Rare-Earth doped Scintillating Silica Fibers for ionizing radiation detection

Scintillating materials find a wide variety of applications in ionizing radiation detection systems, monitoring and imaging, real time dosimetry in the medical field, homeland and industrial security, and high energy physics. In the recent years, the development of new, fast, and performing scintillators has been an active field of research. Scintillating fiber technology freshly raised a lot of interest because its extreme flexibility can provide a powerful tool for innovative detector designs. This thesis focuses on the study of scintillating fibers made of silica glass which show efficient luminescent properties when activated with rare-earth ions, like Cerium and Praseodymium. Both fundamental and practical aspects are discussed, in view of the improvement and optimization of the material performances for application perspectives in the future generation of high energy physics detectors. With this objective, the effects of high dose levels of ionizing radiation on the transparency of the material are studied. The fine-tuning of the activator content incorporated in the silica matrix and of the sol-gel synthesis and fiber drawing processes allow to obtain a good light guiding and a well-controlled optical quality. The feasibility of a simultaneous readout of Cherenkov and scintillation light is demonstrated in high energy calorimetry conditions, probing Ce-doped silica fibers embedded in a small detector prototype exposed to beams of electrons. Silica fibers can be considered as promising candidates in the framework of the dual readout calorimetry approach, which aims at compensating the energy fluctuations, inherent to the detection of hadronic particles. A deep understanding of the factors limiting the scintillation performances is of primary importance for future material engineering: they are found to be mainly related to the presence of point defects, which compete with the luminescent centers in capturing the free carriers created upon irradiation and introduce a delay in the recombination kinetics. A fundamental study of the role of defects in silica fibers, detrimental for the scintillation efficiency, is proposed and discussed. The potential of silica fibers for applications in high energy physics detectors is outlined and further optimization of the material technology is foreseen. This work was performed at the Department of Materials Science at the University of Milano - Bicocca, in collaboration with the European Organization for Nuclear Research (CERN, Switzerland) and with the Lawrence Berkeley National Laboratory (US). Some measurements were carried out in collaboration with Saint Gobain Research (France) and the Institute of Physics of the Czech Academy of Sciences (Czech Republic).