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Over the past several years, the Pacific Northwest National Laboratory (PNNL) has developed an ultra-low-background proportional counter (ULBPC) technology. The resulting detector is the product of an effort to produce a low-background, physically robust gas proportional counter for applications like radon emanation measurements, groundwater tritium, and Ar. In order to fully take advantage of the inherent low-background properties designed into the ULBPC, a comparably low-background dedicated counting system is required. An ultra-low-background counting system (ULBCS) was recently built in the new shallow underground laboratory at PNNL. With a design depth of 30 m water-equivalent, the shallow underground laboratory provides approximately 100× fewer fast neutrons and 6× fewer muons than a surface location. The ULBCS itself provides additional shielding in the form of active anti-cosmic veto (via 2-in-thick plastic scintillator paddles) and passive borated poly (1 in.), lead (6 in.), and copper (~3 in.) shielding. This work will provide details on PNNL's new shallow underground laboratory, examine the motivation for the design of the counting system, and provide results from the characterization of the ULBCS, including initial detector background. [ABSTRACT FROM AUTHOR]

Pacific Northwest National Laboratory uses beta-gamma coincidence detectors in a number of xenon sampling and measurement systems to enable simultaneous, sensitive measurements of 131Xe, 133Xe, 133mXe, and 135Xe for treaty monitoring applications. In recent years, a new style of beta–gamma detector was developed to improve upon the detector module used in the Automated Radioxenon Sampler/Analyzer. The results of an MCNP5 Monte Carlo simulation of the new detector cell are presented, with particular emphasis on the identification of an energy deposition sequence with the potential to introduce significant error into the detector efficiency calibration. This sequence occurs when an 81 keV gamma from 133Xe is absorbed in an inactive region of the CsI(Na) scintillator, followed by emission of a 31 keV X-ray from cesium (or possibly a 28.5 keV X-ray from iodine). These X-rays add excess counts into the 31 keV peak observed in the decay of 133Xe. The impact of this effect on different efficiency calibration techniques is discussed. [ABSTRACT FROM AUTHOR]