Positron motion and trapping in ionic crystals

Measurements of the temperature effect on the time spectra of positrons annihilated in additively colored KCl show that the rate of positron trapping at F centers per unit F-center concentration is temperature independent in the range 7 to 3000K. The discussion of this new result points out that the trapping process in ionic crystals is not diffusion limited, and, therefore, that the hypothesis, on which was based the derivation of published values of positron diffusion constants, is not confirmed. A secondary result of our experiment is that the mean life of the positron--F-center bound system decreases by approximately 5 percent when the temperature changes from 300 to 70K. This effect may be attributed to the volume decrease of the void where the system is localized