The influence of cosmic-ray production on extinct nuclide systems

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 (Leya et al., 2000a). 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.