Ashley, P. N.; (1996) Investigations of positron impact ionization. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes an experimental investigation into the process of ionization of gaseous targets by the impact of positrons. The cross-sections (σi+) for direct, single ionization of He, H and Ar have been measured for incident positron energies (E) in the range 0-1000eV, with particular attention given to the energy dependence of σi+ in the vicinity of the direct ionization threshold (Ei.). An experimental apparatus has been developed and used to determine σi+ close-to-threshold with unprecedented accuracy and detail. In this system, a magnetically confined positron beam is crossed with a gas jet and resultant ions are detected in a time-of-flight technique which distinguishes ions of different specific charge. The electric field extracting the ions is triggered by the arrival of scattered positrons at an end-of-beamline detector. Thus the field is pulsed on after the collision in order to prevent perturbations. The apparatus has been used to measure a. up to 1keV incident positron energy for He, Ar and H2. Some differences between these results and existing data are evident and possible causes of discrepancy are discussed. Comparison with theory is made where possible. Near-threshold studies of He and H2 have been performed and have revealed that σi+ has a different energy dependence from the single ionization cross-section for electron impact, σi-. Differences have also been found between these results and earlier experimental determinations. As predicted, σi+ undercuts σi- in the first few electronvolts above threshold. This is thought to be a result of the direct ionization channel in the positron impact case being suppressed by positronium formation, the dominant ionization process close to Ei. The energy dependence of σi+ has been analysed in terms of existing, conflicting theories, and has been found to be in qualitative agreement with the classical theory of Wannier (1953) and its extensions.

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