The International Monitoring System includes a network of radionuclide detectors operated around the world monitoring for nuclear explosions. A key aspect of the International Monitoring System is the detection of radioxenon with a network of stations and laboratories. Beta-gamma detectors are utilized extensively for the detection of radioxenon, and the beta detection is primarily performed with a plastic scintillator cell. Two areas of improvement for plastic scintillator are the sample carry-over (“memory effect”) and energy resolution. While the scintillator can be coated to remove the memory effect, the energy resolution must be improved with a different detector material. Silicon is the current leading candidate for the future beta cell material due to the much-improved energy resolution compared to plastic scintillators (factor of ~ 3x). PNNL is developing a silicon beta cell for use as a potential modular replacement within Xenon International (a next generation radioxenon detection system currently undergoing acceptance testing for potential inclusion in the International Monitoring System). The beta cell utilizes four different silicon detectors to create an active volume for the radioxenon within an outer gas cell. Since there are four separate beta signals (compared to one for plastic scintillators), data acquisition modifications are required. In this paper, we detail the design, efficiency measurements, and long-term testing of the silicon beta cell and potential improvements in isotopic discrimination.

This document defines the requirements for the new Xenon International radioxenon system. The output of this project will be a Pacific Northwest National Laboratory (PNNL) developed prototype and a manufacturer-developed production prototype. The two prototypes are intended to be as close to matching as possible; this will be facilitated by overlapping development cycles and open communication between PNNL and the manufacturer.

The Radionuclide Aerosol Sampler/Analyzer (RASA) is an automated collection and analysis system designed for aerosol radionuclide monitoring for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The advantages of an automated system include minimal need for human intervention and consistent analytical data. However, maintainability and down time issues threaten this utility, even for systems with over 90 % data availability. Engineering upgrades to the RASA are currently being pursued to address these issues, as well as measures relevant to technical lessons learned from the Fukushima nuclear power plant event. Current work includes a new automation control unit and other potential improvements such as alternative detector cooling and sampling options. This paper presents the current state of upgrades and improvements under investigation.

Detection of radioactive noble gases can provide definitive evidence of a nuclear explosion at large stand-off distances. The Automated Radioxenon Sampler/Analyzer (ARSA) developed at Pacific Northwest National Laboratory (PNNL) measures the relative concentrations of xenon isotopes using a β-γ coincidence system. Typically, a set of plastic scintillating cells are surrounded by a NaI(Tl) scintillator. The cells are evacuated and the background count rate is measured before the cell is filled with sampled air. Any radioxenon present in the air emits β particles, which are detected in the plastic cell, as well as coincident γ rays, which are detected in the NaI(Tl). When a sample count is finished, the cell is again evacuated, and another background count is taken before measuring the next air sample. Previous tests of the ARSA system have shown that latent radioactivity remains in the plastic cells after evacuation of the gases, leading to a "memory effect" in which the background count rate is dependent on the sample history. The increased background results in lower detection sensitivity. Two possible solutions to the memory effect are explored in this work: depositing a thin layer of metal on the plastic cell ("metallization"), and using an inorganic scintillating cell composed of yttrium aluminum perovskite (YAP). In both cases, the presence of inorganic material at the surface is intended to inhibit the diffusion of gases into the cell walls. In the metallization experiments, several different metals (Al, Cu, chrome) were deposited on plastic cells using electron beam lithography. The light collection performance of the cells was evaluated using standard sealed γ-ray sources and compared to a bare plastic cell. The aluminized cell demonstrated comparable light collection performance and good adhesion to the plastic and was chosen for further studies. The aluminized cell, YAP cell, and bare plastic cell were each placed in the void of a CsI(Na) well counter and injected with radioactive xenon and radon. β-γ coincidence measurements were taken before, during, and after injection of radioactive gases. The YAP cell demonstrated little or no observable memory effect, while the aluminum-coated plastic cell showed reduced latent radioactivity relative to the bare cell as expected. Although the memory effect results for the YAP cell are promising, the wall thickness is too large for the escape of the xenon x- rays into the gamma-ray detector, which is required for radioxenon detection. This paper discusses the measurement details and provides recommendations for further research and optimization.