Study and Optimization of the Metrological Performance of a Graphite Calorimeter for Absorbed Dose to Water Measurements in Medium-Energy X-Ray Beams

Abstract This thesis documents the progressive enhancement of the metrological performance of the Italian primary standard for absorbed dose to water, realized through a structured program of experimental investiga- tions. The work was carried out at National Institute of Ionizing Radiation Metrology (ENEA INMRI), and is centered on a graphite calorimeter operating within a water phantom. The measurements re- ported for this investigation took place in ENEA Casaccia Research Center and at the Laboratory of Industrial Measurements (LAMI), of the Department of Electrical and Information Engineering (DIEI) of the University of Cassino and Southern Lazio. The primary objective of this study was to improve the uncertainty budget associated with the calorimeter’s ability to detect resistance variations of the main thermistor under ionizing radiation ex- posure. This required a detailed investigation of the stability of the electronic signal, achieved through the optimization of the acquisition workflow and the refinement of the measurement methodology. In this framework, the experimental campaign involved the optimization of the operating parameters, fol- lowed by an assessment of the repeatability component contributing to the overall uncertainty budget. Part of the investigations was carried out at the Medical Dosimetry Laboratory of the Technical Univer- sity of Denmark (DTU), with the aim of extending the developed methodology to additional graphite calorimeters and supporting its adoption within the wider metrological community. Instead of limiting the analysis to the performance of a single instrument, the study sought to generalize the approach and strengthen the overall understanding of calorimetric systems. The research began with the restoration and verification of the operational functionality of the X-ray calorimeter, addressing both hardware and software aspects before proceeding to the optimization phase. This initial stage was followed by a structured, stepwise methodology in which individual parameters were systematically selected and assessed for their influence on the calorimetric response. After the Introduction in Chapter 1, Chapter 2 provides the metrological framework of the study, highlighting the significance of the Absorbed Dose to Water standard not only from a general metrological perspective but also in view of the intrinsic challenges associated with establishing and maintaining a reliable standard in the demanding medium-energy X-ray range. Chapter 3 outlines the structure of the Rx calorimeter and its measurement chain, introducing the governing model equations and the Wheatstone bridge configurations used to monitor thermistor resis- tance. The electronic acquisition system, based on eight coordinated Keithley instruments, is described together with the calorimeter’s performance under quasi-adiabatic operating conditions. Chapter 4 reports the preliminary results obtained from the initial experimental setup, which form the basis of the optimization methodology developed in this work. Two dedicated setups at DTU were used to assess the influence of selected electrical parameters on calorimetric data acquisition. Chapter 5 presents the analysis of the calorimetric system’s response in terms of linearity and thermal stability, together with a study of the uncertainties associated with the fitting procedure. The chapter also examines the structure and limitations of the existing fitting approach and introduces a revised method- ology aimed at improving robustness and reducing the uncertainty of linear fits applied to calorimetric runs. Chapter 6 describes the electrical calibration procedure of the calorimeter, detailing the correspond- ing experimental setup and the methodology adopted to derive the revised quasi-adiabatic calibration coefficient kqa. The chapter also presents the measurement campaign carried out to analyze the metrics associated with the calibration runs, organized according to their specific objectives. Chapter 7 describes the experiments performed under X-ray beam irradiation and presents the main conclusions of the work. It reports the achieved reduction in the calorimeter’s uncertainty budget, together with the direct calibra- tion in terms of absorbed dose to water using three clinical ionization chambers. Chapter 8 summarizes additional research activities conducted in parallel with the core work, including the irradiation of bio- logical samples for space-radiation protection studies in collaboration with the University of Tor Vergata. v The chapter also presents an analysis of the advantages and limitations of calibrating ionization chambers through a direct calorimetric approach in terms of absorbed dose to water, compared with an indirect calibration based on air-kerma. These considerations were further explored in the poster presented at the BIPM event celebrating the 150th anniversary of the Metre Convention. Appendix A presents the technical activities undertaken to restore the functionality of the Rx calorime- ter, including the work performed to resolve the vacuum-related issue. Appendix B reports the results of the preliminary analysis on voltage data in the nanovolt range. Representative calibration runs, grouped according to their specific objectives, are provided in Appendices C and D, while Appendix E contains the irradiation runs overview performed under X-ray beam irradiation. The final section includes the peer review publications resulting from this work, provided in PDF format.