Utilizing Monte-Carlo radiation transport and spallation cross sections to estimate nuclide dependent scaling with altitude

Cosmogenic Nuclides (CNs) are a critical new tool for geomorphology, allowing researchers to date Earth surface events and measure process rates [1]. Prior to CNs, many of these events and processes had no absolute method for measurement and relied entirely on relative methods [2]. Continued improvements in CN methods are necessary for expanding analytic capability in geomorphology. In the last two decades, significant progress has been made in refining these methods and reducing analytic uncertainties [1,3]. Calibration data and scaling methods are being developed to provide a self consistent platform for use in interpreting nuclide concentration values into geologic data [4]. However, nuclide dependent scaling has been difficult to address due to analytic uncertainty and sparseness in altitude transects. Artificial target experiments are underway, but these experiments take considerable time for nuclide buildup in lower altitudes. In this study, a Monte Carlo method radiation transport code, MCNPX, is used to model the galactic cosmic-ray radiation impinging on the upper atmosphere and track the resulting secondary particles through a model of the Earth’s atmosphere and lithosphere. To address the issue of nuclide dependent scaling, the neutron flux values determined by the MCNPX simulation are folded in with estimated cross-section values [5,6]. Preliminary calculations indicate that scaling of nuclide production potential in free air seems to be a function of both altitude and nuclide production pathway. At 0 g/cm2 (sea-level) all neutron spallation pathways have attenuation lengths within 1% of 130 g/cm2. However, the differences in attenuation length are exacerbated with increasing altitude. At 530 g/cm2 atmospheric height (~5,500 m), the apparent attenuation lengths for aggregate SiO2(n,x)10Be, aggregate SiO2(n,x)14C and K(n,x)36Cl become 149.5 g/cm2, 151 g/cm2 and 148 g/cm2 respectively. At 700 g/cm2 atmospheric height (~8,400m - close to the highest possible sampling altitude), the apparent attenuation lengths become 171 g/cm2, 174 g/cm2 and 165 g/cm2 respectively, a difference of +/-5%. Based on this preliminary data, there may be up to 6% error in production rate scaling. Proton spallation is a small, yet important component of spallation events. This data will be also be presented along with the neutron results. While the differences between attenuation length for individual nuclides are small at sea-level, they are systematic and exacerbate with altitude. Until now, there has been no numeric analysis of this phenomenon, therefore the global scaling schemes for CNs have been missing an aspect of physics critical for achieving close agreement between empiric calibration data and physics based models. [1] T. J. Dunai, "Cosmogenic Nuclides: Principles, Concepts and Applications in the Earth Surface Sciences", Cambridge University Press, Cambridge, 2010 [2] D. Lal, Annual Rev of Earth Planet Sci, 1988, p355-388 [3] J. Gosse and F. Phillips, Quaternary Science Rev, 2001, p1475-1560 [4] F. Phillips et al.,(Proposal to the National Science Foundation), 2003 [5] K. Nishiizumi etal., Geochimica et Cosmochimica Acta, 2009, p2163-2176 [6] R. C. Reedy, personal com.