Model calculations for neutron-induced reactions in meteorites and planetary surfaces

TL;DR

This study uses a simulation tool to model neutron reactions in space rocks and planetary surfaces, comparing predictions with real data.

Contribution

The study improves neutron reaction modeling in planetary objects using GEANT4, achieving better agreement with lunar surface measurements.

Findings

The model accurately predicts 41Ca activity concentrations in meteorites and the lunar surface.

The model fails to predict 60Co activity concentrations and struggles with certain isotope shifts.

Discrepancies in isotope shifts may be linked to neutron spectra shape and self-shielding effects.

Abstract

This study investigates the capabilities of the GEANT4 Monte Carlo toolkit to quantitatively predict neutron production, neutron transport, and nuclide production by neutron capture reactions in cosmochemical relevant objects. The model reproduces neutron densities measured in the lunar surface within the experimental uncertainties, which is a major improvement compared to earlier studies. Since, for many applications in meteorites and planetary surfaces, nuclide production by neutron capture is of importance, the production of \documentclass[12pt]{minimal}
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\begin{document}$$^{41}$$\end{document}41Ca and \documentclass[12pt]{minimal}
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\begin{document}$$^{60}$$\end{document}60Co is studied as an example. In addition, shifts in the stable isotope ratios \documentclass[12pt]{minimal}
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\begin{document}$$^{157}$$\end{document}157Gd/\documentclass[12pt]{minimal}
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\begin{document}$$^{160}$$\end{document}160Gd, \documentclass[12pt]{minimal}
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\begin{document}$$^{158}$$\end{document}158Gd/\documentclass[12pt]{minimal}
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\begin{document}$$^{149}$$\end{document}149Sm/\documentclass[12pt]{minimal}
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\begin{document}$$^{152}$$\end{document}152Sm, and \documentclass[12pt]{minimal}
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\begin{document}$$^{150}$$\end{document}150Sm/\documentclass[12pt]{minimal}
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\begin{document}$$^{152}$$\end{document}152Sm (and combinations thereof) are modeled and compared to experimental data. The model describes experimental \documentclass[12pt]{minimal}
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\begin{document}$$^{41}$$\end{document}41Ca activity concentrations in different types of meteorites and the lunar surface within the uncertainties. In contrast, it fails to describe \documentclass[12pt]{minimal}
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\begin{document}$$^{60}$$\end{document}60Co activity concentrations. In addition, it is difficult to consistently model the isotope shifts \documentclass[12pt]{minimal}
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\begin{document}$$^{160}$$\end{document}160Gd and \documentclass[12pt]{minimal}
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\begin{document}$$^{150}$$\end{document}150Sm/\documentclass[12pt]{minimal}
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\begin{document}$$^{152}$$\end{document}152Sm in Apollo 15 drill core samples. The observed trends depend on the temperature of the irradiated object and are more pronounced for colder temperatures. Since the observed discrepancies are likely related to the shape of the neutron spectra, self-shielding effects by, e.g., \documentclass[12pt]{minimal}
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\begin{document}$$^{56}$$\end{document}56Fe, might be of importance and some of the consequences are discussed.

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