Showing papers on "Positronium published in 1985"
TL;DR: The current status of certain aspects of positron scattering in gases is reviewed in this article and a brief resume of the experimental techniques used in this field is also given, along with a detailed discussion of potential systematic errors which can affect such measurements.
Abstract: The current status of certain aspects of positron scattering in gases is reviewed. A brief resume of the experimental techniques used in this field is also given. Results for total scattering cross sections in a number of gases are presented along with a detailed discussion of potential systematic errors which can affect such measurements. Important features of the cross sections are pointed out and comparisons are made with electron scattering data. Results from experiments which go beyond total cross section determinations are discussed. Emphasis is placed upon recent measurements of positronium formation cross sections. Recent positron lifetime studies in dense gases are reviewed. The subjects covered include positron annihilation in clusters, ortho-positronium annihilation in bubbles and positronium formation in spurs.
132 citations
TL;DR: In this paper, the behavior of positrons in crystalline and amorphous ice has been studied with a beam of monoenergetic positrons with incident energies 0--4.5 keV.
Abstract: The behavior of positrons in crystalline and amorphous ice has been studied with a beam of monoenergetic positrons with incident energies 0--4.5 keV. Positronium (Ps) is formed in the bulk ice and diffuses until it annihilates or escapes from the surface. Measurements were carried out on the fraction of ortho-Ps leaving the surface and of the Doppler broadening of the 511-keV \ensuremath{\gamma} annihilation line. For incident energies 0--60 eV the Ps formation probability shows large variations. These variations are associated with Ps formation in the so-called Ore gaps and reflect the electronic structure of ice as demonstrated by Monte Carlo simulations of the positron slowing-down process. At higher energies, up to about 1 keV, the total Ps yield increases from about 50 to 75 %, which is attributed to Ps formation via spur processes. A large difference is found between the Ps diffusion coefficient in crystalline ice (about 0.2 ${\mathrm{cm}}^{2}$/sec) and in amorphous ice (roughly ${10}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{cm}}^{2}$/sec). From the red shift of the 511-keV annihilation line the Ps work function (affinity) in the crystalline ice is estimated to be -2\ifmmode\pm\else\textpm\fi{}1 eV. Evidence for low-energy-positron diffraction in the crystalline ice is found with scattered intensities higher than 25%. Sputtering of the crystalline ice creates surface damage which strongly reduces the yield of Ps escaping the surface. Cavities of average diameter larger than about 17 A\r{} are found in the as-grown amorphous ice. They anneal out at about 100 K, which is below the crystallization temperature of about 135 K.
60 citations
TL;DR: The needed normalizaton constant is obtained from a very precise and extensive Hylleraas wave function for the three-particle bound state from the bound-state wave function and a plane wave for the final-state waves.
Abstract: The photodetachment cross section of the negative positronium ion is calculated. The description of the initial bound state is simplified by representing it by an asymptotic form whose normalization comes from the most accurate Hylleraas wave function of the ion. The final state is assumed to be a plane wave.
55 citations
TL;DR: Using a high-intensity beam of 200-eV positrons the authors have measured the two-dimensional angular correlation of the 2..gamma.. annihilation radiation from a clean Al(100) surface.
Abstract: Using a high-intensity beam of 200-eV positrons the authors have measured the two-dimensional angular correlation of the 2..gamma.. annihilation radiation from a clean Al(100) surface. The momentum distribution identified with the positron surface state has a nearly isotropic conical shape and a (7.1 +/- 0.5)-mrad full width at half maximum. The data are not consistent with a simple interpretation based on either the usual model of a positron bound in a surface state by its image-correlation potential or a positronium atom weakly bound to the surface. 15 references, 3 figures.
52 citations
TL;DR: In this article, the curvature γ-quanta emitted in the pulsar magnetospheres tangentially to the curved lines of force of the magnetic field is shown to be canalized along the magnetic fields by gradually converting into a mutually bound electron-positron pair, i.e. a positronium atom.
Abstract: The curvature γ-quanta emitted in the pulsar magnetospheres tangentially to the curved lines of force of the magnetic field are shown to be later canalized along the magnetic field—if it is strong enough,B≳0.1Bcr=4×1012G—by gradually converting into a mutually bound electron-positron pair, i.e. a positronium atom. This happens before the photon reaches the threshold of free-pair creation. The positronium thus arising is stable against the ionizing action of the electric field near the pulsar unless it reaches a critical value about 4×107 CGSE forB≃1013G. This prevents the screening of the electric field up to the distances from the pulsar, where the magnetic field is already below the value of 0.1Bcrand the free pair creation may become essential. This effect provides, at least within the Arons model, a higher theoretical estimate for the total luminosity of pulsars whose field at the surfaceBsexceeds 0.1Bcras compared with the conventional one.
51 citations
TL;DR: In this paper, the d-wave contributions to the elastic scattering and positronium formation cross sections for positron scattering by atomic hydrogen in the Ore gap, 6.8-10.2 eV, were calculated using the Kohn variational method.
Abstract: Calculations, using the Kohn variational method, have been made of the d-wave contributions to the elastic scattering and positronium formation cross sections for positron scattering by atomic hydrogen in the Ore gap, 6.8-10.2 eV. Very elaborate trial functions were used and it is believed that the results have converged to within 10% of their exact values. These results have been combined with others by the present authors for s- and p-wave scattering in an investigation of the behaviour of the total elastic scattering cross section in the vicinity of the positronium formation threshold and, also, of the angular distribution of the positronium. The findings of both investigations suggest a possible explanation of the discrepancy between the experimental measurements of the positronium formation cross section by Charlton et al. (1983) and Fornari et al. (1983).
51 citations
TL;DR: In this paper, the authors discuss positron annihilation in the interstellar medium and show that a significant increase of the line intensity, sufficient to explain the observed spectrum, may arise from positron annihilations on grains of interstellar dust.
Abstract: HEAO 3 autumn 1979 ..gamma..-ray observations of the galactic center indicate that the 2..gamma.. 0.511 MeV annihilation line contains: as compared with the observed 3..gamma.. continuum, which is also emitted in the annihilation process: more than 2 times as many photons as can be accounted for by standard models, in which positron annihilation takes place in an ionized interstellar gas. I discuss positron annihilation in the interstellar medium and show that a significant increase of the line intensity, sufficient to explain the observed spectrum, may arise from positron annihilation on grains of interstellar dust. Dust, in spite of its small abundance, is bound to play a significant role in positron annihilation because its projected surface area, approx.10/sup -21/ cm/sup 2/ per gas atom, is three orders of magnitude more than the Thomson cross section, which determines relevant annihilation rates on ionized gas. Approximate analytic expressions for the rates of various annihilation processes occurring in the interstellar medium are given.
47 citations
TL;DR: This work reconsidered this process and finds that the captured ''photon'' is transformed into bound positronium, which can be ionized by the intense electric fields present in the polar cap region, or by thermal radiation from the neutron star surface, whereby the free electrons and positrons are regained that are urgently required for the models of pulsar radio emission.
Abstract: Recently it was pointed out by Shabad and Usov that in the vicinity of pulsars high-energy photons can be captured by the strong magnetic field without creating pairs. We have reconsidered this process and find that the captured "photon" is transformed into bound positronium. This positronium can be ionized by the intense electric fields present in the polar cap region, or by thermal radiation from the neutron star surface, whereby the free electrons and positrons are regained that are urgently required for the models of pulsar radio emission.
36 citations
TL;DR: The maximum yield was estimated to be ≈ 5.7 × 10 −2 excited Ps atoms per stopped positron of energy ≈ 16 eV in H 2, about 14 times greater than previous maximum yields.
32 citations
TL;DR: Positron annihilation and positronium formation have been studied in pure Kr and Xe gases and in mixtures of these gases with other inert and molecular species as discussed by the authors, which suggests that positrons annihilate after self-trapping in clusters in the gas.
Abstract: Positron annihilation and positronium formation have been studied in pure Kr and Xe gases and in mixtures of these gases with other inert and molecular species. Evidence is presented which, at least for Xe, suggests that positrons annihilate after self-trapping in clusters in the gas. Results are presented for the extra annihilation components visible in the low-density lifetime spectra and these are linked to the observed death of normal long-lived ortho-positronium.
31 citations
TL;DR: The first two-dimensional momentum measurements of annihilating positron-electron pairs at a solid surface at 18 keV or 740 eV with distinct asymmetry for the directions parallel and perpendicular to the surface are reported.
Abstract: We report the first two-dimensional momentum measurements of annihilating positron-electron pairs at a solid surface. Positrons of 18 keV or 740 eV impinged on a clean Cu(121) surface. The 740-eV spectrum was resolved into two components, one associated with energetic positronium emission displaced from zero momentum and a second centered on zero momentum with distinct asymmetry for the directions parallel and perpendicular to the surface. The 18-keV spectrum was dominated by positron Bloch-state annihilation.
TL;DR: In this paper, a series of isothermally crystallized polyethylene pairs with known concentrations of chain defects, having a lamellar structure, have been measured by small-angle and wide-angle x-ray scattering techniques.
Abstract: New positron lifetime data in a series of isothermally crystallized polyethylene pairs with known concentrations of chain defects, having a lamellar structure, have been measured. The materials were characterized by small‐angle and wide‐angle x‐ray scattering techniques. Positron lifetime data were computer analyzed and only three components could be resolved. It is shown that the annihilation mechanisms can be understood in terms of average distance between lamellar crystals (long period), crystal thickness, and chain defect concentration. The present results confirm, in consonance with the conclusions of other authors, the longest component to be due to ortho‐positronium pick‐off annihilation. The intermediate component may be ascribed to positronium trapped at the crystal‐amorphous interface. The application of a three‐state trapping model yields a lifetime of about 900 ps for this state. It is suggested that the shortest component may be due to free positron annihilation, para‐positronium self‐annihil...
TL;DR: In this article, the authors observed a statistically significant decrease in the high-energy (511 keV to about 3 MeV) luminosity of the Galactic center region during the fall of 1979 and the spring of 1980.
Abstract: The Galactic center region was observed with the HEAO 3 High Resolution Gamma-Ray Spectrometer during the fall of 1979 and the spring of 1980 Between these epochs there was observed (1) a statistically significant decrease in the high-energy (511 keV to about 3 MeV) luminosity, (2) a decrease in the positron annihilation line intensity, reported previousy, and (3) a low positronium annihilation fraction f = 038 + or 019 during the fall of 1979 If positrons are generated by photon-photon collisions of high-energy photons, then the absence of a detected flux above 511 keV in the spring of 1980 may indicate a time delay between positron production and annihilation 26 references
15 May 1985-Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms
TL;DR: In this paper, the positron beam is pulsed with 10 ns to 3 μs duration at rates up to 1440 pulses per second, with as many as 106 positrons available per pulse.
Abstract: 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 μs duration at rates up to 1440 pulses per second, with as many as 106 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.
TL;DR: The first-Born-approximation results of the 2p-excited-state capture cross section in the case of helium is reported for the first time and the first Born approximation is found to be unsuitable for prediction of the rearrangement processes.
Abstract: A distorted-wave model (Phys. Rev. A 27, 1904 (1983); 28, 2180 (1983)) is applied to calculate the formation of positronium in the n = 2 states in e/sup +/ scattering from hydrogen and helium atoms. The incident wave is represented by a polarized-orbital method. The first-Born-approximation results of the 2p-excited-state capture cross section in the case of helium is reported for the first time. The first Born approximation is found to be unsuitable for prediction of the rearrangement processes. The present total (ground- and excited-state) positronium-formation cross sections have been compared with the corresponding observed values of Fornari et al. (Phys. Rev. Lett. 51, 2276 (1983)) and of Charlton et al. (J. Phys. B 16, L465 (1983)).
TL;DR: The trapping model for the annihilation rate of positrons implanted in solids has been combined with a diffusionmodel for the spatial motion of the implanted positrons prior to annihilation.
Abstract: The trapping model for the annihilation rate of positrons implanted in solids has been combined with a diffusion model for the spatial motion of the implanted positrons prior to annihilation. Results are given for a simple system having traps only at the surface of a sample and allowing no detrapping, and for a general case where both effects are included. Numerical results are given for the example of positrons trapping in an image-potential well (as well as forming positronium) at a surface.
TL;DR: In this paper, the ionization cross section of hydrogen atoms by positron impact has been calculated by using a distorted-wave polarized-orbital method, and two models depending on the choice of the final-channel wave function were employed.
Abstract: The ionization cross section of hydrogen atoms by positron impact has been calculated by using a distorted-wave polarized-orbital method. We have employed two models depending on the choice of the final-channel wave function, and the two sets of results differ dramatically. We have also found that below 100 eV the total positron-impact ionization cross section including positronium formation is appreciably larger than the corresponding results for electron-impact ionization.
TL;DR: The positronium negative ion is treated adiabatically in hyperspherical coordinates and the approximate separation of the wave function has roughly the same validity as in H/sup -/.
Abstract: We treat the positronium negative ion adiabatically in hyperspherical coordinates. The potential curves of the ground state and of the first /sup 1/S doubly excited state are calculated variationally. The corresponding energy levels are in good agreement with previous calculations, showing that the approximate separation of the wave function in hyperspherical coordinates has roughly the same validity as in H/sup -/.
TL;DR: In this paper, free positron lifetimes and positronium formation fractions have been measured for CO2 and SF6 in the temperature range 273-350 K. The free particle lifetimes behave in a similar fashion to observations in other gases close to their critical temperatures and are interpreted as being due to the formation of molecular cluster around the positrons prior to annihilation.
TL;DR: In this paper, the interactions of positronium (Ps) with the paramagnetic Co/sub 2/..mu.. ion are investigated in water at various temperatures by both lifetime spectroscopy and the Doppler broadening of annihilation radiation line shape technique.
Abstract: The interactions of positronium (Ps) with the paramagnetic Co/sub 2/..mu.. ion are investigated in water at various temperatures by both lifetime spectroscopy and the Doppler broadening of annihilation radiation line shape technique. They are shown to arise essentially from spin conversion reactions. However, the consistent fitting of the various data indicates the presence of an additional process, ascribed to Ps complex formation. The relative contribution of the latter increases importantly with temperature while the spin conversion is almost temperature independent. This peculiar behavior is tentatively explained on the basis of weak spin interactions. 32 references, 5 figures, 2 tables.
TL;DR: An anomalously high magnetic quenching of positronium at low field (a few kG) has been revealed in certain polymeric materials as mentioned in this paper, and the existence in the same specimen of two Ps-like centres is suggested.
Abstract: An anomalously high magnetic quenching of positronium at low field (a few kG) has been revealed in certain polymeric materials. To interpret the experimental results, the existence in the same specimen of two Ps-like centres is suggested. The «normal» Ps-like centre is characterized by hyperfine splitting smaller by a factor not greater than 2 than that of free Ps. The «anomalous» one shows hyperfine splitting in an order of magnitude weaker than for free Ps.
TL;DR: Description de l'interaction d'attraction entre atomes comme une energie propre ou les effets de recul et de vitesse relative finie sont traites exactement.
Abstract: The attractive interaction between atoms is described as a self-energy in which the effects of recoil and finite relative velocity are treated exactly. For systems involving positronium significant deviations from the van der Waals potential are found due to both recoil and relative motion.
TL;DR: In this paper, positron annihilation lifetimes and Doppler-broadened annihilation lines have been measured in solid 2-aminopyridine (2-APY), 3-amino-polymorphic polymorphine (3APY) and 4-aminopyryl-polycyclic polycycline (4APYM) compounds.
TL;DR: In this paper, the positron transporters were divided into four distinct categories on the basis of their different positron-transport properties: electronic insulating solids, positronic as well as electronic insulators.
TL;DR: The van der Waals force between atoms and molecules is generally taken to be inversely proportional to the inverse sixth power of the distance, but positronium atoms may interact differently.
Abstract: The van der Waals force between atoms and molecules is generally taken to be inversely proportional to the inverse sixth power of the distance, but positronium atoms may interact differently.
TL;DR: In this article, deviations of the ortho-positronium annihilation rate from the linear dependence on density have been observed in N 2 gas at temperatures 100-170 K. The deviations observed at not so high densities can be reasonably explained by the simple density fluctuation model, but disagreement between this model and observed deviations is found at much higher densities.
TL;DR: In this paper, the change in the fraction of positrons forming positronium in an oscillating electric field is measured in n-pentane, n-hexane and n-decane.
TL;DR: In this article, a large perturbing effect of diamagnetic ions in aqueous solution on the positronium hyperfine splitting has been revealed through magnetic-quenching measurements.
Abstract: A large perturbing effect of diamagnetic ions in aqueous solution on the positronium hyperfine splitting has been revealed through magnetic-quenching measurements. The hyperfine splitting amounts to ΔW=0.57 ΔW
vac (at concentration ≥0.2 mol). This lowering, if interpreted on the basis of the model of Urbanovich, indicates that there exists a bound state of positronium in water and in aqueous solutions of diamagnetic ions.
TL;DR: In this paper, the lifetime and angular correlation curves contain an unusually narrow component, which is associated with positronium formation in the helium bubbles and indicates localisation of the Ps atom, possibly in a surface physisorbed state.
Abstract: Aluminium samples containing helium bubbles produced by 600 MeV proton irradiation at 430 degrees C were investigated by positron annihilation; both lifetime and angular correlation measurements were made. The angular correlation curves contain an unusually narrow component. This component is associated with positronium formation in the bubbles. The width of the component indicates localisation of the Ps atom, possibly in a surface physisorbed state. Sodium segregated at the surfaces of the bubbles is a likely cause of the positronium formation.
01 Jan 1985
TL;DR: In this article, the authors considered the correlation in the polarization of two photons emitted simultaneously from a common source and showed that the polarization correlation has only recently been observed at Stirling.
Abstract: This lecture considers the correlation in the polarization of two photons emitted simultaneously from a common source. The only two processes of this kind on which experiments have been carried out, are the annihilation of para-positronium, in which an electron and positron in a singlet state are converted into two identical photons each with energy 0.511 MeV, and the decay of metastable atomic hydrogen. The former process has been studied experimentally quite extensively and the results are well documented1,2 so we shall concentrate our attention on the decay of metastable atomic hydrogen, for which the polarization correlation has only recently been observed at Stirling3, to illustrate the main features of this type of process and its application to tests of so called “hidden variable” theories and Bell’s inequality. We shall not consider the extensive work4 which has been carried out using atomic cascades in which the two photons are not emitted simultaneously, although many of the conclusions we shall reach also apply to these cascades.