Solar cosmic ray records in lunar rock 64455

Cosmic ray produced Be-10 (half-life = 1.36 x 10(6) yr), Al-26 (7.05 x 10(5) yr), and Cl-36 (3.01 x 10(5) yr) were measured in a depth profile of 19 carefully-ground samples from the glass-coated lunar surface rock 64455. The solar cosmic ray (SCR) produced Al-26 and Cl-36 in this rock are present in high concentrations, which in combination with the low observed erosion rate, <0.5 mm/Myr, provide well defined depth profiles characterizing the SCR component of the cosmic rays. In conjunction with new experimentally determined excitation functions, the Cl-36 concentrations suggest a softer solar-proton spectral shape than that derived from most previous measurements. The fact that no SCR-produced Be-10 activity could be detected in 64455 is in good agreement with observations in 68815 and also indicates a softer SCR spectrum. Comparison of observed SCR profiles in 64455 with theoretical calculations indicates that the average solar-proton spectrum over the past 2 Myr (based on Al-26) has an exponential rigidity parameter (R-0) of about 90 MV with a proton flux (J) of 73 protons/cm(2)/s.4 pi above 10 MeV. Over the last similar to 0.5 Myr (based on Cl-36) R-0 is about 70 MV with a flux of similar to 196 protons/cm(2)/s.4 pi above 10 MeV. These SCR fluxes are consistent with most previous work, (c) 2009 Elsevier Ltd. All rights reserved.