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

摘要

The bombardment of the lunar surface by cosmic rays produces secondary neutrons, some of which are thermalized by the lunar soil and then interact strongly with the isotopes of Cd, Gd and Sm which have high neutron capture cross sections as thermal energies causing changes in their isotopic compositions. We have measured these changes in samples from the Apollo 14, 16 and 17 missions using the mal ionization mass spectrometry. Changes of ~(114)Cd/~(113)Cd of 0.3-0.5% in samples 60501, 105, 65701, 23 and 72161, 73, of 0.4 and 0.8% in ~(158)Gd/~(157)Gd in samples 14163, 848 and 60501, 105 and of 0.8, 1.2 and 0.06% for ~(150)Sm/~(149)Sm in samples 14163,848, and 60501, 105 and 74220, 125 respectively, have been observed. This is the first time that neutron capture on Cd has been detected in lunar samples. Thermal neutrons are captured preferentially at resonance energies of 0.03 eV by ~(155)Gd and (157)Gd, at 0.09 eV by ~(149)Sm and at 0.178 eV by ~(113)Cd. A comparis of the changes in ~(114)Cd/~(113)Cd, ~(156)Gd/~(155)Gd, ~(158)Gd/~(157)Gd and ~(150)Sm/~(149)Sm due to neutron capture can therefore indicate the energy distribution of the neutrons. Previous work has compared changes in ~(158)Gd/~(157)Gd and ~(150)Sm/~(149)Sm, while this study extends the comparison to ~(114)Cd/~(113)Cd, where the resonance energy is significantly higher. This has enabled us to confirm evidence for a harder neutron energy spectrum in lunar soil than is predicted theoretically. This can be seen in significantly higher capture rates than predicted for ~(149)Sm compared to ~(157)Gd. We also present evidence of enhanced capture rates on ~(113)Cd compared with ~(149)Sm and ~(157)Gd. The concentrations of Cd, Gd and Sm in nine lunar samples have also been measured by isotope dilution mass spectrometry with Cd being measured in lunar samples for the first time by this method. While Cd was found to be heterogeneously distributed in some samples. Gd and Sm showed good agreement with previous measurements, except for sample 14163, 848 where it was significantly lower.