Diffusion of Helium Isotopes in Vitreous Silica

Abstract

The diffusion of ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ in vitreous silica was experimentally measured in the temperature range 100-760°C. The ratio of the diffusion coefficients is given by $\frac{D\left({\mathrm{He}}^3\right)}{D\left({\mathrm{He}}^4\right)}=(1.205\mathrm{}\pm 0.003)$ $\exp \left[\frac{-(131\mathrm{}\pm 7\frac{\mathrm{cal}}{\mathrm{g}-\mathrm{a}\mathrm{t}.\mathrm{}})}{\mathrm{RT}}\right]$. These results cannot be explained by classical diffusion theory, which does not predict an isotope effect in the activation energy for diffusion. The quantum corrections proposed by LeClaire for the special case of low-atomic-mass isotope diffusion satisfactorily explain the observed results.