MEASUREMENTS OF TOTAL AND POSITRONIUM-FORMATION CROSS-SECTIONS FOR THE SCATTERING OF POSITRONS BY ALKALI ATOMS

We present a review of our recent measurements of total cross sections (rho(T)'s) for the scattering of positrons by Na, K, and Rb, and posi tronium-formation cross sections (rho(ps)'s) for Na and K. For our tot al cross section measurements, a beam transmission technique has been used. For our rho(ps) measurements, our approach involves setting uppe r and lower limits on rho(ps), using a combination of(1) measuring the transmission of the positron beam with the angular discrimination of the apparatus made as poor as possible, and (2) measuring the 511 keV annihilation gamma rays in coincidence produced by the decay of para-P s formed in the scattering cell. Comparison with theoretical calculati ons shows that our measured rho(T)'s and rho(ps)'s for Na and K agree reasonably well with a close coupling approximation (CCA) calculation which takes into account the formation of Ps in the n = 1 and n = 2 st ates. In the 3-10 eV energy range, this calculation predicts a peak in the rho(T)'s and rho(ps)'s for K which also appears in our measuremen ts. The absence of such a peak in our measured rho(T)'s and rho(ps)'s (preliminary) for Na in this energy range is also consistent with the same theory. Comparisons with five-state CCA calculations of rho(t) wh ich do not take Ps-formation into account also show good agreement wit h our positron-Na, K, and Rb rho(T) measurements for energies above 20 eV, but show dramatic departures from our measurements below 10 eV fo r K and Rb.