Adsorption and excess fission XE: Adsorption of XE on vacuum crushed minerals

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Abstract
We report the results of Xe adsorption studies at low partial pressures of Xe (∼2×10−13 atm) for a terrestrial labradorite and lunar anorthosite 15415 which were crushed under high vacuum. The creation of fresh surfaces resulted in a distribution of surface adsorption potentials, with the most energetic, but least abundant, fractions of the surface dominating adsorption behavior at lower temperatures. Atmospheric exposure tends to eliminate the most energetic surface fractions. For sample 15415, surfaces with an adsorption potential as great as 13.7 kcal/mole were created, but these constituted only about 10−6 of the surface area. Observation of the adsorption behavior of this sample over a period of days revealed the gradual disappearance of the most energetic fractions of the surface, probably because of site competition effects caused by sorption of active gases in our high vacuum manifold. Our results consist of values of the Henry constants of Xe adsorption at various temperatures. We have used extrapolations based upon these data to evaluate the possible role of adsorption in the excess fission Xe effect observed in lunar highland gas‐rich breccias, for a model in which excess fission Xe is concentrated by adsorption while diffusing through the regolith. Our experimental results, and estimations of the sorptive properties of the in situ regolith based on the time scale of lunar surface Rn emanations, indicate that adsorption falls about five orders of magnitude short of accounting for the effect. We also infer that the adsorption characteristics of our vacuum‐crushed lunar sample are reasonable analogs to those of in situ lunar soil, so that the failure of adsorption to account for excess fission Xe cannot be attributed to significant differences in sorptive behavior of these materials.

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