  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.
