Kinetics ofF-center formation and interstitial stabilization mechanism in alkali halides irradiated at 77 K

In pure crystals of KBr electron irradiated at 77 K, the $F$-center concentration is proportional to the 0.8th power of the irradiation dose and the dose rate dependence of the defect formation is very small. A model is proposed in which the $F$ creation rate is fixed by the secondary reactions taking place after the primary Frenkel pair creation: recombination of free interstitials with $F$ centers and interstitial capture by traps. To account for all our experimental results and electron microscopy observations, we show that clusters of $V_4$ centers (di-interstitials) are nucleated inhomogeneously in the proximity of some residual impurities. The $V_4$-center cluster originates from the interaction between a mobile interstitial and another one temporarily trapped by an impurity, and develops then by capture of $H$-center pairs. We put in evidence a dependence of the interaction volume on the cluster size.