Showing papers on "Positronium published in 1995"
TL;DR: In this paper, the polar gap model for pulsars was extended for the case where the surface magnetic field, Bs, of the neutron star is strong, and the stability of such bound pairs against ionization by the parallel electric field, Ell' in the polar cap, and against photoionization was discussed.
Abstract: Modifications to polar-gap models for pulsars are discussed for the case where the surface magnetic field, Bs, of the neutron star is strong. For B ~ 4 X 108 T, the curvature ,-quanta emitted tangentially to the curved force lines of the magnetic field are captured near the threshold of bound pair creation and are channelled along the magnetic field as bound electron-positron pairs (positronium). The stability of such bound pairs against ionization by the parallel electric field,. Ell' in the polar cap, and against photoionization is discussed. Unlike free pairs, bound pairs do not screen Ell near the neutron star. As a consequence, the energy flux in highly relativistic particles and high-frequency (X-ray and/or ,-ray) radiation from the polar gaps can be much greater than in the absence of positronium formation. We discuss this enhancement for (a) Arons-type models, in which particles flow freely from the surface, and find any enhancement to be modest, and (b) Ruderman-Sutherland-type models, in which particles are tightly bound to the surface, and find that the enhancement can be substantial. In the latter case we argue for a self-consistent model in which partial screening of Ell maintains it close to the threshold value for field ionization of the bound pairs, and in which a reverse flux of accelerated particles maintains the polar cap at a temperature such that thermionic emission supplies the particles needed for this screening. This model applies only in a restricted range of periods, P2 Pl.
106 citations
TL;DR: In this article, free-volume hole sizes, fractions, and distributions in a series of polycarbonate-based polymers are measured by using positron annihilation lifetime (PAL) spectroscopy.
Abstract: Free-volume hole sizes, fractions, and distributions in a series of polycarbonate-based polymers are measured by using positron annihilation lifetime (PAL) spectroscopy. Correlations between the obtained free-volume hole properties and gas diffusion coefficients are observed. © 1995 John Wiley & Sons, Inc.
79 citations
KEK1
TL;DR: 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.
Abstract: Poly(aryl-ether-ether ketone) (PEEK) films irradiated with 1-MeV and 2-MeV ${\mathrm{O}}^{+}$ ions were exposed to positron beams to measure the positron annihilation Doppler broadening as a function of the positron energy. The annihilation lines recorded at relatively low positron energies were found to become broader with increasing irradiation dose, suggesting that positronium (Ps) formation is inhibited in the damaged regions. The positron data were compared with the results of dynamic hardness and electron-spin-resonance measurements. The slow-positron Doppler broadening technique is found to be a useful means for damage-depth profiling of Ps-forming polymers.
75 citations
TL;DR: In this article, the authors reported results for positron scattering by ground-state atomic hydrogen in the energy range 0 to 80 eV. The calculations have been performed in an 18-state Ps(1s, 2s, 3s, 4s) approximation where the pseudostates (denoted by a bar) have been taken from Fon et al. (1981).
Abstract: We report results for positron scattering by ground-state atomic hydrogen in the energy range 0 to 80 eV. The calculations have been performed in an 18-state Ps(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d)+H(1s, 2s, 3s, 4s, 2p, 3p, 4p, 3d, 4d) approximation where the pseudostates (denoted by a bar) have been taken from Fon et al. (1981). Cross sections are presented for elastic scattering, positronium formation, total scattering and ionization. The elastic scattering results are in good agreement with accurate variational numbers at low energies and with other sophisticated, but very different, theoretical approximations at higher energies. We estimate that the elastic cross section is now known to better than 10%. The cross section for positronium formation is dominated by capture into the 1s state and is in fairly good agreement with the measurements of Weber et al. (1994). The results for the total cross section are generally consistent with the upper and lower bounds of Zhou et al. (1994) but are a little larger than other theoretical estimates at the higher energies. There is also a general theoretical problem concerning the degree to which the total cross sections for electrons and positrons merge at energies above 31 eV. The calculated ionization cross section is in agreement with the measurements of Jones et al. (1993).
66 citations
TL;DR: In this article, the effects of positron irradiation on polypropylene (PP) and polyethylene (PE) have been studied using the positron source for positron annihilation (PA) experiments.
58 citations
TL;DR: Using many-body perturbation theory to examine the interaction of positrons with atoms, calculations predict positron {ital s}-type bound states with binding energies 0.87, 0.35, and 0.045 eV for Mg, Zn, Cd, and Hg atoms, respectively.
Abstract: We use many-body perturbation theory to examine the interaction of positrons with atoms Our calculations predict positron s-type bound states with binding energies 087, 023, 035, and 0045 eV for Mg, Zn, Cd, and Hg atoms, respectively This binding is due to the positron-atom polarization potential and virtual positronium formation A simple criterion is used to find other atoms that are likely to form bound states with positrons Among the most probable candidates are Ti, V, Cr, Mn, Zr, Nb, and Mo
58 citations
TL;DR: In this paper, the positron-hydrogen and positronium-proton scattering using the close coupling method are reported at energies below the ionization threshold, which gives integrated cross sections accurate to 10%.
Abstract: Calculations of low-energy positron-hydrogen and positronium-proton scattering using the close coupling method are reported at energies below the ionization threshold. The channel space included six hydrogen (1s, 2s, 2p, 3s, 3p and 3d) and six positronium (1s, 2s, 2p, 3s, 4s and 3p) states. Comparisons with more accurate calculations demonstrate that this model should give integrated cross sections accurate to 10%. Cross sections are computed with a fine energy mesh and a total of 11 resonances associated with the H(n=2) and Ps(n=2) thresholds have been identified and the resonance positions and widths determined. Calculations of the total cross section and the positronium formation cross section for positron-hydrogen scattering are consistent with the available experimental data. Cross sections for elastic scattering in the Ps(1s)-p entrance channel are dominated by a strong polarization potential and vary from 1000 pi a02 at threshold to 15 pi a02 at 1.0 Ryd. Cross sections for the Ps(1s)+p to e++H(nl) rearrangement collision are also reported since these provide baseline data relevant to experiments aimed at making antihydrogen atoms. Differential cross sections for elastic Ps(1s)-p and e+-H(1s) scattering and for electron transfer to the hydrogen atom ground state are also reported at selected energies.
53 citations
TL;DR: In this paper, the results of a comprehensive positronium annihilation study of carbon dioxide in porous Vycor glass are presented, showing that on cooling, the gas-liquid phase boundary is raised by 5 K while the liquid-solid transition is depressed by 12 K relative to the bulk.
Abstract: The results of a comprehensive positronium annihilation study of the phase behaviour of carbon dioxide in porous Vycor glass are presented. Isobaric measurements of the 3 gamma :2 gamma annihilation ratio show that on cooling, the gas-liquid phase boundary is raised by ~5 K while the liquid-solid transition is depressed by ~12 K relative to the bulk. The resulting phase diagram suggests the existence of a `triple point` of the confined fluid at a temperature ~10 K and a pressure ~2 bar below the bulk triple point.
52 citations
TL;DR: In this article, the positron mobility in polyethylene samples (67.2% crystalline, glass transition temperatureTg=151 K) was determined in the 64-400 K temperature range by Doppler shift measurements.
Abstract: We have determined the positron mobility (μ+) in polyethylene samples (67.2% crystalline, glass transition temperatureTg=151 K) in the 64–400 K temperature range by Doppler shift measurements. A method based on the simulataneous observation of two γ lines from133Ba and137Cs radioactive sources together with the positron annihilation γ line, was employed to measure the Doppler shift of the 511 keV γ line as a function of the electric field applied to the samples. With this method we were able to measure at the same time the drift velocity of positrons and theS parameter. This parameter is very important in the interpretation of the mobility trend in samples where the positron states change with temperature. The positron mobility was corrected for positronium formation. μ+ at 64 K is 31.7±0.8 cm2 V−1 s−1 then decreases up to 123 K, increases at 148 K and decreases again up to 170 K (μ+=26.9±0.8 cm2 V−s−). This sharp change in mobility is centred around the glass transition temperature of our samples. Then the mobility remains almost constant up to 230 K. From 250 K to 377 K, μ+ increases and reaches the value of 38.4±1.0 cm2 V−1s−1. The corrected experimental data were well fitted by a simple model taking into account scattering and a thermally activated process (hopping mechanism).
32 citations
TL;DR: In this paper, an extension of weak orthogonality (WO) techniques developed by Szalewicz et al. and coupled pair equations derived by Jeziorski et al to positronium compounds are presented.
Abstract: Extension of weak orthogonality (WO) techniques developed by Szalewicz et al. and coupled pair equations derived by Jeziorski et al. to positronium compounds are presented. These methods enable us to calculate many‐electronic positronium compounds with explicitly correlated functions. These methods were applied to positronium hydride (PsH) with Hylleraas‐type functions (HTF’s), and the total energies, the positron‐electron two‐photon annihilation rates, and the positronium binding energies were calculated. Extended coupled pair equations were solved by CEPA(2)‐type, complete coupled pair (CCP) procedures, and independent pair approximation (IPA) of CCP. For the total energies, IPA, CEPA(2), and CCP procedures gave −0.78899, −0.78238, and −0.77471 au, respectively. For the two‐photon annihilation rates, the respective procedure gave 2.088, 2.064, and 1.972 ns−1, respectively.
31 citations
TL;DR: In this paper, a low energy positron-hydrogen scattering using the close coupling approach is reported at low energies, and elastic scattering and positronium formation cross sections in the Ore gap for the J = 0 and J = 1 partial waves are essentially identical with previous variational calculations.
Abstract: Calculations of low energy positron-hydrogen scattering using the close coupling approach are reported at low energies. The channel space includes nine physical hydrogen and positronium states and in addition twelve hydrogen and positronium pseudo-states. For energies below the positronium formation threshold, phase shifts are reported for J = 0 to 6 and are believed to have an absolute accuracy of 0·0015 radian or better. Elastic scattering and positronium formation cross sections in the Ore gap for the J = 0 and J = 1 partial waves are essentially identical with previous variational calculations. Total elastic and positronium formation cross sections are reported at incident energies below the ionisation threshold. Cross sections for the excitation of the H(n=2), H(n=3) and Ps( n=2) levels are also reported over a restricted energy range, and the total reaction cross section has been computed and compared with experiment.
TL;DR: In this paper, the authors present experimental results for impact ionization of molecular hydrogen by electrons and positrons for the range of impact energies from threshold to about 2 keV (0.4-2 keV for e-).
Abstract: We present experimental results for impact ionization of molecular hydrogen by electrons and positrons for the range of impact energies from threshold to about 2 keV (0.4-2 keV for e-). Our electron data agree with the most recent ones measured by others. When we compare our positron results to earlier published single ionization cross sections we find significant differences for impact energies from threshold to about 100 eV with the new cross sections being substantially smaller. In the present study an effort has been made to discriminate against false signals caused by positronium formation and other effects.
TL;DR: The peak cross section for antihydrogen formation from excited positronium targets is much larger than that from a ground-state positronia target at low incident energies.
Abstract: The cross sections for antihydrogen formation in the {ital n}=1, 2, 3, 4, 5, 6, and 7 levels from antiproton-positronium collisions are computed in the unitarized Born approximation (UBA). Twenty-seven physical states of antihydrogen (1{ital s}{r_arrow}7{ital h}) and ten physical states (1{ital s}{r_arrow}4{ital f}) of positronium are included in the UBA basis. The peak cross section for antihydrogen formation from excited positronium targets is much larger than that from a ground-state positronium target at low incident energies. The high-{ital n} antihydrogen levels make a significant contribution to the total antihydrogen formation cross section, especially for incident positronium atoms in the Ps({ital n}=3) and Ps({ital n}=4) levels.
TL;DR: In this article, a two channel version of the Kohn variational method is used with very flexible trial wavefunctions and elaborate helium target wave functions to obtain accurate values of the s-wave elastic and positronium-formation cross sections for positron-helium scattering in the Ore gap.
Abstract: A two channel version of the Kohn variational method is used with very flexible trial wavefunctions and elaborate helium target wavefunctions to obtain accurate values of the s-wave elastic and positronium-formation cross sections for positron-helium scattering in the Ore gap. The positronium-formation cross section displays a similar behaviour to that found in positron-hydrogen scattering, with a rapid rise from threshold followed by a slowly rising plateau at the higher energies. The small magnitude of this cross section is consistent with the findings of a recent analysis of the experimental results. Investigations are also made of the behaviour of the elastic-scattering cross section near to the positronium-formation threshold.
Journal Article•
TL;DR: The functional quantum field theory as discussed by the authors provides the possibility to derive the quantum dynamics of a positronium gas from Coulomb interacting electrons and positrons, by identifying the functional space with an infinite dimensional Euclidean Clifford algebra.
Abstract: The functional quantum field theory, developed by Stumpf, provides the possibility to derive the quantum dynamics of a positronium gas from Coulomb interacting electrons and positrons. By this example, the method will be brought in a Clifford algebraic light, through identifying the functional space with an infinite dimensional Euclidean Clifford algebra.
TL;DR: In this article, the second-order variational perturbation method with partial wave expansion was used to estimate the positronium binding energies for the ground state (2,1 S) of the ground states of the four classes of positron ionization (PsLi, PsF, and PsCl), together with the excited state ( 2,1 P) of PSF and PSCl.
TL;DR: In this article, a low energy positronium-proton scattering using the close coupling approach is reported at energies below the three-body breakup energy of 0·5 Rydberg.
Abstract: Calculations of low energy positronium-proton scattering using the close coupling approach are reported at energies below the three-body breakup energy of 0·5 Rydberg. The channel space includes nine physical hydrogen and positronium states and in addition twelve hydrogen and positronium pseudo-states. Total elastic and electron-transfer cross sections are reported at incident energies below the ionisation threshold. Cross sections for electron transfer to the H(n=2) and H(n=3) levels are also reported.
TL;DR: In this article, the hysteresis behavior of capillary confined CO2 gas is studied through the positron/positronium annihilation technique and the arrangement of the confining medium (VYCOR glass) allowed simultaneous measurement of the bulk and pore confined phase behaviour.
Abstract: The hysteresis behaviour of capillary confined CO2 gas is studied through the positron/positronium annihilation technique. The arrangement of the confining medium (VYCOR glass) allowed simultaneous measurement of the bulk and pore confined phase behaviour. Isobaric temperature cycling showed marked hysteresis at the gas-liquid and liquid-solid phase boundaries for the confined gas. Observations at the gas-liquid transition are in agreement with capillary condensation theories and simple ideas are proposed to explain the liquid-solid transition behaviour.
TL;DR: In this article, a brief review of the theory of the positron diffusivity, D+, in condensed matter with emphasis on the role of e+-polarons was given.
TL;DR: The dependence of the Doppler broadening of the 511 keV photon annihilation line of positrons annihilating in a variety of solid and liquids is studied as a function of the positron age in this paper.
TL;DR: In this paper, the authors used PALS to probe the micro-structural changes in the polymer polycarbonate in terms of the changes in free volume hole size and their content as a function of temperature.
Abstract: Positron Annihilation Lifetime Spectroscopy (PALS) is used to probe the micro-structural changes in the polymer polycarbonate in terms of the changes in free volume hole size and their content as a function of temperature. The measured spectra are best fitted to three lifetime component analysis. The average hole size in the amorphous regions is determined from the measured Ortho Positronium (O-Ps) lifetime τ3 by following the treatment of Nakanishi et al. On the other hand, information about defects in the crystalline regions is revealed by trapped positrons lifetime τ2. In this polymer the average hole radius varies from 2.71 A to 2.77 A. The present study indicates that this polymer has a glass transition temperature of 152°C. Further, we have calculated the trapping rates in the ordered and disordered regions of the polymer based on Goldanskii's kinetic equations and an attempt is made for the first time to estimate the activation energy in the amorphous and crystalline regions separately.
TL;DR: The origin of short-lived components in the annihilation lifetime spectrum of positronium (Ps) is due to fast Ps that is collisionally dissociating at the surfaces of the surrounding confinement cavity as discussed by the authors.
Abstract: The origin of short‐lived components in the annihilation lifetime spectrum of positronium (Ps) is shown to be due to fast Ps that is collisionally dissociating at the surfaces of the surrounding confinement cavity. The results are consistent with a model of fast (10–100 eV) Ps production by backscattered positrons from the incident beam. It is found that the typical lifetime of dissociating Ps scales with the mean free path of the cavity, and the relative formation intensity depends inversely on the incident positron beam energy. This ubiquitous effect will be present in any Ps formation experiment involving a free surface and can only be eliminated at beam energies less than 10 eV. More practical methods of minimizing the undesirable systematic effects of fast Ps quenching in depth‐profiled positron lifetime spectroscopy and in precision Ps decay rate measurements will be discussed.
TL;DR: In this paper, the Coulomb Born Approximation (CBA) was applied to the system e+ + H(2s) and e++ + He+(2s), respectively, with formation of positronium in the ground state.
Abstract: We study the formation of positronium through electron capture in collisions of positrons impacting on mono and dielectronic atoms (ions). We obtain the scattering amplitude of the charge exchange process by using the Coulomb Born Approximation (CBA). We apply the CBA approximation to the system e+ + H(2s) and e+ + He+(2s) with formation of positronium in the ground state. We also study the process e+ + He(1s2) with formation of positronium in 1s, 2s and 2p states analysing the influence of the non-captured electron on the reaction. We compute differential and total cross sections for these reactions. The energy range varies from some eV to hundreds of eV. We compare our results with other theories and recent experimental data.
TL;DR: In this paper, a hyperspherical close-coupling method is used to calculate the elastic, positronium formation and excitation cross sections for positron collisions with atomic hydrogen at energies below the H(n=4) threshold for L=0 and 1 partial waves.
Abstract: A hyperspherical close-coupling method is used to calculate the elastic, positronium formation and excitation cross sections for positron collisions with atomic hydrogen at energies below the H(n=4) threshold for L=0 and 1 partial waves. A new two-dimensional coordinate transformation is used such that the Schrodinger equation at large hyper-radii can be solved accurately. The coupled hyper-radial equations are integrated to a large hyper-radius at which the solution is matched to the dipole states in the outer region. From the extracted K-matrix, the elastic, positronium formation and atomic hydrogen excitation cross sections are computed. Resonance positions, total widths, and partial cross sections are also examined and compared with those from other calculations.
05 May 1995-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, progress in the study of ionization, including electron capture, by positron impact is discussed, in particular with respect to channel-coupling, near threshold and correlation effects.
Abstract: Recent results concerning the interaction of positrons with atoms and molecules are presented. In this context, progress in the study of ionization, including electron capture, by positron impact is discussed, in particular with respect to channel-coupling, near threshold and correlation effects. Novel advances in the field of atomic collisions involving positronium as a projectile are also reported.
TL;DR: In this article, the authors studied the annihilation characteristics of positrons in various types of diamonds by measurements of two-dimensional angular correlation of positron annihilation, those of Doppler Broadening profiles and those of lifetime spectra.
Abstract: Annihilation characteristics of positrons in various types of diamonds were studied by measurements of two‐dimensional angular correlation of positron annihilation, those of Doppler Broadening profiles and those of lifetime spectra. From the measurements, it was found that there is a small amount of defects in type Ib synthesized diamond and the positron lifetime is 115 ps. On the other hand, it was found that positronium is formed in natural diamonds. Results of the double x‐ray measurements suggested that IIa, IIb, and Ia type natural diamonds also may have empty space between crystallites or grain boundaries.
TL;DR: In this article, the authors present results for low-energy elastic S-, P-, and D-wave phase shifts, capture and total cross sections of positron-helium scattering with different basis sets in the close coupling approach using realistic wave functions for He(1s1s), He( 1s21p), and positronium (1s) states.
TL;DR: Positron annihilation lifetime (PAL) measurements on well-characterized pellets of Bi-2212, (Bi,Pb)-2212 and Bi-Pb-2223 superconductors indicate the presence of a very long (around 1500 ps) third lifetime.
TL;DR: In this article, the angular dependence of the electron transfer process Ar(e+, Ps) Ar+ from 0 degrees to 120 degrees with an angular resolution of +or-4 degrees was studied.
Abstract: In an e+-Ar crossed-beam experiment with 75 eV incident positron energy we studied the angular dependence of the electron-transfer process Ar(e+, Ps) Ar+ from 0 degrees to 120 degrees with an angular resolution of +or-4 degrees . For comparison we also studied the angular distribution of secondary electrons from the impact-ionization process Ar(e+, e-e+) Ar+. As anticipated, the angular distribution of the positronium is much more forward-peaked than that of the secondary electrons. For comparison of our relative Ps-formation cross section data with the theoretical angular dependence, we folded the theory of McAlinden and Waiters (1994) with the angular-resolution function of our experiment; the agreement is satisfactory. We also performed Ps measurements at 30 eV incident positron energy for which McAlinden and Waiters predicted a maximum of the differential cross section near 15 degrees . Folded with the angular-resolution function of the experiment, the theoretical curve retains a distinct minimum at 0 degrees which is not apparent in our data.
TL;DR: In this paper, a monoenergetic slow positron beam has been used for the first time to profile porous silicon films, which correspond to positron annihilation within the porous layers and these are attributed to the decay of para-positronium.
Abstract: A monoenergetic slow positron beam has been used for the first time to profile porous silicon films. High values of the Doppler‐broadened line shape parameters are observed, which correspond to positron annihilation within the porous layers and these are attributed to the decay of para‐positronium. After allowing for the reduced density of the porous film, fitted values of thickness were deduced which were in reasonable agreement with values obtained from ellipsometry measurements. Low values of the Doppler parameters observed for the two samples with the thinnest films are attributed to oxide residing at the interface of the porous and bulk silicon regions. Etching the samples in a solution of 48% hydrogen fluoride reduced the porous film thickness significantly, suggesting that a considerable amount of the film consists of SiO2. A four‐component convolution analysis routine is used to analyze the individual 511 keV annihilation peaks, the result of which confirms the formation of positronium within the porous layer. Positron annihilation is shown to be a promising method for the nondestructive investigation of thin porous films.