The global distribution of secondary cosmic rays and applications to cosmogenic dating

Abstract

Methods of surface exposure dating usmg terrestrial cosmogemc nuclides require accurate knowledge of the spatial variability of nuclide production rates. The nucleon component of the secondary cosmic-ray flux is responsible for a major fraction of terres trial nuclide production and this component is particularly sensitive to variations in alti tude and position in the geomagnetic field. To be applied at widely different locations, calibrated production rates must be scaled to account for variations in cosmic-ray intensity due to these two factors. Current scaling models are based on a small number of nuclear emulsion and cloud chamber measurements and data from BF3 proportional counters that are mostly limited to altitudes above 3,000 m. Over the past 50 years, however, there have been numerous alti tude and latitude surveys with neutron monitors as well as additional measurements with proportional counters. These surveys not only describe more precisely how the nucleon flux depends on altitude and geomagnetic position, but also provide valuable data on the energy spectrum and on the effects of solar activity. This work utilizes more recent and more extensive measurements of nucleon intensity along with an improved understanding of cosmic-ray phenomena to derive scaling models for thermal neutron absorption reactions and high-energy spallation reactions. Latitude data are ordered according to effective vertical cutoff rigidity [GV] and altitude data according to mass shielding depth [g cm2 ]. Neutron monitor data are corrected for instrumental biases and parameterized using polynomials. Attenuation lengths for thermal neutrons are greater than for high-energy neutrons by 15% at 3800 m and 14 GV, whereas the difference at sea level is estimated to be negligible at all latitudes