Modeling global cosmogenic nuclide production rates through first principles

Abstract

The work contained in this thesis is focused on utilizing radiation transport code software as the basis for developing a well validated, first-principles model of global terrestrial cosmogenic nuclide production rates. The state-of-the-art radiation transport code, MCNPX, is utilized to model the terrestrial radiation field. Folding the radiation field neutron and proton results with cosmogenic nuclide production cross-sections yields production rates. This comprehensive, first-principles model is used to investigate characteristics of cosmogenic nuclide production. The goal of the work is to constrain uncertainties in cosmogenic nuclides by better understanding production systematics. Greater understanding of cosmogenic nuclide production rate systematics will assist in constraining uncertainties in cosmogenic nuclide production rate scaling, thereby reducing uncertainties in calculations based on sample nuclide concentrations exposure ages, erosion rates, and burial dating. Furthermore, novel uses of cosmogenic nuclides, currently unachievable due to uncertainties, will be enabled by further constraining these. The model is benchmarked against Dr. Paul Goldhagen's ER-2 aircraft neutron monitor measurements, the Knyahinya meteorite in-situ cosmogenic nuclides, the Beacon Heights sandstone core measurements, and estimated sea level production rates. In this work, I examine: the production rates of each commonly used cosmogenic nuclide as a function of altitude and latitude; the angular distribution of nuclide-producing cosmic-ray particles as a function of altitude and latitude; subsurface production rate systematics; and the production of 36Cl in both the atmosphere and the oceans.