Operational, Procedural, and Analytical Factors Influencing the Precision of Measurements with Thermoluminescent Dosimeters

Abstract

This dissertation focuses on improving precision in radiation dosimetry for thermoluminescent dosimeters (TLDs) by analyzing the factors influencing precision and reliability throughout the dosimetry process. Radiation dosimetry involves several critical stages, including annealing, calibration, deployment, readout, and analysis, each of which can introduce variability into measurements. The research presented here investigates major factors that affect precision at these stages, ranging from operational practices to analysis methods, to develop strategies that enhance precision and consistency in dose assessments. Radiation dosimetry calibration sources, such as 137Cs irradiators, require systematic quality assurance and control to ensure reliable dose delivery. This work establishes a Phase I quality control protocol that applies statistical techniques, including Shewhart control charts and Nelson’s Rules, to analyze air kerma rate measurements collected over 24 months. The findings demonstrate the importance of rational subgrouping methods in identifying systematic errors, positional variations, and equipment malfunctions. Building on this foundation, a Phase II protocol incorporates advanced statistical tools, such as exponentially weighted moving average control charts, to quantify uncertainties and detect short-term variations. Establishing a comprehensive quality control program helps identify assignable causes of errors and establish uncertainties in dynamic dosimetry calibration processes, offering a framework that can be applied to monitor and evaluate any value of interest throughout the dosimetry process. Advances in glow curve analysis were made for TLDs by introducing improved algorithms for peak detection and curve fitting. A transition to stochastic gradient descent algorithms and the addition of early stopping mechanisms have significantly enhanced the accuracy of automated peak identification, improving the mean figure of merit by 46%. These improvements facilitate more precise characterization of TLD glow curves, essential for analyzing dosimetric materials' thermal and kinetic properties. A detailed investigation into the effects of heating rates on TLD precision reveals that both noise and thermal effects contribute to variance in dosimetric readings. An optimal heating rate of 4 °C/s was identified, minimizing variance while maintaining accuracy across different TLD materials. The study further highlights the relationship between heating rate, kinetic parameters, and dosimeter properties, providing guidance for optimizing readout conditions in dosimetric applications. This dissertation applies these processes to a practical example, which includes calculating the minimum detectable dose for TLDs while considering the influence of uncertainties from dose calibration, annealing methods, heating rates, and analysis techniques. The minimum detectable dose has varying calculation methods, and six different methods were used in this study to determine how the value changes based on the method. The research reveals a general range of values from 10µGy to 120 µGy, though a broader range exists, illustrating the significant variability introduced by each factor. This study shows that a wide range of values exist for the minimum detectable dose, indicating that each dosimetry program has different approaches that influence both precision and applicability. This emphasizes the need to consider the specific goals of the dosimetry program when considering precision in measurements. It is important to balance required accuracy with desired operational efficiency. The findings presented in this dissertation contribute to the broader understanding of precision in radiation dosimetry. By addressing key challenges in the use and handling of TLDs, this work highlights the relationship between inherent operational practices and measurement precision. It highlights the importance of robust quality control, advanced analytical methods, and methodological rigor in achieving accurate and reliable dose measurements.