Numerical Simulation of CosmogenicNuclide Production in Lunar Rocks
Abstract
The production rates of cosmogenic nuclides depend on the primary cosmicray particles; the irradiatedbody's bulk composition, size, and shape; and the sample's composition and shielding depth. Although much work has been done on some of these dependencies [14], more detailed studies still need be done on others. This work describes the influence of irradiation geometry on nuclide production in lunar rocks. In most cases, computer simulations of cosmogenicnuclide production were restricted to spherical objects irradiated with a 4pi isotropic flux (meteoroids) or in lunar core samples irradiated by a 2pi flux incident on semiinfinite layers or cylinders of huge sizes. Many lunar samples are rocks found on top of the lunar surface. For these rocks, neither of above mentioned models correspond to the real conditions. We present results of our simulations of cosmogenic nuclides production in models simulating the irradiation of rocks sitting on top of the lunar surface. The GCR production profiles in lunar rocks were calculated using the Los Alamos 3D Monte Carlo LAHET Code System (LCS). LCS was earlier used for calculation of production rates in both meteorites and lunar samples and gave the results that are almost always in good agreement with cosmogenicnuclide measurements [5,6]. In this work, the irradiated object was modeled as the union of a sphere with the radius of the Moon and a small hemisphere with radii varying from 10 to 100 g/cm2 simulating the lunar rock. As the particle production and equilibrium spectra are strongly depth dependent, the hemispheres were divided into concentric hemispherical shells with 1g/cm^2 thicknesses. This geometrical model, with the composition of lunar rock 68815, was irradiated by homogenous and isotropic GCRparticle fluxes. Statistical errors using this geometrical model and running 10^6 primary particles were at the level of 810%. Proton and neutron fluxes calculated by LCS for each layer were then multiplied by the relevant cross sections and integrated over energy for ^10Be, ^14C, ^21Ne, ^22Ne, ^26Al, ^36Cl, ^38Ar and ^53Mn. All studied nuclides in the huge sphere have GCR production profiles that increase from the surface of the Moon to a maximum at a depth of ~2050 g/cm^2 and then decrease with increasing depth. The steepness and amount of this increase depends on the reactions making a particular nuclide [6]. In the case of lunarrock models with a range of radii, only production rates in the rock's hemispherical shells were calculated. The increase of all production rates is steeper and the absolute production rates are higher than for the lunar sphere alone. The steepness of the productionrate profile near the surface and the increase in production rates relative to the case with no hemisphere increased with increasing radius of the hemisphere. These differences in the shape and absolute value of production rates can be explained with different conditions for the development of the particle cascade. These calculations for the production of cosmogenic nuclides in lunar rocks by GCR particle show that there are important differences between the results obtained by commonly used geometric irradiation models and the lunarrock models presented in this work. The steeper GCR production profiles for a rock could help to explain the poor agreement for ^10Be in rock 68815, where slab models give GCR profiles flatter than the observed profiles [7]. Acknowledgments. This work was supported by NASA and done under the auspices of the U.S. Dept. of Energy. References: [1] Michel R. et al. (1991) Meteoritics, 26, 221. [2] Masarik J. and Reedy R. C. (1993) LPS XXIV, 937. [3] Bhandari N. et al. (1993) GCA, 57, 2361. [4] Masarik J. and Reedy R. C. (1994) LPS XXV, 843. [5] Reedy R. C. et al. (1993) LPS XXIV, 1195. [6] Reedy R. C. and Masarik J. (1994) LPS XXV, 1119. [7] Nishiizumi K. et al. (1988) LPSC 18th, 79.
 Publication:

Meteoritics
 Pub Date:
 July 1994
 Bibcode:
 1994Metic..29Q.521R
 Keywords:

 Computerized Simulation;
 Cosmology;
 Lunar Rocks;
 Radiation Effects;
 Radioactive Isotopes;
 Cosmic Rays;
 Irradiation;
 Monte Carlo Method;
 Spheres;
 Lunar and Planetary Exploration;
 COSMOGENIC ISOTOPES; GALACTIC COSMIC RAYS; LUNAR ROCK 68815; LUNAR ROCKS; RADIONUCLIDES; SPALLATION