The research described in this thesis was concerned with the experimental determination of total scattering cross-sections, Qt, for the collision of low-energy positrons with noble gas atoms, viz: helium, neon and argon. To ensure the highest accuracy in measurement, considerable effort was directed into the development and generation of intense low-energy positron beams, the effective discrimination against forward scattering and the elimination of all systematic experimental errors. Only since 1969 have successful methods of producing monoenergetic positron beams of low-energies been reported. Since then considerable theoretical and experimental interest in low-energy positron scattering has grown, particularly in the noble gases, and is reviewed in Chapter II. In the present experiment approximately 1 in 106 positrons from a 3 mCi source of 22Na were moderated to near thermal energies when they impinged on an assembly of gold vanes coated with magnesium oxide. The moderated positrons were then accelerated and momentum selected in a 180° magnetic spectrometer, which also acted as the scattering chamber. After momentum selection, the positrons annihilated at a target. The coincident gamma-ray count rate was proportional to the transmitted beam intensity. The reduction in count rate as a function of gas pressure was measured, from which Qt could be obtained. Values of Qt, for positron-helium scattering, were obtained using positrons having energies in the range 13 to 1000 eV and it was demonstrated in initial measurements that cross-sections determined at higher energies (> 400 eV) were significantly lower than values calculated using the Born approximation. Earlier theoretical work had predicted that the Born values were likely to be correct in helium at approximately 400 eV and above. To overcome this discrepancy, which was due to positrons scattered in the forward direction reaching the target, two changes were necessary: a) to position a retarding-field spectrometer in the front of the target to prevent the transmission of inelastically scattered positrons; b) to use narrower resolving slits. These resulted in a progressive increase in values of Q for increasing energies above 200 eV. The final set of results are in good agreement with recent theory and experimentally demonstrated for the first time that the Born approximation is valid in helium above 500 eV. Examination of these results shows them to be consistent with the "sum rule" based on a forward dispersion relation. After the study with helium, neon and argon were studied using positrons having energies in the ranges 10 to 1000 eV and 200 to 1000 eV respectively. The overall impression gained in the study of these gases is that the discrepancy between theory and experiment above 200 eV is now not greater than 10%. Evaluation of the "sum rule" using the neon data shows that the rule again holds.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:636149 |
| Date | January 1979 |
| Creators | Brenton, A. G. |
| Publisher | Swansea University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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