COSMOGENIC CL-36 ACCUMULATION IN UNSTABLE LANDFORMS .1. EFFECTS OF THE THERMAL-NEUTRON DISTRIBUTION

Cosmogenic nuclides produced in situ within minerals at the surface of the Earth are proving to be an effective means of assessing geomorphi c histories. The use of multiple cosmogenic nuclides permits both expo sure times and erosion rates to be determined. However, if two nuclide s are produced only by spallation reactions, the systematic difference s in their accumulation rates depend only on the differences in their production rates and half-lives. The relatively small differences that result require a high degree of analytical precision to yield useful results. In contrast to other spallogenic nuclides, Cl-36 is also prod uced by low-energy neutron absorption, which creates a different patte rn of production as a function of depth. We have measured the thermal flux with depth in a concrete block using He-3-filled neutron detector s. The measured thermal neutron profile agrees well with predictions f rom a simple diffusion-based thermal neutron distribution model. Calcu lations of Cl-36 production using the model suggest that the use of Cl -36 along with a purely spallogenic nuclide to determine erosion rates and exposure times should be less sensitive to analytical error than are determinations from two purely spallogenic nuclides.