The interest in the detection of radioargon isotopes--³⁷Ar, ³⁹Ar, and ⁴²Ar--is increasing important for on-site inspections within the Comprehensive Nuclear-Test-Ban Treaty verification regime. In an underground nuclear explosion ³⁷Ar is produced by ⁴⁰Ca(n,[alpha])³⁷Ar reaction in surrounding soil and rock. With a half-life of 35 days, ³⁷Ar provides a signal useful for confirming the location of an underground nuclear event. The development of detector systems is underway. This work produced radioargon isotopes by three methods for the development and testing of radioargon detection systems. The irradiation of argon gas at natural enrichment in the 3L facility within the Mark II TRIGA reactor facility at The University of Texas at Austin provides a source of ³⁷Ar for the calibration of the ULBPC in development at PNNL. The ⁴¹Ar activity is measured by the gamma activity using an HPGe detector after the sample is removed from the core. Using the ⁴¹Ar/³⁷Ar production ratio and the ⁴¹Ar activity, the amount of ³⁷Ar created is calculated. The ⁴¹Ar decays quickly (half-life of 109.34 minutes) leaving a radioactive sample of high purity ³⁷Ar and only trace levels of ³⁹Ar. The second method was the irradiation of a calcium-containing compound. This option is not the best match for the TRIGA reactor type due to the thermal neutron flux. Therefore, the use of the Cd-lined 3L irradiation canister minimized the thermal activation of impurities while still allowing the majority of the ⁴⁰Ca(n,[alpha])³⁷Ar reactions occur. The third and last irradiation technique was a large volume, in-core gas facility developed at The University of Texas at Austin MARK II TRIGA reactor to produce a sample of ⁴²Ar with an activity above 1 mBq. The method requires a large volume, 1.4 L, of natural argon gas (99.6003% ⁴⁰Ar) at about 1 atm and three-12 hour irradiation periods. The production of ⁴²Ar requires a double capture to be produced from the stable 40Ar isotope. This method produced 940 kBq of ³⁹Ar, 3.08 MBq ³⁷Ar, 114 GBq ⁴¹Ar, and 0.311 Bq ⁴²Ar at the end of the final irradiation period. / text

For verifying the Comprehensive Nuclear-Test-Ban Treaty (CTBT), different monitoring technologies (seismic, infrasound, hydroacoustic, and radionuclide detection) are combined. The monitoring of radioactive xenon isotopes is one of the principal methods for the determination of the nuclear nature of an explosion. After an underground nuclear detonation the radioxenon isotopes [superscript 131m]Xe, [superscript 133m]Xe, ¹³³Xe, and ¹³⁵Xe, and the radioargon isotope ³⁷Ar have an increased probability of detection. In order to effectively utilize these isotopes as indicators of nuclear testing, an accurate background must be calculated. This work examines the fission products produced by spontaneous fission of ²³⁸U, which is naturally present in the earth's crust, and of ²⁴⁰Pu which is present as a product of nuclear weapons and nuclear reactor accidents. These calculations provide a range of production values for radioxenon in a variety of geologies as well as at various historic locations. The activation of geologic calcium and potassium by cosmic ray neutrons is considered for a variety of properties effecting the neutron flux. These calculations provide a range of radioargon production values across a selection of geologies. The impact of latitude and the solar activity cycle are also examined. In order to examine the transport of the isotopes through soil a model of the transport of xenon and argon through various geologies was developed. This model incorporates both the introduction of xenon from the atmosphere and that produced by spontaneous fission. This is then considered in light of what might be observed in an on-site inspection (OSI). What this work finds is that the radioxenon natural background does exceed detection limits in particular locations and geologies, however, a careful examination of the location and the ideal sampling depths can minimize the impact during an OSI. Radioargon, however, has a much larger natural background at shallow depths which are the realm of OSI sampling. Should radioargon sampling be used in an OSI the sampling time is crucial in distinguishing a nuclear explosion from the natural background. In some scenarios the natural background production of radioargon may be sufficient to interfere with the detection of an underground nuclear weapon test. This information may be beneficial in the development of future OSI noble gas monitoring techniques. / text

On Site Inspection is the final measure for verifying compliance of Member States with the Comprehensive Nuclear-Test-Ban Treaty. In order to enable the use of ³⁷Ar as a radiotracer for On Site Inspection, the sources of radioargon background must be characterized and quantified. A radiation transport model of the University of Texas at Austin Nuclear Engineering Teaching Laboratory (NETL) TRIGA reactor was developed to simulate the neutron flux in various regions of the reactor. An activation and depletion code was written to calculate production of ³⁷Ar in the facility based on the results of the radiation transport model. Results showed ³⁷Ar production rates of (6.567±0.31)×10² Bq·kWh⁻¹ in the re- actor pool and the air-filled irradiation facilities, and (5.811±0.40)×10⁴ Bq·kWh⁻¹ in the biological shield. Although ⁴⁰Ca activation in the biological shield was found to dominate the total radioargon inventory, the contribution to the effluent release rate would be diminished by the immobility of Ar generated in the concrete matrix and the long diffusion path of mobile radioargon. Diffusion of radioargon out of the reactor pool was found to limit the release rate but would not significantly affect the integrated release activity. The integrated ³⁷Ar release for an 8 hour operation at 950 kW was calculated to be (1.05±0.8)×10⁷ Bq, with pool emissions continuing for days and biological shield emissions continuing for tens of days following the operation. Sensitivity analyses showed that estimates for the time-dependent concentrations of ³⁷Ar in the NETL TRIGA could be made with the calculated buildup coefficients or through analytical solution of the activation equations for only (n,[gamma]) reactions in stable argon or (n,[alpha]) reactions in ⁴⁰Ca. Analyses also indicated that, for a generalized system, the integrated thermal flux can be used to calculate the buildup due to air activation and the integrated fast flux can be used to calculate the buildup due to calcium activation. Based on the results of the NETL TRIGA, an estimate of the global research reactor source term for ³⁷Ar and an estimate of ground-level ³⁷Ar concentrations near a facility were produced. / text