Solar cosmic ray records in lunar rock 64455

Cosmic ray produced 10Be (half-life = 1.36 × 106 yr), 26Al (7.05 × 105 yr), and 36Cl (3.01 × 105 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 26Al and 36Cl 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 36Cl concentrations suggest a softer solar-proton spectral shape than that derived from most previous measurements. The fact that no SCR-produced 10Be 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 26Al) has an exponential rigidity parameter (R0) of about 90 MV with a proton flux (J) of 73 protons/cm2/s·4π above 10 MeV. Over the last ∼0.5 Myr (based on 36Cl) R0 is about 70 MV with a flux of ∼196 protons/cm2/s·4π above 10 MeV. These SCR fluxes are consistent with most previous work.