Some systematics of positron-vacancy interactions in metals

Abstract

Experimental data describing the equilibrium temperature dependence of positron annihilation in Cu, Ag, Au, Zn, Cd, Al and Pb are considered in terms of the simple trapping model. The results show that there is a correlation between a temperature, Tv, associated with the onset of measurable positron trapping at vacancies and the absolute temperature of melting, Tm, of the metal; Tv approximately 0.6Tm. The correlation may be shown to imply that the product of the specific trapping rate of positrons at vacancies, mu , and the mean positron lifetime, tau , characteristic of the bulk metal, is similar for each of these elements. Values of mu are then extracted from the data using vacancy concentrations based on a present assessment of parameters describing the temperature dependence of the equilibrium vacancy populations of these metals. It is found that values of the product, mu tau , are indeed similar, as indicated by the Tv/Tm correlation. The result can be qualitatively understood in terms of a theory of positron-vacancy interactions which suggests that changes in mu tend to be cancelled by opposite changes in tau .