Development of the Compton-TDCR Method for Scintillator Metrology
The objectives of this PhD thesis lie upstream of the applied domain, specifically in the field of radionuclide metrology. The research aims to obtain essential information for a deeper understanding of scintillation mechanisms. This topic represents a new discipline within the national metrology laboratory, currently nonexistent in other laboratories, and focuses specifically on scintillator metrology. The work will be centered on instrumentation and data analysis, enabling a refined understanding of the underlying physical phenomena. The PhD will be co-supervised by Benoit Sabot (expert in radioactivity metrology) and Christophe Dujardin (expert in scintillation).
One of the primary experimental objectives of this PhD is the development and implementation of the new Compton-TDCR setup [7], designed for the absolute measurement of scintillation yield as a function of electron energy. This system will be designed using 3D printing technology and will integrate high-purity germanium (GeHP) detectors to enhance measurement precision. After characterizing these detectors in terms of energy resolution and efficiency, they will be integrated into the final experimental setup. The PhD candidate will be responsible for signal processing using a digital module generating List-Mode files. The data will then be analyzed using an existing Rust-based software with a Python interface, which is currently limited to four channels. Given that the new setup will incorporate up to three GeHP detectors in addition to three photomultiplier channels, the software must be adapted to ensure optimal processing of the acquired data. Following fine-tuning of the electronics and a series of experimental tests, the required software modifications will be implemented to enable full data exploitation from the platform.
Once this initial phase is completed and the platform is fully operational, the candidate will focus on investigating scintillation phenomena. The first studies will examine standard scintillating materials, such as organic (liquid or plastic) and inorganic scintillators. Subsequently, the research will extend to less explored materials, such as porous scintillators. This phase will involve close collaboration with the University of Lyon, particularly with the Institut Lumière Matière, where complementary measurements will be performed to refine the analysis of scintillation phenomena, complete the laboratory findings, and develop simulations that integrate various experimental approaches.
The ultimate goal of this setup is to establish a metrology methodology for scintillators, enabling access to the response curve of these materials as a function of the energy of electrons interacting within them, as well as their temporal properties. This work will pave the way for new ionizing radiation measurement techniques and will make a significant contribution to the scientific community in this field.
Towards a multimodal photon irradiation platform: foundations and conceptualization
Photonic irradiation techniques exploit the interactions between a beam of high-energy photons and matter to carry out non-destructive measurements. By inducing photonuclear reactions such as photonic activation, nuclear resonance fluorescence (NRF) and photofission, these irradiation techniques enable deep probing of matter. Combining these different nuclear measurement techniques within a single irradiation platform would enable precise, quantitative identification of a wide variety of elements, probing the volume of the materials or objects under study. The high-energy photon beam is generally produced by the Bremsstrahlung phenomenon within a conversion target of a linear electron accelerator. An innovative alternative is to exploit the high-energy electrons delivered by a laser-plasma source, converted by Bremsstrahlung radiation or inverse Compton scattering. A platform based on such a source would open up new possibilities, as laser-plasma sources can reach significantly higher energies, enabling access to new advanced imaging techniques and applications. The aim of this thesis is to establish the foundations and conceptualize a multimodal photonic irradiation platform. Such a device would aim to be based on a laser-plasma source and would enable the combination of photonic activation, nuclear resonance fluorescence (NRF) and photofission techniques. By pushing back the limits of non-destructive nuclear measurements, this platform would offer innovative solutions to major challenges in strategic sectors such as security and border control, radioactive waste package management, and the recycling industry.
Caliste-3D CZT: development of a miniature, monolithic and hybrid gamma-ray imaging spectrometer with improved efficiency in the 100 keV to 1 MeV range and optimised for detection of the Compton effect and sub-pixel localisation
Multi-wavelength observation of astrophysical sources is the key to a global understanding of the physical processes involved. Due to instrumental constraints, the spectral band from 0.1 to 1 MeV is the one that suffers most from insufficient detection sensitivity in existing observatories. This band allows us to observe the deepest and most distant active galactic nuclei, to better understand the formation and evolution of galaxies on cosmological scales. It reveals the processes of nucleosynthesis of the heavy elements in our Universe and the origin of the cosmic rays that are omnipresent in the Universe. The intrinsic difficulty of detection in this spectral range lies in the absorption of these very energetic photons after multiple interactions in the material. This requires good detection efficiency, but also good localisation of all the interactions in order to deduce the direction and energy of the incident photon. These detection challenges are the same for other applications with a strong societal and environmental impact, such as the dismantling of nuclear facilities, air quality monitoring and radiotherapy dosimetry.
The aim of this instrumentation thesis is to develop a versatile '3D' detector that can be used in the fields of astrophysics and nuclear physics, with improved detection efficiency in the 100 keV to 1 MeV range and Compton events, as well as the possibility of locating interactions in the detector at better than pixel size.
Several groups around the world, including our own, have developed hard X-ray imaging spectrometers based on high-density pixelated semiconductors for astrophysics (CZT for NuSTAR, CdTe for Solar Orbiter and Hitomi), for synchrotron (Hexitec UK, RAL) or for industrial applications (Timepix, ADVACAM). However, their energy range remains limited to around 200 keV (except for Timepix) due to the thinness of the crystals and their intrinsic operating limitations. To extend the energy range beyond MeV, thicker crystals with good charge carrier transport properties are needed. This is currently possible with CZT, but several challenges need to be overcome.
The first challenge was the ability of manufacturers to produce thick homogeneous CZT crystals. Advances in this field over the last 20 years mean that we can now foresee detectors up to at least 10 mm thick (Redlen, Kromek).
The main remaining technical challenge is the precise estimation of the charge generated by the interaction of a photon in the semiconductor. In a pixelated detector where only the X and Y coordinates of the interaction are recorded, increasing the thickness of the crystal degrades spectral performance. Obtaining Z interaction depth information in a monolithic crystal theoretically makes it possible overcome the associated challenge. This requires the deployment of experimental methods, physical simulations, the design of readout microelectronics circuits and original data analysis methods. In addition, the ability to localise interactions in the detector to better than the size of a pixel will help to solve this challenge.
Development of a multiphysics stochastic modelling for liquid scintillation measurements
The Bureau international des poids et mesures (BIPM) is developing a new transfer instrument named the "Extension of the International Reference System" (ESIR), based on the Triple-to-Double Coincidence Ratio (TDCR) method of liquid scintillation counting with a specific instrumentation comprising three photomultipliers. The aim is to enable international comparisons of pure beta radionuclides, certain radionuclides that decay by electron capture, and to facilitate international comparisons of alpha emitting radionuclides.
The TDCR method is a primary activity measurement technique used in national laboratories. For the activity determination, its application relies on the construction of a model of light emission requiring knowledge of the energy deposited in the liquid scintillator. Depending on the decay scheme, the combination of different deposited energies can be complex, particularly when it results from electronic rearrangement following electron capture decay. The stochastic approach of the RCTD model is applied by randomly sampling the different ionizing radiation emissions following a radioactive decay. The recent addition of modules for automatically reading nuclear data (such as those available in the Table des Radionucléides) in radiation/matter simulation codes (PENELOPE, GEANT4), means that all possible combinations can be rigorously taken into account. The stochastic approach makes it possible to consider the actual energy deposited in the liquid scintillation vial, taking into account interactions in the instrumentation as a whole.
The aim of this thesis is to develop a multiphysics stochastic approach using the GEANT4 radiation/matter simulation code, to be applied in particular to the BIPM's ESIR system. The choice of the Geant4 code offers the possibility of integrating the transport of ionizing particles and scintillation photons. This development is of great interest for radioactivity metrology, with the aim of ensuring metrological traceability to a larger number of radionuclides with the BIPM's ESIR system. The thesis will be carried out in collaboration with the Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), which already has experience in developing a stochastic model with the GEANT4 code for its instrumentation dedicated to the TDCR method at the Laboratoire National Henri Becquerel (LNE-LNHB).
Development of a ML-based analysis framework for fast characterization of nuclear waste containers by muon tomography
This PhD thesis focuses on developing an advanced analysis framework for inspecting nuclear waste containers using muon tomography, particularly the scattering method. Muon tomography, which leverages naturally occurring muons from cosmic rays to scan dense structures, has proven valuable in areas where traditional imaging methods fail. CEA/Irfu, with expertise in muon detectors, seeks to harness AI and Machine Learning (ML) to optimize muon data analysis, particularly to reduce long exposure times and improve image reliability.
The project will involve familiarizing with muography (muon tomography image) principles, simulating muon interactions with waste containers, and developing ML-based data augmentation and image processing techniques. The outcome should yield efficient tools to interpret muon images, enhance analysis speed, and classify container contents reliably. The thesis aims to improve nuclear waste inspection’s safety and reliability by producing cleaner, faster, and more interpretable muon tomography data through innovative analysis methods.