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Title: Positron induced ionization processes
Author: Szluinska, Marta
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 2003
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In this work, a monoenergetic positron beam obtained from a radioactive source and guided by a magnetic field has been used to study ionization processes, such as positronium formation and annihilation, induced by positron collisions with atoms and molecules. Precise total ionization cross-sections for Ne and Xe have been performed in the course of the present work in order to aid the extraction of the positronium formation cross-sections by subtracting direct ionization cross-sections. In the case of Xe, the energy dependence of the total ionization and positronium formation cross-sections has been scrutinized. No evidence of a structure around 80eV, predicted by a theoretical calculation, has been found. However, the positronium formation cross-sections in the noble gases have revealed a double-peak structure, which might be related to the positronium formation in its various quantum states. The double peak structure has also been confirmed by direct measurements for Ar and Xe described in this thesis. This type of investigation has also been extended to He. The finding of a simple scaling formula for positron- and electron- impact ionization cross-sections, also described in this thesis, has enabled the estimation of the contribution of the excited state positronium formation in the noble gases. These estimates suggest that excited state positronium might occur much more abundantly than previously thought. Despite a comparatively poor energy resolution, low beam intensity and an uncertainty in the absolute energy calibration of the positron beam, the energy dependence of the annihilation cross-sections below the positronium formation threshold in Ne, Xe and O2 has been investigated by measuring coincidence yields between -rays and ions. This study has been prompted by the observation of a residual ion yield in the total ionization cross-sections for Ne below the positronium formation threshold. Whilst in the case of Ne and O2, the annihilation cross-sections suggest some increase significantly below the positronium formation threshold, for Xe the increase is ambiguous. It is anticipated that this work will stimulate further experimental and theoretical investigations.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available