On the experimental study of Helium by positron impact

Abstract

The fundamental thrust of this research is to obtain high resolution absolute cross-section measurements for helium by positron impact, performed under a single scattering regime. This investigation focuses on low energy collisions ({u2264} 60 eV), where theory continues to be hampered by the positronium formation channel. The intention here is to provide our theoretical colleagues with detailed data that will rigorously test their models. Measurements of this calibre (absolute errors {u2264} 10%) form the building blocks to what are commonly referred to in the field of atomic and molecular physics as "benchmark measurements". The aspiration here is nothing less than to use our experimental expertise and theoretical insights of positron-helium interactions as a platform from which to understand better positron interactions with more complex systems. These results are obtained using a transmission based, pulsed positron beam apparatus which can probe matter at high resolution (45 to 90 meV). To date, the measurements presented here amount to the most detailed and extensive study of helium by positron bombardment. These results include benchmark quality measurements of the total scattering cross-section, the positronium formation cross-section and the cross-section which does not include the contribution from the positronium formation channel, the "non-positronium cross-section". Unique to this investigation is the unprecedented level of detailed measurements ({u2264} 50 meV energy mesh) taken around the onset for positronium formation and around the electronic and direct ionisation thresholds. The main motivation behind these experiments was to search for the possible existence of resonances and Wigner cusps, which are prevalent in electron scattering experiments. A novel experimental technique is used to partition from the grand total cross-section, the 2{u00B9}S and the 2{u00B9}P electronic excitation scattering cross-sections, from their respective thresholds up to 38 eV. These measurements have recently been used to gain indirect access into the behaviour of the elastic cross-section above the Ore gap region and as an added bonus, helped expose the contribution coming from the sum of the 3{u00B9}S, 3{u00B9}D and 3{u00B9}P excitation cross-section. Also a detailed study of the non-positronium cross-section has revealed the possible indirect observation of a shape resonance in the 2{u00B9}S excitation cross-section centered at around 21.9 eV and what appears to be a fairly broad Feshbach resonance in the 2{u00B9}P excitation cross-section centered at around 22.6 eV. In addition, much needed absolute measurements of elastic differential cross-sections have been catalogued. These measurements provide a more stringent test for theories in three distinct energy regions which become more complex as more channels become open for scattering. Finally, where ever possible, all measurements are compared with available experimental results and cutting edge theoretical calculations.