Trapping mechanisms and delayed scintillation processes in Ce-doped sol-gel silica fibers

In recent years, scintillators based on glass matrices proved to be a valid alternative to several crystals; in particular, rare-earth doped silica glasses prepared by sol-gel route using high purity precursors turned out to be suitable materials for the realization of scintillating optical fibers , with application perspectives in real-time medical dosimetry as well as high-energy physics experiments. In this framework, Ce-doped silica fibers have recently been explored as candidates for the dual-readout calorimetry approach, exploiting the simultaneous detection of scintillation and Cherenkov light . A fundamental stage in the scintillation process is the transport to the luminescent centers of free carriers generated upon the interaction between ionizing radiation and the scintillating material; it is often largely affected by the presence of trapping sites, which can capture migrating charge carriers and thus either delay their radiative recombination or decrease the overall scintillation efficiency, according to the characteristics of the traps involved . Although preparation technologies are continuously improved, the presence of a certain amount of lattice defects is very common even for a material synthesized in very controlled conditions. In this work, we study the role of trapping defects and their close interplay with luminescent activators in the recombination processes governing the scintillation emission in 0.05 mol% Ce-doped silica optical fibers. In particular, we present a thorough investigation of the tight correlation existing between delayed scintillation processes and defects acting as carrier traps, as well as of the competition between trapping centers and luminescent ions in free carrier capture. To these purposes, steady-state radio-luminescence as a function of both temperature and cumulated X-ray dose is combined with time-resolved scintillation measurements and wavelength-resolved thermally stimulated luminescence (TSL). The presence of a wide distribution of trap levels, typical of amorphous materials, is revealed and the calculated thermal energies turn out to range from 0.1 eV to 0.8 eV. Besides, the delayed components in the scintillation decay become more intense with increasing temperature and can thus be ascribed to the temperature-dependent recombination of carriers from a progressively increasing portion of the continuous trap distribution. Moreover, a detailed analysis of both scintillation decays and TSL signals puts in evidence the additional occurrence of an athermal tunneling mechanism between traps and luminescent centers. A good agreement between the TSL results and the analysis of the delayed contributions of the scintillation process is thus demonstrated. Therefore, the effectiveness of this investigation approach, previously exploited only for crystalline systems, is demonstrated also in amorphous materials.