Characterization of a low background proportional counter for a high throughput Argon-37 collection and measurement system

The measurement of ³⁷Ar has been proposed as a method of detecting underground nuclear testing. The isotope ³⁷Ar is generated by neutron activation of calcium by the reaction, ⁴⁰Ca(n, a)³⁷Ar, and, as a noble gas, is able to migrate more freely underground. Pacific Northwest National Laboratory has developed a high throughput ³⁷Ar collection and measurement system using modified Ultra-Low Background Proportional Counters (ULBPCs). This system is capable of collecting, purifying, and measuring radioactivity in argon from either atmospheric or soil gas samples. This process is automated, with minimal operator intervention. This paper describes quantification of ³⁷Ar and how we account for varying background conditions. An example is provided in which ³⁹Ar has been shown to be a significant background in some sample sets, created by process ³⁹K(n,p)³⁹Ar during an underground nuclear explosion that would also create ³⁷Ar. To account for the large background of ³⁹Ar, we fit the data with a constant plus exponential background model with Gaussian signal model and use the results of a constrained fit to calculate ³⁷Ar activity. We discuss the methods used to purify and count samples. We discuss the impact of increased ³⁹Ar backgrounds on the measurement of ³⁷Ar.