Comparison of near-background concentrations of Argon-37 and Xenon-133 in the atmosphere

Radioisotopes of the noble gases xenon and argon can be 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 ^131mXe, ¹³³Xe, ^133mXe, and ¹³⁵Xe. 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 and with a higher signal to background ratio, depending on the radioxenon background levels. Therefore, inclusion of ³⁷Ar measurement capabilities at atmospheric International Monitoring System (IMS) stations may represent an improvement in the capability to detect a nuclear explosion. At an IMS station location, an understanding of the expected range of background ³⁷Ar activity concentrations is critical to determining what levels would constitute an elevated concentration. This work describes our analysis of atmospheric samples for ³⁷Ar to evaluate the range of background concentrations. Samples were collected at multiple locations withing the United States, with approximately half coming from a sampler co-located with an IMS xenon monitoring station (RN75). The range of ³⁷Ar concentrations measured in atmospheric air samples was relatively narrow; for samples considered detectable, the minimum and maximum measured concentrations were 0.56 and 2.3 mBq/m³, respectively. Comparison of ³⁷Ar and ¹³³Xe concentrations measured at the IMS station indicated some correlation between the measured concentrations. The results presented here demonstrate the capability to detect background concentrations of ³⁷Ar in atmospheric air and provide a basis for potential implementation of ³⁷Ar monitoring at IMS stations.