Fm. Phillips et al., An improved approach to calculating low-energy cosmic-ray neutron fluxes near the land/atmosphere interface, CHEM GEOL, 175(3-4), 2001, pp. 689-701
Low-energy cosmic-ray neutrons play an important role in the production of
the cosmogenic nuclides Cl-36 and Ca-41. Previous approaches to modeling th
e distribution of low-energy neutrons beneath the surface of the earth have
derived the thermal neutrons directly from the high-energy neutron flux. W
e have improved on this model by deriving the thermal neutrons from the mod
eration of the epithermal neutron flux, and the epithermal neutrons from th
e fast neutron flux. Predictions from the improved model agree well with ex
perimental measurements of thermal and epithermal neutron fluxes both above
and below the land/atmosphere interface. Recalibration of the Cl-36 surfac
e production parameters of Phillips et al. [Phillips, F.M., Zreda, M.G., Fl
insch, M.R., Elmore, D., Sharma, P., 1996. A reevaluation of cosmogenic Cl-
36 production rates in terrestrial rocks. Geophys. Res. Lett., 23, pp. 949-
952), incorporating the new approach to simulating the low-energy neutron f
luxes, yielded the following values: P-s,P-Ca 66.8 atoms (g Ca)(-1) year(-1
), P-s,P-K 137 atoms (g K)(-1) year(-1), and P-f(0) 626 neutrons (g air)(-1
) year(-1) (this updated calibration also includes mugenic Cl-36 production
, based on independent work). Comparison of ages of three groups of samples
from sites not included in the calibration data set with independently det
ermined 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%. (C) 2001 Elsevier Science B.V. All rights reserved.