Neutron spectrum variations in the lunar subsurface evaluated from isotopic shifts in Apollo 15 deep drill core samples

Neutron-capture reactions are fundamental processes driving isotopic variations among cosmic-ray-irradiated planetary materials. The energy spectra of neutrons interacting with these planetary materials provide insights into their cosmic-ray exposure conditions and enable quantitative estimates of neutron-induced isotopic shifts in elements of interest. In this study, we measured Yb and Hf isotopic compositions of Apollo15 deep drill core samples, which provide information on higher-energy neutrons compared with conventional neutron indicators of Sm, Gd, and Er. Furthermore, we developed a calculation method to reconstruct a neutron spectrum from isotopic shifts in Sm, Gd, Er, Yb and Hf. Applying the method to Apollo 15 samples revealed significant neutron spectrum variations in the lunar subsurface. The observed epithermal neutron spectrum variations likely reflect depth dependence of energy moderation processes of neutrons. A neutron spectrum at the lunar surface estimated from our data is enriched in lower-energy epithermal neutrons compared with those calculated from numerical calculations in previous studies. The depth-dependent spectrum variations observed in the Apollo 15 samples possibly affect correction calculations of neutron-capture effects for nuclides strongly influenced by epithermal neutrons, such as 176 Hf. This is important in the context of accurate interpretation of radiometric and isotopic studies of lunar samples, which often record significantly high neutron fluences due to long-time cosmic-ray exposure on the lunar surface.