Depth profile of53Mn in the lunar surface

Measurements of cosmic-ray produced⁵³Mn are reported for a series of lunar surface samples down to a depth of 416 g/cm². These results clearly illustrate the decrease in activity with depth as the incident galactic cosmic rays are absorbed. Below 60 g/cm² the production rate decreases exponentially with a mean length, λ, of about 220 g/cm². These results indicate that, at the Apollo 15 site, the lunar regolith has been unmixed, on a meter scale, for the last 5 my. The neutron activation technique for⁵³Mn, which allowed samples smaller than 200 mg to be used for these measurements, is described.