Low energy positron interactions with the rare gases

Abstract

This thesis presents measurements of low-energy positron scattering from the noble gases. Sets of total cross section measurements are presented for Ne-Xe, and are compared with prior experimental and theoretical results. The present results represent a substantial improvement on previous experiments, due to improved statistical and systematic uncertainty, as well as an energy resolution that is better than that achieved in most other experiments. Methods for calculating cross sections in a strong magnetic field are discussed, along with improvements made to the techniques for analysis, and the handling and correction, or minimisation, of systematic errors. Preliminary tests of the application of effective range theory have been conducted using theoretical total cross sections, calculated in the polarized orbital approach. These tests provide estimates of the range of applicability of effective range theory. Using this knowledge, analysis of the low-energy cross section, for energies below about 5 eV, has been carried out, using effective range theory to extract the s-wave scattering length. This work is in preparation for future measurements of low-energy total cross sections from metal vapours, where knowledge of the scattering length will provide a means of inferring the presence of theoretically predicted positron-atom bound states. Measurements of the elastic total cross section for positron scattering from He-Xe, in the region of the Ps formation threshold, are presented. These results indicate the presence of pronounced Wigner cusps in each cross section, resulting from channel coupling between the elastic and Ps formation scattering channels. These measurements are compared and contrasted with other recent experimental measurements. The trend of the channel coupling behaviour is discussed with respect to theoretical predictions.