Accumulation and circulation of gaseous radon between lunar fines

Abstract

During: the lunar night, the temperature of the regolith upper layer is lower than the radon freezing point. Thus random atoms coming from the interior can be trapped at the surface of the cold lunar fires. The 222Rn daughter products, 210Pb and 210Po, are em- bedded in a very thin layer at the surface of the grains. It is therefore possible, by spectrometry, to distinguish between the continuum due to uranium, thorium (and decay products) homogeneously distributed and the narrow peak at 5.3 MeV, due to an excess of 210Po. We have determined a mean dav-and-night concentration of about 3.5 x 10 3 atoms of intergranular 222Rn per gram of superficial fines, corresponding to a continuous flow of 3 atoms min-1 cm2 of soil. To account for such a flow of radon atoms moving in a random way from a 6 m source depth, the pore size of the regolith should be 60 pm. On the other hand, the involved changes in the isotopic composition of the radiogenic lead remain less than 1 %.