A Hybrid Radiation Transport Detector Response Function Methodology for Modeling Contaminated Sites
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Asano, Ethan Akira
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Abstract
To date, guidance for environmental assessment using experimental radiation detection techniques – specifically relating photon detector responses to dose or cancer risk-based radionuclides concentrations in environmental media – exist under the Environmental Protection Agency (EPA) Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA). In recent years, the EPA has undertaken efforts to develop the “U.S. EPA Superfund Counts-Per-Minute (CPM) Electronic Calculator” web-based field screening tool to address this technical challenge. The CPM Calculator predicts photon detector readings in CPM that, if exceeded in the field, may warrant further action (e.g., cleanup) when demonstrating regulatory compliance in the radiological site remediation process. A modeling research effort was undertaken to estimate thousands of sodium iodide thallium-doped (NaI(Tl)) detector total efficiency responses in the development of a methodology to calculate detector response from simulated contamination in various geometries in environmental media.
A detector response function (DRF) methodology was developed by coupling capabilities of the stochastic Monte Carlo (MC) method with the Consistent Adjoint Driven Importance Sampling (CADIS) hybrid radiation transport method to estimate the NaI(Tl) detector efficiency responses for a broad range of wide-area contamination scenarios. The CADIS hybrid method, available through the massively parallel MC code Shift, was harnessed to significantly increase the computational efficiency of thousands of Monte Carlo N-Particle (MCNP) pulse height tally (PHT) simulations involving wide-area, distributed photon source contamination in environmental media (surface source contamination scenarios were evaluated using only MCNP). Total detector efficiency responses were estimated for a breadth of contamination scenarios based on the following parameters: discrete monoenergetic source energy/radionuclide spectrum, contaminated medium type, depth of contamination, modeled cylindrical NaI(Tl) detector configuration, and source-detector distance. Simulated results were post-processed to yield calibration factors in units of CPM per surface or distributed source activity concentration (i.e., CPM/(pCi/〖cm〗^2 ) or CPM/(pCi/g), respectively). At the time of writing, all calibration factor data has been implemented into the CPM Calculator – the final version is expected to be released following physical laboratory validation in partnership with EPA laboratories. In practice, field surveying personnel will be able to quickly compare a measured NaI(Tl) detector net count rate to a threshold value derived from a predetermined dose or risk limit (i.e., calibration factor normalized by an activity concentration derived from a dose or risk limit) for a variety of wide-area contamination scenarios. Ultimately, currently expensive and time-consuming sample collection and laboratory analyses efforts required for demonstrating regulatory compliance in future wide-area contamination assay applications may be reduced.
This work results from a comprehensive study that consists of multiple auxiliary efforts following the development of the DRF methodology described previously, which we refer to as “Shift with CADIS.” A study was conducted to analyze the effectiveness of Shift with CADIS compared to various radiation transport variance reduction (VR) methods for improved modeling in future wide-area contamination assay applications. The latter is intended for publication outside the current study and is therefore only adverted herein. Instead, a follow-up analysis that further explores alternative methods for approximating solutions to the wide-area contamination transport problems of interest is presented. Additionally, a method was developed for estimating the total detector efficiency response contribution from secondary photons arising from electron source emissions (called bremsstrahlung) in soil with MC simulation, which employed the electron transport condensed history method in a computationally feasible manner. Lastly, investigatory measurements were performed with laboratory-scale setups of low-level activity environmental soil contamination with field surrogate photon source terms to validate simulated detector efficiency responses.
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2023-10-27
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Dissertation