Positron states in ionic media

Abstract

The lifetime spectra of positrons being annihilated in various ionic crystals were measured. The value of decay rates and intensities of the spectral components thus obtained were analyzed together with previously published results on alkali halides. This set of data suggested a model whose essential features are: (i) that the positron is annihilated in states where there are different electron densities; (ii) that each positron, when slowed down, populates a single state; (iii) that the electron density required for annihilation in the first state depends entirely on the number of negative ions per unit volume accessible to the positron; (iv) that the transition rates are comparable with annihilation rates. The proposed model also accounts for the distortion of the time annihilation spectrum observed by Dupasquier (1970) in crystals of KCl coloured by F centres.