Advancements in radionuclide monitoring technologies used to detect indications of nuclear explosions

The objective of the research documented in this dissertation was to advance the state-of-the-art radionuclide monitoring technologies used to detect indications of nuclear explosions, which are absolutely prohibited by the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Advancements are made in two areas. The first advancements are in the characterization and optimization of a Si-PIN diode-based radiation spectrometer prototype sensitive to both photons and conversion electrons. A novel peak-fitting algorithm referred to herein as the WiPFA algorithm was developed to support the Si-PIN diode spectrometer prototype characterization efforts. The absolute conversion electron detection efficiency of the prototype was found to be 5.2 ± 0.4 % at conversion electron energies near 150 keV, and the [superscript 131m]Xe, [superscript 133m]Xe, ¹³³Xe, and ¹³⁵Xe Minimum Detectable Concentrations (MDCs) were found to be 1.7, 2.0, 2.1, and 56 mBq-m⁻³, respectively. A series of Monte Carlo N-Particle (MCNP) radiation transport code models were then developed to evaluate the MDCs associated with a series of optimized Si-PIN diode-based spectrometer designs. These optimization studies revealed that coupling Si-PIN diodes available today with thinner, cylindrical spectrometer designs could reduce the [superscript 131m]Xe, [superscript 133m]Xe, and ¹³³Xe, MDCs to 0.48, 0.57, and 0.58 mBq-m⁻³, respectively. Subsequent studies utilizing larger, thicker Si-PIN diodes indicated that additional reductions down to 0.31, 0.37 and 0.37 mBq-m⁻³ might be possible. These small radioxenon MDCs coupled with other perceived advantages of Si-PIN diodes suggest that Si-PIN diode-based radiation spectrometers could serve as attractive alternatives to the high-resolution gamma-ray and beta-gamma coincidence spectrometers currently employed by the verification regime of the CTBT. The second area in which advancements are made is the radionuclide background activity concentration characterization area. The focus here is on CTBT-relevant radioactive particulates and noble gases produced via spontaneous fission and via naturally occurring cosmic-ray induced fission and activation reactions. A new application—the Terrestrial Xenon and Argon Simulator (TeXAS) application—was developed to streamline and automate the creation of high-fidelity MCNP models and dedicated nuclear data libraries required to support detailed, site-specific background activity concentration characterization studies. The capabilities of the TeXAS application are demonstrated and used to develop background activity concentration estimates specific to several layers of the Earth’s atmosphere, several subsurface depths in six geologies prevalent in the Earth’s upper crust, and seawater.