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Formation and scattering of positronium from atoms and molecules

Newson, Donovan Michael; (2023) Formation and scattering of positronium from atoms and molecules. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

The collimated positronium production efficiency has been measured over an extended energy range for helium, neon, argon, krypton and xenon, as well as molecular hydrogen, nitrogen and oxygen. At low energies, the efficiencies for krypton and xenon are found to be superior to that of argon, suggesting alternative production gases for low-energy positronium beam production. Corresponding absolute differential positronium-formation cross-sections have been determined from the collimated positronium production efficiency and structure has been identified for helium and argon in close proximity to the lowest excitation thresholds of the targets and of positronium. The total cross-section measurements for positronium scattering from helium, neon and krypton have been investigated down to low energies and compared with recent theoretical studies. The trend of the experimental results is consistent with recent theoretical approaches that account for virtual target-excitation. The positronium-oxygen total scattering cross-section has also been investigated, and the results support the broad trend of recent theoretical studies although the prediction of resonant peaks at low energies cannot at present be corroborated. Measurements of the positron detection efficiency for a channel electron multiplier have been performed and, in combination with previous results, found to be consistent with a recent statistical description of inelastic collision processes. Additionally, it has been shown that the energy-dependence of the secondary electron yield from clean and unprepared materials also follows a similar statistical model. The origin of unexpected structure in the detection efficiency measurements at low incident energies has been explained with the aid of a simulation of the detector arrangement. The ratio of positron and positronium detection efficiencies, required to determine absolute cross-sections involving the ratio of positron and positronium count rates, is extracted by fitting the statistical model to the channel electron multiplier positron detection efficiency measurements.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Formation and scattering of positronium from atoms and molecules
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2023. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy
URI: https://discovery.ucl.ac.uk/id/eprint/10170059
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