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In this paper we review prior measurements and report new measurements of elastic differential cross sections (DCSs) obtained in our laboratory for positron scattering by the inert gas atoms He, Ne, Ar, Kr, and Xe. The gases (Ne, Ar, Kr, and Xe) for which low-energy measurements have been made below and above the positronium (Ps)-formation thresholds show evidence that the Ps-formation channel is having an observable "absorption" effect on the elastic scattering channel when compared with calculations that only consider the elastic channel. At low energies where only the elastic channel is open, good agreement is found with available calculations. DCS measurements made at high energies (≥200 eV) for He, Ne, and Ar are also found to be consistent with theoretical calculations. At intermediate energies (above the Ps-formation thresholds and below a few hundred eV) theory has not yet been able to match the relative DCSs obtained experimentally owing to the difficulty of including the effects of the most important inelastic channels (i.e., Ps formation and direct ionization). Some initial attempts by our group to obtain absolute elastic DCSs at fixed scattering angles versus positron energy revealed unexpected structure between 55–60 eV for Ar and at 25 and 200 eV for Kr. Our most recent measurements reported here do not confirm the prior observations of these structural features.

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In this letter we present measurements of total cross sections for 1-10 eV positrons scattered from sodium (Na) atoms. Comparisons of these measurements and other recent measurements of total and positronium (Ps) formation cross sections for positron-Na and potassium (K) scattering by our group with the close-coupling approximation calculations by Hewitt et al. (1993) provide rather clear evidence that coupling effects between Ps formation and other scattering channels are playing a very important role in positron scattering by Na and K.

Total cross sections (Qr's) have been measured for I -500-eV positrons and electrons scattered by 02, CH4, and SF6 using a beam-transmission technique. The positron Qr s are compared with the corresponding electron Qr's for each target gas. It is found that the positron Qr's are, in general, lower than the electron results. There are no prominent structures observed for positron scattering at low energies that would be comparable to the narrow shape resonances observed for electrons scattering from various molecules, such as SF6. The positron Qr curve for CH4 reveals a significant increase in the vicinity of its positronium formation threshold, while the 02 and SF6 curves are monotonically increasing in this vicinity. At the highest energies investigated, there are indications of a tendency toward merging of the positron and electron Qr s for these gases. Interesting similarities are found in a comparison of the present positron and electron Qr curves for CH4 with prior comparison measurements for argon.