S. P. Parikh

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 (QTs) for the scattering of positrons by Na, K, and Rb, and positronium-formation cross sections (QPss) 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 measuredQTs andQPss 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 theQTs andQPss for K which also appears in our measurements. The absence of such a peak in our measuredQTs andQPss (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.

Measurements of positronium formation cross sections for positron-ar and-K scattering

We have made preliminary measurements of positronium (Ps) formation cross sections for 9 to 452 eV positrons scattered from Ar atoms and for 1 to 20 eV positrons scattered from K atoms. Our experimental approach involves setting lower and upper limits on Ps formation cross sections using a combination of (1) the detection of the coincidences of 511 keV annihilation gamma rays produced by the decay of para-Ps and by the interaction of ortho-Ps with the wall of the scattering cell in which the Ps is formed, and (2) the determination of scattering cross sections associated with the measured transmission of the positron beam through the gas in our scattering cell with the angular discrimination of our apparatus deliberately made as poor as possible. The constraints placed by these lower and upper limits are used to check for consistency with prior experimental and theoretical results for Ar and to provide the first measurements of Ps formation cross sections for K, which are compared with available theoretical results.

Measurements of positronium formation cross sections for positron–potassium atom scattering

We report preliminary measurements of positronium (Ps) formation cross sections for 1–20 eV positron–potassium atom collisions. Our results are compared with prior theoretical results.

Measurements of positronium formation cross sections for positron–argon scattering

We report preliminary measurements of positronium (Ps) formation cross sections (QP’s) for 9–452 eV positron–argon collisions. The present QP’s lower and upper limits are used to check for consistency with prior measurements.

Measurements of total cross sections for positron-K and-Rb scattering

A modification of the apparatus that we have been using to measure absolute total cross sections (QT′s) for positron and electron scattering by alkali metal atoms has enabled us to more easily obtainQT′s at low energies (<10 eV). Our positron-K and-RbQT measurements now extend from 1 to 98 eV and we have made the correspondingQT measurements for electrons in the same system using the same technique (a beam transmission technique). Our measured positron-K and-RbQT values show a peak in the vicinity of 6 eV and decrease substantially as the positron energy is reduced below 6 eV, in sharp contrast to the results obtained in recent 5-state close-coupling approximation calculations. At higher energies (above 10 eV), we have found that the same calculations are in quite good agreement with our measured positron-K and-RbQT′s. Our measurements of the correspondingQT′s for electrons below 10 eV do not show a peak, but rather continue to increase as the energy is decreased.