The state-of-the-art modeling of cosmogenic Cr isotopes produced in
lunar rocks compared with existing calculations and measurements
Abstract
The distribution of Cr isotopes provides useful information to trace the
source and origin of extraterrestrial samples, but it is usually
influenced by high-energy cosmic rays. Since lunar and terrestrial
materials have quite similar Cr isotope compositions, distinguishing the
effect of cosmic rays in lunar samples is especially important. Those
cosmic radiation particles (primary particles) can react with lunar
materials, creating many secondary particles. Both primary and secondary
particles can produce cosmogenic nuclides on the Moon. Radiation
Environment and Dose at the Moon (REDMoon) is a novel GEANT4 Monte-Carlo
model built to simulate the interactions of space particles with the
lunar surface and subsurface content. Using this model, we simulate the
production of cosmogenic Cr isotopes
($^{50}$Cr,$^{52}$Cr,$^{53}$Cr,$^{54}$Cr) at
different depths of lunar surface, and compare the contribution of
different reactions generating these nuclides. The results suggest that
spallation reactions are the most important process producing cosmogenic
Cr isotopes. We also analyze the relationship between
$^{53}$Cr/$^{52}$Cr and $^{54}$Cr/$^{52}$Cr
predicted by our model and compare it with different Apollo samples. As
previously studied, we also find an approximate linear relationship
between $\varepsilon^{53}$Cr and
$\varepsilon^{54}$Cr (per 10,000 deviation of
$^{53}$Cr/$^{52}$Cr and $^{54}$Cr/$^{52}$Cr
ratios from the standard). Furthermore, we reveal a change of this
linear relationship in different depths of lunar surface. Besides, we
investigate how the slopes can be influenced by exposure age and the
Fe/Cr ratio. With these additional factors carefully considered, the
comparison between our modeled results and the measurements is better
than previous studies.