MEASUREMENTS OF POSITRON SCATTERING BY HYDROGEN, ALKALI-METAL, AND OTHER ATOMS

Recent developments in measurements of total and positronium (Ps) form ation cross sections for positrons (in the energy range of 1-300 eV) s cattered by hydrogen, alkali metal, and other atoms are reviewed. Meas urements and calculations of total and Ps formation cross section (Q(P s)'s) for positrons scattered by hydrogen atoms are in very good agree ment, and for the most part there is also good agreement for sodium, p otassium, and rubidium atoms, but there is a puzzling discrepancy betw een measured and recently calculated Q(Ps)'s for sodium. Preliminary m easurements of Q(Ps)'s for Mg show a very rapid rise to a large maximu m value less than 2 eV above the Ps formation threshold energy (0.8 eV ) which may be related to the proximity of that threshold to zero ener gy. It appears that structure observed in e(+)-Ar and Kr Q(Ps) measure ments may be related to capture of inner-subshell electrons. (C) 1998 Elsevier Science B.V. All rights reserved.