ON THE INFLUENCE OF DEFECTS ON POSITRON ANNIHILATION IN ALKALI-HALIDES

Résumé
La technique d'annihilation positronique par l'utilisation de la durée de vie et la corrélation angulaire est présentée. Les résultats sont présentés pour : 1) Monocristaux de NaCl et KCl colorés additivement ; 2) Conversion F à F^- ; 3) Trempe thermique ; 4) Défaut d'irradiation.

Abstract
The technique of positron annihilation by use of lifetime and angular correlation will be presented. The main characteristic of this technique is that defects are important in the annihilation processes. Results will be presented for : 1) Additively colored NaCl and KCl single crystals ; 2) F to F^- conversion ; 3) Thermal quenching ; 4) Radiation damage. These experiments show that F centers are effective as traps for positrons by the presence of a new 1.1 ns lifetime, and that trapping probability is proportional to the density of F centers, and further that F to F^- conversion decreases this probability. The introduction of defects by thermal treatment causes considerable changes in the annihilation spectra, and this defect influence on stage I coloration is also revealed in angular correlation measurements. From these experiments it is clear that stage I coloration takes place in a thin surface layer (in NaCl) within approximately four hours using a 1 mCi Na²² source.