Book Description

Low energy positron scattering has been conducted for over 40 years and large discrepancies (> 20%) in the measured cross sections have plagued both experimental and theoretical results. The advent of the Surko buffer-gas trap has provided for sufficiently large numbers of positrons, with previously unmatched angular and energy distributions, which has allowed for the measurement of absolute cross sections with absolute uncertainties of less than 10%. Continual improvement of the experimental techniques has allowed for the separation of the partial scattering channels. These results are crucial for testing our theoretical understanding of the scattering process and for modeling of positron transport in complex systems. The experiments reported in this thesis used a scattering apparatus which incorporated a radiative sodium-22 source of positrons that were moderated using solid neon and cooled further by a Surko buffer-gas trap. Those positrons were subsequently scattered from various atomic and molecular gases in a cylindrical scattering cell. Scattered and unscattered positrons emerging from the scattering cell were energy analysed with a retarding potential analyser to determine their energy loss, which enabled the determination of various scattering cross sections. Absolute elastic differential, grand total, total elastic, total inelastic and positronium formation scattering cross sections for energies between 1 and 200 eV are presented for a number of atomic and molecular species. Where possible, knowledge of the experimental limitations are combined with theoretical calculations to estimate the amount of the angular scattering missed due to the finite angular resolution of the experiment. Comparison of both grand total and partial total cross sections are compared to a number of theoretical calculations and previous experimental results, again where possible. A rotating electric field, or 'rotating-wall' electrode, was implemented to increase the central density of the positron beam. The operational phase space was explored in the context of the standard running parameters of this Surko-trap system used for atomic and molecular scattering experiments. Mechanisms for compression are reviewed in the context of applicability to the operational parameters of the trap and in the understanding of the dominant mechanism. The limitations in the implementation are discussed in regard to scattering experiments from atoms and molecules.