Merrison, Jonathan Peter; (1992) Positron moderation and timing. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The development of more intense and better resolved slow positron (e+) beams is one of the major goals in e+ research. One way to achieve this is by improving the efficiency of moderation techniques. Such improvements would allow the application of this unique probe to many areas of research which currently find the technology prohibitively complex and expensive. It may also open the way to more ambitious research projects requiring high intensity beams, for example the formation of anti-hydrogen. A technique employed m e+ physics which is often important m order to improve the quality of the detected signal is that of single particle timing. This technique relies on the fact that generally e+ beams are of low intensity and by resolving interactions in time it becomes possible to reduce spurious (random) background counts. The development of a new method of timing has been carried out which utilises the secondary e+s emitted on β+ impact with the moderator. The success of this method lies m the fact that the same component of the β+ energy spectrum which contributes most to the emitted slow e+ yield (le the those with low energy) is also most efficient at secondary e+ emission. Positron tagging efficiencies of greater than 20% were observed with a signal to background ratio much higher than that obtained with other β tagging techniques. There was also no loss of beam intensity using this technique, unlike that of timing at a remoderator. An investigation was performed on the e+ moderating properties of the rare gas solids (RGS). Neon has recently provided the highest currently quoted moderation efficiencies. The work presented in this thesis showed that the other RGS (Ar, Kr and Xe) could achieve comparable efficiencies and provide greater than a ten fold improvement on the commonly used metal foil or mesh type moderators. Importantly these RGS moderators are easily fabricated, rugged and may be replaced without loss of vacuum. The first observation of electric field assisted e+ extraction was made during the course of this study. This was achieved by surface charging of the RGS film by the trapping of electrons on overlayered oxygen molecules. Enhancement in the moderation efficiency of a factor of three was observed and was attributed to the electric field, of strength approximately 6kV/mm, across the film due to the trapped surface charge. This effect is not only of importance in the development of more efficient slow e+ moderators but may prove to be an interesting new field of research in its own right.

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