Ts. Stein et al., MEASUREMENTS OF TOTAL AND (OR) POSITRONIUM-FORMATION CROSS-SECTIONS FOR POSITRONS SCATTERED BY ALKALI, MAGNESIUM, AND HYDROGEN-ATOMS, Canadian journal of physics, 74(7-8), 1996, pp. 313-333
Recent developments in measurements of total and (or) positronium-form
ation cross sections for positrons (in the range of 1 to 300 eV) scatt
ered by alkali, magnesium, and hydrogen atoms are reviewed. Measuremen
ts of total and positronium (Ps)-formation cross sections for positron
s scattered by sodium, potassium, and rubidium have revealed an intere
sting pattern of differences and similarities between these collision
systems. These measurements, together with recent calculations, provid
e evidence that coupling effects between Ps formation and other scatte
ring channels are very important at low energies for the alkali atoms.
The calculations also indicate that formation of Ps in excited states
in the cases of potassium and rubidium is more likely than formation
in the ground state when the positron energy is greater than about 5 e
V. Measurements of total and Ps-formation cross sections for positrons
scattered by magnesium are in a preliminary stage, but the difference
s between them and the results of available theoretical calculations a
re providing a strong incentive to intensify experimental and theoreti
cal investigations of this collision system. Refinements in a recently
developed technique for measuring total cross sections for positrons
and electrons scattered by atomic hydrogen have led to preliminary new
results for these systems that are in very good agreement with recent
theoretical calculations. These measurements, together with the calcu
lations with which they agree, indicate that as the projectile energy
is increased through the higher energy range (above 50 eV), the total
cross sections for positrons approach the corresponding results for el
ectrons from above, which is the reverse of the relationship between t
he positron and electron total cross sections in this energy range for
all of the room-temperature gases that have been investigated, except
possibly for molecular hydrogen.