Time Distribution of Positron Annihilation in Liquids and Solids

Abstract

The time distribution of the annihilation radiation emerging from liquids and solids, following the absorption by the samples of fast positrons, has been studied by the delayed coincidence method. The mean life of positrons in metals is found to be about 1.5×${10}^{-10\mathrm{}}$ sec, nearly independent of the metal chosen. Some simple crystals behave similarly, but many amorphous substances, both solid and liquid, show a complex time decay in which about $\frac{2}{3}$ of the positrons annihilate with mean life a few times ${10}^{-10\mathrm{}}$ second, and the remaining $\frac{1}{3}$ with mean life from 0.45×${10}^{-9\mathrm{}}$ sec to 3.5×${10}^{-9\mathrm{}}$ sec, depending on the substance. This longer lifetime is shown to decrease, in general, as the sample is cooled. Placing the sample in moderate electric and magnetic fields has no detectable effect. The long-delayed radiation is shown to be similar in energy and angular distribution to the more prompt component. Tentative interpretations of these facts are given, but some points still await a full explanation.