Ionisation studies by positron impact.

Differential studies of positron impact ionisation have been performed to understand the energy shift (Kover et al., 2001) of 1.6eV between the theoretical and measured energy spectrum of the ejected electron in the triply-differential cross-section, d*o +1 dE_dQ_dQ+, at 50eV incident energy on H2. Firstly, an absolute energy calibration has been carried out to check for possible contact potential effects in the determination of Kover et al. (2001). Secondly, D2 and He have been investigated at the same residual kinetic energy as in Kover et al. (2001) to probe the occurrence of molecular dissociation or excitation. Finally, the distribution of kinetic energies of scattered positrons from the same collision system (e+-H2) has been measured for the first time. This work shows an unexpected asymmetry in the energy sharing between electron and positron. Possible reasons for this finding are discussed. An extensive study has been carried out also on water vapour ionisation upon positron collision for the first time. Besides their intrinsic interest, ionisation data are essential to provide a measure of the energy released in living matter during radiobiological applications. Preliminary results for the total, o-,+ , and direct, o *, ionisation cross-sections of water have been obtained for collision energies from the threshold to -IkeV.