DEPTH PROFILE OF CA-41 IN AN APOLLO-15 DRILL CORE AND THE LOW-ENERGY NEUTRON-FLUX IN THE MOON

Systematic measurements of the concentrations of cosmogenic Ca-41 (hal f-life = 1.04 x 10(5) yr) in the Apollo 15 long core 15001-15006 were performed by accelerator mass spectroscopy. Earlier measurements of co smogenic Be-10, C-14, Al-26, Cl-36, and Mn-53 in the same core have pr ovided confirmation and improvement of theoretical models for predicti ng production profiles of nuclides by cosmic ray induced spallation in the Moon and large meteorites. Unlike these nuclides, Ca-41 in the lu nar surface is produced mainly by thermal neutron capture reactions on Ca-40. The maximum production of Ca-41, about 1 dpm/g Ca, was observe d at a depth in the Moon of about 150 g/cm(2). For depths below about 300 g/cm(2), Ca-41 production falls off exponentially with an e-foldin g length of 175 g/cm(2). Neutron production in the Moon was modeled wi th the Los Alamos High Energy Transport Code System, and yields of nuc lei produced by low-energy thermal and epithermal neutrons were calcul ated with the Monte Carlo N-Particle code. The new theoretical calcula tions using these codes are in good agreement with our measured Ca-41 concentrations as well as with Co-60 and direct neutron fluence measur ements in the Moon.