Depth profile of 41Ca in an Apollo 15 drill core and the low-energy neutron flux in the Moon

Abstract

Systematic measurements of the concentrations of cosmogenic 41Ca (half-life = 1.04 × 10 5 yr) in the Apollo 15 long core 15001–15006 were performed by accelerator mass spectroscopy. Earlier measurements of cosmogenic 10Be, 14C, 26Al, 36Cl, and 53Mn in the same core have provided confirmation and improvement of theoretical models for predicting production profiles of nuclides by cosmic ray induced spallation in the Moon and large meteorites. Unlike these nuclides, 41Ca in the lunar surface is produced mainly by thermal neutron capture reactions on 40Ca. The maximum productions of 41Ca, about 1 dpm/g Ca, was observed at a depth in the Moon of about 150 g/cm 2. For depths below about 300 g/cm 2, 41Ca production falls off exponentially with an e-folding length of 175 g/cm 2. Neutron production in the Moon was modeled with the Los Alamos High Energy Transport Code System, and yields of nuclei produced by low-energy thermal and epithermal neutrons were calculated with the Monte Carlo N-Particle code. The new theoretical calculations using these codes are in good agreement with our measured 41Ca concentrations as well as with 60Co and direct neutron fluence measurements in the Moon.