Showing papers on "Positronium published in 1982"

β Decay and the origins of biological chirality: theoretical results

Abstract: A dynamical mechanism is found whereby a dissymmetric molecule and its mirror image are ionized at different rates by longitudinally polarized electrons such as produced by nuclear β decay. An enhancement is predicted for molecules containing heavy atoms. Order-of-magnitude estimates indicate that the asymmetric effect of this mechanism may be detectable by current experiments on positronium formation.

Self-trapped states of positrons and positronium in dense gases in liquids

Abstract: In dense media positrons and positronium atoms are bound in self-trapped states. These states are density fluctuations stabilised by the light quantum particles. The positronium atoms are trapped in a bubble, while positrons are trapped in a cluster. Transitions to these states at changes of the density or temperature of the medium produce an essential effect on the annihilation rate. It can be considered as a local phase transition in the vicinity of the positron (or positronium). The present article is a review of the present state of the physics of positron and positronium self-trapped states in dense gases and liquids. Basic properties of the positronium bubbles and positron clusters are considered, as well as the conditions under which they exist and their effect on the annihilation rate. Experiments in which the self-trapping is considered to be closely related to other similar electronic phenomena, known for dense gases, liquids and other dense media. The main results from the study of positron and positronium interactions with isolated atoms are also presented. The density effects in moderately dense gases anticipating self-trapping are considered.

Beta decay and the origins of biological chirality - Experimental results

Abstract: Preliminary experimental results are presented of an investigation of the possible role of preferential radiolysis by electrons emitted in the beta decay of radionuclides, a parity-nonconserving process, in the universal causation of the optical activity of biological compounds. Experiments were designed to measure the asymmetry in the production of triplet positronium upon the bombardment of an amino acid powder target by a collimated beam of positrons as positron helicity or target chirality is reversed. No asymmetry down to a level of 0.0007 is found in experiments on the D and L forms of cystine and tryptophan, indicating an asymmetry in positronium formation cross section of less than 0.01, while an asymmetry of 0.0031 is found for leucine, corresponding to a formation cross section asymmetry of about 0.04

Positronium formation in s-wave positron–hydrogen scattering

Abstract: A two channel version of the Kohn variational method is used with trial functions containing up to 120 linear variational parameters to calculate the R-matrix elements and the positronium formation cross section for the scattering of s-wave positrons by atomic hydrogen. Results are obtained in the energy range between the positronium formation threshold and the lowest excitation threshold of the atom. The positronium formation cross section rises very steeply to a value of just above the positronium formation threshold and then rises approximately linearly with positron energy to a value of just below the excitation threshold. Owing to the weakness of the coupling between the positron and positronium channels, the elastic scattering cross section for the positron continues smoothly across the positronium formation threshold.

Quantum Electrodynamic Effects in Few-Electron Atomic Systems

Abstract: Publisher Summary The generally good agreement between theory and experiment has provided an important check on the computational techniques of quantum electrodynamics in the presence of strong fields. Equally important confirmations of quantum electrodynamics in the low-energy regime provided by atomic physics have been obtained from studies of the anomalous magnetic moments of the electron and muon and of the properties of positronium and muonium. The study of quantum electrodynamic effects in one-electron systems has now reached a high state of refinement, both theoretically and experimentally. This chapter briefly summarizes the material contained in the older reviews and then discusses the progress that has been made since 1977, in detail. The Lamb shift of light muonic atoms is dominated by the “vacuum-polarization” term, and approximate numerical values using nonrelativistic wave functions are given here. The chapter provides a discussion on two-electron systems. High-precision calculations for two-electron atoms and ions are complicated by the necessity of simultaneously taking into account relativistic, QED, and electron-correlation effects. There is no unique way of specifying an exact relativistic two-electron Hamiltonian analogous to the Dirac Hamiltonian without at the same time including QED effects to all orders. The analysis of QED effects in atomic systems containing more than two electrons is obscured by the lack of high-precision nonrelativistic eigenvalues and relativistic corrections available for two-electron systems. It therefore becomes more difficult to make a reliable subtraction of these contributions from the observed transition frequencies to reveal the specifically QED effects such as the Lamb shift.

Surface Analysis and Atomic Physics with Slow Positron Beams

Abstract: Recent advances in slow positron beam techniques are making it possible to study the interactions of low-energy positrons with gas molecules and solid surfaces and to measure the properties of free positronium atoms. New surface related results include the observation of surfaces with negative positron affinity and the thermionic emission of slow positronium atoms, low-energy positron diffraction measurements, and the sensitive detection of near-surface crystalline imperfections. Two recent successful experiments in atomic physics are the formation of the positronium negative ion and the optical excitation of positronium for high precisin spectroscopy. Prospects for a positron microscope and the study of exotic antimatter systems such as the two-component Fermi gas are based on the imminent possibility of enormous increases in the brightness and instantaneous intensity of positron beams.

Positron–gas cross-section measurements

Abstract: The purpose of this paper is to review the progress in positron–gas cross-section measurements. Total cross-section measurements for 0.3–1000 eV e+–He, Ne, Ar, Kr, Xe, H2, N2, CO2, O2, CH4, and CO are summarized. Interesting shape features in the total cross-section curves, such as Ramsauer–Townsend effects and the onsets of positronium formation are discussed. An effort is made to compare total cross-section measurements from different laboratories to see if there are any consistencies which could provide information on the reliability of the various measurements. Comparisons of total cross-section measurements for positrons and electrons colliding with several of the above gases are presented in an attempt to illustrate the similarities and differences between positron and electron scattering. Some recent positron cross-section investigations pertaining to differential scattering, positronium formation, inelastic scattering, atomic excitation, and resonance searches are also discussed.

The interaction of positrons with large atoms and molecules: theory

Abstract: Calculations of positron–atom and positron–molecule scattering are reviewed. The polarized orbital results of the York group are found to be of the same reliability as recent experimental determinations in the elastic region for the noble gases. The temperature dependence of the annihilation rate is suggested as a sensitive point of contact between theory and experiment. Using a new, direct experimental determination of the positronium formation cross section below the ionization threshold for argon and helium, we have made an improved Ore-type estimate of the positronium fraction of the noble gases. The new estimates agree perfectly with experiment for the three lighter gases. However, for krypton and xenon, the new estimates are much too high, which suggests the existence of an unknown process which suppresses or depletes positronium. Recent calculations on other atoms and on simple diatomic molecules are also reviewed.

The interaction of positrons and positronium with small atoms

Abstract: This is a report on recent progress in the theory of small atomic systems including a positron. I have selected four main topics that seem interesting, instead of trying to cover the entire extensi...

Elastic scattering and annihilation of low-energy positrons by molecular nitrogen

Abstract: A single-centre formalism is used to calculate the integrated elastic cross section in the energy range 0

Hypernetted Chain Theory of Charged Impurity

Abstract: Applying the inhomogeneous hypernetted chain method we formulate the general impurity problem for a correlated Bose or Fermi fluid as a host. In the present paper we apply the method to an impurity in electron gas. From the Euler-Lagrange equation for the induced density one obtains the screening and cusp conditions. It turns out that these two conditions alone are sufficient to give accurate prediction for the positron annihilation in metals. We construct numerical solution for the Euler-Lagrange equation demanding the two conditions to be satisfied. The resulting energies compare well with existing field theoretical calculations for hydrogen and positronium impurities. The uniform limit approximation is used to derive an expression for the dielectric function. In the present theory the linear response to an external field is determined by one characteristic function which is calculable from the ground state quantities alone. Eliminating this function in favour of the dielectric function which is assumed to be known exactly we obtain an equation for the non linear density response which may have more general validity than the approximations made here suggest.

Pseudostate effects on positron-helium excitation cross sections at intermediate energies

Abstract: Results of two five-state close-coupling (5CC) calculations of positron excitation of helium (11S to 21S,21P) at energies from 40 to 200 eV are presented and compared with an earlier 3CC calculation. In contrast to the e- case, inclusion of k1P and 41S pseudostates increases the cross sections (by 40% at 60 eV). For the 21P excitation the effect is almost entirely due to the k1P pseudostate. The e+ cross sections are substantially larger than those for e-. Total cross sections are given, but are poor below 100 eV due to neglect of positronium formation.

Threshold law for positron-atom impact ionisation

Abstract: The threshold law for ionisation of atoms by positron impact is adduced in analogy with our approach to the electron-atom ionization. It is concluded the Coulomb-dipole region of the potential gives the essential part of the interaction in both cases and leads to the same kind of result: a modulated linear law. An additional process which enters positron ionization is positronium formation in the continuum, but that will not dominate the threshold yield. The result is in sharp contrast to the positron threshold law as recently derived by Klar on the basis of a Wannier-type analysis.

Corrections to the Balmer-energy differences in positronium

Abstract: The absolute values of the ground-state (n = 1) levels of positronium and of deltaE(2/sup 3/S/sub 1/-1/sup 3/S/sub 1/) are given through order cap alpha/sup 3/ Ry

Possible effects of positronium negative ion on 0.511 MeV ψ-ray line in astrophysical sources

Abstract: The astrophysical importance of the negative positronium ion, detected recently in the laboratory, has been pointed out. It is found that the presence of Ps− ions will contribute additionally to the width of 0.511 ψ-ray line formed by pair annihilation. The formation of Ps− ion from an aggregate of electrons, positrons and positronium results in a variable positron population in the 0.511 MeV ψ-ray line source.

Positronium formation in gaseous ethane and ethane–Ne mixtures

Abstract: The positronium yields have been measured in gaseous ethane at gas densities in the range 0.1–286 amagat at 256, 294, 306, 307, 310, 323, 343, and 377 K and in ethane–neon mixtures having neon concentrations from about 0.38 to 0.99 at 256, 294, and 306 K. The positronium yields in gaseous ethane are observed to vary with gas density in a complex manner and to remain independent of temperature for the range of temperatures investigated. The presence of neon in ethane–neon mixtures enhances the fraction of positrons forming positronium at low partial densities of ethane. These results are discussed in terms of the models for positronium formation.

Positronium-oxygen interaction in hexane

Abstract: The role played by the O2 molecule in the decay of Ps in air-saturated hexane was investigated by measurements of lifetime and by an experiment of magnetic quenching. The results indicate that a) O2 reduces the overlap probability |φ(O)|2 of the interacting positron-electron pair in Ps by a factor 1.6, b) Ps may react with the O2 molecule forming a short-lived compound with a rate constant k q=5.7·10−11 cm3 s−1 and c) it may undergo exchange collisions with unpaired electrons of the O2 molecule with a rate constant γq = cm3 s−1.

On the Bloch States of Positronium in Alkali Halides

Abstract: An experimental result has been obtained which indicates that, in a low-temperature NaCl-type-alkali-halide crystal, positronium atoms move coherently through the interstices between ions almost without experiencing the exchange forces from the electrons in the ion cores.

Positron binding to metal surfaces

Abstract: Experiment and theory are shown to contradict each other about the image-induced binding energy B of positrons (e+) at metal surfaces, with (i) experimental values ( approximately 0.1 au) inferred from thermally activated positronium (Ps) emission, assuming that the barrier between the e+ and Ps channels is no higher than dictated by energy conservation; and (ii) calculated values (

Study of Imperfections in Ge–Te Semiconductors by the Positron Annihilation Technique

Abstract: The nature of defects in crystal and glass phases and in transition ranges for the system Ge–Te is studied by the positron annihilation technique. Additional evidence of positronium formation in semiconductors is obtained. [Russian Text Ignored].

Positron lifetime studies on thorium oxide powders

Abstract: Positron lifetime spectra have been studied for ThO2. powders, calcined at different temperatures and having different particle sizes. Three lifetime components could be resolved, the longest component being of low intensity. An observed strong dependence on the particle size of the annihilation process and the variation of positronium diffusion constant is explained on the basis of defect density variations in these powders.

Positron annihilation in fine particles of silver and copper

Abstract: Angular correlation measurements have been made on ultrafine silver and copper particles with different particle size. It was found that in fine silver particles positron annihilation mechanisms fall into two categories: annihilation with electrons inside the particles and annihilation from particle surface trapped states and/or positronium states. The positron diffusion constant in silver at room temperature was roughly estimated to be about 0.4 cm2 s-1. In the case of fine copper particles, the phenomenon was complex since the degree of oxidation had direct effects upon the angular correlation curve.