Armitage, Simon Peter; (2002) Positronium ionisation in collision with He atoms. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Positronium is the lightest known atom, consisting of an electron and its antiparticle the positron. Because of its light mass (comparable with that of the electron and positron, rather than conventional atoms), recoil effects are expected to play an important role in its scattering from atomic and molecular targets. In Positronium the centres of charge and mass coincide, leading to a zero-static interaction and enhancing the relative importance of electron-exchange effects. Up until now, Positronium beam experiments have been restricted to total cross-section measurements from simple target atoms and molecules i.e. H2, He and Ar. Significant discrepancies exist among various (theoretical and experimental) determinations of the Positronium-He total cross-section. In addition to their intrinsic interest. Positronium-atom partial-cross-sections are expected to provide a more sensitive test of our understanding of this collision system. In this work, the ionisation cross-section of Positronium has been measured for the first time. A monoenergetic Positronium beam has been created through charge exchange of positrons in a gaseous target and positrons, originating from the break-up of positronium in collision with He atoms, have been detected with a time-of-flight system. Measurements are presented in the energy range 10 - 40eV and absolute break-up cross-section values have been achieved by measuring explicitly both the positron and Positronium detection efficiencies. From the measured times-of-flight, longitudinal energy spreads of the residual positrons have also been obtained. The distributions have been found to be peaked at around 50% of the residual energy, suggesting a strong correlation between the residual particles. The present work is expected to stimulate further theoretical and experimental activity in the study of Positronium-atom interactions. Possible future new directions are discussed.

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