Understanding production rates of cosmogenic nuclides on the surface of the earth can be done effectively using a short depth profile. 10Be surface exposure ages of a few moraine boulders at the Mackenzie Basin in New Zealand were determined using a 10Be depth profile and bash samples. The 10Be depth profiles from the Mackenzie Basin and Macraes Flat [1] were compared. Our mean 10Be surface exposure age of moraine boulders from Mackenzie Basin was determined to be 19.7 ± 1.4 kyr. The erosion rates Mackenzie Basin were obtained using a short depth profile of 10Be and found to be 7 m Myr-1. An observed attenuation length of 180 ± 14 g cm-2 for the 10Be depth profile from newly exposed boulder at Mackenzie Basin suggests that the muon contribution at the surface of the boulder is not significant. This confirms the 10Be surface exposure age of this site is reliable, while the unexpectedly long apparent attenuation lengths at the surface of the Macraes Flat would indicate significant inherited muon component at the surface. Therefore the latter case demands a muon correction for surface samples. Numerical simulation of the 10Be depth profile using standard rock composition at the sampling location, Mackenzie Basin, was done using Monte Carlo N Particle eXtend (MCNPX) code. The production depth profile of this numerical simulation is consistent with the measured 10Be depth profile for this study. The comparison of this measured depth profile with numerical simulation is important in understanding the absolute primary proton flux in space and beneficial to future model calculations for terrestrial environments.
[1] K.J. Kim and P.A.J. Englert, EPSL 223 (2004) 113-126.
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