Prediction Of Sub-Surface Ar-37 Concentrations At Locations In The Northwestern United States

The Comprehensive Nuclear-Test-Ban Treaty, which is intended to prevent nuclear weapon test explosions and any other nuclear explosions, includes a verification regime, which provides monitoring to identify potential nuclear explosions. The presence of elevated Ar-37 is one way to identify subsurface nuclear explosive testing. However, the naturally occurring formation of Ar-37 in the subsurface adds a complicating factor. Prediction of the naturally occurring concentration of Ar-37 can help to determine if a measured Ar-37 concentration is elevated relative to background. The naturally occurring Ar-37 background concentration has been shown to vary between less than 1 mBq/m(3) to greater than 100 mBq/m(3) (Riedmann and Purtschert, 2011). The purpose of this work was to enhance the understanding of the naturally occurring background concentrations of Ar-37, allowing for better interpretation of results. To that end, we present and evaluate a computationally efficient model for predicting the average concentration of Ar-37 at any depth under transient barometric pressures. Further, measurements of Ar-37 concentrations in samples collected at multiple locations are provided as validation of the concentration prediction model. The model is shown to compare favorably with concentrations of Ar-37 measured at multiple locations in the Northwestern United States.