These are representative sample records from Science.gov related to your search topic.
For comprehensive and current results, perform a real-time search at Science.gov.
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

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

3

Positron Annihilation Lifetime Study of Semicrystalline Syndiotactic Polystyrene  

NASA Astrophysics Data System (ADS)

Positron Annihilation lifetime spectroscopy (PALS) measurements were performed on a series of syndiotactic polystyrene (s-PS) samples crystallized from the melt and glassy states. The semicrystalline s-PS samples were of pure ?^', ?^'', and ? conformations with crystallinities between 0--40% as indicated by x-Ray, DSC, and density measurements. These crystalline forms are known to have densities similar to the amorphous phase. A question can be raised whether positronium can form and annihilate in these porous crystals. The PALS measurements of these samples showed 4 distinct lifetime components, of which the longest (?_4=2--3ns) exhibits behavior typical of amorphous polymers with an abrupt change in slope at T_g. The second largest lifetime (?_3 ~0.8ns) showed no dependence on temperature from -100^oC to 170^oC, or crystallinity in the region studied. No evidence of ortho-positronium annihilation in the large channel structure of the ?-form was found. The intensities of I3 and I4 were both independent of crystallinity, contrary to the belief that the intensity is a measure of the amorphous phase, and should therefore decrease with an increasing crystallinity. This implies, that any ortho-positronium formed in the crystalline region quickly diffuses out of the channels and annihilate in the amorphous region. The fact that positronium can quickly traverse the length of the channel in a fraction of a picosecond, with a thermal velocity of the order of 10^4m/s, is consistent with this conclusion.

Olson, Brian G.; Jamieson, Alexander; Prodpran, Thummanoon; Nazarenko, Sergei

2002-03-01

4

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

5

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

2014-05-01

6

Dose effect in gamma-irradiated polyethylene studied by positron annihilation lifetime technique  

NASA Astrophysics Data System (ADS)

Commercial high-density polyethylene (HDPE) samples were irradiated by ?-rays with doses ranging from 10 to 200 kGy. Positron annihilation lifetime measurements were performed to study the effect of ?-radiation on the structure of polyethylene. The lifetime ?3, associated with the pick-off process, in which the positron in o-Ps annihilates an electron from the surrounding material, and its intensity I3 are measured as a function of ?-dose. It can be concluded that the effect of ?-radiation on polyethylene results in intensive formation of new bonds or crosslinking at doses ranging from 10 to 100 kGy. Saturation of I3 for the irradiated HDPE at doses ranging from 100 to 200 kGy may indicate the achievement of a structural stability.

EL-Sayed, A. M. A.; Abdel-Hady, E. E.; Rizk, R. A. M.; M´sen, M.

1994-07-01

7

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

NASA Astrophysics Data System (ADS)

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

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

2012-12-01

8

Positron annihilation lifetime and coincidence Doppler broadening study of ?-irradiated polyethylene  

NASA Astrophysics Data System (ADS)

We performed positron annihilation lifetime and coincidence Doppler broadening measurements on ?-irradiated high-density polyethylene (HDPE) at temperatures of 80, 293 and 373 K as a function of the measurement time. At 293 K, it is shown that more than one order of magnitude increase in free radical concentration (10 18 spins/g) results in a reduction of the ortho-positronium ( o-Ps) yield only by ˜2% for HDPE irradiated in vacuum. Stronger Ps formation inhibition was observed in an HDPE irradiated in air, which is attributed to a larger C dbnd O concentration. After temperature was raised from 293 to 373 K, free radicals were found ineffective to the Ps formation. Because of the stabilization of C dbnd O -e + complex at 80 K, enhanced positron trapping by oxygen was observed.

Yu, R. S.; Suzuki, T.; Djourelov, N.; Kondo, K.; Ito, Yasuo; Shantarovich, V.

2005-06-01

9

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

10

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

SciTech Connect

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

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

2013-05-15

11

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

12

Surface areas by positron annihilation spectroscopy  

SciTech Connect

Positron annihilation spectroscopy (PAS) is a technique for measuring the length of time that a positron exists after being injected into a specific material. Lifetime measurements have been used for studying defects in metals. These imperfections affect the localized electron densities which in turn influence the lifetime of the positron before annihilation occurs. Electron density differences are also associated with surfaces. This paper describes the PAS technique and shows a correlation between positron annihilation intensities and surface areas of some fumed amorphous silicas.

Dale, J.M.; Rosseel, T.M.; Hulett, L.D.; Venkateswaran, K.; Jean, Y.C.; Fuller, E.L.

1985-01-01

13

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

14

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

NASA Astrophysics Data System (ADS)

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

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

2011-01-01

15

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

16

Free-volume structure of fluoropolymer-based radiation-grafted electrolyte membranes investigated by positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

In the field of polymer-electrolyte-membrane (PEM) fuel cell technology, the structures of free-volume holes in the PEMs are very important because they are correlated to the supplied-gas crossover phenomenon, which sometimes deteriorates the cell performance. In this study, we investigated the size and location of free-volume holes in the crosslinked-polytetrafluoroethylene (cPTFE) based radiation-grafted PEMs by positron annihilation lifetime (PAL) spectroscopy. For comparison, the base cPTFE and polystyrene grafted films were also measured. From the analysis of PAL spectra, it was found that there were free-volume holes with different radius of 0.28-0.30 nm and 0.44-0.45 nm. The smaller holes should be located in both PTFE crystallites and poly(styrene sulfonic acid) grafts, while the larger holes are considered to exist in amorphous PTFE phases.

Sawada, S.; Kawasuso, A.; Maekawa, M.; Yabuuchi, A.; Maekawa, Y.

2010-04-01

17

Hydrolytic Degradation of Poly(L-Lactide-co-Glycolide) Studied by Positron Annihilation Lifetime Spectroscopy and Other Techniques  

NASA Astrophysics Data System (ADS)

Changes of the poly(L-lactide-co-glycolide) structure as a function of degradation time in phosphate-buffered saline for 7 weeks were investigated by gel permeation chromatography, differential scanning calorimetry, nuclear magnetic resonance (1H NMR), and positron annihilation lifetime spectroscopy. Surface properties as wettability by sessile drop and topography by atomic force microscopy were also characterized. Chain-scission of polyester bonds in hydrolysis reaction causes a quite uniform decrease in molecular weight, and finally results in an increase in semicrystallinity. Molecular composition of the copolymer and water contact angle do not change considerably during degradation time. Atomic force microscopy studies suggest that the copolymer degrades by "in bulk" mechanism. The average size of the molecular-level free volume holes declines considerably after one week of degradation and remains constant till the sixth week of degradation. The free volume fraction decreases as a function of degradation time.

Pamu?a, E.; Dryzek, E.; Dobrzy?ski, P.

2006-11-01

18

Size of sodium dodecyl sulphate micelles in aqueous NaCl solutions as studied by positron annihilation lifetime spectroscopy  

NASA Astrophysics Data System (ADS)

The changes in size and shape of the micelles formed by 0.28 M sodium dodecyl sulphate in aqueous solution are studied as a function of added sodium chloride concentration at 303 K, by using positron annihilation lifetime spectroscopy (LS). As in a previous work, the trapping of ortho-positronium ( o-Ps) from the aqueous to the organic micellar subphase results in the appearance of two o-Ps states with distinct lifetimes. The LS spectra are analysed on the basis of a time-dependent trapping rate coefficient, which involves two parameters characteristic of the micellar system: the mean core radius, R core, and the mean aggregation number, N ag. On selecting a set of N ag values at various NaCl concentrations from two previous independent experimental works, the LS data provide R core values. A good agreement is observed between the latter values and those expected either from the theoretical geometrical considerations or from the previous experimental data on the mean micelle diffusion coefficient. The agreement is obtained only based on the assumption that Ps exists in the solution in a bubble state and not as a quasi-free particle.

Bockstahl, Frédéric; Pachoud, Eric; Duplâtre, Gilles; Billard, Isabelle

2000-06-01

19

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

20

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

PubMed

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

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

2014-02-01

21

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

22

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

23

Positron annihilation in zeolite 13X  

NASA Astrophysics Data System (ADS)

Results presented in previous papers on positron annihilation in zeolite 13X referred only to fully hydrated or dehydrated samples. In these investigations the dehydration process was studied based on measurements of positron lifetime spectra in zeolite 13X samples. All spectra were resolved into three or four components. Measurements show that water removal is most intensive at the beginning of pumping. For dehydration at higher temperature results suggest that the last molecules of ``zeolitic water'' are removed from the sample at a temperature of about 200 °C. It was found that independent of the stage of dehydration of the sample the intensity (I3+I4) of the two longest components in the spectrum is virtually constant (24%) and is equal to the intensity I3 of the longest component for the fully hydrated sample.

Habrowska, A. M.; Popiel, E. S.

1987-09-01

24

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

NASA Astrophysics Data System (ADS)

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

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

2002-02-01

25

Positron annihilation spectroscopy of sandstone and carbonate rocks  

Microsoft Academic Search

The techniques of positron annihilation spectroscopy have been applied to the study of well-characterized reservoir rock samples. In this article we focus primarily on sandstone and carbonate samples. We report (a) measurements of the Doppler broadening (DB) parameters and mean lifetime values () of dry samples; (b) measurement of the DB parameters as a function of temperature from room temperature

J. M. Urban-Klaehn; C. A. Quarles

1999-01-01

26

Positron annihilation spectroscopy of sandstone and carbonate rocks  

Microsoft Academic Search

The techniques of positron annihilation spectroscopy have been applied to the study of well-characterized reservoir rock samples. In this article we focus primarily on sandstone and carbonate samples. We report (a) measurements of the Doppler broadening (DB) parameters and mean lifetime values (???) of dry samples; (b) measurement of the DB parameters as a function of temperature from room temperature

J. M. Urban-Klaehn; C. A. Quarles

1999-01-01

27

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

28

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

29

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

30

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

E-print Network

). Abstract. 2014 With the aim of extending positron-lifetime measurements to high temperature and avoiding source-specimen systems for high temperature positron annihilation experiments have been recently39 A new way of using positron-lifetime measurements to study lattice defects Ch. Janot, B. George

Boyer, Edmond

31

Defect characterization with positron annihilation. [Review  

SciTech Connect

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

Granatelli, L; Lynn, K G

1980-01-01

32

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

33

Analysis of positron lifetime spectra via a fast Prony algorithm  

NASA Astrophysics Data System (ADS)

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

Kul'Ment'ev, A. I.

2004-03-01

34

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

35

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

NASA Astrophysics Data System (ADS)

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

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

2006-11-01

36

On the method of positron lifetime measurement  

NASA Technical Reports Server (NTRS)

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

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

1983-01-01

37

Electron-positron annihilation under magnetic field  

Microsoft Academic Search

The radiation loss due to the annihilation of electron-positron pair into two ?-quanta in neutron stars is calculated. The\\u000a dependence of radiation loss on the magnetic field, density, temperature, and chemical potentials is explicitly shown for\\u000a relativistic and non-relativistic cases. The reaction cross section is also calculated.

M. El Khishen; M. Toubia

1976-01-01

38

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

39

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

40

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

41

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

E-print Network

dominates for atoms and small molecules. I show that its contribution to the annihilation rate canMechanisms of positron annihilation on molecules G. F. Gribakin* School of Physics, University in low-energy positron annihilation on molecules. The two mechanisms considered are the following: i

Gribakin, Gleb

42

Microemulsion systems studied by positron annihilation techniques  

SciTech Connect

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, that microemulsion formation requires a certain water/oil ratio if the oil is a long-chain aliphatic hydrocarbon such as hexadecane. This ratio is 0.4 in the case of a 1-pentanol- and 0.2 for a 1-hexanol-containing mixture. This minimum water content is strongly reduced if the oil is an aromatic hydrocarbon. The positron annihilation data also sensitively reflect structural rearrangements in these solutions occurring upon further addition of water, such as the transition of spherical aggregates to a disk-like lamellae structure.

Boussaha, A.; Djermouni, B.; Fucugauchi, L.A.; Ache, H.J.

1980-07-02

43

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

SciTech Connect

Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy, show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.

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

2013-08-01

44

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

45

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.

46

Positron Annihilation Spectroscopy of Barnett Shale Core Samples  

NASA Astrophysics Data System (ADS)

Positron annihilation spectroscopy (PAS) is an experimental technique that provides information about the internal structure of an object, specifically the porous spaces or defects that are present within the object. The lifetime of a positron within the sample is measured, which depends upon the volume of the space the positron becomes ``trapped'' in. While PAS has been applied to geological samples in the past, the present project focuses on Barnett Shale core, which has not been studied extensively with PAS. PAS presents a unique opportunity to learn about the micro-pores within the shale. These micro-pores are of critical importance because they contain natural gas, oil, and other organic compounds. Our project has 3 main goals: to determine the average positron lifetimes of a shale sample, to investigate the uniformity of shale core, and to observe the effect on the internal structure of shale after a handheld micro-conical indentation test, known as a ``dimple test,'' has been performed. This dimple test is an application of a small, concentrated force onto the shale, which subsequently fractures the shale (within a small radius around the impact point). Our preliminary results conclude that shale is relatively non-uniform, and that the volume of the micro-pores within the shale sample is significantly affected by the dimple test.

Morgan, Hayden; Enderlin, Milton; Quarles, C. A.

2012-10-01

47

Positron-annihilation radiation from neutron stars.  

NASA Technical Reports Server (NTRS)

Matter accreted on the surfaces of neutron stars consists of energetic particles of a few tens to one or two hundred MeV per nucleon, depending on the neutron-star mass. In addition to heat, such particles produce nuclear reactions with the surface material. It is proposed that the recently observed 473 plus or minus 30 keV spectral feature from the galactic center is gravitationally redshifted positron-annihilation radiation produced at the surfaces of neutron stars. The principal observational tests of the model would be the detection of nuclear gamma-ray lines from the galactic center.

Ramaty, R.; Borner, G.; Cohen, J. M.

1973-01-01

48

Three-photon annihilation of the electron-positron pairs  

E-print Network

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

A. M. Frolov

2008-09-07

49

Positron annihilation investigations on poly(methyl methacrylate)  

NASA Astrophysics Data System (ADS)

Positron lifetime and Doppler broadened annihilation radiation were measured for seven different samples of poly(methyl methacrylate) at room temperature in vacuum. The polymerisation of methyl methacrylate was carried out as a bulk polymerisation in the presence of benzoyl peroxide as an initiator. The effect of the amount of the initiator on the viscosity-average molecular weight was studied. It was found that the viscosity-average molecular weight decreased with increasing amount of the initiator. The average lifetime and intensity of ortho-positronium ( o-Ps) increased with increasing viscosity-average molecular weight up to 6.85 × 10 4 and remained constant after that. The S-parameter showed a similar behaviour as that of the o-Ps intensity.

Mohamed, Hamdy F. M.; Abd-Elsadek, Gomaa G.

2000-06-01

50

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

51

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

52

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

53

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

54

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

55

Momentum Distributions of Photons from Positrons Annihilating in Alkali Halides  

Microsoft Academic Search

The angular correlation of photons from the two-gamma decay of positrons in all sodium halides and all alkali chlorides has been measured. The data yields the momentum distribution of the two gamma rays which is also the momentum distribution of the annihilating electron-positron pairs. It is seen that the positive ion has very little influence on the momentum distribution in

A. T. Stewart; N. K. Pope

1960-01-01

56

Positron Annihilation Studies In Polymer Nano-Composites  

SciTech Connect

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

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

2011-06-01

57

Positron annihilation study of microvoids in centrifugally atomized 304 stainless steel  

NASA Astrophysics Data System (ADS)

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 ?3 component decreased with decreasing number density of 1.5 nm microvoids. The Doppler peak shape was found to be much more strongly influenced by microvoids than by any other defects such as precipitates or grain boundaries. In particular microvoids produced significant narrowing of the Doppler distribution shape.

Kim, J. Y.; Byrne, J. G.

1993-03-01

58

Positron scattering and annihilation on noble-gas atoms  

NASA Astrophysics Data System (ADS)

Positron scattering and annihilation on noble-gas atoms is studied ab initio using many-body theory methods for positron energies below the positronium formation threshold. We show that in this energy range, the many-body theory yields accurate numerical results and provides a near-complete understanding of the positron-noble-gas atom system. It accounts for positron-atom and electron-positron correlations, including the polarization of the atom by the positron and the nonperturbative effect of virtual positronium formation. These correlations have a large influence on the scattering dynamics and result in a strong enhancement of the annihilation rates compared to the independent-particle mean-field description. Computed elastic scattering cross sections are found to be in good agreement with recent experimental results and Kohn variational and convergent close-coupling calculations. The calculated values of the annihilation rate parameter Zeff (effective number of electrons participating in annihilation) rise steeply along the sequence of noble-gas atoms due to the increasing strength of the correlation effects, and agree well with experimental data.

Green, D. G.; Ludlow, J. A.; Gribakin, G. F.

2014-09-01

59

Positrons from dark matter annihilation in the galactic halo: uncertainties  

E-print Network

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 show that, especially in the low energy tail of the positron spectra at Earth, the uncertainty is sizeable and we quantify the effect. Comparison of our predictions with current available and foreseen experimental data are derived.

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

2007-09-13

60

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

61

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.

62

Phase behavior of microemulsion systems studied by positron annihilation techniques  

SciTech Connect

The positron annihilation technique was applied to the study of phase behavior of sodium stearate (or oleate)-alcohol-oil-water microemulsion systems. The positron annihilation parameters revealed a dependence of the water/oil ratio at which microemulsion formation occurs on the hydrocarbon chain length of both alcohol cosurfactant and solvent as well as surfactant concentration. Dynamic laser light scattering has been utilized for substantiating the phase transitions determined in the different microemulsion systems by positron annihilation. The difference in the behavior between saturated and unsaturated surfactants is the most remarkable result of the investigation. Thus, replacing sodium stearate by sodium oleate in the surfactant-1-hexanol-isooctane systems obviated microemulsion formation. This behavior has been rationalized by considering packing and kink presence in microemulsion formation. 26 references.

Serrano, J.; Reynoso, R.; Lopez, R.; Olea, O.; Fucugauchi, L.A.

1983-02-17

63

Analysis of positron lifetime spectra in polymers  

NASA Technical Reports Server (NTRS)

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

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

1988-01-01

64

Positron annihilation in TiBe/sub 2/  

SciTech Connect

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

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

1988-01-01

65

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

66

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

67

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

SciTech Connect

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

Colgate, S.A.

1983-03-17

68

Development of positron annihilation spectroscopy at LEPTA facility  

NASA Astrophysics Data System (ADS)

The Low Energy Positron Toroidal Accumulator (LEPTA) at JINR proposed for generation of positronium in flight can be used for Positron Anihilation Spectroscopy (PAS). This is the sensitive method for microstructure studies of solid body. The structural defects such as vacancies, vacancy clusters and nanovoids with the size 0.1-10 nm can be detected. In this paper the progress in the development of PAS at LEPTA facility will be presented. The description of Doppler broadening of annihilation gamma line technique and the examples of results obtained on slow positron beam will be shown.

Horodek, P.; Bugdol, M.; Kobets, A. G.; Meshkov, I. N.; Orlov, O. S.; Rudakov, A. Yu.; Sidorin, A. A.; Yakovenko, S. L.

2014-09-01

69

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

SciTech Connect

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 sp{sup 2} 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 sp{sup 3} 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. {copyright} 2001 American Institute of Physics.

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

2001-07-01

70

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

71

Positron annihilation spectroscopy of biological tissue in (11)C irradiation.  

PubMed

Positron annihilation spectroscopy (PAS) spectra of biological tissue in (11)C irradiation are reported and spatial resolution coefficient of positron emission tomography (PET) obtained from the PAS spectrum is discussed for (11)C irradiation. A PAS spectrum of the biological tissue with water is the same as that of the water pool phantom in (11)C irradiation. However, a PAS spectrum of the biological tissue with less water differs from that of the water pool phantom. The PET spatial resolution coefficient depends on the kind of biological tissue. However, the PET spatial resolution coefficient, 0.00243? ± ?0.00014, can be used as a common value of maximum limit. PMID:25360543

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

2014-11-21

72

Positron annihilation study of silicon irradiated by different neutron doses  

Microsoft Academic Search

Single crystals of silicon grown in an argon atmosphere were irradiated with three different neutron doses; 6*1016, 3.6*1017 and 1.2*1018 cm-2. Positron annihilation spectroscopy indicates that there are two distinct annealing stages of V-type defects in the three neutron-irradiated silicon samples: one at about 200 degrees C and another at about 500 degrees C. The former is due to the

X. T. Meng; W. Puff

1994-01-01

73

Positron annihilation spectra and core-electron enhancement factors  

E-print Network

$\\gamma$-spectra for positron annihilation with core and valence electrons in the noble gases are calculated using many-body theory (MBT). We show that proper inclusion of core annihilation is crucial to accurately describe the measured spectra [Phys. Rev. Lett. 79, 39 (1997)]. We use the MBT to calculate `exact' enhancement factors $\\bar{\\gamma}_{n\\ell}$ for annihilation on individual $n\\ell$ subshells. They parameterize the important effects of (non-local) short-range electron-positron correlations, including the non-perturbative process of virtual positronium formation. We show that they follow a simple and physically motivated scaling with the subshell ionization energy $I_{n\\ell}$: $\\bar{\\gamma}_{n\\ell}=1+\\sqrt{A/I_{n\\ell}}+(B/I_{n\\ell})^{\\beta}$, where $A$, $B$ and $\\beta$ are positive constants. We suggest that this formula can be used with relatively simple independent-particle-approximation calculations to determine accurate core-annihilation spectra for atoms across the periodic table and in condens...

Green, D G

2014-01-01

74

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

SciTech Connect

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

Howell, R.H.

1981-07-27

75

Detection of vacancy-like defects during Cu diffusion in GaAs by positron annihilation  

NASA Astrophysics Data System (ADS)

The positron annihilation spectroscopy is a method for direct characterization of vacancy-type defects by measuring the positron lifetime. It provides information about open volume and concentration of defects. Such measurements were carried out to study the defect properties of semi-insulating GaAs after copper diffusion. A 30 nm layer of Cu was deposited by evaporation to undoped GaAs samples. The diffusion of Cu was performed during an annealing step at 1100 °C under different arsenic vapor pressures. The samples were quenched into room temperature water. The initial semi-insulating (SI) undoped GaAs sample shows no positron traps in that state. After gentle annealing, a vacancy-type defect complex in addition to shallow positron traps was observed to be an efficient positron trap. After Cu in-diffusion during the annealing process, the shallow positron trap is believed to be the CuGa double acceptor. The exact nature of the vacancy-like defects could not be determined unambiguously. The concentration of these defects exhibits inverse relationship to the arsenic vapor pressure. Thus, the arsenic vacancy is believed to be part of this complex. The temperature-dependent Hall-effect measurements have revealed the presence of an acceptor level at EV + 0.5 eV that is usually attributed to CuGa.

Elsayed, M.; Bondarenko, V.; Petters, K.; Krause-Rehberg, R.

2011-01-01

76

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

77

Positron lifetimes in solids from first principles calculations  

SciTech Connect

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

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

1991-08-07

78

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

NASA Astrophysics Data System (ADS)

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

Idczak, R.; Konieczny, R.

2014-10-01

79

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

80

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.

81

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

82

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

83

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

84

Positrons from particle dark-matter annihilation in the Galactic halo: Propagation Green's functions  

E-print Network

Positrons from particle dark-matter annihilation in the Galactic halo: Propagation Green the propagation of positrons from dark-matter particle annihilation in the Galactic halo in different models of the dark matter halo distribution using our three-dimensional code, and present fits to our numerical

Moskalenko, Igor V.

85

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

86

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

87

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

88

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

89

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

90

Moisture dependence of positron lifetime in Kevlar-49  

NASA Technical Reports Server (NTRS)

Because of filamentary character of Kevlar-49 aramid fibers, there is some concern about the moisture uptake and its effect on plastic composites reinforced with Kevlar-49 fibers. As part of continuing studies of positron lifetime in polymers, we have measured positron lifetime spectra in Kevlar-49 fibers as a function of their moisture content. The long lifetime component intensities are rather low, being only of the order of 2-3 percent. The measured values of long component lifetimes at various moisture levels in the specimens are as follows: 2072 +/- 173 ps (dry); 2013 +/- 193 ps (20.7 percent saturation); 1665 +/- 85 ps (25.7 percent saturation); 1745 +/- 257 ps (32.1 percent saturation); and 1772 +/- 217 ps (100 percent saturation). It is apparent that the long component lifetime at first decreases and then increases as the specimen moisture content increases. These results have been compared with those inferred from Epon-815 and Epon-815/K-49 composite data.

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

1984-01-01

91

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

92

Fragmentation production of charmed hadrons in electron-positron annihilation  

SciTech Connect

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

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

2010-10-15

93

Positron lifetime measurements in chiral nematic liquid crystals  

SciTech Connect

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, J.J.; Eftekhari, A.; Parmar, D.S.

1991-10-01

94

Positron lifetime calculation for the elements of the periodic table  

Microsoft Academic Search

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

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

2007-01-01

95

Low energy positron flux generator for lifetime studies in thin films  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

96

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

97

Doppler-broadening measurements of positron annihilation with high-momentum electrons in pure elements  

Microsoft Academic Search

Doppler-broadening measurements of the electron-positron annihilation line in twenty-seven single-element samples are presented. A coincidence technique has been used to suppress the background and to evidence the contribution of positron annihilation with core electrons. Systematic dependences on the atomic number of the target material are found in ratio curves obtained dividing the measured spectra by the spectrum of a reference

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

2002-01-01

98

Doppler-broadening measurements of positron annihilation with high-momentum electrons in pure elements  

Microsoft Academic Search

Doppler-broadening measurements of the electron–positron annihilation line in twenty-seven single-element samples are presented. A coincidence technique has been used to suppress the background and to evidence the contribution of positron annihilation with core electrons. Systematic dependences on the atomic number of the target material are found in ratio curves obtained dividing the measured spectra by the spectrum of a reference

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

2002-01-01

99

Defect annealing studies on metals by positron annihilation and electrical resitivity measurements  

Microsoft Academic Search

Doppler-broadening positron-annihilation measurements combined with electrical-resistivity measurements were performed on Cu and Al samples, irradiated by high-energy electrons at liquid-helium temperature. A defect-specific parameter R was determined from the shape of the 511-keV annihilation line. The R parameter is found to be independent of the defect concentration C and the positron trapping constant mu within the framework of the two-state

S. Mantl; W. Triftshäuser

1978-01-01

100

Moisture in Composites is Measured by Positron Lifetime  

NASA Technical Reports Server (NTRS)

New technique is expected to measure the moisture content and moisture depth distribution in fiber-reinforced polymeric composites. Technique is based on dependence of positron lifetime on moisture content of composite specimen. None of the previous non-destructive testing techniques measured moisture content and depth distribution simultaneously.

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

1982-01-01

101

The calculation of positron annihilation rates in voids in aluminium as a function of void radius using non-local density functional theory  

SciTech Connect

We have calculated the positron wave functions and annihilation rate in voids in aluminium as functions of void radius using density functional theory with the non-local approximation to the positron-electron correlation potential developed by Jensen and Walker. Our calculations show the gradual evolution of the positron state, as the radius increases, from a state extending over the whole of the defect volume in monovacancies to a state localised at the void surface in large voids. We find that for voids with radii larger than about 5 A the lifetime is independent of void size.

Dunn, G.M. (Physics Department, Royal Holloway Bedford New College, University of London, Egham Hill, Egham, Surrey, TW20 OEX (England)); Jensen, K.O.; Walker, A.B. (School of Physics, University of East Anglia, Norwich, NR4 7TJ (England)); Rice-Evans, P. (Physics Department, Royal Holloway Bedford New College, University of London, Egham Hill, Egham, Surrey, TW20 OEX (England))

1991-02-01

102

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

NASA Astrophysics Data System (ADS)

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

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

2014-08-01

103

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

104

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

105

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

106

Positron Lifetime Calculations of Hexagonal Metals with the True Geometry  

Microsoft Academic Search

hancement factor.1. IntroductionTo our knowledge, theoretical calculations of positron lifetimes for all the hexagonalmetals have never been systematically performed using the true hexagonal structure forthe calculations. Instead of the real hexagonal structure, the f.c.c. structure has beenused in previous works [1, 2, 3]. In these works, the hexagonal structure is convertedinto a f.c.c. one, which has the Wigner-Seitz cell volume

J. M. Campillo; F. Plazaola; M. J. Puska

1998-01-01

107

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

NASA Astrophysics Data System (ADS)

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

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

2013-06-01

108

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

109

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

110

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

111

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

NASA Technical Reports Server (NTRS)

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

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

2008-01-01

112

Positron beam lifetime spectroscopy of atomic scale defect distributions in bulk and microscopic volumes  

SciTech Connect

We are developing a defect analysis capability based on two positron beam lifetime spectrometers: the first is based on a 3 MeV electrostatic accelerator and the second on our high current linac beam. The high energy beam lifetime spectrometer is operational and positron lifetime analysis is performed with a 3 MeV positron beam on thick samples. It is being used for bulk sample analysis and analysis of samples encapsulated in controlled environments for {ital in}{ital situ} measurements. A second, low energy, microscopically focused, pulsed positron beam for defect analysis by positron lifetime spectroscopies is under development at the LLNL high current positron source. This beam will enable defect specific, 3-D maps of defect concentration with sub-micron location resolution and when coupled with first principles calculations of defect specific positron lifetimes it will enable new levels of defect concentration mapping and defect identification.

Howell, R.H.; Cowan, T.E.; Hartley, J.; Sterne, P. [Lawrence Livermore National Lab., CA (United States); Brown, B. [Mount Holyoke Coll., South Hadley, MA (United States)

1996-05-01

113

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

NASA Astrophysics Data System (ADS)

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

Thome, T.; Grynszpan, R. I.

2006-06-01

114

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

NASA Astrophysics Data System (ADS)

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

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

2012-08-01

115

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

Microsoft Academic Search

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

Stefano Profumo; Tesla E. Jeltema

2009-01-01

116

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

E-print Network

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

Xiaoguang Ma; Feng Wang

2012-11-02

117

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

118

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

119

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

PubMed

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

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

2014-08-14

120

Positron annihilation study of defects and Si nanoprecipitation in sputter-deposited silicon oxide films  

NASA Astrophysics Data System (ADS)

Positron annihilation spectroscopy was conducted to study defects and Si nanoprecipitation in sputter-deposited silicon oxide films. For as-deposited SiO 0.9 and SiO 1.9 films, Doppler broadening spectra are strongly influenced by the type of paramagnetic defects ( Pb or E'). However, the disappearance of these defects after annealing at 1050 °C in a vacuum cannot account for the corresponding change of the Doppler broadening. In annealed film of SiO 1.9, positronium formation is reduced from defect-free SiO 2 because of the presence of Si precipitates; meanwhile, in annealed film of SiO 0.9 with nanocrystalline Si (nc-Si), positron annihilation with high momentum electrons is enhanced as a result of efficient positron trapping at the nc-Si surface with oxygen.

Yu, R. S.; Ito, K.; Hirata, K.; Sato, K.; Zheng, W.; Kobayashi, Y.

2003-09-01

121

Positron Annihilation Study of Biopolymer Inulin for Understanding its Structural Organization  

E-print Network

Inulins are nano-meter size semi-crystalline particles, composed of oligomeric fructose units. It has been subjected to fine micro-structural analysis under temperature variations using mainly positron annihilation spectroscopy. The results show a non-monotonous temperature sensitive behaviour of the positron parameters, with considerable variation of its free volume size. The ortho-positronium pick-off component shows a major thermotropic transition at ~320K and a structure loss due to glass transition. Differential scanning calorimetry confirms the onset of the major molecular transition around the same temperature with an enthalpy change of {\\Delta}H ~379J /gm and thermo-gravimetric analysis shows mass loss in the said transition. Keywords: Inulin, fructose units, positron annihilation spectroscopy, microstructure, free volume analysis. thermotropic transition, thermal analysis.

Bichitra Nandi Ganguly; Madhusudan Roy; S. P. Moulik

2014-01-08

122

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

123

Experimental study of jets in electron-positron annihilation  

NASA Astrophysics Data System (ADS)

Data on hadron production by e+e--annihilation at c.m. energies between 30 GeV and 36 GeV are presented and compared with two models both based on first-order QCD but using different schemes for the fragmentation of quarks and gluons into hadrons. In one model the fragmentation proceeds along the parton momenta, in the other along the colour-anticolour axes. The data are reproduced better by fragmentation along the colour axes.

Bartel, W.; Cords, D.; Dittmann, P.; Eichler, R.; Felst, R.; Haidt, D.; Krehbiel, H.; Naroska, B.; O'Neill, L. H.; Steffen, P.; Wenninger, H.; Zhang, Y.; Elsen, E.; Helm, M.; Petersen, A.; Warming, P.; Weber, G.; Bethke, S.; Drumm, H.; Heintze, J.; Heinzelmann, G.; Hellenbrand, K. H.; Heuer, R. D.; von Krogh, J.; Lennert, P.; Kawabata, S.; Matsumura, H.; Nozaki, T.; Olsson, J.; Rieseberg, H.; Wagner, A.; Bell, A.; Foster, F.; Hughes, G.; Wriedt, H.; Allison, J.; Ball, A. H.; Bamford, G.; Barlow, R.; Bowdery, C.; Duerdoth, I. P.; Hassard, J. F.; King, B. T.; Loebinger, F. K.; MacBeth, A. A.; McCann, H.; Mills, H. E.; Murphy, P. G.; Prosper, H. B.; Stephens, K.; Clarke, D.; Goddard, M. C.; Marshall, R.; Pearce, G. F.; Imori, M.; Kobayashi, T.; Komamiya, S.; Koshiba, M.; Minowa, M.; Nozaki, M.; Orito, S.; Sato, A.; Suda, T.; Takeda, H.; Totsuka, Y.; Watanabe, Y.; Yamada, S.; Yanagisawa, C.

1981-04-01

124

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

125

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

SciTech Connect

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

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

2012-12-10

126

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

127

Positron annihilation study of defects and Si nanoprecipitation in sputter-deposited silicon oxide films  

Microsoft Academic Search

Positron annihilation spectroscopy was conducted to study defects and Si nanoprecipitation in sputter-deposited silicon oxide films. For as-deposited SiO0.9 and SiO1.9 films, Doppler broadening spectra are strongly influenced by the type of paramagnetic defects (Pb or E?). However, the disappearance of these defects after annealing at 1050 °C in a vacuum cannot account for the corresponding change of the Doppler

R. S. Yu; K. Ito; K. Hirata; K. Sato; W. Zheng; Y. Kobayashi

2003-01-01

128

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

129

Extragalactic Inverse Compton Light from Dark Matter Annihilation and the Pamela Positron Excess  

E-print Network

We calculate the extragalactic diffuse emission originating from the up-scattering of cosmic microwave photons by energetic electrons and positrons produced in particle dark matter annihilation events at all redshifts and in all halos. We outline the observational constraints on this emission and we study its dependence on both the particle dark matter model (including the particle mass and its dominant annihilation final state) and on assumptions on structure formation and on the density profile of halos. We find that for low-mass dark matter models, data in the X-ray band provide the most stringent constraints, while the gamma-ray energy range probes models featuring large masses and pair-annihilation rates, and a hard spectrum for the injected electrons and positrons. Specifically, we point out that the all-redshift, all-halo inverse Compton emission from many dark matter models that might provide an explanation to the anomalous positron fraction measured by the Pamela payload severely overproduces the observed extragalactic gamma-ray background.

Stefano Profumo; Tesla E. Jeltema

2009-05-30

130

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

SciTech Connect

We calculate the extragalactic diffuse emission originating from the up-scattering of cosmic microwave photons by energetic electrons and positrons produced in particle dark matter annihilation events at all redshifts and in all halos. We outline the observational constraints on this emission and we study its dependence on both the particle dark matter model (including the particle mass and its dominant annihilation final state) and on assumptions on structure formation and on the density profile of halos. We find that for low-mass dark matter models, data in the X-ray band provide the most stringent constraints, while the gamma-ray energy range probes models featuring large masses and pair-annihilation rates, and a hard spectrum for the injected electrons and positrons. Specifically, we point out that the all-redshift, all-halo inverse Compton emission from many dark matter models that might provide an explanation to the anomalous positron fraction measured by the Pamela payload severely overproduces the observed extragalactic gamma-ray background.

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

2009-07-01

131

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

132

Extragalactic Inverse Compton Light from Dark Matter Annihilation and the Pamela Positron Excess  

E-print Network

We calculate the extragalactic diffuse emission originating from the up-scattering of cosmic microwave photons by energetic electrons and positrons produced in particle dark matter annihilation events at all redshifts and in all halos. We outline the observational constraints on this emission and we study its dependence on both the particle dark matter model (including the particle mass and its dominant annihilation final state) and on assumptions on structure formation and on the density profile of halos. We find that for low-mass dark matter models, data in the X-ray band provide the most stringent constraints, while the gamma-ray energy range probes models featuring large masses and pair-annihilation rates, and a hard spectrum for the injected electrons and positrons. Specifically, we point out that the all-redshift, all-halo inverse Compton emission from many dark matter models that might provide an explanation to the anomalous positron fraction measured by the Pamela payload severely overproduces the obs...

Profumo, Stefano

2009-01-01

133

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

134

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

E-print Network

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

Ma, Xiaoguang

2012-01-01

135

The 511 keV emission from positron annihilation in the Galaxy  

SciTech Connect

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

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

2011-07-01

136

Hunting for glueballs in electron-positron annihilation  

SciTech Connect

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

Stanley Brodsky; Alfred Scharff Goldhaber; Jungil Lee

2003-05-01

137

Hunting for Glueballs in Electron-Positron Annihilation  

SciTech Connect

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

Brodsky, Stanley J.

2003-05-28

138

Hunting for glueballs in electron-positron annihilation  

E-print Network

We calculate the cross section for the exclusive production of J^{PC}=0^{++} glueballs G_0 in association with the J/psi in e^+e^- annihilation using the pQCD factorization formalism. The required long-distance matrix element for the glueball is bounded by CUSB data from a search for resonances in radiative Upsilon decay. The cross section for e^+e^- -> J/psi+ G_0 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 2 than that for h=eta_{c}(2S). As the subprocesses gamma^* -> (c c-bar) (c c-bar) and gamma^* -> (c c-bar) (g g) are of the same nominal order in perturbative QCD, it is possible that some portion of the anomalously large signal observed by Belle in e^+ e^- -> J/psi X may actually be due to the production of charmonium-glueball J/psi G_J pairs.

Stanley J. Brodsky; Alfred Scharff Goldhaber; Jungil Lee

2003-05-23

139

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

NASA Astrophysics Data System (ADS)

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

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

2012-07-01

140

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

141

Positron annihilation on defects in silicon irradiated with 15 MeV protons  

NASA Astrophysics Data System (ADS)

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

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

2013-01-01

142

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

NASA Astrophysics Data System (ADS)

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

Cheng, Yongjun; Mitroy, J.

2014-10-01

143

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

NASA Astrophysics Data System (ADS)

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

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

2009-02-01

144

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

145

Free volume studies of various polymeric systems using positron annihilation and PVT-EOS analyses  

NASA Astrophysics Data System (ADS)

The glass transition phenomenon and free volume behavior below and above the glass transition temperature of various polymeric systems have been investigated. Several novel polymeric systems were considered for this study. Two generations of hyperbranched polyols, H40 and H20, were selected due to large number of hydroxyl groups on the periphery and within the bulk. The effect of hydrogen bonds and molecular weight was related with the glass transition and free volume behavior for the whole range of experimental temperature. The free volume behavior was experimentally studied using PVT and PALS to determine occupied volume, fractional free volume and number density of holes. Molecular dynamic simulation was performed to compare atmospheric pressure V-T data and visualize hydrogen bond structures. Linear as well as crosslinked isomeric polymers were selected for the study of isomerism on glass transition and free volume. Isomers were selected based on para and meta substitution on phenylene ring in the polymer repeat unit. In this way the polymer chemical composition was kept the same and only architecture was varied. Two linear polymers based on isomeric repeat unit, polyethylene terephthalate (para) and polyethylene isophthalate (meta), and five sets of epoxy networks prepared using isomeric diamine crosslinkers, 3,3'-DDS and 4,4'-DDS were used. The crosslinked networks followed the same trend of glass transitions and free volume properties as in linear polymers. The glass transition temperatures of para isomer based linear polymers and epoxy-amine networks were higher. It was observed for linear polymers as well as for all networks that para isomer generates structure with higher amount of free volume in the glassy state; however, in the rubbery state they are the same. Free volume studies were further extended for solvent uptake in epoxy-amine networks to correlate hole free volume and van der Waals volume of solvent. The higher frozen in free volume in all para isomers leads to higher solvent uptake as compared to meta isomers. Pressure-volume-temperature (PVT) data were used to calculate occupied volume and fractional free volume using Simha-Somcynsky (S-S) Equation of State (EOS). PALS was utilized to evaluate average hole free volume for a wide range of temperature. Both PALS and PVT were utilized to evaluate occupied volume and hole number density. The PVT fractional free volume was also correlated with positron annihilation lifetime spectroscopy (PALS) hole free volume, v3, and ortho-positronium formation intensity, I 3, to calculate the correlation coefficient, C.

Kaushik, Mukul

146

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

147

Gamma-ray lines from novae. [relationship to radioactive decay and positron annihilation  

NASA Technical Reports Server (NTRS)

An appropriate gamma-ray telescope could detect the gamma-rays associated with radioactive decays. The observable lines would be the annihilation radiation following the positron emission of N-13, O-14, O-15, and Na-22 and the 2.312-MeV line emitted following the O-14 decay and the 1.274-MeV line emitted following the Na-22 decay. The experimental possibility should be borne in mind for the occurrence of novae within a few kiloparsecs.

Clayton, D. D.; Hoyle, F.

1974-01-01

148

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

149

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

NASA Astrophysics Data System (ADS)

This paper deals with the characterization of sub-surface damage caused by the machining of 100Cr6 roll bearing steel. The samples turned using tools with variable flank wears were characterized by two non-destructive techniques sensitive to defects introduced by plastic deformation: magnetic Barkhausen noise and positron annihilation. These techniques were combined with light and electron microscopy, x-ray diffraction and microhardness testing. The results of the experiment showed that damage in the sub-surface region increases with increasing flank wear, but from a certain critical value dynamic recovery takes place. The intensity of Barkhausen noise strongly decreases with increasing flank wear due to the increasing density of the dislocations pinning the Bloch walls and suppressing their motion. This was confirmed by positron annihilation spectroscopy, which enables the determination of the dislocation density directly. Hence, a good correlation between Barkhausen noise emission and positron annihilation spectroscopy was found.

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

2014-11-01

150

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

E-print Network

A large number of bound state properties of the four-body muonium-positronium system MuPs (or $\\mu^{+} e^{-}_2 e^{+}$) are determined to high accuracy. Based on these expectation values we predict that the weakly-bound four-body MuPs system has the `two-body' cluster structure Mu + Ps. The two neutral clusters Mu ($\\mu^{+} e^{-}$) and Ps ($e^{+} e^{-}$) interact with each other by the attractive van der Waals forces. By using our expectation values of the electron-positron delta-functions we evaluated the half-life $\\tau_a$ of the MuPs system against annihilation of the electron-positron pair: $\\tau_a = \\frac{1}{\\Gamma} \\approx 4.076453 \\cdot 10^{-10}$ $sec$. The hyperfine structure splitting of the ground state in the MuPs system evaluated with our expectation values is $\\Delta \\approx$ 23.064(5) $MHz$.

Frolov, Alexei M

2014-01-01

151

Outstanding Timing Resolution of Pure CsBr Scintillators for Coincidence Measurements of Positron Annihilation Radiation  

NASA Astrophysics Data System (ADS)

An outstanding timing resolution of 75 ps at full width at half maximum (FWHM) has been achieved in coincidence measurement of positron annihilation radiation. Pure CsBr crystals characterized by an ultrafast response of less than 70 ps and a considerably low light output of 20 photons/MeV are utilized as ultrafast scintillators by coupling them with microchannel-plate photomultiplier tubes (MCP-PMTs) and a fast digitizer. The achieved timing resolution corresponds to a time-of-flight (TOF) localization of 11 mm in positron emission tomography (PET). The results can serve as a new standard in the development of scintillator materials to achieve a timing resolution better than 100 ps.

Shibuya, Kengo; Saito, Haruo; Koshimizu, Masanori; Asai, Keisuke

2010-08-01

152

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

153

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

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

154

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

155

Structure, electric properties and positron annihilation studies of CuZnFe2O4 doped with BaTiO3  

NASA Astrophysics Data System (ADS)

Composite materials of spinel CuZnFe2O4 ferrite (CZF) and barium titanate BaTiO3 (BT) were prepared by using the high-energy ball milling technique. The X-ray diffraction (XRD) patterns of the composite system confirmed the composite preparation with two piezomagnetic and piezoelectric phases. The results of scanning electron microscope (SEM) measurements show a nearly homogeneous microstructure with good dispersion of BT grains as well as the presence of some pores. The positron annihilation lifetime (PAL) is used to probe the defects and structural changes of the BT-CZF composites. The PAL parameters ( , I1, , I2 and mean lifetime) show that the doped BT content affects the size and concentration of the vacant type defects.

Hemeda, O. M.; Mahmoud, K. R.; Sharshar, T.

2014-08-01

156

Positron lifetime studies of electron-irradiated copper. [4. 5 MeV electron beams  

SciTech Connect

Single-crystal copper was irradiated at 80 K with 4.5 MeV electrons producing simple Frenkel defects as well as a significant concentration of multivacancies. Mean positron lifetime characteristics, which are sensitive to the presence of vacancies and multivacancies in copper, were monitored after isochronal anneals between 80 and 800 K. A study of the dependence of the mean positron lifetime on the total electron fluence was made and compared with existing theories relating these lifetimes to vacancy or multivacancy concentrations. Numerical data from curve-fitting procedures using a conventional trapping model for defect-induced changes in positron lifetimes indicate that about 8 percent of the defects produced by the irradiation were multivacancy units.

Hadnagy, T.D.; Byrne, J.G.; Miller, G.R.

1981-01-01

157

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

158

Effects of transition metal ions on positron annihilation characteristics in epoxies  

NASA Astrophysics Data System (ADS)

Effects of several 3d-transition metal ions on positron annihilation characteristics in MY-720 epoxy resin have been investigated. All metal complexes were added at the mole ratio of 1 metal complex for every 10 MY-720 repeat units. In all cases, it appears that the 3d magnetic electrons are localized on their respective ions. Cr 3+-ions make the strongest impact — both on the probability of formation of positronium atoms as well as their subsequent decay. The effects of Co 2+ and Co 3+ ions are not greatly different, indicating that the final oxidation states of Co x+ -ions in the host epoxy may be the same, namely 2 +. Ni 2+-ions are unique because their coordination geometry remains unchanged when Ni(acac) 2 is introduced in the MY-720 epoxy. In this respect, Ni(acac) 2 is similar to Co(aeac) 2 and both have comparable impacts on positron annihilation characteristics. The effects of Fe 3+-ions are consistent with coordination symmetry change from sp 3d 2 to d 1sp 3 when Fe(acac) 3 is introduced in the epoxy.

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

1987-06-01

159

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

160

Effect of lithium diffusion on the native defects in GaAs studied by positron annihilation spectroscopy  

Microsoft Academic Search

The influence of Li diffusion on GaAs was studied by means of positron annihilation spectroscopy. Positron trapping at both gallium vacancy (VGa) and antisite (GaAs) defects were found to increase after the in-diffusion, and increase further after the out-diffusion of Li. This indicates an increase in the concentrations of negative and neutral defects in both phases. Majority of the Ga

S. Arpiainen; K. Saarinen; J. T. Gudmundsson; H. P. Gislason

1999-01-01

161

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

162

Study of transformation kinetics in the Fe40Ni38Mo4B18 metallic glass by positron annihilation methods  

E-print Network

831- Study of transformation kinetics in the Fe40Ni38Mo4B18 metallic glass by positron annihilation dans le verre métallique. Abstract. - We report on transformations induced in the metallic glass Fe40Ni are trapped in the as-received metallic glass. Revue Phys. Appl. 20 (1985) D�CEMBRE 1985, PAGE 831

Boyer, Edmond

163

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

E-print Network

We discuss complete theory of spin-1/2 electron-positron quantum plasmas, when electrons and positrons move with velocities mach smaller than the speed of light. We derive a set of two fluid quantum hydrodynamic equations consisting of the continuity, Euler, spin (magnetic moment) evolution equations for each species. We explicitly include the Coulomb, spin-spin, Darwin and annihilation interactions. The annihilation interaction is the main topic of the paper. We consider contribution of the annihilation interaction in the quantum hydrodynamic equations and in spectrum of waves in magnetized electron-positron plasmas. We consider propagation of waves parallel and perpendicular to an external magnetic field. We also consider oblique propagation of longitudinal waves. We derive set of quantum kinetic equations for electron-positron plasmas with the Darwin and annihilation interactions. We apply the kinetic theory for the linear wave behavior in absence of external fields. We calculate contribution of the Darwin and annihilation interactions in the Landau damping of the Langmuir waves. We should mention that the annihilation interaction does not change number of particles in the system. It does not related to annihilation itself, but it exists as a result of interaction of an electron-positron pair via conversion of the pair into virtual photon. A pair of the non-linear Schrodinger equations for electron-positron plasmas including the Darwin and annihilation interactions. Existence of conserving helicity in electron-positron quantum plasmas of spinning particles with the Darwin and annihilation interactions is demonstrated. We show that annihilation interaction plays an important role in quantum electron-positron plasmas giving contribution of the same magnitude as the spin-spin interaction.

Pavel A. Andreev

2014-04-18

164

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

165

Positrons from supernovae  

NASA Technical Reports Server (NTRS)

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

Chan, Kai-Wing; Lingenfelter, Richard E.

1993-01-01

166

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

E-print Network

We discuss complete theory of spin-1/2 electron-positron quantum plasmas, when electrons and positrons move with velocities mach smaller than the speed of light. We derive a set of two fluid quantum hydrodynamic equations consisting of the continuity, Euler, spin (magnetic moment) evolution equations for each species. We explicitly include the Coulomb, spin-spin, Darwin and annihilation interactions. The annihilation interaction is the main topic of the paper. We consider contribution of the annihilation interaction in the quantum hydrodynamic equations and in spectrum of waves in magnetized electron-positron plasmas. We consider propagation of waves parallel and perpendicular to an external magnetic field. We also consider oblique propagation of longitudinal waves. We derive set of quantum kinetic equations for electron-positron plasmas with the Darwin and annihilation interactions. We apply the kinetic theory for the linear wave behavior in absence of external fields. We calculate contribution of the Darwin...

Andreev, Pavel A

2014-01-01

167

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

168

Identification of defect properties by positron annihilation in Te-doped GaAs after Cu in-diffusion  

NASA Astrophysics Data System (ADS)

Positron lifetime measurements and Doppler-broadening spectroscopy were combined to investigate the defect properties during Cu diffusion in Te-doped GaAs. The diffusion of Cu was performed during an annealing step at 1100 °C under two different arsenic vapor pressures. The samples were quenched into room temperature water. During a subsequent isochronal annealing experiment, it was found that vacancy clusters were generated and grown, and finally they disappeared. The lifetime results show that, in addition to deep positron traps of vacancy type, positron trapping with a lifetime close to the bulk value of 228 ps occurs. The positron lifetime results give direct evidence of positron localization at shallow traps in GaAs:Te. Due to the Cu contamination during the annealing process, the shallow trap is believed to be the CuGa2- double acceptor. The concentration of shallow traps is determined and found to be in good agreement with the concentration determined by Hall measurement. It decreases up to saturation with increasing annealing. The positron binding energy to these negative nonopen volume trap centers is determined to be 79 meV. It is found to be in agreement with the calculated value. Moreover, coincidence Doppler-broadening spectroscopy shows clearly that Cu atoms are bound in the direct vicinity of the observed vacancy-like defects. Theoretical calculations of momentum distribution predicted that one Cu atom incorporated into a Ga site surrounds the observed open-volume defects.

Elsayed, M.; Krause-Rehberg, R.; Anwand, W.; Butterling, M.; Korff, B.

2011-11-01

169

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

170

Comparison of Exact Results for the Virtual Corrections to Bremsstrahlung in Electron-Positron Annihilation at High Energies  

E-print Network

We have compared the virtual corrections to electron-positron annihilation to fermion pairs with single hard bremsstrahlung as calculated by S. Jadach, M. Melles, B.F.L. Ward and S.A. Yost to several other expressions. The most recent of these comparisons is to the leptonic tensor calculated by J.H. Kuhn and G. Rodrigo for radiative return. Agreement is found to within $10^{-5}$ or better, as a fraction of the Born cross section.

S. A. Yost; C. Glosser; S. Jadach; B. F. L. Ward

2004-09-03

171

Studies of CdI2-Bi3 microstructures with optical methods, atomic force microscopy and positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Results of the comprehensive morphological study of CdI2-BiI3 layered crystals are presented. Direct AFM observations of micro- and nanostructures formed in the volume of the crystals confirm the predictions made on the basis of positron annihilation spectroscopy studies. The model explaining the possible pores formation mechanism is proposed and validated by the results of luminescence measurements at 8 K.

Karbovnyk, Ivan; Bolesta, Ivan; Rovetskii, Ivan; Velgosh, Sergiy; Klym, Halyna

2014-09-01

172

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

E-print Network

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

Ma, X G

2014-01-01

173

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

174

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

175

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

SciTech Connect

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

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

1990-12-20

176

Mn(2+)-induced substitutional structural changes in ZnS nanoparticles as observed from positron annihilation studies.  

PubMed

Zinc sulfide nanoparticles doped with different concentrations of manganese ions (Mn(2+)) were synthesized at various temperatures to investigate the effects of substitution and the associated defect evolution. Positron lifetime and Doppler broadening measurements were used as probes. The initial stage of defect recovery was dominated by the occupation of Zn(2+) vacancies by Mn(2+) ions, bringing in characteristic changes in the positron lifetimes, intensities and Doppler broadened lineshape parameters. Detailed analyses considering the presence of one and two types of defects were carried out to identify the type of defects which trap positrons at the different dopant concentrations. Electron paramagnetic resonance studies indicated increased Mn-Mn interaction and the formation of Mn clusters with further doping. The results are in striking contrast to those for nanorods, where vacancy recombination transformed their interior into regions free of defects. PMID:21694317

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

2008-06-11

177

High energy beam lifetime analysis  

SciTech Connect

We have developed a positron lifetime defect analysis capability based on a 3 MeV electrostatic accelerator. The high energy beam lifetime spectrometer is operational with a 60 mCi {sup 22}Na source providing a current of 7 10{sup 5} positrons per second. Lifetime data are derived from a thin plastic transmission detector providing an implantation time and a BaF{sub 2} detector to determine the annihilation time. Positron lifetime analysis is performed with a 3 MeV positron beam on thick sample specimens at counting rates in excess of 2000 per second. The instrument is being used for bulk sample analysis and analysis of samples encapsulated in controlled environments for in situ measurements.

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

1997-05-01

178

Thermal cycling effects in Cu-Zn-Al shape memory alloy by positron lifetime measurements  

SciTech Connect

This paper presents the results of an investigation on the effects of thermal cycling on the transformation and memory properties of a CuZnAl shape memory alloy. The positron annihilation technique was used to elucidate the micro-mechanism of transformation during thermal cycling. The present paper shows that martensite finish temperature and austenite start temperature of CuZuAl shape memory material will decrease with increasing number of thermal cycles, while martensite start temperature and austenite finish temperature remain constant. On the other hand, the thermal hysteresis will decrease and the shape memory strain will remain constant. The thermal cycling behavior is closely related to the formation of vacancy clusters by vacancy agglomeration.

Lin, G.M. [Zhongshan Univ., Guangzhou (China). Dept. of Physics] [Zhongshan Univ., Guangzhou (China). Dept. of Physics; Lai, J.K.L.; Chung, C.Y. [City Univ. of Hong Kong (Hong Kong). Dept. of Physics and Materials Science] [City Univ. of Hong Kong (Hong Kong). Dept. of Physics and Materials Science

1995-06-01

179

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

SciTech Connect

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

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

2007-12-24

180

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

NASA Astrophysics Data System (ADS)

A compact ensemble of high density nitrogen-vacancy (NV) centers in diamond is essential to sense various external fields with a high precision at the nanoscale. Here, defects in type IIa and type Ib diamonds induced by 28 MeV electron irradiation at 77 K were studied by combining the positron annihilation spectroscopy and first-principles calculations. It is shown that the electron irradiation at 77 K can significantly enhance the NV center formation by directly converting 24% vacancies into the NV centers, indicating that it is an efficient way to produce the high density NV centers in the type Ib diamond.

Tang, Z.; Chiba, T.; Nagai, Y.; Inoue, K.; Toyama, T.; Hasegawa, M.

2014-04-01

181

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

182

Modeling of charged particles trajectories in order to optimize the design of a new, higher resolution, Time of flight- Positron Annihilation Induced Auger Electron Spectroscopy (TOF PAES) System  

NASA Astrophysics Data System (ADS)

Time of Flight Positron Annihilation Induced Auger Electron Spectroscopy (TOF PAES) is a surface analytical technique with high surface selectivity. TOF PAES is used to study elemental composition, surface defects, and various energy loss mechanisms. Positrons incident on the sample surface at low energies can be trapped in an image-potential well just above the surface Prior to annihilation. Consequently it is possible to use positron annihilation related signals to selectively probe the top-most atomic layer. This poster presents the results of modeling of the charge particle beam transport system performed in connection with the optimization of the the design of the new TOF-PAES system currently under construction at U T Arlington. The system will incorporate a 2 m long drift tube in order to achieve better energy resolution than our previous TOF-PAES system design which used a 1 m long drift tube

Joglekar, Prasad; Lim, L.; Satyal, Suman; Kalaskar, Sushant; Shastry, K.; Weiss, Alex

2011-03-01

183

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

184

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

185

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

186

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

187

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

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

188

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

NASA Astrophysics Data System (ADS)

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

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

2000-06-01

189

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

190

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

E-print Network

contribution to the annihilation rate of the positron comes from valence electrons and the contri- bution-550 15 DÃ?CEMBRE 1981,1 Classification Physics Abstracts 78.70B 1. Introduction. - The positron to the variations of the electron density that one can distinguish the complexes of the type Ln Y m with n vacancies

Boyer, Edmond

191

Evaluation of a microchannel-plate PMT as a potential timing detector suitable for positron lifetime measurements  

NASA Astrophysics Data System (ADS)

This paper focuses on the evaluation of a microchannel-plate photomultiplier tube (MCP-PMT) as a candidate detector, suitable for positron lifetime studies. Several properties of MCP-PMTs, such as their fast time response, compact size, low susceptibility to magnetic fields, relatively high gain and the low power consumption make them attractive for positron lifetime spectroscopy. The preliminary tests were performed with a 85001-501 Burle Planacon TM photomultiplier tube assembly. Initial measurements were conducted with a pulsed Picosecond Injection Laser (PiLas) system. The engineering sample of the 85001 exhibits a transit-time-spread (TTS) of 110 ps (FWHM). Further timing experiments showing the suitability of the device as Cherenkov detector are presented. For the first time, a conventional positron lifetime spectrum of a Cz-Si probe measured with a spectrometer, where an MCP-PMT detector is included, has been demonstrated.

Kosev, K.; Butterling, M.; Anwand, W.; Cowan, T.; Hartmann, A.; Heidel, K.; Jungmann, M.; Krause-Rehberg, R.; Massarczyk, R.; Schilling, K. D.; Schwengner, R.; Wagner, A.

2010-12-01

192

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

E-print Network

spectroscopy to study the development of damage and annealing behavior ofneutron-irradiated reactor pressure. 439©1997 Materials Research Society #12;EXPERIMENTAL METHODS ASTM A508 reactor pressure-vessel steel

Motta, Arthur T.

193

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

NASA Technical Reports Server (NTRS)

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

Ruderman, Malvin

1993-01-01

194

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

NASA Astrophysics Data System (ADS)

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

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

2009-01-01

195

Electron-positron annihilation into three pions and the radiative return  

E-print Network

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

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

2005-12-14

196

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

E-print Network

of metal-oxide-semiconductor (MOS) capacitors at room temperature and at 35 K. The centroid shift capacitor within the Si. I. INTRODUCTION The past few years have seen a rapid growth in the studies of Si was used to study hydrogen activation energy, interface density variations, positron mobility in Si

Rubloff, Gary W.

197

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

SciTech Connect

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 temperature independent, the second critical concentration is shifted to slightly lower values at higher temperatures (from approx. 1.4 m to approx. 1.0 m on going from 21 to 51/sup 0/C). The plot of the Arrhenius activation energies of positronium formation as a function of surfactant molality indicates some continuous changes in the properties of the micelles below and above the third concentration.

Boussaha, A.; Ache, H.J.

1981-08-20

198

The positron annihilation and Hall-Petch relation in polycrystalline Fe[sub 78]B[sub 13]Si[sub 9] alloys with ultrafine grains  

Microsoft Academic Search

Nanometer-sized crystalline materials, which are sometimes called nanocrystalline materials', have been reported to exhibit different properties from ordinary crystalline materials. The possibility of affecting the physical or mechanical properties of materials by producing nanocrystalline structures constitutes a major subject of interest in materials science. The positron annihilation and the Hall-Petch behaviors in the nanocrystalline FeBSi alloy prepared using the crystallization

B. Z. Ding; H. Y. Tong; H. G. Jiang; J. T. Wang; W. D. Wei

1993-01-01

199

A program for the interactive analysis of positron lifetime spectra on personal computers with the aid of screen graphics  

NASA Astrophysics Data System (ADS)

A personal computer program has been developed for use in the interactive analysis of positron lifetime spectra with the aid of a graphic display. The multi-exponential function model is used to fit a spectrum by the linear least-squares method. The spectrum displayed on the screen is altered simultaneously as the lifetime components are stripped one by one from the longest-lived to the shortest. The merit of this code is that it provides the user with a visual feedback at any stage of the analysis. Moreover, the obtained model parameters of the spectrum can be used as initial estimates for the POSITRONFIT program used for final analysis.

Dai, G. H.; Fu, J.; Liu, Q. S.

1991-10-01

200

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

NASA Astrophysics Data System (ADS)

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

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

2012-08-01

201

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

NASA Technical Reports Server (NTRS)

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

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

1992-01-01

202

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

NASA Astrophysics Data System (ADS)

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

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

2009-03-01

203

Positron spectroscopy for materials characterization  

SciTech Connect

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

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

1988-01-01

204

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

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

205

Electron-positron annihilation into four charged pions and the a_1-rho-pi Lagrangian  

E-print Network

The excitation curve of e+e- annihilation into four charged pions in the rho(770) region is calculated using three existing models with rho mesons and pions in intermediate states supplemented by Feynman diagrams with the a_1(1260)pi intermediate states. A two-term phenomenological Lagrangian of the a_1-rho-pi interaction is used. The mixing angle is determined by fitting the e+e- --> pi+pi-pi+pi- cross section data of the Novosibirsk CMD-2 collaboration and also its combination with the low-energy part of the BaBar collaboration data. It is shown that the inclusion of the a_1-pi intermediate states succeeds in obtaining a good agreement with the data on both cross section and the rho^0 --> pi+pi-pi+pi- decay width. When moving to energies above 1 GeV, the rho(1450) and rho(1700) resonances are taken into account to get excellent agreement with the BaBar data over the full energy range up to 4.5 GeV.

Peter Lichard; Josef Juran

2006-01-27

206

Observation of Charmed Baryons in Electron-Positron Annihilations at 10GEV Center of Mass Energy.  

NASA Astrophysics Data System (ADS)

The charmed baryons Lambda_ {c} and Sigma_{c } have been observed using the ARGUS detector in the DORIS II e^{+}e ^{-} storage at DESY. The ARGUS experiment is a 4pi magnetic solenoidal detector used to observe e^{+ }e^{-} annihilations in the center-of-mass energy region near 10GeV. The Lambda_{c} and its charge conjugate, were observed in three decay modes. In its decay to pK^{ -}pi^{+} the measured sigma cdot Br was 9.0 +/- 1.2 +/- 1.0 pb, for Lambdapi^ {+}pi^{-}pi^ {+} sigma cdot Br. was observed to be 5.5 +/- 1.3 +/- 0.8, and for K^0 p sigma cdot Br was 5.6 +/- 1.1 +/- 0.7pb. These measurements were all made at a mean centre-of-mass energy of 10.2GeV. The mean of the Lambda_{c} mass measured in these decay channels was 2283.1 +/- 1.7 +/- 2.0MeV/c^2 . Two members of the lowest isospin-triplet charmed baryons, Sigma_{c} were observed. The Sigma_sp{c }{++} and the Sigma _sp{c}{0} were observed to decay to Lambda_{c}^ {+}pi^{+} and Lambda_{c}^{+ }pi^{-} respectively, where the Lambda_{c} was then observed in the decay modes already mentioned. The mass difference between the Sigma_sp {c}{++} and the Lambda _{c} was 168.5 +/- 0.5MeV/c^2, and the Sigma_sp{c}{0} Lambda _{c} mass difference was 167.0 +/- 0.5MeV/c^2. The mean Sigma_{c} Lambda _{c} mass difference was 167.8 +/- 1.8 +/- 2.0. The isospin mass splitting between the Sigma _sp{c}{++} and the Sigma_sp{c}{0} was 1.5 +/- 0.7 +/- 0.3MeV/c^2. Studies were made into the nature of charmed baryon production in e^{+}e ^{-} annihilations. Limits for the direct hadronic production of Lambda _{c}'s from the Upsilon (1S) and Upsilon(2S) resonances, relative to the continuum, of 15% and 20% respectively, were established. The momentum spectrum of the Lambda_{c} was quite hard. Fitting the x_{p}^ectrum of the Lambda_{c} with the well known Peterson splitting function gave a value of epsilon = 0.236 +/- .035. The x_{p}^ectrum for the Sigma_{c} was softer, and had a Peterson epsilon of 0.59 +/- 0.16. The rate of Lambda _{c} production from Sigma _{c} decays was measured to be 39 +/- 12%.

Seywerd, Henry Cornelus Joseph

207

He-implantation induced defects in Si studied by slow positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

Open volume defect profiles have been obtained by performing Doppler broadening measurements with a slow positron beam on p-type Si samples implanted near liquid nitrogen temperature with He ions at 20 keV and at 5×1015 and 2×1016cm-2 fluence. The evolution of the defect profiles was studied as a function of isothermal annealing at 250 °C. The fraction of released He was measured by thermal programmed desorption. The defects could be identified as a coexistence of monovacancies stabilized by He-related defects and divacancies. The number of defects decreases for annealing time of a few minutes, then increases at longer annealing times. The mean depth of the defect profiles in the as-implanted samples was found to be very near the surface. After annealing, the mean depth increases to less than one half of the projected He range. This complex dynamics has been interpreted as due to passivation of vacancies by He during the implantation process and the first annealing step when no appreciable He is lost, and to subsequent depassivation during He desorption.

Brusa, R. S.; Karwasz, G. P.; Tiengo, N.; Zecca, A.; Corni, F.; Calzolari, G.; Nobili, C.

1999-02-01

208

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

PubMed Central

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

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

2012-01-01

209

Positron annihilation study of defects in electron-irradiated single crystal zinc oxide  

NASA Astrophysics Data System (ADS)

Pressurized melt grown zinc oxide (ZnO) single crystals purchased from Cermet Inc. were irradiated by 2MeV electrons with fluence of 6x1017cm-2. Isochronal annealing from 100°C-800°C was performed on the crystals under argon and air ambience. Variable Energy Doppler Broadening Spectroscopy (VEDBS) was carried out on both the as-grown and the irradiated samples at each annealing step. The migration, agglomeration and annealing of grown-in and irradiated-introduced defects were studied. It was observed that the grown-in vacancy-type defects concentration decreased at 300°C and 600 °C. For the irradiated sample annealed in argon, the positron trapping vacancy-type defect concentration decreased at 300°C and 600°C. Further annealing the as-grown and irradiated samples in argon increased the S parameter further. For the irradiated sample annealed in air, the vacancy-type defect concentration decreases at 300°C and 700°C.

To, C. K.; Yang, B.; Beling, C. D.; Fung, S.; Ling, C. C.; Gong, M.

2011-01-01

210

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

NASA Astrophysics Data System (ADS)

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

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

211

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

NASA Astrophysics Data System (ADS)

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

Kovács, Z.; Harko, T.

2011-11-01

212

Observation of the electron ridge Fermi surface in YBa{sub 2}Cu{sub 3}O{sub 7-x} by positron annihilation  

SciTech Connect

Positron annihilation (two-dimensional-angular-correlation) experiments on an untwinned single crystal of metallic YBa{sub 2}Cu{sub 3}O{sub 7{minus}x} sample are reported in the c-projection. The measurements were carried out at room temperature and involved 94 Mcounts. An analysis of the spectra reveals clearly for the first time the presence of the electron ridge Fermi surface associated with the one-dimensional chain bands, and orthorhombic anisotropies in momentum density in good agreement with the band theory predictions.

Smedskjaer, L.C.; Fang, Y.; Bailey, K.G.; Welp, U. [Argonne National Lab., IL (United States); Bansil, A. [Northeastern Univ., Boston, MA (United States). Physics Dept.

1991-04-01

213

A study of double-charm and charm-strange baryons in electron-positron annihilations  

NASA Astrophysics Data System (ADS)

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

Edwards, Adam Jacob

214

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

SciTech Connect

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

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

2009-06-19

215

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

E-print Network

to be largely unaffected by bulk free volume differences in the absence of hydrogen.9 Noting that the free is paramount if one wants to fun- damentally understand the mechanisms controlling fatigue failure. Positron Positron beams with variable low positron implantation energies are also widely used to depth profile near

Gidley, David

216

Voids and other neutron-produced microstructure in Mo and Mo-0. 5 at. % Ti as studied by positron-annihilation techniques  

SciTech Connect

Specimens of Mo and Mo-0.5 at. % Ti which have been irradiated with neutrons (approx. 10/sup 22/ n/cm/sup 2/, E > 0.1 MeV) at temperatures between 425 and 1500/sup 2/C have been studied using both lifetime and Doppler-broadening measurements. Both the shape parameter and the intensity of the lifetime component from positrons trapped at voids define swelling as a function of temperature in a way that is independent of the neutron fluence. The relative swelling as a function of irradiation temperature and the swelling peak (approx. 750/sup 0/C) are well defined, but no information on the magnitude of the void volume is obtainable. In the determination of the shape and peak of the derived swelling curve, the positron analysis is more definitive than similar determinations using transmission electron microscopy.

Snead, Jr, C L; Lynn, K G; Jean, Y; Wiffen, F W; Schultz, P

1980-01-01

217

The kinetics of formation and growth of TiC precipitates in Ti-modified stainless steel studied by positron annihilation spectroscopy  

Microsoft Academic Search

The formation and growth of TiC precipitates in Ti-modified austenitic stainless steel (D-9 alloy) is monitored by positron lifetime spectroscopy. From isochronal annealing studies various recovery stages are identified. TiC precipitates are found to be more stable in 20% cold worked alloy than in a 17.5% cold worked sample. From the isothermal annealing studies, it is found that TiC precipitation

Padma Gopalan; R Rajaraman; B Viswanathan; K. P Gopinathan; S Venkadesan

1998-01-01

218

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

SciTech Connect

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

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

2009-01-01

219

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

NASA Astrophysics Data System (ADS)

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

Abdel-Hamed, M. O.

220

Photoluminescence spectroscopy and positron annihilation spectroscopy probe of alloying and annealing effects in nonpolar m-plane ZnMgO thin films  

NASA Astrophysics Data System (ADS)

Temperature-dependent photoluminescence characteristics of non-polar m-plane ZnO and ZnMgO alloy films grown by metal organic chemical vapor deposition have been studied. The enhancement in emission intensity caused by localized excitons in m-plane ZnMgO alloy films was directly observed and it can be further improved after annealing in nitrogen. The concentration of Zn vacancies in the films was increased by alloying with Mg, which was detected by positron annihilation spectroscopy. This result is very important to directly explain why undoped Zn1-xMgxO thin films can show p-type conduction by controlling Mg content, as discussed by Li et al. [Appl. Phys. Lett. 91, 232115 (2007)].

Yang, A. L.; Song, H. P.; Liang, D. C.; Wei, H. Y.; Liu, X. L.; Jin, P.; Qin, X. B.; Yang, S. Y.; Zhu, Q. S.; Wang, Z. G.

2010-04-01

221

Comparisons of Exact Results for the Virtual Photon Contribution to Single Hard Bremsstrahlung in Radiative Return for Electron-Positron Annihilation  

E-print Network

We compare fully differential exact results for the virtual photon correction to single hard photon bremsstrahlung obtained using independent calculations, both for electron-positron annihilation at high-energy colliders and for radiative return applications. The results are compared using Monte Carlo evaluations of the matrix elements as well as by direct analytical evaluation of certain critical limits. Special attention is given to the issues of numerical stability and the treatment of finite-mass corrections. It is found that agreement on the order of 10^{-5} or better is obtained over most of the range of hard photon energies, at CMS energies relevant to both high energy collisions and radiative return experiments.

S. Jadach; B. F. L. Ward; S. A. Yost

2006-02-21

222

Constraints on Resonant Dark Matter Annihilation  

E-print Network

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

Backovic, Mihailo

2011-07-04

223

Rapid three-dimensional imaging of defect distributions using a high-intensity positron microbeam  

NASA Astrophysics Data System (ADS)

An intense positron microbeam generated by an electron accelerator has been developed for obtaining three-dimensional positron lifetime mappings in a sample to permit visual evaluation of defect distributions. The beam diameter at the sample was 80-100 ?m. The counting rate of the positron annihilation ? rays used to measure positron lifetime was as large as 3×103 s-1. Three-dimensional imaging was demonstrated of positron lifetimes in a SiO2 sample, which was irradiated with ion beams through a mesh mask. The time to obtain a single image (3500 pixels for an area of 2.5×3.5 mm2) was 0.5-1 h.

Oshima, N.; Suzuki, R.; Ohdaira, T.; Kinomura, A.; Narumi, T.; Uedono, A.; Fujinami, M.

2009-05-01

224

Precision measurement of charged pion and kaon differential cross sections in electron-positron annihilation at Q = 10.52 GeV  

E-print Network

Measurements of inclusive differential cross sections for charged pion and kaon production in electron-positron annihilation have been carried out at a center-of-mass energy of Q = 10.52 GeV. The measurements were performed with the Belle detector at the KEKB electron-positron collider using a data sample containing 113 million e+e- -> qqbar events, where q={u,d,s,c}. We present charge-integrated differential cross sections d\\sigma_h+-/dz for h+- = pi+-, K+- as a function of the relative hadron energy z = 2*E_h / sqrt{s} from 0.2 to 0.98. The combined statistical and systematic uncertainties for pi+- (K+-) are 4% (4%) at z ~ 0.6 and 15% (24%) at z ~ 0.9. The cross sections are the first measurements of the z-dependence of pion and kaon production for z > 0.7 as well as the first precision cross section measurements at a center-of-mass energy far below the Z^0 resonance used by the experiments at LEP and SLC.

Belle Collaboration; M. Leitgab; R. Seidl; M. Grosse Perdekamp; A. Vossen; I. Adachi; H. Aihara; D. M. Asner; V. Aulchenko; T. Aushev; A. M. Bakich; B. Bhuyan; A. Bondar; A. Bozek; M. Bra?ko; J. Brodzicka; T. E. Browder; V. Chekelian; A. Chen; P. Chen; B. G. Cheon; K. Chilikin; K. Cho; V. Chobanova; Y. Choi; D. Cinabro; J. Dalseno; Z. Drásal; D. Dutta; S. Eidelman; D. Epifanov; H. Farhat; J. E. Fast; V. Gaur; N. Gabyshev; R. Gillard; F. Giordano; Y. M. Goh; B. Golob; J. Haba; K. Hayasaka; H. Hayashii; Y. Hoshi; W. -S. Hou; Y. B. Hsiung; H. J. Hyun; T. Iijima; A. Ishikawa; R. Itoh; W. W. Jacobs; T. Julius; J. H. Kang; P. Kapusta; E. Kato; T. Kawasaki; H. J. Kim; H. O. Kim; J. B. Kim; J. H. Kim; M. J. Kim; J. Klucar; B. R. Ko; P. Kodyš; R. T. Kouzes; P. Križan; P. Krokovny; R. Kumar; T. Kumita; Y. -J. Kwon; J. S. Lange; S. -H. Lee; Y. Li; Z. Q. Liu; D. Liventsev; D. Matvienko; K. Miyabayashi; H. Miyata; R. Mizuk; A. Moll; N. Muramatsu; E. Nakano; M. Nakao; Z. Natkaniec; M. Nayak; E. Nedelkovska; C. Ng; N. Nellikunnummel; O. Nitoh; A. Ogawa; S. Ogawa; T. Ohshima; S. Okuno; S. L. Olsen; C. Oswald; P. Pakhlov; H. Park; H. K. Park; T. K. Pedlar; R. Pestotnik; M. Petri?; L. E. Piilonen; M. Röhrken; H. Sahoo; Y. Sakai; S. Sandilya; L. Santelj; T. Sanuki; Y. Sato; O. Schneider; G. Schnell; C. Schwanda; K. Senyo; O. Seon; M. E. Sevior; M. Shapkin; C. P. Shen; T. -A. Shibata; J. -G. Shiu; B. Shwartz; A. Sibidanov; F. Simon; P. Smerkol; Y. -S. Sohn; A. Sokolov; E. Solovieva; M. Stari?; M. Sumihama; T. Sumiyoshi; G. Tatishvili; Y. Teramoto; T. Tsuboyama; M. Uchida; T. Uglov; Y. Unno; S. Uno; Y. Usov; C. Van Hulse; G. Varner; V. Vorobyev; M. N. Wagner; C. H. Wang; J. Wang; M. -Z. Wang; P. Wang; M. Watanabe; Y. Watanabe; K. M. Williams; E. Won; Y. Yamashita; V. Zhilich; V. Zhulanov

2013-01-25

225

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

PubMed

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

Townrow, S; Coleman, P G

2014-03-26

226

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

NASA Astrophysics Data System (ADS)

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

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

1991-07-01

227

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

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

228

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

NASA Technical Reports Server (NTRS)

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

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

1982-01-01

229

The kinetics of formation and growth of TiC precipitates in Ti-modified stainless steel studied by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

The formation and growth of TiC precipitates in Ti-modified austenitic stainless steel (D-9 alloy) is monitored by positron lifetime spectroscopy. From isochronal annealing studies various recovery stages are identified. TiC precipitates are found to be more stable in 20% cold worked alloy than in a 17.5% cold worked sample. From the isothermal annealing studies, it is found that TiC precipitation is controlled by dislocations. The limited temperature dependence of dislocation controlled TiC precipitation is governed by an apparent activation energy of 1.6 eV. In 20% cold worked alloy, TiC precipitates are found to be stable against growth even after 1000 h of annealing at 923 K. For higher annealing temperatures, TiC precipitate coarsening occurs due to recrystallisation.

Gopalan, Padma; Rajaraman, R.; Viswanathan, B.; Gopinathan, K. P.; Venkadesan, S.

1998-08-01

230

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

SciTech Connect

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

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

1994-06-01

231

Annihilation explosions in macroscopic polyelectrons. Photon detonation  

E-print Network

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

Alexei M. Frolov

2009-06-05

232

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

NASA Technical Reports Server (NTRS)

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

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

1994-01-01

233

Silicon displacement threshold energy determined by electron paramagnetic resonance and positron annihilation spectroscopy in cubic and hexagonal polytypes of silicon carbide  

NASA Astrophysics Data System (ADS)

Both for electronic and nuclear applications, it is of major interest to understand the properties of point defects into silicon carbide (SiC). Low energy electron irradiations are supposed to create primary defects into materials. SiC single crystals have been irradiated with electrons at two beam energies in order to investigate the silicon displacement threshold energy into SiC. This paper presents the characterization of the electron irradiation-induced point defects into both polytypes hexagonal (6H) and cubic (3C) SiC single crystals by using both positron annihilation spectroscopy (PAS) and electron paramagnetic resonance (EPR). The nature and the concentration of the generated point defects depend on the energy of the electron beam and the polytype. After an electron irradiation at an energy of 800 keV v Si mono-vacancies and v Si-v C di-vacancies are detected in both 3C and 6H-SiC polytypes. On the contrary, the nature of point defects detected after an electron irradiation at 190 keV strongly depends on the polytype. Into 6H-SiC crystals, silicon Frenkel pairs v Si-Si are detected whereas only carbon vacancy related defects are detected into 3C-SiC crystals. The difference observed in the distribution of defects detected into the two polytypes can be explained by the different values of the silicon displacement threshold energies for 3C and 6H-SiC. By comparing the calculated theoretical numbers of displaced atoms with the defects numbers measured using EPR, the silicon displacement threshold energy has been estimated to be slightly lower than 20 eV in the 6H polytype and close to 25 eV in the 3C polytype.

Kerbiriou, X.; Barthe, M.-F.; Esnouf, S.; Desgardin, P.; Blondiaux, G.; Petite, G.

2007-05-01

234

Nuclear Instruments and Methods in Physics Research. Section B; Microstructural Characterization of Semi-Interpenetrating Polymer Networks by Positron Lifetime Spectroscopy  

NASA Technical Reports Server (NTRS)

Thermoset and thermoplastic polyimides have complementary physical/mechanical properties. Whereas thermoset polyimides are brittle and generally easier to process, thermoplastic polyimides are tough but harder to process. It is expected that a combination of these two types of polyimides may help produce polymers more suitable for aerospace applications. Semi-Interpenetrating Polymer Networks (S-IPNs) of thermoset LaRC(Trademark)-RP46 and thermoplastic LARC(Trademark)-IA polyimides were prepared in weight percent ratios ranging from 100:0 to 0: 100. Positron lifetime measurements were made in these samples to correlate their free volume features with physical/mechanical properties. As expected, positronium atoms are not formed in these samples. The second life time component has been used to infer the positron trap dimensions. The "free volume" goes through a minimum at about 50:50 ratio, suggesting that S-IPN samples are not merely solid solutions of the two polymers. These data and related structural properties of the S-IPN samples have been discussed in this paper.

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

1998-01-01

235

Annihilation in Gases and Galaxies  

NASA Technical Reports Server (NTRS)

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

Drachman, Richard J. (editor)

1990-01-01

236

Cosmic rays from dark matter annihilation and big-bang nucleosynthesis  

SciTech Connect

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

Hisano, Junji; Kawasaki, Masahiro [Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582 (Japan); Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa 277-8568 (Japan); Kohri, Kazunori [Physics Department, Lancaster University, Lancaster LA1 4YB (United Kingdom); Moroi, Takeo [Institute for the Physics and Mathematics of the Universe, University of Tokyo, Kashiwa 277-8568 (Japan); Department of Physics, Tohoku University, Sendai 980-8578 (Japan); Nakayama, Kazunori [Institute for Cosmic Ray Research, University of Tokyo, Kashiwa 277-8582 (Japan)

2009-04-15

237

Ubiquitous Nature of Multimode Vibrational Resonances in Positron-Molecule Annihilation A. C. L. Jones, J. R. Danielson, M. R. Natisin, and C. M. Surko  

E-print Network

in which the positron couples directly to a quasicontinuum of multimode vibrational states. A model that reproduces key features of the data. DOI: 10.1103/PhysRevLett.108.093201 PACS numbers: 34.80.Uv, 34.50.Ã?s, 34, we are led to conclude that broad quasicontinuous spectra of multimode vibrations (e.g., combinations

Gribakin, Gleb

238

Portable Positron Measurement System (PPMS)  

SciTech Connect

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

None

2011-01-01

239

Portable Positron Measurement System (PPMS)  

ScienceCinema

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

None

2013-05-28

240

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

NASA Astrophysics Data System (ADS)

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

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

2013-08-01

241

Positron production by pulsars  

NASA Technical Reports Server (NTRS)

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

Sturrock, P.; Baker, K. B.

1979-01-01

242

A general quantum mechanical method to predict positron spectroscopy  

NASA Astrophysics Data System (ADS)

The nuclear-electronic orbital (NEO) method was modified and extended to positron systems for studying mixed positronic-electronic wavefunctions. These methods include: Hartree-Fock (HF); second-order Moller-Plesset perturbation theory (MP2); configuration interaction (CI); complete active space self-consistent field (CASSCF); and full configuration interaction (FCI). The methodology for calculating positron-electron annihilation rates based on NEO-HF and NEO-MP2 wavefunctions was also implemented. Positronic and electronic basis sets were optimized at the NEO-FCI level for the positronium hydride (PsH) system and used to compute NEO-MP2 energies and annihilation rates. The effects of basis set size on correlation energies captured with the NEO-MP2 and NEO-FCI methods are compared and discussed. Equilibrium geometries and vibrational energy levels were computed for the LiX and e+LiX (X = H, F, Cl) systems at the MP2 and NEO-MP2 levels. Anharmonic effects were included by fitting the computed potential energy curves (PECs) to a Morse potential function. It was found that anharmonicity plays a significant role, specifically in the differences between the vibrational energy levels of the LiX and e+LiX systems. The implications of these results with respect to vibrational Feshbach resonances (VFRs) for these systems is discussed. The positron lifetime in pressed samples of K2B12H 12·CH3OH was measured to be 0.2645+/-0.0077 ns. This result is interpreted with quantum mechanical calculations of B 12H122- and e+B12H 122-. Calculations reveal a spherically symmetric positronic wavefunction, with a peak in the positron density at the outside edge of the hydrogen atom cage. The experimentally determined annihilation rate corresponds to an effective number of electrons of 1.88, or 0.94 of the two electrons present in the B12H122- dianion, indicating that there is significant positron density both inside and outside of the B12H122- cage.

Adamson, Paul E.

243

Slow-Positron Generator For Studying Polymer Films  

NASA Technical Reports Server (NTRS)

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

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

1992-01-01

244

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

NASA Astrophysics Data System (ADS)

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

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

2014-06-01

245

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

PubMed

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

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

2014-01-28

246

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

SciTech Connect

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

Martirena, S.G.

1994-04-01

247

Determination of the B lifetime  

NASA Astrophysics Data System (ADS)

We have determined the lifetime of B-hadrons produced in e + e - annihilation into multihadronic final states, by measuring the impact parameter of the track with respect to the centre of the interaction region. Two different analyses of the data were performed and gave compatible results. The lifetime is ? B =1.8{-0.4/+0.5}±0.4 ps.

Bartel, W.; Becker, L.; Cords, D.; Felst, R.; Haidt, D.; Knies, G.; Krehbiel, H.; Laurikainen, P.; Magnussen, N.; Meinke, R.; Naroska, B.; Olsson, J.; Schmidt, D.; Steffen, P.; Dietrich, G.; Hagemann, J.; Heinzelmann, G.; Kado, H.; Kawagoe, K.; Kleinwort, C.; Kuhlen, M.; Petersen, A.; Ramcke, R.; Schneekloth, U.; Weber, G.; Ambrus, K.; Bethke, S.; Dieckmann, A.; Elsen, E.; Heintze, J.; Hellenbrand, K. H.; Komamiya, S.; von Krogh, J.; Lannert, P.; Matsumura, H.; Rieseberg, H.; Spitzer, J.; Wagner, A.; Bowdery, C.; Finch, A.; Foster, F.; Hughes, G.; Nye, J.; Allison, J.; Ball, A. H.; Barlow, R. J.; Chrin, J.; Duerdoth, I. P.; Greenshaw, T.; Hill, P.; Loebinger, F. K.; MacBeth, A. A.; McCann, H.; Mills, H. E.; Murphy, P. G.; Stephens, K.; Warming, P.; Glasser, R. G.; Skard, J. A. J.; Wagner, S. R.; Zorn, G. T.; Cartwright, S. L.; Clarke, D.; Marshall, R.; Middleton, R. P.; Kawamoto, T.; Kobayashi, T.; Takeda, H.; Takeshita, T.; Yamada, S.

1986-09-01

248

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

Microsoft Academic Search

We derive a constraint on the pair density ratio, z+ = n+\\/np, in an electron-positron pair wind flowing away from the central region of an accretion disk around a compact object under the assumption of a coupling between electrons, positrons, and protons. The minimum rate at which positrons are injected into the annihilation volume is given by the observed annihilation

M. D. Moscoso; J. C. Wheeler

1994-01-01

249

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

SciTech Connect

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

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

2013-04-19

250

Gamma ray bursts and the origin of galactic positrons  

E-print Network

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

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

2004-05-01

251

Positrons as imaging agents and probes in nanotechnology  

NASA Astrophysics Data System (ADS)

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

Smith, Suzanne V.

2009-09-01

252

Positron emission tomography wrist detector  

DOEpatents

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

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

2006-08-15

253

Descriptions of positron defect analysis capabilities  

SciTech Connect

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

Howell, R.H.

1994-10-01

254

Constraints on dark matter annihilation from AMS-02 results  

NASA Astrophysics Data System (ADS)

We use recently released data on the positron-to-electron ratio in cosmic rays from the AMS-02 experiment to constrain dark matter annihilation in the Milky Way. Due to the yet unexplained positron excess, limits are generally weaker than those obtained using other probes, especially gamma rays. This also means that explaining the positron excess in terms of dark matter annihilation is difficult. Only if very conservative assumptions on the dark matter distribution in the Galactic center region are adopted, it may be possible to accommodate dark matter annihilating to leptons with a cross section above 10-24cm3/sec?. We comment on several theoretical mechanisms to explain such large annihilation cross sections.

Kopp, Joachim

2013-10-01

255

Mesopore quality determines the lifetime of hierarchically structured zeolite catalysts  

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

256

Simulation of a method for forming a laser-cooled positron plasma A. S. Newbury,* B. M. Jelenkovic,  

E-print Network

efficiency leads to the positron capture rate of 1000 positrons per second, assuming a 100 mCi positron mCi source and a positron moderator to trap 3 104 positrons at a rate exceeding 103 per hour 3 is reduced to 3 10 10 Torr, resulting in a positron lifetime of about one hour. With a 90 mCi positron source

257

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

SciTech Connect

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

Murphy, T.J.

1986-11-01

258

What is the fate of runaway positrons in tokamaks?  

Massive runaway positrons are generated by runaway electrons in tokamaks. The fate of these positrons encodes valuable information about the runaway dynamics. The phase space dynamics of a runaway position is investigated using a Lagrangian that incorporates the tokamak geometry, loop voltage, radiation and collisional effects. It is found numerically that runaway positrons will drift out of the plasma to annihilate on the first wall, with an in-plasma annihilation possibility less than 0.1%. The dynamics of runaway positrons provides signatures that can be observed as diagnostic tools.

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

2014-04-01

259

Damage-depth profiling of an ion-irradiated polymer by monoenergetic positron beams  

SciTech Connect

Poly(aryl-ether-ether ketone) (PEEK) films irradiated with 1-MeV and 2-MeV O{sup +} ions were exposed to positron beams to measure the positron annihilation Doppler broadening as a function of the positron energy. The annihilation lines recorded at relatively low positron energies were found to become broader with increasing irradiation dose, suggesting that positronium (Ps) formation is inhibited in the damaged regions. The positron data were compared with the results of dynamic hardness and electron-spin-resonance measurements. The slow-positron Doppler broadening technique is found to be a useful means for damage-depth profiling of Ps-forming polymers.

Kobayashi, Y.; Kojima, I. [National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305 (Japan)] [National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305 (Japan); Hishita, S. [National Institute for Research in Inorganic Materials, Tsukuba, Ibaraki 305 (Japan)] [National Institute for Research in Inorganic Materials, Tsukuba, Ibaraki 305 (Japan); Suzuki, T. [National Laboratory for High Energy Physics (KEK), Tsukuba, Ibaraki 305 (Japan)] [National Laboratory for High Energy Physics (KEK), Tsukuba, Ibaraki 305 (Japan); Asari, E.; Kitajima, M. [National Research Institute for Metals, Tsukuba, Ibaraki 305 (Japan)] [National Research Institute for Metals, Tsukuba, Ibaraki 305 (Japan)

1995-07-01

260

Dark matter annihilation in the universe  

NASA Astrophysics Data System (ADS)

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

Salati, Pierre

2014-05-01

261

Unthermalized positrons in gamma ray burst sources  

NASA Technical Reports Server (NTRS)

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

Tkaczyk, W.; Karakula, S.

1992-01-01

262

Measurement of Charm Meson Lifetimes  

Microsoft Academic Search

We report measurements of the D0, D+, and D+s meson lifetimes using 3.7 fb-1 of e+e- annihilation data collected near the Upsilon\\\\(4S\\\\) resonance with the CLEO detector. The measured lifetimes of the D0, D+, and D+s mesons are 408.5+\\/-4.1+3.5-3.4 fs, 1033.6+\\/-22.1+9.9-12.7 fs, and 486.3+\\/-15.0+4.9-5.1 fs. The precisions of these lifetimes are comparable to those of the best previous measurements, and

G. Bonvicini; D. Cinabro; R. Greene; L. P. Perera; G. J. Zhou; S. Chan; G. Eigen; E. Lipeles; M. Schmidtler; A. Shapiro; W. M. Sun; J. Urheim; A. J. Weinstein; F. Würthwein; D. E. Jaffe; G. Masek; H. P. Paar; E. M. Potter; S. Prell; V. Sharma; D. M. Asner; A. Eppich; J. Gronberg; T. S. Hill; C. M. Korte; D. J. Lange; R. J. Morrison; H. N. Nelson; T. K. Nelson; D. Roberts; H. Tajima; B. H. Behrens; W. T. Ford; A. Gritsan; H. Krieg; J. Roy; J. G. Smith; J. P. Alexander; R. Baker; C. Bebek; B. E. Berger; K. Berkelman; V. Boisvert; D. G. Cassel; D. S. Crowcroft; M. Dickson; S. von Dombrowski; P. S. Drell; D. J. Dumas; K. M. Ecklund; R. Ehrlich; A. D. Foland; P. Gaidarev; L. Gibbons; B. Gittelman; S. W. Gray; D. L. Hartill; B. K. Heltsley; S. Henderson; P. I. Hopman; N. Katayama; D. L. Kreinick; T. Lee; Y. Liu; T. O. Meyer; N. B. Mistry; C. R. Ng; E. Nordberg; M. Ogg; J. R. Patterson; D. Peterson; D. Riley; A. Soffer; J. G. Thayer; P. G. Thies; B. Valant-Spaight; A. Warburton; C. Ward; M. Athanas; P. Avery; C. D. Jones; M. Lohner; C. Prescott; A. I. Rubiera; J. Yelton; J. Zheng; G. Brandenburg; R. A. Briere; A. Ershov; Y. S. Gao; D. Y.-J. Kim; R. Wilson; T. E. Browder; Y. Li; J. L. Rodriguez; H. Yamamoto; T. Bergfeld; B. I. Eisenstein; J. Ernst; G. E. Gladding; G. D. Gollin; R. M. Hans; E. Johnson; I. Karliner; M. A. Marsh; M. Palmer; C. Plager; C. Sedlack; M. Selen; J. J. Thaler; J. Williams; K. W. Edwards; A. Bellerive; R. Janicek; P. M. Patel; A. J. Sadoff; R. Ammar; P. Baringer; A. Bean; D. Besson; D. Coppage; R. Davis; S. Kotov; I. Kravchenko; N. Kwak; L. Zhou; S. Anderson; Y. Kubota; S. J. Lee; R. Mahapatra; J. J. O'Neill; R. Poling; T. Riehle; A. Smith; M. S. Alam; S. B. Athar; Z. Ling; A. H. Mahmood; S. Timm; F. Wappler; A. Anastassov; J. E. Duboscq; K. K. Gan; C. Gwon; T. Hart; K. Honscheid; H. Kagan; R. Kass; J. Lee; J. Lorenc; H. Schwarthoff; A. Wolf; M. M. Zoeller; S. J. Richichi; H. Severini; P. Skubic; A. Undrus; M. Bishai; S. Chen; J. Fast; J. W. Hinson; N. Menon; D. H. Miller; E. I. Shibata; I. P. Shipsey; S. Glenn; Y. Kwon; A. L. Lyon; E. H. Thorndike; C. P. Jessop; K. Lingel; H. Marsiske; M. L. Perl; V. Savinov; D. Ugolini; X. Zhou; T. E. Coan; V. Fadeyev; I. Korolkov; Y. Maravin; I. Narsky; R. Stroynowski; J. Ye; T. Wlodek; M. Artuso; S. Ayad; E. Dambasuren; S. Kopp; G. Majumder; G. C. Moneti; S. Schuh; T. Skwarnicki; S. Stone; A. Titov; G. Viehhauser; J. C. Wang; S. E. Csorna; K. W. McLean; S. Marka; Z. Xu; R. Godang; K. Kinoshita; I. C. Lai; P. Pomianowski; S. Schrenk

1999-01-01

263

Electroweak fragmentation functions for dark matter annihilation  

E-print Network

Electroweak corrections can play a crucial role in dark matter annihilation. The emission of gauge bosons, in particular, leads to a secondary flux consisting of all Standard Model particles, and may be described by electroweak fragmentation functions. To assess the quality of the fragmentation function approximation to electroweak radiation in dark matter annihilation, we have calculated the flux of secondary particles from gauge-boson emission in models with Majorana fermion and vector dark matter, respectively. For both models, we have compared cross sections and energy spectra of positrons and antiprotons after propagation through the galactic halo in the fragmentation function approximation and in the full calculation. Fragmentation functions fail to describe the particle fluxes in the case of Majorana fermion annihilation into light fermions: the helicity suppression of the lowest-order cross section in such models cannot be lifted by the leading logarithmic contributions included in the fragmentation f...

Cavasonza, Leila Ali; Pellen, Mathieu

2014-01-01

264

Positron Beam Characteristics at NEPOMUC Upgrade  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

265

Resolvability of positron decay channels  

SciTech Connect

Many data analysis treatments of positron experiments attempt to resolve two or more positron decay or exist channels which may be open simultaneously. Examples of the need to employ such treatments of the experimental results can be found in the resolution of the constituents of a defect ensemble, or in the analysis of the complex spectra which arise from the interaction of slow positrons at or near the surfaces of solids. Experimental one- and two-dimensional angular correlation of annihilation radiation experiments in Al single crystals have shown that two defect species (mono- and divacancies) can be resolved under suitable conditions. Recent experiments at LLNL indicate that there are a variety of complex exit channels open to positrons interacting at surfaces, and ultimely these decay channels must also be suitably resolved from one another. 6 refs., 4 figs.

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

1985-03-07

266

Bound states of positrons with atoms and molecules: Theory  

Microsoft Academic Search

Calculations of the binding energies and annihilation rates of bound atomic and molecular systems which contain a positron are reviewed. Emphasis is placed on methods of calculation and the quality of the numerical results. In this article we limit our attention to positrons interacting with atoms, diatomic molecules, and their ions.

D. M. Schrader

1998-01-01

267

Positron spectroscopy in atomic and solid state physics  

Microsoft Academic Search

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

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

2003-01-01

268

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

SciTech Connect

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

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

1985-01-01

269

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

NASA Astrophysics Data System (ADS)

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

Guessoum, Nidhal

2014-05-01

270

Positrons in the Galaxy: Their Births, Marriages and Deaths  

NASA Technical Reports Server (NTRS)

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

Skinner, Gerald K.

2010-01-01

271

Improved source and transport of monoenergetic MeV positrons  

NASA Astrophysics Data System (ADS)

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

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

2000-08-01

272

Positron microscopy  

SciTech Connect

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

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

1995-02-01

273

Positron mobilities in isooctane, n-hexane and hexafluorobenzene  

Microsoft Academic Search

Positron mobilities in three nonpolar liquids, isooctane (2, 2, 4-trimethylpentane), n-hexane and hexafluorobenzene, were measured by observing the drift velocity of free positrons in the presence of an external electric field. The Doppler shift of the 511 keV annihilation line was measured as a function of the electric field up to 26 kV cm?1 at room temperature. The free positron

C. L Wang; Y. Kobayashi; K. Hirata

2000-01-01

274

Kaluza-Klein dark matter and the positron excess  

SciTech Connect

The excess of cosmic positrons observed by the High-Energy Antimatter Telescope (HEAT) experiment may be the result of Kaluza-Klein dark matter annihilating in the galactic halo. Kaluza-Klein dark matter annihilates dominantly into charged leptons that yield a large number and hard spectrum of positrons per annihilation. Given a Kaluza-Klein dark matter particle with a mass in the range of 300 GeV-400 GeV, no exceptional substructure or clumping is needed in the local distribution of dark matter to generate a positron flux that explains the HEAT observations. This is in contrast to supersymmetric dark matter which requires unnaturally large amounts of dark substructure to produce the observed positron excess. Future astrophysical and collider tests are outlined that will confirm or rule out this explanation of the HEAT data.

Hooper, Dan [Astrophysics Department, University of Oxford, OX1 3RH Oxford (United Kingdom); Kribs, Graham D. [School of Natural Sciences, Institute for Advanced Study, Princeton, New Jersey 08540 (United States)

2004-12-01

275

Measurement of the tau lepton lifetime  

Microsoft Academic Search

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

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

1996-01-01

276

Measurement of Charm Meson Lifetimes  

NASA Astrophysics Data System (ADS)

We report measurements of the D0, D+, and D+s meson lifetimes using 3.7 fb-1 of e+e- annihilation data collected near the ?\\(4S\\) resonance with the CLEO detector. The measured lifetimes of the D0, D+, and D+s mesons are 408.5+/-4.1+3.5-3.4 fs, 1033.6+/-22.1+9.9-12.7 fs, and 486.3+/-15.0+4.9-5.1 fs. The precisions of these lifetimes are comparable to those of the best previous measurements, and the systematic errors are very different. In a single experiment we find that the ratio of the D+s and D0 lifetimes is 1.19+/-0.04.

Bonvicini, G.; Cinabro, D.; Greene, R.; Perera, L. P.; Zhou, G. J.; Chan, S.; Eigen, G.; Lipeles, E.; Schmidtler, M.; Shapiro, A.; Sun, W. M.; Urheim, J.; Weinstein, A. J.; Würthwein, F.; Jaffe, D. E.; Masek, G.; Paar, H. P.; Potter, E. M.; Prell, S.; Sharma, V.; Asner, D. M.; Eppich, A.; Gronberg, J.; Hill, T. S.; Korte, C. M.; Lange, D. J.; Morrison, R. J.; Nelson, H. N.; Nelson, T. K.; Roberts, D.; Tajima, H.; Behrens, B. H.; Ford, W. T.; Gritsan, A.; Krieg, H.; Roy, J.; Smith, J. G.; Alexander, J. P.; Baker, R.; Bebek, C.; Berger, B. E.; Berkelman, K.; Boisvert, V.; Cassel, D. G.; Crowcroft, D. S.; Dickson, M.; von Dombrowski, S.; Drell, P. S.; Dumas, D. J.; Ecklund, K. M.; Ehrlich, R.; Foland, A. D.; Gaidarev, P.; Gibbons, L.; Gittelman, B.; Gray, S. W.; Hartill, D. L.; Heltsley, B. K.; Henderson, S.; Hopman, P. I.; Katayama, N.; Kreinick, D. L.; Lee, T.; Liu, Y.; Meyer, T. O.; Mistry, N. B.; Ng, C. R.; Nordberg, E.; Ogg, M.; Patterson, J. R.; Peterson, D.; Riley, D.; Soffer, A.; Thayer, J. G.; Thies, P. G.; Valant-Spaight, B.; Warburton, A.; Ward, C.; Athanas, M.; Avery, P.; Jones, C. D.; Lohner, M.; Prescott, C.; Rubiera, A. I.; Yelton, J.; Zheng, J.; Brandenburg, G.; Briere, R. A.; Ershov, A.; Gao, Y. S.; Kim, D. Y.-J.; Wilson, R.; Browder, T. E.; Li, Y.; Rodriguez, J. L.; Yamamoto, H.; Bergfeld, T.; Eisenstein, B. I.; Ernst, J.; Gladding, G. E.; Gollin, G. D.; Hans, R. M.; Johnson, E.; Karliner, I.; Marsh, M. A.; Palmer, M.; Plager, C.; Sedlack, C.; Selen, M.; Thaler, J. J.; Williams, J.; Edwards, K. W.; Bellerive, A.; Janicek, R.; Patel, P. M.; Sadoff, A. J.; Ammar, R.; Baringer, P.; Bean, A.; Besson, D.; Coppage, D.; Davis, R.; Kotov, S.; Kravchenko, I.; Kwak, N.; Zhou, L.; Anderson, S.; Kubota, Y.; Lee, S. J.; Mahapatra, R.; O'Neill, J. J.; Poling, R.; Riehle, T.; Smith, A.; Alam, M. S.; Athar, S. B.; Ling, Z.; Mahmood, A. H.; Timm, S.; Wappler, F.; Anastassov, A.; Duboscq, J. E.; Gan, K. K.; Gwon, C.; Hart, T.; Honscheid, K.; Kagan, H.; Kass, R.; Lee, J.; Lorenc, J.; Schwarthoff, H.; Wolf, A.; Zoeller, M. M.; Richichi, S. J.; Severini, H.; Skubic, P.; Undrus, A.; Bishai, M.; Chen, S.; Fast, J.; Hinson, J. W.; Menon, N.; Miller, D. H.; Shibata, E. I.; Shipsey, I. P.; Glenn, S.; Kwon, Y.; Lyon, A. L.; Thorndike, E. H.; Jessop, C. P.; Lingel, K.; Marsiske, H.; Perl, M. L.; Savinov, V.; Ugolini, D.; Zhou, X.; Coan, T. E.; Fadeyev, V.; Korolkov, I.; Maravin, Y.; Narsky, I.; Stroynowski, R.; Ye, J.; Wlodek, T.; Artuso, M.; Ayad, S.; Dambasuren, E.; Kopp, S.; Majumder, G.; Moneti, G. C.; Mountain, R.; Schuh, S.; Skwarnicki, T.; Stone, S.; Titov, A.; Viehhauser, G.; Wang, J. C.; Csorna, S. E.; McLean, K. W.; Marka, S.; Xu, Z.; Godang, R.; Kinoshita, K.; Lai, I. C.; Pomianowski, P.; Schrenk, S.

1999-06-01

277

Annihilating cold dark matter  

PubMed

Structure formation with cold dark matter (CDM) predicts halos with a central density cusp, which are observationally disfavored. If CDM particles have an annihilation cross section sigmav approximately 10(-29)(m/GeV) cm(2), then annihilations will soften the cusps. We discuss plausible scenarios for avoiding the early Universe annihilation catastrophe that could result from such a large cross section. The predicted scaling of core density with halo mass depends upon the velocity dependence of sigmav, and s-wave annihilation leads to a core density nearly independent of halo mass, which seems consistent with observations. PMID:11030890

Kaplinghat; Knox; Turner

2000-10-16

278

Characterization of dynamics in complex lyophilized formulations: II. Analysis of density variations in terms of glass dynamics and comparisons with global mobility, fast dynamics, and Positron Annihilation Lifetime Spectroscopy (PALS)  

PubMed Central

Amorphous HES/disaccharide (trehalose or sucrose) formulations, with and without added polyols (glycerol and sorbitol) and disaccharide formulations of human growth hormone (hGH), were prepared by freeze drying and characterized with particular interest in methodology for using high precision density measurements to evaluate free volume changes and a focus on comparisons between “free volume” changes obtained from analysis of density data, fast dynamics (local mobility), and PALS characterization of “free volume” hole size. Density measurements were performed using a helium gas pycnometer, and fast dynamics was characterized using incoherent neutron scattering spectrometer. Addition of sucrose and trehalose to hGH decreases free volume in the system with sucrose marginally more effective than trehalose, consistent with superior pharmaceutical stability of sucrose hGH formulations well below Tg relative to trehalose. We find that density data may be analyzed in terms of free volume changes by evaluation of volume changes on mixing and calculation of apparent specific volumes from the densities. Addition of sucrose to HES decreases free volume, but the effect of trehalose is not detectable above experimental error. Addition of sorbitol or glycerol to HES/trehalose base formulations appears to significantly decrease free volume, consistent with the positive impact of such additions on pharmaceutical stability (i.e., degradation) in the glassy state. Free volume changes, evaluated from density data, fast dynamics amplitude of local motion, and PALS hole size data generally are in qualitative agreement for the HES/disaccharide systems studied. All predict decreasing molecular mobility as disaccharides are added to HES. Global mobility as measured by enthalpy relaxation times, increases as disaccharides, particularly sucrose, are added to HES. PMID:23623797

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

2013-01-01

279

Dark Matter Annihilation and the PAMELA, FERMI and ATIC Anomalies  

E-print Network

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

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

2009-03-29

280

A new annihilator in the Galactic center region?  

NASA Astrophysics Data System (ADS)

Following the detection of a possible double backscattered line from 511 keV photons in EXS 1737-2952 (Grindlay et al. 1993) which makes this source a possible new black hole candidate, the authors searched at radio (mm) wavelengths for a molecular cloud in the vicinity of this source. They detected a cloud very similar to the one associated with 1E 1740-2942. It has the required density (105cm-3) to annihilate the positrons possibly emitted by EXS and contribute to the Galactic center narrow annihilation line.

Durouchoux, P.; Vilhu, O.; Wallyn, P.; Huovelin, J.; Bally, J.

281

Hypernovae as possible sources of Galactic positrons  

E-print Network

INTEGRAL/SPI has recently observed a strong and extended emission resulting from electron-positron annihilation located in the Galactic center region, consistent with the Galactic bulge geometry, without any counterpart at high gamma-ray energies, nor in the 1809 keV $^{26}$Al decay line. In order to explain the rate of positron injection in the Galactic bulge, estimated to more than 10$^{43}$ s$^{-1}$, the most commonly considered positron injection sources are type Ia supernovae. However, SN Ia rate estimations show that those sources fall short to explain the observed positron production rate, raising a challenging question about the nature of the Galactic positron source. In this context, a possible source of Galactic positrons could be supernova events of a new type, as the recently observed SN2003dh/GRB030329, an exploding Wolf-Rayet star (type Ic supernova) associated with a hypernova/gamma-ray burst; the question about the rate of this kind of events remains open, but could be problematically low. In this paper, we explore the possibility of positron production and escape by such an event in the framework of an asymmetric model, in which a huge amount of $^{56}$Ni is ejected in a cone with a very high velocity; the ejected material becomes quickly transparent to positrons, which spread out in the interstellar medium.

Stephane Schanne; Michel Casse; Bertrand Cordier; Jacques Paul

2004-04-26

282

Evidence Of Dark Matter Annihilations In The WMAP Haze  

E-print Network

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

Dan Hooper; Douglas P. Finkbeiner; Gregory Dobler

2007-05-24

283

KENO lifetimes  

SciTech Connect

When performing k-eigenvalue solutions with KENO-V.a, two different prompt neutron lifetimes are estimated - a system lifetime and a neutron generation time. The meaning of these two lifetimes has been ascertained by comparing values of various neutron lifespans/lifetimes predicted by MCNP and DANTSYS based on the neutron-balance theory. The system lifetime in KENO-Va corresponds to the unweighted removal lifetime calculated by both MCNP and DANTSYS. The unweighted removal lifetime is the average time between removal events resulting from a neutron absorption or a neutron leakage. The generation time in KENO-V.a corresponds to the fission lifespan calculated by MCNP, where the fission lifespan in MCNP represents the average time for a newly born neutron to cause another fission. As such, the generation time in KENO-Va does not represent the generation time that appears in the point kinetic model. The generation time in the point kinetic model is the adjoint-weighted removal lifetime divided by k{sub eff}, which is identically equal to the adjoint-weighted neutron production rate. In small bare systems operating in the vicinity of delayed critical, the difference between the adjoint-weighted neutron generation time and the fission lifespan can be as small as a few percent. However, in reflected systems, the difference between these two quantities can be several orders of magnitude. In conclusion, the prompt neutron generation time predicted by KENO-Va corresponds to the fission lifespan of a prompt neutron in a given system. The fission lifespan is the average time from birth-to-fission and, in general, is not a good approximation for the adjoint-weighted neutron generation time that appears in the point-kinetic model.

Petrie, L. [Oak Ridge National Lab., TN (United States); Parsons, D.K.; Spriggs, G.D. [Los Alamos National Lab., NM (United States)

1997-01-30

284

Electroweak fragmentation functions for dark matter annihilation  

E-print Network

Electroweak corrections can play a crucial role in dark matter annihilation. The emission of gauge bosons, in particular, leads to a secondary flux consisting of all Standard Model particles, and may be described by electroweak fragmentation functions. To assess the quality of the fragmentation function approximation to electroweak radiation in dark matter annihilation, we have calculated the flux of secondary particles from gauge-boson emission in models with Majorana fermion and vector dark matter, respectively. For both models, we have compared cross sections and energy spectra of positrons and antiprotons after propagation through the galactic halo in the fragmentation function approximation and in the full calculation. Fragmentation functions fail to describe the particle fluxes in the case of Majorana fermion annihilation into light fermions: the helicity suppression of the lowest-order cross section in such models cannot be lifted by the leading logarithmic contributions included in the fragmentation function approach. However, for other classes of models like vector dark matter, where the lowest-order cross section is not suppressed, electroweak fragmentation functions provide a simple, model-independent and accurate description of secondary particle fluxes.

Leila Ali Cavasonza; Michael Krämer; Mathieu Pellen

2014-09-29

285

Cosmic Ray Positrons from Cosmic Strings  

E-print Network

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

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

2009-01-22

286

The positron excess and supersymmetric dark matter  

E-print Network

Using a new instrument, the HEAT collaboration has confirmed the excess of cosmic ray positrons that they first detected in 1994. We explore the possibility that this excess is due to the annihilation of neutralino dark matter in the galactic halo. We confirm that neutralino annihilation can produce enough positrons to make up the measured excess only if there is an additional enhancement to the signal. We quantify the `boost factor' that is required in the signal for various models in the Minimal Supersymmetric Standard Model parameter space, and find that a boost factor >30 provides good fits to the HEAT data. Such an enhancement in the signal could arise if we live in a clumpy halo.

Edward A. Baltz; Joakim Edsjo; Katherine Freese; Paolo Gondolo

2002-11-12

287

Neutron Lifetime  

NASA Astrophysics Data System (ADS)

The recent neutron lifetime experiment [1] has provided the value 878.5 ± 0.8 s. It differs by 6.5 standard deviations from the world average value 885.7 ± 0.8 s quoted by the particle data group (PDG) in 2006 [2]. In determination of the world average value of the neutron lifetime there is rather dramatic situation. On the one hand a new value of neutron lifetime from work [1] cannot be included in the world average value because of the big difference of results. On the other hand until this major disagreement is understood the present world average value for the neutron lifetime must be suspect. So the situation on PDG page devoted to the neutron lifetime is formulated [2] in view of this controversy. The only way out of the present situation is to carry out new more precise experiments. More detailed analysis of the previous experiments and search of possible systematic error is also reasonable. In this connection the analysis and Monte Carlo simulation of experiments [3] and [4] is carried out. Systematic errors of about -6 s are found in each of the experiments. The summary table for the neutron lifetime measurements after corrections and additions is given. A new world average value for the neutron lifetime 879.9 ± 0.9 s is presented. The value |Vud| = 09743(7), calculated for the new world average value for the neutron lifetime 879.9(9) s and gA = 12750(9) [5], agrees with both |Vud| = 097419(22) from the unitarity of the CKM matrix elements [2] and |Vud| = 097425(22), measured from the superallowed 0^+ -> 0^+ nuclear ?-decays, caused by pure Fermi transitions only [5,6]. The analysis of neutron ?-decay with new world average neutron lifetime demonstrates reasonable agreement in frame of Standard Model. [4pt] [1] A. Serebrov et al., Phys. Lett. B 605, 72 (2005); A.P. Serebrov et al., Phys. Rev. C 78, 035505 (2008). [0pt] [2] C. Amsler et al. (Particle Data Group), Phys. Lett. B 667, 1 (2008). [0pt] [3] S. Arzumanov et al., Phys. Lett. B 483, 15 (2000). [0pt] [4] W. Mampe et al., Phys. Rev. Lett. 63, 593 (1989). [0pt] [5] H. Abele, Prog. Part. Nucl. Phys. 60, 1 (2008). [0pt] [6] J. C. Hardy, I.S. Towner, Phys. Rev. C 79, 055502 (2009).

Serebrov, Anatolii

2010-11-01

288

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

NASA Astrophysics Data System (ADS)

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

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

2014-09-01

289

PhytoBeta imager: a positron imager for plant biology  

SciTech Connect

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

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

2012-06-01

290

PhytoBeta imager: a positron imager for plant biology.  

PubMed

Several positron emitting radioisotopes such as (11)C and (13)N can be used in plant biology research. The (11)CO(2) tracer is used to facilitate plant biology research toward optimization of plant productivity, biofuel development and carbon sequestration in biomass. Positron emission tomography (PET) imaging has been used to study carbon transport in live plants using (11)CO(2). Because plants typically have very thin leaves, little medium is present for the emitted positrons to undergo an annihilation event. The emitted positrons from (11)C (maximum energy 960 keV) could require up to approximately 4 mm of water equivalent material for positron annihilation. Thus many of the positrons do not annihilate inside the leaf, resulting in limited sensitivity for PET imaging. To address this problem we have developed a compact beta-positive, beta-minus particle imager (PhytoBeta imager) for (11)CO(2) leaf imaging. The detector is based on a Hamamatsu H8500 position sensitive photomultiplier tube optically coupled via optical grease to a 0.5 mm thick Eljen EJ-212 plastic scintillator. The detector is equipped with a flexible arm to allow its placement and orientation over or under the leaf to be studied while maintaining the leaf's original orientation. To test the utility of the system the detector was used to measure carbon translocation in a leaf of the spicebush (Lindera benzoin) under two transient light conditions. PMID:22684043

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

2012-07-01

291

Positron Physics  

NASA Technical Reports Server (NTRS)

I will give a review of the history of low-energy positron physics, experimental and theoretical, concentrating on the type of work pioneered by John Humberston and the positronics group at University College. This subject became a legitimate subfield of atomic physics under the enthusiastic direction of the late Sir Harrie Massey, and it attracted a diverse following throughout the world. At first purely theoretical, the subject has now expanded to include high brightness beams of low-energy positrons, positronium beams, and, lately, experiments involving anti-hydrogen atoms. The theory requires a certain type of persistence in its practitioners, as well as an eagerness to try new mathematical and numerical techniques. I will conclude with a short summary of some of the most interesting recent advances.

Drachman, Richard J.

2003-01-01

292

Microstructural Characterization of Polymers with Positrons  

NASA Technical Reports Server (NTRS)

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

Singh, Jag J.

1997-01-01

293

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

NASA Astrophysics Data System (ADS)

Vacancy-type defects introduced by the grinding of Czochralski-grown Si wafers were studied using monoenergetic positron beams. Measurements of Doppler broadening spectra of the annihilation radiation and the lifetime spectra of positrons showed that vacancy-type defects were introduced in the surface region (<98 nm), and the major defect species were identified as (i) relatively small vacancies incorporated in dislocations and (ii) large vacancy clusters. Annealing experiments showed that the defect concentration decreased with increasing annealing temperature in the range between 100 and 500 °C. After 600-700 °C annealing, the defect-rich region expanded up to about 170 nm, which was attributed to rearrangements of dislocation networks, and a resultant emission of point defects toward the inside of the sample. Above 800 °C, the stability limit of those vacancies was reached and they started to disappear. After the vacancies were annealed out (900 °C), oxygen-related defects were the major point defects and they were located at <25 nm.

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

2014-10-01

294

Measurement of Charm Meson Lifetimes  

SciTech Connect

We report measurements of the D{sup 0} , D{sup +} , and D{sup +}{sub s} meson lifetimes using 3.7 fb{sup {minus}1} of e{sup +}e{sup {minus}} annihilation data collected near the {Upsilon}(4S) resonance with the CLEO detector. The measured lifetimes of the D{sup 0} , D{sup +} , and D{sup +}{sub s} mesons are 408.5{plus_minus}4.1{sup +3.5}{sub {minus}3.4} fs , 1033.6{plus_minus}22.1{sup +9.9}{sub {minus}12.7} fs , and 486.3{plus_minus}15.0{sup +4.9}{sub {minus}5.1} fs . The precisions of these lifetimes are comparable to those of the best previous measurements, and the systematic errors are very different. In a single experiment we find that the ratio of the D{sup +}{sub s} and D{sup 0} lifetimes is 1.19{plus_minus}0.04 . {copyright} {ital 1999} {ital The American Physical Society}

Bonvicini, G.; Cinabro, D.; Greene, R.; Perera, L.P.; Zhou, G.J. [Wayne State University, Detroit, Michigan 48202 (United States)] [Wayne State University, Detroit, Michigan 48202 (United States); Chan, S.; Eigen, G.; Lipeles, E.; Schmidtler, M.; Shapiro, A.; Sun, W.M.; Urheim, J.; Weinstein, A.J.; Wuerthwein, F. [California Institute of Technology, Pasadena, California 91125 (United States)] [California Institute of Technology, Pasadena, California 91125 (United States); Jaffe, D.E.; Masek, G.; Paar, H.P.; Potter, E.M.; Prell, S.; Sharma, V. [University of California, San Diego, La Jolla, California 92093 (United States)] [University of California, San Diego, La Jolla, California 92093 (United States); Asner, D.M.; Eppich, A.; Gronberg, J.; Hill, T.S.; Korte, C.M.; Lange, D.J.; Morrison, R.J.; Nelson, H.N.; Nelson, T.K.; Roberts, D.; Tajima, H. [University of California, Santa Barbara, California 93106 (United States)] [University of California, Santa Barbara, California 93106 (United States); Behrens, B.H.; Ford, W.T.; Gritsan, A.; Krieg, H.; Roy, J.; Smith, J.G. [University of Colorado, Boulder, Colorado 80309-0390 (United States)] [University of Colorado, Boulder, Colorado 80309-0390 (United States); Alexander, J.P.; Baker, R.; Bebek, C.; Berger, B.E.; Berkelman, K.; Boisvert, V.; Cassel, D.G.; Crowcroft, D.S.; Dickson, M.; von Dombrowski, S.; Drell, P.S.; Dumas, D.J.; Ecklund, K.M.; Ehrlich, R.; Foland, A.D.; Gaidarev, P.; Gibbons, L.; Gittelman, B.; Gray, S.W.; Hartill, D.L.; Heltsley, B.K.; Henderson, S.; Hopman, P.I.; Katayama, N.; Kreinick, D.L.; Lee, T.; Liu, Y.; Meyer, T.O.; Mistry, N.B.; Ng, C.R.; Nordberg, E.; Ogg, M.; Patterson, J.R.; Peterson, D.; Riley, D.; Soffer, A.; Thayer, J.G.; Thies, P.G.; Valant-Spaight, B.; Warburton, A.; Ward, C. [Cornell University, Ithaca, New York 14853 (United States)] [Cornell University, Ithaca, New York 14853 (United States); Athanas, M.; Avery, P.; Jones, C.D.; Lohner, M.; Prescott, C.; Rubiera, A.I.; Yelton, J.; Zheng, J. [University of Florida, Gainesville, Florida 32611 (United States)] [University of Florida, Gainesville, Florida 32611 (United States); Brandenburg, G.; Briere, R.A.; Ershov, A.; Gao, Y.S.; Kim, D.Y.; Wilson, R. [Harvard University, Cambridge, Massachusetts 02138 (United States)] [Harvard University, Cambridge, Massachusetts 02138 (United States); Browder, T.E.; Li, Y.; Rodriguez, J.L.; Yamamoto, H. [University of Hawaii at Manoa, Honolulu, Hawaii 96822 (United States)] [University of Hawaii at Manoa, Honolulu, Hawaii 96822 (United States); Bergfeld, T.; Eisenstein, B.I.; Ernst, J.; Gladding, G.E.; Gollin, G.D; and others

1999-06-01

295

Cosmic ray anomalies and dark matter annihilation to muons via a Higgs portal hidden sector  

NASA Astrophysics Data System (ADS)

Annihilating dark matter (DM) models based on a scalar hidden sector with Higgs portal-like couplings to the standard model are considered as a possible explanation for recently observed cosmic-ray excesses. Two versions of the model are studied, one with nonthermal DM as the origin of the boost factor and one with Sommerfeld enhancement. In the case of nonthermal DM, four hidden sector scalars which transform under a U(1)X symmetry are added. The heaviest scalars decouple and later decay to DM scalars, thus providing the boost factor necessary to explain the present DM annihilation rate. The mass of the annihilating scalars is limited to ?600GeV for the model to remain perturbative. U(1)X breaking to Z2 at the electroweak transition mixes light O(100) MeV hidden sector scalars with the Higgs. The DM scalars annihilate to these light scalars, which subsequently decay to two ?+?- pairs via Higgs mixing, thus generating a positron excess without antiprotons. Decay to ?+?- rather than e+e- is necessary to ensure a fast enough light scalar decay rate to evade light scalar domination at nucleosynthesis. In the version with Sommerfeld enhancement only three new scalars are necessary. TeV scale DM masses can be accommodated, allowing both the higher energy electron plus positron excess and the lower energy positron excess to be explained. DM annihilates to 2?+?- pairs as in the nonthermal model. This annihilation mode may be favored by recent observations of the electron plus positron excess by FERMI and HESS.

Kohri, Kazunori; McDonald, John; Sahu, Narendra

2010-01-01

296

Cosmic ray anomalies and dark matter annihilation to muons via a Higgs portal hidden sector  

SciTech Connect

Annihilating dark matter (DM) models based on a scalar hidden sector with Higgs portal-like couplings to the standard model are considered as a possible explanation for recently observed cosmic-ray excesses. Two versions of the model are studied, one with nonthermal DM as the origin of the boost factor and one with Sommerfeld enhancement. In the case of nonthermal DM, four hidden sector scalars which transform under a U(1){sub X} symmetry are added. The heaviest scalars decouple and later decay to DM scalars, thus providing the boost factor necessary to explain the present DM annihilation rate. The mass of the annihilating scalars is limited to < or approx. 600 GeV for the model to remain perturbative. U(1){sub X} breaking to Z{sub 2} at the electroweak transition mixes light O(100) MeV hidden sector scalars with the Higgs. The DM scalars annihilate to these light scalars, which subsequently decay to two {mu}{sup +{mu}-} pairs via Higgs mixing, thus generating a positron excess without antiprotons. Decay to {mu}{sup +{mu}-} rather than e{sup +}e{sup -} is necessary to ensure a fast enough light scalar decay rate to evade light scalar domination at nucleosynthesis. In the version with Sommerfeld enhancement only three new scalars are necessary. TeV scale DM masses can be accommodated, allowing both the higher energy electron plus positron excess and the lower energy positron excess to be explained. DM annihilates to 2{mu}{sup +{mu}-} pairs as in the nonthermal model. This annihilation mode may be favored by recent observations of the electron plus positron excess by FERMI and HESS.

Kohri, Kazunori; McDonald, John; Sahu, Narendra [Cosmology and Astroparticle Physics Group, University of Lancaster, Lancaster LA1 4YB (United Kingdom)

2010-01-15

297

Low-energy, high-intensity positron beam experiments with a linac  

SciTech Connect

Previous experiments with positrons from radionuclides have demonstrated that positron beams are a rich source of information about the surface condition of solids. We have now demonstrated the possibility of producing very intense beams at the Lawrence Livermore 100 MeV electron linac and installed an apparatus that produces a variable energy positron beam at energies between 500 eV and 20 keV with sufficient intensity to perform a variety of new positron experiments. The positron beam is pulsed with 10 ns to 3 microseconds duration at rates up to 1440 pulses per second, with as many as 10/sup 6/ positrons available per pulse. Experiments that require either pulsed or steady currents are possible in an ultrahigh vacuum environment. For the first time two-dimensional angular correlation spectra of the surface positron and positronium annihilation at a single crystal sample have been obtained for copper. 6 references, 4 figures, 1 table.

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

1984-09-01

298

Power Corrections in Electron-Positron Annihilation: Experimental Review  

E-print Network

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

Stefan Kluth

2006-06-20

299

Quark flavor identification in electron-positron annihilation  

SciTech Connect

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

Kaye, H.S.

1983-09-01

300

Compact conscious animal positron emission tomography scanner  

DOEpatents

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

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

2006-10-24

301

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

E-print Network

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

Cao, Qing-Hong; Gong, Ti

2014-01-01

302

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

E-print Network

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

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

2014-09-25

303

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

SciTech Connect

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

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

2013-08-21

304

Reduced explicitly correlated Hartree-Fock approach within the nuclear-electronic orbital framework: applications to positronic molecular systems.  

PubMed

In the application of the nuclear-electronic orbital (NEO) method to positronic systems, all electrons and the positron are treated quantum mechanically on the same level. Explicit electron-positron correlation can be included using Gaussian-type geminal functions within the variational self-consistent-field procedure. In this paper, we apply the recently developed reduced explicitly correlated Hartree-Fock (RXCHF) approach to positronic molecular systems. In the application of RXCHF to positronic systems, only a single electronic orbital is explicitly correlated to the positronic orbital. We apply NEO-RXCHF to three systems: positron-lithium, lithium positride, and positron-lithium hydride. For all three of these systems, the RXCHF approach provides accurate two-photon annihilation rates, average contact densities, electronic and positronic single-particle densities, and electron-positron contact densities. Moreover, the RXCHF approach is significantly more accurate than the original XCHF approach, in which all electronic orbitals are explicitly correlated to the positronic orbital in the same manner, because the RXCHF wavefunction is optimized to produce a highly accurate description of the short-ranged electron-positron interaction that dictates the annihilation rates and other local properties. Furthermore, RXCHF methods that neglect or approximate the electronic exchange interactions between the geminal-coupled electronic orbital and the regular electronic orbitals lead to virtually identical annihilation rates and densities as the fully antisymmetric RXCHF method but offer substantial advantages in computational tractability. Thus, NEO-RXCHF is a promising, computationally practical approach for studying larger positron-containing systems. PMID:23883006

Sirjoosingh, Andrew; Pak, Michael V; Swalina, Chet; Hammes-Schiffer, Sharon

2013-07-21

305

Brane annihilations during inflation  

SciTech Connect

We investigate brane inflation driven by two stacks of mobile branes in a throat. The stack closest to the bottom of the throat annihilates first with antibranes, resulting in particle production and a change of the equation of state parameter w. We calculate analytically some observable signatures of the collision; related decays are common in multi-field inflation, providing the motivation for this case study. The discontinuity in w enters the matching conditions relating perturbations in the remaining degree of freedom before and after the collision, affecting the power-spectrum of curvature perturbations. We find an oscillatory modulation of the power-spectrum for scales within the horizon at the time of the collision, and a slightly redder spectrum on super-horizon scales. We comment on implications for staggered inflation.

Battefeld, Diana; Battefeld, Thorsten [Department of Physics, Princeton University, Princeton, NJ 08544 (United States); Firouzjahi, Hassan; Khosravi, Nima, E-mail: dbattefe@princeton.edu, E-mail: tbattefe@princeton.edu, E-mail: firouz@ipm.ir, E-mail: nima@ipm.ir [School of Physics, Institute for Research in Fundamental Sciences (IPM), P.O. Box 19395-5531, Tehran (Iran, Islamic Republic of)

2010-07-01

306

The NEPOMUC upgrade and advanced positron beam experiments  

NASA Astrophysics Data System (ADS)

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

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

2012-05-01

307

High Energy Positrons and the WMAP Haze from Exciting Dark Matter  

E-print Network

We consider the signals of positrons and electrons from "exciting" dark matter (XDM) annihilation. Because of the light (m_phi ~haze," focusing on light (m_phi ~< 2 m_pi) phi bosons, and find that significant signals can be found for all three, although significant signals generally require high dark matter densities. We find that measurements of the positron fraction are generally insensitive to the halo model, but do suffer significant astrophysical uncertainties. We discuss the implications for upcoming PAMELA results.

Ilias Cholis; Lisa Goodenough; Neal Weiner

2008-02-20

308

K-shell ionization in the positron decay of /sup 65/Zn  

SciTech Connect

The probability for K-shell ionization in the positron decay of /sup 65/Zn, P/sub K/(..beta../sup +/) has been studied by recording coincidences between the annihilation radiation produced subsequent to the positron decay and the Cu K x rays produced when the K-shell vacancy is filled. This probability was found to be (16.1 +- 3.0) x 10/sup -4/, which is approximately three times larger than recent theoretical predictions of Law and Suzuki.

Nha, S.K.; Schupp, G.; Nagy, H.J.

1983-06-01

309

K-shell electron shakeoff in the positron decay of /sup 58/Co  

SciTech Connect

The probability for K-shell electron shakeoff in the positron decay of /sup 58/Co has been measured by recording coincidences between the annihilation radiation produced subsequent to the positron decay and the Fe K x rays produced when the K-shell vacancy is filled. This probability was found to be (13.8 +- 2.4) x 10/sup -4/, which is approximately 2 times larger than recent theoretical predictions by Law and Suzuki.

Nha, S.K.; Schupp, G.; Nagy, H.J.

1983-03-01

310

Absolute electron and positron fluxes from PAMELA/Fermi and dark matter  

SciTech Connect

We extract the positron and electron fluxes in the energy range 10–100 GeV by combining the recent data from PAMELA and Fermi LAT. The absolute positron and electron fluxes thus obtained are found to obey the power laws: E{sup ?2.65} and E{sup ?3.06} respectively, which can be confirmed by the upcoming data from PAMELA. The positron flux appears to indicate an excess at energies E ?> 50 GeV even if the uncertainty in the secondary positron flux is added to the Galactic positron background. This leaves enough motivation for considering new physics, such as annihilation or decay of dark matter, as the origin of positron excess in the cosmic rays.

Balázs, C. [School of Physics, Monash University, Melbourne, Victoria 3800 (Australia); Sahu, N.; Mazumdar, A., E-mail: csaba.balazs@sci.monash.edu.au, E-mail: n.sahu@lancaster.ac.uk, E-mail: a.mazumdar@lancaster.ac.uk [Physics Department, University of Lancaster, Lancaster LA1 4YB (United Kingdom)

2009-07-01

311

Ultrafast dynamics of singlet-singlet and singlet-triplet exciton annihilation in poly(3- 2' -methoxy- 5' octylphenyl)thiophene films  

NASA Astrophysics Data System (ADS)

Singlet-singlet (S-S) and singlet-triplet (S-T) exciton annihilation was studied in poly(3- 2' -methoxy- 5' octylphenyl)thiophene films. For the S-S exciton annihilation studies, transient absorption spectroscopy at excitation laser pulse fluences of 1.2×1013-4.4×1014photons/cm2 and 2.5kHz pulse repetition rate was applied. The obtained kinetics demonstrate a typical nonexponential character with intensity-dependent amplitudes and lifetimes. In time-resolved fluorescence experiments, low excitation pulse fluences of 1.6×109-2.2×1012photons/cm2 at high repetition rates of 0.4, 0.8, 4, and 81MHz lead to S-T exciton annihilation as a result of triplet exciton accumulation. S-T annihilation kinetics results in monoexponential decay of the fluorescence kinetics and manifests itself as a decrease of the singlet exciton lifetime. The calculated time-independent S-S and S-T exciton annihilation rates strongly support the conclusion that the processes are controlled by the interchain diffusion of singlet excitons. Despite the low efficiency of S-T annihilation compared to that of S-S annihilation, it has a substantial effect on the singlet exciton lifetime due to a relatively long triplet lifetime (60?s) . Thus, even optical excitation with low fluence at high pulse repetition rate creates a significant concentration of triplet states that efficiently quenches singlet excitons.

Zaushitsyn, Yuri; Jespersen, Kim G.; Valkunas, Leonas; Sundström, Villy; Yartsev, Arkady

2007-05-01

312

Annihilation radiation in cosmic gamma-ray bursts  

NASA Astrophysics Data System (ADS)

The emission features observed in the energy spectra of cosmic gasmma-ray bursts imply the existence of two radiation components of comparable intensity. The softer component is similar to the continua of featureless bursts. The fast decrease in the intensity of this radiation with increasing photon energy is apparently due to the neutron star's magnetosphere being opague to hard photons because of the formation of electron-positron pairs in single-photon (gamma, B) and two-photon (gamma, gamma) processes. The hard component originates from the annihilation of electron-positron pairs, its spectrum representing a broad line with an extended power-law wing. Such a shape of the spectrum is apparently due to either thermal broadening in a source with a spatially inhomogeneous and rapidly time-varying plasma temperature, or nonthermal energy distribution of particles in their motion along the magnetic field lines. It is assumed that the sources of these components are spatially separated, the annihilation radiation escaping from the polar regions of a strongly magnetized neutron star in a collimated beam without appreciable attenuation.

Golenetskii, S. V.; Mazets, E. P.; Aptekar, R. L.; Gurian, Iu. A.; Ilinskii, V. N.

1986-07-01

313

A fiber-optically coupled positron-sensitive surgical probe  

SciTech Connect

Positron-emitting radiopharmaceuticals such as {sup 18}F-labeled 2-deoxy-D-glucose (FDG) have considerable utility in the noninvasive imaging of cancers due to their rapid and excellent tumor-localizing properties. In addition, the relatively short range of positrons in tissue facilitates the precise delineation of FDG-avid tumors. Therefore, FDG used in conjunction with a positron-sensitive probe may be capable of guiding surgical procedures. Many of the current probe systems, however, are sensitive to the intense flux of background photons produced by positron annihilation. The authors describe the design, manufacture and initial in vitro and in vivo testing of a probe well-suited to the detection of positron-emitting isotopes in a high-photon background. The device consists of a small piece of plastic scintillator coupled by fiber-optic cable to a photomultiplier tube. Measurements of resolution and detector sensitivity were obtained. In addition, the reduction in resolution caused by the effects of various levels of background photon flux was determined. These measurements indicate that resolution is degraded minimally ({approximately}5% with a background-to-source ratio of 2:1) due to annihilation photon background. Sensitivity for positrons is good, detecting amounts of radioactivity as low as 10.2 nCi of FDG in vitro. In rats given FDG subcutaneously, lymph nodes containing as little as 11 nCi of FDG could be detected above the background activity levels present in normal surrounding tissues. A plastic scintillator probe system has been devised which may be highly suitable for intraoperative FDG-guided (or other positron or beta emitting-tracer) surgery. 29 refs., 7 figs.

Raylman, R.R.; Wahl, R.L. [Univ. of Michigan Medical Center, Ann Arbor, MI (United States)

1994-05-01

314

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

E-print Network

The beaming pattern of thermal annihilation radiation is broader than the beaming pattern produced by isotropic nonthermal electrons and positrons in the jets of radio-emitting active galactic nuclei which Compton scatter photons from an external isotropic radiation field. Thus blueshifted thermal annihilation radiation can provide the dominant contribution to the high-energy radiation spectrum at observing angles theta > 1/Gamma, where Gamma is the bulk Lorentz factor of the outflowing plasma. This effect may account for the spectral features of MeV blazars discovered with the Compton Telescope on the Compton Gamma Ray Observatory. Coordinated gamma-ray observations of annihilation line radiation to infer Doppler factors and VLBI radio observations to measure transverse angular speeds of outflowing plasma blobs can be used to determine the Hubble constant.

J. G. Skibo; C. D. Dermer; R. Schlickeiser

1997-01-28

315

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

SciTech Connect

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

Burrows, P.N.

1997-03-01

316

An anomalous positron abundance in cosmic rays with energies 1.5-100GeV  

Microsoft Academic Search

Antiparticles account for a small fraction of cosmic rays and are known to be produced in interactions between cosmic-ray nuclei and atoms in the interstellar medium, which is referred to as a `secondary source'. Positrons might also originate in objects such as pulsars and microquasars or through dark matter annihilation, which would be `primary sources'. Previous statistically limited measurements of

O. Adriani; G. C. Barbarino; G. A. Bazilevskaya; R. Bellotti; M. Boezio; E. A. Bogomolov; L. Bonechi; M. Bongi; V. Bonvicini; S. Bottai; A. Bruno; F. Cafagna; D. Campana; P. Carlson; M. Casolino; G. Castellini; M. P. de Pascale; G. de Rosa; N. de Simone; V. di Felice; A. M. Galper; L. Grishantseva; P. Hofverberg; S. V. Koldashov; S. Y. Krutkov; A. N. Kvashnin; A. Leonov; V. Malvezzi; L. Marcelli; W. Menn; V. V. Mikhailov; E. Mocchiutti; S. Orsi; G. Osteria; P. Papini; M. Pearce; P. Picozza; M. Ricci; S. B. Ricciarini; M. Simon; R. Sparvoli; P. Spillantini; Y. I. Stozhkov; A. Vacchi; E. Vannuccini; G. Vasilyev; S. A. Voronov; Y. T. Yurkin; G. Zampa; N. Zampa; V. G. Zverev

2009-01-01

317

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

E-print Network

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

Ko, P

2014-01-01

318

Neutralino annihilation beyond leading order  

NASA Astrophysics Data System (ADS)

High-precision measurements of the relic dark matter density and the calculation of dark matter annihilation branching fractions in the sun or the galactic halo today motivate the computation of the neutralino annihilation cross section beyond leading order. We consider neutralino annihilation via squark exchange and parameterize the effective annihilation vertex as a dimension-six operator suppressed by two powers of the squark mass and related to the divergence of the axial vector current of the final-state quarks. Since the axial vector current is conserved at tree level in the limit of massless quarks, this dimension-six operator contains a suppression by the quark mass. The quark mass suppression can be lifted in two ways: (1) by corrections to the dimension-six operator involving the anomalous triangle diagram, and (2) by going to dimension-eight. We address the first of these possibilities by evaluating the anomalous triangle diagram, which contributes to neutralino annihilation to gluon pairs. We relate the triangle diagram via the anomaly equation to the decay of a pseudoscalar into two gluons and use the Adler-Bardeen theorem to extract the next-to-leading order (NLO) QCD corrections to ???gg from the known corrections to pseudoscalar decay. The strong dependence of the dominant ???qqbar cross section on the relative velocity of the neutralinos makes these NLO corrections unimportant at ? decoupling but significant today.

Barger, Vernon; Keung, Wai-Yee; Logan, Heather E.; Shaughnessy, Gabe; Tregre, Adam

2006-02-01

319

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

NASA Astrophysics Data System (ADS)

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

Anto, C. Varghese; Rajaraman, R.; Rao, G. Venugopal; Abhaya, S.; Parimala, J.; Amarendra, G.

2012-06-01

320

Semi-annihilation of dark matter  

E-print Network

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

D’Eramo, Francesco

321

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

NASA Astrophysics Data System (ADS)

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

De Cesare, G.

2011-09-01

322

Positrons from quantum evaporation of primordial black-holes  

NASA Technical Reports Server (NTRS)

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

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

1997-01-01

323

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

DOEpatents

A method and means is provided for characterizing interfacial electron states in solid heterostructures using a variable energy positron beam to probe the solid heterostructure. The method includes the steps of directing a positron beam having a selected energy level at a point on the solid heterostructure so that the positron beam penetrates into the solid heterostructure and causes positrons to collide with the electrons at an interface of the solid heterostructure. The number and energy of gamma rays emitted from the solid heterostructure as a result of the annihilation of positrons with electrons at the interface are detected. The data is quantified as a function of the Doppler broadening of the photopeak about the 511 keV line created by the annihilation of the positrons and electrons at the interface, preferably, as an S-parameter function; and a normalized S-parameter function of the data is obtained. The function of data obtained is compared with a corresponding function of the Doppler broadening of the annihilation photopeak about 511 keV for a positron beam having a second energy level directed at the same material making up a portion of the solid heterostructure. The comparison of these functions facilitates characterization of the interfacial states of electrons in the solid heterostructure at points corresponding to the penetration of positrons having the particular energy levels into the interface of the solid heterostructure. Accordingly, the invention provides a variable-energy non-destructive probe of solid heterostructures, such as SiO[sub 2]/Si, MOS or other semiconductor devices.

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

1993-04-06

324

A novel source of MeV positron bunches driven by energetic protons for PAS application  

NASA Astrophysics Data System (ADS)

This paper proposes a novel methodology of MeV positrons generation for PAS application. Feasibility of this proposal analyzed by G4Beamline and Transport have shown reasonable success. Using 2 Hz, 1.6 GeV, 100 ns and 1.5 ?C/bunch proton bunches for bombarding a graphite target, about 100 ns e+ bunches are generated. Quasi-monochromatic positrons in the range of 1-10 MeV included in these bunches have a flux of >107/s, peak brightness of 1014/s. A magnetic-confinement beamline is utilized to transport the positrons and a “Fast Beam Chopper” is unprecedentedly extended to chop those relativistic bunches. The positron beam can be finally characterized by the energy range of 1-10 MeV and bunch width from one hundred ps up to 1 ns. Such ultrashort bunches can be useful in tomography-type positron annihilation spectroscopy (PAS) as well as other applications.

Tan, Zongquan; Xu, Wenzhen; Liu, Yanfen; Xiao, Ran; Kong, Wei; Ye, Bangjiao

2014-11-01

325

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

SciTech Connect

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

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

1990-01-01

326

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

SciTech Connect

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

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

1990-10-15

327

Positron-rubidium scattering  

NASA Technical Reports Server (NTRS)

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

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

1990-01-01

328

Simulation of tail distributions in electron-positron circular colliders  

SciTech Connect

In addition to the Gaussian shaped core region, particle bunches in electron-positron circular colliders have a rarefied halo region of importance in determining beam lifetimes and backgrounds in particle detectors. A method is described which allows simulation of halo particle distributions.

Irwin, J.

1992-02-01

329

Pressure from dark matter annihilation and the rotation curve of spiral galaxies  

NASA Astrophysics Data System (ADS)

The rotation curves of spiral galaxies are one of the basic predictions of the cold dark matter paradigm, and their shape in the innermost regions has been hotly debated over the last decades. The present work shows that dark matter annihilation into electron-positron pairs may affect the observed rotation curve by a significant amount. We adopt a model-independent approach, where all the electrons and positrons are injected with the same initial energy E0˜mdmc2 in the range from 1 MeV to 1 TeV and the injection rate is constrained by INTEGRAL, Fermi and HESS data. The pressure of the relativistic electron-positron gas is determined by solving the diffusion-loss equation, considering inverse Compton scattering, synchrotron radiation, Coulomb collisions, bremsstrahlung and ionization. For values of the gas density and magnetic field that are representative of the Milky Way, it is estimated that pressure gradients are strong enough to balance gravity in the central parts if E0 < 1 GeV. The exact value depends somewhat on the astrophysical parameters, and it changes dramatically with the slope of the dark matter density profile. For very steep slopes, as those expected from adiabatic contraction, the rotation curves of spiral galaxies would be affected on ˜kpc scales for most values of E0. By comparing the predicted rotation curves with observations of dwarf and low surface brightness galaxies, we show that the pressure from dark matter annihilation may improve the agreement between theory and observations in some cases, but it also imposes severe constraints on the model parameters (most notably, the inner slope of halo density profile, as well as the mass and the annihilation cross-section of dark matter particles into electron-positron pairs).

Wechakama, M.; Ascasibar, Y.

2011-05-01

330

Electron-positron interaction in jellium  

NASA Astrophysics Data System (ADS)

The problem of a positron in jellium is solved in an approach involving self-consistent perturbation of a Jastrow-type state. The merits of this approach are the following: (1) The one-electron wave functions are allowed to be nonorthogonal, (2) the formalism is indifferent with regard to uti- lizing the Pauli exclusion principle, and (3) numerical calculations are shorter by a factor of the order of 100 in comparison with other theories. The first two points are of special importance in view of the difficulties encountered both by the Kahana formalism and the approach of Lowy and Jackson. The screening cloud obtained in this work reproduces quite well the recent results of Rubaszek and Stachowiak, as do the partial annihilation rates. A comparison with the results of other theories and with experiment is also made.

Stachowiak, Henryk

1990-06-01

331

Positrons for linear colliders  

SciTech Connect

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

Ecklund, S.

1987-11-01

332

Upper Limits to the Annihilation Radiation Luminosity of Centaurus a  

NASA Technical Reports Server (NTRS)

A high resolution observation of the active nucleus galaxy Centaurus A (NGC 5128) was made by the GSFC low energy gamma-ray spectrometer (LEGS) during a balloon flight on 1981 November 19. The measured spectrum between 70 and 500 keV is well represented by a power law of the form 1.05 x 10 (-4) (E/100 keV) (-1.59) ph/sq cm /s with no breaks or line features observed. The 98% confidence (2 sigma) flux upper limit for a narrow ( 3 keV) 511-keV positron annihilation line is 9.9 x 10 (-4) ph/ sq cm /s. Using this upper limit, the ratio of the narrow-line annihilation radiation luminosity to the integral or = 511 keV luminosity is estimated to be 0.09 (2 sigma upper limit). This is compared with the measured value for our galactic center in the Fall of 1979 of 0.10 to 0.13, indicating a difference in he emission regions in the nuclei of the two galaxies.

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

1983-01-01

333

Upper limits to the annihilation radiation luminosity of Centaurus A  

NASA Technical Reports Server (NTRS)

A high resolution observation of the active nucleus galaxy Centaurus A (NGC 5128) was made by the GSFC low energy gamma-ray spectrometer (LEGS) during a balloon flight on 1981 November 19. The measured spectrum between 70 and 500 keV is well represented by a power law of the form 1.05 x 10 (-4) (E/100 keV) (-1.59) ph/sq cm/s with no breaks or line features observed. The 98 percent confidence (2 sigma) flux upper limit for a narrow (3 keV) 511-keV positron annihilation line is 9.9 x 10 (-4) ph/sq cm/s. Using this upper limit, the ratio of the narrow-line annihilation radiation luminosity to the integral or = 511 keV luminosity is estimated to be 0.09 (2 sigma upper limit). This is compared with the measured value for our Galactic center in the Fall of 1979 of 0.10 to 0.13, indicating a difference in the emission regions in the nuclei of the two galaxies.

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

1983-01-01

334

Radio and gamma-ray constraints on dark matter annihilation in the Galactic center  

NASA Astrophysics Data System (ADS)

We determine upper limits on the dark matter (DM) self-annihilation cross section for scenarios in which annihilation leads to the production of electron—positron pairs. In the Galactic center, relativistic electrons and positrons produce a radio flux via synchroton emission, and a gamma-ray flux via bremsstrahlung and inverse Compton scattering. On the basis of archival, interferometric and single-dish radio data, we have determined the radio spectrum of an elliptical region around the Galactic center of extent 3° semimajor axis (along the Galactic plane) and 1° semiminor axis and a second, rectangular region, also centered on the Galactic center, of extent 1.6°×0.6°. The radio spectra of both regions are nonthermal over the range of frequencies for which we have data: 74 MHz-10 GHz. We also consider gamma-ray data covering the same region from the EGRET instrument (about GeV) and from HESS (around TeV). We show how the combination of these data can be used to place robust constraints on DM annihilation scenarios, in a way which is relatively insensitive to assumptions about the magnetic field amplitude in this region. Our results are approximately an order of magnitude more constraining than existing Galactic center radio and gamma-ray limits. For a DM mass of m?=10GeV, and an Navarro-Frank-White profile, we find ??Av??few×10-25cm3s-1.

Crocker, R. M.; Bell, N. F.; Balázs, C.; Jones, D. I.

2010-03-01

335

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

NASA Astrophysics Data System (ADS)

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

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

2014-11-01

336

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

SciTech Connect

Annihilation of dark matter can result in the production of stable Standard Model particles including electrons and positrons that, in the presence of magnetic fields, lose energy via synchrotron radiation, observable as radio emission. Galaxy clusters are excellent targets to search for or to constrain the rate of dark matter annihilation, as they are both massive and dark matter dominated. In this study, we place limits on dark matter annihilation in a sample of nearby clusters using upper limits on the diffuse radio emission, low levels of observed diffuse emission, or detections of radio mini-halos. We find that the strongest limits on the annihilation cross section are better than limits derived from the non-detection of clusters in the gamma-ray band by a factor of {approx}3 or more when the same annihilation channel and substructure model, but different best-case clusters, are compared. The limits on the cross section depend on the assumed amount of substructure, varying by as much as two orders of magnitude for increasingly optimistic substructure models as compared to a smooth Navarro-Frenk-White profile. In our most optimistic case, using the results of the Phoenix Project, we find that the derived limits reach below the thermal relic cross section of 3 Multiplication-Sign 10{sup -26} cm{sup 3} s{sup -1} for dark matter masses as large as 400 GeV, for the b b-bar annihilation channel. We discuss uncertainties due to the limited available data on the magnetic field structure of individual clusters. We also report the discovery of diffuse radio emission from the central 30-40 kpc regions of the groups M49 and NGC 4636.

Storm, Emma; Jeltema, Tesla E.; Profumo, Stefano [Department of Physics, University of California, 1156 High St., Santa Cruz, CA 95064 (United States); Rudnick, Lawrence [Minnesota Institute for Astrophysics, School of Physics and Astronomy, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455 (United States)

2013-05-10

337

Electron-Positron Flows around Magnetars  

NASA Astrophysics Data System (ADS)

The twisted magnetospheres of magnetars must sustain a persistent flow of electron-positron plasma. The flow dynamics is controlled by the radiation field around the hot neutron star. The problem of plasma motion in the self-consistent radiation field is solved using the method of virtual beams. The plasma and radiation exchange momentum via resonant scattering and self-organize into the "radiatively locked" outflow with a well-defined, decreasing Lorentz factor. There is an extended zone around the magnetar where the plasma flow is ultra-relativistic; its Lorentz factor is self-regulated so that it can marginally scatter thermal photons. The flow becomes slow and opaque in an outer equatorial zone, where the decelerated plasma accumulates and annihilates; this region serves as a reflector for the thermal photons emitted by the neutron star. The e ± flow carries electric current, which is sustained by a moderate induced electric field. The electric field maintains a separation between the electron and positron velocities, against the will of the radiation field. The two-stream instability is then inevitable, and the induced turbulence can generate low-frequency emission. In particular, radio emission may escape around the magnetic dipole axis of the star. Most of the flow energy is converted to hard X-ray emission, which is examined in an accompanying paper.

Beloborodov, Andrei M.

2013-11-01

338

ELECTRON-POSITRON FLOWS AROUND MAGNETARS  

SciTech Connect

The twisted magnetospheres of magnetars must sustain a persistent flow of electron-positron plasma. The flow dynamics is controlled by the radiation field around the hot neutron star. The problem of plasma motion in the self-consistent radiation field is solved using the method of virtual beams. The plasma and radiation exchange momentum via resonant scattering and self-organize into the 'radiatively locked' outflow with a well-defined, decreasing Lorentz factor. There is an extended zone around the magnetar where the plasma flow is ultra-relativistic; its Lorentz factor is self-regulated so that it can marginally scatter thermal photons. The flow becomes slow and opaque in an outer equatorial zone, where the decelerated plasma accumulates and annihilates; this region serves as a reflector for the thermal photons emitted by the neutron star. The e {sup ±} flow carries electric current, which is sustained by a moderate induced electric field. The electric field maintains a separation between the electron and positron velocities, against the will of the radiation field. The two-stream instability is then inevitable, and the induced turbulence can generate low-frequency emission. In particular, radio emission may escape around the magnetic dipole axis of the star. Most of the flow energy is converted to hard X-ray emission, which is examined in an accompanying paper.

Beloborodov, Andrei M., E-mail: amb@phys.columbia.edu [Physics Department and Columbia Astrophysics Laboratory, Columbia University, 538 West 120th Street New York, NY 10027 (United States)

2013-11-10

339

Four-pion production in tau decays and e+e- annihilation: an update  

E-print Network

An improved description of four-pion production in electron-positron annihilation and in tau lepton decays is presented. The model amplitude is fitted to recent data from BaBar which cover a wide energy range and which were obtained exploiting the radiative return. Predicting tau decay distributions from e+e- data and comparing these predictions with ALEPH and CLEO results, the validity of isospin symmetry is confirmed within the present experimental errors. A good description of two- and three-pion sub-distributions is obtained. Special emphasis is put on the predictions for omega pi (-> pi+pi-pi0) in e+e- annihilation and in tau decay. The model amplitude is implemented in the Monte Carlo generator PHOKHARA.

Henryk Czyz; Johann H. Kuhn; Agnieszka Wapienik

2008-04-01

340

Plasmon Annihilation into Kaluza-Klein Graviton: New Astrophysical Constraints on Large Extra Dimensions  

E-print Network

In large extra dimensional Kaluza-Klein (KK) scenario, where the usual Standard Model (SM) matter is confined to a 3+1-dimensional hypersurface called the 3-brane and gravity can propagate to the bulk (D=4+d, d being the number of extra spatial dimensions), the light graviton KK modes can be produced inside the supernova core due to the usual nucleon-nucleon bremstrahlung, electron-positron and photon-photon annihilations. This photon inside the supernova becomes plasmon due to the plasma effect. In this paper, we study the energy-loss rate of SN 1987A due to the KK gravitons produced from the plasmon-plasmon annihilation. We find that the SN 1987A cooling rate leads to the conservative bound $M\\_D$ > 22.9 TeV and 1.38 TeV for the case of two and three space-like extra dimensions.

Prasanta Kumar Das; V H Satheeshkumar; P. K. Suresh

2008-01-08

341

The Neutron Lifetime  

E-print Network

The decay of the free neutron into a proton, electron, and antineutrino is the prototype semileptonic weak decay and the simplest example of nuclear beta decay. The nucleon vector and axial vector weak coupling constants G_V and G_A determine the neutron lifetime as well as the strengths of weak interaction processes involving free neutrons and protons that are important in astrophysics, cosmology, solar physics and neutrino detection. In combination with a neutron decay angular correlation measurement, the neutron lifetime can be used to determine the first element of the CKM matrix Vud. Unfortunately the two main experimental methods for measuring the neutron lifetime currently disagree by almost 4 sigma. I will present a brief review of the status of the neutron lifetime and prospects for the future.

Wietfeldt, F E

2014-01-01

342

WIMP Annihilation and Cooling of Neutron Stars  

E-print Network

We study the effect of WIMP annihilation on the temperature of a neutron star. We shall argue that the released energy due to WIMP annihilation inside the neutron stars, might affect the temperature of stars older than 10 million years, flattening out the temperature at $\\sim 10^4$ K for a typical neutron star.

Chris Kouvaris

2007-08-17

343

Electroweak bremsstrahlung in dark matter annihilation  

SciTech Connect

A conservative upper bound on the total dark matter annihilation rate can be obtained by constraining the appearance rate of the annihilation products which are hardest to detect. The production of neutrinos, via the process {chi}{chi}{yields}{nu}{nu}, has thus been used to set a strong general bound on the dark matter annihilation rate. However, standard model radiative corrections to this process will inevitably produce photons which may be easier to detect. We present an explicit calculation of the branching ratios for the electroweak bremsstrahlung processes {chi}{chi}{yields}{nu}{nu}Z and {chi}{chi}{yields}{nu}eW. These modes inevitably lead to electromagnetic showers and further constraints on the dark matter annihilation cross section. In addition to annihilation, our calculations are also applicable to the case of dark matter decay.

Bell, Nicole F.; Jacques, Thomas D. [School of Physics, University of Melbourne, Victoria 3010 (Australia); Dent, James B.; Weiler, Thomas J. [Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235 (United States)

2008-10-15

344

Static pairwise annihilation in complex networks.  

PubMed

We study static annihilation on complex networks, in which pairs of connected particles annihilate at a constant rate during time. Through a mean-field formalism, we compute the temporal evolution of the distribution of surviving sites with an arbitrary number of connections. This general formalism, which is exact for disordered networks, is applied to Kronecker, Erdös-Rényi (i.e., Poisson), and scale-free networks. We compare our theoretical results with extensive numerical simulations obtaining excellent agreement. Although the mean-field approach applies in an exact way neither to ordered lattices nor to small-world networks, it qualitatively describes the annihilation dynamics in such structures. Our results indicate that the higher the connectivity of a given network element, the faster it annihilates. This fact has dramatic consequences in scale-free networks, for which, once the "hubs" have been annihilated, the network disintegrates and only isolated sites are left. PMID:16196639

Laguna, M F; Aldana, M; Larralde, H; Parris, P E; Kenkre, V M

2005-08-01

345

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

E-print Network

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

Douglas P. Finkbeiner

2004-09-02

346

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

SciTech Connect

The profiles of positrons implanted from the radioactive source {sup 22}Na into a stack of foils and plates are the subject of our experimental and theoretical studies. The measurements were performed using the depth scanning of positron implantation profile method, and the theoretical calculations using the phenomenological multi-scattering model (MSM). Several stacks consisting of silver, gold and aluminum foils, and titanium and germanium plates were investigated. We notice that the MSM describes well the experimental profiles; however when the stack consisting of silver and gold foils, the backscattering and linear absorption coefficients differ significantly from those reported in the literature. We suggest the energy dependency of the backscattering coefficient for silver and gold. In the stacks which comprise titanium and germanium plates, there were observed the features, which indicate the presence of the “accumulation effect” in the experimental implantation profile. This effect was previously detected in implantation profiles in Monte Carlo simulations using the GEANT4 tool kit, and it consists in higher localization of positrons close the interface. We suppose that this effect can be essential for positron annihilation in any heterogeneous materials.

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

2013-12-14

347

Alternative positron-target design for electron-positron colliders  

SciTech Connect

Current electron-positron linear colliders are limited in luminosity by the number of positrons which can be generated from targets presently used. This paper examines the possibility of using an alternate wire-target geometry for the production of positrons via an electron-induced electromagnetic cascade shower. 39 refs., 38 figs., 5 tabs.

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

1991-04-01

348

Positron flight in human tissues and its influence on PET image spatial resolution.  

PubMed

The influence of the positron distance of flight in various human tissues on the spatial resolution in positron emission tomography (PET) was assessed for positrons from carbon-11, nitrogen-13, oxygen-15, fluorine-18, gallium-68 and rubidium-82. The investigation was performed using the Monte Carlo code PENELOPE to simulate the transport of positrons within human compact bone, adipose, soft and lung tissue. The simulations yielded 3D distributions of annihilation origins that were projected on the image plane in order to assess their impact on PET spatial resolution. The distributions obtained were cusp-shaped with long tails rather than Gaussian shaped, thus making conventional full width at half maximum (FWHM) measures uncertain. The full width at 20% of the maximum amplitude (FW20M) of the annihilation distributions yielded more appropriate values for root mean square addition of spatial resolution loss components. Large differences in spatial resolution losses due to the positron flight in various human tissues were found for the selected radionuclides. The contribution to image blur was found to be up to three times larger in lung tissue than in soft tissue or fat and five times larger than in bone tissue. For (18)F, the spatial resolution losses were 0.54 mm in soft tissue and 1.52 mm in lung tissue, compared with 4.10 and 10.5 mm, respectively, for (82)Rb. With lung tissue as a possible exception, the image blur due to the positron flight in all human tissues has a minor impact as long as PET cameras with a spatial resolution of 5-7 mm are used in combination with (18)F-labelled radiopharmaceuticals. However, when ultra-high spatial resolution PET cameras, with 3-4 mm spatial resolution, are applied, especially in combination with other radionuclides, the positron flight may enter as a limiting factor for the total PET spatial resolution--particularly in lung tissue. PMID:14551751

Sánchez-Crespo, Alejandro; Andreo, Pedro; Larsson, Stig A

2004-01-01

349

Constraining dark matter late-time energy injection: decays and p-wave annihilations  

NASA Astrophysics Data System (ADS)

We use the latest cosmic microwave background (CMB) observations to provide updated constraints on the dark matter lifetime as well as on p-wave suppressed annihilation cross sections in the 1 MeV to 1 TeV mass range. In contrast to scenarios with an s-wave dominated annihilation cross section, which mainly affect the CMB close to the last scattering surface, signatures associated with these scenarios essentially appear at low redshifts (zlesssim50) when structure began to form, and thus manifest at lower multipoles in the CMB power spectrum. We use data from Planck, WMAP9, SPT and ACT, as well as Lyman-? measurements of the matter temperature at z ~ 4 to set a 95% confidence level lower bound on the dark matter lifetime of ~ 4 × 1025 s for m? = 100 MeV. This bound becomes lower by an order of magnitude at m? = 1 TeV due to inefficient energy deposition into the intergalactic medium. We also show that structure formation can enhance the effect of p-wave suppressed annihilation cross sections by many orders of magnitude with respect to the background cosmological rate, although even with this enhancement, CMB constraints are not yet strong enough to reach the thermal relic value of the cross section.

Diamanti, Roberta; Lopez-Honorez, Laura; Mena, Olga; Palomares-Ruiz, Sergio; Vincent, Aaron C.

2014-02-01

350

Development of a sample chamber with humidity control for an atmospheric positron probe microanalyzer  

NASA Astrophysics Data System (ADS)

In order to perform positron lifetime measurements on thin films under atmospheric conditions, a slow positron microbeam was extracted into air using silicon nitride thin films (30 nm and 200 nm) as a vacuum window. Even the thinner window (30 nm) was found to reliably withstand a differential pressure of 1 atm under various stress tests. By placing the sample in an enclosed chamber through which gas with a fixed, controllable relative humidity (RH) was continuously passed, the RH dependence of the ortho-positronium lifetime for bulk fused silica was examined.

Zhou, W.; Oshima, N.; Chen, Z.; Ito, K.; O'Rourke, B. E.; Kuroda, R.; Suzuki, R.; Yanagishita, H.; Tsutsui, T.; Uedono, A.; Hayashizaki, N.

2013-06-01

351

Positron excitation of neon  

NASA Technical Reports Server (NTRS)

The differential and total cross section for the excitation of the 3s1P10 and 3p1P1 states of neon by positron impact were calculated using a distorted-wave approximation. The results agree well with experimental conclusions.

Parcell, L. A.; Mceachran, R. P.; Stauffer, A. D.

1990-01-01

352

The Japanese Positron Factory  

NASA Astrophysics Data System (ADS)

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

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

1999-06-01

353

Dark matter annihilations in the causal diamond  

NASA Astrophysics Data System (ADS)

We investigate the implications of dark matter annihilations for cosmological parameter constraints using the causal entropic principle. In this approach cosmologies are weighted by the total entropy production within a causally connected region of spacetime. We calculate the expected entropy from dark matter annihilations within the causal diamond and investigate the preferred values of the cosmological constant and the mass and annihilation cross section of the annihilating dark matter and their dependence on the assumptions in the models. For realistic values of the cross section, we typically find preferred values of ? on the order of 10-5 of the present value assuming dark matter annihilations are the primary source of entropy production. The greatest amount of entropy production from dark matter within the causal diamond is likely to occur with light keV scale dark matter with a low annihilation cross section. We also investigate the effect of combining this entropy with the entropy production from stars and show that if the primary source of entropy production is from stars varying the dark matter cross section directly produces a preferred value of ?m in excellent agreement with observations.

Scacco, Andrew; Albrecht, Andreas

2014-02-01

354

Constraining the origin of the rising cosmic ray positron fraction with the boron-to-carbon ratio  

NASA Astrophysics Data System (ADS)

The rapid rise in the cosmic ray positron fraction above 10 GeV, as measured by PAMELA and AMS, suggests the existence of nearby primary sources of high energy positrons, such as pulsars or annihilating/decaying dark matter. In contrast, the spectrum of secondary positrons produced through the collisions of cosmic rays in the interstellar medium is predicted to fall rapidly with energy, and thus is unable to account for the observed rise. It has been proposed, however, that secondary positrons could be produced and then accelerated in nearby supernova remnants, potentially explaining the observed rise, without the need of primary positron sources. Yet, if secondary positrons are accelerated in such shocks, other secondary cosmic ray species (such as boron nuclei and antiprotons) will also be accelerated, leading to rises in the boron-to-carbon and antiproton-to-proton ratios. The measurements of the boron-to-carbon ratio by the PAMELA and AMS collaborations, however, show no sign of such a rise. With this new data in hand, we revisit the secondary acceleration scenario for the rising positron fraction. Assuming that the same supernova remnants accelerate both light nuclei (protons, helium) and heavier cosmic ray species, we find that no more than ˜25% of the observed rise in the positron fraction can result from this mechanism (at the 95% confidence level).

Cholis, Ilias; Hooper, Dan

2014-02-01

355

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

PubMed

Positron range impairs resolution in PET imaging, especially for high-energy emitters and for small-animal PET. De-blurring in image reconstruction is possible if the blurring distribution is known. Furthermore, the percentage of annihilation events within a given distance from the point of positron emission is relevant for assessing statistical noise. This paper aims to determine the positron range distribution relevant for blurring for seven medically relevant PET isotopes, (18)F, (11)C, (13)N, (15)O, (68)Ga, (62)Cu and (82)Rb, and derive empirical formulas for the distributions. This paper focuses on allowed-decay isotopes. It is argued that blurring at the detection level should not be described by the positron range r, but instead the 2D projected distance ? (equal to the closest distance between decay and line of response). To determine these 2D distributions, results from a dedicated positron track-structure Monte Carlo code, Electron and POsitron TRANsport (EPOTRAN), were used. Materials other than water were studied with PENELOPE. The radial cumulative probability distribution G(2D)(?) and the radial probability density distribution g(2D)(?) were determined. G(2D)(?) could be approximated by the empirical function 1 - exp(-A?(2) - B?), where A = 0.0266 (E(mean))(-1.716) and B = 0.1119 (E(mean))(-1.934), with E(mean) being the mean positron energy in MeV and ? in mm. The radial density distribution g(2D)(?) could be approximated by differentiation of G(2D)(?). Distributions in other media were very similar to water. The positron range is important for improved resolution in PET imaging. Relevant distributions for the positron range have been derived for seven isotopes. Distributions for other allowed-decay isotopes may be estimated with the above formulas. PMID:22643300

Jødal, L; Le Loirec, C; Champion, C

2012-06-21

356

Slow positron beam and nanoindentation study of irradiation-related defects in reactor vessel steels  

NASA Astrophysics Data System (ADS)

In order to understand the nature of the hardening after radiation in reactor vessel steels, China A508-3 steels were implanted by proton with an energy of 240 keV up to 2.5 × 1016, 5.5 × 1016, 1.1 × 1017, and 2.5 × 1017 ions cm-2, respectively. Vacancy type defects were detected by energy-variable positron beam Doppler broadening technique and then nanoindentation measurements were performed to investigate proton-induced hardening effects. The results showed that S-parameter increased as a function of positron incident energy after irradiation, and the increasing rate of the S-parameter near the surface was larger than that in the bulk due to radiation damage. The size of vacancy type defects increased with dose. Irradiation induced hardening was shown that the average hardness increased with dose. Moreover a direct correlation between positron annihilation parameter and hardness was found based on Kasada method.

Liu, Xiangbing; Wang, Rongshan; Jiang, Jing; Wu, Yichu; Zhang, Chonghong; Ren, Ai; Xu, Chaoliang; Qian, Wangjie

2014-08-01

357

Neutron LifetimeNeutron Lifetime IUCF Colloquium April 13,  

E-print Network

Neutron LifetimeNeutron Lifetime IUCF Colloquium April 13, 2007 Albert Steyerl Department 940 878.5±0.8 885.7±0.8 new result neutronlifetime(),s year world average Neutron lifetime data #12 world average Neutron lifetime data A. Serebrov et al. 2005Storage of ultra-cold neutrons878.5 ±±±± 0

Steyerl, Albert

358

FERMI CONSTRAINS DARK-MATTER ORIGIN OF HIGH-ENERGY POSITRON ANOMALY  

SciTech Connect

Fermi measurements of the high-latitude {gamma}-ray background strongly constrain a decaying-dark-matter origin for the 1-100 GeV Galactic positron anomaly measured with PAMELA. Inverse Compton scattering of the microwave background by the emergent positrons produces a bump in the diffuse 100-200 MeV {gamma}-ray background that would protrude from the observed background at these energies. The positrons are thus constrained to emerge from the decay process at a typical energy between {approx}100 GeV and {approx}250 GeV. By considering only {gamma}-ray emission of the excess positrons and electrons, we derive a minimum diffuse {gamma}-ray flux that, apart from the positron spectrum assumed, is independent of the actual decay modes. Any {gamma}-rays produced directly by the dark-matter decay leads to an additional signal that makes the observational limits more severe. A similar constraint on the energy of emergent positrons from annihilation in dark-matter substructures is argued to exist, according to recent estimates of enhancement in low-mass dark-matter substructures, and improved simulations of such substructure will further sharpen this constraint.

Pohl, Martin [Institut fuer Physik und Astronomie, Universitaet Potsdam, 14476 Potsdam-Golm (Germany); DESY, 15738 Zeuthen (Germany); Eichler, David [Physics Department, Ben-Gurion University, Beer-Sheva 84105 (Israel)], E-mail: pohlmadq@gmail.com, E-mail: eichler@bgumail.bgu.ac.il

2010-03-20

359

Measurement of the B0 and B+ Meson Lifetimes with Fully Reconstructed Hadronic Final States  

Microsoft Academic Search

The B0 and B+ meson lifetimes have been measured in e+e- annihilation data collected in 1999 and 2000 with the BABAR detector at center-of-mass energies near the Upsilon(4S) resonance. Events are selected in which one B meson is fully reconstructed in a hadronic final state while the second B meson is reconstructed inclusively. A combined fit to the B0 and

B. Aubert; D. Boutigny; J.-M. Gaillard; A. Hicheur; Y. Karyotakis; J. Lees; P. Robbe; V. Tisserand; A. Palano; G. Chen; J. Chen; N. Qi; G. Rong; P. Wang; Y. Zhu; G. Eigen; P. Reinertsen; B. Stugu; B. Abbott; G. Abrams; A. Borgland; A. Breon; D. Brown; J. Button-Shafer; R. Cahn; A. Clark; M. Gill; A. Gritsan; Y. Groysman; R. Jacobsen; R. Kadel; J. Kadyk; L. Kerth; S. Kluth; Yu. Kolomensky; J. Kral; C. Leclerc; M. Levi; T. Liu; G. Lynch; A. Meyer; M. Momayezi; P. Oddone; A. Perazzo; M. Pripstein; N. Roe; A. Romosan; M. Ronan; V. Shelkov; A. V. Telnov; W. Wenzel; P. Bright-Thomas; T. Harrison; C. Hawkes; D. Knowles; S. O'Neale; R. Penny; A. Watson; N. Watson; T. Deppermann; K. Goetzen; H. Koch; J. Krug; M. Kunze; B. Lewandowski; K. Peters; H. Schmuecker; M. Steinke; J. Andress; N. Barlow; W. Bhimji; N. Chevalier; P. Clark; W. Cottingham; N. De Groot; N. Dyce; B. Foster; J. McFall; D. Wallom; F. F. Wilson; K. Abe; C. Hearty; T. Mattison; J. McKenna; D. Thiessen; S. Jolly; A. McKemey; J. Tinslay; V. Blinov; A. Bukin; D. Bukin; A. Buzykaev; V. Golubev; V. Ivanchenko; A. A. Korol; E. Kravchenko; A. Onuchin; A. A. Salnikov; S. I. Serednyakov; Yu. I. Skovpen; V. I. Telnov; A. Yushkov; D. Best; A. Lankford; M. Mandelkern; S. McMahon; D. Stoker; A. Ahsan; K. Arisaka; C. Buchanan; S. Chun; J. Branson; D. MacFarlane; S. Prell; Sh. Rahatlou; G. Raven; V. Sharma; C. Campagnari; B. Dahmes; P. Hart; N. Kuznetsova; S. Levy; O. Long; A. Lu; J. Richman; W. Verkerke; M. Witherell; S. Yellin; J. Beringer; D. Dorfan; A. Eisner; A. Frey; A. A. Grillo; M. Grothe; C. Heusch; R. Johnson; W. Kroeger; W. Lockman; T. Pulliam; H. Sadrozinski; T. Schalk; R. Schmitz; B. Schumm; A. Seiden; M. Turri; W. Walkowiak; D. Williams; M. Wilson; E. Chen; G. Dubois-Felsmann; A. Dvoretskii; D. Hitlin; S. Metzler; J. Oyang; F. Porter; A. Ryd; A. Samuel; M. Weaver; S. Yang; R. Zhu; S. Devmal; T. Geld; S. Jayatilleke; G. Mancinelli; B. Meadows; M. Sokoloff; T. Barillari; P. Bloom; M. Dima; S. Fahey; W. Ford; D. Johnson; U. Nauenberg; A. Olivas; P. Rankin; J. Roy; S. Sen; J. Smith; W. van Hoek; D. Wagner; J. Blouw; J. Harton; M. Krishnamurthy; A. Soffer; W. Toki; R. Wilson; J. Zhang; T. Brandt; J. Brose; T. Colberg; G. Dahlinger; M. Dickopp; R. Dubitzky; E. Maly; R. Müller-Pfefferkorn; S. Otto; K. Schubert; K. R. Schubert; B. Spaan; L. Wilden; L. Behr; D. Bernard; G. Bonneaud; F. Brochard; J. Cohen-Tanugi; S. Ferrag; E. Roussot; S. T'Jampens; C. Thiebaux; G. Vasileiadis; M. Verderi; A. Anjomshoaa; R. Bernet; A. Khan; F. Muheim; S. Playfer; J. Swain; M. Falbo; C. Borean; C. Borean; S. Dittongo; M. Folegani; L. Piemontese; E. Treadwell; F. Anulli; R. Baldini-Ferroli; A. Calcaterra; R. de Sangro; D. Falciai; G. Finocchiaro; P. Patteri; I. Peruzzi; I. M. Peruzzi; Y. Xie; A. Zallo; S. Bagnasco; A. Buzzo; R. Contri; G. Crosetti; P. Fabbricatore; S. Farinon; M. Lo Vetere; M. Macri; M. Macri; R. Musenich; M. Pallavicini; R. Parodi; S. Passaggio; F. Pastore; C. Patrignani; M. Pia; C. Priano; E. Robutti; A. Santroni; M. Morii; R. Bartoldus; T. Dignan; U. Mallik; J. Cochran; H. Crawley; P.-A. Fischer; J. Lamsa; W. Meyer; E. Rosenberg; M. Benkebil; G. Grosdidier; C. Hast; A. Höcker; H. Lacker; V. LePeltier; A. Lutz; S. Plaszczynski; M. Schune; S. Trincaz-Duvoid; A. Valassi; G. Wormser; R. Bionta; V. Brigljevi?; D. Lange; M. Mugge; X. Shi; K. van Bibber; T. Wenaus; D. Wright; C. Wuest; M. Carroll; J. Fry; E. Gabathuler; R. Gamet; M. George; M. Kay; D. Payne; R. Sloane; C. Touramanis; M. Aspinwall; D. Bowerman; P. Dauncey; U. Egede; I. Eschrich; N. Gunawardane; J. Nash; P. Sanders; D. Smith; D. Azzopardi; J. Back; P. Dixon; P. Harrison; R. Potter; H. Shorthouse; P. Strother; P. Vidal; M. Williams; G. Cowan; S. George; M. Green; A. Kurup; C. Marker; P. McGrath; T. McMahon; S. Ricciardi; F. Salvatore; I. Scott; G. Vaitsas; C. Davis; J. Allison; R. Barlow; J. Boyd; A. Forti; J. Fullwood; F. Jackson; G. Lafferty; N. Savvas; E. Simopoulos; J. Weatherall; A. Farbin; A. Jawahery; V. Lillard; J. Olsen; D. Roberts; J. Schieck; G. Blaylock; S. Hertzbach; R. Kofler; T. Moore; H. Staengle; S. Willocq; B. Brau; R. Cowan; G. Sciolla; F. Taylor; R. Yamamoto; M. Milek; P. Patel; J. Trischuk; F. Lanni; F. Palombo; J. Bauer; M. Booke; L. Cremaldi; V. Eschenburg; R. Kroeger; J. Reidy; D. Sanders; D. Summers; J. Martin; J. Nief; R. Seitz; P. Taras; V. Zacek; H. Nicholson; C. Sutton; C. Cartaro; N. Cavallo; G. De Nardo; F. Fabozzi; C. Gatto; L. Lista; P. Paolucci; D. Piccolo; C. Sciacca; J. LoSecco; J. Alsmiller; T. Gabriel; T. Handler; J. Brau; R. Frey; M. Iwasaki; N. Sinev; D. Strom; F. Colecchia; F. Dal Corso; A. Dorigo; F. Galeazzi; M. Margoni; G. Michelon; M. Morandin; M. Posocco; M. Rotondo; F. Simonetto; R. Stroili; E. Torassa; C. Voci; M. Benayoun; H. Briand; J. Chauveau; P. David; C. De la Vaissière; L. Del Buono; O. Hamon; F. Le Diberder; Ph. Leruste; J. Lory; L. Roos; J. Stark; S. Versillé

2001-01-01

360

Cardiac positron emission tomography  

SciTech Connect

Positron emission tomography (PET) is a new technique for noninvasively assessing myocardial metabolism and perfusion. It has provided new insight into the dynamics of myocardial fatty acid and glucose metabolism in normal subjects, patients with ischemic heart disease and those with cardiomyopathies, documenting regionally depressed fatty acid metabolism during myocardial ischemia and infarction and spatial heterogeneity of fatty acid metabolism in patients with cardiomyopathy. Regional myocardial perfusion has been studied with PET using water, ammonia and rubidium labeled with positron emitters, permitting the noninvasive detection of hypoperfused zones at rest and during vasodilator stress. With these techniques the relationship between perfusion and the metabolism of a variety of substrates has been studied. The great strides that have been made in developing faster high-resolution instruments and producing new labeled intermediates indicate the promise of this technique for facilitating an increase in the understanding of regional metabolism and blood flow under normal and pathophysiologic conditions. 16 references, 9 figures, 2 tables.

Geltman, E.M.

1985-12-01

361

Positrons from pulsar winds  

E-print Network

Pulsars, or more generally rotation powered neutron stars, are excellent factories of antimatter in the Galaxy, in the form of pairs of electrons and positrons. Electrons are initially extracted from the surface of the star by the intense rotation induced electric fields and later transformed into electron-positron pairs through electromagnetic cascading. Observations of Pulsar Wind Nebulae (PWNe) show that cascades in the pulsar magnetosphere must ensure pair multiplicities of order $10^{4}-10^{5}$. These pairs finally end up as part of the relativistic magnetized wind emanating from the pulsar. The wind is slowed down, from its highly relativistic bulk motion, at a termination shock, which represents the reverse shock due to its interaction with the surrounding ejecta of the progenitor supernova. At the (relativistic) termination shock, acceleration of the pairs occurs, as part of the dissipation process, so that the cold wind is transformed into a plasma of relativistic non-thermal particles, plus a potent...

Blasi, Pasquale

2010-01-01

362

Lifetime of fluorescence from light-harvesting chlorophyll a/b proteins. Excitation intensity dependence.  

PubMed Central

The fluorescence from a purified, aggregate form of the light-harvesting chlorophyll a/b protein has a lifetime of 1.2 +/- 0.5 ns at low excitation intensity, but the lifetime decreases significantly when the intensity of the 20-ps, 530-nm excitation pulse is increased above about 10(16) photons/cm2. A solubilized, monomeric form of the protein, on the other hand, has a fluorescence lifetime of 3.1 +/- 0.3 ns independent of excitation intensity from 10(14)-10(18) photons/cm2/pulse. We interpret the lifetime shortening in the aggregates and the lack of shortening in monomers in terms of exciton annihilation, facilitated in the aggregate by the larger population of interacting chlorophylls. PMID:7025929

Nordlund, T M; Knox, W H

1981-01-01

363

B Lifetimes and Mixing  

SciTech Connect

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

Evans, Harold G.; /Indiana U.

2009-05-01

364

Stimulated positron emission for 3-D tomographic imaging and bone studies; Part I; method feasibility and system considerations  

SciTech Connect

The authors discuss the feasibility and inherent performance parameters of a new method of 3-D tomographic imaging studied analytically. A cross section in the patient's body is excited by high energy X-rays to produce positron-electron pairs. The resulting annihilation quanta are detected in coincidence by two detectors placed on opposite sides of the irradiated slice. Following a coincidence, the annihilation point is determined as the intersection of the line defined by the annihilation pair and the irradiated plane. Since the photon cross section for pair production interaction is proportional to the square of the atomic number of the absorber, the image thus formed will be sensitive to atomic number and density of tissues in the irradiated slice. This technique is unique among other tomographic imaging modalities in its direct 3-D imaging capability.

Benjamin, M. ((IL)); Macovski, A. (Stanford Univ., CA (USA). Dept. of Electrical Engineering)

1989-06-01

365

The ATLAS Positron Experiment -- APEX  

SciTech Connect

APEX -- the ATLAS Positron Experiment -- is designed to measure electrons and positrons emitted in heavy-ion collisions. Its scientific goal is to gain insight into the puzzling positron-line phenomena observed at the GSI Darmstadt. It is in operation at the ATLAS accelerator at Argonne National Lab. The assembly of the apparatus is finished and beginning 1993 the first positrons produced in heavy-ion collisions were observed. The first full scale experiment was carried out in December 1993, and the data are currently being analyzed. In this paper, the principles of operation are explained and a status report on the experiment is given.

Ahmad, I.; Back, B.B.; Betts, R.R.; Dunford, R.; Kutschera, W.; Rhein, M.D.; Schiffer, J.P.; Wilt, P.; Wuosmaa, A. [Argonne National Lab., IL (United States). Physics Div.; Austin, S.M.; Kashy, E.; Winfield, J.S.; Yurkon, J.E. [Michigan State Univ., East Lansing, MI (United States). NSCL; Bazin, D. [GANIL, Caen (France); Calaprice, F.P.; Young, A. [Princeton Univ., NJ (United States). Physics Dept.; Chan, K.C.; Chisti, A.; Chowhury, P.; Greenberg, J.S.; Kaloskamis, N.; Lister, C.J. [Yale Univ., New Haven, CT (United States). Wright Nuclear Structure Lab.; Fox, J.D.; Roa, E. [Florida State Univ., Tallahassee, FL (United States). Physics Dept.; Freedman, S.; Maier, M.R. [Lawrence Berkeley Lab., CA (United States); Freer, M. [Univ. of Birmingham (United Kingdom). Dept. of Physics; Gazes, S. [Univ. of Chicago, IL (United States). Dept. of Physics; Hallin, A.L.; Liu, M. [Queen`s Univ., Kingston, Ontario (Canada). Physics Dept.; Happ, T. [GSI, Darmstadt (Germany); Perera, A.; Wolfs, F.L.H. [Univ. of Rochester, NY (United States). NSRL; Trainor, T. [Univ. of Washington, Seattle, WA (United States). Nuclear Physics Lab.; Wolanski, M. [Argonne National Lab., IL (United States). Physics Div.]|[Univ. of Chicago, IL (United States). Dept. of Physics

1994-03-01

366

Measurement of the positron-electron reaction which produces a gamma ray pair in the region psi/3684/  

NASA Technical Reports Server (NTRS)

An experiment was conducted to measure gamma ray pairs produced in the electron-positron reaction in the region of psi(3684) resonance. The apparatus consisted of two identical spectrometers mounted in a collinear configuration about the beam interaction region. Graphs are presented for the coplanarity angle between reconstructed gamma ray directions, for the scattering and energy deposition of gamma rays in the two crystals used, and for the observed rate of annihilation reactions as a function of center of mass energy.

Hughes, E. B.; Beron, B. L.; Carrington, R. L.; Ford, R. L.; Hofstadter, R.; Liberman, A. D.; Martin, T. W.; Oneill, L. H.; Simpson, J. W.; Hilger, E.

1976-01-01

367

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

NASA Astrophysics Data System (ADS)

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

Strasburger, K.; Chojnacki, H.

1998-02-01

368

21 CFR 892.1110 - Positron camera.  

...Positron camera. (a) Identification. A positron camera is a device intended to image the distribution of positron-emitting radionuclides in the body. This generic type of device may include signal analysis and display equipment,...

2014-04-01

369

21 CFR 892.1110 - Positron camera.  

Code of Federal Regulations, 2011 CFR

...Positron camera. (a) Identification. A positron camera is a device intended to image the distribution of positron-emitting radionuclides in the body. This generic type of device may include signal analysis and display equipment,...

2011-04-01

370

21 CFR 892.1110 - Positron camera.  

Code of Federal Regulations, 2012 CFR

...Positron camera. (a) Identification. A positron camera is a device intended to image the distribution of positron-emitting radionuclides in the body. This generic type of device may include signal analysis and display equipment,...

2012-04-01

371

21 CFR 892.1110 - Positron camera.  

Code of Federal Regulations, 2013 CFR

...Positron camera. (a) Identification. A positron camera is a device intended to image the distribution of positron-emitting radionuclides in the body. This generic type of device may include signal analysis and display equipment,...

2013-04-01

372

21 CFR 892.1110 - Positron camera.  

Code of Federal Regulations, 2010 CFR

...Positron camera. (a) Identification. A positron camera is a device intended to image the distribution of positron-emitting radionuclides in the body. This generic type of device may include signal analysis and display equipment,...

2010-04-01

373

Microscopic approaches to nucleon-antinucleon annihilation  

SciTech Connect

The problem of nucleon-antinucleon (N anti N) annihilation is discussed in terms of the underlying quark-gluon dynamics of QCD. It is shown how recently observed selection rules exert a strong constraint on the effective quark-antiquark (Q anti Q) creation/destruction operator which enters in the annihilation process. The data on two meson modes suggest that a Q anti Q operator with one gluon quantum numbers is inadequate, and that a strong coupling approach to the treatment of the gluonic degrees of freedom is more appropriate. 25 refs., 1 fig.

Dover, C.B.

1986-01-01

374

Concepts for the design of an antimatter annihilation rocket  

Microsoft Academic Search

Matter-antimatter annihilation is considered for spacecraft propulsion. Annihilation produces considerably more energy per unit mass of propellant than any other known means of energy production. An antimatter annihilation rocket requires several systems and components that are unique to its nature. Among these are an antimatter storage system, a means to extract the antimatter from storage, a system to transport the

D. L. Morgan Jr.

1982-01-01

375

Observation of e+e- Annihilation into the C=+1 Hadronic Final States rho0rho0 and varphirho0  

Microsoft Academic Search

We report the first observation of e+e- annihilation into states of positive C parity, namely, rho0rho0 and varphirho0. The two states are observed in the pi+pi-pi+pi- and K+K-pi+pi- final states, respectively, in a data sample of 225fb-1 collected by the BABAR experiment at the Positron-Electron Project II e+e- storage rings at energies near s=10.58GeV. The distributions of cosf theta*, where

B. Aubert; R. Barate; M. Bona; D. Boutigny; F. Couderc; Y. Karyotakis; J. P. Lees; V. Poireau; V. Tisserand; A. Zghiche; E. Grauges; A. Palano; J. C. Chen; N. D. Qi; G. Rong; P. Wang; Y. S. Zhu; G. Eigen; I. Ofte; B. Stugu; G. S. Abrams; M. Battaglia; D. N. Brown; J. Button-Shafer; R. N. Cahn; E. Charles; M. S. Gill; Y. Groysman; R. G. Jacobsen; J. A. Kadyk; L. T. Kerth; Yu. G. Kolomensky; G. Kukartsev; G. Lynch; L. M. Mir; P. J. Oddone; T. J. Orimoto; M. Pripstein; N. A. Roe; M. T. Ronan; W. A. Wenzel; P. Del Amo Sanchez; M. Barrett; K. E. Ford; T. J. Harrison; A. J. Hart; C. M. Hawkes; S. E. Morgan; A. T. Watson; K. Goetzen; T. Held; H. Koch; B. Lewandowski; M. Pelizaeus; K. Peters; T. Schroeder; M. Steinke; J. T. Boyd; J. P. Burke; W. N. Cottingham; D. Walker; T. Cuhadar-Donszelmann; B. G. Fulsom; C. Hearty; N. S. Knecht; T. S. Mattison; J. A. McKenna; A. Khan; P. Kyberd; M. Saleem; D. J. Sherwood; L. Teodorescu; V. E. Blinov; A. D. Bukin; V. P. Druzhinin; V. B. Golubev; A. P. Onuchin; S. I. Serednyakov; Yu. I. Skovpen; E. P. Solodov; K. Yu Todyshev; D. S. Best; M. Bondioli; M. Bruinsma; M. Chao; S. Curry; I. Eschrich; D. Kirkby; A. J. Lankford; P. Lund; M. Mandelkern; R. K. Mommsen; W. Roethel; D. P. Stoker; S. Abachi; C. Buchanan; S. D. Foulkes; J. W. Gary; O. Long; B. C. Shen; K. Wang; L. Zhang; H. K. Hadavand; E. J. Hill; H. P. Paar; S. Rahatlou; V. Sharma; J. W. Berryhill; C. Campagnari; A. Cunha; B. Dahmes; T. M. Hong; D. Kovalskyi; J. D. Richman; T. W. Beck; A. M. Eisner; C. J. Flacco; C. A. Heusch; J. Kroseberg; W. S. Lockman; G. Nesom; T. Schalk; B. A. Schumm; A. Seiden; P. Spradlin; D. C. Williams; M. G. Wilson; J. Albert; E. Chen; A. Dvoretskii; F. Fang; D. G. Hitlin; I. Narsky; T. Piatenko; F. C. Porter; A. Ryd; A. Samuel; G. Mancinelli; B. T. Meadows; M. D. Sokoloff; F. Blanc; P. C. Bloom; S. Chen; W. T. Ford; J. F. Hirschauer; A. Kreisel; U. Nauenberg; A. Olivas; W. O. Ruddick; J. G. Smith; K. A. Ulmer; S. R. Wagner; J. Zhang; A. Chen; E. A. Eckhart; A. Soffer; W. H. Toki; R. J. Wilson; F. Winklmeier; Q. Zeng; D. D. Altenburg; E. Feltresi; A. Hauke; H. Jasper; A. Petzold; B. Spaan; T. Brandt; V. Klose; H. M. Lacker; W. F. Mader; R. Nogowski; J. Schubert; K. R. Schubert; R. Schwierz; J. E. Sundermann; A. Volk; D. Bernard; G. R. Bonneaud; P. Grenier; E. Latour; Ch. Thiebaux; M. Verderi; D. J. Bard; P. J. Clark; W. Gradl; F. Muheim; S. Playfer; A. I. Robertson; Y. Xie; M. Andreotti; D. Bettoni; C. Bozzi; R. Calabrese; G. Cibinetto; E. Luppi; M. Negrini; A. Petrella; L. Piemontese; E. Prencipe; F. Anulli; R. Baldini-Ferroli; A. Calcaterra; R. de Sangro; G. Finocchiaro; S. Pacetti; P. Patteri; I. M. Peruzzi; M. Piccolo; M. Rama; A. Zallo; A. Buzzo; R. Capra; R. Contri; M. Lo Vetere; M. M. Macri; M. R. Monge; S. Passaggio; C. Patrignani; E. Robutti; A. Santroni; S. Tosi; G. Brandenburg; K. S. Chaisanguanthum; M. Morii; J. Wu; R. S. Dubitzky; J. Marks; S. Schenk; U. Uwer; W. Bhimji; D. A. Bowerman; P. D. Dauncey; U. Egede; R. L. Flack; J. A. Nash; M. B. Nikolich; W. Panduro Vazquez; X. Chai; M. J. Charles; U. Mallik; N. T. Meyer; V. Ziegler; J. Cochran; H. B. Crawley; L. Dong; V. Eyges; W. T. Meyer; S. Prell; E. I. Rosenberg; A. E. Rubin; A. V. Gritsan; M. Fritsch; G. Schott; N. Arnaud; M. Davier; G. Grosdidier; A. Höcker; F. Le Diberder; V. Lepeltier; A. M. Lutz; A. Oyanguren; S. Pruvot; S. Rodier; P. Roudeau; M. H. Schune; A. Stocchi; W. F. Wang; G. Wormser; C. H. Cheng; D. J. Lange; D. M. Wright; C. A. Chavez; I. J. Forster; J. R. Fry; E. Gabathuler; R. Gamet; K. A. George; D. E. Hutchcroft; D. J. Payne; K. C. Schofield; C. Touramanis; A. J. Bevan; F. di Lodovico; W. Menges; R. Sacco; G. Cowan; H. U. Flaecher; D. A. Hopkins; P. D. Jackson; T. R. McMahon; S. Ricciardi; F. Salvatore; A. C. Wren; C. L. Davis; J. Allison; N. R. Barlow; R. J. Barlow; Y. M. Chia; C. L. Edgar; G. D. Lafferty; M. T. Naisbit; J. C. Williams; J. I. Yi; C. Chen; W. D. Hulsbergen; A. Jawahery; C. K. Lae; D. A. Roberts; G. Simi; G. Blaylock; C. Dallapiccola; S. S. Hertzbach; X. Li; T. B. Moore; S. Saremi; H. Staengle; R. Cowan; G. Sciolla; S. J. Sekula; M. Spitznagel; F. Taylor; R. K. Yamamoto; H. Kim; P. M. Patel; S. H. Robertson; A. Lazzaro; V. Lombardo; F. Palombo; J. M. Bauer; L. Cremaldi; V. Eschenburg; R. Godang; R. Kroeger; D. A. Sanders; D. J. Summers; H. W. Zhao; S. Brunet; D. Côté; P. Taras; F. B. Viaud; H. Nicholson; N. Cavallo; G. de Nardo; F. Fabozzi; C. Gatto; L. Lista; D. Monorchio; P. Paolucci; D. Piccolo; C. Sciacca; M. Baak; G. Raven; H. L. Snoek; C. P. Jessop; J. M. Losecco; T. Allmendinger; G. Benelli; K. K. Gan; K. Honscheid; D. Hufnagel; H. Kagan; R. Kass; A. M. Rahimi; R. Ter-Antonyan; Q. K. Wong; N. L. Blount; J. Brau; R. Frey; O. Igonkina; M. Lu; C. T. Potter; R. Rahmat; N. B. Sinev; D. Strom; J. Strube; E. Torrence; F. Galeazzi; A. Gaz; M. Margoni

2006-01-01

376

Positron Emission Tomography (PET)  

SciTech Connect

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

Welch, M.J.

1990-01-01

377

Future directions in high energy electron-positron experimentation  

SciTech Connect

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

Trilling, G.H.

1988-09-01

378

Gamma ray lines from dark matter annihilation  

SciTech Connect

If direct annihilation of dark matter particles into a pair of photons occurs in the galactic halo, a narrow {gamma}-ray line can be discovered at future {gamma}-ray detectors sensitive to the GeV region. The signals predicted by different dark matter candidates are analyzed. 16 refs., 3 figs.

Giudice, G.F.

1989-08-01

379

Cosmic ray propagation time scales: lessons from radioactive nuclei and positron data  

E-print Network

We take a fresh look at high energy radioactive nuclei data reported in the 90's and at the positron data recently reported by PAMELA. Our aim is to study the model independent implications of these data for the propagation time scales of cosmic rays in the Galaxy. Considering radioactive nuclei, using decaying charge to decayed charge ratios -- the only directly relevant data available at relativistic energies -- we show that a rigidity independent residence time is consistent with observations. The data for all nuclei can be described by f_{s,i}=(t_i/100 Myr)^{0.7}, where f_{s,i} is the suppression of the flux due to decay and t_i is the observer frame lifetime for nucleus specie i. Considering positron measurements, we argue that the positron flux is consistent with a secondary origin. Comparing the positron data with radioactive nuclei at the same energy range, we derive an upper bound on the mean electromagnetic energy density traversed by the positrons, \\bar U_T<1.25 eV/cm^3 at a rigidity of R=40 GV. Charge ratio measurements within easy reach of the AMS-02 experiment, most notably a determination of the Cl/Ar ratio extending up to R\\sim100 GV, will constrain the energy dependence of the positron cooling time. Such constraints can be used to distinguish between different propagation scenarios, as well as to test the secondary origin hypothesis for the positrons in detail.

Kfir Blum

2010-10-14

380

Surface conditioning of synchrotron radiation source to improve beam lifetime  

SciTech Connect

Surface cleanliness is of prime importance to achieve the desirable vacuum condition for long beam lifetime in all synchrotron radiation sources. The Photon Factory at KEK resolved the severe gas load from synchrotron radiation by in-situ Ar glow discharging the whole storage ring. Sufficient experience had been gained in the past two years operation on the synchrotron radiation source, especially on the vuv ring of the National Synchrotron Light Source (NSLS), to allow us to decide to glow discharge the vuv ring. In hoping to obtain higher circulating current and to reveal the cause of lifetime limitation, a better understanding of the beam loss mechanism is needed. Photo desorption is the major gas load in all existing electron (positron) storage rings. It is generally believed that these photodesorbed species (neutral and charged) provide the major loss mechanism to lessen the beam lifetime. The interaction of these species to the beam give rise to the various loss mechanisms, such as multiple Coulomb scattering, single scattering, bremsstrahlung and ion trapping. All these mechanisms has been under intense study at NSLS. This report only describes the result of surface condition and beam lifetime.

Chou, T.S.

1985-01-01

381

Fluorescence lifetime distributions in proteins.  

PubMed Central

The fluorescence lifetime value of tryptophan residues varies by more than a factor of 100 in different proteins and is determined by several factors, which include solvent exposure and interactions with other elements of the protein matrix. Because of the variety of different elements that can alter the lifetime value and the sensitivity to the particular environment of the tryptophan residue, it is likely that non-unique lifetime values result in protein systems. The emission decay of most proteins can be satisfactorily described only using several exponential components. Here it is proposed that continuous lifetime distributions can better represent the observed decay. An approach based on protein dynamics is presented, which provides fluorescence lifetime distribution functions for single tryptophan residue proteins. First, lifetime distributions for proteins interconverting between two conformations, each characterized by a different lifetime value, are derived. The evolution of the lifetime values as a function of the interconversion rate is studied. In this case lifetime distributions can be obtained from a distribution of rates of interconversion between the two conformations. Second, the existence of a continuum of energy substates within a given conformation was considered. The occupation of a particular energy substate at a given temperature is proportional to the Boltzmann factor. The density of energy states of the potential well depends upon the width of the well, which determines the degree of freedom the residue can move in the conformational space. Lifetime distributions can be obtained by association of each energy substate with a different lifetime value and assuming that the average conformation can change as the energy of the substate is increased. Finally, lifetime distributions for proteins interconverting between two conformations, each characterized by a quasi-continuum of energy substates, are presented. The origin of negative components of the lifetime distribution is also discussed. In the companion paper that will follow (Alcala, J. R., E. Gratton, and F. J.Prendergast, 1987, Biophys. J., in press) lifetime distributions obtained here are used to fit experimental data. PMID:3580486

Alcala, J. R.; Gratton, E.; Prendergast, F. G.

1987-01-01

382

Fluorescence lifetime distributions in proteins.  

PubMed

The fluorescence lifetime value of tryptophan residues varies by more than a factor of 100 in different proteins and is determined by several factors, which include solvent exposure and interactions with other elements of the protein matrix. Because of the variety of different elements that can alter the lifetime value and the sensitivity to the particular environment of the tryptophan residue, it is likely that non-unique lifetime values result in protein systems. The emission decay of most proteins can be satisfactorily described only using several exponential components. Here it is proposed that continuous lifetime distributions can better represent the observed decay. An approach based on protein dynamics is presented, which provides fluorescence lifetime distribution functions for single tryptophan residue proteins. First, lifetime distributions for proteins interconverting between two conformations, each characterized by a different lifetime value, are derived. The evolution of the lifetime values as a function of the interconversion rate is studied. In this case lifetime distributions can be obtained from a distribution of rates of interconversion between the two conformations. Second, the existence of a continuum of energy substates within a given conformation was considered. The occupation of a particular energy substate at a given temperature is proportional to the Boltzmann factor. The density of energy states of the potential well depends upon the width of the well, which determines the degree of freedom the residue can move in the conformational space. Lifetime distributions can be obtained by association of each energy substate with a different lifetime value and assuming that the average conformation can change as the energy of the substate is increased. Finally, lifetime distributions for proteins interconverting between two conformations, each characterized by a quasi-continuum of energy substates, are presented. The origin of negative components of the lifetime distribution is also discussed. In the companion paper that will follow (Alcala, J. R., E. Gratton, and F. J.Prendergast, 1987, Biophys. J., in press) lifetime distributions obtained here are used to fit experimental data. PMID:3580486

Alcala, J R; Gratton, E; Prendergast, F G

1987-04-01

383

On the theory of Gamma Ray Amplification through Stimulated Annihilation Radiation (GRASAR)  

NASA Technical Reports Server (NTRS)

The theory of photon emission, absorption, and scattering in a relativistic plasma of positrons, electrons, and photon was studied. Expressions for the emissivities and absorption coefficients of pair annihilation, pair production, and Compton scattering are given and evaluated numerically. The conditions for negative absorption were investigated. In a system of photons and e(+) - e(-) pairs, an emission line at at approximately 0.43 MeV can be produced by grasar action provided that the pair chemical potential exceeds approximately 1 MeV. At a temperature of approximately 10 to the 9th power. This requires a pair density approximately 10 to the 30th power cm to the (-3) power a value much larger than the thermodynamic equilbrium pair density at this temperature. This emission line could account without a gravitational redshift for the observed lines at this energy from gamma ray bursts.

Ramaty, R.; Mckinley, J. M.; Jones, F. C.

1981-01-01

384

Cosmic Ray Positrons from Pulsars  

NASA Technical Reports Server (NTRS)

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

Harding, Alice K.

2010-01-01

385

Positron emission particle tracking using a modular positron camera  

NASA Astrophysics Data System (ADS)

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

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

2009-06-01

386

Electromagnetic cascades in the magnetosphere of a very young pulsar - A model for the positron production near the Galactic center  

NASA Technical Reports Server (NTRS)

A detailed model for positron production by a young pulsar is presented. It is shown that electromagnetic cascades can develop in a young pulsar's magnetosphere, and the model results are applied to the pulsar which is hypothesized to lie near the Galactic center. It is found that such a pulsar would be expected to produce relatively low energy electron-positron pairs with an efficiency rating high enough to explain the observed luminosity of the Galactic center annihilation line. Virtually all of the gamma ray continuum radiation produced in the cascades would be beamed along the magnetic poles of the neutron star, and therefore probably would not be observed from earth. Some observational predictions generated by the proposed model for the Galactic center positron source are given.

Mastichiadis, Apostolos; Brecher, Kenneth; Marscher, Alan P.

1987-01-01

387

Nambu--Goldstone Dark Matter and Cosmic Ray Electron and Positron Excess  

SciTech Connect

We propose a model of dark matter identified with a pseudo-Nambu-Goldstone boson in the dynamical supersymmetry breaking sector in a gauge mediation scenario. The dark matter particles annihilate via a below-threshold narrow resonance into a pair of R-axions each of which subsequently decays into a pair of light leptons. The Breit-Wigner enhancement explains the excess electron and positron fluxes reported in the recent cosmic ray experiments PAMELA, ATIC and PPB-BETS without postulating an overdensity in halo, and the limit on anti-proton flux from PAMELA is naturally evaded.

Ibe, Masahiro; /SLAC; Nakayama, Yu; /UC, Berkeley /LBL, Berkeley; Murayama, Hitoshi; /UC, Berkeley /LBL, Berkeley /Tokyo U., IPMU; Yanagida, Tsutomu T.; /Tokyo U. /Tokyo U., IPMU

2009-06-19

388

AMS-02 positron excess: New bounds on dark matter models and hint for primary electron spectrum hardening  

NASA Astrophysics Data System (ADS)

The data collected by ATIC, CREAM and PAMELA all display remarkable cosmic ray nuclei spectrum hardening above the magnetic rigidity ?240 GV. One natural speculation is that the primary electron spectrum also gets hardened (possibly at ?80 GV) and the hardening partly accounts for the electron/positron total spectrum excess discovered by ATIC, HESS and Fermi-LAT. If it is the case, the increasing behavior of the subsequent positron-to-electron ratio will get flattened and the spectrum hardening should be taken into account in the joint fit of the electron/positron data otherwise the inferred parameters will be biased. Our joint fits of the latest AMS-02 positron fraction data together with the PAMELA/Fermi-LAT electron/positron spectrum data suggest that the primary electron spectrum hardening is needed in most though not all modelings. The bounds on dark matter models have also been investigated. In the presence of spectrum hardening of primary electrons, the amount of dark-matter-originated electron/positron pairs needed in the modeling is smaller. Even with such a modification, the annihilation channel ????+?- has been tightly constrained by the Fermi-LAT Galactic diffuse emission data. The decay channel ???+?- is found to be viable.

Feng, Lei; Yang, Rui-Zhi; He, Hao-Ning; Dong, Tie-Kuang; Fan, Yi-Zhong; Chang, Jin

2014-01-01

389

Effect of positron range on spatial resolution.  

PubMed

The effect of beta+ range on spatial resolution of imaging systems employing the detection of 511-keV annihilation radiation was determined by measuring the variation in the line-spread functions (LSFs) of positron-emitting radionuclides of 64Cu, 11C, and 15O as compared with the 514-keV gamma-ray emitter 85Sr. These radionuclides have maximum beta+ energies of 0.656, 0.960, and 1.72 MeV, respectively. The LSFs were measured in a tissue-equivalent phantom with high-resolution (approximately 2.4 mm FWHM) and low-resolution (approximately 8.8 mm FWHM) straightbore collimators coupled to a NaI(Tl) detector. Theoretical LSFs for the beta+ ranges were also calculated and convolved with the 85Sr LSF to yield the predicted LSFs for 11C and 15O. The high-resolution study showed a 0% and 2.3% increase in the full-width half-maximum (FWHM) and full-width tenth-maximum (FWO.1M) for the low-energy beta+ of 64Cu and a 37% (FWHM) and 52% (FWO.1M) increase for the high energy beta+ of 15O as compared with 85Sr. However, when the system resolution was decreased to 8.8 mm FWHM, the 64Cu showed no change at FWHM or FWO.1M and the 15O showed a 2.3% (FWHM) and 7.8% (FWO.1M) relative to 85Sr. The predicted LSFs were in good agreement with the experimental. These data indicate that the effect of beta+ range on spatial resolution is minimal unless the beta+ energy is larger than or equal to 1.5 MeV and the system resolution is on the order of a few millimeters. PMID:1151485

Phelps, M E; Hoffman, E J; Huang, S C; Ter-Pogossian, M M

1975-07-01

390

Numerically Stable Calculations of Radiative Corrections to Bremsstrahlung in Electron-Positron Annihilation  

E-print Network

We discuss techniques for obtaining a numerically-stable evaluation of the fully differential cross section for the virtual photon correction to single hard photon bremsstrahlung in two different computational schemes. We also compare the role of finite mass corrections in these schemes.

S. A. Yost; B. F. L. Ward

2006-10-13

391

Measurements of charm meson production in 10.5 GeV electron-positron annihilations  

NASA Astrophysics Data System (ADS)

A study of charm meson production in e+ e- ?* --> cc¯ events at s = 10.5 GeV is presented. Included are measurements of the fragmentation distributions of D+s and D*+s , the vector to pseudoscalar production ratio for Ds, and the D*+ spin alignment. A description of the new trigger designed for the CLEO III detector is also provided.

Johnson, Edward Eric

392

A search for single electron production in electron positron annihilation at E = 29 GeV  

SciTech Connect

This thesis presents experimental results from the ASP detector which took data on e{sup +}e{sup -} interactions in the PEP storage ring at SLAC. Its design was particularly suitable for searching for production of supersymmetric particles. The motivations for and phenomenology of Supersymmetry are discussed. In particular, the production of a single supersymmetric electron ( selectron'', {tilde e}) in combination with a supersymmetric photon ( photino'', {tilde {gamma}}) would result in events in which a single electron and no other particles are observed in the detector at an e{sup +}e{sup -} collider such as PEP, provided the masses of these particles are not too large. Such events would also result from the production of a single supersymmetric W-boson ( wino'', {tilde W}) in combination with a supersymmetric neutrino ( sneutrino'', {tilde {nu}}). These processes make it possible to search for electrons and winos with masses greater than the beam energy. Observation of these unusual events would distinctly indicate the production of new particles. The ASP detector was designed to be hermetic and to provide efficient event reconstruction for low multiplicity events. The detector is described and its performance is evaluated; it is found to be well-suited to this study. The data sample collected with the detector was thoroughly analyzed for evidence of single-electron events. The various possible background processes are considered and Monte Carlo calculations of the distributions from single selectron and single wino production are presented. Using this information an efficient off-line event selection process was developed, and it is described in detail. 82 refs., 41 figs., 4 tabs.

Steele, T.R.

1989-09-01

393

Assessment of the fatigue transformation zone in bulk metallic glasses using positron annihilation spectroscopy  

E-print Network

where the crack grew at different stress intensities. The presence of the FTZ is independent crystalline metals and alloys. One property which has been perceived as a limitation for BMGs has been low with different sizes, often exhibiting well-defined medium range order MRO on a scale of 1­2 nm.11­14 Like

Gidley, David

394

Short and medium range order in two-component silica glasses by positron annihilation spectroscopy  

NASA Astrophysics Data System (ADS)

The dependence of chemical composition on the average sizes of subnanometer-scale intrinsic structural open spaces surrounded by glass random networks in two-component silica-based glasses was investigated systematically using positronium (Ps) confined in the open spaces. The average sizes of the open spaces for SiO2-B2O3 and SiO2-GeO2 glasses are only slightly dependent on the chemical compositions because the B2O3 and GeO2 are glass network formers that are incorporated into the glass network of the base SiO2. However, the open space sizes for all SiO2-R2O (R = Li, Na, K) glasses, where R2O is a glass network modifier that occupies the open spaces, decrease rapidly with an increase in the R2O concentration. Despite the large difference in the ionic radii of the alkali metal (R) atoms, the open space sizes decrease similarly for all the alkali metal atoms studied. This dependence of the chemical composition on the open space sizes in SiO2-R2O observed by Ps shows that the alkali metal atoms do not randomly occupy the structural open spaces, but filling of the open spaces by R2O proceeds selectively from the larger to the smaller open spaces as the R2O concentrations are increased.

Inoue, K.; Kataoka, H.; Nagai, Y.; Hasegawa, M.; Kobayashi, Y.

2014-05-01

395

DARK MATTER DECAY AND ANNIHILATION IN THE LOCAL UNIVERSE: CLUES FROM FERMI  

SciTech Connect

We present all-sky simulated Fermi maps of {gamma}-rays from dark matter (DM) decay and annihilation in the local universe. The DM distribution is obtained from a constrained cosmological simulation of the neighboring large-scale structure provided by the CLUES project. The DM fields of density and density squared are then taken as an input for the Fermi observation simulation tool to predict the {gamma}-ray photon counts that Fermi would detect in 5 years of an all-sky survey for given DM models. Signal-to-noise ratio (S/N) sky maps have also been obtained by adopting the current Galactic and isotropic diffuse background models released by the Fermi Collaboration. We point out the possibility for Fermi to detect a DM {gamma}-ray signal in local extragalactic structures. In particular, we conclude here that Fermi observations of nearby clusters (e.g., Virgo and Coma) and filaments are expected to give stronger constraints on decaying DM compared to previous studies. As an example, we find a significant S/N in DM models with a decay rate fitting the positron excess as measured by PAMELA. This is the first time that DM filaments are shown to be promising targets for indirect detection of DM. On the other hand, the prospects for detectability of annihilating DM in local extragalactic structures are less optimistic even with extreme cross-sections. We make the DM density and density squared maps publicly available online.

Cuesta, A. J.; Zandanel, F.; Prada, F. [Instituto de Astrofisica de Andalucia (CSIC), E-18008 Granada (Spain); Jeltema, T. E. [UCO/Lick Observatories, Santa Cruz, CA 95064 (United States); Profumo, S.; Primack, J. [Department of Physics, University of California, Santa Cruz, CA 95064 (United States); Yepes, G. [Universidad Autonoma de Madrid, Grupo de Astrofisica, E-28049 Madrid (Spain); Klypin, A. [Department of Astronomy, New Mexico State University, Las Cruces, NM 88003-0001 (United States); Hoffman, Y. [Racah Institute of Physics, Hebrew University, Jerusalem 91904 (Israel); Gottloeber, S. [Astrophysical Institute Potsdam, 14482 Potsdam (Germany); Sanchez-Conde, M. A. [Instituto de Astrofisica de Canarias, E-38200 La Laguna, Tenerife (Spain); Pfrommer, C., E-mail: ajcv@iaa.es, E-mail: fabio@iaa.es, E-mail: tesla@ucolick.org [Canadian Institute for Theoretical Astrophysics, Toronto, ON M5S 3H8 (Canada)

2011-01-01

396

Design of a Pulsed Flux Concentrator for the ILC Positron Source  

SciTech Connect

The Positron Source for the International Linear Collider requires an optical matching device after the target to increase the capture efficiency for positrons. Pulsed flux concentrators have been used by previous machines to improve the capture efficiency but the ILC has a 1 ms long pulse train which is too long for a standard flux concentrator. A pulsed flux concentrator with a 40 ms flat top was created for a hyperon experiment in 1965 which used liquid nitrogen cooling to reduce the resistance of the concentrating plates and extend the lifetime of the pulse. We report on a design for a 1 ms device based on this concept.

Gronberg, J; Abbott, R; Brown, C; Javedani, J; Piggott, W T; Clarke, J

2010-05-17

397

Sintering of zirconia-based nanomaterials studied by variable-energy slow-positron beam  

NASA Astrophysics Data System (ADS)

A variable-energy slow-positron beam was applied to the investigations of the tetragonal yttria-stabilised zirconia (YSZ), the YSZ co-doped with small amount of Cr2O3. The initial nanopowders exhibiting the mean particle size of ? 20 nm were prepared by co-precipitation technique. Prior the sintering, the nanopowders were calcined and compacted using a pressure of 500 MPa. The ordinary shape parameters of the Doppler-broadened annihilation peak and the relative positronium 3?-fractions were determined as functions of positron energy. The results are consistent with a remarkable sintering-induced grain growth and disappearance of porosity which is driven out from the sample interior toward a thin subsurface layer.

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

2014-04-01

398

Corrosion-related defects in Zircaloys: a preliminary study with slow positron beam  

NASA Astrophysics Data System (ADS)

Corrosion-related microstructure and defects in Zircaloy-4 and N18 alloys were investigated by variable energy positron annihilation spectroscopy. The specimens were corroded in 0.01mol/L LiOH aqueous solution at 360 °C/18.6 MPa and in super heated steam at 400 °C/10.3 MPa, respectively. Defect profiles were analyzed by measuring the S parameter as a function of incident positron energy from 0.25 to 27 keV. Results indicated that Zircaloys corroded in LiOH aqueous solution contained more defects in the oxide layer than that in superheated steam, which implies that formation of defects in oxide layer may relate to the effects of Li+ ions in corrosion solution.

Zhu, Z. J.; Yao, M. Y.; Xue, X. D.; Wu, Y. C.; Zhou, B. X.

2014-04-01

399

Cosmic positron excess: is the dark matter solution a good bet?  

E-print Network

The recent observation by the PAMELA satellite of a rising positron fraction up to $\\sim$ 100 GeV has triggered a considerable amount of putative interpretations in terms of dark matter (DM) annihilation or decay. Here, we make a critical reassessment of such a possibility, recalling the elementary conditions with respect to the standard astrophysical background that would make it likely, showing that they are not fulfilled. Likewise, we argue that, as now well accepted, DM would need somewhat contrived properties to contribute significantly to the observed positron signal, even when including e.g. clumpiness effects. This means that most of natural DM candidates arising in particle physics beyond the standard model are not expected to be observed in the cosmic antimatter spectrum, unfortunately. However, this does not prevent them from remaining excellent DM candidates, this only points towards the crucial need of developing much more complex detection strategies (multimessenger, multiwavelength, multiscale ...

Lavalle, Julien

2009-01-01

400

Annihilation in Charmless Nonleptonic B Decays  

SciTech Connect

We show that the leading contributions to annihilationamplitudes in nonleptonic B\\to M_1 M_2 decays (where M_1,2 are charmlessnon-isosinglet mesons) of order \\alpha_s(m_b)\\, \\Lambda/m_b are real andare determined by distribution functions that already occur in the lowestorder factorization theorem~;\\cite{Arnesen:2006vb}. A complexnonperturbative parameter from annihilation first appears at \\cal O[\\alpha_s2(\\sqrt\\Lambda m_b) \\Lambda/m_b]. "Chirally enhanced"contributions are also factorizable and real at lowest order. Thus,incalculable strong phases are suppressed in annihilation amplitudes,unless the \\alpha_s(\\sqrt\\Lambda m_b) expansion breaks down. Modeling thedistribution functions, we find that (11\\pm 9)\\ percent and (15\\pm 11)\\percent of the absolute values of the measured \\bar B0\\to K^-\\pi^+ andB^-\\to K^-K0 penguin amplitudes come from annihilation. This isconsistent with the expected size of power corrections.

Ligeti, Zoltan

2006-08-08

401

Constraints on Cosmological Dark Matter Annihilation from the Fermi-LAT Isotropic Diffuse Gamma-Ray Measurement  

E-print Network

The first published Fermi large area telescope (Fermi-LAT) measurement of the isotropic diffuse gamma-ray emission is in good agreement with a single power law, and is not showing any signature of a dominant contribution from dark matter sources in the energy range from 20 to 100 GeV. We use the absolute size and spectral shape of this measured flux to derive cross section limits on three types of generic dark matter candidates: annihilating into quarks, charged leptons and monochromatic photons. Predicted gamma-ray fluxes from annihilating dark matter are strongly affected by the underlying distribution of dark matter, and by using different available results of matter structure formation we assess these uncertainties. We also quantify how the dark matter constraints depend on the assumed conventional backgrounds and on the Universe's transparency to high-energy gamma-rays. In reasonable background and dark matter structure scenarios (but not in all scenarios we consider) it is possible to exclude models proposed to explain the excess of electrons and positrons measured by the Fermi-LAT and PAMELA experiments. Derived limits also start to probe cross sections expected from thermally produced relics (e.g. in minimal supersymmetry models) annihilating predominantly into quarks. For the monochromatic gamma-ray signature, the current measurement constrains only dark matter scenarios with very strong signals.

The Fermi-LAT collaboration; :; A. A. Abdo; M. Ackermann; M. Ajello; L. Baldini; J. Ballet; G. Barbiellini; D. Bastieri; K. Bechtol; R. Bellazzini; B. Berenji; R. D. Blandford; E. D. Bloom; E. Bonamente; A. W. Borgland; A. Bouvier; J. Bregeon; A. Brez; M. Brigida; P. Bruel; T. H. Burnett; S. Buson; G. A. Caliandro; R. A. Cameron; P. A. Caraveo; S. Carrigan; J. M. Casandjian; C. Cecchi; O. C. Elik; A. Chekhtman; C. C. Cheung; J. Chiang; S. Ciprini; R. Claus; J. Cohen-Tanugi; J. Conrad; S. Cutini; C. D. Dermer; A. de Angelis; F. de Palma; S. W. Digel; E. do Couto e Silva; P. S. Drell; R. Dubois; D. Dumora; Y. Edmonds; C. Farnier; C. Favuzzi; S. J. Fegan; W. B. Focke; P. Fortin; M. Frailis; Y. Fukazawa; P. Fusco; F. Gargano; D. Gasparrini; N. Gehrels; S. Germani; N. Giglietto; F. Giordano; T. Glanzman; G. Godfrey; J. E. Grove; L. Guillemot; S. Guiriec; M. Gustafsson; D. Hadasch; A. K. Harding; D. Horan; R. E. Hughes; A. S. Johnson; W. N. Johnson; T. Kamae; H. Katagiri; J. Kataoka; N. Kawai; M. Kerr; J. Knodlseder; M. Kuss; J. Lande; L. Latronico; M. Llena Garde; F. Longo; F. Loparco; B. Lott; M. N. Lovellette; P. Lubrano; A. Makeev; M. N. Mazziotta; J. E. McEnery; C. Meurer; P. F. Michelson; W. Mitthumsiri; T. Mizuno; C. Monte; M. E. Monzani; A. Morselli; I. V. Moskalenko; S. Murgia; P. L. Nolan; J. P. Norris; E. Nuss; T. Ohsugi; N. Omodei; E. Orlando; J. F. Ormes; D. Paneque; J. H. Panetta; D. Parent; V. Pelassa; M. Pepe; M. Pesce-Rollins; F. Piron; S. Raino; R. Rando; A. Reimer; O. Reimer; T. Reposeur; A. Y. Rodriguez; M. Roth; H. F. W. Sadrozinski; A. Sander; P. M. Saz Parkinson; J. D. Scargle; A. Sellerholm; C. Sgro; E. J. Siskind; P. D. Smith; G. Spandre; P. Spinelli; J. L. Starck; M. S. Strickman; D. J. Suson; H. Takahashi; T. Tanaka; J. B. Thayer; J. G. Thayer; D. F. Torres; Y. Uchiyama; T. L. Usher; V. Vasileiou; N. Vilchez; V. Vitale; A. P. Waite; P. Wang; B. L. Winer; K. S. Wood; T. Ylinen; G. Zaharijas; M. Ziegle

2010-02-24

402

Runtime verification of object lifetime specifications  

E-print Network

This thesis reports on the implementation of a runtime verification system for object lifetime specifications. This system is used to explore and evaluate the expressiveness object lifetime specifications. Object lifetime ...

Benjamin, Zev (Zev A.)

2009-01-01

403

Measurement of the tau lifetime  

SciTech Connect

If the tau lepton couples to the charged weak current with universal strength, its lifetime can be expressed in terms of the muon's lifetime, the ratio of the masses of the muon and the tau, and the tau's branching ratio into e anti nu/sub e/ nu/sub tau/ as tau/sub tau/ = tau/sub ..mu../ (m/sub ..mu..//m/sub tau/)/sup 5/ B(tau ..-->.. e anti nu/sub e/nu/sub tau/) = 2.8 +- 0.2 x 10/sup -13/ s. This paper describes the measurement of the tau lifetime made by the Mark II collaboration, using a new high precision drift chamber in contunction with the Mark II detector at PEP. The results of other tau lifetime measurements are summarized.

Jaros, J.A.

1982-10-01

404

On Sunspot and Starspot Lifetimes  

NASA Astrophysics Data System (ADS)

We consider the lifetimes of spots on the Sun and other stars from the standpoint of magnetic diffusion. While normal magnetic diffusivity predicts lifetimes of sunspots that are too large by at least two orders of magnitude, turbulent magnetic diffusivity accounts for both the functional form of the solar empirical spot-lifetime relation and for the observed sunspot lifetimes, provided that the relevant diffusion length is the supergranule size. Applying this relation to other stars, the value of turbulent diffusivity depends almost entirely on supergranule size, with very weak dependence on other variables such as magnetic field strength and density. Overall, the best observational data for other stars is consistent with the extension of the solar relation, provided that stellar supergranule sizes for some stars are significantly larger than they are on the Sun.

Bradshaw, S. J.; Hartigan, P.

2014-11-01

405

Lifetime Measurements in 120Xe  

E-print Network

Lifetimes for the lowest three transitions in the nucleus $^{120}$Xe have been measured using the Recoil Distance Technique. Our data indicate that the lifetime for the $2_{1}^{+} \\to 0_{1}^{+}$ transition is more than a factor of two lower than the previously adopted value and is in keeping with more recent measurements performed on this nucleus. The theoretical implications of this discrepancy and the possible reason for the erroneous earlier results are discussed. All measured lifetimes in $^{120}$Xe, as well as the systematics of the lifetimes of the 2$_{1}^{+}$ states in Xe isotopes, are compared with predictions of various models. The available data are best described by the Fermion Dynamic Symmetry Model (FDSM).

J. C. Walpe; B. F. Davis; S. Naguleswaran; W. Reviol; U. Garg; Xing-Wang Pan; Da Hsuan Feng; J. X. Saladin

1995-03-25

406

Vacancy-type defects in In{sub x}Ga{sub 1-x}N alloys probed using a monoenergetic positron beam  

SciTech Connect

Native defects in In{sub x}Ga{sub 1-x}N grown by plasma-assisted molecular beam epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured, and these were compared with results obtained using first-principles calculation. The defect concentration increased with increasing In composition x and reached the maximum at x = 0.44{approx}0.56. A clear correlation between the line-width of photoluminescence and the defect concentration was obtained. The major defect species detected by positron annihilation was identified as cation vacancies coupled with multiple nitrogen vacancies (V{sub N}s), and their introduction mechanism is discussed in terms of the strain energy due to bond-length/angle distortions and the suppression of the V{sub N} formation energy by neighboring In atoms.

Uedono, A.; Watanabe, T. [Division of Applied Physics, Faculty of Pure and Applied Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573 (Japan); Ishibashi, S. [Nanosystem Research Institute (NRI) 'RICS,' National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568 (Japan); Wang, X. Q.; Liu, S. T.; Chen, G.; Shen, B. [State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871 (China); Sang, L. W.; Sumiya, M. [Wide Bandgap Material Group, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044 (Japan)

2012-07-01

407

Concepts for the design of an antimatter annihilation rocket  

NASA Technical Reports Server (NTRS)

Matter-antimatter annihilation is considered for spacecraft propulsion. Annihilation produces considerably more energy per unit mass of propellant than any other known means of energy production. An antimatter annihilation rocket requires several systems and components that are unique to its nature. Among these are an antimatter storage system, a means to extract the antimatter from storage, a system to transport the antimatter to the rocket engine, and the engine wherein annihilation occurs and thrust is produced. Design concepts of these systems and components are presented and discussed.

Morgan, D. L., Jr.

1982-01-01

408

Radiative lifetimes of neutral erbium  

Microsoft Academic Search

Radiative lifetimes have been measured for 123 levels of neutral erbium using time-resolved laser-induced fluorescence on a slow beam of erbium atoms. Of the 123 levels, 56 are even parity and range in energy from 26 993 to 40 440 cm?1 and 67 are odd parity ranging from 16 070 to 38 401 cm?1. This set of Er i lifetimes

E A Den Hartog; J P Chisholm; J E Lawler

2010-01-01

409

Radiative lifetimes of neutral erbium  

Microsoft Academic Search

Radiative lifetimes have been measured for 123 levels of neutral erbium using time-resolved laser-induced fluorescence on a slow beam of erbium atoms. Of the 123 levels, 56 are even parity and range in energy from 26 993 to 40 440 cm-1 and 67 are odd parity ranging from 16 070 to 38 401 cm-1. This set of Er i lifetimes

E. A. Den Hartog; J. P. Chisholm; J. E. Lawler

2010-01-01

410

Radiative lifetimes of neutral cerium  

Microsoft Academic Search

Radiative lifetimes, accurate to ±5%, have been measured for 153 levels of neutral cerium using time-resolved laser-induced fluorescence (TRLIF) on a slow beam of cerium atoms. Of the 153 levels studied, 150 are even parity and 3 are odd parity. The levels range in energy from 16 869 to 28 557 cm-1. This set of Ce I lifetimes is much

E. A. Den Hartog; K. P. Buettner; J. E. Lawler

2009-01-01

411

Radiative lifetimes of neutral cerium  

Microsoft Academic Search

Radiative lifetimes, accurate to ±5%, have been measured for 153 levels of neutral cerium using time-resolved laser-induced fluorescence (TRLIF) on a slow beam of cerium atoms. Of the 153 levels studied, 150 are even parity and 3 are odd parity. The levels range in energy from 16 869 to 28 557 cm?1. This set of Ce I lifetimes is much

E A Den Hartog; K P Buettner; J E Lawler

2009-01-01

412

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

NASA Astrophysics Data System (ADS)

The development of a positron annihilation spectroscopy laboratory at the HH-NIPNE Bucharest-to be used for material studies and applications was started in the last 10 years. In the framework of a national research project extended over the last 3 years, was designed a low energy positron accelerator, as a high-vacuum dedicated beam line with two options: a 25 mCi 22NaCl source and in line with the NIPNE-cyclotron or a new intense compact cyclotron. The construction of the beam line was planned as a sequence of modules: source- moderator system; magnetical filter for fast positrons in order to select the positrons energies in the range 0.8-1 keV; a modular system for focusing, transport and acceleration of monoenergetic positrons in the energy range 0.8-50 keV and a CDBS analysis chamber. The moderator proposed-is tungsten as a foil of about 3 ?m prepared at the Optoelectronics Institute were put into a thermal treatment vacuum chamber and bombarded with electrons from a 100 W electron gun After the treatment, they were tested for changes of elemental composition of the surface and structure at the Polytechnic University. The structure tests were performed on a DRON 3 M diffractometer, with a Co tube (?K? = 1.7903 A)-the angular regions studied were around 34° (1 0 0) and 69° (2 0 0). In the present time, the trajectories of the positron are going to be simulated with dedicated software (an ion and electron optics simulator). For the coincidence measurements (CDBS) set-up we used a home-made 22NaCl source, by separation without carrier from a metallic Mg target irradiated with 12 MeV protons and separated by columnar cation exchange. A home- made biparametric system for CDBS measurements will be reported, also.

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

2009-03-01

413

Status and Perspectives for a Slow Positron Beam Facility at the HH—NIPNE Bucharest  

NASA Astrophysics Data System (ADS)

The development of a positron annihilation spectroscopy laboratory at the HH-NIPNE Bucharest-to be used for material studies and applications was started in the last 10 years. In the framework of a national research project extended over the last 3 years, was designed a low energy positron accelerator, as a high-vacuum dedicated beam line with two options: a 25 mCi 22NaCl source and in line with the NIPNE-cyclotron or a new intense compact cyclotron. The construction of the beam line was planned as a sequence of modules: source- moderator system; magnetical filter for fast positrons in order to select the positrons energies in the range 0.8-1 keV; a modular system for focusing, transport and acceleration of monoenergetic positrons in the energy range 0.8-50 keV and a CDBS analysis chamber. The moderator proposed—is tungsten as a foil of about 3 ?m prepared at the Optoelectronics Institute were put into a thermal treatment vacuum chamber and bombarded with electrons from a 100 W electron gun After the treatment, they were tested for changes of elemental composition of the surface and structure at the Polytechnic University. The structure tests were performed on a DRON 3 M diffractometer, with a Co tube (?K? = 1.7903 A)—the angular regions studied were around 34° (1 0 0) and 69° (2 0 0). In the present time, the trajectories of the positron are going to be simulated with dedicated software (an ion and electron optics simulator). For the coincidence measurements (CDBS) set-up we used a home-made 22NaCl source, by separation without carrier from a metallic Mg target irradiated with 12 MeV protons and separated by columnar cation exchange. A home- made biparametric system for CDBS measurements will be reported, also.

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

2009-03-01

414

Triplet-Charge Annihilation versus Triplet-Triplet Annihilation in Organic Semiconductors  

SciTech Connect

A triplet can annihilate with a charge or a triplet, generating triplet-charge annihilation (TCA) or triplet-triplet annihilation (TTA) in organic semiconductors. On one hand, the TCA and TTA are critical issues to improve optoelectronic responses by using triplet states. On the other hand, the TCA and TTA are important spin-dependent processes to generate magneto-optoelectronic responses. Our experimental studies find that the TCA is a dominant process over TTA in organic semiconductors. Specifically, we separately confine triplets with charges or with triplets towards the generation of TCA and TTA by adjusting triplet density, charge confinement, and charge/exciton ratio based on organic light-emitting diodes. We then use magnetic field effects of electroluminescence (MFEEL), as an experimental tool, to study the generation of TCA and TTA. We observe that the electroluminescence can show negative response to applied magnetic field, generating a negative MFEEL, when triplets and charges are simultaneously confined within close proximity by using interfacial confinement with unbalanced charge/exciton ratio. On contrast, the electroluminescence only exhibits a positive MFEEL when triplets are confined within close proximity by using interfacial confinement without unbalanced charge/exciton ratio. Therefore, it can be concluded from our MFEEL results that the TCA is a major process to annihilate triplets over than TTA. Clearly, this experimental finding provides a new understanding on controlling triplets-related optoelectronic and magneto-optoelectronic processes in organic semiconductors.

Shao, Ming [ORNL; Yan, Liang [ORNL; Li, Mingxing [ORNL; Ivanov, Ilia N [ORNL; Hu, Bin [ORNL

2013-01-01

415

Positron-alkali atom scattering  

NASA Technical Reports Server (NTRS)

Positron-alkali atom scattering was recently investigated both theoretically and experimentally in the energy range from a few eV up to 100 eV. On the theoretical side calculations of the integrated elastic and excitation cross sections as well as total cross sections for Li, Na and K were based upon either the close-coupling method or the modified Glauber approximation. These theoretical results are in good agreement with experimental measurements of the total cross section for both Na and K. Resonance structures were also found in the L = 0, 1 and 2 partial waves for positron scattering from the alkalis. The structure of these resonances appears to be quite complex and, as expected, they occur in conjunction with the atomic excitation thresholds. Currently both theoretical and experimental work is in progress on positron-Rb scattering in the same energy range.

Mceachran, R. P.; Horbatsch, M.; Stauffer, A. D.; Ward, S. J.

1990-01-01

416

Positron emitter labeled enzyme inhibitors  

SciTech Connect

This invention involves a new strategy for imaging and mapping enzyme activity in the living human and animal body using positron emitter-labeled suicide enzyme inactivators or inhibitors which become covalently bound to the enzyme as a result of enzymatic catalysis. Two such suicide inactivators for monoamine oxidase have been labeled with carbon-11 and used to map the enzyme subtypes in the living human and animal body using PET. By using positron emission tomography to image the distribution of radioactivity produced by the body penetrating radiation emitted by carbon-11, a map of functionally active monoamine oxidase activity is obtained. Clorgyline and L-deprenyl are suicide enzyme inhibitors and irreversibly inhibit monoamine oxidase. When these inhibitors are labeled with carbon-11 they provide selective probes for monoamine oxidase localization and reactivity in vivo using positron emission tomography.

Fowler, J.S.; MacGregor, R.R.; Wolf, A.P.; Langstrom, B.

1990-04-03

417

Bayesian reliability analysis for fuzzy lifetime data  

Microsoft Academic Search

Lifetime data are important in reliability analysis. Classical reliability estimation is based on precise lifetime data. It is usually assumed that observed lifetime data are precise real numbers. However, some collected lifetime data might be imprecise and are represented in the form of fuzzy numbers. Thus, it is necessary to generalize classical statistical estimation methods for real numbers to fuzzy

Hong-zhong Huang; Ming J. Zuo; Zhan-quan Sun

2006-01-01

418

High-resolution positron Q-value measurements and nuclear-structure studies far from the stability line. Progress report  

SciTech Connect

Extensive data analysis and theoretical analysis has been done to complete the extensive decay scheme investigation of /sup 206/ /sup 208/Fr and the level structures of /sup 206/ /sup 208/Rn. A final version of a journal article is presented in preprint form. Extensive Monte Carlo calculations have been made to correct the end point energies of positron spectra taken with intrinsic Ge detectors for annihilation radiation interferences. These calculations were tested using the decay of /sup 82/Sr which has previously measured positron branches. This technique was applied to the positron spectra collected at the on-line UNISOR isotope separator. The reactions used were /sup 60/Ni(/sup 20/Ne;p2n)/sup 77/Rb and /sup 60/Ni(/sup 20/Ne;pn)/sup 78/Rb. Values for 5, ..gamma..-..beta../sup +/ coincidence positron end point energies are given for the decay of /sup 77/Rb. The implied Q-value is 5.075 +- 0.010 MeV. A complete paper on the calculated corrections is presented. A flow chart of a more complete program which accounts for positrons scattering out of the detector and for bremsstralung radiation is also presented. End-point energies of four ..beta../sup +/ branches in /sup 77/Rb are given as well as a proposed energy level scheme of /sup 75/Kr based on ..gamma..-..gamma.. coincidence data taken at UNISOR.

Avignone, F.T. III.

1981-02-28

419

Photo-detectors for time of flight positron emission tomography (ToF-PET).  

PubMed

We present the most recent advances in photo-detector design employed in time of flight positron emission tomography (ToF-PET). PET is a molecular imaging modality that collects pairs of coincident (temporally correlated) annihilation photons emitted from the patient body. The annihilation photon detector typically comprises a scintillation crystal coupled to a fast photo-detector. ToF information provides better localization of the annihilation event along the line formed by each detector pair, resulting in an overall improvement in signal to noise ratio (SNR) of the reconstructed image. Apart from the demand for high luminosity and fast decay time of the scintillation crystal, proper design and selection of the photo-detector and methods for arrival time pick-off are a prerequisite for achieving excellent time resolution required for ToF-PET. We review the two types of photo-detectors used in ToF-PET: photomultiplier tubes (PMTs) and silicon photo-multipliers (SiPMs) with a special focus on SiPMs. PMID:22163482

Spanoudaki, Virginia Ch; Levin, Craig S

2010-01-01

420

Photo-Detectors for Time of Flight Positron Emission Tomography (ToF-PET)  

PubMed Central

We present the most recent advances in photo-detector design employed in time of flight positron emission tomography (ToF-PET). PET is a molecular imaging modality that collects pairs of coincident (temporally correlated) annihilation photons emitted from the patient body. The annihilation photon detector typically comprises a scintillation crystal coupled to a fast photo-detector. ToF information provides better localization of the annihilation event along the line formed by each detector pair, resulting in an overall improvement in signal to noise ratio (SNR) of the reconstructed image. Apart from the demand for high luminosity and fast decay time of the scintillation crystal, proper design and selection of the photo-detector and methods for arrival time pick-off are a prerequisite for achieving excellent time resolution required for ToF-PET. We review the two types of photo-detectors used in ToF-PET: photomultiplier tubes (PMTs) and silicon photo-multipliers (SiPMs) with a special focus on SiPMs. PMID:22163482

Spanoudaki, Virginia Ch.; Levin?, Craig S.

2010-01-01

First Page Previous Page 1 2 3 4 5 6 7 8 9 10