Simulation of Galactic Cosmic Ray Interactions with Regolith: Implications for Cosmogenic Nuclide and Planetary Surface Studies

Details Simulation of Galactic Cosmic Ray Interactions with Regolith: Implications for Cosmogenic Nuclide and Planetary Surface Studies Simulation of Galactic Cosmic Ray Interactions with Regolith: Implications for Cosmogenic Nuclide and Planetary Surface Studies

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.205b&link_type=abstract

Meteoritics, vol. 27, no. 3, volume 27, page 205

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In planetary, asteroidal, and cometary surfaces the development of cosmic ray produced secondary particle cascades depends very much on the chemical composition. Most important is the abundance and distribution of neutron moderators, such as hydrogen and carbon. Their presence influences the distribution and flux of neutrons in extraterrestrial surfaces. Consequently, the distribution and activity of neutron produced cosmogenic nuclides, such as the neutron capture products ^36Cl (3.01x10^5y), ^41Ca (1.03x10^5y), ^59Ni (7.6x10^4 y) and ^60Co (5.271y), and the medium energy products such as ^22Na (2.605 y, from Al), ^26Al (7.4x10^5 y, from Si), ^53Mn (3.7x10^6, from Fe), etc., will be dependent on the development of the secondary neutron fluxes. Three thick target bombardments of artificial planetary surface soil (martian) [1,2,3] that measured the secondary particle cascade as well as surface neutron flux and leakage spectra were performed at the Los Alamos Meson Physics Facility's Weapons Neutron Research Laboratory. They provide data to study qualitatively and quantitatively the relation between bombarding galactic cosmic ray particles, the development of neutrons inside a surface and of neutrons emitted from a planetary surface. Past thick target bombardments were limited to only determining the secondary particle cascades or the emitted gamma radiation [4,5,6]. The experiments show that the second target, which was two times larger than the target for the first experiment, was holding more neutrons inside the target. The third irradiation utilized a polyethylene sheet in front of the second target to simulate the presence of water or ice on a planetary surface. Threshold monitors were placed both along the axis of symmetry and radially displaced axes. The monitors included foils of Ti, Mn, Fe, Co, Ni, and Au. The induced radioactivity was measured at Los Alamos by conventional gamma-ray spectrometry approximately 48 hours at the end of irradiation. Both the influence of volume and of moderation via hydrogen and/or carbon on secondary low and high energy neutron fluences in very extended targets under charged particle bombardment are now measured. In combination with model calculations, they can be used to establish secondary low energy particle fluxes in planetary surfaces. The neutron distributions measured will help to open up a new area of extraterrestrial cosmogenic nuclides studies. Especially interesting is the effect of surface ice on low energy neutron fluxes. It was noticed that the "ice-coverage" caused a low energy neutron flux increase at the surface by almost an order of magnitude. The effect of permanent, seasonal, and historical ice coverage on planetary surfaces, such as that of Mars, or other bodies, may now become a subject for interpretation of cosmogenic nuclide production rates in specimens from sample return missions. REFERENCES: [1] Englert, P., S.G. Bobias, et al. (1990), Meteoritics, 361. [2] Englert, P., et al. (1990), Lunar Planet. Sci. XXI, 325. [3] Englert, P., L.J. Harris, S.G. Bobias, et al. (1991), Lunar Planet. Sci. XXII, 353. [4] Englert, P., R.C. Reedy, and J.R. Arnold (1987) Nucl. Instr. Meth. A262, 496-502. [5] Reedy, R.C. And P. Englert (eds.) (1986), LPI Technical Report 86 06, (80 pages). [6] Metzger, A.E., R.H. Parker, and J. Yellin (1986), J. Geophys. Res. 91, No. B4. D495-D504.

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