The lunar neutron energy spectrum inferred from the isotope compositions of rare-earth elements and hafnium in Apollo samples

Abstract

The isotopic abundances of Sm, Gd, Dy, Er, Yb, and Hf have been measured in nine lunar samples by MC-ICP-MS. The data were corrected for both instrumental mass bias and natural isotope fractionation. We used the data to calculate the total flux and energy spectrum of the neutrons absorbed by the rocks. We write the constitutive equations of the isotopic changes for these elements induced by neutrons and solve the inverse problem by computing local energy averages. Resonant absorption peaks can be used as convenient kernels to define the spectrum of epithermal neutrons. We find that 149Sm and 157Gd anomalies correlate with neutron flux density for E<0.015 eV (r2>0.98) and E≈0.13 eV (r2>0.85), while no significant correlation exists between the ratio of these anomalies and the epithermal/thermal flux ratio at any value of energy. Neutron flux density variations can be used to trace the proportions of neutrons scattered out of the samples. The spectrum in the thermal region follows the expected E−1/2 dependence but with ‘notches’ corresponding to neutron absorption. A major notch at the lowest end of the epithermal neutron spectrum (0.2–0.8 eV) is possibly due to absorption of neutrons by 151Eu, 167Er, and 149Sm. In general, we find a rather good correlation between the neutron flux density at specific energies and the exposure age, which suggests a mean residence time of the samples at the surface of the regolith of 2–300 Ma. Another correlation of epithermal neutrons with sample wt% FeO + TiO2 is consistent with orbital reflectance observations.