Low-temperature diffusion, solubility, and interaction of helium in potassium bromide crystals with cation defects

The solubility and diffusion of helium in potassium bromide single crystals doped by calcium were studied by thermal desorption from crystals prelimin arily saturated in the gas phase at temperatures 400-590 K and pressures up to 3.2 MPa. Three temperature regions of solubility are each described by the c = c(0)exp(-E-s/kT). Arrhenius dependence were observed for two sample s with different calcium contents. The middle regions with anomalously low energies of solution (-1.0 and 0.8 +/- 0.1 eV) were proved to correspond to the solution of helium atoms in cationic vacancies formed in the dissociat ion of vacancy-impurity clusters. The results were substantiated by the exp erimental data on diffusion of helium in crystals and the results of comput er simulation of the decomposition of clusters in the approximation of rigi d ions combined with the shell model. Interaction between helium and crysta ls was shown to be chemical in character.