Kinetic theory of positron-impact ionization in gases

Abstract

A kinetic theory model is developed for positron-impact ionization (PII) with neutral rarefied gases. Particular attention is given to the sharing of available energy between the postionization constituents. A simple model for the energy-partition function that qualitatively captures the physics of high-energy and near-threshold ionization is developed for PII, with free parameters that can be used to fit the model to experimental data. By applying the model to the measurements of Kover and Laricchia [Phys. Rev. Lett. 80, 5309 (1998)] for positrons in ${\mathrm{H}}_{2}$, the role of energy partitioning in PII for positron thermalization is studied. Although the overall thermalization time is found to be relatively insensitive to the energy partitioning, the mean energy profiles at certain times can differ by more than an order of magnitude for the various treatments of energy partitioning. This can significantly impact the number and energy distribution of secondary electrons.