Determination of the energy spectrum of secondary neutron flux for lunar samples and its application for meteoritics

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The isotopic compositions of Sm and Gd in seven lunar samples from the Apollo 12 and 15 drill cores were determined to discuss the effects of neutron capture near the lunar surface. Neutron fluence values calculated from the isotopic shifts of A- 12 and A- 15 samples were (1.77 - 2.17) x10ィイD116ィエD1ncmィイD1-2ィエD1 and (3.02 - 4.68)x10ィイD116ィエD1 n cmィイD1-2ィエD1, respectively. Variations of εィイD2SmィエD2/εィイD2GdィエD2 with depth are interpreted as due to variations in the neutron energy spectrum. Our data suggest that the neutron is more thermalized at the lower layers than it is at the upper layers (Hidaka et al., in press).This approach was applied for meteoritics. Large isotopic deviations of ィイD1149ィエD1Sm-ィイD1150ィエD1Sm and ィイD1157ィエD1Gd-ィイD1158ィエD1Gd derived from neutron capture effects were observed in five enstatite achondrites (aubrites). The isotopic variations of these aubrites correspond to irradiation with neutron fluence of (1.17-3.99)x10ィイD116ィエD1 n cmィイD1-2ィエD1 which is within the … More range of estimated neutron fluence values of some lunar samples. These results indicate that aubrites may have been located near the surface of their parent body for several hundreds of Ma. Furthermore, the neutron energy spectrum obtained from the combination of Sm and Gd isotope systematics (εィイD2SmィエD2/εィイD2GdィエD2) suggests that the energy of thermalized neutrons in five aubrites was in the range from 200 〜 400 K, and was lower than those in A-12 and A-15 lunar samples (Hidaka et al., 1999). Isotopic compositions of Sm and Gd in eight chondrites show an evidence of neutron capture effects in the chondrites, correspond to thermal neutron fluences of (1.03 〜 5.17)x10ィイD115ィエD1 n cmィイD1-2ィエD1. The Sm and Gd isotopic results of four of eight chondrites were compared with information on epi-thermal and fast neutron fluences calculated from literature Ne and Kr isotopic data. Thermal neutron fluence values of them are in agreement with epi-thermal neutron fluences (Hidaka et al., to be submitted). Less