Radioisotopes of noble gases, primarily xenon and argon, are important indicators of underground nuclear explosions. The Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes monitoring capabilities to identify potential nuclear explosions conducted in violation of the CTBT. This monitoring currently focuses on measurement of the xenon isotopes 133Xe, 135Xe, 131mXe and 133mXe. However, it is predicted that within 100 days of an underground nuclear explosion (UNE) 37Ar would be released to the atmosphere at higher concentrations than xenon isotopes (Haas et al. 2010). Therefore, inclusion of 37Ar measurement capabilities at atmospheric International Monitoring System (IMS) stations could improve the capability to detect a nuclear explosion.
At an IMS station, it could be difficult to determine what constitutes an elevated concentration as a result of an UNE without first understanding the expected range of background concentrations. This work describes our analysis of atmospheric samples for 37Ar to evaluate the range of background concentrations. The range of 37Ar concentrations measured in atmospheric air samples was relatively narrow ranging between less than 1 mBq/m3 to greater than 2 mBq/m3. The average atmospheric concentration of all samples was 0.98 mBq/m3, with a standard deviation of 0.5 mBq/m3. This is consistent with previous published results. The results presented here demonstrate the capability to detect background concentrations of 37Ar in atmospheric air.