THE INFLUENCE OF COSMIC-RAY PRODUCTION ON EXTINCT NUCLIDE SYSTEMS

Abstract

Variations in the atomic abundances of 53Cr, 92Zr, 98Ru, 99Ru, and 182W in meteorites and lunar samples relative to terrestrial values may imply the early decay of radioactive 53Mn, 92Nb, 98Tc, 99Tc and 182Hf, respectively. From this one can deduce nucleosynthetic sites and early solar system timescales. Because these effects are very small, production and consumption of the respective isotopes by cosmic-ray interactions is a concern. It has recently been demonstrated that 182W production by neutron capture reactions on 181Ta is crucial for most lunar samples . In this study the neutron fluence of each sample was estimated from its nominal cosmic-ray exposure age as deduced from noble gas data. This approach overestimates the true cosmogenic isotopic shift for samples that might have been irradiated very close to the regolith surface. Here we therefore combine our model calculations with the neutron dose proxies 157Gd/158Gd and 149Sm/150Sm. This allows us to accurately correct the measured W isotopic data for cosmic-ray induced shifts without the explicit knowledge of the exposure age or the shielding depth of the sample simply by measuring 157Gd/158Gd and/or 149Sm/150Sm in an aliquot. In addition we present new model results for the GCR-induced effects on 53Mn-53Cr, 92Nb-92Zr and 98Tc-99Tc-98Ru-99Ru. For each of these systems, except Tc-Ru, a proper cosmic-ray dose proxy is given, permitting the accurate correction of measured isotopic ratios for cosmogenic contributions.