Kinetic Theory of Positron-Impact Ionization in Gases

TL;DR

This paper develops a kinetic theory model for positron-impact ionization in gases, focusing on energy sharing between particles, and explores its effects on positron thermalization and secondary electron production.

Contribution

It introduces a simple, adjustable energy-partition function model for PII that captures high-energy and near-threshold physics, aiding understanding of positron thermalization.

Findings

Energy-partitioning significantly affects mean energy profiles during thermalization.

Thermalization time is relatively insensitive to energy-sharing details.

Secondary electron production is influenced by energy-partitioning choices.

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 post-ionization 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 [1] for positrons in H2, the role of energy-partitioning in PII for positron thermalisation is studied. Although the overall thermalisation 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.