Spatial and temporal distribution of secondary cosmic-ray nucleon intensities and applications to in situ cosmogenic dating

Abstract

Cosmogenic nuclide production rates depend critically on the spatio-temporal distribution of cosmic-ray nucleon fluxes. Since the 1950s, measurements of the altitude, latitude and solar modulation dependencies of secondary cosmic-ray fluxes have been obtained by numerous investigators. However, until recently there has been no attempt to thoroughly evaluate the large body of modern cosmic-ray literature, to explain systematic discrepancies between measurements or to put these data into a rigorous theoretical framework appropriate for cosmogenic dating. The most important parameter to be constrained is the dependence of neutron intensity on atmospheric depth. Our analysis shows that effective nucleon attenuation lengths measured with neutron monitors over altitudes 0–5000 m range from 128 to 142 g cm−2 at effective vertical cutoff rigidities of 0.5 and 14.9 GV, respectively. Effective attenuation lengths derived from thermal neutron data are somewhat higher, ranging from 134 to 155 g cm−2 at the same cutoff rigidities and over the same altitudes. We attribute the difference to a combination of two factors: the neutron monitor is more sensitive to the higher end of the nucleon energy spectrum, and the shape of the nucleon energy spectrum shifts towards lower energies with increasing atmospheric depth. We have derived separate scaling models for thermal neutron reactions and spallation reactions based on a comprehensive analysis of cosmic-ray survey data. By assuming that cosmic-ray intensity depends only on atmospheric depth and effective vertical cutoff rigidity, these models can be used to correct production rates for temporal changes in geomagnetic intensity using paleomagnetic records.