Radon-222 emanation from rocks, soils, and lunar dust
Barretto, Paulo Marcos de C
Adams, John A. S.
Master of Arts
The investigation of the Radon-222 emanation rate and the escape-to-production rate ratio by means of alpha counting was undertaken for rocks, soils and a sample from lunar dust. The material was analysed for different grain sizes, moisture, contents and temperatures. Radon escape-to-production rate ratios from 1% - 25% were found for rocks crushed to identical grain size classes and distinctly higher values, from 20% - 70% were observed in soils. The results allow to subdivide the natural materials at the earth's surface into three categories: strong emanators, soils at 40% radon escape to production rate; moderate emanators, igneous rocks at 10% - 15%; poor emanators, sediments and some metamorphic rocks at = 5%. The factors controlling the emanation rate are: grain size, nature matrix or groundmass, uranium distribution within the crystalline structure, crystal damage resulting from radiation and/or weathering. Radiation damage has been put forward as a plausible mechanism to explain the phenomena on fresh grains because diffusion through the crystal structure alone cannot account for the observed values. Daily temperature variations have no effect on the escape rates as no appreciable adsorption or release of radon was observed in the temperature range of -15° to 96° C. The general effect of moisture is a slight increase in the emanation rate. An escape-to-production rate ratio of 45 ± 20% was observed for 0.4 g of an Apollo 12 dust. The radon isotopes diffusion through the surface rubble and lunar "atmosphere" and the consequent "fallout" of lead isotopes over the moon surface could provide a mechanism for lead enrichment and depletion which might be related to the uranium-lead discordant ages for fines compared to associated rocks.