Positron lifetime in silver bromide

Measurements of positron lifetime in AgBr crystals grown by different techniques are presented. The time spectra for annealed samples are analyzed in the frame of a 'two-state' model which takes into account the polarization by the positron's charge and the contribution from the silver d-shell electrons. The annihilation rates from the two states turn out to be 4.35x109s-1 and 3.16x109s-1 respectively. The transition rate varies from 109s-1 to 5x1010s-1 for the crystals investigated. From massspectrographic analysis and from the influence of doping or illumination on the positron lifetime it is concluded that the transition rate cannot be attributed to trapping positrons at cation vacancies. However, a systematic correlation between the mean lifetime and the occurrence of the molecular ions AgBr+ and Br+2 was noted in the massspectrographic studies. (Auth.)