Isotopic Composition of Cosmic Importance

Abstract

IN 1964 I showed¹ that approximately 60 per cent of terrestrial ¹⁸⁷Os should be regarded as having resulted from the decay of ¹⁸⁷Re and that most of that decay occurred before the formation of the solar system. Measurements of the neutron capture cross-sections for the isotopes of several elements have now confirmed² the correctness of the s-process theory on which that conclusion depended, and although the specific measurements of the cross-sections for the osmium isotopes have not yet been made there is little doubt that ¹⁸⁷Os must be primarily radiogenic. This compelling conclusion is, however, awkward for current cosmological thought because the age of the ¹⁸⁷Re seems to exceed the age of the universe. Recent chronological models (refs. 3, 4 and unpublished work of Hohenberg) for the natural radioactivities have therefore tended to ignore ¹⁸⁷Re in favour of the short lived species ²³⁵U, ¹²⁹I and ²⁴⁴Pu, and although the resulting models suggest significant enrichment shortly before the solar system formed, they do little to clarify when nucleosynthesis actually began. Here I wish to outline a potentially important factor for the ¹⁸⁷Re age along with an isotopic measurement capable of clarifying the situation.