Measurements of total and positronium-formation cross sections for the scattering of positrons by alkali atoms

Abstract

We present a review of our recent measurements of total cross sections (QT's) for the scattering of positrons by Na, K, and Rb, and positronium-formation cross sections (QPs's) for Na and K. For our total cross section measurements, a beam transmission technique has been used. For ourQPs measurements, our approach involves setting upper and lower limits onQPs 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-Ps formed in the scattering cell. Comparison with theoretical calculations shows that our measuredQT's andQPs's for Na and K agree reasonably well with a close coupling approximation (CCA) calculation which takes into account the formation of Ps in then=1 andn=2 states. In the 3–10 eV energy range, this calculation predicts a peak in theQT's andQPs's for K which also appears in our measurements. The absence of such a peak in our measuredQT's andQPs's (preliminary) for Na in this energy range is also consistent with the same theory. Comparisons with five-state CCA calculations ofQT which do not take Ps-formation into account also show good agreement with our positron-Na, K, and RbQT measurements for energies above 20 eV, but show dramatic departures from our measurements below 10 eV for K and Rb.