Cosmogenic nuclides produced in situ within minerals at the surface of the Earth are proving to be an effective means of assessing geomorphic histories. The use of multiple cosmogenic nuclides permits both exposure times and erosion rates to be determined. However, if two nuclides are produced only by spallation reactions, the systematic differences 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, 36Cl is also produced by low-energy neutron absorption, which creates a different pattern of production as a function of depth. We have measured the thermal flux with depth in a concrete block using 3He-filled neutron detectors. The measured thermal neutron profile agrees well with predictions from a simple diffusion-based thermal neutron distribution model. Calculations of 36Cl production using the model suggest that the use of 36Cl 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. ¿ American Geophysical Union 1994