Positron annihilation in the alkali halides

Abstract

Position lifetime spectra and Doppler broadening spectra have been measured for 17 alkali halides. Three significant lifetime components are found in almost all cases. The results are interpreted in terms of a model in which a fraction of the positrons form positronium and the remainder are initially in a state from which either annihilation or transitions may occur. The annihilation rate from the initial state is shown to depend on the distance separating negative ions, which suggests that the positron is in a bound state.Deconvoluted Doppler broadening data indicate that at least two momentum components are present.

A Bound State of Positrons with Sulphate Ions

“…On the contrary, z, decreases from 400 to 255ps, although not linearly. The behavior of z1 in ionic crystals is an important feature of these solids: The lifetime increase with the crystal density [13]. The sulphates studied in this work are listed in Table 1 in the order of increasing density.…”

“…The possibility of the annihilation of these positrons from a bound state [13] has also been considered, but it has never been given a clear picture of the details, although that state should be associated to a shallow trap.…”

Positron annihilation in colourless centres in heavily gamma-irradiated NaCl

“…The time spectrum of unirradiated samples has been fitted with two exponential decay components; the values obtained agree with those reported for this material by Linderoth et a1 (1986). A three-terms fit, as suggested by other authors (Kerr et al 1978) yields a very low value for 23 (less than 1%) and does not substantially improve the variance of the fit. However, for irradiated samples a third component (q = 1000 ps, Z3 = 6%) has been necessary in order to fit the data.…”