samples were irradiated for approximately 45 minutes to allow for the saturation of fission products. The first method used the beam port shutter and allowed for longer counting and irradiation times, but was unsuitable for examining fission products with half-lives below 10 seconds. The on/off method used a cycle of equal irradiation and counting times of one minute. The second method is able to measure track the production of fission products with half-lives of less than 10 seconds. This method used a borated aluminum wheel beam chopper to stop the irradiation of the sample during counting. The beam chopper was set to cycle for approximately one second of counting following half a second of irradiation. The spectra from both methods were analyzed and the peaks were assigned to the appropriate fission products. The majority of the peaks were composed of gamma-rays from multiple nuclides. The peaks created by gamma-rays from decays of a single nuclide were used to calculate the detection limits of the system. Using the beam chopper system, 21 peaks would be above the detection limits of our system 95% of the time for uranium samples of less than one kilogram. / text

Accurate estimates for the molecular transport coefficients are critical to predicting the movement of gases in geological media. Here I present a novel methodology for using prompt gamma activation analysis to measure the effective diffusivity of noble gases in a porous medium. I also present a model to estimate the connectivity parameter of a soil from measurements of its saturated conductivity, macro porosity, and pore volume and pore surface fractal dimensions. Experiments with argon or xenon diffusing through a nitrogen saturated geological media were conducted. The noble gas concentration variations at its source were measured using prompt gamma activation analysis and later compared to a numerical diffusion model to estimate the effective diffusion coefficient. Numerical simulations using the estimated diffusivity and the experimental argon data produced results with a correlation parameter R² = 0.98. However, neglecting transport mechanisms other than diffusion largely under-predicted the xenon depletion rates observed during the first hours of experiment. To explain these results, a second model was developed which included the effect of pressure gradients and bulk convection that might arise from the faster molecular migration of the light species in a non-equimolar system and gravitational currents. Finally, the fractal model developed for this dissertation was used to estimate the connectivity parameters and walking fractal dimension of a group of geological samples that were previously characterized. This model successfully predicted positive connectivity factors and walking fractal dimensions between two and three for every sample analyzed. / text