Correlated annealing of radiation defects in alkali halide crystals

Abstract

The kinetics of the correlated annealing of pairs of neutral (F-H) and charged ( alpha -I) Frenkel defects in alkali halides is treated theoretically, taking into account defect diffusion, annihilation at short distances, and elastic or Coulomb interactions. It is shown that elastic interactions considerably affect the annealing kinetics and survival probability even for defect pairs that are third or fourth neighbours in the crystalline lattice. A widely used description of the correlated annealing in terms of the pseudo-first-order reaction fails for close defects, yielding effective energies E* for which in fact differ essentially from the activation energy Ea of diffusion, even when it is corrected by an interaction energy. The effect of the defect initial distribution over relative distances is studied. It is demonstrated that pulsed creation of spatially well correlated defects can lead to the non-exponential decay of concentrations usually expected for an isothermal annealing. The theory developed is compared with actual experimental data for KBr. It is concluded that the several-stage annealing of defect concentrations often observed in thermostimulated experiments does not necessarily mean recombination of dose Frenkel defects which differ in their initial distances.