Silica-based scintillating fibers for ionizing radiation sensing

Luminescent rare-earth ions (RE) can be incorporated in silica up to considerable concentrations, despite their different ionic radius and different valence with respect to silicon. Active fiber devices could therefore be developed, like erbium-doped fiber amplifiers and ytterbium-doped laser fibers. In recent years, a further application for luminescent RE doped fibers was proposed, consisting in their employment as sensors of ionizing radiation fields. Such application exploits the phenomenon of radio-luminescence (RL), i.e. the prompt luminescence generated by ionizing radiation absorbed by the material. To this scope, the incorporation features of RE ions in sol-gel silica were widely investigated. Structural and vibrational studies, coupled to optical investigations, allowed to find the most suitable RE concentrations and synthesis conditions for optimizing scintillation efficiency and RE dispersion, avoiding the formation of aggregates. Nano-aggregates are indeed formed for RE concentrations exceeding 1 mol%. Amorphous clusters are detected for Gd, Tb, and Yb doping. Moreover, cluster formation is sensitive to glass sintering conditions: CeO2 nano-crystals or amorphous Ce-based clusters are formed in oxidizing or reducing sintering atmosphere, respectively [1, 2]. Crystalline and highly luminescent Eu2Si2O7 nano-aggregates are found in SiO2:Eu [3]. In this presentation, after the description of advantages and challenges for the use of optical fiber based dosimeters during medical radiation therapy and diagnostic irradiations, results obtained by using Ce, Eu, and Yb doped optical fibers are described [4]. Moreover, additional perspectives and open problems for the employment of silica fibers in other fields are also discussed [5, 6]. In particular, the dual Cherenkov and scintillation light emitted by silica-based fibers potentially offers applications in high-energy physics calorimetry as well as in radiation monitoring in medicine, security, and industrial controls. The response of the fibers, embedded in a tungsten-copper absorber block to obtain a Spaghetti-like geometry in a high-energy physics environment, has been investigated through a test beam campaign at the CERN Super Proton Synchrotron (SPS) facility. The discrimination of Cherenkov and scintillation light is demonstrated and discussed in details, along with a detailed investigation of the scintillation properties of the material. Finally, preliminary results concerning the scintillation time decay dependence upon the type of ionizing field, including x-rays with various energies and electrons, are presented and discussed by considering the different interactions mechanisms of such radiations in the silica matrix.