Julien Mekki

We investigated the impact of different types of radiation (protons, gamma rays and neutrons) on the electro-optical performance of blue, green and red light-emitting GaN and GaAs diodes (LEDs). In addition to the Total Ionizing Dose (TID) effects, the displacement damage dose (DDD) was calculated using SR-NIEL and PHITS simulations, in GaN and GaAs bulks. γ-rays irradiation up to 100 krad [air] induces no significant LED degradation, whereas 15 MeV protons at 10¹¹,10¹² and 10¹³ p/cm²only slightly degrade the LED efficiencies, the red GaAs-based ones being the most affected. For neutron irradiation at GANIL-NFS, we reached fluences between 3.44×10¹³ and 1.65×10¹⁵n/cm². A higher degradation is visible at lower currents (below 0.5 A), this is attributed to trap-induced generation which have a dominant effect in lower injected current region. At the highest fluence, blue and green LEDs have their External Quantum Efficiency (EQE) degraded by 90 to 85% but remain operational, whereas red LEDs have an EQE of 0. In the emission spectra of the LEDs irradiated at this fluence, a second blue band appears at ~430 nm and a small redshift shift of 3 nm is observed for blue LEDs. Green LEDs experienced an even more significant redshift of about 10 nm. This red shift phenomenon is attributed to heterogeneity in indium concentration and the creation of indium clusters by displacement damage. However, its impact on the color of blue and green LEDs, remains low. By summarizing all the experimental results, the main radiation effect remains the efficiency of the LEDs, which drops significantly, after a DDD of 1.0×10¹¹MeV/g for blue and green GaN-based LEDs and a DDD of 1.0×10¹⁰MeV/g for the red GaAs-based LED technology. We performed bias annealing on blue and green LEDs irradiated at a fluence of 3.79×10¹⁴n/cm², as well as on red LEDs irradiated at 10¹²p/cm². The results revealed a 10% increase in efficiency for both blue and red LEDs following annealing. In contrast, the green LEDs exhibited a more modest 6% efficiency recovery.

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.

Since landing in 2012, the Radiation Assessment Detector (RAD) onboard the Mars science laboratory (MSL) Curiosity rover has encountered different local landscapes, which have induced variations in the local surface environment. The aim of this study is to include realistic and site‐specific local topography for any position in a radiation transport model, and to validate this modeling strategy by comparison with MSL/RAD results. We present a method that integrates complex planetary surface models into neutron transport simulations to assess how surface heterogeneities influence secondary neutron environments—combining flexible terrain representation with radiation transport modeling. By modeling the topography of the main sites traversed by the rover, we show and quantify the dependence of downward neutron intensity on topography, in particular on the blocking zenith angle, across the different energy ranges: thermal, epithermal, fast neutrons. Using a method involving the visible surface from an observation point, we reconstruct the additional albedo neutron environment created by the terrain as seen by MSL/RAD, using the Atmospheric RAdiation Model for Ionizing spectra on martian Surface (ARAMIS). In particular, we show that the terrain‐generated thermal neutron spectrum increases with greater topographic relief, while the flux of high‐energy neutrons (>1 MeV) decreases.

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.

A distributed dosimetry system based on phosphorus-doped optical fiber interrogated by an embedded Optical Time-Domain Reflectometer (OTDR) operated around 1610 nm is investigated for its performance, accuracy, repeatability and reliability in radiation environments corresponding to space applications. We analyzed the metrological properties of the system, showing an accuracy of ±6% on dose measurement using the minimum pulse width, and characterized the evolution of measurement uncertainty according to the sampling length. We demonstrated the system ability to measure Total Ionizing Dose under γ- and X-rays, at four dose rates ranging from 5.8 μGy(SiO₂)/s to 5038 μGy(SiO₂)/s, and with doses up to 344 Gy(SiO₂). Finally, the device reliability under 350 MeV and 480 MeV proton irradiation was tested. No Single-Event Latchups (SEL) have been observed up to a fluence of ∼4.2×10⁹ cm⁻² and the Single-Event Functional Interrupt (SEFI) cross-section was on the order of ∼10^-8 cm², equivalent to ∼11.5 days in low Earth Orbit.

We investigate the occurrence of SET in the programmable logic of a 16-nm FinFET SoC using SPA laser testing. Results indicate sensitive locations decorrelated from CRAM errors and a significant difference between SET and SEU thresholds. Correlation of these observations with available heavy ion results on the same technology is discussed. TCAD simulation results of SET in a single inverter are presented. Implications for SET rate prediction are discussed.

We use an open-source simulation software to design III-V multi-junction solar cells containing a dilute nitride subcell. We discuss the different modeling techniques in terms of physical models and numerical methods, and provide an educated guess for a GaInP$_{2}$ - GaAs - GaInAsN - Ge lattice-matched 4-junction solar cell performance assessment. We suggest a way to ensure an effective computation of the main output values of an open-source Poisson-Drift-Diffusion solver for multi-junction solar cell analysis. An optimization of a multi-junction cell including a n-i-p subcell is performed, starting from basic theoretical considerations, to the use of a double-layer anti-reflective coating to target the most limiting subcell, and then with an optimization of the n-i-p architecture in the 1eV subcell with respect to the GaInAsN layer properties. We show that the detrimental effects of low diffusion lengths in the GaInAsN layer can be alleviated by taking advantage of field-assisted collection.

Dosimetry holds significant importance across various applications such as radiotherapy, nuclear power plants, accelerators, and space research domains. Indeed, ensuring accurate dosimetry calibration is crucial for radioprotection or operations. In the context of challenging environments such as space, temperature calibration of radiation sensors plays an important role in ensuring their performances. This article discusses the temperature calibration of a cerium (Ce)-doped silica (SiO2)-based Optical Fiber (OF). The calibration covers a temperature range, during 40 keV X-rays irradiation, varying from - 80 °C to +100 °C, meeting the requirements for space applications and allowing for the determination of the temperature dependence of the Radiation Induced Luminescence (RIL). The tests were repeated on three identical 1 cm-long Ce-doped OFs. Our experiments reveal a temperature dependence of the RIL, particularly noticeable at low temperatures, where RIL decreases significantly as the temperature drops. Consequently, a temperature sensitivity curve for RIL is proposed, to calibrate the variations in RIL responses across the investigated temperature range. However, in complex operation conditions, with varying dose rates and temperatures, other phenomena could alter the detector performances, in particular the thermoluminescence observed during strong positive temperature gradients. This work contributes to highlight the potential of RIL-based sensors under varying temperature conditions, crucial for improving the reliability and accuracy, especially in a harsh environment as space.