The Neutron Energy Spectra of Lunar Meteorites Evaluated from Sm and Er Isotopic Compositions

Cosmic-ray exposure records of 13 lunar meteorites, Dhofar 081, Dhofar 910, Dhofar 911, Northwest Africa (NWA) 482, NWA 2995, NWA 2996, NWA 3136, NWA 3163, NWA 4472, NWA 4734, NWA 4884, NWA 4932, and NWA 4936, were characterized from the abundances of spallogenic ( 10 Be and 26 Al) and neutron-captured ( 36 Cl, 41 Ca, 150 Sm, and 168 Er) nuclides produced by cosmic-ray irradiation. Assuming a single-stage irradiation model for individual meteorites, 11 of the 13 meteorites had resided at shallow depths in the range of 55 to 330 g cm −2 from the lunar surface and experienced cosmic-ray irradiations for 140–870 Ma on the Moon. In contrast, 2 of the 13 meteorites, Dhofar 911 and NWA 4932, cannot be simply explained by a single-stage irradiation, but need at least two-stage irradiation on the Moon. Furthermore, the neutron fluences of thermal and epithermal energy regions for individual meteorites were quantified from a combination of the isotopic shifts of 149 Sm– 150 Sm and 167 Er– 168 Er, respectively. Our estimates gave 8–11 times higher epithermal neutron fluences (1.7–13.7 × 10 1 7 neutrons cm −2 ) than the thermal neutron fluences (0.65–13.8 × 10 16 neutrons cm −2 ) for 9 of the 13 meteorites, which are consistent with those from the lunar regolith materials in our previous study. This result also supports the long cosmic-ray irradiation of most lunar meteorites on the surface of the Moon.