Graduate Thesis Or Dissertation

A Two-element CZT-based Radioxenon Detection System for Nuclear Weapon Test Monitoring

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  • Detection of xenon radioisotopes (radioxenons) has proven to be an important method for detecting nuclear explosions and is particularly well suited for detecting undeclared underground testing. The radioxenon isotopes ¹³¹mXe (t₁/₂ = 11.934 d), ¹³³mXe (t₁/₂ = 2.19 d), ¹³³Xe (t₁/₂ = 5.243 d) and ¹³⁵Xe (t₁/₂ = 9.14 h) are produced in significant amounts in nuclear explosions and are of particularly high-value in identifying such events and thus are the focus of current radioxenon detection systems. The main objective of this dissertation is to design, build and test a new radioxenon detection system to study the response of CdZnTe (CZT) detectors to xenon radioisotopes in order to support the CTBT for discovering clandestine nuclear weapon tests. This prototype detection system was intended to be small and compact with minimal number of channels which reduces complexity, power and size and still can achieve good energy resolution at room temperature compared with other scintillator-based radioxenon detectors. The prototype design uses two CZT crystals for this purpose, with the ultimate goal of using six CZT crystals for optimum geometric efficiency. The system measures xenon radioisotopes through beta-gamma coincidence detection between the two detection elements. The CZT-based detection system was characterized with radioactive lab sources and four radioxenons produced in the OSU's TRIGA reactor. The detection system offers excellent energy resolution and background count rate compared with scintillator-based beta-gamma coincidence detectors currently in operation at the IMS stations. The detection system was also simulated using MCNP to understand the response of the system to radioxenons of interest. PTRAC card was used for this purpose to find the track of beta/conversion electrons and gamma/X-rays in each CZT detector. The MCNP simulations were compared with the measurement results and shows very good consistency. The minimum detectable concentration (MDC) of this system for ¹³³Xe is estimated to be less than the requirement set by the IMS (1 mBq/m³). Our estimations also show that by increasing the number of CZT crystals to 6, the MDC of all radioxenons will be improved to less than 1 mBq/m³ which is comparable with the most sensitive radioxenon detection systems currently in operation.
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