Radioxenon background at high northern latitudes

[ 1] As part of the International Noble Gas Experiment (INGE) two stations were deployed in remote regions at high northern latitudes: Longyearbyen, Spitsbergen, Norway (78.2 degrees N), and Yellowknife, Canada (62.5 degrees N). With one exception, both stations are 2000 km or more from any single known stationary nuclear facility. Nevertheless, the short-lived anthropogenic radionuclide Xe-133 (t(1/2) = 5.24 days) has been detected on a regular basis at both stations, and also Xe-131m ( t(1/2) = 11.84 days) was found at the Yellowknife station. Measuring these very low concentrations ( similar to 0.1 mBq/m(3)) has been possible because of the introduction of new and sensitive equipment developed specifically for the verification of the Comprehensive Nuclear-Test Ban Treaty. By means of atmospheric transport modeling, it was determined that the measurements at both sites are broadly consistent with reported releases from reactors in North America and Europe and that the Spitsbergen station is much more susceptible to this influence than the Yellowknife station. Especially at Spitsbergen, the simplified assumption of a continuous bulk release across Europe could well explain the month-to-month variation of the time series. A future radioxenon event classification scheme for treaty verification purposes thus needs to consider the actual meteorological situation and large-scale transport processes.