The isotopic composition of Gd and the neutron capture effects in samples from Apollo 11

The isotopic composition of Gd in six rocks, one breccia, two core samples, and soil has been determined. Isotopic variations due to capture of low energy neutrons produced by cosmic ray interactions have been found in all samples except for two rocks. Using cosmic ray exposure ages, the effective thermal neutron fluxes were calculated using a nominal value of ¹⁵⁷σ = 2.62 × 10⁵ barns. They vary from less than 0.27 n/cm² sec to 1.1 n/cm² inidicating variable shielding for the samples studied. The neutron dosage of the top and the bottom of core #2 is the same. This means that the soil has been mixed to 13 cm at least once in the last 100 my. A mixing depth of 23 to 47 m can be calculated based on the integrated effective thermal neutron dosage of 9 × 10¹⁵ n/cm² for the soil and an assumed surface flux of 0.2 n/cm² sec. For rock #17 the barium isotopic composition was determined and found to be the same as terrestrial to within 0.1% for the five abundant isotopes.