Sviluppo e Applicazioni di nuovi materiali per la dosimetria delle radiazioni ionizzanti

The Research Activities carried out during the Three-year International PhD Course in Applied Physics of the candidate Salvatore Gallo had two main topics related to dosimetry of ionizing radiations (IR): the analysis of 3D gel dosimeters and the analysis of solid state dosimeters through electron spin resonance. Both these activities are focused on a careful study of new materials for the IR Dosimetry, both in terms of physical characterization of these new materials and of the optimization of readout methods for dosimetric measurements. The study performed through various techniques such as UV-Vis absorption spectroscopy, NMR Relaxometry, Magnetic Resonance Imaging (MRI) and Electron Spin Resonance (ESR) spectroscopy. The materials studied are Fricke Xylenol Gels (FXGs) based on agarose for 3D dosimetry and for ESR dosimetry conventional and unconventional organic materials for solid state ESR dosimetry. The application of Fricke gels for ionizing radiation dosimetry is continuously increasing worldwide due to their many favorable properties. Fricke Xylenol Gels (FXGs) dosimetric system is based on the radiation induced oxidation of ferrous (Fe2+) to ferric (Fe3+) ions. The oxidation of ferrous ions also causes a reduction of the longitudinal nuclear magnetic relaxation time T1 which can be measured by means of Nuclear Magnetic Resonance Relaxometry (NMR) and Magnetic Resonance Imaging (MRI). The results here presented are related to an experimental investigations performed on Fricke Gels characterized by gelatinous matrix of agarose. This work describes the characterization of various Fricke-Agarose-Xylenol gels dosimeters using various techniques. The photon sensitivity of the FXGs was measured in terms of NMR relaxation rates; its dependence on radiation dose was determined as a function of ferrous ammonium sulfate contents (from 0.5 mM to 5 mM). Furthermore, the stability of the NMR signal was monitored over several days after irradiation. These measurements were aided by Magnetic Resonance Imaging (MRI) scans which allowed three-dimensional (3D) dose mapping. In order to maximize the MRI response, a systematic study was performed to optimize acquisition sequences and parameters. In particular, we analyzed the dependence of MRI signal on the repetition time TR and on the inversion time TI using inversion recovery sequences. The results are reported and discussed from the point of view of the dosimeter use in clinical radiotherapy. The dose calibration curves are reported and discussed from the point of view of the dosimeter use in clinical radiotherapy. This work highlights that the optimization of additive content inside gel matrix is fundamental for optimizing photon sensitivity of these detectors. We can conclude that FXG dosimeters with optimal ferrous ammonium sulfate content can be regarded as a valuable dosimetric tool to achieve fast information of spatial dose distribution. In parallel, studies through Electron Spin Resonance (ESR) spectroscopy on conventional and unconventional materials for solid state dosimetry were carried out. ESR is a spectroscopic method for investigating the structure and dynamics of such paramagnetic species. Free radicals are known to be produced when a compound is irradiated with ionizing radiations. The concentration of radiation-induced free radicals is proportional to the energy released inside the medium and this allows for dosimetric measurements through ESR technique which is able to quantitatively determine the concentration of free radicals. The use of alanine as a dosimetric material gave the possibility to apply ESR spectroscopy for high-dose measurements and dose control in radiation processing. The ESR dosimetric method has many advantages such as simple and rapid dose evaluation, the readout procedure is non-destructive, linear response of many organic and inorganic compounds. ESR detectors show a behavior that suggest possible applications for various kinds of beams used for radiation therapy. Nowadays, the most widely organic compound used as a dosimeter is the alanine. However, many researches are in progress with the aim at improving sensitivity of ESR dosimetry for doses much smaller than 1 Gy. More sensitive materials than alanine are needed to make the ESR dosimeter competitive with other dosimetry systems. In this work the results of an investigation of the ESR response of some phenols compounds (octadecyl-3-(3,5-di-tert.butyl-4-hydroxyphenyl)-propionate or commercial name IRGANOX 1076) ) for possible ESR dosimetric applications suitable features, such as high efficiency of radiationmatter energy transfer and radical stability at room temperature, are reported. The dosimetric features of these ESR dosimeters such as dependence on microwave power and modulation amplitude, their response after gamma and electron irradiations, dependence on beam type and energy, detection limits for various type beam typologies, signal stability after irradiation were investigated. The characteristics observed suggest these organic compounds as promising materials for ESR dosimetry. The Research Activities carried out in this work by the Candidate are part of the projects “Neutron dOsimetry and Radiation quality Measurements by ESR and TL” (NORMET) (Project Leader: Dr. Maurizio Marrale), supported by the Group V of the National Institute of Nuclear Physics (INFN) and Project of National Interest (PRIN) 2011 entitled “Development and application of new materials for dosimetry of ionizing radiations” (Project Leader: Prof. Francesco d’Errico) funded by the Ministry of Education (MIUR).