New formulation of the modeling of radiation-induced microstructure evolution in alkali halides

Abstract

Recent experimental results on heavily irradiated natural and synthetic NaCl crystals give a strong evidence for the formation of large vacancy voids, which cannot be explained by the Jain—Lidiard model used up to date for description of metal colloids and dislocation loops formed during earlier stages of irradiation. We propose a new mechanism of dislocation climb, which involves production of cation vacancies (with a trapped hole) as a results of absorption of extra H centers. We consider an additional set of reactions between experimentally observed extended defects (dislocations, metal colloids, gas bubbles and voids) and point defects. The latter include F and H centers that are the primary defects produced by irradiation, and cation vacancies that are secondary defects produced in the process of dislocation climb. We show that highly overpressurized bubbles of fluid halogen are strongly biased for absorption of H centers, which makes them grow via punching out interstitial dislocation loops. The loops grow and produce cation vacancies that are subsequently trapped at the surfaces of extended defects, such as the incoherent colloids, and recombine with extra F centers giving rise to the void formation. Elastic interaction between extended defects and point defects is shown to play a major role, since it determines the bias factors of extended defects, which is a major driving force of the microstructural evolution under irradiatioin.