Title: Development of an Advanced Radioxenon Detector for Nuclear Explosion Monitoring

Chair: Sara Pozzi

Abstract: The Comprehensive Nuclear-Test-Ban Treaty was opened for signature in 1996 and seeks to ban nuclear weapons testing worldwide. The International Monitoring System (IMS) was established to verify treaty compliance, and consists of four technologies: seismic, infrasound, hydroacoustic, and radionuclide. The radionuclide component of the IMS conducts atmospheric monitoring to identify radioactive particles and gases associated with nuclear testing, such as radioxenon. As a noble gas, the radioxenon produced in an underground nuclear explosion can be released into the atmosphere, for subsequent detection by the IMS. Radioxenon is also produced by fission-based civilian processes, such as nuclear reactors and medical isotope production facilities, requiring discrimination between these sources. The focus of this work is to improve the resolution and sensitivity of radioxenon monitoring systems.

Radioxenon is measured using beta-gamma coincidence techniques, typically with scintillating plastic and NaI(Tl) detectors; however, the poor energy resolution of the plastic results in isotopic interference, complicating the analysis. Additionally, radon emits decay energies that interfere with those from radioxenon, requiring complex gas- processing systems to filter it from the sample. Furthermore, radioxenon diffuses into the plastic detectors, which increases the background of subsequent measurements; this phenomenon is known as the memory effect. To mitigate these issues, this thesis demonstrated 1) an anticoincidence analysis method to better identify metastable isotopes, 2) a validated MCNPX-PoliMi simulation tool to analyze new detector systems and produce training spectra for analysis testing, and 3) a prototype radioxenon detector system based on stilbene.

Stilbene cell prototypes have been developed, tested, and compared with a traditional plastic scintillator cell. The results show that the stilbene cell has similar response to the plastic cell with an improved energy resolution, full-width at half-maximum decreased by 2.2 keV at 129 keV. The stilbene cell is capable of pulse shape discrimination allowing for radon mitigation through alpha identification. The analysis presented reduced the minimum detectable concentration of Xe-135 by 1% and could be used for environmental monitoring. The stilbene cell was shown to have 0.043% residual activity compared to 4.5% residual activity for the plastic cell, demonstrating significantly improved memory effect. The results presented in the thesis allow for better identification of metastable isotopes, improved simulation techniques, and improved detection sensitivity which could lead to improved source discrimination strengthening the Comprehensive Nuclear-Test-Ban Treaty verification regime.