An improved approach to calculating low-energy cosmic-ray neutron fluxes near the land/atmosphere interface

Low-energy cosmic-ray neutrons play an important role in the production of the cosmogenic nuclides 36Cl and 41Ca. Previous approaches to modeling the distribution of low-energy neutrons beneath the surface of the earth have derived the thermal neutrons directly from the high-energy neutron flux. We have improved on this model by deriving the thermal neutrons from the moderation of the epithermal neutron flux, and the epithermal neutrons from the fast neutron flux. Predictions from the improved model agree well with experimental measurements of thermal and epithermal neutron fluxes both above and below the land/atmosphere interface. Recalibration of the 36Cl surface production parameters of Phillips et al. [Phillips, F.M., Zreda, M.G., Flinsch, M.R., Elmore, D., Sharma, P., 1996. A reevaluation of cosmogenic 36Cl production rates in terrestrial rocks. Geophys. Res. Lett., 23, pp. 949–952), incorporating the new approach to simulating the low-energy neutron fluxes, yielded the following values: Ps,Ca 66.8 atoms (g Ca)−1 year−1, Ps,K 137 atoms (g K)−1 year−1, and Pf(0) 626 neutrons (g air)−1 year−1 (this updated calibration also includes mugenic 36Cl production, based on independent work). Comparison of ages of three groups of samples from sites not included in the calibration data set with independently determined ages gave an average absolute error of 6.6% for all three data sets and coefficients of variation among the samples in the groups ranging from 5% to 14%.