~(14)C depth profiles in Apollo 15 and 17 cores and lunar rock 68815

摘要

Accelerator mass spectrometry (AMS) was used to measure the activity vs. depth profiles of ~(14)C produced by both solar cosmic rays (SCR) and galactic cosmic rays (GCR) in Apollo 15 lunar cores 15001-6 and 15008, Apollo 17 core 76001, and lunar rock 68815. Calculated GCR production rates are in good agreement with ~(14)C measurements at depths below ～10 cm. Carbon-14 produced by solar protons was observed in the top few cm of the Apollo 15 cores and lunar rock 68815, with near-surface values as high as 66 dpm/kg in 68815. Only low levels of SCR-produced ~(14)C were observed in the Apollo 17 core 76001. New cross sections for production of ~(14)C by proton spallation on O, Si, Al, Mg, Fe, and Ni were measured using AMS. These cross sections are essential for the analysis of the measured ~(14)C depth profiles. The best fit to the activity-depth profiles for solar-proton-produced ~(14)C measured in the tops of both the Apollo 15 cores and 68815 was obtained for an exponential rigidity spectral shape R_0 of 110-115 MV and a 4π flux (J_(10), Ep > 10 MeV) of 103-108 protons/cm~2/s. These values of R_O are higher, indicating a harder rigidity, and the solar-proton fluxes are higher than those determined from ~(10)Be, ~(26)Al, and ~(53)Mn measurements.