Positron annihilation on the F'-centers in ionic crystals

The Pekar-Tomasevich and Simpson-Wills approximations were used to calculate the lifetime τ2γ with respect to self-annihilation of a positron bound to the F'-center (the so-called F double-prime +-center, that is, the polyelectron Wheeler system e-2e+ 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 τ2γ enable the overall lifetimes τs,tFdouble-prime+ of the F double-prime +-centers with account for annihilation in crystals, the so-called open-quotes pick-offclose quotes -annihilation, to be more accurately calculated. Formulas were derived for calculating the angular correlation of annihilation radiation (ACAR) and the halfwidths Γ of ACAR curves for the case of F double-prime +-center self-annihilation. The obtained estimates led to the conclusion that positron annihilation from the F double-prime +-centers [for example, in alkali halide crystals (AHC) most likely contributes to the intensities of the τ2 ∼6x10-10 s and τ3∼10-9 s components in the experimental time-resolved annihilation spectra