@article{oai:nagoya.repo.nii.ac.jp:02001407,
 author = {Hidaka, Hiroshi and Mizutani, Yuki and Yoneda, Shigekazu},
 issue = {2},
 journal = {The Astrophysical Journal},
 month = {Dec},
 note = {Thermalized neutrons arising at the surface of solar planets are produced from the interaction of cosmic rays with the nucleus consisting of surficial materials. The neutron energy spectrum in the range between thermal and epithermal regions at the lunar surface was investigated based on the combination of the isotopic variations of Sm and Gd caused by the thermal neutron-capture reactions in our previous study, with those of Dy, Er, and Yb caused by the epithermal neutron-capture reactions in this study. The detailed comparison of the systematic isotopic variations among Sm, Gd, Dy, Er, and Yb helps to construct a neutron energy spectrum at the surface of the Moon. Seven kinds of lunar soils at different depths of the drill core recovered from the Apollo 15 landing site (A-15) were used in this study. Isotopic variations of 164Dy/161Dy, 168Er/167Er, and 168Yb/174Yb were newly found in the A-15 samples, and showed the depth dependence caused by the interaction with cosmic-ray irradiation. In particular, the combination of the isotopic shifts of 168Er/167Er and 150Sm/149Sm could be effectively used to evaluate the epithermal neutron fluences of 5.4–8.1 × 10^17 n cm^−2 that were more than 10 times higher than thermal neutron fluences of 0.48–0.69 × 10^17 n cm^−2 reestimated in this study.},
 title = {Estimation of Thermal and Epithermal Neutron Fluences at the Lunar Surface from Isotopic Compositions of Rare Earth Elements},
 volume = {904},
 year = {2020}
}