THE DIFFUSION OF "ATTACHED" INERT-GAS ACTIVITY

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Abstract
It was shown previously that, when an inert gas is irradiated with neutrons in the presence of a powdered solid, between 0.1 and 14% of the induced inert-gas activity is firmly "attached" to the solid. The manner in which attached inert-gas activity is released at temperatures between 50 and 800 °C has now been studied. Release data were assembled for a number of systems, and it was noted that the release was reproducible and apparently a diffusion phenomenon. Several theoretical treatments based on diffusion theory were then explored. Agreement between theory and observation was poor if a constant activation enthalpy was assumed. Agreement was good, on the other hand, with theory that took account of the damaged condition of the solid, and the existence of composition anomalies, in the surface region where the inert-gas activity was attached. The damage and composition anomalies were assumed to lead to a spectrum of activation enthalpies and thence to a simplified diffusive motion involving a single rate-controlling jump. The activation-enthalpy spectra for the systems Ar–Nb and Xe–Nb were found to be identical, extending from about 25 to 47 kcal. The spectrum for Xe–Nb2O5 extended from about 28 to 76 kcal, and was, in general, similar to the spectra for Xe–SiO2, Ar–TiO2, and Xe–TiO2.

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