DETERMINATION OF THE FLUX-DISTRIBUTION AND ENERGY-DISTRIBUTION OF ENERGETIC SOLAR PROTONS IN THE PAST 2-MYR USING LUNAR ROCK-68815

Cosmogenic Ne-21, Ne-22, Ar-38, and He-3 produced by nuclear interacti ons of energetic (>10 MeV) solar protons were determined in 11 depth s amples of lunar rock 68815. Concentrations of these proton-produced, S CR nuclides smoothly decrease from the rock surface down to 4.3 cm, wh ere a galactic cosmic ray (GCR) component dominates. The cosmogenic Ne -21/Ne-22 isotopic ratio shows a systematic change with depth, from th e characteristic GCR value of 0.79 at 4.9 cm to a mixed SCR + GCR rati o of 0.66 near the surface. The surface exposure age for 68815 calcula ted from both SCR and GCR components for Ne-21, Ne-22, and Ar-38 agree s with the reported Kr-81-Kr exposure age of 2.04 Myr. Theoretical SCR + GCR depth profiles were calculated from cross-section data for diff erent assumed spectra of energetic solar protons (R(0), or rigidity, v alues of 50-125 MV) and for different rock surface erosion rates (0-3 mm/Myr). These theoretical SCR profiles were added to GCR profiles and statistically compared with measured data by minimizing the standard deviation of the least squares statistical fit and by requiring the GC R component in the 4.3 cm sample to be >90% of the measured concentrat ion. SCR Ne-21, Ne-22, and Ar-38 give the following results for energe tic solar protons over the last 2 Myr. For a preferred erosion rate of 1 mm/Myr and R(0) of 80-90 MV, J(4 pi, E > 10 MeV) similar or equal t o 58-73 p/cm(2)/s; for the broader possible range of R(0) values of 70 -100 MV, J(4 pi, E, 10 MeV) similar or equal to 49-92 p/cm(2)/s. These proton fluxes increase by similar to 12 and similar to 24% for erosio n rates of 2 mm/Myr and 3 mm/Myr, respectively. These same data analys is techniques were also applied to literature depth profiles for SCR r adionuclides in rock 68815. Al-26 and Mn-53 give proton fluxes compara ble to those obtained from neon and argon at R(0) values of 70-85 MV, but give somewhat higher J values at larger values of R(0). Proton flu xes characterized by Kr-81 and C-14 tend to be higher compared to the other nuclides. The most likely reason for different J values determin ed from different SCR nuclides is errors in cross section data, althou gh measurement errors and temporal variations in J cannot be excluded.