14C DEPTH PROFILES IN APOLLO 15 AND 17 CORES AND LUNAR ROCK 68815 - REVISED PRODUCTION RATES USING NEW CROSS SECTION MEASUREMENTS

Abstract

Accelerator mass spectrometry (AMS) was used to measure the activity vs. depth profiles of 14C 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 14C 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 14C were observed in the Apollo 17 core 76001. New cross sections for production of 14C 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 14C depth profiles. The best fit to the activity-depth profiles for solar-proton-produced 14C measured in the tops of both the Apollo 15 cores and 68815 was obtained for an exponential rigidity spectral shape R0 of 110-115 MV and a 4π flux (J10, Ep > 10 MeV) of 103-108 protons/cm2/s. These values of R0 are higher, indicating a harder rigidity, and the solar-proton fluxes are higher than those determined from 10Be, 26Al, and 53Mn measurements.