Self-Diffusion of Tritium in Natural and Synthetic Ice Monocrystals

The self-diffusion of tritium in artificially and naturally grown ice monocrystals, parallel and perpendicular to the optical axis, was studied between −2.5° and −35.9°C. The plane-source solution to Fick's second law was used in treating the data. An anisotropy of ≈ 12% was observed. The activation energy was found to be 0.62 eV for all cases and D0 ≈ 10 cm2/sec. Based on the experimental data, it is concluded that the diffusion takes place by a vacancy mechanism, and that entire H2O molecules are diffusing, i.e., molecular diffusion occurs. Theoretical calculations using the atomic-diffusion theory and Zener's theory for D0 are in excellent agreement with the experimentally determined diffusion coefficient.