Brawley, S.J.; (2009) Collisions of positronium with atoms and molecules. Doctoral thesis , UCL (University College London).

Abstract

Positronium, Ps, is the atom-like bound state of an electron and its antimatter counterpart, the positron. This work encompasses the experimental techniques required to produce a collimated beam of Ps atoms and recent investigations of Ps collisions on a variety of atomic and molecular targets to determine Ps total and fragmentation cross-sections. For the former experiment, the present work is compared to theory - for which there is now very good agreement for low energy Ps–He collisions - and to other projectiles where the present data shows that total cross-sections for equivelocity Ps and electrons are nearly identical for a wide range of targets and velocities. The Ps fragmentation measurements proceed via the detection of ejected electrons or positrons for Ps collisions on Xe at 18 and 30 eV. Detection of ejected positrons is a unique signature of Ps fragmentation and the recent integrated measurements for this process agree well with theory, as do the shapes of the differential cross-sections with respect to the longitudinal energy of the ejected positrons. Concerning these, the energy dependence of the positron distributions appear similar for the present measurements on Xe and those previously measured for a He target. When detecting ejected electrons, the experiment is sensitive to reactions in which the projectile and/or target are ionised, and the integrated cross-sections indicate that target-ionisation is significant at only ∼3 eV above threshold in Xe.

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