Ozen, Aysun; (1999) Positronium beam scattering from He and positron moderation from rare gas solids. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Positronium is the bound state of an electron and a positron (antimatter counterpart of the electron). In this work, the efficiency with which a positron beam is converted into a positronium beam, via charge-exchange, in helium gas was investigated over a range of pressures and positronium energies. Total cross-sections of ortho-positronium scattering from helium have been obtained for the positronium energy range 10 to 110 eV and compared to theoretical calculations, where possible. Additionally, the effect of forward scattering of positronium atoms has been investigated and has been evaluated by varying the detection acceptance angles. High intensity slow positron beams can be obtained by moderating + particles in rare gas solids (RGS). Since RGS require ultra high vacuum (UHV) in order to ensure long term stability of their moderation efficiency, a vacuum system capable of reaching UHV has been designed, installed and tested. The increase in the slow positron beam intensity resulting from RGS moderators is reflected by a corresponding increase of the positronium beam flux. Solid argon and neon have been investigated and different moderator growing conditions (especially temperature and pressure) have been explored. The results are compared with previous experiments. In addition to their efficiency as positron moderators other aspects of the RGS are also discussed, for example characteristics of the ensuing beam, its long term stability and energy spread.

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