Cosmic ray effects induced in a rock exposed on the moon or in free space: contrast in patterns for 'tracks' and 'isotopes'

Abstract

Analytical studies are reported here for two cosmogenic effects due to low energy particles in extraterrestrial samples:(i) Formation of latent chemically etchable tracks in crystalline materials due to solid state damage as a result of ionisation losses suffered by multicharged cosmic ray nuclei, and (ii) Production of low threshold isotopes due to nuclear interactions of solar cosmic ray particles. The present analytical treatment is different from those previously reported and is more directly applicable to recent studies of low energy cosmogenic effects in meteorites and in lunar samples. We consider irradiation of ellipsoidal rocks in space and on the Moon. In the latter case, different irradiation geometries corresponding to different burials in the regolith are also considered. It is shown that results of irradiation of an object on the surface of a parent body differ from that of an object in free space in more complex manner than a uniform reduction by a factor of two due to the change over from 2π to 4π irradiation. Isocontours for tracks or isotopes are found to be markedly different in the two cases. Thus, the irradiation geometry must be explicitly taken into account in interpreting low-energy cosmogenic effects in lunar rocks. Simultaneous analyses of tracks and radioisotopes of different half-lives should allow one to establish principal irradiation geometries both for meteorites and lunar samples.