Cosmic-Ray Produced Radionuclides in the Barwell and Saint-Séverin Meteorites

Abstract

The radionuclides ³²P, ³⁶Cl, ³⁷Ar, and ³⁹Ar have been determined in separate mineral phases of the Barwell and Saint-Séverin meteorites. Hence, not only the activities of these isotopes in the bulk sample are known, but the contributions of specific neutron-induced reactions to the total activity can be evaluated as well. The activities in the total meteorite samples are, where comparable, rather lower than normal. Of special interest is the ratio ³⁷Ar/39Ar in metal and a comparison of the ³²S(n, p)-produced ³²P with the ⁴⁰Ca(n, α)-produced ³⁷Ar and the ³⁹K(n, p)-produced ³⁹Ar.

The ³⁷Ar/³⁹Ar ratio in metal is a direct measure of the intensity ratio of cosmic-ray particles with E ≳ 200 MeV in the close vicinity of the earth, to that averaged over many revolutions of the meteoroid. At least in the case of Saint-Séverin this ratio is slightly higher than the one obtained from target experiments.

³²P and ³⁹Ar have very different half-lives, while their production cross-sections by (n, p) reactions are almost identical over the entire energy range of interest. The ratio of their activities is the same as that of the neutron fluxes within the meteorite in the close vicinity of the earth to that averaged over many revolutions of the meteoroid.

The consequences of the results are discussed.