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An experimental study of the electric field drift of positrons in solid media and its application to the development of field assisted positron moderators

Simpson, Richard Iain; (1990) An experimental study of the electric field drift of positrons in solid media and its application to the development of field assisted positron moderators. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

The primary aim of this work was to investigate the possibility of developing more intense laboratoiy-based slow positron (e+) beams. Conventionally, these are formed by moderating β+ particles in vacuo using metal foils and meshes, e+ thermalise in the metal bulk and a fraction subsequently diffuse to a surface where they are re-emitted with eV energies. It is proposed that a new type of moderator could be developed based on the electric field drift of e+ in semiconductors or insulators. A solution to the diffusion equation is derived, which includes terms describing positron implantation, drift and annihilation. Using published mobility data for Si, the expression is used to estimate the fraction of e+ drifted to a surface or interface. For a 50μm Si crystal cooled to 80K, efficiencies of up to 10% are predicted. It is suggested that the trapping of e+ in defects in the thin contact required to generate the electric field can be reduced by using an epitaxially grown NiSi2 layer on a Si(l 11) substrate. A new method of measuring positron mobility, μ+, using a modified e+ lifetime spectrometer was developed. Changes in the lifetime spectra are related to the electric field drift of e+ from the Si bulk into an annihilation state associated with the Au contact or Au-Si interface. Values of μ calculated from the data are in good agreement with other published measurements. The re-emission of e+ from clean and oxygen exposed Si(111) and Si(l00) surfaces bombarded with β+ particles was measured in ultra high vacuum. The data suggest that the Si(111) and Si(l00) samples studied have positive positron work functions and the small yield of slow e+ measured is attributed to the escape of non-thermal e+ from the surface. Measurements of positron yield versus oxygen exposure are consistent with a changing surface dipole arising from oxygen adsorption. It is concluded that Si is not suitable as a positron moderator and that other materials need to be evaluated to establish whether a practical field assisted moderator can be developed.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: An experimental study of the electric field drift of positrons in solid media and its application to the development of field assisted positron moderators
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Pure sciences; Positrons
URI: https://discovery.ucl.ac.uk/id/eprint/10112783
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