Spallation and Neutron-Capture Produced Cosmogenic Nuclides in Aubrites

70th Annual Meteoritical Society Meeting (2007) 5054.pdf

SPALLATION AND NEUTRON-CAPTURE PRODUCED COSMOGENIC NUCLIDES IN AUBRITES. J. Masarik1, K. Nishiizumi2 and K. C. Welten2, 1Department of Nuclear Physics, Comenius University Bratislava, Slovakia, E-mail: ma- [email protected], 2Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA,

Introduction: A purely physical model for the simulation of cosmic-ray-particle interactions with matter was used to investi- gate the production rates of cosmogenic nuclides in aubrites with radii ranging from 5 cm to 120 cm. Production rates of spallogenic and neutron-capture produced nuclides were investigated and compared with measured cosmogenic nuclide concentrations to constrain the complex exposure histories of aubrites. Calculational Model: The numerical simulation of interactions of primary and secondary cosmic-ray particles was done with the LAHET Code System (LCS) [1] which uses MCNP [2] for transport of low energy neutrons. The investigated objects were spheres with various radii that were divided into spherical layers. We used the spectrum of the galactic-cosmic-ray particles corresponding to solar modulation parameter Φ = 550 MeV and a flux of 4.8 protons/s·cm2. The statistical errors of the LCS calculated fluxes were 3–5%. The production rates of nuclides were calculated by integrating over energy the product of these fluxes and cross sections for the nuclear reactions making the investigated nuclide. For cross sections of spallogenic products, we relied on the values evaluated by us and tested by earlier calculations [e.g., 3]. Previous calculations of the 53Mn production rate in aubrites showed good agreement with measured 53Mn concentrations in the Norton County aubrite [4]. The production rates of nuclides produced by neutron capture reactions were calculated using the excitation functions from neutron evaluated data files ENDF/B-VI [5], which were previously used for calculating neutron-capture 41Ca in the lunar surface [6]. In our previous work, the calculated production rates of neutron capture 41Ca showed excellent agreement with the measured depth profile of 41Ca in the Apollo 15 drill core [6]. Results and Discussion: The depth profiles depend on the shape of the excitation functions producing a nuclide and the particle spectra. These spectra depend on the shielding of the sample, i.e., its location inside the object, and the object’s size. In this work we will compare neutron-capture produced 41Ca and spallation produced 10Be and 26Al measured in various aubrites [7] with calculated production rates to constrain the pre- atmospheric size of these aubrites and depth of the measured samples within these objects. In addition, we will compare measured concentrations of neutron-capture 41Ca, 150Sm and 158Gd [7,8] with calculated production rates to constrain the 2π and 4π exposure histories of aubrites. References: [1] Prael R. E. and Lichtenstein H. 1989. LA- UR-89-3014. [2] Briesmeister J. F. 1993. LA-12625-M. [3] Ma- sarik J. and Reedy R. C. 1994. Geochimica et Cosmochimica Acta 58:5307-5317. [4] Englert P. A. J. et al. 1995. Geochimica et Cosmochimica Acta 59:825-830. [5] McLane V. E. et al. 1991. BNL-NCS-17541. [6] Nishiizumi K. et al. 1997. Earth and Planetary Science Letters 148:545-552. [7] Welten K. C. et al. 2004. Meteoritics & Planetary Science 39:A113. [8] Hidaka H. et al. 1999. Earth and Planetary Science Letters 173:41-51.