Aziz Boukenter

To enable fiber Bragg gratings (FBGs) to operate effectively as cryogenic temperature sensors, metals with high thermal expansion coefficients should be deposited onto the surface of an optical fiber. In this work, targeting aerospace applications, a copper–indium bimetallic-coated FBG designed for operation at around 850 nm was manufactured using sputtering and electrodeposition techniques. Furthermore, the influence of bonding materials, specifically Kapton tape and Apiezon N grease, as well as fixation methods, on the performance of the coated FBG was investigated during cryogenic temperature calibration. The results demonstrate that with Kapton tape bonding, the Bragg wavelength shift (BWS) was approximately 1.6 times greater than that obtained with Apiezon N grease at 77 K during sensor calibrations. However, in order to have efficient thermal coupling between the sample holder and the metal-coated sensor in harsh environments such as cryogenic and vacuum environments, the optimal choice of bonding material as well as its architecture is mandatory to achieve high sensor sensitivity. Therefore, this study gives insights into the fact that metal-coated FBGs’ calibration accuracy and real-time stability during sensor deployments can be affected by the fixing techniques and the bonding materials used.

We investigate the performances of a dosimeter exploiting the radiation induced attenuation (RIA) of phosphosilicate multimode optical fiber in the visible domain. For this, we investigate the visible RIA under various irradiation conditions: dose up to 15 Gy, dose rates from 675 μGy/s to 50 mGy/s and temperatures of irradiation from -40 °C to 80 °C. This study was supplemented by an extensive analysis of the effect of photobleaching on the RIA and its impact on the dosimetry applications. Using optical fibers with larger cores enable to mitigate the undesirable effects of photobleaching on dosimetry performances observed with single-mode fibers. The results have enabled the development of a mathematical tool for simulating fiber performance based on its intrinsic parameters and the dose deposited.

We tested the capability of a cerium activated lithium silicate glass scintillator, typically employed for thermal neutron monitoring, to act as a 14 MeV neutrons flux and fluence monitor. A flux correlation of a fibered online sensor based on radioluminescence phenomenon is obtained from a first irradiation run and subsequently validated on a second dataset, within a 17% agreement. Cumulative dose effects are investigated under X-rays, revealing a significant contribution from radiation-induced attenuation, which reduces radiation sensitivity in preirradiated sensors, unlike what has been observed in cerium-doped fibers. Geant4 simulations provide an overview of potential applications in choosing between a Ce-doped fiber or a lithium-based scintillator as dosimeter. Finally, a preliminary spectroscopic study highlights a characteristic photoluminescence lifetime of 38 ns associated with the emission peak centered at 395 nm. The comparison of emission spectra under different excitations further suggests the involvement of additional ionization processes requiring deeper investigation.

This study investigates the impact of pre-irradiation on radiation-induced luminescence (RIL) based optical fiber dosimeters and compares the performances of Copper (Cu)-, Cerium (Ce)- and Cu/Ce-co-doped silica optical fibers under ionizing radiations. A total of 112 optical fiber probes were tested under 40 keV mean energy fluence X-rays over a wide dose rate range (5 mGy/s to 1.5 Gy/s) after different levels of pre-irradiation treatments (up to 2 MGy). Spectral analyses were also conducted to study the impact of total ionizing dose (TID) on spectral emission profiles. Results show that the pre-irradiation improves the RIL linearity with the dose rate, particularly for Cu-doped optical fibers. Cu/Ce-co-doped optical fibers show higher radiation sensitivity but also a greater probe-to-probe dispersion. However, this latter remains below 30% for all the investigated samples. Normalized RIL spectra show that Ce-doped optical fibers maintain a stable emission spectrum with increasing dose, while Cu- and Cu/Ce-co-doped optical fibers show spectral changes, through a blue shift of the RIL peak. These results underline the benefit of pre-irradiating the radioluminescent optical fibers to optimize their RIL dosimetry performances. For the most challenging applications, an individual calibration remains today needed for accurate dosimetry.

We studied the radiation response of two different Erbium-Doped Fiber Amplifiers based on radiation hardened optical fibers when an external LED lighting is used to photobleach these fibers to limit the radiation-induced attenuation (RIA) and then the amplifier gain degradation (GD). Noticeable results in terms of RIA and GD reduction are achieved by using LED lighting; however, the best results were achieved through the combination of blue LED lighting with pump injection into the tested fibers.

Measuring the swelling of fuel cladding is a critical parameter in research nuclear reactors. In this framework, a fiber-optic, contactless sensor known as the confocal chromatic sensor is currently under development. This sensor is planned for installation in an irradiation rig within the future Jules Horowitz research reactor in France. The sensor must be resistant to pressurized water reactor conditions and to be small enough to be positioned between the cladding and the test device wall within the reactor core. To overcome the lack of identified optical materials that meet these severe requirements, particularly in terms of chromatic generation for measurement range and corrosion resistance for the sensor window, an experimental campaign was conducted at the Belgium Reactor 2 at SCK•CEN. Custom measurement modules were developed to quantify optical transmission losses and optical path changes of materials such as silica and sapphire. The irradiation campaign lasted approximately one month, leading to a total gamma dose of ~5 GGy and neutron fluence ~1 to 2 10¹⁹ n/cm² while temperature during the run reached 300°C. Post-irradiation (after 1-year delay) measurements were also performed on irradiated samples. The results of this study provide valuable insights into the performance of candidate materials for the sensor, paving the way for their potential implementation in nuclear research reactors.

This study investigates the influence of pre‐irradiation and numerical aperture (NA) on radiation‐induced luminescence (RIL) and radiation‐induced attenuation (RIA) of pure silica core (PSC) multimode (MM) optical fibers (OFs) under steady state X‐rays (40 keV mean energy‐fluence). The RIL properties of MM PSC OFs with identical core compositions and geometries but having different NAs (0.12, 0.15, and 0.22) have been characterized as well as the influence of pre‐irradiation doses at 250 kGy, 1 and 2 MGy. RIL were measured under dose rates varying from 40 mGy(SiO ₂ )/s to 1.1 Gy/s. RIL spectral analysis shows two main emission bands correlated to Oxygen Deficient Centers (ODC(II), ∼460 nm) and NonBridging Oxygen Hole Centers (NBOHC, ∼650 nm). The spatial distribution of NBOHC, confined in the fiber core, has been measured through confocal photoluminescence microscopy. RIA was carried out up to a total ionizing dose (TID) up to 200 kGy and no major effect of NAs was observed. RIL intensity increases with both NA and pre‐irradiation dose due to defect generation within the PSC MM optical fiber. Based on these results, the potential of these PSC MM fibers to serve as radiation sensitive probes or as transport fibers for dosimetry systems is discussed.

We demonstrate the influence of the ambient lightning on the X‐ray radiation‐induced attenuation (RIA) in the visible to infrared domains of germanosilicate optical fibers (OFs). RIA spectra and kinetics are then discussed depending on the illumination conditions in terms of point defects and also from a practical point of view. In the presence of an external illumination, the RIA levels can change and therefore lead to misinterpretations during irradiation campaigns. This study highlights the role of the GeY defect in infrared Ge‐doped OF RIA and its significant sensitivity to photobleaching.

Dosimetry techniques at high dose levels in the kGy-MGy range are challenging and still insufficiently explored. One of the emerging candidates for high level applications is silver doped metaphosphate glass, commercially known as FD-7 radiophotoluminescence (RPL) dosimeters. In this study, a free-space optical setup is used to enable dedicated study for online (during irradiation) characterization of RPL dosimeters. Transmission spectra are measured online as a function of wavelength, dose and dose rates, with additional monitoring of post-irradiation recovery kinetics. The present work focuses on the near-infrared (NIR) analysis, comparing it with previous studies in the visible range and verify the feasibility of extending the detection range of FD-7 dosimeters to higher doses by probing it with longer wavelengths. For the first time, radiation-induced attenuation (RIA) has been reported in the NIR domain and with dose deposition up to 862 kGy. RIA shows monotonic increase with dose up to few kGys and a saturation trend is observed at higher doses. The sensitivity response is dependent on the wavelengths and aligns with the previous studies in the visible domain. Minor recovery is observed in RIA after the irradiation end. Additionally, dose rate effects on RIA kinetics is tested by changing the dose rates from 1.77 Gy/s down to 0.36 Gy/s.