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.

The absolute cross section for the fragmentation of positronium in collision with He atoms has been measured. The results are compared with available theories. The longitudinal energy distributions of positrons resulting from fragmentation have also been determined and are found to display a peak situated just below half the residual energy. This is suggestive of the occurrence of "electron loss to the continuum" in which the two residual charged particles lie in a low relative-velocity Coulomb-continuum state.

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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, θ, 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 θ → 0) for Ps-He, Ar and H 2 have been obtained. Where possible, the data are compared with available theoretical calculations.

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