Background Sources of Ar-37

Argon-37 is a radioactive isotope of argon that is naturally produced in both the atmosphere and the subsurface. It has a half-life of 35.04 days and decays via electron capture, emitting Auger electrons and x-rays as it decays. In both the atmosphere and the shallow subsurface, 37Ar is produced primarily by cosmic neutron reactions, with the highest production rate occurring at approximately two meters underground. The shallow subsurface production rate of 37Ar is dependent primarily on geology, but other factors such as location (latitude), altitude, and the solar cycle can also have an impact on the production rate. Many measurements have been made of the 37Ar concentration in both atmospheric and shallow subsurface air and a selection of these will be presented. Additionally, potential production mechanisms deep underground will be examined. This presentation was used for the Low-Radioactivity Underground Argon Workshop held at Pacific Northwest National Laboratory in Richland, Washington on March 19 - 20, 2018