Showing papers on "Positronium published in 1983"
TL;DR: In this paper, the absolute cross sections for the formation of positronium in positron collisions with He, Ar, and ${\mathrm{H}}_{2}$ have been measured for incident positron energies up to 76.3 eV.
Abstract: Absolute cross sections for the formation of positronium in positron collisions with He, Ar, and ${\mathrm{H}}_{2}$ have been measured for incident positron energies up to 76.3 eV. The results are markedly different, in magnitude and energy dependence, from those of Charlton et al. The measurements in He and ${\mathrm{H}}_{2}$ agree well with distorted-wave and charge-exchange calculations.
123 citations
TL;DR: In this article, the positronium negative ion Ps(-, the system composed of two electrons and a positron, has been reinvestigated theoretically using a Hylleraas wave function with two nonlinear parameters.
Abstract: The positronium negative ion Ps(-), the system composed of two electrons and a positron, has been reinvestigated theoretically. Using a Hylleraas wave function with two nonlinear parameters and more than 200 linear terms, excellent values of binding energy and annihilation lifetime of the particle-stable 1Se ground state have been obtained. In addition, the question of stability of the 3Pe state discussed by Mills has been examined and it is agreed with his conclusion that the state is probably not stable against breakup into Ps(n = 2) + e(-). Improved limits on the critical 'positron' mass for binding the 3Pe state have also been obtained.
94 citations
TL;DR: A theoretical study of positronium negative ions is carried out in this article, where Hylleraas-type wavefunctions are employed to describe the system and an improved ground-state energy is reported.
Abstract: A theoretical study of positronium negative ions is carried out. Hylleraas-type wavefunctions are employed to describe the system and an improved ground-state energy is reported. In addition, the two-photon annihilation rate and the lifetime against two-photon annihilation are also calculated, as well as the two-photon angular correlation function.
62 citations
TL;DR: In this article, the positronium formation cross sections of all inert gases at energies up to 150 eV were measured and shown to have a pronounced structure in the cross sections in the form of secondary peaks and plateaus above the ionisation potentials.
Abstract: New measurements of positronium formation cross sections are reported for the inert gases at energies up to 150 eV. These data show pronounced structure in the cross sections of all gases in the form of secondary peaks and plateaus above the ionisation potentials.
61 citations
61 citations
TL;DR: In this article, a method for calculating radiative corrections to positronium decay is presented that is direct, and allows for a systematic extension applicable to the calculation of corrections beyond the first (o(..cap alpha..)) correction.
58 citations
TL;DR: In this article, the authors analyzed the free volume distribution of polycarbonate polyethersulphone and polysuloxide over the temperature range 260-370 K. The data were analyzed into three components, the longest lifetime component associated with the pick-off mechanism; a linear variation of that component with temperature was observed.
41 citations
TL;DR: In this article, positronium (Ps) forms in the ice and diffuses as a neutral particle until annihilation or escape from the surface, and the fraction of $o$-Ps leaving the surface reveals two distinct contributions to the total Ps-formation probability (both Ore-and spur-type) as well as the Ps diffusion coefficient, 0.09
Abstract: Crystalline and amorphous ice were studied with monoenergetic positrons with incident energies $0lEl5$ keV. Positronium (Ps) forms in the ice and diffuses as a neutral particle until annihilation or escape from the surface. Measurement of the fraction of $o$-Ps leaving the surface reveals two distinct contributions to the total Ps-formation probability (both Ore- and spur-type) as well as the Ps diffusion coefficient, 0.17\ifmmode\pm\else\textpm\fi{}0.09 ${\mathrm{cm}}^{2}$/sec in crystalline ice. In addition, positronium trapping is observed in sputtered (crystalline) ice and voids (\ensuremath{\gtrsim} 17 \AA{} diameter) in amorphous ice.
39 citations
TL;DR: In this article, the proton-electron-positron system has been treated within the frame of hyperspherical coordinates and the total binding energy obtained in this work isE0≃−0.128506 a.u.
Abstract: The proton-electron-positron system has been treated within the frame of hyperspherical coordinates. Adiabatic potentials and coupling matrix elements which depend on the electron-positron mean-square radius have been obtained forS-states. From these data we have calculated elastic positron-hydrogen scattering phase shifts. No stable ground state and no resonance below the threshold for positronium formation in the ground state has been found. Our investigation however predicts a resonance slightly below the threshold forH(n=2). Its total binding energy obtained in this work isE
0≃−0.128506 a.u.
35 citations
TL;DR: Free positron and ortho-positronium lifetimes have been measured, along with the positronium formation fractions, for the three molecular gases H2, CO2 and CH4 in the intermediate density range as discussed by the authors.
Abstract: Free positron and ortho-positronium lifetimes have been measured, along with the positronium formation fractions, for the three molecular gases H2, CO2 and CH4 in the intermediate density range. The measurements are compared with other existing data where possible and significant discrepancies between earlier results and those presented have been found. The complex behaviour observed for all three parameters has been discussed in terms of current models where applicable.
34 citations
TL;DR: In this article, an explicit form of the solution of the one-dimensional hyperspherical matrix equation corresponding to the three-body Coulomb problem is found and used to obtain Schroedinger and Faddeev bound state wavefunctions, correlation integrals, and probabilities of different states.
TL;DR: In this paper, the positronium-formation cross sections in positron-helium scattering have been calculated with the use of a distorted-wave polarizedorbital method from the threshold to 100 eV.
Abstract: The positronium-formation cross sections in positron-helium scattering have been calculated with the use of a distorted-wave polarized-orbital method from the threshold to 100 eV. The results with and without the matrix elements involving the distorted target wave functions are found to differ appreciably. The results of the first Born approximation are not expected to be correct even at the incident-positron energy 100 eV. The measured values at 20 eV are found to be less than (1/2) of the present predicted values. The sharp rise of the formation cross section within the ore-gap region as observed by Charlton et al. has also been noticed by us. The minimum in the differential cross section has been found at all energies as in the case of hydrogen atom.
TL;DR: In this article, the positron surface interactions with clean tungsten (111) single crystals were investigated and the observed yield was 41% for 1-keV incident beams, and high yields were expected for incident radioactive sources.
Abstract: We present new experimental results on positron interactions with clean tungsten (111) single crystals. The measured value of the positron work function is ${\ensuremath{\varphi}}_{+}=\ensuremath{-}2.59(10)$ eV, which implies a negative work function for positronium and its negative ion as well. The observed yield, which is defined as the fraction of slow positrons reemitted from the sample, is 41% for 1-keV incident beams, and high yields are expected for incident radioactive source ${\ensuremath{\beta}}^{+}$ because of the small ${\ensuremath{\beta}}^{+}$ penetration depth. The yield is limited by positronium formation and trapping into a surface state bound by ${E}_{B}=2.81(10)$ eV. Positron surface interactions for W(111) appear similar to other clean crystalline metals with the exception of the broad energy distribution of emitted positrons which appears not to be caused by surface contamination. Difficulties associated with this energy width can apparently be overcome by growing thin epitaxial Cu films on the W(111) surface, as suggested by Lynn and Lutz [Phys. Rev. B 22, 4143 (1980)], and may result in an efficient, bright, slow-positron source.
TL;DR: The positronium-formation cross section in positron-hydrogen scattering has been calculated by using the distorted-wave approximation in which the total incident wave function is obtained by the method of polarized orbitals.
Abstract: The positronium-formation cross section in positron-hydrogen scattering has been calculated by using the distorted-wave approximation in which the total incident wave function is obtained by the method of polarized orbitals The two variants of the polarized-orbital method, one due to Temkin and Lamkin and the other due to Callaway et al, have been employed to estimate their relative importance The present results have been compared with those obtained by elaborate variational methods The results obtained by using the potential of Temkin and Lamkin have been found to be encouraging
01 Jan 1983
TL;DR: In this article, the authors report the details of the first experiment to excite optically active positronium from the n=1 triplet ground state to the 23S1 state.
Abstract: We report here the details of the first experiment to excite optically Ps from the n=1 triplet ground state to the 23S1 state[1]. The accuracy of our measurement of the 23S1 - 13S1 interval is approximately 0.8 ppm. Since the experiment was done with thermal positronium in vacuum, straightforward extension of the now proven technology should allow us to improve the accuracy of this measurement by several orders of magnitude.
TL;DR: In this paper, a model that displays the essential environmental effects that influence the self-annihilation rate and hyperfine coupling constant of positronium in condensed matter is described, which is exactly solvable, is based on harmonic forces.
Abstract: A model that displays the essential environmental effects that influence the self-annihilation rate and hyperfine coupling constant of positronium in condensed matter is described. The model, which is exactly solvable, is based on harmonic forces. Ways in which the model can be extended are discussed.
TL;DR: In this article, the authors calculated the rate for orthopositronium to decay to five photons using the lowest-order OOR decay rate, where OOR = (0.0189(11)).
Abstract: In this report we calculate GAMMA/sub 5/, the rate for orthopositronium to decay to five photons. We find that GAMMA/sub s/ = (0.0189(11))..cap alpha../sup 2/GAMMA/sub LO/, where GAMMA/sub LO/ is the lowest-order orthopositronium decay rate. The relevance of our result to the experimental situation is discussed.
TL;DR: In this article, temperature dependence of the momentum distribution of positronium in KCl single crystals has been investigated in a temperature range from 5 K to 300 K, where the distribution is sharp at low temperatures indicating that the positronium is in a delocalized state, but it is rapidly broadened when the specimen is heated up through a narrow temperature range around 65 K indicating the localization of the positronitum at sufficiently high temperatures.
Abstract: Temperature dependence of the momentum distribution of positronium in KCl single crystals has been investigated in a temperature range from 5 K to 300 K. The distribution is sharp at low temperatures indicating that the positronium is in a delocalized state, but it is rapidly broadened when the specimen is heated up through a narrow temperature range around 65 K indicating the localization of the positronitum at sufficiently high temperatures. Such a phenomenon is similar to that previously observed for NaF though the temperature range for the broadening is much lower and narrower than that for NaF. A model proposed for NaF which assumes meta-stable self-trapping of positronium is found to explain the present data fairly well; the analysis suggests that the energy level of the possible self-trapped state is located about 0.05 eV higher than the bottom of the delocalized band.
TL;DR: In this article, the positronium formation was investigated as a function of a static electric field in various polymers and the experimental results can be well framed by a compound model which assumes that positron formation occurs partly in the Ore gap and partly in a positron spur.
Abstract: The positronium formation was investigated as a function of a static electric field in various polymers. With increasing field strength the Ps relative yield at first declines, passes through a minimum which can be very flat, then rises rapidly, exceeds unity and approaches the saturation value. The experimental results can be well framed by a compound model which assumes that Ps formation occurs partly in the Ore gap and partly in the positron spur. This model enabled us to obtain mean values of the rates for positron elastic scattering and energy loss and for Ps formation above the threshold energy. The relative contribution of the two processes to Ps formation at zero field is also obtained together with the mean distance between positron and electron in the spur.
TL;DR: In this article, the elastic and vibrational excitation cross-sections for the positronium-H2 molecule scattering were investigated in the context of multichannel scattering theory for identical particles.
Abstract: A detailed account is reported of a perturbative calculation of the elastic and vibrational excitation cross-sections for the positronium-H2 molecule scattering. The problem is studied in the context of the multichannel scattering theory for identical particles; the first-order exchange amplitude and the second-order direct amplitude are evaluated for energy from zero to the positronium excitation threshold. The lowest-order Pade approximant is also constructed and this suggests the occurrence of a strong enhancement in the vibrational cross-section and in the related energy loss in matter at the positronium excitation threshold. In comparison with a short preliminary account presented elsewhere, various improvements are introduced both in the physical approximations and in the numerical calculations.
Journal Article•
TL;DR: In this paper, the authors describe the formation of structures of type clathrate in solutions diluees avec THF, in which the THF tendance de l'eau a s'inclure dans des complexes THF-H 2 O 2 O.
Abstract: Formation de structures de type clathrate en solutions diluees avec THF. Mesures par celerite ultrasonore et durees de vie moyenne de Ps. En solutions plus concentrees de THF tendance de l'eau a s'inclure dans des complexes THF-H 2 O
TL;DR: In this article, a monolayer of 4 He adsorbed on the powder surfaces was found to reduce the quenching rate of orthopositronium in the void regions of fine SiO 2 powder.
TL;DR: In this article, the enhancement of the positronium formation in polyethylene on the application of an electric field above 0.35 MV cm-1, measured by Bisi et al., is discussed in terms of the spur model of positron formation.
TL;DR: In this paper, the transition probability amplitudes for absorption of two photons in positronium are computed from the 1 /sup 3/S/sub 1/ ground state to the excited states 2 /sup 2 S/sub S 1/S 1/ and 3 /sup S/S 2/S1/S sub 1/ as functions of the frequencies of the photons.
Abstract: The transition-probability amplitudes for absorption of two photons in positronium are computed from the 1 /sup 3/S/sub 1/ ground state to the excited states 2 /sup 3/S/sub 1/ and 3 /sup 3/S/sub 1/as functions of the frequencies of the photons. Sharp resonances are found in correspondence to the intermediate states and transparencies occur between each two resonances and also before the first resonance in the 1 /sup 3/S/sub 1hyphen/3 /sup 3/S/sub 1/ transition. The detailed structure of the transition probability in proximity of the intermediate /sup 3/P states reproduces the excited-state structure of positronium with inclusion of relativistic and radiative corrections.
TL;DR: In this paper, a new quantum mechanical formalism is considered for the e+-H system; it explicitly introduces into the scattering picture the positronium (Ps) formation and breakup channels.
Abstract: A new quantum mechanical formalism, is considered for the e+-H system; it explicitly introduces into the scattering picture the positronium (Ps) formation and breakup channels. In its lowest-order form the scattering T matrix of the theory (T(1)) contains two terms. The first of these (TB) is the conventional first Born approximation, while the second (TS), involving a non-local potential, can be interpreted as a screening of of the bare Coulomb interactions. Numerical applications seem to indicate that the new theory is characterised by much faster convergence properties than more conventional quantum mechanical treatments. In particular, the presence of the new Ts term, in the T(1) approximation, leads to differential and total cross sections comparable with distorted-wave results of the literature.
TL;DR: In this paper, the electron and positron work functions of submonolayer contaminated single crystal surfaces of Cr(100) in ultra high vacuum were measured by measuring the spectrum of slow positrons reemitted by the surface when it is bombarded with keV energy positrons.
TL;DR: In this paper, the Doppler broadened annihilation radiation and angular correlation technique was used for the detection of positronium (Ps) formation in cyclo hexane (CCl4) solutions.
TL;DR: In this article, the positron surface states on the three principal surfaces of Al and Cu were accurately reproduced and the anisotropy of the binding energies and the lateral diffusion constant were predicted.
Abstract: We report on the results of an atomistic, discrete-lattice calculation of positron surface states on the three principal surfaces of Al and Cu. We are able to (i) accurately reproduce the observed values and anisotropy of the binding energies, and (ii) predict the surface state life times. Furthermore, we calculate (iii) the positron lateral diffusion constant, and find it considerably enhanced over the bulk value. We also investigate (iv) the positron trapping at surface vacancies, and (v) the effect of ordered chemisorbed monolayers of oxygen. We find that the oxidation lowers the binding energy and makes the surface state unstable with respect to positronium emission on Al (100) and Al (111). Implications to surface studies are discussed.
TL;DR: In this paper, general scaling laws for matrix elements of r/sup n/, transition strengths, transition probabilities and other quantities of interest for hydrogen-like systems (hydrogen, its isoelectronic sequence, excitons, positronium, muonium, and other exotic atoms) in a magnetic field of arbitrary strength are presented.
Abstract: General scaling laws for matrix elements of r/sup n/, transition strengths, transition probabilities, and other quantities of interest for hydrogen-like systems (hydrogen, its isoelectronic sequence, excitons, positronium, muonium, and other exotic atoms) in a magnetic field of arbitrary strength are presented. Analytic approximations for these quantities for hydrogen (that is, Z = 1 and an infinitely massive nucleus) are given for a complete range of magnetic fields from B = 0 to Bapprox.10/sup 13/ gauss. The scaling laws can then be used to obtain numerical values of transition probabilities, etc., for any other hydrogen-like system in an arbitrary magnetic field.
01 Jan 1983
TL;DR: In this article, the authors observed positronium atoms generated in proton-carbon collisions at a proton energy 70 GeV, and obtained an estimate of the ratio rho of the number of A/sub 2e/ to the amount of gamma rays emitted in the same solid angle.
Abstract: We have observed ultrarelativistic positronium atoms generated in proton-carbon collisions at a proton energy 70 GeV. The principal source of the positronium is decays of ..pi../sup 0/ mesons produced in the target into a ..gamma.. ray and positronium (A/sub 2e/). We recorded 185 +- 30 positronium atoms and obtained an estimate of the ratio rho of the number of A/sub 2e/ to the number of ..gamma.. rays emitted in the same solid angle.