POSITRON-ANNIHILATION ON THE F-CENTERS IN IONIC-CRYSTALS

The Pekar-Tomasevich and Simpson-Wills approximations were used to calculate the lifetime tau2gamma with respect to self-annihilation of a positron bound to the F'-center (the so-called F+''-center, that is, the polyelectron Wheeler system e2-e+ in an anion vacancy) in a defect ionic crystal. It turned out that positron lifetimes for various ionic crystals most likely fall into the nanosecond range in the time-resolved annihilation spectra. The obtained values of tau2gamma enable the overall lifetimes tau(F+'')s,t of the F+''-centers with account for annihilation in crystals, the so-called ''pick-off''-annihilation, to be more accurately calculated. Formulas were derived for calculating the angular correlation of annihilation radiation (ACAR) and the halfwidths GAMMA of ACAR curves for the case of F+''-center self-annihilation. The obtained estimates led to the conclusion that positron annihilation from the F+''-centers [for example, in alkali halide crystals (AHC)] most likely contributes to the intensities of the tau2 approximately 6 x 10(-10) s and tau3 approximately 10(-9) s components in the experimental time-resolved annihilation spectra.