An investigation was made of Xe release from single crystals. Diffusion at low gas concentrations, and negligible radiation damage levels, was measured by growing single crystals which contained radioactive 133I; which then decayed to 133Xe. Both the time rate of release, and the temperature dependence of the rare gas diffusion coefficient were consistent with classical diffusion solutions. The data for 150 to 500C could be described by:

D (cm2/s) = 0.57 exp[-1.01(eV)/kT]

Crystals which contained high concentrations of defects exhibited trapping of the gas, and anomalous diffusion kinetics. Trapped gas atoms tended to stabilize the defects and prevented their annealing-out during heating. The diffusion of fission gas which had been recoiled into the specimen surface, from an external fissionable source, obeyed classical diffusion solutions at low fission recoil concentrations. At high concentrations, radiation damage created traps which decreased the gas diffusion rates. These traps differed significantly from natural defects with regard to concentration, gas-atom binding energy and annealing characteristics. Increasing the gas concentration, independently of radiation damage, also lowered the rare-gas diffusion coefficient; thus showing that the formation of small gas atom clusters also produced trapping.

Influence of Defects on Rare-Gas Diffusion in Solids. T.S.Elleman, C.H.Fox, L.D.Mears: Journal of Nuclear Materials, 1969, 30[1-2], 89-106

Table 198

Diffusivity of 22Na in CsI