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

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 $_{}^{41}$ Ca and $_{}^{60}$ Co is studied as an example. In addition, shifts in the stable isotope ratios $_{}^{157}$ Gd/ $_{}^{160}$ Gd, $_{}^{158}$ Gd/ $_{}^{160}$ Gd, $_{}^{149}$ Sm/ $_{}^{152}$ Sm, and $_{}^{150}$ Sm/ $_{}^{152}$ Sm (and combinations thereof) are modeled and compared to experimental data. The model describes experimental $_{}^{41}$ Ca activity concentrations in different types of meteorites and the lunar surface within the uncertainties. In contrast, it fails to describe $_{}^{60}$ Co activity concentrations. In addition, it is difficult to consistently model the isotope shifts $_{}^{157}$ Gd/ $_{}^{160}$ Gd and $_{}^{150}$ Sm/ $_{}^{152}$ Sm 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., $_{}^{56}$ Fe, might be of importance and some of the consequences are discussed.