Neutron capture effects on Sm-Er isotopes in meteorites

Abstract

The quantitative evaluation of the effect of neutron capture reactions generated by cosmic rays on the isotopic composition of meteorites and returned samples is important for radiometric dating of these samples. Furthermore, since the abundance ratio of secondary neutrons between thermal and epithermal neutrons (ep/th) highly depends on the hydrogen content in the vicinity of the sample, neutron-capture-induced isotopic variations can be used to evaluate the presence of water on the surface of the object. In this study, we measured the isotope ratios of Sm, which easily captures thermal neutrons, and Er, which easily captures epithermal neutrons, using five eucrite samples and the mesosiderite meteorite Vaca Muerta.Both Sm and Er showed isotopic anomalies, and the ep/th was obtained from calculations. We also attempted to reproduce the calculated ep/th using the Monte Carlo code Phits (Ver.3.34). Simulating cosmic-ray irradiation on the parent body of eucrite showed that the observed ep/th could not be reproduced if secondary neutrons captured by these meteorite samples were generated within eucritic crust. This suggests that there were light elements such as hydrogen near the surface of the parent body that more effectively generate thermal neutrons. This interpretation is consistent with the observational results of the dawn mission (McCord et al., 2012).