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1

Tomographic Positron Annihilation Lifetime Spectroscopy  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

2

Position-resolved Positron Annihilation Lifetime Spectroscopy  

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

3

Investigation of nanocrystalline CoFe2O4 by positron annihilation lifetime spectroscopy  

Microsoft Academic Search

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

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

2003-01-01

4

Study of Chemical Carcinogens by Positron Annihilation Lifetime Spectroscopy  

NASA Astrophysics Data System (ADS)

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

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

2013-11-01

5

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

Microsoft Academic Search

A new positron annihilation lifetime spectrometer consisting of a start gamma-ray detector, a stop gamma-ray detector, a digital oscilloscope, and a positron detector, which is a plastic scintillator coupled to a photomultiplier tube, is described. A 22Na source is placed between the positron detector and a sample to be studied. gamma-ray signals related to positrons annihilating in the positron detector

Masato Yamawaki; Yoshinori Kobayashi; Kanehisa Hattori; Yoshihiro Watanabe

2011-01-01

6

Positron annihilation lifetime spectroscopy of ZnO bulk samples  

SciTech Connect

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

Zubiaga, A.; Plazaola, F.; Garcia, J. A.; Tuomisto, F.; Munoz-Sanjose, V.; Tena-Zaera, R. [Fisika Aplikatua II Saila, Euskal Herriko Unibertsitatea, Posta Kutxatila 644, 48080 Bilbao (Spain); Laboratory of Physics, Helsinki University of Technology, P.O. Box 1100, 02015 TKK, Espoo (Finland); Departament de Fisica Aplicada i Electromagnetisme, c/ Doctor Moliner 50, E-46100 Burjassot, Valencia (Spain)

2007-08-15

7

Photon-induced positron annihilation lifetime spectroscopy using an S-band compact electron linac  

NASA Astrophysics Data System (ADS)

A new photon-induced positron annihilation lifetime spectroscopy approach has been developed using an S-band compact electron linac at the National Institute of Advanced Industrial Science and Technology (AIST). The high energy (<42MeV), intense (105 photons pulse-1), and ultra-short pulse (3 ps pulse width) photon beam creates positrons throughout an entire sample via pair production. A positron lifetime spectrum can be obtained by measuring the time difference between the accelerator's RF frequency and the detection time of the annihilation gamma rays. The positron lifetimes for lead and yttria-stabilized zirconia samples have been successfully measured.

Taira, Y.; Kuroda, R.; Tanaka, M.; Kumaki, M.; Oshima, N.; O'Rourke, B. E.; Suzuki, R.; Toyokawa, H.

2014-02-01

8

Nanoscopic properties of silica filled polydimethylsiloxane by means of positron annihilation lifetime spectroscopy  

Microsoft Academic Search

Positron annihilation lifetime spectroscopy (PALS) was performed on a series of polydimethylsiloxane (PDMS)\\/fumed silicon dioxide (SiO2) composites at temperatures between ?185 and 100°C to study the effect of filler content and filler particle size on the free volume properties and the positron annihilation characteristics. The glass transition behavior of the PDMS\\/SiO2 composites was determined with differential scanning calorimetry. A clear

Petra Winberg; Morten Eldrup; Frans H. J. Maurer

2004-01-01

9

Various types of polysiloxanes studied by positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

In the work, linear poly(vinylsiloxanes) with regular distributions of vinyl groups along chains, the networks obtained by their hydrosilylation with various hydrogensiloxanes, products of pyrolysis of such networks as well as the networks prepared via reaction of low-molecular siloxanes have been studied by positron anihilation lifetime spectroscopy (PALS).

Nyczyk-Malinowska, Anna; Dryzek, Ewa; Hasik, Magdalena; Dryzek, Jerzy

10

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

11

Electron beam and gamma irradiation effects on high density polyethylene studied via positron annihilation lifetime spectroscopy  

Microsoft Academic Search

Positron annihilation lifetime spectroscopy is applied to the study of electron beam and gamma irradiation effects on high density polyethylene (HDPE). The electron and ?-irradiations were carried out in air, with a flux of 80 kGy s?1 and 26 10?3 kGy s?1, respectively. In four-component analyses of the spectra, two long-lived states are found, with lifetimes ?3=1100 ps and ?4=2570

A. Badia; G. Duplâtre

1999-01-01

12

Positron-annihilation-lifetime response and broadband dielectric relaxation spectroscopy: Diethyl phthalate  

Microsoft Academic Search

We report the results of a combined phenomenological analysis of the data from positron-annihilation-lifetime spectroscopy (PALS) and the relaxation data from broadband dielectric spectroscopy (BDS) on diethyl phthalate (DEP). The ortho-positronium ( o -Ps) lifetime, tau3 , as a function of temperature over a temperature range from 67K up to 300K is compared with the spectral features and the relaxation

J. Bartos; A. Alegría; O. Sausa; M. Tyagi; D. Gómez; J. Kristiak; J. Colmenero

2007-01-01

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

Investigation on CO 2-induced plasticization in polycarbonate membrane using positron annihilation lifetime spectroscopy  

Microsoft Academic Search

In this study, the microstructural changes in polycarbonate (PC) membranes induced by the CO2 sorption were investigated using the CO2 sorption isotherm data and the positron annihilation lifetime spectroscopy (PALS). The effects of the PC exposure to CO2 at different temperatures and gas pressures on the free-volume distribution were discussed. The distribution at ambient conditions or 5atm He pressure displayed

Chia-Hao Lo; Wei-Song Hung; Manuel De Guzman; Shu-Hsien Huang; Chi-Lan Li; Chien-Chieh Hu; Yan-Ching Jean; Kueir-Rarn Lee; Juin-Yih Lai

2010-01-01

15

On determining the entrance size of cage-like pores in mesoporous silica films by positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

Pore entrance size of cage-like pores in mesoporous silica films is difficult to be determined by conventional techniques. A simple expedient is proposed by using positron annihilation lifetime spectroscopy (PALS) based on a slow positron beam. Because of the nature of positronium (Ps, the bound state of a positron and an electron) in mesoporous silica, almost no Ps annihilates in the smaller connecting channels of cages. By trimethylsilylation of the silica, an appreciable fraction of Ps can be trapped and annihilate in the channels, which renders the possibility to estimate the pore entrance size from Ps lifetime in it.

He, Chunqing; Xiong, Bangyun; Mao, Wenfeng; Kobayashi, Yoshinori; Oka, Toshitaka; Oshima, Nagayasu; Suzuki, Ryoichi

2013-12-01

16

Microcomputer program for analysis of positron annihilation lifetime spectra  

Microsoft Academic Search

A user friendly program (LT) for analysis of the lifetime spectra has been designed for personal computers. The program is compared with other existing programs such as POSITRONFIT, CONTIN and MELT. LT enables one to analyse both discrete and continuous spectra as well as mixed- partially linear and partially continuous spectra. The searched parameters are found by using a fully

J. Kansy

1996-01-01

17

Structural studies of spinel manganite ceramics with positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

The new transition-metal manganite Cu0.1Ni0.8Co0.2Mn1.9O4 ceramics for temperature sensors with improved functional reliability are first proposed. It is established that the amount of additional NiO phase in these ceramics extracted during sintering play a decisive role. This effect is well revealed only in ceramics having a character fine-grain microstructure, while the monolithization of ceramics caused by great amount of transferred thermal energy reveals an opposite influence. The process of monolitization from the position of evolution of grain-pore structure was studied in these ceramics using positron annihilation lifetime spectroscopy.

Klym, H.; Ingram, A.; Shpotyuk, O.; Filipecki, J.; Hadzaman, I.

2011-04-01

18

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

19

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

NASA Astrophysics Data System (ADS)

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

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

2011-01-01

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

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

22

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

23

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

24

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

PubMed

PALS in comparison with FTIR studies have been applied to investigate the structure of different oxide glasses. Three components of the positron lifetime ? (?1 para- and ?3 ortho-positronium and ?2 intermediate lifetime component) and their intensities were obtained. The results of the calculation of mean values of positron lifetimes for the investigated glasses showed the existence of a long-living component on the positron annihilation lifetime spectra. From the Tao-Eldrup formula we can estimate the size of free volume. On the basis of the measurements we can conclude that the size and fraction of free volume reaches the biggest value for the fused silica glass. The degree of network polymerisation increases void size. PMID:24815814

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

2014-08-14

25

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

26

Positron-annihilation-lifetime response and broadband dielectric relaxation spectroscopy: Diethyl phthalate  

NASA Astrophysics Data System (ADS)

We report the results of a combined phenomenological analysis of the data from positron-annihilation-lifetime spectroscopy (PALS) and the relaxation data from broadband dielectric spectroscopy (BDS) on diethyl phthalate (DEP). The ortho-positronium ( o -Ps) lifetime, ?3 , as a function of temperature over a temperature range from 67K up to 300K is compared with the spectral features and the relaxation parameters of the BDS spectra decomposed into the primary ? and the secondary ? processes in the temperature range from 140K up to 380K by using the Williams-Watts scheme. Phenomenological model-free analysis of the ?3-T plot provides the three characteristic PALS temperatures, where the two most pronounced ones at TgPALS=185K and Tb2=245K=1.32TgPALS are related to the glass-liquid transition and the onset of a quasiplateau region, respectively. In the case of a weaker bend effect at Tb1=210K=1.14TgPALS , a number of new coincidences with changes in the dielectric ? process have been found. They concern the changes in width parameter of the distribution function for the ? relaxation time and the activation energy of the ?eff process, a crossover from the Arrhenius to the non-Arrhenius type of temperature dependence as well as with the onset of a short-time tail of the ? relaxation time distribution and finally, with changes in the relaxation strength of the ? and ?eff processes. All these findings indicate a close connection of the o -Ps annihilation parameters and relaxation characteristics of BDS response for the DEP matrix.

Bartoš, J.; Alegría, A.; Šauša, O.; Tyagi, M.; Gómez, D.; Krištiak, J.; Colmenero, J.

2007-09-01

27

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

Microsoft Academic Search

Focused ion beam (FIB) microscopy was used to obtain the time dependent transformation fraction and positron annihilation lifetime spectroscopy (PALS) was employed to analyze the vacancy-type defects in electroplated copper (Cu) during room-temperature microstructrual evolution. It was found that PALS is more sensitive than FIB to show the room-temperature microstructual evolution of electroplated Cu at the first stage of self-annealing.

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

2008-01-01

28

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

NASA Astrophysics Data System (ADS)

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

Gregory, Roger B.

1991-05-01

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

2013-08-01

30

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

SciTech Connect

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

MacQueen, R.C.

1992-01-01

31

Positron Annihilation in Metals  

Microsoft Academic Search

An adequate description of positron annihilation rates in metals is obtained by extension of the high-density limit, for annihilation in an electron gas, to realistic densities. A short-range electron-positron force is obtained in the high-density limit and is used to treat accurately the two-body correlations between the positron and metallic valence electrons. This description proves to be satisfactory for metals

S. Kahana

1963-01-01

32

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

NASA Astrophysics Data System (ADS)

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

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

2002-09-01

33

A study of annealing stages in Al–Mn (3004) alloy after cold rolling using positron annihilation lifetime technique and Vickers microhardness measurements  

Microsoft Academic Search

Positron annihilation lifetime (PAL), scanning electron microscope (SEM) and Vickers microhardness (Hv) measurements have been performed to study the isochronal annealing of Al–Mn (3004) alloy in the temperature range from RT to 823K after cold rolling at RT with various deformations of 7 and 23%. A positive correlation has been found between average lifetime (?av) of positrons and Vickers microhardness.Three

E. Gomaa; M. Mohsen; A. S. Taha; M. M. Mostafa

2003-01-01

34

Positron annihilation in germanium  

Microsoft Academic Search

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

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

1975-01-01

35

Phase dependence of positron annihilation in tristearin.  

NASA Technical Reports Server (NTRS)

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

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

1972-01-01

36

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

NASA Astrophysics Data System (ADS)

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

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

2004-07-01

37

Tracking of the viability of Stenotrophomonas maltophilia bacteria population in polyvinylalcohol nanofiber webs by positron annihilation lifetime spectroscopy.  

PubMed

Polyvinylalcohol (PVA) fiber web containing embedded bacteria was prepared by electrospinning technique. From the point of the complex functionality of such potential delivery systems, it will be of impact how bacteria can survive in such artificial biotopes. The present study suggests a possible fast method for the tracking of the viability of the embedded bacteria based on the changes of the supramolecular structure of the polymeric delivery system caused by the metabolic product of the bacteria. Positron annihilation lifetime spectroscopy (PALS) was applied to track the free volume changes of the system in the course of storage. The PALS method sensitively detected the free volume changes, thus the viability of the bacteria in the polymeric fiber web. PMID:22449412

Vajdai, Attila; Szabó, Barnabás; Süvegh, Károly; Zelkó, Romána; Ujhelyi, Gabriella

2012-06-15

38

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

NASA Astrophysics Data System (ADS)

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

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

2008-03-01

39

Positron annihilation processes update  

NASA Technical Reports Server (NTRS)

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

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

1997-01-01

40

Ion induced modification in free volume in PN-6 and PES polymers by positron annihilation lifetime studies  

NASA Astrophysics Data System (ADS)

The irradiation of polymeric materials with swift heavy ions (SHI) results in a change of their free volume properties which have strong correlation with their macroscopic properties. Positron annihilation lifetime spectroscopy (PALS) has been developed into a powerful characterization tool for the study of free volume and free volume fraction in polymers. Polyamide nylon-6 (PN-6) and polyethersulphone (PES) films of thickness of 250 ?m were irradiated with C 5+ ions of energy 70 MeV from 15 UD Pelletron accelerator at Inter University Accelerator Centre (IUAC), New Delhi, India. PN-6 films were irradiated to the fluences of 10 11, 10 12 and 10 13 ions/cm 2 whereas PES films were irradiated to the fluences of 9.3 × 10 11, 9.3 × 10 12 and 1.2 × 10 13 ions/cm 2. Characterization of the effect of ion irradiation on free volume has been done by PALS. The average free volume and fractional free volume obtained from long lived component, attributed to ortho-positronium ( o-Ps) lifetime, are found to decrease with the fluence in both the cases. With increasing fluence, scissioned segments cross-link randomly, resulting in a decrease of average free volume due to overlapping of tracks.

Kumar, Rajesh; Prasad, Rajendra

2007-03-01

41

Positron annihilation lifetime and Doppler broadening study in 50 MeV Li 3+ 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 Li 3+ ions of energy 50 MeV from 15 UD Pelletron accelerators at Inter University Accelerator Centre (IUAC), New Delhi, India to the fluences of 10 11, 10 12 and 10 13 ions/cm 2. 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 10 13 ions/cm 2. 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

42

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

NASA Astrophysics Data System (ADS)

Self-assembled amphiphile systems are utilized in a wide variety of applications including drug delivery and energy storage. Nano-scale physical and chemical interactions govern the packing of self-assembled amphiphilic molecules, resulting in thermodynamically stable phases of defined geometries. Possible phases include micellar, hexagonal, cubic, lamellar and sponge phases. The internal nano-structure of the amphiphile self assembly materials play an important role in the properties of these systems and their application. To date small angle x-ray scattering (SAXS) has been the most common technique used to characterise their structure. Positron annihilation lifetime spectroscopy (PALS) offers a possible alternative technique as it is sensitive to both the internal cavities and the intermolecular forces and in combination with SAXS, may provide more detailed structural information such as trends with composition and temperature variations. The phase behaviour of a bulk phytantriol sample, consisting of 33 % w/w water was explored using PALS, and it was found that PALS was sensitive to phase transitions from bicontinuous cubic (Pn3m) to reversed hexagonal (H2) to reversed micellar (L2) phases. These boundaries agreed well with SAXS data. Trends observed for the PALS parameters ?3 and I3 as a function of temperature largely supports the concept that the ortho-positronium is annihilating in the organic regions of the self-assembled structure. However, further investigation is required. We have also developed an innovative data analysis technique to analyse PALS spectra for pore information, with the aim of minimising operator bias and error, which leads to better quantitative comparison of PALS results between laboratories.

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

2007-12-01

43

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

Microsoft Academic Search

A relatively new method of materials structure analysis --- positron annihilation spectroscopy (PAS) --- is reviewed. Measurements of positron lifetimes, the determination of positron 3gamma- and 2gamma-annihilation probabilities, and an investigation of the effects of different external factors on the fundamental characteristics of annihilation constitute the basis for this promising method. The ways in which the positron annihilation process operates

Viktor I. Grafutin; Evgenii P. Prokop'ev

2002-01-01

44

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

45

Detection of atomic scale changes in the free volume void size of three-dimensional colorectal cancer cell culture using positron annihilation lifetime spectroscopy.  

PubMed

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

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

2014-01-01

46

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

PubMed Central

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

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

2014-01-01

47

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

NASA Astrophysics Data System (ADS)

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

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

1999-08-01

48

Free volume anomalies in mixed-cation glasses revealed by positron annihilation lifetime spectroscopy (PALS).  

PubMed

PALS experiments reveal a minimum in ortho-positronium (o-Ps) lifetimes and a maximum in the corresponding intensities that emerge when mixed-cation (Li/Na) borate glasses are heated from ambient temperatures up to 473 K. These free volume 'anomalies' appear to be a true manifestation of the mixed alkali effect (MAE). They are consistent with a mechanism of ion transport involving cooperation between hops of unlike cations, resulting in increased disturbance of the glass network. The result lends support to the dynamic structure model. PMID:19787916

Ingram, Malcolm D; Pas, Steven J; Cramer, Cornelia; Gao, Yong; Hill, Anita J

2005-04-21

49

The annihilation of galactic positrons  

NASA Technical Reports Server (NTRS)

The annihilation of galactic positrons is studied in order to evaluate the probabilities of various channels of annihilation and to calculate the spectrum of the resulting radiation. The narrow width (FWHM less than 3.2 keV) of the 0.511-MeV line observed from the galactic center by Leventhal, McCallum, and Stang (1978) implies that a large fraction of positrons should annihilate in a medium of temperature less than 100,000 K and ionization fraction greater than 0.05. H II regions at the galactic center could be possible sites of annihilation.

Bussard, R. W.; Ramaty, R.; Drachman, R. J.

1979-01-01

50

Nuclear excitation by positron annihilation  

SciTech Connect

Experimental and theoretical studies of the excitation of low-lying states of the nuclei /sup 113//sup ,//sup 115/In, /sup 111/Cd, /sup 103/Rh, /sup 107//sup ,//sup 109/Ag, /sup 176/Lu, and /sup 197/Au by the annihilation of positrons with atomic shell electrons are reviewed. Neutron emission from the /sup 9/ Be nucleus due to nonradiative annihilation of positrons is considered. A new phenomenon: excitation of the daughter nucleus by positron annihilation in ..beta../sup +/ decay: is discussed.

Vishnevskii, I.N.; Zheltonozhskii, V.A.; Kolomiets, V.M.

1988-03-01

51

Effect of Voids on Angular Correlation of Positron Annihilation Photons in Molybdenum  

Microsoft Academic Search

POSITRON annihilation investigations of defects in crystals have shown that for sufficiently high defect concentrations (typically above about 10-6) all positrons become trapped in the defects before annihilation, thus changing the characteristics of the annihilation process. For example, trapping of positrons may result in the increase in the positron lifetime, a narrowing of the 2-gamma angular correlation distribution, and a

O. Mogensen; K. Petersen; R. M. J. Cotterill; B. Hudson

1972-01-01

52

Effect of interfacial interaction on free volumes in phenol-formaldehyde resin-carbon nanotube composites: positron annihilation lifetime and age momentum correlation studies.  

PubMed

The phenol-formaldehyde-carbon nanotube composites were characterized for their free volume properties and interfacial interactions between nanotubes and the polymer matrix. The base polymeric material was a novolac type phenol-formaldehyde (PF) condensation resin cross-linked with para-toluene sulfonic acid. Multi-wall carbon nanotubes (MWCNTs) were synthesized using a catalytical chemical vapor deposition method and characterized using high-resolution transmission electron microscopy. The PF resin-carbon nanotubes composites having 2, 5, 10 and 20% (w/w%) MWCNTs were prepared. The crystallinity and morphology of the samples were characterized using X-ray diffraction and scanning electron microscopy. The free volume size in the polymer nanocomposites was observed to increase with the increase in nanotube content. Positron age momentum correlation (AMOC) studies revealed the electronic environment around different positron annihilation sites. The studies showed that ortho-positronium principally annihilates from interfacial regions of polymer and nanotubes in the nanocomposite. The positron lifetime studies together with AMOC measurements indicate an increase in the free volumes at the interface of polymer and MWCNTs in the composite. The free positron intensities showed that the polymer and nanotubes are weakly interacting in this system. PMID:22688656

Sharma, S K; Prakash, J; Sudarshan, K; Maheshwari, P; Sathiyamoorthy, D; Pujari, P K

2012-08-21

53

Iodine-doped polyvinylalcohol using positron annihilation spectroscopy  

Microsoft Academic Search

Iodine-doped polyvinylalcohol, doped up to 52 wt %, was studied using positron annihilation lifetime technique and Doppler broadening of annihilation radiation. Three component, free fit analysis was used for the lifetime spectra. It is found that I3 drops sharply on initial iodine doping, whereas decrease in tau3 is small, indicating inhibition of positronium formation by the dopant. The S parameter,

Blaise Lobo; M. R. Ranganath; T. S. G. Ravi Chandran; G. Venugopal Rao; V. Ravindrachary; S. Gopal

1999-01-01

54

Time dependent diffusion and annihilation of positrons  

SciTech Connect

The time-dependent diffusion equation of positrons implanted epithermally in a semi-infinite solid has been solved using a Green's function method. Subsequent lifetime spectra, deriving from the annihilation of free thermal and surface trapped positrons, and from para-Ps, have been modeled. The resulting curves resemble a sum of two exponential components. Because of the very short times involved, the contribution due to non-thermal positrons reaching the surface is much less significant than for steady-state models. At room temperature reflection of thermal positrons at the surface has very little effect on the solution to the diffusion equation. For strongly reflecting surfaces the sink and annihilation rates are qualitatively similar to those for a transparent surface, although strongly reduced.

Britton, D.T. (Institut fuer Nukleare Festkoerperphysik, Universitaet der Bundeswehr, Muenchen, D-8014 Neubiberg (Germany))

1991-02-01

55

Positron Annihilation in Aqueous Solutions  

Microsoft Academic Search

The angular correlation of 2-quantum emission from the annihilation of positrons in different materials has been measured in an apparatus with 8 photon counters providing coincidences for 16 output channels. As positron targets were chosen indium, water and aqueous solutions of 5 paramagnetic salts and 10 other substances. It was confirmed that the amount of singlet positronium formed is influenced

Georg Trumpy

1960-01-01

56

Positron Annihilation - A Nondestructive Probe for Materials Science.  

National Technical Information Service (NTIS)

This paper presents information on the use of positron annihilation in studying defects in metals and ionic compounds. The major emphasis is on lifetimes and comparatively little attention is devoted to angular correlation, which gives a considerable deta...

J. T. Waber

1982-01-01

57

Infrared, Raman, 1H NMR, thermal and positron annihilation lifetime studies of Pb(II), Sn(II), Sb(III), Bi(III)-barbital complexes  

NASA Astrophysics Data System (ADS)

Metal complexes of Pb(II), Sn(II), Sb(III) and Bi(III) with a barbital sodium were synthesized and characterized by several techniques, including elemental analysis (C, H and N), molar conductance measurements, infrared, Raman, 1H NMR, positron annihilation lifetime and thermogravimetric analysis. Reactions of barbital sodium (NaL) with salts of Pb(NO3)2, SnCl2?2H2O, SbCl3 and BiCl3 affords four novel mononuclear complexes [Pb(HL)2], [Sn(L)(H2O)], [Sb(HL)(L)] and [Bi(HL)(L)]. In complexes of Pb(II), Sb(III) and Bi(III), the molar ratio of metal-to-ligand is 1:2. Both of Sb(III) and Bi(III) complexes have six coordination via two molecules of barbital (HL and L), one of them deprotonated NH. The Pb(II) complex has a central metal ion adopts tetradentate fashion which surrounded by two (HL) barbital moieties. The elemental analysis shows that Sn(II) complex is tetradentate 1:1 ratio, chelated through oxygen sbnd O of (ONa), deprotonated (sbnd NH) and one coordinated water molecule. The positron annihilation lifetime parameters were found to be dependent on the structure, electronic configuration and molecular weight of metal complexes.

Refat, Moamen S.; Sharshar, T.

2012-05-01

58

Positron annihilation spectroscopy to study nanoprecipitations in aluminum alloys  

NASA Astrophysics Data System (ADS)

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

Meyendorf, Norbert; Dlubek, Gunter; Surkov, Alexander

2004-07-01

59

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

60

Low-temperature positron annihilation  

NASA Technical Reports Server (NTRS)

For a satisfactory understanding of astrophysical annihilation radiation, especially that observed from the galactic center direction, the interaction of positrons with the ambient medium must be carefully investigated. Although hot, ionized regions may be important sources of annihilation radiation, the simpler processes occurring in low-temperature neutral hydrogen gas are mainly addressed. The goal is to set limits on conditions in the annihilation region by using the predictions of atomic theory compared with the observed gamma-ray line width, continuum strength and time dependence.

Drachman, R. J.

1983-01-01

61

Positron lifetime setup based on DRS4 evaluation board  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

62

Positron Annihilation in Diatomic Crystals  

Microsoft Academic Search

The angular correlation between the two photons created by annihilating positron-electron pairs in lattices with a basis is derived in various approximations. The effects of temperature as introduced through the momentum distribution of the positrons and through lattice vibrations are examined. Self-consistent electron wave functions with exchange are calculated for the LiH crystal in the cell approximation. The resulting x-ray

Werner Brandt; Leslie Eder; Stig Lundqvist

1966-01-01

63

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

64

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

65

Applications of positron annihilation spectroscopy in materials research  

NASA Technical Reports Server (NTRS)

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

Singh, Jag J.

1988-01-01

66

Positron annihilation induced Auger electron emission  

Microsoft Academic Search

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

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

1988-01-01

67

Positron annihilation in the interstellar medium  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

68

Positron annihilation spectroscopy with magnetically analyzed beams  

NASA Technical Reports Server (NTRS)

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

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

1982-01-01

69

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

70

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

71

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

72

TEMPERATURE EFFECT ON POSITRON ANNIHILATION IN CONDENSED MATTER  

Microsoft Academic Search

An experimental investigation has been made of the temperature effect on ; the angular distribution of radiation from positron annihilation in matter. The ; materials chosen were Tefion, fused quartz, naphthalene, and water, for which the ; most comprehensive studies or the long-lifetime (Tâ) component and its ; intensily under various conditions have previously been made. The correlation ; between

R. L. de Zafra; W. T. Joyner

1958-01-01

73

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

74

Positron annihilation induced Auger electron spectroscopy  

NASA Technical Reports Server (NTRS)

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

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

1990-01-01

75

Nondestructive examination using neutron activated positron annihilation  

DOEpatents

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

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

2001-01-01

76

Positron annihilation study of microvoids in centrifugally atomized 304 stainless steel  

Microsoft Academic Search

Positron trapping in microvoids was studied by positron-lifetime and positron Doppler line-shape measurements of centrifugally atomized 304 stainless-steel powder, which was hot-isostatically-press consolidated. This material contained a concentration of several times 1023\\/m3 of 1.5-nm-diam microvoids. Positron annihilation was strongly influenced by the microvoids in that a very long lifetime component ?3 of about 600 ps resulted. The intensity of the

J. Y. Kim; J. G. Byrne

1993-01-01

77

Positron annihilation in gamma-ray bursts  

NASA Technical Reports Server (NTRS)

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

Harding, Alice K.

1990-01-01

78

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

79

Positron Annihilation Study of Y1Ba2Cu306+delta Superconducting Ceramics.  

National Technical Information Service (NTIS)

The positron annihilation lifetime spectra of Y1Ba2Cu3O(6 + delta) (0 < delta < 1.0) samples with different oxygen contents were measured. The relationship between positron lifetime and the oxygen vacancy in the ceramics was discussed, based on the positr...

A. Chen Y. Zhi B. R. Li X. L. Zhang X. H. Li S. J. Wang

1992-01-01

80

Review Positron annihilation in fine-grained materials and fine powders—an application to the sintering of metal powders  

Microsoft Academic Search

We consider the specific problem of the influence of an inhomogeneous distribution of defects in solids on positron annihilation characteristics. In detail, we investigate the effect of micro-structure, i.e. dislocations, vacancies, vacancy clusters, grain and subgrain boundaries, pores or inner surfaces, on positron lifetime spectroscopy. Only few materials show such small grain sizes that positron annihilation is affected. One example

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

1999-01-01

81

Theory of Positron Annihilation on Molecules  

NASA Astrophysics Data System (ADS)

Recently there has been a rapid progress in understanding enhanced positron annihilation on polyatomic molecules. Building on the hypothesis about the role of vibrational Feshbach resonances [1] and their first observations in alkanes [2], positron binding energies have been determined for many molecules [3]. Also, first calculations of resonant annihilation have been performed and showed excellent agreement with the measured annihilation rates in methyl halides [4]. I will review the current theoretical understanding of the two annihilation mechanisms, direct and resonant. While the complete problem of positron-molecule annihilation is very complex, its various aspects can be modeled by relatively simple means. Thus, by using zero-range potentials we can study the scaling of the positron binding energy with the size of the molecule. For small polyatomics with infrared active modes (e.g., methyl halides), a complete calculation of resonant annihilation can be done, in good agreement with experiment. Applying this theory to other molecules highlights the role of overtones and combination vibrations, and ultimately, intramolecular vibrational redistribution. -3pt-12pt *G. F. Gribakin, Phys. Rev. A 61, 022720 (2000). *S. J. Gilbert et al., Phys. Rev. Lett. 88, 043201 (2002); L. D. Barnes, S. J. Gilbert, and C. M. Surko, Phys. Rev. A 67, 032706 (2003). *L. D. Barnes, J. A. Young, and C. M. Surko, Phys. Rev. A 74, 012706 (2006). *G. F. Gribakin and C. M. R. Lee, Phys. Rev. Lett. 97, 193201 (2006).

Gribakin, Gleb

2007-10-01

82

Observations of Galactic Positron Annihilation Radiation  

NASA Astrophysics Data System (ADS)

Positron annihilation radiation has been detected from the direction of the galactic center for over 20 years. Early observations indicated that the emission was time variable, suggesting that the positrons were produced by a compact source. By the late 1980's it became clear that there was a significant diffuse source of positron annihilation radiation, in addition to a possible compact source. Observations by the OSSE instrument on NASA's Gamma Ray Observatory have greatly increased our understanding of the source and distribution of galactic positrons. The annihilation radiation is found to follow a 2-component, nova-like distribution consisting of a galactic disk and a nuclear bulge component. The disk component, which represents only ~ 15-20% of the total flux, may be completely due to the radioactive decay of \\Al26; however, the source of the bulge component is uncertain. The same 2-component distribution is also found to be consistent with most of the earlier observations by other instruments, suggesting that the emission may be entirely diffuse in origin. A description of the distribution of galactic positron annihilation radiation, limits to variability of the emission and a discussion of the possible sources of the bulge positrons will be presented.

Purcell, W. R.; Grabelsky, D. A.; Matz, S. M.; Skibo, J. G.; Dixon, D. D.; Johnson, W. N.; Kinzer, R. L.; Kurfess, J. D.

1995-12-01

83

Studies of light alloys by positron annihilation techniques  

Microsoft Academic Search

The potential of positron annihilation spectroscopy (PAS) in the study of light alloys is illustrated with special regards to age-hardening, severe plastic deformation, fatigue and fracture in aluminium- and magnesium-based alloys. First, the physical grounds of PAS sensitivity to open-volume defects are explained. Then the main conventional variants of PAS, lifetime spectroscopy and Doppler-broadening spectroscopy, are introduced. State-of-the-art equipment, based

A.. Dupasquier; G. Koegel; A. Somoza

2004-01-01

84

Positron annihilation in polyvinylalcohol doped with CuCl 2  

Microsoft Academic Search

Positron annihilation lifetime spectra were measured forpolyvinylalcohol (PVA) doped with CuCl2 (0.5 to 5.0 wt%) at temperature range from room temperature to 160°C. For a fresh pure PVA (without annealing) 3 belowT\\u000ag was larger in the heating runs than in the cooling runs, but aboveT\\u000ag, 3 was the same for both runs. The larger 3 values in the

Hamdy F. M. Mohamed; Y. Ito; A. M. A. El-Sayed; E. E. Abdel-Hady

1996-01-01

85

Positron annihilation study on age-hardened Al alloys.  

National Technical Information Service (NTIS)

Positron annihilation measurements are used to characterize the precipitation phenomena during the age hardening process of the 7012, 2219 and 2014 Al alloys. The response of the positron annihilation parameters during the ageing process can be explained ...

P. Bartolomei M. Biasini M. Valli S. Abis A. Dupasquier

1991-01-01

86

Increased Elemental Specificity of Positron Annihilation Spectra  

Microsoft Academic Search

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

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

1996-01-01

87

Positron annihilation at the Galactic center  

NASA Technical Reports Server (NTRS)

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

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

1987-01-01

88

Positron Annihilation Studies of Mesoporous Silica MCM-41  

NASA Astrophysics Data System (ADS)

Positron annihilation has been used to study the mesoporous silica MCM-41. Lifetime spectra of evacuated MCM-41 indicate a significant contribution from 3? annihilation events with ?4 = 116 ns and I4 = 24.5 %. This is supported by measurements of the full energy distribution, where MCM-41 shows enhanced counts in the low energy region (below 511 keV) relative to a pure 2? sample. MCM-41 was also studied under air and oxygen atmospheres. The presence of atmosphere has a significant effect on both the lifetime and Doppler patterns, with both the lifetime data (?4 and I4) and the 3?-fraction decreasing with increasing oxygen concentration. This is indicative of paramagnetic quenching of o-Ps by oxygen.

Williams, J. F.; Guagliardo, P.; Sudarshan, K.; Ranganathaiah, C.; Koutsantonis, G.; Hondow, N.; Samarin, S.

2013-06-01

89

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

NASA Astrophysics Data System (ADS)

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

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

1985-09-01

90

Positron annihilation gamma rays from novae  

NASA Technical Reports Server (NTRS)

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

Leising, Mark D.; Clayton, Donald D.

1987-01-01

91

Moisture dependence of positron annihilation rates in molecular substances  

NASA Technical Reports Server (NTRS)

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

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

1982-01-01

92

Moisture dependence of positron annihilation rates in molecular substances  

SciTech Connect

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

Singh, J.J. (NASA, Langley Research Center, Hampton, VA); Holt, W.H. (U.S. Navy, Naval Surface Weapons Center, Dahlgren, VA)

1982-03-01

93

Microstructural Characterization of Thin Polyimide Films by Positron Lifetime Spectroscopy  

NASA Technical Reports Server (NTRS)

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

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

1996-01-01

94

Free volumes and holes near the polymer surface studied by positron annihilation  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime and Doppler broadening of annihilation radiation experiments have been performed in a polystyrene film using the mono-energetic slow positron probe as a function of implantation energy. Significant variations of positron annihilation signals are observed at a short distance from the surface (?200 Å). The ortho-positronium lifetime in the polymer increases near the surface, while its intensity decreases. The intensity results are interpreted in terms of a free-volume hole model for positronium formation. The lifetime results are interpreted as an expansion of local hole volume near the polymer surface. The free-volume and hole distribution near the surface is found to be broader than in the bulk. Applications of slow positrons to industrial problems, such as degradation of polymer coatings and membrane separations, are promising.

Cao, H.; Yuan, J.-P.; Zhang, R.; Sundar, C. S.; Jean, Y. C.; Suzuki, R.; Ohdaira, T.; Nielsen, B.

1999-08-01

95

Study on radiation aging of polymeric insulating material by positron annihilation  

Microsoft Academic Search

Structure changes of fluoropolymer, a heat-resistant insulating material, after ?-ray irradiation, were investigated using positron annihilation. The correlation between structure changes and dielectric breakdown strength was studied. The dielectric breakdown strength is found to decrease with increasing dose. Positron lifetime data showed that crystallinity increased with irradiation. As the crystalline region is increased by irradiation, it is easy to accelerate

S. Fujita; M. Baba; F. Noto; T. Suzuki; H. Kitahara

1993-01-01

96

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

97

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

PubMed Central

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

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

2013-01-01

98

Slow positron beam generator for lifetime studies  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

99

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

100

Positron annihilation radiation from solar flares  

NASA Technical Reports Server (NTRS)

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

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

1983-01-01

101

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

102

Positron scattering and annihilation from hydrogenlike ions  

SciTech Connect

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

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

2004-05-01

103

Microemulsion systems studied by positron annihilation techniques  

Microsoft Academic Search

The formation of thermalized positronium atoms is greatly reduced if increasing amounts of water become solubilized in reversed micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate in apolar solvents. Similar observations have been made if the surfactant is Triton X-100. The application of the positron annihilation technique to the study of microemulsions consisting of potassium oleate-alcohol-oil-water mixtures indicates, consistent with previous results,

Ali Boussaha; Belkacem Djermouni; Luz Alicia Fucugauchi; Hans J. Ache

1980-01-01

104

Positron annihilation study of P implanted Si  

SciTech Connect

High-energy ion implantation (above 200 keV) is now commonly used in a variety of VLSI processes. The high energy required for these implants is often achieved by implanting multiply charged ions, which inevitably brings in the problem of low-energy ion contamination. The low-energy contamination is difficult to diagnose and detect. Positron annihilation spectroscopy is used to examine the defect distributions in these high energy implants with varying degrees of contamination.

Asoka-Kumar, P.; Au, H.L.; Lynn, K.G. (Brookhaven National Lab., Upton, NY (United States)); Sferlazzo, P. (Eaton Corp., Beverly, MA (United States). SED Division)

1992-01-01

105

Positron annihilation study of P implanted Si  

SciTech Connect

High-energy ion implantation (above 200 keV) is now commonly used in a variety of VLSI processes. The high energy required for these implants is often achieved by implanting multiply charged ions, which inevitably brings in the problem of low-energy ion contamination. The low-energy contamination is difficult to diagnose and detect. Positron annihilation spectroscopy is used to examine the defect distributions in these high energy implants with varying degrees of contamination.

Asoka-Kumar, P.; Au, H.L.; Lynn, K.G. [Brookhaven National Lab., Upton, NY (United States); Sferlazzo, P. [Eaton Corp., Beverly, MA (United States). SED Division

1992-12-01

106

Positron annihilation radiation from the Galactic center region  

NASA Technical Reports Server (NTRS)

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

Ramaty, R.; Lingenfelter, R. E.

1991-01-01

107

Defects in FZ-silicon after neutron irradiation—A positron annihilation and photoluminescence study  

Microsoft Academic Search

Float-zone (FZ) Si irradiated with 1 MeV neutrons was investigated by means of positron annihilation lifetime spectroscopy and photoluminescence. Three types of defects were observed: di-vacancies, small vacancy clusters and an unknown defect with the defect-related lifetime of (285 ± 5) ps that contribute to positron trapping only at low temperatures. Two annealing stages were observed: one at 350°C and

V. Bondarenko; R. Krause-Rehberg; H. Feick; C. Davia

2004-01-01

108

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

Microsoft Academic Search

Positron annihilation lifetime and Doppler-broadening measurements were performed on n-dotriacontane (n-C32H66) and n-tritriacontane (n-C33H68) between 80 K and their melting points. The annihilation parameters plotted as a function of temperature show irregularities above the room temperature, which reflect solid–solid phase transitions in these materials. The transition temperatures derived from the positron annihilation measurements are in excellent agreement with literature values

B. Le´vay; M. Lalovic´; H. J. Ache

1989-01-01

109

Evaluation of the reactor pressure vessel steels by positron annihilation  

NASA Astrophysics Data System (ADS)

This paper presents a comparison of commercially used German and Russian reactor pressure vessel steels from the positron annihilation spectroscopy (PAS) point of view, having in mind knowledge obtained also from other techniques from the last decades. The second generation of Russian RPV steels seems to be fully comparable with German steels and their quality allows prolongation of NPP operating lifetime over projected 40 years. The embrittlement of CrMoV steels is relatively low due to effect of higher temperature which implies partial in situ annealing of primary microstructural point defects and therefore delays the degradation processes caused by neutron irradiation.

Sluge?, V.; Hein, H.; Sojak, S.; Simeg Veterníková, J.; Petriska, M.; Sabelová, V.; Pavúk, M.; Hinca, R.; Stacho, M.

2013-11-01

110

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

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

111

Positron Lifetime Measurements in Natural Rubber with Different Fillers  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectra (PLAS) have been measured for natural rubber polymer with different fillers (Titenium dioxide, Nanosilica and Nanoclay) as a function of filler concentration to investigate how these fillers affect the microstructure of free volume of natural rubber. The lifetime spectra is analyzed by using LT9.0 and the longest lived component(?o-Ps) is attributed to the pick- off annihilation of o-Ps in free volume sites, available mostly in the amorphous region of polymer. On the basis of the ?o-Ps values the radii of the free volume holes (Rh) are calculated. The PALS results show that o-Ps lifetime as well as the size of free volume decreases with the increase of filler concentration.

Mandal, A.; Mukherjee, S.; Pan, S.; Sengupta, A.

112

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

113

Positron annihilation radiation from neutron stars  

NASA Technical Reports Server (NTRS)

Balloon observations of a low energy flux line at 2 x 0.001 photons cm/2 s/1 from the galactic center region around 470 keV are interpreted as the positron annihilation radiation that occurs on the surface of old neutron stars and is redshifted by their gravitational fields. An astrophysical model is formulated to explain the observed flux that provides for about 0.2 neutron stars per 3 x 10 to the 49th power cubic meters assuming that the disk thickness is about 1.5 x 10 to the 19th power meters.

Borner, G. A.

1973-01-01

114

Applications of positron annihilation spectroscopy to polymeric membranes  

Microsoft Academic Search

Positron annihilation spectroscopy (PAS) coupled with a slow positron beam has been used to study the freevolume depth profile (0–10 ?m) in polymeric membrane systems prepared by the interfacial polymerization method. Doppler broadening energy parameters of annihilation radiation vary as a function of the depth from the surface through multi-layers in polyamide asymmetric membranes prepared under different experimental parameters, such

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

2008-01-01

115

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

116

Studying the recovery of as-received industrial Al alloys by positron annihilation spectroscopy  

Microsoft Academic Search

Positron annihilation lifetime (PAL) spectroscopy, Doppler broadening of annihilation radiation (DBAR) spectroscopy and Vickers microhardness (Hv) measurements were performed to study the micro- and macro-structure variations during isochronal annealing from room temperature (RT) to 500°C of commercial pure Al (1100), Al–Mn–Mg (3004) and Al–Mg–Si (6201) alloys. Three annealing stages of microstructures have been identified as recovery, partial recrystallization and complete

E. E. Abdel-Hady; A. Ashry; H. Ismail; S. El-Gamal

2006-01-01

117

Role of vibrational dynamics in resonant positron annihilation on molecules.  

PubMed

Vibrational Feshbach resonances are dominant features of positron annihilation for incident positron energies in the range of the molecular vibrations. Studies in relatively small molecules are described that elucidate the role of intramolecular vibrational energy redistribution into near-resonant multimode states, and the subsequent coupling of these modes to the positron continuum, in suppressing or enhancing these resonances. The implications for annihilation in other molecular species, and the necessary ingredients of a more complete theory of resonant positron annihilation, are discussed. PMID:23767720

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

2013-05-31

118

Positron annihilation in cardo-based polymer membranes.  

PubMed

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

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

2014-06-01

119

PATFIT-88: A DATA-PROCESSING SYSTEM FOR POSITRON ANNIHILATION SPECTRA ON MAINFRAIME AND PERSONAL COMPUTERS  

Microsoft Academic Search

A data processing system has been developed for analyzing positron annihilation lifetime and angular correlation spectra on main- frame and Personal Computers (PCs). The system is based on the PATFIT programs previously developed for use on mainframe computers. It con- sists of the three fitting programs POSITRONFIT, RESOLUTION and ACARFIT and three associated programs for easy editing of the input

Peter Kirkegaard; Niels Jørgen Pedersen; Morten Eldrup

2003-01-01

120

Radiation effect on polypropylene studied by the relaxational behaviour at low temperature using positron annihilation  

Microsoft Academic Search

The irradiation effect of ?-rays on polypropylene (PP) has been studied through relaxational behaviour, using the positron annihilation lifetime (PAL) method. At low temperature, the intensity of a long-lived component of positronium, I3, for unirradiated PP samples increased due to a termination of the local thermal motion of polymer structures. At 100 K, the increase in I3 of 1 MGy

Yasuo Ito; Y OKI; M NUMAJIRI; T MIURA; K KONDO; N OSHIMA; Y ITO

1996-01-01

121

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

SciTech Connect

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

Chakraborty, B.

1982-06-01

122

Development of positron annihilation spectroscopy to test accelerated weathering of protective polymer coatings  

Microsoft Academic Search

A variable mono-energetic positron beam with a computer-controlled system has recently been constructed at the University of Missouri–Kansas City for weathering studies of polymeric coatings. The beam is designed to measure the S-parameter from Doppler-broadening energy spectra and the sub-nanometer defect properties from positron annihilation lifetimes (PAL). Significant variations of S-parameter and ortho-positronium intensity in coatings, as obtained from the

R. Zhang; H. Cao; H. M. Chen; P. Mallon; T. C. Sandreczki; J. R. Richardson; Y. C. Jean; B. Nielsen; R. Suzuki; T. Ohdaira

2000-01-01

123

Irradiation-induced defects in ZnO studied by positron annihilation spectroscopy  

Microsoft Academic Search

We have used positron annihilation spectroscopy to study the point defects induced by 2 MeV electron irradiation (fluence 6 × 1017 cm-2) in single crystal n-type ZnO samples. The positron lifetime measurements have shown that the zinc vacancies in their doubly negative charge state, which act as dominant compensating centers in the as-grown material, are produced in the irradiation and

F. Tuomisto; K. Saarinen; D. C. Look

2004-01-01

124

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

125

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

126

Defects in electron-irradiated Si studied by positron-lifetime spectroscopy  

Microsoft Academic Search

Differently doped and undoped silicon was irradiated with electrons to study the formation of nonequilibrium defects and their annealing behavior. The annealing curves, measured by positron lifetime and also partly by the shape parameters of the Doppler-broadened annihilation line, depend strongly on the doping concentration and the oxygen content. In addition, temperature-dependent positron-lifetime measurements starting at 15 K were performed

A. Polity; F. Börner; S. Huth; S. Eichler; R. Krause-Rehberg

1998-01-01

127

Iodine-doped polyvinylalcohol using positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Iodine-doped polyvinylalcohol, doped up to 52 wt %, was studied using positron annihilation lifetime technique and Doppler broadening of annihilation radiation. Three component, free fit analysis was used for the lifetime spectra. It is found that I3 drops sharply on initial iodine doping, whereas decrease in ?3 is small, indicating inhibition of positronium formation by the dopant. The S parameter, on iodine doping, initially shows a drastic increase due to the decrease in crystallinity of the doped sample. Differential scanning calorimetry (DSC) and x-ray diffraction curves confirm that doping results in decreased crystallinity of the sample. ?3 increases beyond 23 wt % doping level due to structural changes induced by formation of polyiodide complexes, which is confirmed by UV-Vis spectra, for moderately and highly doped samples. The drop in ?3 at higher levels of doping (>44 wt %) suggests chemical quenching of positronium, probably due to iodine aggregation. DSC confirms formation of iodine aggregates, as revealed by a large endotherm at 185 °C, for doping levels beyond 47 wt %.

Lobo, Blaise; Ranganath, M. R.; Chandran, T. S. G. Ravi; Venugopal Rao, G.; Ravindrachary, V.; Gopal, S.

1999-06-01

128

Positron annihilation studies of moisture in graphite-reinforced composites  

NASA Technical Reports Server (NTRS)

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

1980-01-01

129

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 (Cr 2O 3) 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

130

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 × 10 18 n/cm 2 at about 150°C or with 3 MeV electrons up to a dose of 1 × 10 18 e/cm 2 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.; Fujinami, M.; Ito, Y.; Sunaga, H.; Okada, S.; Yamaguchi, S.

1996-08-01

131

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

132

The Correlation Between Dislocations and Vacancy Defects Using Positron Annihilation Spectroscopy  

NASA Astrophysics Data System (ADS)

An analysis program for positron annihilation lifetime spectra is only applicable to isolated defects, but is of no use in the presence of defective correlations. Such limitations have long caused problems for positron researchers in their studies of complicated defective systems. In order to solve this problem, we aim to take a semiconductor material, for example, to achieve a credible average lifetime of single crystal silicon under plastic deformation at different temperatures using positron life time spectroscopy. By establishing reasonable positron trapping models with defective correlations and sorting out four lifetime components with multiple parameters, as well as their respective intensities, information is obtained on the positron trapping centers, such as the positron trapping rates of defects, the density of the dislocation lines and correlation between the dislocation lines, and the vacancy defects, by fitting with the average lifetime with the aid of Matlab software. These results give strong grounds for the existence of dislocation-vacancy correlation in plastically deformed silicon, and lay a theoretical foundation for the analysis of positron lifetime spectra when the positron trapping model involves dislocation-related defects.

Pang, Jinbiao; Li, Hui; Zhou, Kai; Wang, Zhu

2012-07-01

133

Moisture dependence of positron annihilation spectra in nylon-6  

NASA Technical Reports Server (NTRS)

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

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

1984-01-01

134

Studies of light alloys by positron annihilation techniques  

SciTech Connect

The potential of positron annihilation spectroscopy (PAS) in the study of light alloys is illustrated with special regards to age-hardening, severe plastic deformation, fatigue and fracture in aluminium- and magnesium-based alloys. First, the physical grounds of PAS sensitivity to open-volume defects are explained. Then the main conventional variants of PAS, lifetime spectroscopy and Doppler-broadening spectroscopy, are introduced. State-of-the-art equipment, based on intense positron sources and energy-controlled beams, is also described, in view of applications where microscopic spatial resolution and sub-nanosecond time resolution are combined. Various examples of PAS studies in the field of light alloys, mainly based on the latest experience of the authors, are presented. It is shown how PAS detects structural changes in age-hardenable alloys, helps to describe the solute aggregation kinetics and gives information on vacancy-solute interactions. PAS characterisation of internal surfaces (misfit interfaces and grain boundaries) in terms of local structure (degree of disorder, chemistry) is also discussed. Lastly, recent advances in the study of fatigue by positron microscopy are reported.

Dupasquier, A. [INFM and Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milan (Italy)]. E-mail: alfredo.dupasquier@polimi.it; Koegel, G. [Institut fuer Nukleare Festkoerperphysik, Universitaet der Bundeswehr Muenchen, 85577 Neubiberg (Germany); Somoza, A. [Instituto de Fisica de Materiales Tandil, Universidad Nacional del Centro de la Provincia de Buenos Aires and Comision de Investigaciones Cientificas de la Provincia de Buenos Aires, Pinto 399, B7000GHG Tandil (Argentina)

2004-09-20

135

Positron annihilation characteristics in mesostructural silica films with various porosities  

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

136

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

NASA Astrophysics Data System (ADS)

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 × 1017 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.

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

2012-08-01

137

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

2012-08-15

138

Direct positron annihilation and positronium formation in thermal plasmas  

NASA Technical Reports Server (NTRS)

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

Gould, Robert J.

1989-01-01

139

Primary cosmic ray positrons and galactic annihilation radiation  

NASA Technical Reports Server (NTRS)

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

Lingenfelter, R. E.; Ramaty, R.

1980-01-01

140

Ion acceleration and positron production and annihilation in solar flares  

NASA Astrophysics Data System (ADS)

I first review: a) the current state of knowledge of ion acceleration in solar flares; b) the physics of positron production and annihilation; and c) recent RHESSI data on solar flare annihilation radiation. I then show how the modeling of the positron production and annihilation in the chromosphere, coupled with the newly available high-resolution data on the 511 keV annihilation line, can have important physical implications w. r. t. the models: a) information on the temperature and density of the chromosphere; b) constraints on some of the physical characteristics of the flare and to some extent on the acceleration process.Although I do mention past instruments (SMM and Yohkoh), this review focuses on the RHESSI satellite, considering the quantum leap it has constituted in the quality of the data it is providing and consequently the constraints it can place on models (of ion acceleration, annihilation environment, etc.).

Guessoum, Nidhal

141

Positron Annihilation Studies In Polymer Nano-Composites  

NASA Astrophysics Data System (ADS)

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.; Yang, J.; Lee, L. James

2011-06-01

142

Positron annihilation in pivalic acid. Temperature dependence of angular correlation curves  

NASA Astrophysics Data System (ADS)

Positron annihilation angular correlation curves have been measured as a function of temperature for trimethylacetic (pivalic) acid in both the brittle and plastic phases. A simple fitting of the data to a sum of three gaussians shows the presence of a narrow component due to para-positronium ( p-Ps) annihilation. In the brittle phase the intensity of the narrow component is inconsistent with previous positron lifetime data. A more detailed analysis, requiring consistency with the lifetime data, results in the determination of the shapes of the angular correlation components for free positron-, pick-off-, and p-Ps intrinsic annihilation. The p-Ps component has a width (fwhm) of 3.75 mrad in the brittle phase, probably due to Ps self-trapping or trapping in defects smaller than molecular vacancies. In the plastic phase the width (fwhm) is 3.25 mrad which is ascribed to Ps localization in vacancies and divacancies in accordance with positron lifetime data.

Jain, P. C.; Eldrup, M.; Pedersen, N. J.; Sherwood, J. N.

1986-07-01

143

Energy-resolved positron annihilation rates for molecules  

SciTech Connect

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

Barnes, L. D.; Young, J. A.; Surko, C. M. [Department of Physics, University of California, San Diego, California 92093-0319 (United States)

2006-07-15

144

Positron annihilation spectroscopic studies of solvothermally synthesized ZnO nanobipyramids and nanoparticles  

NASA Astrophysics Data System (ADS)

Zinc oxide (ZnO) samples in the form of hexagonal-based bipyramids and particles of nanometer dimensions were synthesized through solvothermal route and characterized by x-ray diffraction and transmission electron microscopy. Positron annihilation experiments were performed to study the structural defects such as vacancies and surfaces in these nanosystems. From coincidence Doppler broadening measurements, the positron trapping sites were identified as Zn vacancies or Zn-O-Zn trivacancy clusters. The positron lifetimes, their relative intensities, and the Doppler broadened lineshape parameter S all showed characteristic changes across the nanobipyramid size corresponding to the thermal diffusion length of positrons. In large nanobipyramids, vacancies within the crystallites also trapped positrons and the effects of agglomeration of such vacancies due to increased temperatures of synthesis were reflected in the variation of the annihilation parameters with their base diameters. The sizes of the nanoparticles used were all in the limit of thermal diffusion length of positrons and the annihilation characteristics were in accordance with the decreasing contribution from surfaces with increasing particle size.

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

2008-02-01

145

Resolving Nuclear Reactor Lifetime Extension Questions: A Combined Multiscale Modeling and Positron Characterization approach  

Microsoft Academic Search

The objective of this work is to determine the chemical composition of nanometer precipitates responsible for irradiation hardening and embrittlement of reactor pressure vessel steels, which threaten to limit the operating lifetime of nuclear power plants worldwide. The scientific approach incorporates computational multiscale modeling of radiation damage and microstructural evolution in Fe-Cu-Ni-Mn alloys, and experimental characterization by positron annihilation spectroscopy

B Wirth; P Asoka-Kumar; A Denison; S Glade; R Howell; J Marian; G Odette; P Sterne

2004-01-01

146

Positron annihilation spectroscopy and specific heat study of Neon ion irradiated MgB 2  

NASA Astrophysics Data System (ADS)

Specific heat studies under magnetic field and positron annihilation spectroscopy were carried out on 160 MeV Ne ion irradiated polycrystalline MgB 2 samples. There is an unusual decrease in positron lifetime in the irradiated sample which may be due to neon ion implantation. This was also indicated by change in cell volume. Coincidence Doppler Broadening Spectra of Mg, B, irradiated and unirradiated MgB 2 show that positrons primarily annihilate in boron sublattice in the unirradiated sample whereas there is some similarity of the spectrum of the irradiated sample with that of Mg. There is Mg deficiency in the unirradiated sample whereas predominantly boron vacancies exist in Ne ion irradiated MgB 2 sample. Specific heat measurements show that there is a small increase in electronic part of the specific heat and electron-phonon coupling constant.

Talapatra, A.; Bandyopadhyay, S. K.; Nambissan, P. M. G.; Sen, Pintu; Ganesan, V.

2008-02-01

147

Positron irradiation effects on polypropylene and polyethylene studied by positron annihilation  

Microsoft Academic Search

Positron irradiation effects on polypropylene (PP) and polyethylene (PE) have been studied using the positron source for positron annihilation (PA) experiments. From the intensities (I3) of the long-lived component of ortho-positronium (o-Ps), it has been shown that the irradiation effect is larger in PP than in PE and that the samples with high crystallinity receive larger effect. The positron irradiation

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

1995-01-01

148

Positron annihilation study of dopant effects on proton-irradiation defect in silicon  

Microsoft Academic Search

Positron annihilation lifetime and doppler broadening measurements have been performed to examine the effects of B, Al and Ga dopants of Czochralski-grown Si (CZ-Si) on defects induced by 10MeV proton irradiations at room temperature with a total dose from 3×1012p\\/cm2 to 5×1014p\\/cm2. Isochronal annealing experiments were also carried out after the irradiations. In spite of the high-energy proton irradiation, a

F. Hori; T. Chijiiwa; R. Oshima; T. Hisamatsu

1999-01-01

149

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

150

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

NASA Astrophysics Data System (ADS)

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

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

151

Low-temperature positron annihilation. [galactic center radiation  

NASA Technical Reports Server (NTRS)

For a satisfactory understanding of astrophysical annihilation radiation, especially that observed from the Galactic center direction, the interaction of positrons with the ambient medium must be carefully investigated. Although hot, ionized regions may be important sources of annihilation radiation, the simpler processes occurring in low-temperature neutral hydrogen gas are mainly addressed. The goal is to set limits on conditions in the annihilation region by using the predictions of atomic theory compared with the observed gamma-ray line width, continuum strength and time dependence.

Drachman, R. J.

1983-01-01

152

Positron lifetime spectroscopy in thin polymer coatings  

NASA Technical Reports Server (NTRS)

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

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

1990-01-01

153

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

SciTech Connect

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

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

2005-08-15

154

Defect study on deformed Fe-Si alloys by positron annihilation techniques  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime and coincidence Doppler broadening measurements have been performed on ultra low carbon steel and Fe-Si alloys with different Si contents (from 3.32 up to 11.1 wt%) in the annealed state and after a deformation of 5% thickness reduction. It was seen that above 5.6 wt% Si the nature of the defects trapping the positron changes. This coincides with the concentration for the onset of long range D03 ordering in the Fe-Si alloys changing the nature of the dislocation structure.

Agundez, M.; Ruiz, D.; van Petegem, S.; de Baerdemaeker, J.; Chou, K. W.; Segers, D.; Houbaert, Y.

2005-07-01

155

Diffuse Galactic Positron Annihilation Radiation and the Underlying Continuum  

Microsoft Academic Search

In this thesis we explore the origins of the diffuse Galactic positron annihilation line radiation at 0.511 MeV and the underlying gamma ray continuum. In order to determine the sources of the positrons responsible for the 0.511 MeV line emission we construct models for the spatial distribution of this emission. We find that it is not possible to model the

Jeffrey George Skibo

1993-01-01

156

Annihilation in positron-atom collisions - A new approach  

NASA Technical Reports Server (NTRS)

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

Drachman, R. J.; Sucher, J.

1979-01-01

157

Positron annihilation study on ZnO-based scintillating glasses  

Microsoft Academic Search

Positron lifetime of ZnO-based scintillating glasses (55?x)SiO2–45ZnO–xBaF2 (x=5, 10, 15mol%) were measured with a conventional fast–fast spectrometer. Three positron lifetime components ?1, ?2, and ?3 are ?0.23ns, ?0.45ns, and ?1.6ns, respectively. All the three positron lifetime components first increase with increasing BaF2 concentration from 5mol% to 10mol%, then decreases as BaF2 further increases to 15mol%. The result suggests that the

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

2009-01-01

158

Positron annihilation in the hydrogenated granular superconductor of YBa 2Cu 3O 7-?  

NASA Astrophysics Data System (ADS)

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

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

1996-02-01

159

Positron annihilation studies of recrystallization in the subsurface zone induced by friction in magnesium—effect of the inhomogeneity on measured positron annihilation characteristics  

NASA Astrophysics Data System (ADS)

The discussion of the positron annihilation studies of crystal structure defects, like vacancies, dislocations, grain boundaries and the defect depth profile, is presented. The role of the positron implantation depth and positron diffusion in such studies has been considered in detail. For description of the measured annihilation characteristics the proposed theoretical models take into account both effects. The annealing studies of defects created in pure magnesium by compression or dry sliding-wear were used for demonstration of the discussed thesis. The positron lifetime measurements were applied for monitoring open volume defects behavior. It was demonstrated that annealing at the temperature of about 300 °C removes the defects created by compression. Application of the proposed model to description of the data obtained allows to determine the activation energy of the grain boundary mobility in pure magnesium equal to Q=0.56±0.18 eV. However, defects created by the dry sliding are not completely annealed up to the temperature of 500 °C. The defect depth profile induced by dry sliding evolves with the annealing temperature in such a way that at the worn surface concentration of defects gradually decreases but at the depth between 60 and 100 ?m the generation of new defects takes place at temperature of 150 and 225 °C. Above 300 °C the defects still are extended up to the depth of about 80 ?m.

Dryzek, Jerzy

2014-02-01

160

Positron annihilation studies of an icosahedral quasicrystal and the cubic R phase of Al-Li-Cu  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime and Doppler-broadening measurements have been performed for Bridgman-grown single-grained icosahedral quasicrystal and cubic R phase of Al-Li-Cu. The results are consistent with the existence of triacontahedral clusters with the vacant center in the icosahedral phase as well as in the R phase.

Ohata, T.; Kanazawa, I.; Iwashita, T.; Kishi, K.; Takeuchi, S.

1990-10-01

161

Positron annihilation in and compressibility of water-organic mixtures: Aqueous solutions of formamide and ethylene glycol  

Microsoft Academic Search

The positron annihilation lifetime and ultrasonic velocity were measured in water+ formamide and water+ethylene glycol systems. The comparison between the concentration and temperature dependences of the results suggests that hydrophylic (substitutional) hydration plays dominating role in structuring the systems under investigation.

A. Baranowski; K. Jerie; J. Gli?ski; K. Orzechowski

1995-01-01

162

Probing the Fermi surface by positron annihilation and Compton scattering  

NASA Astrophysics Data System (ADS)

Positron annihilation and Compton scattering are important probes of the Fermi surface. Relying on conservation of energy and momentum, being bulk sensitive and not limited by short electronic mean-free-paths, they can provide unique information in circumstances when other methods fail. Using a variety of examples, their contribution to knowledge about the electronic structure of a wide range of materials is demonstrated.

Dugdale, S. B.

2014-04-01

163

Positron annihilation spectroscopy study of materials for reactor vessels  

SciTech Connect

Steels used in the nuclear industry have been experimentally studied by positron annihilation spectroscopy. Analysis of the experimental results and their comparison with the existing data make it possible to reveal vacancy defects, in particular, those caused by neutron radiation, and to determine their size.

Grafutin, V. I.; Prokop'ev, E. P. [Institute for Theoretical and Experimental Physics (Russian Federation); Krsjak, V.; Burcl, R.; Haehner, P.; Zeman, A. [Joint Research Centre of the European Commission, Institute for Energy (Netherlands); Ilyukhina, O. V., E-mail: ilyukhina@itep.ru [Institute for Theoretical and Experimental Physics (Russian Federation); Erak, D. [Russian Research Centre Kurchatov Institute (Russian Federation); Mogilevskyi, M. A.; Myasischeva, G. G.; Funtikov, Yu. V. [Institute for Theoretical and Experimental Physics (Russian Federation)

2011-02-15

164

Positron Annihilation and Electronic Lattice Structure in Insulator Crystals  

Microsoft Academic Search

The dimensions of the electronic lattice structure of the hexagonal quartz crystal and the fcc CaF2 crystal have been measured by resolving characteristic markings on the angular-correlation curves as observed for the two gamma quanta emitted in the annihilation of positrons with electrons in these crystals.

Werner Brandt; Gérard Coussot; Robert Paulin

1969-01-01

165

Structure of Copper Electrodeposits by the Positron Annihilation Method  

Microsoft Academic Search

The hydrogen state in copper electroplated in a wide range of current densities is studied by a positron annihilation method. Hydrogen atoms are located in vacancy clusters and dislocation cores (mobile hydrogen). In microcavities, hydrogen atoms form molecules, which are removed microcavities at the copper layer recrystallization temperatures.

I. I. Bardyshev; Yu. M. Polukarov

2004-01-01

166

Study of Precipitation Phenomena in Aluminum Alloys by Positron Annihilation.  

National Technical Information Service (NTIS)

To assess the sensitivity of positron annihilation (PA) to precipitation phenomena, the Doppler broadened PA lineshape was monitored as a function of isochronal annealing steps on a series of quenched aluminum alloys. A study of the Al--Cu system, includi...

W. R. Wampler W. B. Gauster

1979-01-01

167

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

NASA Technical Reports Server (NTRS)

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

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

1984-01-01

168

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

169

Positron Annihilation Spectroscopy for Chemical Analysis  

Microsoft Academic Search

The prediction and discovery of the existence of positron (e) and positronium atom (Ps) led rapidly to the development of a new spectroscopic technique for the study of the structure and composition of matter. The technique quickly expanded across interdisciplinary from physics to chemistry, metallurgy, biochemistry and recently to superconductors. As an early non-destructive and in situ testing tool, positron

K. L. Cheng; Y. C. Jean; X. H. Luo

1989-01-01

170

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

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

171

Phase transitions in physisorbed ethane investigated by positron-annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Data are presented for the lifetime and Doppler broadening parameters for positrons annihilating in ethane physisorbed on grafoil. The coverage dependence of these parameters has been investigated up to about 2.5 monolayers at 108 K. Both the positron lifetimes and Doppler broadening parameters reflect significant variations in the surface electron density as a function of adsorbate coverage, with clear evidence for minimum electron density occurring at coverages of ~0.8 and 2. The temperature dependencies of the lifetime and Doppler broadening data, studied for coverages of 0.4, 0.9, and 1.5 monolayers, show structural phase transitions in the adsorbate. We propose a mechanism for this structural phase transition at 39 K.

Jain, P. C.; Lee, S. K.; Hozhabri, N.; Sharma, S. C.

1999-07-01

172

Positron-electron annihilation radiation from the Galactic Center  

NASA Technical Reports Server (NTRS)

Evidence and arguments are presented supporting the theory that the 0.511 MeV line observed from the Galactic Center stems from photon-photon pair production near a black hole with a mass not greater than approximately 500 solar masses. Observations are reviewed and the subsequent implications on the annihilation site and the positron source are discussed. Because of the variations and line width of the e(+) - e(-) annihilation radiation from the Galactic Center, positrons must essentially be produced by a single source and annihilate in an ambient gas of density greater than 10 to the 5th/cu cm, an ionization fraction greater than 10 percent, a temperature less than 5 x 10 to the 4th K, and be confined to a region of size less than 10 to the 18th cm. Such conditions may exist in warm clouds and other IR sources within the central parsec of the Galaxy.

Ramaty, R.; Lingenfelter, R. E.

1983-01-01

173

Recent progress in understanding positron annihilation on molecules  

NASA Astrophysics Data System (ADS)

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

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

2012-10-01

174

Positron annihilation in argon intercalated n-alkanes at high pressure  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy was used to observe the effects of argon intercalation in some solid long-chain alkanes at high pressure. The ortho-Ps lifetime rises with argon pressure, which means increase of free volumes in the alkane structure. The range of pressures in which the rotator phase exists increases, comparing to pure alkane. In n-heptadecane, n-nonadecane, and possibly n-heneicosane, a stepwise change of ortho-Ps lifetime and intensity at ?12 MPa is observed, suggesting the transition to a new kind of the rotator phase. The transition rate is low, final lifetime value is ?3.3 ns. Despite a large size of free volumes corresponding to such a lifetime, their compressibility is found negligible up to the pressure of 90 MPa. At low pressures the compressibility of free volumes in the rotator phase is negative.

Zgardzi?ska, B.; Wawryszczuk, J.; Goworek, T.

2006-01-01

175

Positron annihilation response and broadband dielectric spectroscopy: salol.  

PubMed

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

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

2011-09-01

176

Monte Carlo investigation of positron annihilation in medical positron emission tomography  

NASA Astrophysics Data System (ADS)

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

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

2007-09-01

177

Nanostructure of thin amorphous hydrogenated carbon films studied by positron annihilation and photoluminescence  

NASA Astrophysics Data System (ADS)

We deposited polymer-like a-C:H films using the plasma-enhanced chemical vapor deposition technique and characterized film microstructure by variable-energy positron lifetime spectroscopy, photoluminescence (PL), and UV-visible absorption spectroscopy. It was confirmed that PL occurs from a chromophore in a sp2 cluster as a result of fast recombination of a photoexcited electron-hole pair. Positron annihilation lifetime spectroscopy showed that positronium (Ps) formation takes place via electron-positron recombination in the sp3 matrix. The lifetime of ortho-positronium (o-Ps) in our a-C:H films was similar to that in polyethylene, indicating their polymer-like nature. The relative PL efficiency increased by about an order of magnitude with increasing film band gap from 1.3 to 3.4 eV, which can be related to the decreasing concentration of nonradiative centers. On the other hand, Ps formation was much less influenced by the band gap and nonradiative centers. Comparison of this result with that for polyethylene mixed with carbon-black nanoparticles, where a considerable reduction in Ps formation was observed, showed that nonradiative centers were of a different nature from the defects on the carbon nanoparticle surface. This work demonstrated the usefulness of positron lifetime spectroscopy combined with optical measurements to study the nanostructure of a-C:H.

Wang, C. L.; Kobayashi, Y.; Katoh, R.; Suzuki, R.; Ohdaira, T.

2001-07-01

178

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 ?400 ps in bulk as-grown specimens. Such a large lifetime suggests that the positron annihilation sites in these materials are dominated by defects. Moreover, analyzing the electron momenta via coincidence Doppler broadening measurements suggests a strong presence of large open-volume defects, likely to be vacancy clusters or voids.

Abdul-Jabbar, N. M.; Bourret-Courchesne, E. D.; Wirth, B. D.

2012-08-01

179

HEAO 3 measurements of the atmospheric positron annihilation line  

NASA Technical Reports Server (NTRS)

The atmospheric positron annihilation line at 511 keV was the first gamma-ray line observed which originates from processes of astrophysical importance. Measurements of the properties of a positron annihilation line make it possible to determine many of the physical conditions present at the source of emission. An investigation was conducted of the 511-keV line measured with the HEAO C-1 experiment. The HEAO C-1 experiment consisted of a cluster of four, large, high-purity germanium detectors surrounded by an active anticoincidence CsI shield. A strong atmospheric line was measured at 511.07 + or - 0.10 keV. The net atmospheric emission has been accurately determined as a function of geomagnetic latitude and varies from 0.01 photons/sq cm-sec-sr near the magnetic equator to 0.035 photons/sq cm-sec-sr at a latitude of 55 deg.

Mahoney, W. A.; Ling, J. C.; Jacobson, A. S.

1981-01-01

180

Variable positron annihilation radiation from the galactic center region  

NASA Technical Reports Server (NTRS)

HEAO 3 Cosmic Gamma-Ray Spectrometer evidence is presented for the existence of a time-varying, unshifted, narrow 511 keV line emission from the vicinity of the galactic center. Although uncertainties exist regarding the spatial extent of the features as well as its centroid, all data are consistent with emission from a single point source located at the galactic center. This interpretation would require a source luminosity of 2 x 10 to the 37th ergs/sec, and a positron annihilation rate of about 10 to the 43rd/sec. It is concluded that a variable source of positrons which could generate such an annihilation figure might be a massive black hole at the galactic center, as has been suggested by IR observations.

Riegler, G. R.; Ling, J. C.; Mahoney, W. A.; Wheaton, W. A.; Willett, J. B.; Jacobson, A. S.; Prince, T. A.

1981-01-01

181

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

182

Modeling the momentum distributions of annihilating electron-positron pairs in solids  

Microsoft Academic Search

Measuring the Doppler broadening of the positron annihilation radiation or the angular correlation between the two annihilation gamma quanta reflects the momentum distribution of electrons seen by positrons in the material. Vacancy-type defects in solids localize positrons and the measured spectra are sensitive to the detailed chemical and geometric environments of the defects. However, the measured information is indirect and

I. Makkonen; M. Hakala; M. J. Puska

2006-01-01

183

Semiexclusive production in electron-positron annihilation  

NASA Astrophysics Data System (ADS)

We present a thorough analysis of the direct semiexclusive production in quantum chromodynamics (QCD) of single, highly isolated mesons from electron-positron or two-photon initial states. Corrections of higher order and of subleading twist are considered, and potential divergences in the naive calculation are contained. Monte Carlo methods are used to relate the QCD calculations to experimentally measureable quantities. We find that the study of semiexclusive production is the most sensitive experimental probe of the structure of mesons in the valence (qq¯) state at energies ?s >~10 GeV.

Hyer, Thomas

1994-10-01

184

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

Microsoft Academic Search

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

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

1996-01-01

185

Effects of process parameters on the defects in graphene oxide-polyaniline composites investigated by positron annihilation spectroscopy.  

PubMed

Graphene oxide (GO)-polyaniline (PANI) composites were prepared with different relative abundance of PANI and GO by in situ polymerization of aniline in the presence of GO and ammonium persulphate at different temperatures. In the process, GO also got reduced to graphene. Positron lifetimes and coincidence Doppler broadening of the electron-positron annihilation gamma ray spectra originating from the composite samples were measured and the results are reported. The positron lifetimes indicated the presence of very large size defects in the form of vacancy clusters within the samples. Another interesting observation was the increase of relative intensity of the defect specific positron lifetime component when an increase in relative abundance of PANI led to increased reduction of GO to graphene. The reduction also shrank the volume occupied by GO and the free volume thereby released added to the overall defect concentration, resulting in a simultaneous increase of the intensity of the positron lifetime component. The variation of the positron lifetime and its intensity with the synthesis temperature suggested an optimum temperature suitable for the process. The above observations are corroborated by other experimental investigations like electron microscopy, X-ray diffraction and electrical conductivity. PMID:24413400

Rana, Utpal; Nambissan, P M G; Malik, Sudip; Chakrabarti, Kuntal

2014-02-21

186

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

187

Ubiquitous nature of multimode vibrational resonances in positron-molecule annihilation.  

PubMed

Positron annihilation on many molecules occurs via positron capture into vibrational Feshbach resonances, with annihilation rates often further enhanced by energy transfer to vibrational excitations weakly coupled to the positron continuum. Data presented here uncover another scenario in which the positron couples directly to a quasicontinuum of multimode vibrational states. A model that assumes excitation and escape from a statistically complete ensemble of multimode vibrations is presented that reproduces key features of the data. PMID:22463631

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

2012-03-01

188

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

Microsoft Academic Search

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

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

1997-01-01

189

Gadolinium substitution induced defect restructuring in multiferroic BiFeO3: case study by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Positron annihilation spectroscopy (PAS) comprising of the measurements of positron lifetime and coincidence Doppler broadening spectra has been carried out to understand and monitor the evolution of the vacancy-type defects arising from the ionic deficiencies at lattice points of the multiferroic perovskite bismuth ferrite (BiFeO3) doped with 1, 5 and 10 at% gadolinium (Gd3+) ions. Negatively charged defects in the form of Bi3+ monovacancies (V_{Bi}^{3-} ) were present in the undoped nanocrystallites, which strongly trapped positrons. During the successive doping by Gd3+ ions, the positron trapping efficiency decreased while the doped ions combined with the vacancies to form complexes, which became neutral. A fraction of the positrons got annihilated at the crystallite surfaces too, being evident from the very large positron lifetimes obtained and confirming the nano-size-specific characteristics of the samples. Further, the intercrystallite regions provided favourable sites for orthopositronium formation, although in minute concentrations. The dopant ion-complex formation was also depicted clearly by the defect characteristic S-W plot. Also, the large change of electrical resistivity with Gd concentration has been explained nicely by invoking the defect information from the PAS study. The study has demonstrated the usefulness of an excellent method of defect identification in such a novel material system, which is vital information for exploiting them for further technological applications.

Mukherjee, A.; Banerjee, M.; Basu, S.; Nambissan, P. M. G.; Pal, M.

2013-12-01

190

Morphology of Thermoset Polyimides by Positron Annihilation Spectroscopy  

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

191

Fatigue damage in superalloys determined using Doppler broadening positron annihilation  

NASA Technical Reports Server (NTRS)

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

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

1990-01-01

192

GRIS observations of positron annihilation radiation from the Galactic center  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

193

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.

194

Design and Characterization of a Three-Dimensional Positron Annihilation Spectroscopy System Using a Low-Energy Positron Beam.  

National Technical Information Service (NTIS)

Previous forays by AFIT into positron research have resulted in the development of a new positron annihilation spectroscopy (PAS) technique and the procurement of a low-energy positron beam system. The primary purpose of this research was to design and bu...

S. M. Jimenez

2012-01-01

195

Development of positron annihilation spectroscopy for characterizing neutron irradiated tungsten  

SciTech Connect

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

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

2013-05-01

196

Positron annihilation and pressure-induced electronic s-d transition  

SciTech Connect

The polycrystalline, partial annihilation rates for positrons in compressed cesium have been calculated using the linear muffin-tin orbitals method. These results suggest that the pressure-induced electronic s-d transition in Cs should be directly observable by momentum sensitive positron annihilation experiments.

McMahan, A.K.; Skriver, H.L.

1985-06-01

197

Investigations of neutron irradiated iron by positron annihilation and correlated internal friction measurements  

NASA Astrophysics Data System (ADS)

Positron annihilation experiments have been performed on iron after low temperature neutron irradiation and during isochronal annealing. A good correlation exists between internal friction and positron annihilation data for iron with interstitial impurities, proving that these impurities react with vacancies during annealing (complex-formation).

Weller, M.; Diehl, J.; Triftshäuser, W.

1975-11-01

198

Interface detection in poly-ethylene terephthalate–metal laminates using variable energy positron annihilation  

Microsoft Academic Search

Thin coatings of poly-ethylene terephthalate (PET) on metal (“laminates”) have been studied with a variable energy positron annihilation technique. A correlation between PET crystallinity and the positron annihilation parameter S related to the free volume in the polymer is found. It is shown that buried interfaces in these systems may be detected provided the S parameter of the polymer coating

R. Escobar Galindo; H. Schut; A. van Veen; R. Rastogi; W. P. Vellinga; H. E. H. Meijer

2005-01-01

199

Defects in nitride-based semiconductors probed by positron annihilation  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

200

Transport theory for low-energy positron thermalization and annihilation in helium  

NASA Astrophysics Data System (ADS)

A transport theory that explicitly incorporates loss of flux due to annihilating collisions is developed and applied to low-energy positron diffusion and annihilation. The use of more complete momentum transfer and annihilation cross sections for helium has resulted in improved descriptions of the time dependence of for positrons injected into gaseous helium. Similarly, the variation of versus E /n0 for experiments where the annihilation region is immersed in an electric field is in closer agreement with experimental data. Inclusion of loss of flux due to annihilation was found to have a very small effect on the derived for helium.

Boyle, G. J.; Casey, M. J. E.; White, R. D.; Mitroy, J.

2014-02-01

201

Positron annihilation spectrum from the Galactic Centre region observed by SPI\\/INTEGRAL  

Microsoft Academic Search

The electron-positron annihilation spectrum observed by SPI\\/INTEGRAL during deep Galactic Centre region exposure is reported. The line energy (510.954+\\/-0.075 keV) is consistent with the unshifted annihilation line. The width of the annihilation line is 2.37 +\\/- 0.25 keV (full width at half-maximum), while the strength of the ortho-positronium continuum suggests that the dominant fraction of positrons (94 +\\/- 6 per

E. Churazov; R. Sunyaev; S. Sazonov; M. Revnivtsev; D. Varshalovich

2005-01-01

202

One-photon annihilation of thermal positrons with bound atomic electrons  

NASA Technical Reports Server (NTRS)

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

Jung, Young-Dae

1994-01-01

203

Electrolytic solutions in ethylene glycol: ultrasonic and positron annihilation studies  

NASA Astrophysics Data System (ADS)

Electrolytic solutions (KCl, KBr, tetra- n-butylammonium bromide-TBABr and hydrated cerium(III) chloride-CeCl 3·6H 2O) in ethane-1,2-diol (ethylene glycol) were studied using sound velocity, density and positron annihilation technique. For simple electrolytes (KCl, KBr, TBABr), effects similar to those known for respective aqueous solutions were observed: the slope of the dependence of sound velocity on salt concentration decreases with increasing size of solvated ion; for TBABr this dependence is even negative. It was also observed that in the latter system the adiabatic compressibility coefficient (calculated from density and sound velocity) is not linear with concentration, suggesting either structural transformations of the bulk solvent or easy formation of contact ionic pairs. Cerium chloride solutions were found to behave different. An anomaly occurs at CeCl 3·6H 2O mole fraction between 0.001 and 0.002, again interpreted either in terms of formation of ionic pairs of cations bonded by glycol molecules or cooperative interactions between glycol molecules in the close environment of the cerium ion (in inner solvation sphere) and those in the bulk. Positron annihilation measurements confirm well the conclusions arising from the ultrasonic ones.

Jerie, Kazimierz; Baranowski, Andrzej; Przybylski, Jan; Gli?ski, Jacek

2004-03-01

204

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

205

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

PubMed

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

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

2013-08-01

206

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

207

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

SciTech Connect

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

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

2005-12-15

208

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

SciTech Connect

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

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

2012-11-15

209

Slow Positron Beam Generator for Lifetime Studies.  

National Technical Information Service (NTIS)

A slow positron beam generator using well-annealed polycrystalline tungsten moderators and a Na-22 positron source was developed. A 250 micro c source, deposited on a 2.54 micrometer thick aluminized mylar, is sandwiched between two (2.54 cm x 2.54 cm x 0...

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

1989-01-01

210

Modeling enhancement and suppression of vibrational Feshbach resonances in positron annihilation on molecules  

NASA Astrophysics Data System (ADS)

Experiments have shown that positrons can attach to molecules via vibrational Feshbach resonances. This leads to increased annihilation rates, the magnitudes of which depend upon molecular structure. Presented here is a simplified rate-equation model to describe the competition between annihilation while the positron is attached to the molecule, positron ejection from the entrance state, and diffusion of the vibrational energy to multimode states followed by similar ejection due to vibrational deexcitation. The latter ejection process can involve vibrations more strongly coupled to the positron continuum, producing suppression of the annihilation, or those more weakly coupled to the continuum, resulting in enhanced annihilation rates. This model elucidates the role that mode coupling can play in determining resonant annihilation amplitudes. Simple limits are obtained and compared with experimental results for selected molecules.

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

2013-12-01

211

Positron lifetime studies in thermoplastic polyimide test specimens  

NASA Technical Reports Server (NTRS)

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

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

1982-01-01

212

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

213

BATSE Observations of Galactic Positron-Annihilation Radiation  

NASA Astrophysics Data System (ADS)

Bright, day-long transient emission features near 0.5 MeV were reported by the Sigma telescope on the Granat spacecraft from the Crab, ``microquasar'' 1E 1740.7-2942, and Nova Muscae, an x-ray nova and black hole candidate (Gilfanov et al., Ap. J. Suppl., 92, 411, 1994). We have searched data from the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory from June 1991 to December 1993 for such transients, and do not find any candidate events. Two of the specific events reported by Sigma were not seen, to better than twice Sigma's sensitivity. We have, however, observed the permanent 511 keV positron-annihilation line from the direction of the Galactic Center, and will report on the progress of an effort to test this line for variability and to map its distribution in Galactic longitude.

Smith, D. M.; Leventhal, M.; Cavallo, R.; Gehrels, N.; Tueller, J.; Fishman, G.

1995-12-01

214

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

215

Positron lifetime calculation for defects and defect clusters in graphite  

NASA Astrophysics Data System (ADS)

Calculations of positron lifetime have been made for vacancy type defects in graphite and compared with experimental results. Defect structures were obtained in a model graphite lattice after including relaxation of whole lattice as determined by the molecular dynamics method, where the interatomic potential given by Pablo Andribet, Dominguez-Vazguez, Mari Carmen Perez-Martin, Alonso, Jimenez-Rodriguez [Nucl. Instrum. and Meth. 115 (1996) 501] was used. For the defect structures obtained via lattice relaxation positron lifetime was calculated under the so-called atomic superposition method. Positron lifetimes 204 and 222 ps were obtained for the graphite matrix and a single vacancy, respectively, which can be compared with the experimental results 208 and 233 ps. For planar vacancy clusters, e.g., vacancy loops, lifetime calculation was also made and indicated that lifetime increases with the number of vacancies in a cluster. This is consistent with the experimental result in the region of higher annealing temperature (above 1200°C), where the increase of positron lifetime is seen, probably corresponding to the clustering of mobile vacancies.

Onitsuka, T.; Ohkubo, H.; Takenaka, M.; Tsukuda, N.; Kuramoto, E.

2000-12-01

216

Positron annihilation spectrum from the Galactic Centre region observed by SPI/INTEGRAL revisited: annihilation in a cooling ISM?  

NASA Astrophysics Data System (ADS)

We analyse SPI/INTEGRAL data on the 511-keV line from the Galactic Centre, accumulated over ˜6 yr of observations. We decompose the X-ray and soft gamma-ray emission of the central part of the Milky Way into relatively compact 'bulge' and more extended 'disc' components and report their spectral properties. The bulge component shows a prominent 511-keV line and essentially no flux at 1.8 MeV, while the disc component on the contrary contains a prominent 1.8-MeV line and a very weak annihilation line. We show that the spectral shape of the annihilation radiation (the narrow 511-keV line and the associated ortho-positronium continuum) is surprisingly well described by a model of annihilation of hot positrons in a radiatively cooling interstellar medium (ISM). The model assumes that positrons are initially injected into a hot (˜106 K), volume-filling ISM, which is allowed to freely cool via radiative losses. The annihilation time in such a medium is longer than the cooling time for temperatures higher than a few 104 K. Thus, most of the positrons annihilate only after the gas has cooled down to ˜105 K, giving rise to annihilation emission characteristic of a warm, ionized ISM.

Churazov, E.; Sazonov, S.; Tsygankov, S.; Sunyaev, R.; Varshalovich, D.

2011-03-01

217

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

SciTech Connect

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

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

2011-12-01

218

Study of the effect of electric field on positron annihilation parameters in polymers  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime measurements have been performed to study the effect of an external electric field in several polymers. The application of the electric field on polytetrafluoroethylene (PTFE) and ultrahigh-molecular weight polyethylene (UHMWPE) noticeably decreased the ortho-positronium ( o-Ps) intensity. The o-Ps intensity increased with increasing electric field strength in the poly(ethylene terephthalate), PET, poly(ethylene naphthalate), PEN, and poly(aryl-ether-ether-ketone), PEEK samples. The data are consistent with a hypothesis that nonpolar polymers (PTFE and UHMWPE) show a decrease in the o-Ps intensity with increasing electric field, while the effect seems to be opposite in polar polymers (PEN, PET and PEEK).

Mohamed, Hamdy F. M.

2003-10-01

219

Positron lifetime measurements in chiral nematic liquid crystals  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

220

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

NASA Astrophysics Data System (ADS)

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

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

2010-04-01

221

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

NASA Technical Reports Server (NTRS)

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

Turner, Michael S.; Wilczek, Frank

1989-01-01

222

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

PubMed

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

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

2012-09-01

223

Assessment of Positron Annihilation as a Potential Non-Destructive Examination Technique.  

National Technical Information Service (NTIS)

The positron annihilation technique can provide a sensitive measure of defect density in metals. In this program the technique has been used to monitor defects generated during plastic deformation by cold work or fatigue cycling. The primary goals have be...

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

1979-01-01

224

The dominant contributions of the inner valence electrons to the positron annihilation process in methanol  

NASA Astrophysics Data System (ADS)

The positron–electron annihilation gamma-ray spectra of methanol have been studied in the present work. The contributions of the bound electrons to the Doppler-shift of gamma-ray spectra have been analysed as well. These bound electrons are divided into three groups: core, inner valence and outer valence rather than the conventional two groups: core and valence in the positron annihilation process in the present work. The results obtained show a dominance of the inner valence electrons of methanol rather than the electrons occupied in the highest occupied molecular orbital (HOMO) in the positron–electron annihilation process. These inner valence electrons occupied in 3a? and 4a? orbitals consist of over 80% outermost atomic 2s electrons in oxygen and carbon atoms. That the positron prefers to annihilate with these outermost s electrons in atoms is suggested.

Ma, Xiaoguang; Wang, Lizhi; Yang, Chuanlu

2014-03-01

225

Microstructural Characterization of Polymers by Positron Lifetime Spectroscopy  

NASA Technical Reports Server (NTRS)

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

Singh, Jag J.

1996-01-01

226

A simplified digital positron lifetime spectrometer based on a fast digital oscilloscope  

NASA Astrophysics Data System (ADS)

A simplified digital positron lifetime spectrometer is presented and tested. This system is only composed of two scintillation detectors, a time delayer and a digital oscilloscope. Both online and offline analysis of the detector signals can be optionally chosen to construct positron lifetime spectrum by histogramming the time difference between the positron birth signal of 1.27 MeV ?-ray and one positron annihilation signal of 0.511 MeV ?-rays. Several pulse discrimination methods and timing numerical algorithms are proposed and well applied to perform pulse energy discrimination and digital constant fraction timing (DCFT). A combination discrimination of pulse amplitude, pulse risetime and ratio of pulse area to pulse amplitude with DCFT is found able to achieve the best time resolution of 208 ps full-width at half-maximum (FWHM), which is much better over the analog spectrometer using the same detectors. This newly developed apparatus except for having better performance is particularly easy to be constructed and implemented.

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

2011-01-01

227

Positron lifetime studies in vinyl polymers of medical importance  

NASA Astrophysics Data System (ADS)

Vinyl polymers used as artificial lens implants in ophthalmology were investigated by positron lifetime spectroscopy. The structure of these polymers with free volumes offers the possibility of charging them with anti-inflammatory drugs for sustained release. A correlation between the amount of normalised free volume and the ratio of the methyl methacrylate to ethyl-hexyl-acrylate, used as polymerisation monomers, was found.

Ferreira Marques, M. F.; Gordo, P. M.; Gil, C. Lopes; Kajcsos, Zs.; Gil, M. H.; Mariz, M. J.; de Lima, A. P.

2003-10-01

228

Nanostructure and Dynamics of Polymers and Thin Polymer Films: Studies by Positron Annihilation Spectroscopy  

NASA Astrophysics Data System (ADS)

The relaxational, mechanical and transport properties of glassy polymers are strongly influenced by the nanostructural and dynamical characteristics of each material. In very thin polymer films such characteristics may be affected by the presence of a free surface or a substrate. Positron Annihilation Lifetime Spectroscopy (PALS) is a useful and in some ways unique tool for probing these important characteristics. Conventional PALS on several bulk polymers over an extended temperature range are used to illustrate how these characteristics are obtained(HA Hristov, B Bolan, AF Yee, L Xie, and DW Gidley, accepted by Macromolecules.). A new technique, which we shall call "beam-PALS", and the results of its application on nm-thick polystyrene films supported on one side by a Si substrate are described. In beam-PALS the lifetime, ? _3, and formation fraction, I_3, of triplet positronium decaying in the void volume near the polymer surface are measured versus the positron implantation energy, E. The strong E dependence of I3 supports a spur-electron capture model of Ps formation with deduced spur sizes ranging from 200 to 660 ÅThin film measurements indicate that the mean probe depth can be much smaller, given mainly by the average positron implantation distance, Z(E)(L Xie, GB DeMaggio, WE Frieze, J DeVries, DW Gidley, HA Hristov and AF Yee, PRL 74, 4947 (1995).). The thermal expansion behaviors of thin, Si-supported polystyrene films near the glass transition temperature, Tg were also measured. A reduction in void volume expansion is correlated with a reduction in the apparent Tg as film thickness decreases. Our results can be fitted using a 3-layer model incorporating a 50 Åconstrained layer at the Si interface and a 20 Åsurface region with reduced T_g(GB DeMaggio, WE Frieze, DW Gidley, M Zhu, HA Hristov, and AF Yee, accepted by PRL.).

Yee, Albert F.

1997-03-01

229

Weathering durability of commercial polymeric coatings tested by positron annihilation spectroscopy  

Microsoft Academic Search

A series of commercial coatings were prepared according to the industrial specifications and were exposed to Florida natural weathering and controlled UVA irradiation. The Doppler broadening energy spectra (DBES) of positron annihilation were measured as a function of incident positron energy at different periods of weathering. A significant decrease in the S parameter was observed and it was used as

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

2003-01-01

230

Polarized hadron pair production from electron-positron annihilation  

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

231

Timelike Virtual Compton Scattering from Electron-Positron Radiative Annihilation  

SciTech Connect

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

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

2009-01-01

232

Timelike Virtual Compton Scattering from Electron-Positron Radiative Annihilation  

SciTech Connect

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

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

2009-03-31

233

Timelike virtual compton scattering from electron-positron radiative annihilation  

SciTech Connect

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

Afanasev, Andrei [Department of Physics, Hampton University, Hampton, Virginia 23668, USA, and Theory Center, Thomas Jefferson National Accelerator Facility, 12000 Jefferson Avenue, Newport News, Virginia 23606 (United States); Brodsky, Stanley J. [SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94309 (United States); Carlson, Carl E. [Department of Physics, College of William and Mary, Williamsburg, Virginia 23187 (United States); Mukherjee, Asmita [Department of Physics, Indian Institute of Technology, Powai, Mumbai 400076 (India)

2010-02-01

234

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

235

Optimisation of the thickness of the moderator for positron annihilation process study in Ar gas  

NASA Astrophysics Data System (ADS)

To investigate the deceleration and annihilation processes of positron in noble gas, we attempted to optimise the thickness of the positron moderator. The results of the decreasing trend of free positron intensity and ortho-positronium intensity with the thickness, and the annihilation rate suggest that the energy of almost all the positrons passed through the metal foils was much above the positronium formation threshold of 9 eV. Total thickness of 0.1 g cm-2 of the moderator and the Ar gas was required to stop almost all the positron in Ar gas. Contribution to the Topical Issue "Electron and Positron Induced Processes", edited by Michael Brunger, Radu Campeanu, Masamitsu Hoshino, Oddur Ingólfsson, Paulo Limão-Vieira, Nigel Mason, Yasuyuki Nagashima and Hajime Tanuma.

Oka, Toshitaka; Sano, Yosuke; Kino, Yasushi; Sekine, Tsutomu

2014-06-01

236

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

237

Positrons from dark matter annihilation in the galactic halo: Theoretical uncertainties  

SciTech Connect

Indirect detection signals from dark matter annihilation are studied in the positron channel. We discuss in detail the positron propagation inside the galactic medium: we present novel solutions of the diffusion and propagation equations and we focus on the determination of the astrophysical uncertainties which affect the positron dark matter signal. We find dark matter scenarios and propagation models that nicely fit existing data on the positron fraction. Finally, we present predictions both on the positron fraction and on the flux for already running or planned space experiments, concluding that they have the potential to discriminate a possible signal from the background and, in some cases, to distinguish among different astrophysical propagation models.

Delahaye, T.; Salati, P. [Laboratoire d'Annecy-le-Vieux de Physique Theorique LAPTH, CNRS-SPM and Universite de Savoie 9, Chemin de Bellevue, B.P.110 74941 Annecy-le-Vieux (France); Lineros, R.; Donato, F.; Fornengo, N. [Dipartimento di Fisica Teorica, Universita di Torino and Istituto Nazionale di Fisica Nucleare, via P. Giuria 1, I-10125 Torino (Italy)

2008-03-15

238

First-principles calculation of coincidence Doppler broadening of positron annihilation radiation  

Microsoft Academic Search

We report a first-principles method for calculating the momentum density of positron-electron pairs in materials, which can be accurately measured, in a wide momentum range, by means of coincidence Doppler broadening (CDB) of positron annihilation radiation. The calculation is based on the two-component density-functional theory within the local-density approximation. The electron and positron wave functions are calculated by means of

Z. Tang; M. Hasegawa; Y. Nagai; M. Saito; Y. Kawazoe

2002-01-01

239

Pair annihilation effects on a surface ion cyclotron wave in semibounded electron-positron-ion plasmas  

SciTech Connect

Electron-positron pair annihilation effects on a surface ion cyclotron wave are investigated in magnetized electron-positron-ion plasmas in atmospheres of neutron stars. The dispersion relation of the surface ion cyclotron wave is obtained by the specular reflection boundary condition with the plasma dielectric function. It is shown that the high- and low-frequency modes of the surface ion cyclotron wave could exist in electron-positron-ion plasmas. For the high-frequency mode, the pair annihilation enhances the wave frequency in large wave-number domains. However, the pair annihilation effects are found to be negligible for the low-frequency mode. It is also found that an increase of the electron temperature or a decrease of the positron temperature strongly suppresses the wave frequency. It is shown that an increase of the magnetic field strongly enhances the wave frequency.

Jung, Young-Dae [National Institute for Fusion Science, Toki, Gifu 509-5292 (Japan); Department of Applied Physics, Hanyang University, Ansan, Kyunggi-Do 426-791 (Korea, Republic of); Kato, Daiji [National Institute for Fusion Science, Toki, Gifu 509-5292 (Japan)

2008-09-15

240

Solid Eu(III) complexes studied by positron annihilation, optical and Mössbauer spectroscopies: insights on the positronium formation mechanism.  

PubMed

In this work, positron annihilation lifetime (PALS), Doppler broadening annihilation radiation lineshape (DBARL), Mössbauer and optical spectroscopies measurements were performed in Eu(III) dipivaloylmetanate complex, Eu(dpm)(3), at 295 and 80 K. The Eu(dpm)(3) complex is not luminescent at 298 K and does not form positronium. On the other hand, it is highly luminescent at 80K, but still does not form positronium. The absence of positronium formation at 80K cannot be explained by a ligand/metal charge transfer process. We found strong evidences that the electronic delocalization does not occur at both temperatures. Despite the Mössbauer results being inconclusive regarding the Eu(III)/Eu(II) reduction hypothesis, previous results showing positronium formation in other Eu(III) complexes suggest that this process is not occurring. Thus, more studies are needed to explain the absence of positronium in Eu(III) complexes. PMID:22466013

Fulgêncio, F; de Oliveira, F C; Ivashita, F F; Paesano, A; Windmöller, D; Marques-Netto, A; Magalhães, W F; Machado, J C

2012-06-15

241

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

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

242

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

243

The Effects of Surface Reconstruction and Electron-Positron Correlation on the Annihilation Characteristics of Positrons Trapped at Semiconductor Surfaces  

SciTech Connect

Experimental positron annihilation induced Auger electron spectroscopy (PAES) data from Ge(100) and Ge(111) surfaces display several strong Auger peaks corresponding to M{sub 4,5}N{sub 1}N{sub 2,3}, M{sub 2,3}M{sub 4,5}M{sub 4,5}, M{sub 2,3}M{sub 4,5}V, and M{sub 1}M{sub 4,5}M{sub 4,5} Auger transitions. The integrated peak intensities of Auger transitions have been used to obtain experimental annihilation probabilities for the Ge 3d and 3p core electrons. The experimental data were analyzed by performing theoretical studies of the effects of surface reconstructions and electron-positron correlations on image potential induced surface states and annihilation characteristics of positrons trapped at the reconstructed Ge(100) and Ge(111) surfaces. Calculations of positron surface states and annihilation characteristics have been performed for Ge(100) surface with (2x1), (2x2), and (4x2) reconstructions, and for Ge(111) surface with c(2x8) reconstruction. Estimates of the positron binding energy and annihilation characteristics reveal their sensitivity to the specific atomic structure of the topmost layers of the semiconductor and to the approximations used to describe electron-positron correlations. The results of these theoretical studies are compared with the ones obtained for the reconstructed Si(100)-(2x1) and Si(111)-(7x7) surfaces.

Fazleev, N. G. [Department of Physics, University of Texas at Arlington, Arlington, Texas 76019 (United States); Department of Physics, Kazan State University, Kazan 420008 (Russian Federation); Jung, E.; Weiss, A. H. [Department of Physics, University of Texas at Arlington, Arlington, Texas 76019 (United States)

2009-03-10

244

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

PubMed

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

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

2013-02-01

245

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

NASA Astrophysics Data System (ADS)

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

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

2006-03-01

246

The temperature dependence of positron lifetimes in solid pivalic acid  

NASA Astrophysics Data System (ADS)

The temperature dependence of positron lifetimes in both the brittle and plastic phases of trimethylacetic (pivalic) acid has been examined using the positron lifetime technique. In the plastic phase two long-lived components attributable to ortho-positronium (ortho-Ps) decay are observed. The longer of these (?2.8 ns) increases with temperature and is believed to be characteristic of ortho-Ps trapped at defects, probably mono- and di-vacancies. A shorter lifetime component (?1 ns) which shows little temperature dependence is also present in the brittle phase. Additionally in the brittle phase and close to the transition region a longer lifetime is detected. This may also be associated with trapping of ortho-Ps in thermally created defects, probably vacancies. In the plastic phase the intensity of a short-lived component associated with para-Ps is confirmed to be approximately one third of the total intensity of the two long-lived components associated with ortho-Ps, but in the brittle phase it higher than one third. On using data for several plastic crystals a relationship is established between the lifetime of ortho-Ps trapped in a vacancy and the vacancy volume. Trimethylacetic acid; 2,2-dimethylpropanoic acid.

Eldrup, M.; Lightbody, D.; Sherwood, J. N.

1981-12-01

247

Charging-induced defect formation in LixCoO2 battery cathodes studied by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Charging-induced formation of vacancy-type defects in LixCoO2 battery cathodes was studied by the defect-specific techniques of positron lifetime spectroscopy and Doppler broadening of positron-electron annihilation radiation. The regime of reversible charging is dominated by vacancy-type defects on the Li+-sublattice the size of which increases with increasing Li+-extraction. Indication is found that Li+-reordering which occurs at the limit of reversible Li+-extraction (x = 0.55) causes a transition from two-dimensional agglomerates into one-dimensional vacancy chains. Degradation upon further Li+-extraction is accompanied by the formation of vacancy complexes on the Co- and anion sublattice.

Parz, P.; Fuchsbichler, B.; Koller, S.; Bitschnau, B.; Mautner, F.-A.; Puff, W.; Würschum, R.

2013-04-01

248

Understanding the swelling of poly ( N -isopropyl acrylamide) gels through the study of free volume hole size distributions using positron annihilation spectroscopy  

Microsoft Academic Search

Poly (N-isopropyl acrylamide) gels were prepared by UV polymerization with different degree of cross-linking in different solvents.\\u000a Ethylene glycol dimethacrylate and penta-erythritol tetra-acrylate were used as cross-linkers, and methanol and dimethyl formamide\\u000a (DMF) were used as solvents for gel preparation. The free volume fraction and hole size distribution in the dry gels were\\u000a measured using positron annihilation lifetime spectroscopy. The

Pushkar N. Patil; Sudarshan Kathi; Dhanadeep Dutta; Pradeep K. Pujari

2010-01-01

249

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

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

250

Interpretation of positron-annihilation data with respect to the electron-positron enhancement factors. II. Applications  

NASA Astrophysics Data System (ADS)

The theoretical considerations of the properties of positron-annihilation characteristics in simple metals, presented in the previous paper, are verified for the alkali metals and Mg. Results are based on a local-density approach to electron-positron correlations and band-structure calculations are performed within the linear-muffin-tin-orbitals-average-spheres approximation. The validity of the average electron density approximation is discussed and the effective densities, determined by electron-positron correlation effects, are presented for valence electrons in simple metals. Methods of extracting the full shape of the electron-positron momentum density from experimental curves are recommended. The proposed analysis of experimental data applied to simple metals allows more reliable verificaton of the shape of electron-positron enhancement factors near the Fermi surface.

Kontrym-Sznajd, G.; Rubaszek, A.

1993-03-01

251

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

252

Positron-annihilation-induced ion desorption from TiO2(110)  

NASA Astrophysics Data System (ADS)

We have investigated the positron-stimulated desorption of ions from a TiO2(110) surface. Desorbed O+ ions were detected in coincidence with the emission of annihilation ? rays. The energy dependence of the ion yields shows that the O+ ions were detected at energies much lower than the previously reported threshold for electron impact desorption corresponding to the excitation energy of Ti(3p) core electrons. These results provide evidence that core-hole creation by positron annihilation with electrons in the core levels leads to ion desorption.

Tachibana, T.; Hirayama, T.; Nagashima, Y.

2014-05-01

253

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

254

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

NASA Astrophysics Data System (ADS)

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

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

2013-12-01

255

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

256

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

NASA Astrophysics Data System (ADS)

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

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

257

Positron lifetime reveals the nano level packing in complex polysaccharide-rich hydrolysate matrixes.  

PubMed

Positron annihilation lifetime spectroscopy (PALS) was used to quantify the free volume and molecular packing in hydrolysate and hemicellulose-based barriers films, derived from process streams during wood processing operations. These hydrolysate films, comprising a fair share of lignin coexisting with poly- and oligo-saccharides, have very low but variable oxygen permeability but differ among themselves with respect to barrier performance as well as molecular weight, degree of branching, and monosaccharide residue main chain composition. From PALS measurements on hydrolysates, the free volume hole radius (r(h)), radius distributions (n(r(h))), volume-weighted hole sizes ((v)), and hole volume distributions (g(v(h))) were calculated showing that the hydrolysate matrixes are very densely packed with small holes. The results show a clear relationship between hydrolysate molecular architecture and composition, the nanolevel molecular packing, and the ability of suppressing the diffusion of oxygen through the film. PMID:22455471

Edlund, Ulrica; Yu, Yang; Ryberg, Yingzhi Zhu; Krause-Rehberg, Reinhard; Albertsson, Ann-Christine

2012-04-17

258

High-temperature positron annihilation and vacancy formation in refractory metals  

Microsoft Academic Search

By means of the new technique of sealed–source specimens the Doppler broadening of the positron–annihilation ?M line has been investigated in the refractory metals V, Kb, Ta, Mo and W between 4·2 K and the melting points. Tho ' high-temperature S ' characteristic for positrons trapped at vacancies in thermal equilibrium was observed for Mo and W. In V, Nb

K. Maier; M. Peo; B. Saile; H. E. Schaefer; A. Seeger

1979-01-01

259

Positron-Annihilation Momentum Profiles in Aluminum: Core Contribution and the Independent-Particle Model  

Microsoft Academic Search

The momentum profiles from positron annihilation with core electrons in Al have been observed with a recently developed two-detector Doppler-broadening technique. Independent-particle-model (IPM) calculations are in good agreement with the experimental results beyond twice the Fermi momentum and suggest the absence of strong positron-electron correlation effects in this region. Comparisons of calculations with properly normalized momentum profiles indicate that high-momentum

K. G. Lynn; J. R. MacDonald; R. A. Boie; L. C. Feldman; J. D. Gabbe; M. F. Robbins; E. Bonderup; J. Golovchenko

1977-01-01

260

Positron annihilation spectroscopic studies of solvothermally synthesized ZnO nanobipyramids and nanoparticles  

Microsoft Academic Search

Zinc oxide (ZnO) samples in the form of hexagonal-based bipyramids and particles of nanometer dimensions were synthesized through solvothermal route and characterized by x-ray diffraction and transmission electron microscopy. Positron annihilation experiments were performed to study the structural defects such as vacancies and surfaces in these nanosystems. From coincidence Doppler broadening measurements, the positron trapping sites were identified as Zn

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

2008-01-01

261

Positron annihilation and relaxation dynamics from dielectric spectroscopy: poly(vinylmethylether).  

PubMed

We report on the temperature dependence of the lifetime of the ortho-positronium (o-Ps), ?(3), annihilation in amorphous polymer poly(vinylmethylether) (PVME) from positron annihilation lifetime spectroscopy (PALS). We show that the behavior of ?(3)(T) can be divided into five regions, each of them having a linear temperature dependence, and that the crossover PALS temperatures situated at T(b1)(G), 0.76T(g)(PALS), T(b1)(L) = 1.14T(g)(PALS) and T(b2)(L) = 1.37T(G)(PALS), and marking the discontinuity in the free volume microstructure are related to various dynamic features from neutron scattering (NS) and broadband dielectric spectroscopy (BDS). First, a slight change in the PALS response in the glassy PVME at T(b1)(G) is related to the onset of the excess wing in an apparent correspondence with the fast secondary ? motion from NS. A further slight bend in the liquid state at T(b1)(L) is related to a high-frequency tail of the primary ? process as well as to the slow secondary ? relaxation from BDS. In addition, it lies also in the vicinity of the crossover temperature, T(B)(?KWW), in the spectral dispersion of the primary ? process, indicating a connection of the change in the o-Ps lifetime with the variation in the width of the primary ? relaxation times distribution. Finally, the ?(3) value at T(b2)(L) is close to the mean relaxation time of the primary ? process, ?(?), in coincidence with the crossover in the secondary effective ? process between two regimes in the liquid PVME. All these relationships point to very close connections between the PALS response and the dynamic behavior of PVME, which can be explained in terms of the temperature dependence of the probability function of the liquid-like and the solid-like domains, as obtained from the two-order parameter (TOP) model description of the liquid to glass transition in glass-formers. PMID:22436554

Bartoš, J; Iskrová-Miklošovi?ová, M; Cangialosi, D; Alegría, A; Šauša, O; Švajdlenková, H; Arbe, A; Krištiak, J; Colmenero, J

2012-04-18

262

Advanced fitting algorithms for analysing positron annihilation lifetime spectra  

NASA Astrophysics Data System (ADS)

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

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

2009-05-01

263

A Simulation Study of a Method to Reduce Positron Annihilation Spread Distributions Using a Strong Magnetic Field in Positron Emission Tomography  

Microsoft Academic Search

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.

H. Iida; I. Kanno; S. Miura; M. Murakami; K. Takahashi; K. Uemura

1986-01-01

264

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

265

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

SciTech Connect

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

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

2013-04-19

266

Installation for Measuring Angular Correlation of Two-Photon Annihilation of Positrons in a Condensed Medium.  

National Technical Information Service (NTIS)

An installation with the linearly-slit geometry for detecting angular distribution of annihilation gamma quanta arising in interaction of positrons with the investigated medium is developed and constructed. The length of each installation arm is 2.5 m., g...

V. P. Komlev V. I. Kudinov Y. V. Obukhov Y. V. Funtikov

1982-01-01

267

Structure of Aqueous Solutions of Acetonitrile Investigated by Acoustic and Positron Annihilation Measurements  

Microsoft Academic Search

We report the results of acoustic and positron annihilation measurements in aqueous solutions of acetonitrile (CH3CN). Hydrophobicity of the solute is discussed, as well as the possibility of describing the title system in terms of hydrophobic solvation. The concept of Levay et al. of calculating the \\

K. Jerie; A. Baranowski; S. Koziol; A. Burakowski

2005-01-01

268

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

PubMed

Doping in ceria (CeO2) nanoparticles with europium (Eu) of varying concentrations (0, 0.1, 0.5, …, 50 atom%) is studied using complementary experimental techniques and novel observations were made during the investigation. The immediate observable effect was a distinct reduction in particle sizes with increasing Eu concentration attributed to the relaxation of strain introduced due to the replacement of Ce(4+) ions by Eu(3+) ions of larger radius. However, this general trend was reversed in the doping concentration range of 0.1-1 atom% due to the reduction of Ce(4+) to Ce(3+) and the formation of anion vacancies. Quantum confinement effects became evident with the increase of band gap energy when the particle sizes reduced below 7-8 nm. Positron annihilation studies indicated the presence of vacancy type defects in the form of vacancy clusters within the nanoparticles. Some positron annihilation was also seen on the surface of crystallites as a result of diffusion of thermalized positrons before annihilation. Coincidence Doppler broadening measurements indicated the annihilation of positrons with electrons of different species of atoms and the characteristic S-W plot showed a kink-like feature at the particle sizes where quantum confinement effects began. PMID:24247546

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

2014-01-01

269

Positrons annihilation in water and ice with an external magnetic field  

Microsoft Academic Search

Summary  The angular distribution of 2? annihilation of positrons in water and in ice has been measured with a 15 kG magnetic field.\\u000a Measurements show that the magnetic field enhances the narrow component of the angular distribution in water. The angular\\u000a distribution in ice seems to be not influenced by the magnetic field.

G. Iaci; I. F. Quercia; E. Turrisi

1962-01-01

270

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

271

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

Microsoft Academic Search

The irradiation effects on five kinds of polyethylenes (PEs) were studied from the viewpoints of mechanical properties, free radicals and positron annihilation. The degradation of the mechanical strength of samples irradiated by 60Co ?-rays was evaluated based on the elongation at break and the tensile strength. Fully stretched PE with a crystallinity larger than 99% was greatly affected by ?-irradiation.

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

1995-01-01

272

The effect of microvoid size on positron annihilation characteristics and residual resistivity in metals  

Microsoft Academic Search

The annihilation characteristics of a positron trapped in spherical clusters of 1., 50 vacancies in Al and Mo are calculated. The results are found to depend sensitively on the void size up to a radius of about 5 Å, beyond which the size dependence disappears. The residual resistivity is shown to be nearly independent of the microvoid size during initial

P. Hautojärvi; J. Heiniö; M. Manninen; R. Nieminen

1977-01-01

273

Internal motion of massive charmed quarks in double charmonium production in electron-positron annihilation  

SciTech Connect

The cross section for double charmonium production in electron-positron annihilation at the interaction energy of {radical}s = 10.6 GeV is estimated. Both the form of the charmonium wave function and a nonzero charmed-quark mass are taken into account.

Berezhnoy, A. V., E-mail: aber@ttk.r [Moscow State University, Institute of Nuclear Physics (Russian Federation)

2008-10-15

274

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

275

Spatially resolved positron annihilation spectroscopy on friction stir weld induced defects  

Microsoft Academic Search

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

Karin Hain; Christoph Hugenschmidt; Philip Pikart; Peter Böni

2010-01-01

276

Determination of the Vacancy Formation Enthalpy in Chromium by Positron Annihilation.  

National Technical Information Service (NTIS)

Doppler broadening of the positron annihilation lineshape in 99.99 at. % pure chromium was measured over the temperature range 296 to 2049 exp 0 K. The chromium sample was encapsulated in sapphire owing to its high vapor pressure near melting. Saturation-...

G. D. Loper L. C. Smedskjaer M. K. Chason R. W. Siegel

1985-01-01

277

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

PubMed

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

Rusz, J; Biasini, M; Czopnik, A

2004-10-01

278

Chemical analysis of precipitates in metallic alloys using coincidence Doppler broadening of positron annihilation radiation  

Microsoft Academic Search

Two-detector coincidence Doppler broadening (CDB) method of positron annihilation radiation is applied to the study of precipitates in metals. As the first step, the CDB spectra for many kinds of elements are measured to obtain the “fingerprint” of each element for the chemical analysis of the precipitates in metals. Utilizing the fingerprint of Cu, we made chemical analysis of nano-particles

Y. Nagai; Z. Tang; M. Hasegawa

2000-01-01

279

Comparison of the Free Volume of LiCl-Added SPPO Membrane and SPPO-PES Blend Membrane by Positron Annihilation Method  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy (PALS) is a powerful technique for the study of free volume in polymers. The lifetime of ortho-positronium (o-Ps), a bound state of an electron and a positron, can be used to assess the pore size, while the intensity can be used to characterize the number of pores. Based on the values of the long-lived o-Ps components in the lifetime spectra, the radii and fractional free volumes of sulfonated poly (2,6-dimethyl-1,4-phenyleneoxide) (SPPO) membranes with added LiCl and SPPO-PES (Polyethersulfone) blend were compared. Free volume radii in both kinds of membranes are discussed.

Htwe Htwe, Yin; Huang, Huan; Tang, Shibiao; Ma, Qingli

2007-10-01

280

Lattice relaxation at V(-), NV(-), and NVN(-) defects in diamond investigated by positron annihilation.  

PubMed

Experimental positron lifetime data for three vacancy-related defects in synthetic type Ib diamonds with well known structures are uniquely suitable for comparison with theoretical investigations of lattice relaxation. PMID:21825427

Dannefaer, S

2009-04-29

281

Study of gamma irradiation effect on positron annihilation mechanism in PFA  

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

282

Investigation of the annealed copper surface by positron annihilation induced Auger electron spectroscopy  

NASA Astrophysics Data System (ADS)

A facility for positron annihilation induced Auger electron spectroscopy (PAES) was built up at the slow positron beam of the Technical University of Munich. The positron beam (intensity 10 4 e +/s, energy some 10 eV) is magnetically guided to the analysis chamber and then electrically focused onto the sample. Released Auger electrons are detected by an electrostatic hemispherical energy analyser with an energy resolution of about 3 eV. First measurements on annealed copper are presented and compared to conventional AES.

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

2003-10-01

283

Investigation of nanoscopic free volume and interfacial interaction in an epoxy resin/modified clay nanocomposite using positron annihilation spectroscopy.  

PubMed

Epoxy/clay nanocomposites are synthesized using clay modified with the organic modifier N,N-dimethyl benzyl hydrogenated tallow quaternary ammonium salt (Cloisite 10A). The purpose is to investigate the influence of the clay concentration on the nanostructure, mainly on the free-volume properties and the interfacial interactions, of the epoxy/clay nanocomposite. Nanocomposites having 1, 3, 5 and 7.5 wt. % clay concentrations are prepared using the solvent-casting method. The dispersion of clay silicate layers and the morphologies of the fractured surfaces in the nanocomposites are studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The observed XRD patterns reveal an exfoliated clay structure in the nanocomposite with the lowest clay concentration (?1 wt. %). The ortho-positronium lifetime (?(3)), a measure of the free-volume size, as well as the fractional free volume (f(v)) are seen to decrease in the nanocomposites as compared to pristine epoxy. The intensity of free positron annihilation (I(2)), an index of the epoxy-clay interaction, decreases with the addition of clay (1 wt. %) but increases linearly at higher clay concentrations. Positron age-momentum correlation measurements are also carried out to elucidate the positron/positronium states in pristine epoxy and in the nanocomposites. The results suggest that in the case of the nanocomposite with the studied lowest clay concentration (1 wt. %), free positrons are primarily localized in the epoxy-clay interfaces, whereas at higher clay concentrations, annihilation takes place from the intercalated clay layers. PMID:23129045

Patil, Pushkar N; Sudarshan, Kathi; Sharma, Sandeep K; Maheshwari, Priya; Rath, Sangram K; Patri, Manoranjan; Pujari, Pradeep K

2012-12-01

284

Satellite Observations of Annihilation of Positrons Produced at the Sun, the Earth, and Center of our Galaxy  

Microsoft Academic Search

Positrons are created in nuclear interactions that produce beta +-unstable nuclei and pi+ mesons. Satellites remotely observe positron production when they annihilate with electrons yielding the characteristic line at 511 keV. Radiation detectors such as the germanium diodes on the Ramaty High-Energy Solar Spectrocopic Imager (RHESSI) observe this line from positrons by nuclei activated in the spacecraft by proton interactions

G. H. Share; R. J. Murphy; R. P. Lin

2007-01-01

285

Measuring electron-positron annihilation radiation from laser plasma interactions.  

PubMed

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. PMID:23126935

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

2012-10-01

286

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

287

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

PubMed

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

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

2014-07-01

288

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

NASA Astrophysics Data System (ADS)

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

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

2008-03-01

289

Positron lifetime spectroscopy for investigation of thin polymer coatings  

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

290

The study of defects in crystals by positron annihilation  

Microsoft Academic Search

The study of defects in crystals through their interaction with thermalized positrons is usually based on the trapping model.\\u000a The theoretical foundation of the trapping model, based on the theory of diffusion-limited reactions, is reviewed with particular\\u000a emphasis on short-range interactions between positrons and trapping defects. For this case the analytical properties of the\\u000a trapping model are developed in some

Alfred Seeger

1974-01-01

291

Recent progress in positron annihilation research at Argonne National Laboratory  

NASA Astrophysics Data System (ADS)

The present work describes the most recent research activity in the positron group at Argonne. Our efforts have been centered around bringing our 2-D ACRR system on line, and a short description of its properties are given here. In addition, recent positron measurements on boron doped Ni3Al, done in collaboration with A. DasGupta from the Metals and Ceramics Division, Oak Ridge National Laboratory, will be presented as well.

Smedskjaer, L. C.; Siegel, R. W.

1985-09-01

292

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

NASA Astrophysics Data System (ADS)

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

Lewicka, Sylwia; Dryzek, Jerzy

2013-12-01

293

Positron annihilation radiation from the galactic center region  

NASA Technical Reports Server (NTRS)

Measurements of 0.511 MeV radiation from the vicinity of the galactic center performed by the high resolution gamma ray spectrometer aboard the HEAO 3 spacecraft are discussed. The spectrometer is described and an exploded view of it exhibited, and scans in the 50-244 keV region as well as 0.511 MeV are shown. The intensity from the Crab nebula remained constant while that of the Cygnus source increased considerably, and there was a decrease in the radiation from the vicinity of the galactic center. The flux emanating from the center implies an annihilation rate of 10 to the 43rd per second. The apparent time variation and the spatial extent observed tend to limit the source to a single object and the annihilation region to a size less than about one light year.

Jacobson, A. S.

1981-01-01

294

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

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

295

Enhancement models of momentum densities of annihilating electron-positron pairs: The many-body picture of natural geminals  

NASA Astrophysics Data System (ADS)

The correlated motion of a positron surrounded by electrons is a fundamental many-body problem. We approach this by modeling the momentum density of annihilating electron-positron pairs using the framework of reduced density matrices, natural orbitals, and natural geminals (electron-positron pair wave functions) of the quantum theory of many-particle systems. We find that an expression based on the natural geminals provides an exact, unique, and compact expression for the momentum density. The natural geminals can be used to define and to determine enhancement factors for enhancement models going beyond the independent-particle model for a better understanding of the results of positron annihilation experiments.

Makkonen, Ilja; Ervasti, Mikko M.; Siro, Topi; Harju, Ari

2014-01-01

296

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

297

Successful implementation of fast preamplifiers in a positron lifetime spectrometer  

NASA Astrophysics Data System (ADS)

A method to improve the long-term stability of fast coincidence apparatuses, like a positron lifetime spectrometer, is reported. Ageing of the photomultiplier tubes (PMT), i.e., nonreversible degradation of the gain, can be slowed down by lowering the supply voltages over the PMTs and by compensating the lower gain with fast preamplifiers set at the anodes. The timing characteristics of the PMTs can be preserved by using voltage dividers with which the voltage in the input optics of the PMTs remains high enough for good photoelectron collection efficiency (for XP2020 above 300 V). With this setup, the anode current and the rate of gain degradation can be reduced at least by a factor of 20 with no loss in the time resolution of the spectrometer.

Nissilä, J.; Rytsölä, K.; Saarinen, K.; Hautojärvi, P.

2002-04-01

298

Trapping of positrons at grain boundaries in nanoparticle systems  

Microsoft Academic Search

The positron annihilation in nanoparticle materials has been discussed in terms of the thermal diffusion of positrons at grain surfaces and trapping at the grain interfaces and interfacial defects. The diffusion trapping model has been applied to obtain the mean lifetime of positrons as a function of grain size and temperature and the S-parameter of the annihilation radiation as a

V Thakur; S B Shrivastava; M K Rathore

2004-01-01

299

Trapping of positrons at grain boundaries in nanoparticle systems  

Microsoft Academic Search

The positron annihilation in nanoparticle materials has been discussed in terms of the thermal diffusion of positrons at grain surfaces and trapping at the grain interfaces and interfacial defects. The diffusion trapping model has been applied to obtain the mean lifetime of positrons (\\\\bar {\\\\tau }) as a function of grain size and temperature and the S-parameter of the annihilation

V. Thakur; S. B. Shrivastava; M. K. Rathore

2004-01-01

300

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

301

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

302

Correlation between ferromagnetism and defects in MgO nanocrystals studied by positron annihilation  

NASA Astrophysics Data System (ADS)

High purity MgO nanopowders were pressed into pellets and annealed in air from 100 to 1400 °C. Variation of the microstructures was investigated by X-ray diffraction and positron annihilation spectroscopy. Annealing induces an increase in the MgO grain size from 27 to 60 nm with temperature increasing up to 1400 °C. Positron annihilation measurements reveal vacancy defects including Mg vacancies, vacancy clusters, microvoids and large pores in the grain boundary region. Rapid recovery of Mg monovacancies and vacancy clusters was observed after annealing above 1200 °C. Room temperature ferromagnetism was observed for MgO nanocrystals annealed at 100, 700, and 1000 °C. However, after 1400 °C annealing, MgO nanocrystals turn into diamagnetic. Our results suggest that the room temperature ferromagnetism in MgO nanocrystals might originate from the interfacial defects.

Wang, D. D.; Chen, Z. Q.; Li, C. Y.; Li, X. F.; Cao, C. Y.; Tang, Z.

2012-07-01

303

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

NASA Technical Reports Server (NTRS)

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

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

2005-01-01

304

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

NASA Technical Reports Server (NTRS)

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

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

2005-01-01

305

Some applications of perturbed angular correlation and positron annihilation to materials science  

Microsoft Academic Search

The perturbed angular correlation (PAC) and positron annihilation spectroscopy (PAS) that use nuclear probes to characterize\\u000a the micro-structure of materials are briefly described. Three examples are given to show their partial applications. The first\\u000a example is the study of radiation damage in Si irradiated by fast neutrons of 1.45·1020 cm?2 and 178W heavy ions of 5·1011 cm?2, respectively. The PAC

Shengyun Zhu; Yongnan Zheng; Yi Zuo; Dongmei Zhou; Daqing Yuan; Anli Li; Zhiqiang Wang; Xiao Duan; Meng Liu; Yong Li

2007-01-01

306

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

Microsoft Academic Search

We are beginning development of a Transition Edge Sensor (TES) based gamma-ray detector for use in Coincidence-Doppler-Broadening\\u000a (CDB) Positron-Annihilation-Spectroscopy (PAS) studies. We are specifically motivated to asses and study damage to nuclear\\u000a Reactor Pressure Vessels (RPVs) which results from neutron irradiation during reactor operation. In this paper we provide\\u000a a brief introduction to CDB-PAS and discuss our detector design.

Steven W. Leman; Hiroyuki Takahashi

2008-01-01

307

Microemulsion formation in sodium stearate-alcohol-hexadecane-water mixtures: studies by positron annihilation techniques  

Microsoft Academic Search

The study of micelle formation in sodium stearate-alcohol-hexadecane-water mixtures used positron annihilation techniqes. The water\\/oil ratio (WOR) at which reversed-micelle formation occurs was determined and correlated with properties such as the chain lengths of the cosurfactant. The radio trend found suggests a dependence of the WOR at which microemulsions are formed on the chain length of the cosurfactant alcohol, i.e.,

A. Boussaha; H. J. Ache

1980-01-01

308

Microemulsion formation in sodium stearate-alcohol-hexadecane-water mixtures: studies by positron annihilation techniques  

SciTech Connect

The study of micelle formation in sodium stearate-alcohol-hexadecane-water mixtures used positron annihilation techniqes. The water/oil ratio (WOR) at which reversed-micelle formation occurs was determined and correlated with properties such as the chain lengths of the cosurfactant. The radio trend found suggests a dependence of the WOR at which microemulsions are formed on the chain length of the cosurfactant alcohol, i.e., the ratio decreases with increasing chain length of the alcohol. 16 references.

Boussaha, A.; Ache, H.J.

1980-11-01

309

Structure of aqueous solutions of tetraethylammonium chloride investigated by positron annihilation and ultrasonic methods  

Microsoft Academic Search

The structure of aqueous solutions of tetraethylammonium chloride was investigated using compressibility and density measurements\\u000a and positron annihilation methods. The experimental results are different from those obtained earlier for systems where hydrophobic\\u000a hydration dominates, although some evidences for formation of cage-like hydrates in liquid phase were observed. The results\\u000a are interpreted, among others, in terms of competition among different hydrates

K. Jerie; A. Baranowski; Gy. Jákli; J. Gli?ski

1999-01-01

310

Event shapes and jet rates in electron-positron annihilation at NNLO  

NASA Astrophysics Data System (ADS)

This article gives the perturbative NNLO results for the most commonly used event shape variables associated to three-jet events in electron-positron annihilation: Thrust, heavy jet mass, wide jet broadening, total jet broadening, C parameter and the Durham three-to-two jet transition variable. In addition the NNLO results for the jet rates corresponding to the Durham, Geneva, Jade-E0 and Cambridge jet algorithms are presented.

Weinzierl, Stefan

2009-06-01

311

Observation of Native Ga Vacancies in GaN by Positron Annihilation  

Microsoft Academic Search

Positron annihilation experiments were performed to identify native point defects in n-type GaN bulk crystals as well as in epitaxial layers. The results show that Ga vacancies are present at concentrations 1017-1018 cm-3 in both GaN bulk crystals and layers. The Ga vacancies are negatively charged, and their concentration correlates with the intensity of the yellow luminescence. We conclude that

K. Saarinen; T. Laine; S. Kuisma; J. Nissilä; P. Hautojärvi; L. Dobrzynski; J. M. Baranowski; K. Pakula; R. Stepniewski; M. Wojdak; A. Wysmolek; T. Suski; M. Leszczynski; I. Grzegory; S. Porowski

1997-01-01

312

Detection of 511 keV positron annihilation radiation from the galactic center direction  

Microsoft Academic Search

A balloon-borne germanium ..gamma..-ray telescope was flown over Alice Springs, Australia, in an attempt to detect spectral features from the galactic center (GC) direction. A 511 keV positron annihilation line was detected at a flux level of (1.22 +- 0.22) x 10⁻³ photons s⁻¹ cm⁻². Suggestive evidence for the detection of the three-photon positronium (ps) continuum is presented. A brief

M. Leventhal; C. J. MacCallum; P. D. Stang

1978-01-01

313

Emission computer assisted tomography with single-photon and positron annihilation photon emitters  

Microsoft Academic Search

Computed transverse section emission tomography using \\/sup 99m\\/Tc with the Anger camera is compared to positron annihilation coincident detection using a ring of crystals and ⁶⁸Ga. The single-photon system has a line spread function (LSF) of 9 mm full width at half maximum (FWHM) at the collimator and gives a transverse section reconstruction LSF of 11 mm FWHM with 144

Thomas F. Budinger; Stephen E. Derenzo; Grant T. Gullberg; William L. Greenberg; Ronald H. Huesman

1977-01-01

314

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

SciTech Connect

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

Stern, S.H.

1982-01-01

315

Application of MÖssbauer Spectroscopy and Positron Annihilation Spectroscopy For Testing Of Neutron-Irradiated Reactor Steels  

Microsoft Academic Search

\\u000a The study is focused on the application of Mössbauer spectroscopy (MS) and positron annihilation spectroscopy (PAS) in the\\u000a evaluation of the microstructure parameters of materials used in nuclear industry. The practical applications of these methods\\u000a are documented on the evaluation of degradation processes currently in practise in nuclear power plant (NPP) reactor pressure\\u000a vessel (RPV) steels. The samples originating from

V. Sluge?; J. Lipka; J. Haš?íK; R. Gröne; I. Tóth; P. Uváíik; A. Zeman; K. Vitázek

316

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

317

On the origin of the positron annihilation radiation from the direction of the galactic center  

NASA Technical Reports Server (NTRS)

Physical constraints on the possible configuration of the positron annihilation region observed at the galactic center with gamma ray instrumentation focused on 511 keV emission are discussed. The line width is less than 2.5 keV, implying that the positrons annihilate in a gas that features some ionization. The line width also limits velocities to less than 700 km/sec, while the line center indicated a bulk velocity along the line of sight of between -90 to 200 km/sec. Data from emissions less than 511 keV suggest a production of 511 keV photons/positron ratio of 0.65 to account for the continuum emission detected. The annihilation region requires a temperature of at least 50,000 K and an ionization fraction of at least 10 percent, a density near 100,000 H/cu cm, and a diameter less than 10 to the 18th cm. Relativitistic considerations support pair production around an accreting black hole or the collision of collimated photon beams as possible sources of the emissions.

Lingenfelter, R. E.; Ramaty, R.

1982-01-01

318

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

SciTech Connect

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

Thomas W. Walters

2011-04-01

319

Effect of a finite-energy spread of the positron beam on the threshold behavior of the positron annihilation cross section  

SciTech Connect

In a recent paper [Phys. Rev. Lett. 88, 163202 (2002)] we established the threshold behavior of the cross section of positron-atom annihilation into two {gamma} quanta near the positronium (Ps)-formation threshold. Here, the near-threshold behavior of the positron 3{gamma} annihilation cross section and its relation to the ortho-Ps-formation cross section are determined. We also analyze the feasibility of observing these effects by examining the effect of the finite-energy resolution of a positron beam on the threshold behavior.

Ludlow, J.; Gribakin, G.F. [Department of Applied Mathematics and Theoretical Physics, Queen's University, Belfast BT7 1NN, Northern Ireland (United Kingdom)

2002-12-01

320

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

SciTech Connect

Positron annihilation induced Auger electron spectroscopy (PAES) measurements from the surface of an oxidized Cu(100) single crystal show a large increase in the intensity of the annihilation induced Cu M2,3VV Auger peak as the sample is subjected to a series of isochronal anneals in vacuum up to annealing temperature 300 deg. C. The PAES intensity then decreases monotonically as the annealing temperature is increased to {approx}550 deg. C. Experimental positron annihilation probabilities with Cu 3p and O 1s core electrons are estimated from the measured intensities of the positron annihilation induced Cu M{sub 2,3}VV and O KLL Auger transitions. PAES results are analyzed by performing calculations of positron surface states and annihilation probabilities of the surface-trapped positrons with relevant core electrons taking into account the charge redistribution at the surface and various surface structures associated with low and high oxygen coverages. The variations in atomic structure and chemical composition of the topmost layers of the oxidized Cu(100) surface are found to affect localization and spatial extent of the positron surface state wave function. The computed positron binding energy and annihilation characteristics reveal their sensitivity to charge transfer effects, atomic structure and chemical composition of the topmost layers of the oxidized Cu(100) surface. Theoretical positron annihilation probabilities with Cu 3p and O 1s core electrons computed for the oxidized Cu(100) surface are compared with experimental ones. The obtained results provide a demonstration of thermal reduction of the copper oxide surface after annealing at 300 deg. C followed by re-oxidation of the Cu(100) surface at higher annealing temperatures presumably due to diffusion of subsurface oxygen to the surface.

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

2011-06-01

321

Donor-doping effect in BaTiO 3 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 BaTiO 3 (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

322

Hunting for glueballs in electron-positron annihilation.  

PubMed

We calculate the cross section for the exclusive production of J(PC)=0(++) glueballs G0 in association with the J/psi in e(+)e(-) annihilation using the perturbative QCD factorization formalism. The required long-distance matrix element for the glueball is bounded by CUSB data from a search for resonances in radiative Upsilon decay. The cross section for e(+)e(-)-->J/psi+G0 at sqrt[s]=10.6 GeV is similar to exclusive charmonium-pair production e(+)e(-)-->J/psi+h for h=eta(c) and chi(c0), and is larger by a factor of 2 than that for h=eta(c)(2S). As the subprocesses gamma(*)-->(cc)(cc) and gamma(*)-->(cc)(gg) are of the same nominal order in perturbative QCD, it is possible that some portion of the anomalously large signal observed by Belle in e(+)e(-)-->J/psiX may actually be due to the production of charmonium-glueball J/psiG(J) pairs. PMID:14525415

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

2003-09-12

323

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

324

Gamma-ray spectra of hexane (C6H14) in positron–electron annihilation process  

NASA Astrophysics Data System (ADS)

Theoretical gamma-ray spectra of molecule hexane have been calculated and compared with the experimental results in both gas (Iwata et al., 1997a) and liquid (Kerr et al., 1965) phases. The present study reveals that in gas phase not all valence electrons of hexane contribute the same degree in the electron–positron annihilation of hexane. The electrons which dominate the positron–electron annihilation of molecules are called positrophilic electrons. The positrophilic electrons are predominately found to involve the electrons on the lowest occupied valence orbital (LOVO) of a free molecule in gas phase. When hexane is confined in liquid phase, however, the intermolecular interactions ultimately eliminate the free molecular orientation and selectivity for the positrophilic electrons in the gas phase. As a result, the gamma-ray spectra of hexane become an "averaged" contribution from all valence electrons, which is again in agreement with liquid phase measurement. The roles of valence electrons in annihilation process for gas and liquid phases of hexane have been recognized for the first time in the present study.

Ma, Xiaoguang; Wang, Feng

2013-08-01

325

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

PubMed

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

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

2013-01-23

326

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

SciTech Connect

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

Hugenschmidt, Christoph; Legl, Stefan [Technische Universitaet Muenchen, ZWEFRM-II, Lichtenbergstrasse 1, 85747 Garching (Germany) and Physik-Department E21, Technische Universitaet Muenchen, James-Franck-Strasse, 85748 Garching (Germany); Physik-Department E21, Technische Universitaet Muenchen, James-Franck-Strasse, 85748 Garching (Germany)

2006-10-15

327

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

328

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

NASA Astrophysics Data System (ADS)

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

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

1999-02-01

329

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

SciTech Connect

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

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

2009-10-09

330

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

PubMed

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

McDonald, John

2009-10-01

331

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

NASA Technical Reports Server (NTRS)

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

Rudaz, S.; Stecker, F. W.

1988-01-01

332

Microstructural origin of the dielectric breakdown strength in alumina: A study by positron lifetime spectroscopy  

Microsoft Academic Search

The dielectric breakdown strengths of two series of sintered alumina samples of low and high impurity content (with Si being the dominant element) and single crystal of low impurity level are compared with positron lifetime measurements. It is found that, in sintered alumina, the breakdown strength increases linearly with increasing concentration of positron traps at grain boundaries. These traps are

A. Si Ahmed; J. Kansy; K. Zarbout; G. Moya; J. Liebault; D. Gœuriot

2005-01-01

333

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

SciTech Connect

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

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

1982-03-01

334

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

335

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

SciTech Connect

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

Nascimento, E. do; Vanin, V. R.; Maidana, N. L.; Silva, T. F.; Rizzutto, M. A. [Instituto de Fisica, Universidade de Sao Paulo, Sao Paulo, SP (Brazil); Fernandez-Varea, J. M. [Facultat de Fisica (ECM), Universitat de Barcelona, Diagonal 645, E-08028 Barcelona (Spain)

2013-05-06

336

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

NASA Astrophysics Data System (ADS)

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

Stern, Stanley Hy.

337

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

338

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

339

Chemical analysis of precipitates in metallic alloys using coincidence Doppler broadening of positron annihilation radiation  

NASA Astrophysics Data System (ADS)

Two-detector coincidence Doppler broadening (CDB) method of positron annihilation radiation is applied to the study of precipitates in metals. As the first step, the CDB spectra for many kinds of elements are measured to obtain the "fingerprint" of each element for the chemical analysis of the precipitates in metals. Utilizing the fingerprint of Cu, we made chemical analysis of nano-particles formed in the initial stage of thermal aging in a Fe-Cu alloy, and found that the particles are consisting of Cu only and free from vacancies, which demonstrates the usefulness of this method in the study of the precipitates.

Nagai, Y.; Tang, Z.; Hasegawa, M.

2000-06-01

340

Vacancy evolution in Ni during irradiation at high temperatures studied by in situ positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

We present experimental results for in situ study of atomic-vacancy production and its evolution occurring during irradiation by using a slow-positron beam. Thermal stability of the vacancy produced during irradiation is investigated under elevated temperature conditions. An annealed Ni specimen was irradiated with 400 keV He ions at three different temperatures of 296 (RT), 368 and 713 K. Doppler broadenings of positron-annihilation ?-rays were measured and variation of the line-shape parameter S was observed under beam-on (during irradiation) and beam-off (non-irradiation) conditions. Results indicate that variation of the S depends on the specimen temperature, showing that vacancy cluster consisting of about 15 vacancies is formed predominantly via thermal evolution of atomic-sized vacancies under irradiation at high temperatures. We found that formation of the activated vacancies occurs during irradiation, which leads to vacancy clustering.

Tsuchida, H.; Iwai, T.; Kasai, S.; Tanaka, H.; Oshima, N.; Suzuki, R.; Yoshiie, T.; Itoh, A.

2011-01-01

341

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

342

Positron annihilation in (Ga, Mn)N: A study of vacancy-type defects  

SciTech Connect

The vacancy-type defects in (Ga,Mn)N films grown by metal organic chemical vapor deposition were studied by positron annihilation technique. Doppler broadening spectra were measured for the films. Compared to the undoped GaN film, the positron trapping defects in the (Ga,Mn)N films have been changed to a new type defects and its concentration increases with the increasing Mn concentration. By analyzing the S-W correlation plots and our previous results, we identify this type defects in the (Ga,Mn)N as V{sub N}-Mn{sub Ga} complex. This type of defects should be considered when understand the magnetic properties in a real (Ga,Mn)N system.

Yang, X. L.; Zhu, W. X.; Wang, C. D.; Fang, H.; Yu, T. J.; Yang, Z. J.; Zhang, G. Y. [State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871 (China); Qin, X. B.; Yu, R. S.; Wang, B. Y. [Key Laboratory of Nuclear Analysis Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China)

2009-04-13

343

New Limits on Dark Matter Annihilation from Alpha Magnetic Spectrometer Cosmic Ray Positron Data  

NASA Astrophysics Data System (ADS)

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

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

2013-10-01

344

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

345

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

346

Effect of temperature on the properties of mixed normal micelles in sodium octanoate-1-pentanol-water systems studied by positron annihilation technique  

Microsoft Academic Search

Positron annihilation technique was applied to the study of structural properties of mixed normal micelles in sodium octanoate-1-pentanol-water systems. Measurements of positron annihilation parameters over a wide range of temperature and surfactant concentrations demonstrate the extreme sensitivity of the technique toward the structural changes associated with the second and third critical concentrations. While the third critical concentration appears to be

Ali Boussaha; Hans J. Ache

1981-01-01

347

Positron Annihilation Studies of the Electronic Structure of Selected High-Temperature Cuprate and Organic Superconductors.  

NASA Astrophysics Data System (ADS)

The understanding of the electronic structure of the high-T_{c} superconductors could be important for a full theoretical description of the mechanism behind superconductivity in these materials. In this thesis, we present our measurements of the positron -electron momentum distributions of the cuprate superconductors Bi_2Sr_2CaCu _2O_8, Tl _2Ba_2Ca _2Cu_3O_ {10}, and the organic superconductor kappa-(BEDT)_2Cu(NCS) _2. We use the positron Two-dimensional Angular Correlation of Annihilation Radiation technique to make the measurements on single crystals and compare our high-statistics data with band structure calculations to determine the existence and nature of the respective Fermi surfaces. The spectra from unannealed Bi _2Sr_2CaCu _2O_8 exhibit effects of the superlattice modulation in the BiO_2 layers, and a theoretical understanding of the modulation effects on the electronic band structure is required to interpret these spectra. Since the present theory does not consider the modulation, we have developed a technique to remove the modulation effects from our spectra, and the resultant data when compared with the positron -electron momentum distribution calculation, yield features consistent with the predicted CuO_2 and BiO_2 Fermi surfaces. In the data from unannealed Tl_2Ba _2Ca_2Cu_3 O_{10}, we only observe indications of the TlO Fermi surfaces, and attribute the absence of the predicted CuO_2 Fermi surfaces to the poor sample quality. In the absence of positron-electron momentum calculations for kappa-(BEDT)_2Cu(NCS) _2, we compare our data to electronic band structure calculations, and observed features suggestive of the predicted Fermi surface contributions from the BEDT cation layers. A complete positron-electron calculation for kappa-(BEDT)_2 Cu(NCS)_2 is required to understand the positron wavefunction effects in this material.

Chan, Lie Ping

348

Electronic structure calculations of positron lifetimes in nuclear materials: SiC and UO2  

NASA Astrophysics Data System (ADS)

We present first-principles calculations of positron lifetimes of vacancy-type defects in two nuclear materials: SiC and UO2. We use a self-consistent positron lifetime calculation scheme based on the two-component density functional theory. Full defect relaxation due to both the creation of the vacancy and the presence of the positron was taken into account. Our results for SiC differ strongly from those published in literature up to now [G. Brauer et al. Phys. Rev. B 54, 2512 (1996)]. This is mostly due to the effect of the relaxation, that was not taken into account before. We also present the first calculated positron lifetimes obtained for UO2 in the DFT+U approach. Results are compared with the experimental data.

Wiktor, Julia; Jomard, Gérald; Freyss, Michel; Bertolus, Marjorie

2014-06-01

349

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

SciTech Connect

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

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

2011-04-15

350

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

351

An investigation of molecular structure of copolymers using positron annihilation spectroscopy  

NASA Technical Reports Server (NTRS)

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

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

1985-01-01

352

Polymeric membrane studied using slow positron beam  

Microsoft Academic Search

A radioisotope slow positron beam has been built at the Chung Yuan Christian University in Taiwan for the research and development in membrane science and technology. Doppler broadening energy spectra and positron annihilation lifetime have been measured as a function of positron energy up to 30keV in a polyamide membrane prepared by the interfacial polymerization between triethylenetetraamine (TETA) and trimesoyl

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

2008-01-01

353

Annealing process in quenched Al-Sn alloys:?A positron annihilation study  

NASA Astrophysics Data System (ADS)

The precipitation of tin atoms and vacancy-tin interaction in the diluted (800 at. ppm) Al-Sn alloy were studied by means of positron-lifetime and coincidence Doppler broadening techniques. In particular, the specimen subjected to the solution treatment at 525 °C/2 h finished by the rapid quenching down to room temperature was examined and, furthermore, the stability of the as-quenched microstructure at room temperature was tested. At room temperature we observed Sn atoms bound to quenched-in excess vacancies. Subsequently, the specimen was isochronally annealed up to the temperature of the solution treatment and the processes, which take place when increasing the temperature, were investigated. Among others, we could monitor the Sn nanoclusters’ formation that starts at about 130 °C. Positron measurements were complemented by calculations of several positron characteristics of Al, Sn, and related defects in order to ensure the consistent interpretation of experimental data.

?ížek, Jakub; Melikhova, Oksana; Procházka, Ivan; Kuriplach, Jan; Stulíková, Ivana; Vostrý, Petr; Faltus, Ji?í

2005-02-01

354

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

Microsoft Academic Search

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

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

1983-01-01

355

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

356

Positron Annihilation Studies of Cyclic Fatigue Damage in Metals and Aging in Polymers  

Microsoft Academic Search

Positron lifetime measurements have been performed on fatigued nickel samples. Both ex-situ type of source (^{22}Na source deposited on a nickel foil in a sandwich geometry) and in-situ source (^{58}Co) (produced by proton irradiation of the nickel sample through the reaction ^{61}Ni(p,alpha )^{58}Co) were used. Specimens were both flexurally and axially fatigued. Spectra were analyzed by resolving into three and

Nokuleswar Panigrahi

1987-01-01

357

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

SciTech Connect

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

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

1998-01-01

358

Bulk materials analysis using high-energy positron beams  

NASA Astrophysics Data System (ADS)

This article reviews some recent materials analysis results using high-energy positron beams at Lawrence Livermore National Laboratory. We are combining positron lifetime and orbital electron momentum spectroscopic methods to provide electron number densities and electron momentum distributions around positron annihilation sites. Topics covered include the correlation of positron annihilation characteristics with structural and mechanical properties of bulk metallic glasses and compositional studies of embrittling features in nuclear reactor pressure vessel steel.

Glade, S. C.; Asoka-Kumar, P.; Nieh, T. G.; Sterne, P. A.; Wirth, B. D.; Dauskardt, R. H.; Flores, K. M.; Suh, D.; Odette, G. R.

2003-08-01

359

Positron-annihilation studies of cyclic fatigue damage in metals and aging in polymers  

SciTech Connect

Positron-lifetime measurements were performed on fatigued nickel samples. Both ex-situ type of source (/sup 22/Na source deposited on a nickel foil in a sandwich geometry) and in-situ source (/sup 58/Co) (produced by proton irradiation of the nickel sample through the reaction /sup 61/Ni(p,..cap alpha..)/sup 58/Co) were used. Specimens were both flexurally and axially fatigued. Spectra were analyzed by resolving into three and four exponentials. In contrast to other studies, positrons are found to be sensitive to defects formed during the cyclic fatiguing. These data were analyzed in terms of various trapping models. The results could be explained by assuming the detrapping of positrons from defect sites. In both types of fatigued specimens the lifetimes of the defect clusters decreased with increasing intensities, showing either the proliferation of smaller clusters or the formation of the new relaxed microstructures. Advantage of using in-situ source for the study of fatigue cycling is stressed. Quenching experiments were performed on polystyrene and polyvinyl acetate samples. In the former the long lifetime (tau/sub 3/) increased with aging, while the intensity decreased. These indicate that the cavities in the polymers getting bigger while becoming fewer in number.

Panigrahi, N.

1987-01-01

360

A slow positron beam generator for lifetime studies  

Microsoft Academic Search

A slow positron beam generator using well-annealed polycrystalline tungsten moderators and a Na-22 positron source was developed. A 250 micro c source, deposited on a 2.54 micron thick aluminized mylar, is sandwiched between two (2.54 cm x 2.54 cm x 0.0127 cm) tungsten pieces. Two (2.54 cm x 2.54 cm x t cm) test polymer films insulate the two tungsten

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

1989-01-01

361

A slow positron beam generator for lifetime studies  

SciTech Connect

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

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

1989-04-01

362

A slow positron beam generator for lifetime studies  

NASA Technical Reports Server (NTRS)

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

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

1989-01-01

363

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

NASA Astrophysics Data System (ADS)

The room temperature positron annihilation lifetime spectra and 57Fe Mössbauer spectra were measured for pure Fe as well as for iron-based Fe1-xRex, Fe1-xOsx, Fe1-xMox, and Fe1-xCrx solid solutions, where x is in the range between 0.01 and 0.05. The measurements were performed in order to check if the known from the literature, theoretical calculations on the interactions between vacancies and solute atoms in iron can be supported by the experimental data. The vacancies were created during formation and further mechanical processing of the iron systems under consideration so the spectra mentioned above were collected at least twice for each studied sample synthesized in an arc furnace— after cold rolling to the thickness of about 40 ?m as well as after subsequent annealing at 1270 K for 2 h. It was found that only in Fe and the Fe-Cr system the isolated vacancies thermally generated at high temperatures are not observed at the room temperature and cold rolling of the materials leads to creation of another type of vacancies which were associated with edge dislocations. In the case of other cold-rolled systems, positrons detect vacancies of two types mentioned above and Mössbauer nuclei "see" the vacancies mainly in the vicinity of non-iron atoms. This speaks in favour of the suggestion that in iron matrix the solute atoms of Os, Re, and Mo interact attractively with vacancies as it is predicted by theoretical computations and the energy of the interaction is large enough for existing the pairs vacancy-solute atom at the room temperature. On the other hand, the corresponding interaction for Cr atoms is either repulsive or attractive but smaller than that for Os, Re, and Mo atoms. The latter is in agreement with the theoretical calculations.

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

2014-03-01

364

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

365

OSSE Mapping of Galactic 511 keV Positron Annihilation Line Emission  

NASA Astrophysics Data System (ADS)

The Oriented Scintillation Spectrometer Experiment (OSSE) observations of the Galactic plane and Galactic center region have been combined with scanning observations by the Transient Gamma-Ray Spectrometer (TGRS) and Solar Maximum Mission (SMM) instruments to produce maps of the Galactic narrow 511 keV positron annihilation line radiation. Two different mapping methods, singular value decomposition and maximum entropy, have been applied to the data. In both cases, the resulting maps show evidence for three distinct features: (1) a central bulge, (2) emission in the Galactic plane, and (3) an enhancement or extension of emission at positive latitudes above the Galactic center. Modeling of the data confirmed the existence of these features. The derived distribution is found to be in good qualitative agreement with nearly all of the historical observations of narrow 511 keV line emission from the Galactic center region. No evidence of time variability is found. Various possible production mechanisms for the observed positrons, including the positive-latitude enhancement, are presented. It is found that supernovae are capable of producing positrons at the required rate to account for the intensity and morphology of the observed 511 keV line emission.

Purcell, W. R.; Cheng, L.-X.; Dixon, D. D.; Kinzer, R. L.; Kurfess, J. D.; Leventhal, M.; Saunders, M. A.; Skibo, J. G.; Smith, D. M.; Tueller, J.

1997-12-01

366

Behavior of structure defects and hydrogen in neutron-irradiated stainless steels studied by positron annihilation method  

Microsoft Academic Search

Measurements of angular distributions of annihilation photons are applied to investigating structural defects and hydrogen behavior in annealed, plastically deformed, and irradiated stainless steels. It is determined that the whole cycle of investigations performed by positron diagnostics helped in tracing the formation and evolution of the defect structure and hydrogen behavior in different kinds of steels being subjected to complex

K. M. Mukashev; B. A. Tronin; F. F. Umarov

2009-01-01

367

Next-to-Next-to-Leading Order Corrections to Three-Jet Observables in Electron-Positron Annihilation  

SciTech Connect

I report on a numerical program, which can be used to calculate any infrared safe three-jet observable in electron-positron annihilation to next-to-next-to-leading order in the strong coupling constant {alpha}{sub s}. The results are compared to a recent calculation by another group. Numerical differences in three color factors are discussed and explained.

Weinzierl, Stefan [Institut fuer Physik, Universitaet Mainz, D-55099 Mainz (Germany)

2008-10-17

368

Moments of event shapes in electron-positron annihilation at next-to-next-to-leading order  

SciTech Connect

This article gives the perturbative next-to-next-to-leading order results for the moments of the most commonly used event shape variables associated to three-jet events in electron-positron annihilation: thrust, heavy jet mass, wide jet broadening, total jet broadening, C parameter and the Durham three-to-two-jet transition variable.

Weinzierl, Stefan [Institut fuer Physik, Universitaet Mainz, D-55099 Mainz (Germany)

2009-11-01

369

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

Microsoft Academic Search

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

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

1992-01-01

370

Defects in ZnO thin films grown on ScAlMgO4 substrates probed by a monoenergetic positron beam  

Microsoft Academic Search

Zinc oxide (ZnO) thin films grown on ScAlMgO4 substrates were characterized by means of positron annihilation. We measured Doppler broadening spectra of annihilation radiation and photoluminescence spectra for the ZnO films deposited by laser molecular-beam epitaxy and single-crystal ZnO. Although the lifetime of positrons in single-crystal ZnO was close to the lifetime of positrons annihilated from the free state, the

A. Uedono; T. Koida; A. Tsukazaki; M. Kawasaki; Z. Q. Chen; Sf. Chichibu; H. Koinuma

2003-01-01

371

Positron Annihilation Studies of Cyclic Fatigue Damage in Metals and Aging in Polymers.  

NASA Astrophysics Data System (ADS)

Positron lifetime measurements have been performed on fatigued nickel samples. Both ex-situ type of source (^{22}Na source deposited on a nickel foil in a sandwich geometry) and in-situ source (^{58}Co) (produced by proton irradiation of the nickel sample through the reaction ^{61}Ni(p,alpha )^{58}Co) were used. Specimens were both flexurally and axially fatigued. Spectra were analyzed by resolving into three and four exponentials. In contradistinction to other studies, positrons are found to be sensitive to defects formed during the cyclic fatiguing. These data were analyzed in terms of various trapping models. The results could be explained by assuming the detrapping of positrons from defect sites. In both types of fatigued specimens the lifetimes of the defect clusters decreased with increasing intensities, showing either the proliferation of smaller clusters or the formation of the new "relaxed" microstructures. Advantage of using in-situ source for the study of fatigue cycling is stressed. Also in the analysis of the results in the third set of the flexurally fatigued samples, rich details of data could be obtained which occurred beyond the saturation of the average lifetime. Quenching experiments were performed on polystyrene and polyvinyl acetate samples. In the former the long lifetime (tau_3) increased with aging, while the intensity decreased. These indicate that the cavities in the polymers getting bigger while becoming fewer in number. In the preliminary results of the PVAC, the tau_3 showed oscillations with aging. A 2-Dimensional angular correlation system was set up. A 2D spectrum could have been taken, but for the lack of a 2D-ADC, an 1D spectrum for nickel was acquired. A model calculation showing the relative merits of the 1D -angular correlation, Doppler Broadening and 2D-angular correlation is presented. Suggestions for improvement are made.

Panigrahi, Nokuleswar

1987-09-01

372

Location of solubilizate in water-AOT-isooctane systems using positron lifetime spectroscopy  

Microsoft Academic Search

In the present work, positrons have been used as a probe to locate the interaction site of the additive molecules in a membrane mimetic system similar to reverse micelles. The systems investigated are water-AOT Aerosol OT [Sodium bis(2-ethylhexyl) sulfosuccinate)-isooctane solutions. Positron lifetime measurements have been performed in water-AOT-isooctane solutions containing chloroform and dichloromethane, respectively, as additives. The choice of these

M. Zaboli; P. C. Jain; A. N. Maitra

1991-01-01

373

Free volume and phase transitions of 1-butyl-3-methylimidazolium based ionic liquids from positron lifetime spectroscopy.  

PubMed

Positron annihilation lifetime spectroscopy (PALS) was used to study a series of ionic liquids (ILs) with the 1-butyl-3-methylimidazolium cation ([C(4)MIM](+)) but different anions [Cl](-), [BF(4)](-), [PF(6)](-), [OTf](-), [NTf(2)](-), and [B(hfip)(4)](-) with increasing anion volumes. Changes of the ortho-positronium (o-Ps) lifetime parameters with temperature were observed for crystalline and amorphous (glass, supercooled, and normal liquid) states. Evidence for distinct phase transitions, e.g. melting, crystallization and solid-solid transitions, was observed in several PALS experiments. The o-Ps mean lifetime ??(3)? showed smaller values in the crystalline phase due to dense packing of the material compared to the amorphous phase. The o-Ps lifetime intensity I(3) in the liquid state is clearly smaller than in the crystallized state. This behaviour can be attributed to a solvation of e(+) by the anions, which reduces the Ps formation probability in the normal and supercooled liquid. These phenomena were observed for the first time when applying the PALS technique to ionic liquids by us in one preliminary and in this work. Four of the ionic liquids investigated in this work ([BF(4)](-), [NTf(2)](-), [PF(6)](-) and [Cl](-) ILs) exhibit supercooled phases. The specific hole densities and occupied volumes of those ILs were obtained by comparing the local free volume with the specific volume from pressure-volume-temperature (PVT) experiments. From the o-Ps lifetime, the mean size ?v(h)? of free volume holes of the four samples was calculated and compared with that calculated according to Fürth's hole theory. The hole volumes from both methods agree well. From the Cohen-Turnbull fitting of viscosity and conductivity against PALS/PVT results, the influence of the free volume on molecular transport properties was investigated. PMID:22472912

Yu, Yang; Beichel, Witali; Dlubek, Günter; Krause-Rehberg, Reinhard; Paluch, Marian; Pionteck, Jürgen; Pfefferkorn, Dirk; Bulut, Safak; Friedrich, Christian; Pogodina, Natalia; Krossing, Ingo

2012-04-01

374

Positron annihilation studies of fluorine-vacancy complexes in phosphorus- and fluorine-implanted germanium  

NASA Astrophysics Data System (ADS)

The formation of FnV2 complexes, with n = 5 ± 1, near the end-of-range damage region in germanium implanted with 30 keV phosphorus and 40 keV fluorine ions, after annealing to 400 °C, has been observed using variable-energy positron annihilation spectroscopy in conjunction with secondary ion mass spectrometry. Phosphorus ions were implanted at 6 × 1013 and 1015, F at 1015 cm-2. Complexes—at lower concentrations—have also been observed at shallower depths in samples implanted with P at 1015 cm-2. The complexes break up and their components diffuse away at 450 and 500 °C for the higher and lower P dose samples, respectively.

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

2014-03-01

375

Are mildly active galaxies sources of electron-positron annihilation radiation?  

NASA Technical Reports Server (NTRS)

Mildly active galaxies are examined as possible sources of 511-keV line radiation resulting from electron-positron annihilations. Observations of seven galaxies of various types are made using an HEAO-3 gamma-ray spectrometer. It is shown that none of the galaxies are detected in the 511 keV line and therefore the proposed scaling law for 511-keV line emission from the galactic nuclei is not valid. The results indicate that the scaling overestimates the 511-keV line flux by a factor of 3 or more. Possible causes of this discrepancy are considered. The expected values of and the observed 2.3-sigma upper limits to the 511-keV line flux for each source and the Galactic Center are presented in a table.

Marscher, A. P.; Brecher, K.; Wheaton, W. A.; Ling, J. C.; Mahoney, W. A.; Jacobson, A. S.

1983-01-01

376

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

377

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

378

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

NASA Astrophysics Data System (ADS)

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

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

2013-11-01

379

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

380

A superstring Z{prime} boson in radiative electron-positron annihilation  

SciTech Connect

The radiative electron-positron annihilation e{sup -}e{sup +} {yields} {gamma}X is studied in the SU(2) x U(1) and SU(2) x U(1) x U{prime}(1) gauge models. The effective cross section and various polarization characteristics [such as right-left asymmetry A{sub RL}, the effect N({lambda}{sub 2}) of positron-beam polarization, and the degree P of photon polarization] are expressed in terms of structure functions for arbitrary polarizations of the electron-positron pair and the circular polarization of photons. Structure functions are found for the processes e{sup -}e{sup +} {yields} {gamma}f{bar f}, e{sup -}e{sup +} {yields} {gamma} + hadrons, e{sup -}e{sup +} {yields} {gamma}{bar f}{bar f}, e{sup -}e{sup +} {yields} {gamma}H{sup +}H{sup -}, e{sup -}e{sup +} {yields} {gamma}H{sub s}{sup o}H{sub p}{sup o}, e{sup -}e{sup +} {yields} {gamma}{bar H}{sup +}{bar H}, and e{sup -}e{sup +} {yields} {gamma} {bar H}{sub s}{sup o}{bar H}{sub p}{sup o}, and electroweak asymmetries are analyzed in detail. The study of polarization characteristics of the processes e{sup -}e{sup +} {yields} {gamma}X is shown to provide valuable information about the properties of an extra superstring Z{prime} boson. 30 refs., 6 figs.

Abdullaev, S.K.; Mukhtarov, A.I. [Baku State Univ. (Azerbaijan)

1995-07-01

381

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

NASA Technical Reports Server (NTRS)

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

Ray, Asit K.

1995-01-01

382

Positron annihilation investigations of vacancies in InP produced by electron irradiation at room temperature  

SciTech Connect

Positron lifetime investigations were done on a series of InP samples irradiated to various doses with 2.5 MeV electrons. In n-type materials, positron lifetimes of 265{plus_minus}5 and 338{plus_minus}15 ps are attributed to indium vacancy{endash}interstitial complexes and divacancy{endash}interstitial complexes, respectively. In p-type materials these defects were not observed. Thermal annealing took place up to 200{degree}C for both defect types. Introduction rates were estimated to be 0.1 cm{sup {minus}1} for V{sub In}{center_dot}In{sub I} and {approximately}0.05 cm{sup {minus}1} for the divacancies. The divacancies showed a temperature dependence of the trapping rate, which suggests a thermally activated process. No evidence for V{sub P} vacancies could be found in neither p-type nor n-type materials. {copyright} {ital 1997 American Institute of Physics.}

Bretagnon, T.; Dannefaer, S.; Kerr, D. [University of Winnipeg, Winnipeg, Manitoba R3B 2E9 (Canada)] [University of Winnipeg, Winnipeg, Manitoba R3B 2E9 (Canada)

1997-04-01

383

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

SciTech Connect

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

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

2004-09-01

384

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

SciTech Connect

The recent data from PAMELA and Fermi-LAT can be interpreted as evidence of new astrophysical sources of high energy positrons. In that case, such astrophysical positrons constitute an additional background against the positrons from dark matter annihilation. In this paper, we study the effect of that background on the prospects for the detection of a positron dark matter signal in future experiments. In particular, we determine the new regions in the (mass, <{sigma}v>) plane that are detectable by the AMS-02 experiment for several dark matter scenarios and different propagation models. We find that, due to the increased background, these regions feature annihilation rates that are up to a factor of 3 larger than those obtained for the conventional background. That is, an astrophysical interpretation of the present data by PAMELA and Fermi-LAT implies that the detection of positrons from dark matter annihilation is slightly more challenging than previously believed.

Choi, Ki-Young [Departamento de Fisica Teorica and Instituto de Fisica Teorica UAM-CSIC, Universidad Autonoma de Madrid, Cantoblanco, E-28049 Madrid (Spain); Department of Physics, Pusan National University, Busan 609-735 (Korea, Republic of); Yaguna, Carlos E. [Departamento de Fisica Teorica and Instituto de Fisica Teorica UAM-CSIC, Universidad Autonoma de Madrid, Cantoblanco, E-28049 Madrid (Spain)

2010-01-15

385

Positron annihilation, X-ray and ESR studies of iron ion implanted crystals of potassium bromide and quartz  

Microsoft Academic Search

The association of electron spin resonance. X-ray diffraction and positron annihilation techniques has been applied to the study of radiation-induced defects in iron implanted single crystals of KBr and quartz with the implant dose of 5 × 1015 ions\\/cm2. Two kinds of defect creation processes due to the ion bombardment are found in these crystals. In the ionic crystal KBr,

Wang Guang-Hou; Sang Hai; Pang Guo-Giang; Chen Ling; Dou Lie; Shen de-Xun; Teng Ming-Kang; Li Siang-Jin; Wang Ke-Ming; Wang Yi-Hua; Liu Jing-Tian; Jin Tong-Zhen; Hang She-Ing

1988-01-01

386

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

387

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

Microsoft Academic Search

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

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

1973-01-01

388

Study of the forces responsible for binding of polar substrates in the cyclohexaamylose cavity by positron annihilation techniques  

Microsoft Academic Search

The complex formation constants for a series of inclusion complexes of p-, m-, and o-nitrophenol and p-, m-, and o-nitrobenzyl alcohol with cyclohexaamylose were determined by positron annihilation techniques. The formation constants, K\\/sub c\\/, were found to be 341, 54, 8, 28, 25, and 9 M⁻¹, respectively. The results seem to suggest that hydrogen bonding and\\/or London dispersion forces are

Eugene S. Hall; Hans J. Ache

1979-01-01

389

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

390

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

391

Hadron production in electron-positron annihilation computed from the gauge-gravity correspondence  

SciTech Connect

We provide a nonperturbative expression for the hadron production in electron-positron annihilation at zero temperature in a strongly coupled, large-N{sub c} SU(N{sub c}) field theory with N{sub f}<

Patino, Leonardo; Toledo Sanchez, G. [Departamento de Fisica, Facultad de Ciencias, Universidad Nacional Autonoma de Mexico, Mexico D.F. 04510 (Mexico); Instituto de Fisica, Universidad Nacional Autonoma de Mexico, Mexico D.F. 04510 (Mexico)

2009-12-15

392

Effect of ?-PARTICLE Energies on CR-39 Line-Shaped Parameters Using Positron Annihilation Technique (pat)  

NASA Astrophysics Data System (ADS)

Polyallyl diglycol carbonate "CR-39" is widely used in etched track-type particle detectors. Doppler-broadening positron annihilation (DBPAT) provides direct information about core and valence electrons in CR-39 due to radiation effects. It provides a non-destructive and non-interfering probe having a detecting efficiency. This paper reports the effect of irradiation ?-particle intensity emitted from an 241Am (5.486 MeV) source on the line-shaped S- and W-parameters for the CR-39 samples. The behavior of the line-shaped S- and W-parameters can be related to the different phases. Modification of the CR-39 samples due to irradiation were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The XRD chart reveals a new peak that starts to appear at 2.86 MeV ?-particle energy. The appearance of this peak might be related to the phase transition. The phase transition in the CR-39 polycarbonate remains complex.

Abdel-Rahman, M. A.; Abdel-Rahman, M.; Lotfy, Yahia A.; Badawi, Emad A.; Abo-Elsoud, M.; Eissa, M.

393

Post-irradiation annealing behavior of microstructure and hardening of a reactor pressure vessel steel studied by positron annihilation and atom probe tomography  

NASA Astrophysics Data System (ADS)

Post-irradiation annealing (PIA) behavior of irradiation-induced microstructural changes and hardening of an A533B (0.16 wt.% Cu) steel after neutron-irradiation of 3.9 × 1019 n cm-2 (0.061 displacement per atom (dpa)) at 290 °C was studied by positron annihilation spectroscopy (PAS), atom probe tomography (APT) and Vickers microhardness measurements. Coincidence Doppler broadening and positron lifetime measurements clearly reveal two recovery stages; (i) as-irradiated state to annealing at 450 °C and (ii) annealing from 450 to 600 °C. The first stage is due to annealing out of the most of irradiation-induced vacancy-related defects, while the second stage corresponds to dissolving of irradiation-induced solute nanoclusters (SCs). APT observations reveal that the SCs are enriched with Cu, Mn, Ni and Si and that their number densities decrease with increasing annealing temperature without coarsening to give almost complete recovery at 550 °C. The experimental hardening is almost twice the SC hardening estimated by the Russell-Brown model below 350 °C, whereas it is almost the same as that estimated in the range 400-550 °C.

Kuramoto, A.; Toyama, T.; Takeuchi, T.; Nagai, Y.; Hasegawa, M.; Yoshiie, T.; Nishiyama, Y.

2012-06-01

394

Thermal vacancies and positron-lifetime measurements in Fe sub 76. 3 Al sub 23. 7  

SciTech Connect

The formation of thermal vacancies in the intermetallic compound Fe{sub 76.3}Al{sub 23.7} was investigated between 20 and 1060 {degree}C by positron-lifetime measurements. The positron lifetime {tau}{sub {ital f}}=112 ps at 20 {degree}C indicates that no structural vacancies can be detected. The increase of the mean positron lifetime {bar {tau}} due to thermal vacancy formation starts at relatively low temperatures ({ital T}{sub 3}=475 {degree}C). The fit of a simple two-state trapping model to the temperature variation of {bar {tau}} yields an effective vacancy formation enthalpy {ital H}{sup {ital F}}=1.18{plus minus}0.04 eV, which suggests a thermal vacancy concentration at the melting temperature of several atomic percent which is much higher than in pure metals and similar to that found in some intermetallic compounds with a {ital B}2 structure. From a comparison with self-diffusion results, a vacancy migration enthalpy of {ital H}{sup {ital M}}=1.4--1.6 eV is deduced. The phase transitions in the present alloy are discussed in terms of vacancy formation and migration.

Schaefer, H.; Wuerschum, R. (Institut fuer Theoretische und Angewandte Physik, Universitaet Stuttgart, Postfach 801140, D-7000 Stuttgart 80 (Federal Republic of Germany)); Sob, M.; Zak, T. (Institute of Metallurgy, Czechoslovak Academy of Sciences, Zizkova 22, CS-61662 Brno (Czechoslovakia)); Yu, W.Z. (Department of Physics, Qinghua University, Beijing 100084 (People's Republic of China)); Eckert, W. (Institut fuer Theoretische und Angewandte Physik, Universitaet Stuttgart, Postfach 801140, D-7000 Stuttgart 80 (Federal Republic of Germany)); Banhart, F. (Institut fuer Physik, Max-Planck-Institut fuer Metallforschung, D-7000 Stuttgart 80 (Federal Republic of Germany))

1990-06-15

395

Revealing the nano-level molecular packing in chitosan-NiO nanocomposite by using positron annihilation spectroscopy and small-angle X-ray scattering.  

PubMed

Chitosan-NiO nanocomposite (CNC) is shown to be a potential dielectric material with promising properties. CNCs containing NiO nanoparticles (0.2, 0.6, 1, 2, 5 wt?%) are prepared through chemical methods. The inclusion of NiO nanoparticles in the chitosan matrix is confirmed by scanning electron microscopy (SEM) and X-ray diffraction. The morphology of the NiO nanoparticles and the nanocomposites is investigated by transmission electron microscopy and SEM, respectively. Positron annihilation lifetime spectroscopy (PALS) and the coincidence Doppler broadening (CDB) technique are used to quantify the free volume and molecular packing in the nanocomposites. The triplet-state positronium lifetime and the corresponding intensity show the changes in nanohole size, density, and size distribution as a function of NiO loading. Small-angle X-ray scattering indicates that the NiO aggregates are identical in all the CNCs. The momentum density distribution obtained from CDB measurements excludes the possibility of a contribution of vacant spaces (pores) available in NiO aggregates to the free volume of nanocomposites upon determination by using PALS. The results show systematic variation in free-volume properties and nano-level molecular packing as a function of NiO loading, which is presumed to play a vital role in determining the various properties of the nanocomposites. PMID:23418038

Sharma, Sandeep K; Bahadur, Jitendra; Patil, Pushkar N; Maheshwari, Priya; Mukherjee, Saurabh; Sudarshan, Kathi; Mazumder, Subhasish; Pujari, Pradeep K

2013-04-01

396

On the Morphology of the Electron-Positron Annihilation Emission as Seen by Spi/integral  

NASA Astrophysics Data System (ADS)

The 511 keV positron annihilation emission remains a mysterious component of the high energy emission of our Galaxy. Its study was one of the key scientific objectives of the SPI spectrometer on board the International Gamma-Ray Astrophysics Laboratory satellite. In fact, a lot of observing time has been dedicated to the Galactic disk with a particular emphasis on the central region. A crucial issue in such an analysis concerns the reduction technique used to treat this huge quantity of data, and more particularly the background modeling. Our method, after validation through a variety of tests, is based on detector pattern determination per ~6 month period, together with a normalization variable on a few hour timescale. The Galactic bulge is detected at a level of ~70?, allowing more detailed investigations. The main result is that the bulge morphology can be modeled with two axisymmetric Gaussians of 3fdg2 and 11fdg8 FWHM and respective fluxes of 2.5 and 5.4 × 10^{-4} photons cm^{-2} s^{-1}. We found a possible shift of the bulge center toward negative longitude at l = -0fdg6 ± 0fdg2. In addition to the bulge, a more extended structure is detected significantly with flux ranging from 1.7 to 2.9 × 10^{-3} photons cm^{-2} s^{-1} depending on its assumed geometry (pure disk or disk plus halo). The disk emission is also found to be symmetric within the limits of the statistical errors. Based on observations with INTEGRAL, an ESA project with instruments and science data center funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Spain, and Switzerland), the Czech Republic, and Poland, with the participation of Russia and USA.

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

2010-09-01

397

Temperature dependence of the free volume from positron lifetime experiments and its relation to structural dynamics: phenylphthalein-dimethylether.  

PubMed

Positron annihilation lifetime spectroscopy (PALS) was used to study the microstructure of the free volume in the temperature range between 103 K and 393 K in phenylphthalein-dimethylether (PDE), a low-molecular-weight glass former. Using the routine LIFETIME9.0, the ortho-positronium (o-Ps) lifetime distribution was analyzed, and from this, the volume distribution gn(vh) of subnanometer-size holes was calculated. From a comparison of PALS and specific volume data, the number density and the volume fraction of holes were estimated. These free-volume data, as a function of temperature, were used to test the validity of the Cohen-Turnbull (CT) free-volume theory. It was found that the structural relaxation from dielectric spectroscopy can be described by the CT theory after introducing a corrected free volume (Vf-DeltaV), where DeltaV=0.014 cm3/g. The extended free-volume theory of Cohen and Grest can be fitted to the dielectric-relaxation and free-volume data, but the parameters of both fits are not consistent. PDE shows some peculiar features. The "knee" in the o -Ps lifetime expansion and crossover in temperature dependence of the frequency of the primary dielectric relaxation process occur at different temperatures. In addition, the change in the Vogel-Fulcher-Tammann parameters at TB/Tg=1.1 has no observable effect on the mean free volume vh (or Vf). The size of the smallest representative freely fluctuating subsystem, VSV estimated from the standard deviation sigmah of gn(vh), decreases from 4.1 nm3 to 2.6 nm3 when the temperature increases from T/Tg=1.0 to 1.15. Correspondingly, the length of dynamic heterogeneity, xi=VVS1/3, decreases from 1.6 nm to 1.4 nm. It is concluded that at T/Tg approximately 1.10=TB/Tg the system transforms from a heterogeneous to a homogeneous (true) liquid. PMID:19113134

Dlubek, Günter; Shaikh, Muhammad Qasim; Rätzke, Klaus; Faupel, Franz; Paluch, Marian

2008-11-01

398

Temperature dependence of the free volume from positron lifetime experiments and its relation to structural dynamics: Phenylphthalein-dimethylether  

NASA Astrophysics Data System (ADS)

Positron annihilation lifetime spectroscopy (PALS) was used to study the microstructure of the free volume in the temperature range between 103K and 393K in phenylphthalein-dimethylether (PDE), a low-molecular-weight glass former. Using the routine LIFETIME9.0, the ortho-positronium ( o -Ps) lifetime distribution was analyzed, and from this, the volume distribution gn(vh) of subnanometer-size holes was calculated. From a comparison of PALS and specific volume data, the number density and the volume fraction of holes were estimated. These free-volume data, as a function of temperature, were used to test the validity of the Cohen-Turnbull (CT) free-volume theory. It was found that the structural relaxation from dielectric spectroscopy can be described by the CT theory after introducing a corrected free volume (Vf-?V) , where ?V=0.014cm3/g . The extended free-volume theory of Cohen and Grest can be fitted to the dielectric-relaxation and free-volume data, but the parameters of both fits are not consistent. PDE shows some peculiar features. The “knee” in the o -Ps lifetime expansion and crossover in temperature dependence of the frequency of the primary dielectric relaxation process occur at different temperatures. In addition, the change in the Vogel-Fulcher-Tammann parameters at TB/Tg=1.1 has no observable effect on the mean free volume ?vh? (or Vf ). The size of the smallest representative freely fluctuating subsystem, ?VSV? estimated from the standard deviation ?h of gn(vh) , decreases from 4.1nm3to2.6nm3 when the temperature increases from T/Tg=1.0 to 1.15. Correspondingly, the length of dynamic heterogeneity, ?=?VVS?1/3 , decreases from 1.6nmto1.4nm . It is concluded that at T/Tg?1.10=TB/Tg the system transforms from a heterogeneous to a homogeneous (true) liquid.

Dlubek, Günter; Shaikh, Muhammad Qasim; Rätzke, Klaus; Faupel, Franz; Paluch, Marian

2008-11-01

399

A feature in the X-ray spectrum of Cygnus X-1 - A possible positron annihilation line  

NASA Technical Reports Server (NTRS)

The paper presents a 10 keV to 1 MeV X-ray spectrum of Cyg X-1 in its low state, accumulated over 3 months in 1977 and 1978. The spectrum is smooth up to 300 keV. The excess at higher energy may be interpreted as a broad 511 keV emission line from the annihilation of positrons. The width and centroid of the line indicate that the temperature of the line-emitting region is 20-200 keV. The feature could also be caused by Comptonized emission from a 165 keV region.

Nolan, P. L.; Matteson, J. L.

1983-01-01

400

Jet Rates in Electron-Positron Annihilation at O({alpha}{sub s}{sup 3}) in QCD  

SciTech Connect

We compute production rates for two, three, four, and five jets in electron-positron annihilation at the third order in the QCD coupling constant. At this order, three-jet production is described to next-to-next-to-leading order in perturbation theory while the two-jet rate is obtained at next-to-next-to-next-to-leading order. Our results yield an improved perturbative description of the dependence of jet multiplicity on the jet resolution parameter y{sub cut}, particularly at small values of y{sub cut}.

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

2008-05-02

401

X-ray photoelectron spectroscopy and positron annihilation spectroscopy analysis of surfactant affected FePt spintronic films  

NASA Astrophysics Data System (ADS)

This paper reports the effects of surfactant Bi atomic diffusion on the microstructure evolution and resulted property manipulation in FePt spintronic films by the quantitative studies of X-ray photoelectron spectroscopy and positron annihilation spectroscopy. The defect density in the FePt layer, which was tunable by varying the thermal treatment temperatures, was found to be remarkably enhanced correlated with the Bi atomic diffusion behavior. The observed defect density evolution substantially favors Fe(Pt) atomic migrations and lowers the energy barrier for atomic ordering transition, resulting in a great improvement of hard magnet property of the films.

Feng, Chun; Li, Xujing; Liu, Fen; Wang, Qiang; Yang, Meiyin; Zhao, Chongjun; Gong, Kui; Zhang, Peng; Wang, Bao-Yi; Cao, Xing-Zhong; Yu, Guanghua

2014-07-01

402

A position-sensitive ?-ray detector for positron annihilation 2D-ACAR based on metal package photomultiplier tubes  

Microsoft Academic Search

A new position-sensitive ?-ray detector to be used in a two-dimensional angular correlation of positron annihilation radiation (2D-ACAR) apparatus has been developed. It consists of 36 compact position-sensitive photomultiplier tubes (PS-PMT: HAMAMATSU R5900-00-C8), a light guide, and 2676 Bi4Ge3O12 (BGO) scintillator pieces of size 2.6mm×2.6mm×18mm. A high detection efficiency for 511keV ?-ray is achieved with the length of BGO scintillators

Koji Inoue; Haruo Saito; Yasuyuki Nagashima; Toshio Hyodo; Yasuyoshi Nagai; Shinichi Muramatsu; Shota Nagai; Keisuke Masuda

2002-01-01

403

Characterization of Vacancy Defects in Electroplated Cu Films by Positron Annihilation and its Impact on Stress Migration Reliability  

SciTech Connect

Positron annihilation was used to evaluate vacancy concentrations in electroplated Cu films with different kinds of electrolytes. The influence of various electrolytes on the impurities, grain boundaries, and micro-voids were also investigated. We found a higher impurity concentration and larger micro-voids were observed in copper films with higher vacancy concentrations. We reduced the failure rate in our stress migration results using a copper film with a higher concentration of vacancy and impurity. The stress migration performance improved because impurity nucleated vacancy clusters act as effective traps for diffusing vacancies.

Suzuki, T.; Nakamura, T.; Mizushima, Y. [Fujitsu Laboratories LTD. Akiruno Technology Center 50 Fuchigami, Akiruno, Tokyo, 197-0833 (Japan); Kouno, T. [Fujitsu LTD. Akiruno Technology Center 50 Fuchigami, Akiruno, Tokyo, 197-0833 (Japan); Uedono, A. [Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573 (Japan); Tsuchikawa, H. [Fujitsu Laboratories LTD. Akiruno Technology Center 50 Fuchigami, Akiruno, Tokyo, 197-0833 (Japan); Graduate School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555 (Japan)

2006-02-07

404

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

405

Adsorption site identification for oxygen molecules on Au/Si(100) by positron annihilation induced Auger electron spectroscopy(PAES)  

NASA Astrophysics Data System (ADS)

The top layer selectivity of PAES (positron annihilation induced Auger electron spectroscopy) was applied to study the differential adsorption of oxygen molecules on Au/Si(100) surfaces. On appropriately prepared Au/Si surfaces, exposure to 400-500 L of O 2 at elevated sample temperature (100°C) produces significant decreases of the Si (LVV) peak intensity (30-40% decrease compared with as-deposited samples), while the PAES intensity at energies corresponding to the Au (O 2,3 VV transition) undergoes only a small change (? ± 10%). These results strongly suggest that the O is adsorbed preferentially on top of Si atoms and that consequently the adsorbed gas is displacing the positron wavefunction away from the Si atoms but not away from the Au atoms. Evidence for changes in the Au overlayer morphology induced by thermal treatment of the Au/Si surface and the corresponding differences in adsorption behavior of oxygen molecules will also be discussed.

Yang, G.; Kim, J. H.; Yang, S.; Weiss, A. H.

1995-01-01

406

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

NASA Technical Reports Server (NTRS)

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

Singh, Jag J.; Eftekhari, Abe

1996-01-01

407

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

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

408

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

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

409

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

1998-09-04

410

Electron-positron annihilation and the cosmic X-ray background  

NASA Technical Reports Server (NTRS)

The possibility that the processes responsible for the cosmic X-ray background (CXB) would also produce an e(-)-e(+) annihilation feature is examined. Under the assumption that these processes are thermal, the absence of a strong e(-)-e(=) annihilation feature places constraints on the compactness (L/R ratio) of these sources. Observations favor souces of small compactness ratio.

Kazanas, D.; Shafer, R. A.

1983-01-01

411

Neutron-irradiated model alloys and pressure-vessel steels studied using positron spectroscopy  

Microsoft Academic Search

We have used positron-annihilation-lifetime spectroscopies to examine microstructural evolution of pressure vessel steels and model alloys that have systematically varied amounts of copper, nickel, and phosphorus during neutron irradiation and post-irradiation annealing. The objective of this work was to characterize the neutron-irradiation induced microstructural features that cause the embrittlement of nuclear reactor pressure-vessel steel. We used positron annihilation lifetime spectroscopy

Stephen Eric Cumblidge

2002-01-01

412

A positron lifetime study of the electron-beam-induced polymerization of 1,6-hexanediol diacrylate  

Microsoft Academic Search

Positron lifetime measurements have been used to characterize the electron-beam-induced polymerization of 1,6-hexanediol diacrylate (HDODA). Lifetimes were measured as a function of radiation dose over a range of 0.5–7.0 Mrad and analyzed into three components. All three components exhibited some variation with radiation dose. Variations in the longest lifetime component have been interpreted in terms of changes in free volume.

William W. Walker; James W. Harrell

1991-01-01

413

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

NASA Astrophysics Data System (ADS)

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

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

2013-04-01

414

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

SciTech Connect

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

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

2004-11-01

415

Neutrino signals from annihilating/decaying dark matter in the light of recent measurements of cosmic ray electron/positron fluxes  

SciTech Connect

The excess of cosmic ray electron and positron fluxes measured by the PAMELA satellite and ATIC balloon experiments may be interpreted as the signals of the dark matter annihilation or decay into leptons. In this paper, we show that the dark matter annihilation/decay which reproduces the electron/positron excess may yield a significant amount of high-energy neutrinos from the Galactic center. In that case, future kilometer-square size experiments may confirm such a scenario, or even the Super-Kamiokande results already put constraints on some dark matter models.

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)

2009-02-15

416

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

NASA Astrophysics Data System (ADS)

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

Rusz, Ján; Biasini, Maurizio

2007-06-01

417

Photo-degradation of Lexan polycarbonate studied using positron lifetime spectroscopy  

SciTech Connect

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

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

2013-02-05

418

Application possibilities of Acqiris digital card DP240 for positron lifetime measurement  

NASA Astrophysics Data System (ADS)

Based on existing knowledge, a software for the digital positron lifetime (LT) spectrometer has been designed at the Slovak University of Technology. As a digitizer, Acqiris DP240 card was used. Start and stop timing signals have been digitised separately with sampling rate 1GS/s in 8 bit resolution. This sampling rate was not sufficient; therefore a joined delayed channel mode with 2GS/s sampling rate was used. Next applications for the study of advanced materials for nuclear industry are foreseen.

Petriska, M.; Sluge?, V.; Kršjak, V.; Zeman, A.

2011-01-01

419

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

NASA Technical Reports Server (NTRS)

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

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

1993-01-01

420

Mössbauer and positron annihilation study of tin-vacancy interaction during the recovery of a dilute Al-Sn alloy  

NASA Astrophysics Data System (ADS)

Tin-vacancy interaction was studied in a dilute Al(Sn) alloy, quenched from 613°C to -70°C and isochronally heat treated up to 250°C, using parallel positron lifetime and Mössbauer spectroscopy. For the interpretation of the results, a simple model was developed including tin-monovacancy tin-divacancy and tin-tetravacancy complexes as well as more complicated tin-vacancy agglomerates as important participants of the recovery process. According to this model, the changes observed in the positron lifetime and Mössbauer spectra can be explained by the gradual transformation of these complexes to simpler ones through transitional configurations. Only two lines could be observed in the Mössbauer spectra and the results suggest that a single vacancy has minor effect on the isomer shift of the line corresponding to tin in substitutional lattice site.

Homonnay, Z.; Süvegh, K.; Szeles, Cs.; Vértes, A.

1989-03-01

421

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

NASA Technical Reports Server (NTRS)

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

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

1996-01-01

422

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

NASA Astrophysics Data System (ADS)

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 ? which then decay into ?+?- or e+e-. 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 ? and show that this indeed gives a rather spectacular broad peak in E2d?/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 SgrA*. 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.

Bergström, Lars; Bertone, Gianfranco; Bringmann, Torsten; Edsjö, Joakim; Taoso, Marco

2009-04-01

423

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

424

Positron lifetime studies of the dose dependence of nanohole free volumes in ion-irradiated conducting poly-(ethylene-oxide) salt polymers  

NASA Astrophysics Data System (ADS)

Polymer based ion conducting materials have potential applications as an electrolyte and separator in the field of lithium batteries. Solid polymer electrolytes for lithium batteries are one of the best applications. The irradiation of polymeric materials with swift heavy ions results into the change of their free volume properties which have strong correlation with their macroscopic properties. Poly-ethylene-oxide (PEO)-salt polymers were prepared using solution-cast method. Irradiation of the films with 95 MeV oxygen (O 6+) ions from the pelletron accelerator at IUAC, New Delhi, India, to different fluences up to 10 13 ions/cm 2 was carried out under high vacuum of the order of 4 × 10 -6 Torr. Nanosized free volume parameters in PEO-salt polymer complex have been studied by positron annihilation lifetime spectroscopy (PALS) and Doppler broadening spectroscopy (DBS). From orthopositronium ( o-Ps) lifetime, free volume hole radius, free volume of micro voids and fractional free volume are computed. Free volume changes with the fluence are studied. The variation of o-Ps lifetime, mean free volume and fractional free volume with the ion fluence is studied. o-Ps lifetime, free volume radius, mean free volume and fractional free volume decrease for the fluence 10 10 and 10 11 ions/cm 2 and then increase with fluences of 10 12 and 10 13 ions/cm 2. The S parameter showed a continuous decrease with increasing fluence of irradiation. The intermediate lifetime ?2 also showed a similar decrease. These results indicate the occurrence of scission in the polymer chains and the fragmentation of larger free volumes into smaller ones.

Kumar, Rajesh; De, Udayan; Nambissan, P. M. G.; Maitra, M.; Ali, S. Asad; Middya, T. R.; Tarafdar, S.; Singh, F.; Avasthi, D. K.; Prasad, Rajendra

2008-04-01

425

Observation of AsGrown Defects in Zn-Doped GaAs by Positron Lifetime Spectra  

Microsoft Academic Search

Positron lifetime spectra were measured for the Zn-doped p-type GaAs. In comparing the horizontal-Bridgman-method-grown and the floating-zone-method grown p-type GaAs with the liquid-encapsulation-Czochralski-grown p-type GaAs samples, positron trapping into vacancy type defects was observed in the former two grown p-type GaAs. Shallow positron traps were detected, and the dominant ones were attributed to acceptor the in p-type GaAs. Bibtex entry

Zhu Wang; Shao-Jie Wang; Zhi-Quan Chen

2000-01-01

426

Observation of As-Grown Defects in Zn-Doped GaAs by Positron Lifetime Spectra  

NASA Astrophysics Data System (ADS)

Positron lifetime spectra were measured for the Zn-doped p-type GaAs. In comparing the horizontal-Bridgman-method-grown and the floating-zone-method grown p-type GaAs with the liquid-encapsulation-Czochralski-grown p-type GaAs samples, positron trapping into vacancy type defects was observed in the former two grown p-type GaAs. Shallow positron traps were detected, and the dominant ones were attributed to acceptor the in p-type GaAs.

Wang, Zhu; Wang, Shao-Jie; Chen, Zhi-Quan

2000-11-01

427

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

428

H passivation of Li on Zn-site in ZnO: Positron annihilation spectroscopy and secondary ion mass spectrometry  

NASA Astrophysics Data System (ADS)

The interaction of hydrogen (H) with lithium (Li) and zinc vacancies (VZn) in hydrothermally grown n-type zinc oxide (ZnO) has been investigated by positron annihilation spectroscopy (PAS) and secondary ion mass spectrometry. Li on Zn-site (LiZn) is found to be the dominant trap for migrating H atoms, while the trapping efficiency of VZn is considerably smaller. After hydrogenation, where the LiZn acceptor is passivated via formation of neutral LiZn-H pairs, VZn occurs as the prime PAS signature and with a concentration similar to that observed in nonhydrogenated Li-poor samples. Despite a low efficiency as an H trap, the apparent concentration of VZn in Li-poor samples decreases after hydrogenation, as detected by PAS, and evidence for formation of the neutral VZnH2 complex is presented.

Johansen, K. M.; Zubiaga, A.; Tuomisto, F.; Monakhov, E. V.; Kuznetsov, A. Yu.; Svensson, B. G.

2011-09-01

429

Search for highly ionizing particles in electron positron annihilations at sq rt 1 = 50-52 GeV  

NASA Astrophysics Data System (ADS)

The paper reports the first result of a search for highly ionizing particles in electron positron annihilations at the KEK storage ring TRISTAN. CR-39 and UG-5 dielectric track detectors were exposed to 4.8 pb integrated luminosity at total energies of 50-52 GeV in the center of mass. This search is sensitive to Dirac magnetic monopoles and yields 95-percent confidence-level upper limits of 8 x 10 to the 37th and 1.3 x 10 to the 35th sq cm on the cross section for production monopoles with magnetic charges g = 68.5e = gD and g = 2gD and respective maximum masses 24.1 and 22.0 GeV/c-squared.

Kinoshita, K.; Fujii, M.; Nakajima, K.; Price, P. B.; Tasaka, S.

1988-04-01

430

Characterization of radiation-induced lattice vacancies in intermetallic compounds by means of positron-lifetime studies  

NASA Astrophysics Data System (ADS)

In the present paper a characterization of atomic vacancies in intermetallic compounds is given by means of positron-lifetime measurements after electron irradiation and comparison with the states after preparation, after long-time annealing, or in high-temperature equilibrium. In TiAl, Ti3Al, and Ni3Al no structural vacancies (detection limit CV=10-6) are observed at ambient temperature. This confirms that in these compounds slight deviations from stoichiometry are compensated by antisite atoms. In the Al-poor B2 alloys FeAl and NiAl, on the other hand, remnant vacancies exist due to the high thermal equilibrium vacancy concentrations and their slow diffusivities. The kinetics of vacancy elimination in FeAl and NiAl is discussed. A substantial temperature dependence of the positron lifetime in vacancies is detected in close-packed intermetallics which is attributed to an increased atomic relaxation or partial positron detrapping at high temperatures. In contrast to that, the temperature dependence of the positron lifetime in vacancies is small in the open-structured B2 aluminides. The lifetimes ?f of free delocalized positrons in transition-metal aluminides and in NiZr and NiTi can be correlated to those of the pure components, taking into account the densities of valence electrons. For the positron lifetimes ?1 of vacancies in intermetallic compounds, values of ?1/?f=1.5-1.7 are observed similar as in the pure metals. Annealing studies of B2-FeAl after electron irradiation yield time constants for the disappearance of vacancies identical to those deduced recently for the equilibration of thermal vacancies. In electron-irradiated Ti aluminides annealing processes at 250 K and 450 K are observed where the latter process is tentatively attributed to long-range migration of vacancies.

Würschum, R.; Badura-Gergen, K.; Kümmerle, E. A.; Grupp, C.; Schaefer, H.-E.

1996-07-01

431

Doppler Width of the Annihilation Line from Positrons Stopped in Oxygen.  

National Technical Information Service (NTIS)

The Doppler-broadened portion of the 511 keV annihilation line in oxygen was measured with a germanium detector to have an r.m.s. width (1.27 plus or minus 0.11) keV which is consistent with the angular width deduced from two-quantum angular correlation w...

J. E. Brimhall L. A. Page

1965-01-01

432

Investigation of density inhomogeneities in liquids by positron annihilation. Micellar solutions and liquid hydrocarbons.  

National Technical Information Service (NTIS)

The case of positronium diffusion and annihilation in micellar solutions as well as in liquid normal alkanes is discussed. The traps are assumed to be the structural sparse density regions in these liquids. The traps in micellar solutions are the micelles...

S. Vass

1990-01-01

433

Subnanometre size free volumes in amorphous Verapamil hydrochloride: a positron lifetime and PVT study through T(g) in comparison with dielectric relaxation spectroscopy.  

PubMed

Positron annihilation lifetime spectroscopy (PALS), a method well established for the study of polymers, is employed to characterize the temperature dependence of the free volume through T(g) in the amorphous pharmaceutical Verapamil hydrochloride. From the PALS spectra analyzed with the routine LifeTime9.0 the size (volume) distribution of local free volumes (subnanometre-size holes), its mean, v(h), and mean dispersion, sigma(h), were calculated. A comparison with the macroscopic volume from PVT-experiments delivered the hole density and the hole free volume fraction and in that way a complete characterization of the free volume microstructure. These data are used in correlation with structural (alpha-) relaxation data from broad-band dielectric spectroscopy in terms of the Cohen-Turnbull free volume model. An extension of this model, distinctions in the free volume behaviour of the glassy and supercooled-liquid state and different ways of extrapolating the equilibrium part of the free volume into the temperature range of the glass are discussed. The potential of the PALS method for the study of pharmaceuticals is briefly reviewed and some recently developed applications (analysis of density fluctuations) are illuminated. PMID:20638474

Dlubek, Günter; Shaikh, Muhammad Q; Rätzke, Klaus; Pionteck, Jürgen; Paluch, Marian; Faupel, Franz

2010-10-01

434

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

NASA Astrophysics Data System (ADS)

Pure nickel and very dilute Ni-S systems (containing 50-340 at.ppm sulphur), irradiated by 5 MeV electrons at 270 K, were studied by positron annihilation spectroscopy (PAS) and the electrical resistivity measurements. The