Measurements of ambient radioxenon levels using the automated radioxenon sampler/analyzer (ARSA)

The Pacific Northwest National Laboratory has developed an Automated Radioxenon Sampler/Analyzer (ARSA) in support of the Comprehensive Nuclear-Test-Ban-Treaty (CTBT) to measure four radioxenon isotopes: Xe-131m, Xe-133m, Xe-133g, and Xe-135g. This system uses a beta-gamma coincidence counting detector to produce two-dimensional plots of gamma-energy versus beta-energy. Betas and conversion electrons (CE) are detected in a cylindrical plastic scintillation cell and gamma and X-rays are detected in a surrounding NaI(TI) scintillation detector. The ARSA has been field tested at several locations to measure the radioxenon concentrations. Most recently it has been deployed at the Institut fur Atmospharische Radioaktivitat in Freiburg, Germany. During the first 4 months of 2000 the measured Xe-133 concentrations have varied between 0.0 +/-0.1 and 110 +/- 10 mBq/m(3) air. The longer lived Xe-131m (T-1/2 = 11.9 d) and Short lived Xe-135 (T-1/2 = 9.1 h) have also been detected in small quantities, while Xe-133m concentrations have been consistent with zero. Minimum detectable concentration (MDC) calculations for Xe-133g fell well below the 1 mBq per standard-cubic-meter of air requirement adopted by the CTBT preparatory Commission.(1) A description of the radioxenon detector, the concentration and MDC calculations and preliminary results of the field test in Germany are presented.