We have investigated the conversion of a positron beam into an ortho-Ps beam via charge exchange in gaseous , He and Ar at energies up to 120 eV and gas pressures up to . has been found to be the most efficient converter at energies up to 90 eV. At higher energies, Ar has been found to be the best. Total cross sections of ortho-Ps scattering from and He have also been measured. A comparison with available theories suggests that electron exchange at lower energies and target inelastic collisions at higher energies are playing a major role in the collisions.

Detailed measurements of the total ionization cross-sections for Ne, Ar, Kr and Xe by positron impact are presented in the energy range from threshold up to ~1 keV. From these and other available data, the cross-sections for positronium formation are extracted. Comparison is made with other results where available. Structure in the positronium formation cross-section has been found which appears to be associated with positronium formation in excited states. Upper and lower limits on the contribution from these are estimated.

Positronium (Ps), a hydrogen-like atom composed of an electron and its antimatter partner, the positron, is formed in considerable quantities whenever positrons interact with matter. It has unexpectedly been found to scatter from a wide variety of atoms and molecules in a way very similar to that of a bare electron moving at the same velocity, despite Ps being neutral and twice the mass.

A critical requirement for high gain and low noise avalanche photodiodes is the single-carrier avalanche multiplication. We propose that the single-carrier avalanche multiplication can be achieved in materials with a limited width of the conduction or valence band resulting in a restriction of kinetic energy for one of the charge carriers. This feature is not common to the majority of technologically relevant semiconductors, but it is observed in chalcogenides, such as Selenium and compound I 2-II-IV-VI 4 alloys.

Measurements of the single and the ratio of double to single ionization cross sections for positron impact on neon, krypton and xenon are presented from near threshold to 1000 eV. By multiplying the values of each target by the appropriate , new data for the corresponding double ionization cross sections have also been obtained. The ratio between positron and electron maxima is generally seen to be lower for higher target atomic number, as expected from the static interaction between the projectile and undistorted target. Comparison with available data suggests that Ps formation accompanied by the production of singly or doubly charged ions is a significant channel in positron - atom scattering.