Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.541m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 541-542
Other
Anomalies, Isotopic, Isotopes, Cosmogenic, Moon, Neutrons, Thermal, Temperature
Scientific paper
High precision W, Nd, and Sm isotopic analyses [1,2] used for precise age determination of the earliest episodes of planetary differentiation require an understanding of possible contributions from neutron-capture reactions to the production of the investigated isotopes. Low-energy neutrons can also be used to study the surface composition of the planets [3,4]. Neutron-capture production profiles, which are very different from those for tracks or from nuclides made by energetic cosmic ray particles, can be used for unfolding the cosmic-ray exposure history of meteorites [5]. We did Monte Carlo numerical simulations of the influence of chemical composition, temperature and water content on neutron fluxes and production of cosmogenic isotopes. The LAHET Code System [6] was used to numerically simulate the irradiation of various objects by galactic-cosmic-ray particles and to calculate neutron fluxes and production rates of various W, Sm, Nd, Gd isotopes and 59Ni, 60Co, 36Cl, 41Ca, 80Kr and 82Kr. The advantage of these calculations is that the physical model applied to the investigation of particle production and transport uses only basic physical quantities and parameters without including any free parameters and assumptions about the neutron source term, as was necessary in older approaches [7,8]. Our simulations started by selecting the energy and direction of the primary particle that starts the particle cascade. As neutrons produced in the cascade are followed down to the thermal energies, we are able to determine the main sources of observed differences in capture rates. The calculations were validated by modeling [9] ^(60)Co [10] and 41Ca [11] measured in lunar samples. For the surface temperature variations during the lunar day, which range from about 120 K to 400 K, we found that the effect on production rates is very small. Temperature influences only relative capture rates of isotopes whose thermal capture cross sections differ from a 1/v dependence. For all studied isotopes, this effect was below 10%. No change in depth dependence of capture rates for temperature variations was found. The effect of total macroscopic neutron capture cross section on production processes was studied by varying its value within a range from 0.5 to 2 times the Apollo 15 value, and both the absolute value and shape of the neutron spectra were strongly influenced. The resulting production rate was shown to be approximately inversely proportional to the thermal neutron macroscopic capture cross section. The effects of small variations in water content in the investigated body was studied also. Even a little water added to the original dry chemical composition leads to an observable increase in production rates. Results obtained for chemistries with high water content together with their physical interpretations were reported in [4]. The new calculated production rates and their dependences on various parameters can contribute to the more accurate interpretation of existing lunar and meteoritic neutron-capture-produced cosmogenic isotopes, as well as in other planetary studies involving low-energy neutrons. Acknowledgments: Work at Los Alamos supported by NASA and done under auspices of the U.S. Dept. of Energy. References: [1] Harper C. L. and Jacobsen S. B. (1994) LPS XXV, 509-510. [2] Harper C. L. and Jacobsen S. B. (1992) Nature, 360, 728-732. [3] Reedy R. C. et al. (1973) JGR, 78, 5847-5860. [4] Masarik J. and Reedy R. C. (1995) JGR, submitted. [5] Honda M. et al. (1982) EPSL, 57, 101-109. [6] Prael R. E. and Lichtenstein H. (1989) Los Alamos Report LA-UR-89-3014. [7] Eberhardt P. et al. (1961) Helv. Phys. Acta, 34, 460-464. [8] Spergel M. S. et al. (1986) Proc. LPSC 16th, in JGR, 91, D483-D494. [9] Masarik J. and Reedy R. C. (1995) LPS XXVI, 899-900. [10] Wahlen M. R. (1975) EPSL, 19, 315-320. [11] Nishiizumi K. et al. (1990) LPS XXI, 893-894.
Masarik Jozef
Reedy Robert C.
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