MEASUREMENTS OF TOTAL AND (OR) POSITRONIUM-FORMATION CROSS-SECTIONS FOR POSITRONS SCATTERED BY ALKALI, MAGNESIUM, AND HYDROGEN-ATOMS

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.