Nucleogenic 36Cl, 236U and 239Pu in uranium ores

The nucleogenic isotopes Cl-36, U-236 and Pu-239 are produced naturally in subsurface environments via neutron capture of thermal and epithermal neutrons. Concentrations are, however, very low and accelerator mass spectrometry (AMS) is required for quantitative measurements. A particular challenge is presented by the measurement of U-236/U-238 ratios down to the level of 10(-13) that is expected from rocks with low uranium concentration. Here, we present the AMS methodology that has been developed at the ANU for measuring U-236/U-238 ratios at this level. The more established methodologies for Cl-36 and Pu-239 measurements are also summarised. These capabilities are then used to characterize the Cl-36, U-236 and Pu-239 concentrations in a range of uranium ores. A simple model of the neutron production and capture processes in subsurface environments has been developed and is presented. It is shown that nucleogenic Cl-36, U-236 and Pu-239 can be used to determine both thermal and epithermal neutron fluxes in subsurface environments. Potential applications include uranium exploration and monitoring of the environmental impact of uranium mining. (C) 2008 Elsevier B.V. All rights reserved.