J. Moxom

Ionization of rare gases by particle - antiparticle impact

New data for single and double ionization of the rare gases Ne, Ar, Kr and Xe, by positron and antiproton impact are presented and compared with existing data for electron and proton impact. It is found that the current understanding of these processes obtained from studies involving low-Z targets only extends to high-Z atoms provided that certain target-dependent effects are taken into consideration. In particular, the static interaction between the projectile and the undistorted target is found to play a crucial role in the case of the lighter projectiles.

Ionization of CO2 by positron impact

Abstract The total ion yield by positron impact on CO 2 has been measured, for the first time, in the range 3–20 eV incident energy. An enhancement of this yield is observed at around 11 eV consistently with formation of positronium simultaneous to target excitation. An estimate of the total ionization cross section is obtained.

Total ionization cross sections of He, H2 and Ar by positron impact

Total positron impact ionization cross sections, including contributions from positronium formation, have been measured for He, H2 and Ar from below the positronium formation threshold, Eps, up to 60 eV, 31 eV and 41 eV respectively. These cross sections have been subtracted from corresponding total cross section measurements and a small decrease in the elastic cross section between Eps and the threshold for excitation has been deduced for all three targets.

Ejected-electron energy spectra in low energy positron-argon collisions

Energy spectra of electrons ejected from an argon target by positron impact have been measured and compared with theoretical expectations. No major cusp structure was observed in any of these spectra. Instead a small ridge is present in some cases at energies compatible with electron capture to continuum (ECC) states of positronium. Its magnitude suggests that ECC makes a minor contribution to ionization by a light positively charged projectile.

Ionization of He, Ar and H2 by position impact at intermediate energies

Total ionization cross sections for positron impact on He, Ar and H2 targets from threshold to around 300 eV are presented and compared with values obtained by summing available partial cross section results. Reasonable accord is found with these data. The experimental method and apparatus is described in detail together with the systematic reliability checks.

Near threshold effects in positron-atom (molecule) scattering

The energy dependence of the total cross-sections for ionization by positron impact has been measured for He, Ne, Ar, CO2 and O2. The interplay between this and other scattering channels has been investigated. The data for He, Ne and Ar has been interpreted using a combination ofR-matrix and threshold theories in the neighbourhood of the Ps formation threshold. In CO2, an enhancement of the Ps formation probability is found when the remnant ion is left in an excited state. In O2, structure has been observed which indicates channel-coupling with excitation to the Schumann-Runge continuum.

Positron impact ionisation of atoms

Abstract Experimental data are presented for the single (σ+i) and the ratio of double to single (R2) ionisation cross sections for e+ impact on Ne, Kr and Xe from near threshold to 1000 eV. The ratio between e+ and e− σ+i maxima was generally seen to be lower for targets with higher atomic number (Z), as would be expected from the effect of the increasing static interaction between the projectile and undistorted target. For double ionisation, the importance of inner-shell ionisation followed by Auger decay is manifest at high velocities for the higher Z targets. Preliminary measurements of the total ionisation cross section (σ+T) in e+–D collisions are also presented and are found to disagree with recent results.

An electrostatic beam for positron–gas scattering measurements

Abstract The details of an electrostatic beam, designed for investigating positron–atom (molecule) scattering processes, are presented. The performance of the system has been tested by measuring differential elastic e + –Ar scattering cross sections. The preliminary data have been found to be consistent with previous results.

A gated positron beam incorporating a scattering cell and novel ion extractor

Abstract A gated positron beam technique has been developed and employed to measure total ion yields for a range of gases. The technique and apparatus are described and some recent results discussed.

Electron emission spectra in low energy positron-atom collisions

Energy spectra of electrons ejected from an Ar target, in the forward direction, by a low energy positron beam have been measured and compared with theory. Structures are present in these spectra at energies compatible with electron capture to continuum states of the scattered positron (ECC) and with binary collisions.