Kavli Affiliate: T. A. Shutt
| Summary:
We estimate the amount of $^37$Ar produced in natural xenon via cosmic ray-induced spallation, an inevitable consequence of the transportation and storage of xenon on the Earth’s surface. We then calculate the resulting $^37$Ar concentration in a 10-tonne payload~(similar to that of the LUX-ZEPLIN experiment) assuming a representative schedule of xenon purification, storage and delivery to the underground facility. Using the spallation model by Silberberg and Tsao, the sea level production rate of $^37$Ar in natural xenon is estimated to be 0.024~atoms/kg/day. Assuming the xenon is successively purified to remove radioactive contaminants in 1-tonne batches at a rate of 1~tonne/month, the average $^37$Ar activity after 10~tonnes are purified and transported underground is 0.058–0.090~$μ$Bq/kg, depending on the degree of argon removal during above-ground purification. Such cosmogenic $^37$Ar will appear as a noticeable background in the early science data, while decaying with a 35~day half-life. This newly-noticed production mechanism of $^37$Ar should be considered when planning for future liquid xenon-based experiments.
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