From Eu3+ incorporation in sol-gel silica to real time monitoring of therapy beams by Eu3+ doped scintillating fibers

Significant improvements have continuously been made in radiation therapy technologies, and therefore innovative detectors must also be developed to ensure the beam quality of these new irradiation systems and to allow in vivo dosimetry measurements. Optical fiber based radioluminescence (RL) dosimeters are a promising option for these purposes. They can enable real time dose measurement and their small size can be exploited in small radiation field dosimetry. A major difficulty involved in marketing these systems is the spurious luminescence, generally known as stem effect, that is intrinsically present in this type of detector. The possible mechanisms causing the stem effect during irradiation are fluorescence phenomena and especially Cerenkov light, produced in the undoped fiber portion. In this study we have faced the problem by making use of a spectral discrimination method exploiting the sharp 5D0-7F2 line emission of Eu3+ [1]. First, the incorporation of Eu3+ ions in sol-gel silica was investigated as a function of dopant concentration and synthesis parameters. Preliminary structural (TEM), vibrational (Raman, FTIR) and optical absorption studies [2, 3] have allowed to find the most suitable rare-earth (RE) concentrations and synthesis conditions for optimizing both RL efficiency and RE dispersion, avoiding the formation of aggregates. Indeed, clusters of few tens of nanometers were observed already for SiO2:3 mol% Eu3+. The 10 mol% doped sample contains also aggregates of much larger dimensions, whose diffraction patterns are in agreement with the presence of rare earth crystalline oxide silicates. Eu3+-doped fibers were realized by drawing cluster-free silica with 600 ppm doping level. The spectral emission of composite fibers made of a small portion (1 cm) of doped silica fiber coupled to a long (15 m) undoped fiber for remote signal transport has then been investigated under irradiation with photons and electrons of different energies, field sizes and orientations, in order to discover the origin of the stem effect and to evaluate its influence on the RL spectral shape. The possibility of an efficient discrimination between the RL dosimetric signal and the spurious one is proved and discussed.