Ck. Kwan et al., MEASUREMENTS OF TOTAL AND POSITRONIUM-FORMATION CROSS-SECTIONS FOR THE SCATTERING OF POSITRONS BY ALKALI ATOMS, Hyperfine interactions, 89(1-4), 1994, pp. 33-45
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.