Modeling of the radiation-induced microstructural evolution in ionic crystals

Results of experimental and theoretical investigations are presented on hea vily irradiated natural and synthetic NaCl crystals in the temperature rang e where anion defects are mobile. They give a strong evidence for the forma tion of vacancy voids, which cannot be explained by the Jain-Lidiard model used up to date for description of metal colloids and dislocation loops for med in ionic crystals during earlier stages of irradiation. We consider an additional set of reactions between experimentally observed extended defect s (metal colloids, gas bubbles and voids) and point defects. The latter inc lude F and H centers that are the primary defects produced by irradiation, and cation vacancies (with a trapped hole) that are secondary defects, prod uced in the process of dislocation climb due to absorption of extra H cente rs. We show that highly overpressurized bubbles of fluid halogen are strong ly biased for absorption of H centers, which makes them grow via punching o ut interstitial dislocation loops. The loops grow and produce cation vacanc ies that are subsequently trapped at the incoherent colloids together with extra F centers giving rise to the colloid-void transition. Elastic interac tion between extended defects and point defects is shown to play a major ro le, since it determines the bias factors of extended defects, which is a ma jor driving force of the microstructural evolution under irradiation. A qua ntitative comparison of the new model for radiation damage in NaCl with exp erimental data is presented. Mean sizes and volume fractions of all types o f observed defects are calculated. It is shown that voids formed due to agg lomeration of F centers and cation vacancies can grow to the dimensions exc eeding the mean distance between colloids and bubbles, eventually absorbing them, hence, bringing the halogen gas and metal to a back reaction. Impuri ties play a major role in the void development with increasing irradiation dose, which strongly affects the radiation stability of NaCl. (C) 1999 Else vier Science B.V. All rights reserved.