Fundamental studies of positron scattering processes from simple targets

Abstract

Positron scattering is useful in the investigation of positron interactions with atoms and molecules, which in turn helps with the classification of fundamental quantum processes. Methods for measuring differential cross sections as well as ionisation cross sections on the Australian National University (ANU) positron beamline are discussed, as well as the measurement of the positronium formation differential cross section (DCS) at the University College London (UCL) positronium formation beamline.

This thesis reports the first absolute elastic differential cross section results for positron scattering on neon. DCS measurements were made for the energies 1,2,3 and 5 eV and were compared to three theoretical methods; relativistic optical potential (ROP), convergent close coupling (CCC), and many body theory (MBT). CCC and ROP were found to be a better match to the data over most angles for the lowest energies measured. The higher energy DCS deviate significantly from theory despite agreement in the total cross section (TCS), and possible reasons for this are also discussed.

Also presented are near threshold ionisation measurements, which are the first results produced within 2 eV above the ionisation energy, with high energy resolution. These measurements test the Wannier threshold law, where the behaviour of the cross section is dependent on the interactions of the escaping particles. This leads to a semiclassical approximation for the cross section in which a strict escape geometry is predicted for ionisation near threshold. It was found that using either positrons or electrons, and hence changing the projectile sign, did not appear to have a significant effect on the measured exponent. This is despite theoretical predictions the exponent would be much higher for positron impact than the corresponding electron scattering process.

The progress towards a positron reaction microscope is also reported, with the eventual goal to measure fully differential cross sections from positron ionisation of the noble gases. Scattered positrons, ejected electrons, and residual ions are all detected in coincidence, with the ionisation event coincidence detection rate increased by a factor of 30 compared to previous work, and an overall $\approx 1$ million triple candidates collected. This suggests viability for a difficult experiment, given the low yields of positrons compared to electrons.

Positronium formation in the forward direction was measured using the University College London positronium beamline. The results presented are forward formation differential cross sections for positrons on neon, and further the investigation into viable targets for efficient positronium beam formation. The measurements taken for neon were compared to measurements made for helium and argon previously on the UCL beamline, with the forward collimation of positronium found to have similar trends for neon and argon, with the greatest yield from helium.