Finch, Rowan Matthew; (1997) Differential cross section measurements in positron-gas collisions. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

An electrostatic system has been used to measure Differential Cross-Sections (DCS) in positron-gas collisions. Single DCS were measured using a Retarding Field Analyzer (RFA). Double DCS were measured using a tandem 30° Parallel Plate Analyzer (PPA). The energy dependencies of the DCS for elastic scattering, positronium formation and direct single ionization in positron-argon collisions at 60° and 30° (not elastic scattering) have been measured in the energy range 40-150eV. Of particular interest was the energy range 55-60eV where Dou et al (Phys. Rev. Lett. 68 (1992) 2913), from the Detroit group, reported a drop in the elastic scattering cross section by a factor of two. This was tentatively attributed to a cross-channel coupling effect with an open inelastic scattering channel. Our measurements showed no structure in any of the cross sections. This agrees with the results of new experiments in which the Detroit group have not been able to reproduce the previously reported structures. Double differential ionization cross-section measurements were performed at 30° for 60 and 100eV impact energies to investigate the phenomena known as Electron Capture to the Continuum (ECC). ECC cusps have been observed in heavy-ion impact collisions but not in the equivalent positron impact studies, despite the many theoretical calculations predicting their existence. Our measurements showed no evidence of ECC. In addition, at the higher scattered positron energies, results of the Classical Trajectory Monte-Carlo calculations of Sparrow and Olson (J. Phys. B 27 (1994) 2647) exceeds our measurements. This discrepancy was first observed in the equivalent measurements of Kover et al (J. Phys. B 27 (1994) L613) using a RFA. The fact that this discrepancy was observed from two independent experiments, using very different detection instruments, suggests that it is a genuine physical effect and that further work, both experimental and theoretical, is necessary to understand it.

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