Electron, positron and positronium cross sections

The work in this thesis concerns cross section measurements involving electron, positron, and positronium (Ps) projectiles and their interactions with different atomic and molecular targets. Two different positron beam lines have been employed for these studies: an electrostatic system for generating an electron beam and a magnetic system for producing a Ps beam. The UCL electrostatic beam has been used to measure the total cross section for electron scattering from water vapour, with an angular acceptance of ∼ 1º), in the energy range (3 - 100) eV. These results are presented and compared with previous experimental and theoretical determinations. The effect and significance of forward elastic scattering have also been investigated, with results consistent with previous measurements of similar angular resolutions. The magnetic positron beam was utilized to determine the collimated Ps production efficiency, differential Ps formation and Ps total scattering cross section. The Ps beam production efficiency has been measured for He and Ne in the energy range (7 - 60) eV and (9 - 150) eV, respectively. The total cross section has been measured for (Ps + He) scattering in the energy range (10 - 75) eV. The current results of the two, along with previous data have been used to extract the differential Ps formation cross section near zero degrees for He and Ne in the positron energy range (7 - 110) eV and (10 - 100) eV, respectively. The electron-like behaviour of equivelocity Ps has also been examined for a variety of targets, including molecular oxygen which displays a significant deviation at the lowest energies investigated (below 30 eV)

Low-Energy Positronium Scattering from O2

The total cross-section of positronium scattering from molecular oxygen has been measured in the velocity range 0.27 − 1.50 a.u. (energy range 2 − 61 eV) and found to be close to the corresponding equivelocity electron cross-section above 0.87 a.u. (20 eV), as previously found by Brawley et al. Science 330 789 (2010). However, below this value the cross-section for positronium is observed to exceed that for electrons by up to a factor of 4 at the lowest energy. Measurements are compared to the predictions of low-energy resonant peaks in the elastic scattering cross-section calculated within a free-electron-gas model refined by applying corrections to the correlation energy for the interaction between Ps and the electron gas. Additionally, cross-sections for O− 2 formation and positronium break-up have been calculated using a classical trajectory Monte Carlo approach. Comparisons are made with earlier calculations and discussed in terms of both experimental and theoretical uncertainties