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Positron impact ionization processes in gases

Jones, Geraint Owain; (1991) Positron impact ionization processes in gases. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

The two experiments that are discussed in this work are both studies of ionization phenomena in gases. The main body of the work involved the development of an experiment to measure the positron impact ionization cross-section of atomic hydrogen. This experiment utilizes the crossed-beam method wherein a magnetically guided positron beam crosses a diverging jet of highly dissociated hydrogen gas. The suitability of this system to measure relative ionization cross-sections with both positrons and electrons has been demonstrated by measuring the impact ionization cross-sections of molecular hydrogen. Results obtained agree with previously published data to within experimental error. Preliminary studies have also been made with the atomic hydrogen source in operation and these revealed that high fluxes of photons, protons and electrons are emitted from the gas discharge and detected. These would make it impossible to detect ions at the considerably lower count-rates that are anticipated and their effects have had to be circumvented. This has been done successfully and it is concluded that the measurement of the positron impact ionization cross-section of atomic hydrogen seems possible with this apparatus. The second experiment has been a study of the times-of-flight of electrons emitted in positron impact ionization of Argon. The purpose of the experiment is to assess the significance of electron capture to the continuum (ECC) as a possible outcome of the ionizing collisions of positrons. Electrons which have been captured into a continuum state of a positron are expected to have a velocity vector equal to that of the positron, one would therefore expect them to have a particular time-of-flight to a detector. A broad peak was observed around the characteristic flight time at all energies indicating that ECC does occur in positron-Ar collisions but quantitative estimates of its likelihood were not possible.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Positron impact ionization processes in gases
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest
Keywords: Pure sciences; Atomic hydrogen
URI: https://discovery.ucl.ac.uk/id/eprint/10106541
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