THE LIFETIMES OF POSITRONS IN METALS

Abstract

The lifetimes of positrons annihilating in some 21 metals have been measured comparatively with the lifetime in aluminum. Significant variations have been found, as large as 30% of the absolute lifetime in aluminum, and the lifetime shows a more marked dependence on the atomic density than on the valence electron density of the metal. By comparison of lifetime and angular distribution data a mechanism of the annihilation process is proposed. In metals with large lattice spacings the annihilation is primarily with valence electrons, but slightly enhanced by pick-off annihilations with outer electrons of the ions; as the spacing decreases the pick-off probability increases, resulting in larger high momentum tails to the parabolic angular distributions and shorter lifetimes. The annihilation rate increases with valence electron density but levels off at higher densities at a rate similar to the theoretical value for positronium ions. With very reduced lattice spacing the annihilation with ionic electrons becomes comparable with that with valence electrons, so that bell-shaped angular distributions and shorter lifetimes result.