Positron Lifetimes in Metals

Abstract

Positron decay curves in various metals have been measured, using careful sample preparation techniques and a delayed-coincidence system with 0.30-nsec (full width at half-maximum) prompt time resolution and ${10}^{4}$:1 peak-to-background ratio. Evidence was found that the annihilation in pure metals under ideal conditions is characterized by a single exponential decay, the second lifetime usually observed being an artifact of sample preparation. In the case of Mg with ${\mathrm{Na}}^{22}$ produced in situ by the ($p, \ensuremath{\alpha}$) reaction, a single exponential decay over approximately four decades was observed. In a plot of annihilation rate versus conduction electron density, the observed rates for the nearly-free-electron metals fell close to the theoretical curve of Kahana, approaching the spin-averaged positronium rate at low densities. Compared with these rates, annihilation rates at a given valence-electron density were found to be higher in the transition metals and lower in Be, Bi, and in the semiconductors Si and Ge, in agreement with qualitative ideas about the one-positron many-electron system in real metals. Within the experimental error (\ifmmode\pm\else\textpm\fi{}2.5%), no difference in lifetime was observed between intrinsic and heavily $n$-type Si, and similarly for Ge.