A. J. Garner

Ps beam production and scattering from gaseous targets

We have investigated the conversion of a positron beam into an ortho-Pa beam via charge exchange in gaseous H-2, He and Ar at energies up to 120 eV and gas pressures up to 14 mu mHg. H-2 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 H-2 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.

POSITRONIUM BEAM SCATTERING FROM ATOMS AND MOLECULES

Abstract Production techniques and applications of positronium beams in the energy range ∼10–100 eV are reviewed. Recent positronium–gas cross-section results for He, Ar, H 2 and O 2 measured at UCL are discussed and compared to theoretical calculations and other relevant experimental data.

The effect of forward-angle scattering on positronium-gas total cross sections

An investigation into forward-angle scattering of Ps atoms from atomic and molecular targets is reported. This effect has been evaluated by varying the angular acceptance of the detector, , in the range ±1.5° to ±6.4°. By assuming an average differential cross section over this range, values of the total cross section (extrapolated to 0) for Ps-He, Ar and H2 have been obtained. Where possible, the data are compared with available theoretical calculations.

Rare gas solid moderator for Ps beam at UCL

Abstract A rare gas solid moderator has been installed into the positronium beamline at UCL. The new system, which has been constructed in order to provide an ultra high vacuum environment and to accommodate the moderator assembly, is described. Solid argon has been investigated as a moderator and different growing conditions, especially with respect to temperature and pressure, have been explored. The results of these investigations are presented and compared with those of previous experiments.

Low-energy positron-argon elastic scattering at 30°

The energy dependence of the cross section for positrons scattering elastically at 30° from argon in the vicinity of the positronium (Ps) formation threshold has been investigated. Together with previous experimental and theoretical results, the present data indicate the influence of channel-coupling effects on elastic scattering from Ps formation and suggest a similar effect from positron impact excitation.

From Positron to Positronium Scattering

Collisions involving positron and positronium are contributing to the understanding of general atomic physics phenomena as well as illuminating specific processes and interactions such as annihilation and exchange. It is hoped that the considerable effort presently aimed towards the production of large ensembles of cold positrons86 will result in beams of extremely well defined energy (≤kT) which should enable the detailed study of near-threshold phenomena, resonances, annihilation and positronic compounds.

An electrostatic beam for positron–gas scattering measurements

Abstract The details of an electrostatic beam, designed for investigating positron–atom (molecule) scattering processes, are presented. The performance of the system has been tested by measuring differential elastic e + –Ar scattering cross sections. The preliminary data have been found to be consistent with previous results.