Interactions of positrons & positronium with matter

The aim of this work is to help and advance the understanding of the interaction of positrons and positronium (Ps) with matter. Knowledge of these interactions has a wide range of applications in many fields including medical imaging, galactic astronomy and condensed matter physics. An electrostatic beam of positrons has been used with a tungsten mesh moderator and a tungsten foil remoderator. The beam has been characterised to have a beam divergence of ∼ 1 degree and energy resolution of 1%. The system has been applied to the measurements of the positron total cross section for He. Despite improved absolute angular resolution and a truly field-free interaction region, similarity with previous magnetic beam measurements with poorer angular resolution and theories has been found, in contrast with the only previous electrostatic beam measurements. The collimated Ps production efficiency from Ar and Xe has been measured down to 1.4eV, enabling Ps scattering experiments to be performed in a new energy range. The total cross sections for Ps scattering from Ar, Xe, O2 and CF4 have been measured between 1.4 and 120eV. In comparison with the total cross section for equivelocity electrons, an enhancement in the Ps scattering is observed centred at ∼ 5 eV. For Ar and Xe, in contrast to theoretical predictions, the Ps total cross section is found to be not inconsistent with the Ramsauer-Townsend minima for electrons. The absolute differential Ps formation cross section near 0 degrees has been determined for Ar, Kr and Xe and compared with theory. In analogy with a semi-empirical scaling previously found for the integral Ps formation cross section, similarities in shape of the scaled differential Ps formation cross sections have been found between Ar, Kr and Xe and, separately, from those of He and H2 . An exponential relationship has been found between the maximum differential Ps formation cross section and the Ps formation threshold.