RECOIL-EMANATION THEORY APPLIED TO RADON RELEASE FROM MINERAL GRAINS

In this paper the α-recoil emanation theory is presented as a major mechanism for Rn release from solid materials. The basic theory is developed for Rn emanation from geometrical shapes: spheres and cylinders having variable thickness of precursor-Ra containing material. Subsequently, a sphere-pore model of emanation from mineral grains (like those in rocks, soils, etc.) is developed. In this model a surface of the grain is constructed as a superposition of in- and out-directed spherical caps and cylindrical pores. The model incorporates processes like Rn embedding, pore overlap, water effect and emanation from the edges. Using such a geometrical model it is shown how the Rn emanating-power measurements can give a clue to the Ra distribution and surface properties of the samples. In particular, it is shown that for grains reasonably larger than the recoil range and for surface Ra distribution, the emanating power is independent of the grain size but the total Ra concentration varies with the reciprocal of the grain size. The opposite is true for Ra distributed uniformly throughout the volume of the grain. It is shown how the Rn emanating power is related to the surface porosity of the grain. Also, implications are made with respect to U transport in nature, and with respect to low emanating power from lunar soil. Finally, a failure of the emanation theory is discussed to reproduce emanation curves for the samples having high specific surface areas and Ra distributed throughout the volume. © 1990.