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.

Ab initio variational calculations are made of the elastic scattering and positronium formation cross sections for positron-helium scattering below the first excitation threshold of the target. Three variants of the Kohn variational method are used with very flexible trial wavefunctions and a very accurate helium wavefunction. The elastic scattering and positronium formation cross sections are believed to have converged to within 5% and 10%, respectively. The calculated positronium formation cross section displays a similar energy dependence to that of recent experimental results but with a 25% difference in magnitude. However, the theoretical total cross section, both above and below the positronium formation threshold, agrees with the experimental measurements to within 10%. Threshold effects in the s-wave cross sections manifest themselves as a significant feature in the total cross section. Similarities with the corresponding results for positron-hydrogen scattering are discussed.

An analysis is made of the behaviour of the elastic scattering and positronium formation cross sections for s-, p-and d-wave low-energy positron-hydrogen scattering in the vicinity of the positronium formation threshold. The accurate cross sections, obtained using the Kohn variational method with elaborate trial functions containing many Hylleraas correlation functions, reveal interesting threshold structures which arise from the coupling between the positronium formation and the elastic scattering channels. Wigner's R-matrix method is used to provide a qualitative analysis of these structures. One of the most interesting features of the present results is the presence of a 'rounded cusp' in the s-wave elastic scattering cross section at the threshold itself, as predicted by Wigner's threshold theory. Remarkably good fits to the partial-wave cross sections for e + -H elastic scattering, positronium formation and Ps-p elastic scattering are obtained using a five-parameter fit for each partial wave.

Detailed measurements of the total ionization cross-section by positron impact on He from threshold up to 850 eV are reported. Both the present total ionization cross-section and the direct ionization cross-section of Moxom et al (1996) and Ashley et al (1996) have been normalized to the latest electron-impact ionization cross-sections. The positronium formation cross-section for this target is thus extracted and compared with the existing experimental and theoretical data.

A detailed investigation is made of low energy positronium–hydrogen s-wave elastic scattering, using the Kohn variational method, with very flexible scattering wavefunctions including all inter-particle correlations. The singlet and triplet scattering lengths are calculated and found to be a+ = 4.311a 0 and a− = 2.126a 0 respectively. The phase shifts are believed to be within less than 0.5% of the exact values, and they are in good agreement with the results of previous calculations.