Determination of the flux and energy distribution of energetic solar protons in the past 2 Myr using lunar rock 68815

Journal Article · Sat Oct 01 00:00:00 EDT 1994 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(94)90275-5· OSTI ID:6589168

Rao, M N; Garrison, D H ^[1]; Bogard, D D ^[2]; Reedy, R C ^[3]

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1. NASA/Johnson Space Center, Houston, TX (United States) Lockheed-ESC, Houston, TX (United States)
2. NASA/Johnson Space Center, Houston, TX (United States)
3. Los Alamos National Lab., NM (United States)

Cosmogenic [sup 21]Ne, [sup 22]Ne, [sup 38]Ar, and [sup 3]He produced by nuclear interactions of energetic (>10 MeV) solar protons were determined in 11 depth samples of lunar rock 68815. Concentrations of these proton-produced, SCR nuclides smoothly decrease from the rock surface down to 4.3 cm, where a galactic cosmic ray (GCR) component dominates. The cosmogenic [sup 21]Ne/[sup 22]Ne isotopic ratios shows a systematic change with depth, from the characteristic GCR value of 0.79 at 4.9 cm to a mixed SCR + GCR ratio of 0.66 near the surface. The surface exposure age for 68815 calculated from both SCR and GCR components for [sup 21]Ne, [sup 22]Ne, [sup 38]Ar agrees with the reported [sup 81]Kr-Kr exposure age of 2.04 Myr. Theoretical SCR + GCR depth profiles were calculated from cross-section data for different assumed spectra of energetic solar protons (R[sub 0], or rigidity, values 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 GCR component in the 4.3 cm sample to be >90% of the measured concentration. SCR [sup 21]Ne, [sup 22]Ne, and [sup 38]Ar give the following results for energetic solar protons over the last 2 Myr. For a preferred erosion rate of 1 mm/Myr and R[sub 0] of 80-90 MV, J(4[pi], E > 10 MeV) [approx equal] 58-73 p/cm[sup 2]/s; for the broader possible range of R[sub 0] values of 70-100 MV, J(4[pi], E > 10 MeV) [approx equal] 49-92 p/cm[sup 2]/s. These proton fluxes increase by [approximately] 12 and [approximately] 24% for erosion rates of 2 mm/Myr and 3 mm/Myr, respectively. These same data analysis techniques were also applied to literature depth profiles for SCR radionuclides in rock 68815.

• OSTI ID: 6589168
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 58:19; ISSN GCACAK; ISSN 0016-7037
• Country of Publication: United States
• Language: English

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