Physics-based modeling of cosmogenic nuclides part I – Radiation transport methods and new insights

• Radiation transport code, MCNPX, is used to simulate cosmic-ray particle flux.
• Neutron and proton flux are folded with cross sections to predict production rates.
• Modeled neutron spectra are compared with neutron spectrum measurements.
• Nuclide production ratios change with both altitude and latitude.
• Production rates for commonly used nuclides are provided as electronic supplement.

We present a comprehensive, nuclear-physics-based cosmogenic nuclide production rate model combining radiation transport modeling with excitation functions for commonly measured nuclides. This model allows investigation of factors influencing nuclide production, such as the energy spectrum and angular distribution of the incident radiation that cannot be easily isolated in calibration measurements on natural samples. We present neutron and proton fluxes over a range of atmospheric depths and cut-off rigidities. Calculated production rates for 3He, 10Be, 14C, 21Ne, 26Al, and 36Cl based on these fluxes are presented. The model predicts that production rates for these nuclides diverge from one another with altitude, hence that production ratios depend on altitude. Compared to existing scaling schemes, the model predicts a larger difference between sea-level production rates at low and high latitude.