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

Abstract

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: ^131mXe, ^133mXe, ^133gXe, and ^135gXe. 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(Tl) 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 für Atmosphärische Radioaktivität in Freiburg, Germany. During the first 4 months of 2000 the measured ¹³³Xe oncentrations have varied between 0.0±0.1 and 110±10 mBq/m³ air. The longer lived ^131mXe (T _1/2 = 11.9 d) and short lived ¹³⁵Xe (T _1/2 = 9.1 h) have also been detected in small quantities, while ^133mXe concentrations have been consistent with zero. Minimum detectable concentration (MDC) calculations for ^133gXe 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.