Radioactive xenon and argon production and transport in the environment
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Verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes both atmospheric and on-site sampling and detection of radioactive noble gases that are produced in a nuclear explosion, particularly radioactive isotopes of xenon and argon. However, other sources of these radioactive noble gases can complicate the analysis by introducing backgrounds which may either mask a signal or may alter the isotopic ratios that are used to distinguish the origin of the gases. A series of field experiments were conducted in order to better understand background sources of these noble gases and their potential impact on CTBT verification activities. Two sampling campaigns were conducted near the Chalk River Laboratories (CRL) medical isotope production facility. During these sampling campaigns air samples were collected from both the atmosphere and the shallow subsurface (1-2 m) using proposed on-site inspection sampling techniques. The Subsurface Transport over Multiple Phases (STOMP) simulator was then used to make predictions about the expected imprinting during the experimental campaign and the results were compared to the experimental results. The combined results of these experiments provided first of their kind measurements of imprinted radioxenon and provided clear evidence that imprinting of atmospheric radioxenon gas into the subsurface does occur as predicted by transport models. Two potential sources of background 37Ar were also examined. First, the results of 37Ar and 41Ar concentration measurements from around The University of Texas at Austin's research reactor were used to make a more general estimate of the 37Ar release rates from the reactor and atmospheric transport modeling was used to estimate the impact of such releases. Atmospheric transport modeling of release rates predicted by this work indicates that research reactors do not release 37Ar in concentrations measurable by proposed OSI argon detection equipment. Second, natural production of 37Ar was examined as a potential subsurface background source by analyzing samples of subsurface air from a high calcium limestone geology for their 37Ar concentration. The final results of two, one-day samples taken from an 18-in deep hole which was tarped at the surface found 37Ar concentrations of 1.6 +- 1 mBq/m3 on the first day, and 0.98 +- 0.17 mBq/m3 on the second day. From the experience gained during this experiment, recommendations were made to improve future sampling in hard geologies.