Camborde; Christopher J. Thompson; Dylan Togane; Nan Zhang; Andrew Reader

2004-01-01

398

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

399

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

NASA Astrophysics Data System (ADS)

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. We have initiated a study of normal brain section and brain section with glioma derived from a rat glioma model. PALS lifetime runs were made with the samples soaked in formalin, and there was not significant evaporation of formalin during the runs. While early results suggested a small decrease in 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. 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.

Quarles, C. A.; Ballmann, Charles; Yang, S. H.

2009-04-01

400

Heat treatment effects on positron doppler broadening in YBa2Cu3O7-x  

NASA Astrophysics Data System (ADS)

Positron annihilation Doppler lineshape parameters for YBa2Cu3O7-x have been demonstrated experimentally to be sensitive to heat treatment conditions and to vary almost linearly with oxygen vacancy concentration. Positron annihilation spectroscopy (PAS) can be used to monitor the beheviors of oxygen vacancies in YBaCuO-like ceramic.

Zhang, Hong; Wang, Xiaogang

1988-10-01

401

Third-Order Coulomb Wave Function and Single Quantum Annihilation  

Microsoft Academic Search

An expression for the third-order Coulomb wave function, correct to order (alphaZ)3 for all momenta and energies, is derived. Using this wave function the total cross section for single-photon emission resulting from a positron annihilating with a K-shell electron (single quantum annihilation), valid to relative order (alphaZ)2, is calculated numerically. The results for lead are compared to the exact answer

C. O. Carroll; R. F. O'Connell

1963-01-01

402

Damage-depth profiling of an ion-irradiated polymer by monoenergetic positron beams  

NASA Astrophysics Data System (ADS)

Poly(aryl-ether-ether ketone) (PEEK) films irradiated with 1-MeV and 2-MeV O+ ions were exposed to positron beams to measure the positron annihilation Doppler broadening as a function of the positron energy. The annihilation lines recorded at relatively low positron energies were found to become broader with increasing irradiation dose, suggesting that positronium (Ps) formation is inhibited in the damaged regions. The positron data were compared with the results of dynamic hardness and electron-spin-resonance measurements. The slow-positron Doppler broadening technique is found to be a useful means for damage-depth profiling of Ps-forming polymers.

Kobayashi, Yoshinori; Kojima, Isao; Hishita, Shunichi; Suzuki, Takenori; Asari, Eiji; Kitajima, Masahiro

1995-07-01

403

Radiative processes in dark matter photino annihilation  

Microsoft Academic Search

It is shown that radiative corrections to the photino annihilation process g~gg~g-->e+e- in the galactic halo can be enormous, with the radiative process being several orders of magnitude larger than the lowest order rate. It is argued that this may give a source of observable high-energy photons (and maybe also positrons) in forthcoming space-borne experiments. The full box diagrams for

Lars Bergström

1989-01-01

404

High Precision Measurement of the Neon-19 Lifetime  

NASA Astrophysics Data System (ADS)

The lifetime of 19Ne is an important parameter in precision tests of the Standard Model. Improvement in the uncertainty of experimental observables of this and other T = ½ mirror isotopes would allow for an extraction of Vud at a similar precision to that obtained by superallowed 0+ ? 0 + Fermi decays. We report on a new high precision measurement of the lifetime of 19Ne, performed at the Kernfysich Versneller Instituut (KVI) in Groningen, the Netherlands. A 10.5 MeVA 19F beam was used to produce 19Ne using inverse reaction kinematics in a H2 gas target. Contaminant productions were eliminated using the TRImuP magnetic isotope separator. The 19Ne beam was implanted into a thick aluminum tape, which was translated to a shielded detection region by a custom tape drive system. Collinear annihilation radiation from the emitted decay positrons were detected by two high purity germanium (HPGe) detectors. Event pulse waveforms were digitized and stored using a CAEN V1724 Digitizer. Systematic studies were performed to characterize rate-dependent data acquisition effects, diffusion, backgrounds, and contamination from the separator. We have obtained the result for the lifetime of tau = 24.9344 +/- 0.0073(stat) +/- 0.0083( sys) seconds.

Broussard, Leah Jacklyn

405

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

SciTech Connect

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

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

2009-03-10

406

Cluster Analysis of Hadronic 3-Jet Events of the Electron-Positron Annihilation in the C.M. Energy Range from 27.0 GeV to 31.6 GeV.  

National Technical Information Service (NTIS)

A multijet analysis of hadronic final states from e exp + e exp - - annihilation is presented. The data have been taken with the detector PLUTO in the energy range 9.4 GeV <= Esub(c)sub(m) +- 31.6 GeV. The analysis uses a cluster method to identify the je...

H. J. Daum

1981-01-01

407

Resolution of the orthopositronium lifetime puzzle  

NASA Astrophysics Data System (ADS)

A new precision measurement of the vacuum decay rate lambdaT of orthopositronium (o-Ps), the spin-one bound state of an electron and a positron, is presented as a unique test of quantum electrodynamics. This experiment uses a beam of positrons to form positronium in the four nanometer diameter pores of a one micron thick nanoporous silica film. The positronium subsequently diffuses into an evacuated dual-chambered confinement cavity. The first time-of-flight measurement of the energy spectrum of orthopositronium emitted into vacuum from these films confirms that it is nearly therinalized, thus minimizing the effect of orthopositronium collisions with the cavity walls on the decay rate. The primary systematic effect on the fitted decay rate, high-energy positronium formed from backscattered positrons that subsequently annihilates on the cavity walls, is accounted for by stripping a suitably normalized spectrum of pure backscattered positronium, acquired from a non-porous SiO2 film, from the lifetime spectrum. The final decay rate is an extrapolation in the fractional intensity of the backscattered positronium component. After a total correction of only ˜500 ppm the final value is lambdaT(vac) = 7.0401(10) mus-1 where the total error is the combination of the 0.0008 mus-1 statistical and 0.0006 mus -1 systematic errors. This new result is the most precise to date and is in excellent agreement with the current theoretical calculation and a recent measurement using fine-grained insulating powders. The new vacuum decay rate supersedes the higher 1990 decay rate using a similar vacuum technique that did not fully account for backscattered positronium. The remaining discrepancy with the 1989 measurement that formed positronium in a buffer gas is addressed by a complete reanalysis of that experiment. Two new measurements, positronium thermalization in gases using a time-resolved doppler broadening technique and the temperature (velocity) dependence of positronium quenching in gases, implicate incomplete positronium thermalization as the major systematic problem in precision decay rate measurements using gases. A correction to the 1989 experiment that accounts for half of the discrepancy vitiates that decay rate result. The new vacuum decay rate result, in combination with the gas correction, definitively resolves the "orthopositronium lifetime puzzle".

Vallery, Richard Sterling

408

Treatment of resonances in the scattering of a heavy positron by H that are due to interaction with vibrationally excited quasibound states  

Microsoft Academic Search

For a positron with wave number k, the rate of annihilation when scattered by an atom or molecule is proportional to Z{sub eff}(k), the effective number of electrons in the target that are available to the positron for annihilation. There is currently great interest in the very large positron annihilation rates, and hence values of Z{sub eff}(k), that have been

Armour

2010-01-01

409

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

NASA Astrophysics Data System (ADS)

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.; Ballmann, C.; Quarles, C. A.

2009-03-01

410

Radiative proton-antiproton annihilation to a lepton pair  

SciTech Connect

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

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

2010-11-01

411

Low-energy positron interactions with atoms and molecules  

Microsoft Academic Search

This paper is a review of low-energy positron interactions with atoms and molecules. Processes of interest include elastic scattering, electronic and vibrational excitation, ionization, positronium formation and annihilation. An overview is presented of the currently available theoretical and experimental techniques to study these phenomena, including the use of trap-based positron beam sources to study collision processes with improved energy resolution.

C M Surko; G F Gribakin; S J Buckman

2005-01-01

412

Bound states of positrons with atoms and molecules: Theory  

Microsoft Academic Search

Calculations of the binding energies and annihilation rates of bound atomic and molecular systems which contain a positron are reviewed. Emphasis is placed on methods of calculation and the quality of the numerical results. In this article we limit our attention to positrons interacting with atoms, diatomic molecules, and their ions.

D. M. Schrader

1998-01-01

413

Positron spectroscopy in atomic and solid state physics  

Microsoft Academic Search

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

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

2003-01-01

414

Positron age-momentum correlation studies of free volumes in toughened cyanate resin networks  

NASA Astrophysics Data System (ADS)

Positron age-momentum correlation (AMOC) spectroscopy is applied to study the associations between the momentum distributions and positron annihilation in the toughened CE networks. The Doppler profile of the positron annihilation process is much broader for the toughened CE than pure CE network. This is due to positron trapping on high polar groups of the added elastomers or nano-CaCO3 and annihilation with the electrons of high momenta. The broadening momentum distribution of ortho-positronium annihilation process indicates that the pick-off annihilation process is strongly influenced by the added high polar elastomers or nano-CaCO3. The potential of AMOC spectroscopy for free volume analysis in polymer blends was demonstrated.

Minfeng, Zeng; Xudong, Sun; Cuiyun, Lu; Zhuoxin, Li; Baoyi, Wang; Chenze, Qi

2012-06-01

415

Coincidence summing corrections for positron emitters in germanium gamma spectrometry  

NASA Astrophysics Data System (ADS)

For positron emitters, 511 keV annihilation quanta are in coincidence with other gamma rays in the decay scheme. If the positrons are not localized at the point of decay, annihilation quanta will be produced at a site some distance from the point of emission. The magnitude of the summing coincidence effect will depend upon the position of annihilation. A method for determining the magnitude of the summing effect for a single gamma of energy E in coincidence with the annihilation gammas from non-localized positrons has been developed which makes use of the counting data for the full energy peaks for both the gamma ray (E) and the 511 keV annihilation gammas. With this data and efficiency calibration data one can determine the average total efficiency for the annihilation positions from which 511 keV gammas originate, and thereby obtain the summing correction factor, SCF, for gamma ray (E). Application of the method to a 22Na NIST standard gave excellent agreement of observed emission rates for the 1275 keV gamma with the NIST value for wide ranging degrees of positron localization having summing correction factors ranging from 1.021 to 1.505. The method was also applied successfully to 58Co in neutron-irradiated nickel foils. The method shows promise as a check on the accuracy of the efficiency calibration for a particular detector geometry at the 511 keV energy and energies for other gammas associated with positron emission.

Richardson, A. E.; Sallee, W. W.

1990-12-01

416

Positron implantation in solids  

NASA Astrophysics Data System (ADS)

The Monte Carlo technique for modeling positron prior to annihilation and electron implantation in semi-infinite metals is described. Particle implantation is modelled as a multistep process, a series of collisions with the atoms of the host material. In elastic collisions with neutral atoms there is no transfer of energy. The particle loses energy by several different channels, excitation of the electron gas, ionization of the ion cores, or, at low energies, by phonon excitation. These competing scattering mechanisms have been incorporated into the Monte Carlo framework and several different models are being used. Brief descriptions of these Monte Carlo schemes, as well as an analytic model for positron implantation are included. Results of the Monte Carlo simulations are presented and compared with experimental data. Problems associated with modeling positron implantation are discussed and the need for more experimental data on energy-loss in different materials is stressed. Positron implantation in multilayers of different metals is briefly described and extensions of this work to include a study of multilayers and heterostructures is suggested.

Ghosh, V. J.; Lynn, K. G.; Welch, D. O.

417

2D ACAR momentum density study of the nature of the positron surface state on Al(100)  

SciTech Connect

The two-dimensional angular correlation of the 2..gamma.. annihilation radiation (2D ACAR) has been measured from an Al(100) surface bombarded by 200-eV positrons. After removing the contribution of fast para-positronium annihilation, the spectrum from positrons annihilating at the surface exhibits a nearly isotropic conical shape with a (7.1 +- 0.5) mrad FWHM. 5 refs., 6 figs.

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

1985-01-01

418

Positron trapping at grain boundaries  

SciTech Connect

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

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

1993-10-01

419

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

420

Study of the effect of magnetic field in positron range using GATE simulation toolkit  

NASA Astrophysics Data System (ADS)

In simultaneous PET-MR systems, the emitted positrons trajectory is influenced by the magnetic field. The aim of this study is to define that influence to the positron annihilation distribution. Monte Carlo methods have been applied, using GATE. Several isotopes were studied, in various types of materials and with different magnetic field strengths. The results showed variations in the positron range between different components and especially between the lung and water. Measurements of the 1-D positron annihilation distance indicated a reduction of the mean positron annihilation distance for 82Rb of ~25%, 68Ga of 19% and 18F of 3.5%, at 3 Tesla. When the magnetic field was increased to 9.5 Tesla, the reduction was significant for all isotopes, and mainly for 68Ga and 82Rb, with approximately 41% reduction of the mean positron annihilation distance in water. Finally, the positron annihilation distribution varies according to the alignment with the magnetic field lines. The results of this study could be used to improve positron annihilation correction algorithms for simultaneous PET-MR acquisition, by taking under consideration the non-isotropic distribution.

Soultanidis, G.; Karakatsanis, N.; Nikiforidis, G.; Loudos, G.

2011-09-01

421

Measurement of the tau lifetime  

SciTech Connect

We have used a high-resolution drift chamber in the Mark II detector at the SLAC storage ring PEP to measure the lifetime of tau leptons produced in e/sup +/e/sup -/ annihilations at 29 GeV. Based on the flight-path distribution of 807 three-prong tau decays, the lifetime is found to be (2.88 +- 0.16 +- 0.17) x 10/sup -13/ sec, in agreement with expectations for e-..mu..-tau universality.

Amidei, D.; Trilling, G.H.; Abrams, G.S.; Baden, A.R.; Boyer, J.; Butler, F.; Gidal, G.; Gold, M.K.; Goldhaber, G.; Golding, L.; and others

1988-04-01

422

Instrumentation in positron emission tomography  

SciTech Connect

Positron emission tomography (PET) is a three-dimensional medical imaging technique that noninvasively measures the concentration of radiopharmaceuticals in the body that are labeled with positron emitters. With the proper compounds, PET can be used to measure metabolism, blood flow, or other physiological values in vivo. The technique is based on the physics of positron annihilation and detection and the mathematical formulations developed for x-ray computed tomography. Modern PET systems can provide three-dimensional images of the brain, the heart, and other internal organs with resolutions on the order of 4 to 6 mm. With the selectivity provided by a choice of injected compounds, PET has the power to provide unique diagnostic information that is not available with any other imaging modality. This is the first five reports on the nature and uses of PET that have been prepared for the American Medical Association's Council on Scientific Affairs by an authoritative panel.

Not Available

1988-03-11

423

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

424

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

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

2013-04-25

425

Comparison of the Electron Momentum Distribution of Solid and Liquid Ni as Measured by Positrons.  

National Technical Information Service (NTIS)

This paper compares the measured electron momentum densities seen by positrons annihilating in the defect-free Ni lattice, in vacancies and in liquid Ni. The experimental techniques and analysis method are also described. (ERA citation 06:027355)

M. J. Fluss L. C. Smedskjaer M. K. Chason

1980-01-01

426

LONG LIFETIME OF POSITRONIUM IN LIQUID HELIUM  

Microsoft Academic Search

The long orthopositronium lifetime observed by Paul and Graham and ; Wackerle and Stump in liquid helium is evidence of a pickoff annihilation rate ; smaller by more than one order of magnitude than the rate to be expected on the ; basis of the average electron density. This discrepancy is removed by taking ; into account the repulsive positronium-helium

Richard Ferrell

1957-01-01

427

Continuum gamma-ray emission from light dark matter positrons and electrons  

Microsoft Academic Search

The annihilation of light dark matter was recently advocated as a possible explanation of the large positron injection rate at the Galactic center deduced from observations by the SPI spectrometer aboard INTEGRAL. The modelling of internal Bremsstrahlung and in-flight annihilation radiations associated to this process drastically reduced the mass range of this light dark matter particle. We estimate critically the

P. Sizun; M. Casse; S. Schanne

2007-01-01

428

Continuum ?-ray emission from light dark matter positrons and electrons  

Microsoft Academic Search

The annihilation of light dark matter was recently advocated as a possible\\u000aexplanation of the large positron injection rate at the Galactic center deduced\\u000afrom observations by the SPI spectrometer aboard INTEGRAL. The modelling of\\u000ainternal Bremsstrahlung and in-flight annihilation radiations associated to\\u000athis process drastically reduced the mass range of this light dark matter\\u000aparticle. We estimate critically the

P. Sizun; M. Cassé; S. Schanne

2006-01-01

429

Neutralinos in an extension of the minimal supersymmetric standard model as the source of the PAMELA positron excess  

SciTech Connect

We consider a scenario within the minimal supersymmetric standard model (MSSM) extended by a singlet chiral superfield, in which neutralino dark matter annihilates to light singletlike Higgs bosons, which proceed to decay to either electron-positron or muon-antimuon pairs. Unlike neutralino annihilations in the MSSM, this model can provide a good fit to the PAMELA cosmic ray positron fraction excess. Furthermore, the singletlike Higgs bosons can induce a large Sommerfeld enhancement and provide an annihilation rate sufficient to accommodate the observed positron excess.

Hooper, Dan [Center for Particle Astrophysics, Fermi National Accelerator Laboratory, Batavia, Illinois 60510-0500 (United States); Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60637-1433 (United States); Tait, Tim M. P. [Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208 (United States); HEP Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States)

2009-09-01

430

New results for positron-molecule Feshbach resonances and bound states.  

NASA Astrophysics Data System (ADS)

Monoenergetic positrons from a trap-based beam have been used to measure the first energy resolved positron-on-molecule annihilation spectra [1,2]. Strong peaks in annihilation rate are observed at energies just below the vibrational modes of various molecules. These peaks are due to vibrational Feshbach resonances (VFR) and provide evidence of positron- molecule binding. In this paper, the properties of these VFR are further explored. The dependence on target morphology is studied for the ring hydrocarbons, benzene, cyclohexane and cyclopropane. A comparison is presented of positron-annihilation and infrared-absorption spectra. Finally, evidence is presented for a second, ``positronically excited'' bound state in large alkane molecules. [1] S. J. Gilbert, et al., Phys. Rev. Lett., 88, 043201 (2002). [2] L. D. Barnes, et al., Phys. Rev. A 67, 032706 (2003).

Young, J. A.

2005-05-01

431

The three photon yield from e+ annihilation in biological liquids  

Microsoft Academic Search

Positrons annihilate either by the emission of 2-511 keV photons or 3-photons (from the decay of positronium in the triplet state.) The fraction of the 3-photon decay depends on the chemical environment and notably on the concentration of O2. Consequently, 3-photon event detection has been proposed as a mean to measure hypoxia, a condition prevailing in cancer. The (delayed) three-photon

R. Laforest; K. Mercurio; P. Zerkel; L. G. Sobotka; R. J. Charity

2008-01-01

432

Fast neutrons from annihilation  

NASA Astrophysics Data System (ADS)

We analyzed experimental data on fast neutron production in ¯p+64Cu and ¯p+238U annihilation and the ratio of231Th/231Ac isotopes from ¯p+232Th annihilation at rest. The contributions from standard reaction channels are calculated by cascade model. The annihilation pion rescattering and absorption in nuclear medium as well as the preequilibrium nucleon emission were analyzed. The calculation based on the assumption that neutron density distribution ?uals the proton one multiplied by factor N/Z does not reproduce the experimental data. We discussed the influence of the neutron density distribution on fast neutron and isotope production. It was found that the hypothesis of the neutron halo allows one to get a reasonable description of the experimental data.

Sibirtsev, A.

1994-09-01

433

KENO lifetimes  

SciTech Connect

When performing k-eigenvalue solutions with KENO-V.a, two different prompt neutron lifetimes are estimated - a system lifetime and a neutron generation time. The meaning of these two lifetimes has been ascertained by comparing values of various neutron lifespans/lifetimes predicted by MCNP and DANTSYS based on the neutron-balance theory. The system lifetime in KENO-Va corresponds to the unweighted removal lifetime calculated by both MCNP and DANTSYS. The unweighted removal lifetime is the average time between removal events resulting from a neutron absorption or a neutron leakage. The generation time in KENO-V.a corresponds to the fission lifespan calculated by MCNP, where the fission lifespan in MCNP represents the average time for a newly born neutron to cause another fission. As such, the generation time in KENO-Va does not represent the generation time that appears in the point kinetic model. The generation time in the point kinetic model is the adjoint-weighted removal lifetime divided by k{sub eff}, which is identically equal to the adjoint-weighted neutron production rate. In small bare systems operating in the vicinity of delayed critical, the difference between the adjoint-weighted neutron generation time and the fission lifespan can be as small as a few percent. However, in reflected systems, the difference between these two quantities can be several orders of magnitude. In conclusion, the prompt neutron generation time predicted by KENO-Va corresponds to the fission lifespan of a prompt neutron in a given system. The fission lifespan is the average time from birth-to-fission and, in general, is not a good approximation for the adjoint-weighted neutron generation time that appears in the point-kinetic model.

Petrie, L. [Oak Ridge National Lab., TN (United States); Parsons, D.K.; Spriggs, G.D. [Los Alamos National Lab., NM (United States)

1997-01-30

434

Positron states in KCL crystals before and after radiation exposure  

NASA Astrophysics Data System (ADS)

The features of positron annihilation in KCl crystals before and after radiation exposure is investigated. A significant increase in the efficiency of the “ortho-vapor” process of positronium state conversion is detected. A model is proposed for taking part in the “ortho-vapor” conversion of conduction electrons drawn into the conduction band by a positron field that affords a qualitative interpretation of the results obtained and permits values of the electron concentration in a spur to be obtained.

Aref'ev, K. P.; Boev, O. V.; Bondarenko, A. L.

1988-07-01

435

TOPICAL REVIEW: Low-energy positron interactions with atoms and molecules  

Microsoft Academic Search

This paper is a review of low-energy positron interactions with atoms and molecules. Processes of interest include elastic scattering, electronic and vibrational excitation, ionization, positronium formation and annihilation. An overview is presented of the currently available theoretical and experimental techniques to study these phenomena, including the use of trap-based positron beam sources to study collision processes with improved energy resolution.

C. M. Surko; G. F. Gribakin; S. J. Buckman

2005-01-01

436

Construction of a Positron Emission Tomograph with 2.4 Mm Detectors  

Microsoft Academic Search

One-quarter of one ring of a positron tomograph has been constructed. The positron annihilation gamma rays are detected by polished plastic scintillators which direct scintillation light by internal reflection to optical fibers for transmission to the photomultiplier tubes. By viewing each scintillator with four sets of optical fibers, the number of photomultipliers required for an eight ring tomograph with 1024

John A. McIntyre; Ronald L. Spross; Ken-Hsi Wang

1986-01-01

437

REVIEW ARTICLE: Self-trapped states of positrons and positronium in dense gases in liquids  

Microsoft Academic Search

In dense media positrons and positronium atoms are bound in self-trapped states. These states are density fluctuations stabilised by the light quantum particles. The positronium atoms are trapped in a bubble, while positrons are trapped in a cluster. Transitions to these states at changes of the density or temperature of the medium produce an essential effect on the annihilation rate.

I. T. Iakubov; A. G. Khrapak

1982-01-01

438

Vacancy-Type Defects Introduced by Gas Cluster Ion-Implantation on Si Studied by Monoenergetic Positron Beams  

NASA Astrophysics Data System (ADS)

Vacancy-type defects in Ar and B gas cluster ion-implanted Si were probed by monoenergetic positron beams. The acceleration energy of the clusters ranged between 20--60 keV, and the mean cluster size was 2× 103 atoms. Doppler broadening spectra of the annihilation radiation were measured, and the vacancy-rich region was found to localize at a depth of 0--13 nm. Measurements of the lifetime spectra of positrons revealed that two different defect species coexisted in the damaged region introduced by Ar gas cluster implantation, and these were identified as divacancy-type defects and large vacancy clusters filled with Ar. The formation of the vacancy clusters was attributed to extremely high temperature and its rapid transients in impact regions of Ar cluster ions. For B gas cluster ion implanted Si, the detected momentum distribution of electrons in the damaged region was broader than that in the defect free Si, suggesting that the trapping of positrons by open spaces adjacent of B clusters.

Uedono, Akira; Moriya, Tsuyoshi; Tsutsui, Takuro; Kimura, Shogo; Oshima, Nagayasu; Suzuki, Ryoichi; Ishibashi, Shoji; Matsui, Hidefumi; Narushima, Masaki; Ishikawa, Yoichi; Graf, Michael; Yamashita, Koji

2012-11-01

439

Annihilation of antimatter meteors  

Microsoft Academic Search

Antimatter meteors probably enter the Earth's atmosphere. If they have the ability to escape complete vaporization during their infall flight, it may be possible, that a fraction of their original mass could survive for short or long time, depending on the mechanisms of ablation. In case of ablation through the annihilation process only, the lifetimedt of such an object is

P. M. Papaelias

1993-01-01

440

A laser-cooled positron plasma  

NASA Astrophysics Data System (ADS)

We present results on trapping and cooling of positrons in a Penning trap. A few thousand positrons are trapped and lose energy through Coulomb collisions (sympathetic cooling) with laser-cooled ^9Be^+ ions. By imaging the ^9Be^+ laser-induced fluorescence, we observed centrifugal separation of the ^9Be^+ ions and positrons, with the positrons coalescing into a ``dark'' column along the trap axis. This indicated that the positrons had densities up to ~4 × 10^9 cm-3 which is ~50 times greater than the highest positron density previously achieved(R. G. Greaves and C. M. Surko, Phys. Rev. Lett. 85), 1883 (2000). By comparing the observed centrifugal separation with a modified theoretical model(T. M. O'Neil, Phys. Fluids 24), 1447 (1981). we place an upper limit on the positron temperature for motion parallel to the magnetic field of ~5 K. The positron lifetime was greater than 8 days in our room temperature vacuum of 10-8 Pa. Cold positron plasmas are useful as a source for cold beams of high brightness, for positron-normal matter interaction studies and for anti-hydrogen production.

Jelenkovi?, B. M.; Bollinger, J. J.; Newbury, A. S.; Mitchell, T. B.; Itano, W. M.

2001-05-01

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