A mesoscopic approach to radiation-induced defect aggregation in alkali halides stimulated by the elastic interaction of mobile Frenkel defects

The radiation-induced aggregation of Frenkel defects in alkali halides is studied in terms of a mesoscopic approach. The asymmetry in elastic interactions between mobile interstitials (I-I) and between interstitials and vacancies (I-V) is shown to play a decisive role in the aggregation of similar defects. Results obtained with a clear-cut approximation of the elastic interaction are compared with those obtained with a more correct long-range (r⁻³) potential. The conditions for defect aggregation are studied in detail for NaCl crystals. The critical irradiation intensity (dose rate) for aggregation is calculated as a function of the temperature as well as the aggregation rate as a function of temperature and dose rate. Furthermore, the role of deep traps (like impurities and di-vacancies), reducing the mobility of interstitials, and the role of dislocations serving as sinks for interstitials are studied. Aggregation appears to reach a maximum at a distinct temperature which is in agreement both with experiment and the Jain-Lidiard theory. The model predicts a shift of this maximum towards lower temperatures if the dose rate is decreased.