Isotopic fractionation of Kr and Xe implanted in solids at very low temperatures

We report results on the implantation of Kr and Xe in W under closed system conditions at very low energies (50-500 eV). Investigation of the fraction of gas trapped as a function of time reveals the existence of competing trapping and release mechanisms, and analysis of recovered trapped gas and residual gas phases shows that both elemental and isotopic fractionation result from these mechanisms. We determined the mass dependence for the overall implantation process to be at or near m1, with heavier isotopes enriched in the implanted gas. This mass dependence is inferred to result from implantation and a combination of diffusive and gas sputtering release mechanisms. Our results reaffirm the conclusion of Bernatowicz and Fahey (1986) that previously observed isotopic fractionation of Kr and Xe in carbonaceous material synthesized in electrical discharges owes its origin to low energy ion implantation, and also suggest that this process may be relevant to incorporation of noble gases in early solar system materials. We also discuss the implication of our results for noble gas mass spectrometry. (author)