Production Rates of Cosmogenic Nuclides Deep in the Moon

Details Production Rates of Cosmogenic Nuclides Deep in the Moon Production Rates of Cosmogenic Nuclides Deep in the Moon

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.564r&link_type=abstract

Meteoritics, vol. 30, no. 5, page 564

Statistics

1

Cosmic Rays, Galactic, Isotopes, Cosmogenic, Moon, Cosmogenic Nuclides, Nuclides, Spallogenic

Scientific paper

Measurements of radionuclide activities in Apollo samples extend to depths of only about 400 g/cm2 in the Moon. Some Apollo samples and lunar meteorites probably have significant concentrations of cosmogenic nuclides made while they were buried at depths greater than 400 g/cm2. We report here initial calculations for production rates of cosmogenic nuclides for depths in the Moon down to 1500 g/cm2. The LAHET Code System (LCS) [1] was used to numerically simulate the irradiation of the Moon by galactic-cosmic-ray particles and to calculate particle fluxes and production rates of various cosmogenic nuclides. The advantage of these calculations is that the LCS physical model used for the production and transport of particles in the Moon has no free parameters. Some previous models, such as Reedy and Arnold (1972) [2], had semi-empirical or adopted parameters that never were tested below about 400 g/cm2. Our simulations started by selecting the energy and direction of the primary particle that starts a particle cascade and following all particles until they are removed by nuclear interactions or escape from the Moon. The calculations were validated by modeling production of 10Be, 26Al, 36Cl, and 53Mn in the Apollo 15 drill core and getting good agreement with measurements to depths of about 400 g/cm2 [3]. We used the composition of the Apollo 15 deep drill core for these calculations, assumed a density of 1.5 g/cm3, and did calculations to a depth of 1000 cm (1500 g/cm2). We ran 5 million incident particles to try to get good statistics at great depths. Statistical uncertainties for depths below about 1000 g/cm2 were about 10%, so we splined the results for depths below 750 g/cm2. We reproduced our original results [3] down to 500 g/cm2, extended the calculated production rates of those four radionuclides to 1500 g/cm2, and also calculated production rates of 14C, 21Ne, 22Ne, and 38Ar from 0 to 1500 g/cm2. As reported earlier [3], production rates for various cosmogenic nuclides vary in the top 300 g/cm2 of the Moon, depending on the reactions making the nuclide. For example, there is very little increase in the production rate of 10Be from the surface to its maximum production rate. Nuclides made by lower-energy particles have greater increases in their production rates and their maximum rates are at greater depths than nuclides such as 10Be made mainly by higher-energy particles [3]. However, for depths greater than about 300 g/cm2, all nuclides showed essentially identical profiles that decrease very exponentially down to 1500 g/cm2 with e-folding lengths of about 182 g/cm2. The trend for spallogenic nuclides deep in iron meteorites [4] is different with production profiles not yet parallel at much greater depths. Our calculations show that the production rates of all cosmogenic nuclides made by spallation reactions in the Moon decrease exponentially with greater depths below about 300 g/cm2 with e-folding lengths of about 182 g/cm2. The fact that production ratios essentially are constant below 300 g/cm2 will make it very hard to determine depths in the Moon below 300 g/cm2 using ratios for spallogenic nuclides. We suspect that this trend continues to much greater depths in the Moon but need to check the trend in production rates below 1500 g/cm2 with additional calculations. This trend in the Moon is different from that in large iron objects. Acknowledgments: Work at Los Alamos supported by NASA and done under auspices of the U.S. Dept. of Energy. References: [1] Masarik J. and Reedy R. C. (1994) GCA, 58, 5307-5317. [2] Reedy R. C. and Arnold J. R. (1972) JGR, 77, 537-555. [3] Reedy R. C. and Masarik J. (1994) LPS XXV, 1119-1120. [4] Michlovich E. S. et al. (1994) JGR, 99, 23187-23194.

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