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Proceedings of the Physical Society Section A

Gaseous Reactions involving Positronium

H. S. W. Massey, C. B. O. Mohr

Published Aug 1, 1954• 153 citations
Publisher

Abstract

The problems involved in providing a detailed interpretation of the processes involved in the ultimate fate of positrons slowed down in a gas are examined by considering the idealized case of a gas of atomic hydrogen. The processes leading to the formation and dissociation of positronium are considered as well as those which lead to the slowing down of a positronium pair in a gas and to the conversion of ortho- into para-positronium. It is shown that the rate of the latter process probably depends strongly on the kinetic energy of the pair. Elastic collisions of a pair, in its ground state, with gas atoms are determined almost completely by electron exchange interaction, but this is so strong that it cannot be taken account of in a weak coupling treatment.

DOI
10.1088/0370-1298/67/8/306

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The problems involved in providing a detailed interpretation of the processes involved in the ultimate fate of positrons slowed down in a gas are examined by considering the idealized case of a gas of atomic hydrogen. The processes leading to the formation and dissociation of positronium are considered as well as those which lead to the slowing down of a positronium pair in a gas and to the conversion of ortho- into para-positronium. It is shown that the rate of the latter process probably depends strongly on the kinetic energy of the pair. Elastic collisions of a pair, in its ground state, with gas atoms are determined almost completely by electron exchange interaction, but this is so strong that it cannot be taken account of in a weak coupling treatment.
By using a kinetic approach, we study the evolution of a gas composed of interacting ortho- and para-positronium atoms. We calculate the total lifetime of the gas and study the ortho-positronium quenching effect induced by the bi-atomic scattering mechanisms with spin exchange. We analyze a realistic situation where the positronium is formed by highly energetic positrons impinging on a solid surface. In the case of a spin-polarized source of positrons, the spinpolarization time of the final positronium gas is estimated.
The total elastic scattering and ortho-para conversion cross sections of low-energy ortho-positronium with hydrogen atoms have been calculated in the static exchange approximation including the l=0, 1 and 2 partial waves with the incident positronium wavenumber k ranging from 0 to 3.0 (in atomic units) for l=0 and from 0 to 1.2 for l=1 and 2. The scattering lengths are a (+) =7.275 a 0 and a (-) =2.476 a 0 for the space-symmetric and anti-symmetric cases respectively. A resonance is found in the anti-symmetric s-wave scattering at k approximately=2.07, but its real existence is of course questionable because of neglected effects.
We have calculated the rates for the processes that result from collisions of positrons with Rydberg atoms in strong magnetic fields. This is the first step in a two-stage charge transfer, which is being explored as a mechanism for the formation of cold anti-hydrogen. Unlike previous theoretical explorations we have also investigated the cases when the energy of the positrons can be comparable to or larger than the binding energy of the atom. We have also examined how big an electric field is needed to destroy the resulting positronium. We find that the charge transfer does not scale as for n = 40 states for positron temperatures above T ∼ 25 K. Also, we find that the positronium atoms are more easily destroyed by electric fields if they emerge from the charge transfer along the magnetic field lines, contrary to what might be expected for the effective electric field. Both results have implications for the formation of anti-hydrogen atoms.
The stability of antimatter in contact with matter has been investigated. The interaction between hydrogen and antihydrogen is considered as the prototype reaction. We have focused interest on the rates for proton-antiproton and/or electron-positron annihilation during hydrogen-antihydrogen collisions at low energies. In particular, we have concentrated on the calculation of the rates for the rearrangement reaction leading to formation of protonium and positronium, ending inevitably in particle-antiparticle annihilation. The cross section for the rearrangement collision has been calculated in a fully quantum-mechanical treatment. Additionally, we have calculated cross sections for direct annihilation during the collision process, which was found to be comparable to the rearrangement cross section. The elastic cross section and its low-energy limit, given by the scattering length, have been calculated, allowing, by comparison to the inelastic processes, a prediction for the efficiency of cooling antihydrogen via collisions with ultracold hydrogen atoms.
When positrons are injected into a gas, 75% of the positronium (Ps) is likely to be formed as long-lived ortho-Ps. The main decay mechanisms for the ortho-Ps have been assumed to be natural decay of ortho-Ps and pickoff annihilation of the positron during Ps-atom collisions. A third possibility for annihilation is $\mathrm{o}\mathrm{r}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{\text{\ensuremath{-}}}\mathrm{P}\mathrm{s}\ensuremath{\rightarrow}\mathrm{p}\mathrm{a}\mathrm{r}\mathrm{a}\mathrm{\text{\ensuremath{-}}}\mathrm{P}\mathrm{s}$ conversion due to the spin-orbit interaction between the atom and colliding Ps. This extra quenching mechanism may explain a number of phenomena observed in the annihilation spectrum of Kr and Xe, including the very small Ps fraction of 3% seen for Xe.
The positronium spin conversion reaction induced by spin-orbit interaction is investigated. We obtain the reaction rates during positronium-Xe and positronium-Kr collisions by using the Zeeman mixing of positronium states. At thermal energies corresponding to room temperature, the reaction rate for spin conversion due to spin-orbit interaction is found to be almost twice that for the positronium pick-off reaction. We also study the energy dependence of the reaction rate. The mean energy of positronium is controlled by changing the gas temperature and using positronium in thermal equilibrium. We found that the reaction rate increases with the collision energy.