Measurements of neutron capture effects on Cd, Sm and Gd in lunar samples with implications for the neutron energy spectrum

The bombardment of the lunar surface by cosmic rays produces secondary neut rons, some of which are thermalized by the lunar soil and then interact str ongly with the isotopes of Cd, Gd and Sm which have high neutron capture cr oss sections at thermal energies causing changes in their isotopic composit ions. We have measured these changes in samples from the Apollo 14, 16 and 17 missions using thermal ionization mass spectrometry. Changes of Cd-114/C d-113 of 0.3-0.5% in samples 60501,105, 65701,23 and 72161,73, of 0.4 and 0 .8% in Gd-158/Gd-157 in samples 14163,848 and 60501,105 and of 0.8, 1.2 and 0.06% for Sm-150/Sm-149 in samples 14163,848 and 60501,105 and 74220,125 r espectively, have been observed. This is the first time that neutron captur e on Cd has been detected in lunar samples. Thermal neutrons are captured p referentially at resonance energies of 0.03 eV by Gd-155 and Gd-157, at 0.0 9 eV by Sm-149 and at 0.178 eV by Cd-113. A comparison of the changes in Cd -114/Cd-113, Gd-156/Gd-155, Gd-158/Gd-157 and Sm-150/Sm-149 due to neutron capture can therefore indicate the energy distribution of the neutrons. Pre vious work has compared changes in Gd-158/Gd-157 and Sm-150/Sm-149, while t his study extends the comparison to Cd-114/Cd-113, where the resonance ener gy is significantly higher. This has enabled us to confirm evidence for a h arder neutron energy spectrum in lunar soil than is predicted theoretically . This can be seen in significantly higher capture rates than predicted for Sm-149 compared to Gd-157. We also present evidence of enhanced capture ra tes on Cd-113 compared with Sm-149 and Gd-157. The concentrations of Cd, Gd and Sm in nine lunar samples have also been measured by isotope dilution m ass spectrometry with Cd being measured in lunar samples for the first time by this method. While Cd was found to be heterogeneously distributed in so me samples, Gd and Sm showed good agreement with previous measurements, exc ept for sample 14163,848 where it was significantly lower. (C) 2001 Elsevie r Science B.V. All rights reserved.