Radiation damage in NaCl: General model of nucleation and aggregation processes in doped NaCl

Abstract

The model presented here offers an extensive description of the nucleation and aggregation processes in irradiated NaCl. In particular the effect of impurities on the production of radiation damage has been studied. As compared to theoretical models in the literature the approach shows the following refinements:

(1) The nucleation behaviour for aggregation of primary defects is included. Dislocation loops are formed by consecutive trapping of H-centres at impurity sites. Colloid nucleation is described by the coagulation of F-centres into higher order clusters. With the proposed mechanism we are able to consider the effects of the mobility of the F- and H-centre and the stability of the clusters during the nucleation process.

(2) The precipitation of molecular halogen centres near the dislocation lines results in a rapidly increasing recombination of F-centres at high damage levels. The production of colloid becomes less-than-linear and the amount of radiation damage approaches a saturation value, determined by the maximum number of halogen molecular centres which can precipitate in the crystal.

The model provides a general scheme for understanding the production of radiation damage in NaCl over a large range of doses (0–15 Grad) and temperatures (20-200°C). Within one framework the model is able to account for the major features of both the low dose F-centre colouring and the high dose colloid production. The competition between the formation and thermal dissociation of unstable clusters explains the experimental observation of quadratic relationships between the F- and the M-centres and between the F-centres and the impurities. The well known three stages in F-colouring as well as recent results on the variation of the colloid production for different impurity concentrations at high doses can be explained satisfactorily by the dominant role of impurities in the nucleation stage.

Key Words:

• Radiation damage
• colloid
• NaCl
• nucleation
• aggregation
• impurities