Franck Mady

Objective. Due to the emergence of ultra-high dose rate proton therapy techniques, the need for advanced in-vivo dosimetry tools able to monitor such beams has become more critical. This study evaluates the potential of silica optical fibers as real-time scintillating detectors for quality assurance of scattered and scanned pulsed proton beams at high and standard dose rates. Approach. The radioluminescence (RL) of short pieces of thin Gd ³⁺ -, Ce ³⁺ -, and N-doped silica fibers was studied focusing on their response to dose rate and energy, and their ability to perform depth-dose measurements, and for pencil beam scanning proton beams, spot size evaluation and real-time beam monitoring. The fibers response was evaluated in a continuous scattered proton beam and a synchrocyclotron scanned pulsed proton beam. Main results. Up to 130 Gy·s ⁻¹ , fibers response was linear as a function of dose rate. For both proton beams, range was measured with less than 1.1 mm deviation with the three fibers, despite scintillation quenching (SQ) from 63 to 226 MeV. With the same data, spot size was estimated with less than 0.3 mm difference from the reference, taking advantage of the real-time RL acquisitions for scanned beams. Also, characterization of synchrocyclotron specific dose delivery by repainting, was investigated. Fibers were able to measure the relative dose contribution of each painting, according to the system log-files (less than 2% difference). Moreover, the ability of the system to resume properly the irradiation after beam interruption was validated in real-time through fibers acquisitions and were correlated to the system log-files. Significance. RL-based silica fiber sensors appeared to be viable detectors for proton beam quality assurance (proton range, spot size) at standard and high dose rates despite showing SQ and showed potential for real-time dosimetry for scanned proton beams and log-files QA, in addition to high dose rate proton beams.

Contact radiotherapy is an effective technique for treating superficial skin tumors, delivering highly localized doses while sparing healthy tissues. However, anatomical complexity in regions such as the nasolabial fold, eyelids, and auricular structures hinders applicator positioning and causes dosimetric uncertainties. This article evaluates germanium‐doped silica optical fibers (Ge‐doped OFs) as innovative radioluminescent in vivo dosimeters for real‐time dose monitoring. Four fibers with different germanium content (0.1%–9.5%) are fabricated using modified chemical vapor deposition and tested under 50–95 kV X‐rays on polymethyl methacrylate and anthropomorphic phantoms. The 9.5% Ge‐doped OF exhibits the highest sensitivity, excellent dose linearity ( R ² = 0.9992), repeatability (0.4%), and reproducibility (2.6%). Its response closely follows that of a 0.02 cc ionization chamber across all field sizes and energies, with deviations <3%. On anthropomorphic phantoms, dose deviations of 1.8% and 6.8% in simple and complex configurations are shown, respectively. The latter value is reduced to 1.3% after correction using the inverse square law of distance. These findings support the use of Ge‐doped radioluminescent OFs as compact, high‐resolution detectors for precise in vivo dosimetry in contact radiotherapy.

In this paper, an experimental and theoretical study was undertaken to assess the impact of rare-earth co-doping of silica glasses on the light emission under X-rays. To this aim, radioluminescence (RL), phosphorescence (PP), and thermoluminescence (TL) signals of Ce³⁺/Gd³⁺ co-doped silica glasses have been successively measured and combined at different dose rates and irradiation temperatures. The RL response of the weakly co-doped sample was found to be temperature-independent between 273 K and 353 K. This result suggests that, based on this RL response, it is possible to design ionizing radiation sensors independent of the irradiation temperature in the corresponding range. Moreover, a model that considers the electron–hole pair generation, the charge carrier trapping–detrapping, and the electron–hole recombination in the localized and delocalized bands has been developed to reproduce these optical signals. The theoretical model also explains the temperature independence of the RL response between 273 K and 353 K for the weakly co-doped sample and, therefore, the operating principle of an X-ray sensor independent of the irradiation temperature.

In the ionizing radiation dosimetry domain, there is still an important demand for a compact, reliable, and accurate sensor. Currently, among the dosimeters based on optical techniques, the fibered systems are the most promising solutions. Optical dosimeters usually exploit one of the physical processes, which is radioluminescence (RL), thermoluminescence (TL) or radiation-induced attenuation (RIA). In this paper, we present, to our knowledge, a new approach to measuring the dose of ionizing radiations that combines radioluminescence, phosphorescence, and thermoluminescence processes. To our knowledge, the integrated signals originating from these three processes have been collected at different temperatures through an optical fiber spliced to a Gd³⁺-doped silica glass rod for the first time. Using this fibered setup, the experimental study shows that the integrated response, originating from the three processes, is linear versus dose at least up to 300 Gy and temperature-independent between 153 K and 353 K. These experimental results have been explained and reproduced by a theoretical model, which is also able to predict the dosimeter response under various conditions. Moreover, as another novelty, to our knowledge, simultaneous optically stimulated luminescence (OSL) and RL measurements allowed the achievement of a real-time and temperature-independent dosimeter.

Radioluminescent silica-based fiber dosimeters offer great advantages for designing miniaturized realtime sensors for high dose-rate dosimetry. Rise and fall kinetics of their response must be properly understood to better assess their performances in terms of measurement speed and repeatability. A standard model of radioluminescence (RL) has already been quantitatively validated for doped silica glasses, but beyond conclusive comparisons with specific experiments, a comprehensive understanding of the processes and parameters determining transient and equilibrium kinetics of RL is still lacking. We analyze in detail the kinetics inherent in the standard RL model. Several asymptotical regimes in the RL growth are demonstrated in the case of a pristine sample (succesive quadratic, linear and power-law time dependencies before the plateau is reached). We show how this situation is modified when a pre-irradiation partly fills traps beforehand. RL growth is then greatly accelerated because of the pre-formation of recombination centers (RCs) from dopant ions, but not due to pre-filling of trapping levels. In all cases, the RL intensity eventually tends to a constant level equal to the pair generation rate, long before all carrier densities themselves reach equilibrium. This occurs late under irradiation, when deep traps get to saturation. The fraction of dopants converted into RCs is then ‘frozen’ at a lower level the smaller the density of deep traps. Controlling RL kinetics through the engineering of material traps is not an option. Pre-irradiation appears to be the simplest way to obtain accelerated and repeatable kinetics.

Cerium-doped-silica glasses are widely used as ionizing radiation sensing materials. However, their response needs to be characterized as a function of measurement temperature for application in various environments, such as in vivo dosimetry, space and particle accelerators. In this paper, the temperature effect on the radioluminescence (RL) response of Cerium-doped glassy rods was investigated in the 193–353 K range under different X-ray dose rates. The doped silica rods were prepared using the sol-gel technique and spliced into an optical fiber to guide the RL signal to a detector. Then, the experimental RL levels and kinetics measurements during and after irradiation were compared with their simulation counterparts. This simulation is based on a standard system of coupled non-linear differential equations to describe the processes of electron-hole pairs generation, trapping-detrapping and recombination in order to shed light on the temperature effect on the RL signal dynamics and intensity.

Silica-based optical fibers made by Modified Chemical Vapor Deposition (MCVD) technique have been doped with different concentrations of germanium (Ge). After characterization of the thermoluminescence (TL) process in each of these fibers, their real-time responses were studied by radioluminescence (RL) under X-rays. First, the same counterintuitive trend was observed for the intensity of the two phenomena as a function of the Ge content, namely the decrease in TL and RL signals with increasing Ge content. This behavior was explained by the existence of thermally disconnected traps. Then, the RL response as a function of the dose-rate of the fiber doped with 1% Ge was judiciously chosen and investigated in detail. The results show that up to Total Ionizing Dose (TID) of 1.9 kGy(SiO ₂ ), this response is not only insensitive to the accumulated dose effect but also exhibits an afterglow decay at the end of the radiation much faster than that of a Ce-doped fiber, a very useful feature where irradiations are repetitive as in the medical field of flash-therapy. In addition to the medical field where the Ge-doped fiber seems to be quite suitable, slightly larger core diameters would also allow this same fiber to meet the challenge of high sensitivity in the environmental radiation monitoring around nuclear power plants or radioactive waste storage areas.

Rare-earth-doped silica glasses are promising materials for ionizing radiation dosimetry. In this paper, a bulk glassy silica doped with Ce ions, was prepared via the sol-gel technique and drawn at about 2000 °C into a cylindrical capillary rod. Under X-rays, this sample presents a radioluminescence (RL) signal that can be exploited for dose rate real-time monitoring with remarkable linear response spanning 6 decimal orders of magnitude. In order to elucidate RL signal dynamics associated with free carriers trapping-detrapping and recombination during and after X-ray irradiation, we studied the obtained RL signals using a kinetic model involving one or several trapping states and one recombination center. With this model and using appropriate sets of trapping parameters, extracted from the thermoluminescence data, the RL signal was numerically simulated, along with the populations of the relevant traps and centers. Several experimentally observed characteristics of the RL signals were explained using the model.

We investigated the influence of the ambient light on the Radiation-Induced Attenuation (RIA) of Ge-doped optical fibers measured at the telecommunication wavelengths when exposed to a ⁶⁰ Co source, and in the visible and Near-InfraRed (NIR) domains when exposed to X-rays. From our results, it is possible to observe that this class of optical fibers are sensitive to the ambient light. Indeed, in all cases, their RIA value changes depending on the illumination conditions. In particular, the γ-ray irradiations revealed jumps in the measured RIA when changing the lamp of the irradiation chamber, whereas the X-ray irradiations reported that the relative photobleaching effect caused by the ambient light increases with the wavelength, up to ~1300 nm. The spectral study revealed no direct correlation between the absorption bands of the known Ge-related point defects and the external photobleaching phenomenon.

We investigated in situ the temperature influence on X-ray radiation-induced attenuation (RIA) levels and kinetics of an Al-doped single-mode optical fiber (OF) in the visible (vis) and near-infrared (NIR) spectral domains (400 nm– $2~\mu \text{m}$ , from room temperature (RT) up to 300 °C). The RIA spectra in the visible show no significant variation up to 200 °C. Above this temperature, the induced losses begin to decrease. The NIR RIA, on the other hand, appears to be more temperature-dependent, showing a monotonic decrease with increasing temperature throughout the whole investigated range. By studying the RIA growth kinetics at 1550 nm at the various temperatures, a linear dependence was found between the RIA levels and doses at which a steady state is reached, suggesting a direct generation process of the induced defects. Finally, the combination of the measurements led to the attribution of the RIA in the NIR domain to a single absorption component, whose generation process is correlated with the defects absorbing in the visible and characterized by a continuum of absorption states.