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.

http://iopscience.iop.org/article/10.1088/0034-4885/48/6/001/pdf

Experimental studies of positrons scattering in gases

Charge exchange in energetic ion-atom collisions

We have investigated the conversion of a positron beam into an ortho-Pa beam via charge exchange in gaseous H-2, He and Ar at energies up to 120 eV and gas pressures up to 14 mu mHg. H-2 has been found to be the most efficient converter at energies up to 90 eV. At higher energies, Ar has been found to be the best. Total cross sections of ortho-Ps scattering from H-2 and He have also been measured. A comparison with available theories suggests that electron exchange at lower energies and target inelastic collisions at higher energies are playing a major role in the collisions.

https://iopscience.iop.org/article/10.1088/0953-4075/29/23/035/pdf

Ps beam production and scattering from gaseous targets

Positron-atom and positron-molecule collisions

Publisher Summary This chapter reviews many of the interesting comparisons, primarily experimental measurements that can be made between positron and electron scattering by the same atoms and molecules with the hope that these comparisons may help to stimulate a better overall understanding of electron scattering processes. The chapter explains most of the experimental results. The e+ and e– cross section measurements of a given group have been made in the same experimental system with the same technique. The relative comparisons between e+ and e– measurements will be more meaningful than the actual quantitative values because many of the potential experimental errors would be the same for e+ and e– measurements, and as a result not affect the relative comparisons. An important motivation for investigating the scattering of positrons by atoms and molecules provides a better understanding of the scattering of electrons by atoms and molecules, the latter being of importance in many different fields of science and technology such as plasma physics, laser development, gaseous electronics, astrophysics, and aeronomy. As positrons differ from electrons only by the signs of their electric charge, comparison measurements of the scattering of positrons and electrons by the same atoms and molecules can reveal interesting similarities and differences that arise from the basic interactions contributing to e+ ,– scattering.

Comparisons of Positron And Electron Scattering By Gases

The distorted-wave approximation is applied to investigate the rearrangement collision problem of positronium formation in the ground state with positron scattering from hydrogen and helium atoms for the positron energy ranges 8.704-200 eV and 20-100 eV respectively.

Positronium formation in positron scattering from hydrogen and helium atoms: the distorted-wave approximation

The two-state approximation is applied to positron--lithium-atom collisions considering only the elastic scattering and the ground-state positronium formation. Taking the effect of the adiabatic polarization potential into account in both the direct and rearrangement channels, results for the differential, total, and momentum-transfer cross sections are computed at incident positron energies 0.5--10 eV. While for the elastic scattering the present data show satisfactory agreement with other available calculations, for positronium formation the present results are quite new and give us an idea of the order of the cross section. It is found that the differential cross section for both the elastic scattering and positronium formation shows a deep minimum at low energies of positron impact in agreement with available earlier findings.

Positron--lithium-atom collisions using the two-state approximation

Calculations are performed within the scope of the pseudopotential formalism to predict the cross section for positronium formation during collisions of positrons with the alkali atoms Na, K, Rb and Cs employing the first Born approximation and the distorted-wave approximation. This maiden attempt allows the authors to obtain an idea of the order of magnitude of the positronium formation cross section for these atoms. Retaining both the post and the prior forms of the interaction potential in the calculation, results for the differential, total and momentum-transfer cross sections are computed at several incident positron energies in the range 0.5-20 eV.

https://iopscience.iop.org/article/10.1088/0022-3700/13/9/021/pdf

Model potential approach for positron-atom collisions. I. Positronium formation in ground state in alkali atoms Na, K, Rb and Cs using the distorted-wave approximation

For pt.I see ibid., vol.13, no.9, p.1919 (1980). Cross sections are computed for positronium formation into an arbitrary excited s state during positron scattering from the alkali atoms using the first Born approximation. Considering both the post and the prior forms of the interaction potential in the calculation, the results are presented for the incident positron energies from 5 to 50 eV. From these results it is found that for the alkali atoms the Ps formation cross section satisfies the inverse n3 law consistently at all positron energies.

Model potential approach for positron-atom collisions. II. Positronium formation in excited ns states in Na, K, Rb and Cs using the first Born approximation

The scattering amplitude for positronium formation into arbitrary excited S states in positron-helium collisions in which the residual helium ion is left in the ground state, is obtained in a closed form using the first Born approximation and the first-order exchange approximation. Retaining both the post and the prior forms of the interaction potential, cross sections are computed for several incident positron energies in the range 20 --2000 eV. For these energies the asymptotic limit of n/sup 3/ times the cross section as n ..-->.. infinity is attained roughly within 0.1% beyond the excited state 16S.

Sunanda Guha

Papers

Positronium formation in positron scattering from hydrogen and helium atoms: the distorted-wave approximation

Positron--lithium-atom collisions using the two-state approximation

Model potential approach for positron-atom collisions. I. Positronium formation in ground state in alkali atoms Na, K, Rb and Cs using the distorted-wave approximation

Model potential approach for positron-atom collisions. II. Positronium formation in excited ns states in Na, K, Rb and Cs using the first Born approximation

Positron-helium collisions: Positronium formation into an arbitrary excited S state