Mesure de la réponse intra-pixel de détecteur infrarouge à base de HgCdTe avec des rayons X pour l’astrophysique
In the field of infrared astrophysics, the most commonly used photon sensors are detector arrays based on the HgCdTe absorbing material. The manufacturing of such detectors is a globally recognized expertise of CEA/Leti in Grenoble. As for the Astrophysics Department (DAp) of CEA/IRFU, it holds renowned expertise in the characterization of this type of detector. A key characteristic is the pixel spatial response (PSR), which describes the response of an individual pixel in the array to the point-like generation of carriers within the absorbing material at various locations inside the pixel. Today, this detector characteristic has become a critical parameter for instrument performance. It is particularly crucial in applications such as measuring galaxy distortion or conducting high-precision astrometry. Various methods exist to measure this quantity, including the projection of point light sources and interferometric techniques. These methods, however, are complex to implement, especially at the cryogenic operating temperatures of the detectors.
At the DAp, we propose a new method based on the use of X-ray photons to measure the PSR of infrared detectors. By interacting with the HgCdTe material, the X-ray photon generates carriers locally. These carriers then diffuse before being collected. The goal is to derive the PSR by analyzing the resulting images. We suggest a two-pronged approach that integrates both experimental methods and simulations. Data analysis methods will also be developed. Thus, the ultimate objective of this thesis is to develop a new, robust, elegant, and fast method for measuring the intra-pixel response of infrared detectors for space instrumentation. The student will be based at the DAp. This work also involves collaboration with CEA/Leti, combining the instrumental expertise of the DAp with the technological knowledge of CEA/Leti.
Development and characterization of a reliable 13.5 nm EUV OAM carrying photon beamline
The Extreme UltraViolet (EUV) photon energy range (10-100 nm) is crucial for many applications spanning from fundamental physics (attophysics, femto-magnetism) to applied domains such as lithography and nanometer scale microscopy. However, there are no natural source of light in this energy domain on Earth because photons are strongly absorbed by matter, requiring thus vacuum environment. People instead have to rely on expensive large-scale sources such as synchrotrons, free electron lasers or plasmas from large lasers. High order laser harmonic generation (HHG), discovered 30 years ago and recognized by the Nobel Prize in Physics in 2023, is a promising alternative as a laboratory scale EUV source. Based on a strongly nonlinear interaction between an ultrashort intense laser and an atomic gas, it results in the emission of EUV pulses with femto to attosecond durations, very high coherence properties and relatively large fluxes. Despite intensive research that have provided a clear understanding of the phenomenon, it has up to know been mostly limited to laboratories. Breaching the gap towards applied industry requires increasing the reliability of the beamlines, subjects to large fluctuations due to the strong nonlinearity of the mechanism, and developing tools to measure and control their properties.
CEA/LIDYL and Imagine Optic have recently joined their expertise in a join laboratory to develop a stable EUV beamline dedicated to metrology and EUV sensors. The NanoLite laboratory, hosted at CEA/LIDYL, is based on a high repetition rate compact HHG beamline providing EUV photons around 40eV. Several EUV wavefront sensors have been successfully calibrated in the past few years. However, new needs have emerged recently, resulting in the need to upgrade the beamline.
The first objective of the PhD will be to install a new HHG geometry to the beamline to enhance its overall stability and efficiency and to increase the photon energy to 92eV, a golden target for lithography. He will then implement the generation of a EUV beam carrying orbital angular momentum and will upgrade Imagine Optic’s detector to characterize its OAM content. Finally, assisted by Imagine Optic engineers, he will develop a new functionality to their wavefront sensors in order to enable large beam characterization.
Advanced characterization of ferroelectric domains in hafnia-based thin films
Les mémoires ferroélectriques à accès aléatoire (FeRAM en anglais) à base d'oxyde d’hafnium et de zirconium (HZO) sont intrinsèquement ultra-faibles en consommation grâce au mécanisme de changement de tension, au potentiel de mise à l'échelle du HZO en dessous de 10 nm et à la compatibilité CMOS complète. De plus, elles présentent une faible latence nécessaire à une grande variété d'applications de logique et de mémoire. La compréhension des mécanismes sous-jacents et de la cinétique du ‘switching’ des domaines ferroélectriques est essentielle pour une conception intelligente des FeRAMs avec des performances optimales.
Cette thèse porte sur la caractérisation complète des domaines ferroélectriques (FE) dans des films HZO ultra-minces. L'étudiant utilisera plusieurs techniques d'imagerie de surface (microscopie à force piézoélectrique, PFM, microscopie électronique à basse énergie, LEEM, et microscopie électronique à photoémission de rayons X, PEEM) combinées à des méthodes avancées de caractérisation operando (détection résolue dans le temps couplée au rayonnement synchrotron). Ce projet marquera une avancée importante dans la recherche fondamentale des mécanismes de basculement de polarisation des couches FE ultra-minces à base d'hafnium, en élucidant les effets spécifiques de l'interface électrode métallique/couche FE dans le comportement électrostatique des condensateurs étudiés. Il permettra à terme une avancée significative dans le développement industriel des mémoires émergentes ferroélectriques, essentielles pour les applications d'intelligence artificielle (IA) à grande échelle.
Effect of water radiolysis on the hydrogen absorption flux by austenitic stainless steels in the core of a nuclear pressurized water reactor
In pressurized water nuclear reactors, the core components are exposed to both corrosion in the primary medium, pressurized water at around 150 bar and 300°C, and to neutron flux. The stainless steels in the core are damaged by a combination of neutron bombardment and corrosion. In addition, radiolysis of the water can have an impact on the mechanisms and kinetics of corrosion, the reactivity of the medium and, a priori, the mechanisms and kinetics of hydrogen absorption by these materials. This last point, which has not yet been studied, may prove problematic, as hydrogen in solid solution in steel can lead to changes in (and degradation of) the mechanical properties of the steel and induce premature cracking of the part. This highly experimental thesis will focus on the study of the impact of radiolysis phenomena on the corrosion and hydrogen uptake mechanisms of a 316L stainless steel exposed to the primary medium under irradiation. Hydrogen will be traced by deuterium, and neutron irradiation simulated by electron irradiation on particle accelerators. An existing permeation cell will be modified to allow in operando measurement by mass spectrometry of the deuterium permeation flux through a sample exposed to the simulated primary water under radiolysis conditions. The distribution of hydrogen in the material, as well as the nature of the oxide layers formed, will be analysed in detail using state-of-the-art techniques available at the CEA and in partner laboratories. The doctoral student will ultimately be required to (i) identify the mechanisms involved (corrosion and hydrogen entry), (ii) estimate their kinetics and (iii) model the evolution of hydrogen flux in the steel in connection with radiolysis activity.
Study of the influence of the microstructure of a 316L steel produced by the L-PBF process on its mechanical properties: characterization and modeling of creep and fatigue behavior
Research into additive manufacturing for the nuclear industry shows that the production of 316L austenitic steel components using laser powder bed fusion (L-PBF) presents technical challenges, including process control, material properties, qualification and prediction of mechanical behaviour under service conditions. The final properties differ from traditional processes, often exhibiting anisotropy that challenges existing design standards.
These differences are linked to the unique microstructure resulting from the L-PBF process. Controlling the manufacturing chain, from consolidation to qualification, requires an understanding of the interactions between process parameters, microstructure and mechanical properties.
The aim of this thesis is to study the relationships between the microstructure, texture and mechanical properties of 316L steel manufactured by the L-PBF process, under static or cyclic loading. This includes the influence on creep and fatigue properties, and the development of a model to predict mechanical behaviour. Using samples of 316L steel with specific microstructures consolidated by L-PBF, the proposed study aims to establish links between microstructure and mechanical properties to better predict in-service behaviour.
Thermally activated glide of screw dislocations in bcc metals
Thermally activated glide of dislocation is a key point for understanding the plastic deformation of metals. The screw dislocation in bcc metals is an archetypical case for which a large quantity of experimental data has been published in the scientific literature. It is then possible to compare these data to the theoretical predictions realized from the Vineyard statistical theory [1,2]. Such a theory is an essential tool allowing to perform a scale transition from atomistic computations toward macroscopic scale at which are realized the deformation tests.
The aim of our research will be to test Vineyard theory in comparison with molecular dynamics simulations [3]. Some preliminary computations have shown a significant discrepancy that is not present when we repeat the comparison for point-like defect as vacancies or self-interstitial atoms.
[1] Vineyard G.H., J. Phys. Chem. Solids 3, 121 (1957).
[2] Proville L., Rodney D., Marinica M-C., Nature Mater. 11, 845 (2012).
[3] Proville L., Choudhury A., Nature Mater. 23, 47 (2024).
Influence of ionization density in water on fluorescent solutes. Application to the detection of alpha radiation
The location and rapid identification, at a distance, of sources of alpha and beta particle emissions on surfaces or in wet cavities or solutions, in nuclear facilities undergoing decommissioning or to be cleaned up, is a real challenge.
The aim of the proposed PhD project is to develop a concept for the remote detection of fluorescence light from water radiolysis processes on molecules or nano-agents. Temporal characterization using fluorescence lifetime measurements will enable detection to be attributed to a type of radiation, depending on its linear energy transfer (LET). In the Bragg peak of alpha radiation, where the TEL is maximal, the ionization density due to this TEL influences the fluorescence lifetime. However, dose rate effects also need to be considered.
Molecules and nanoparticles that are candidates for forming fluorescent products and are sensitive to the ionization density and radicals produced in traces at very short times will be identified by guided bibliography work, then tested and compared by measurements. Spectral measurements (absorption and fluorescence) and fluorescence lifetimes of the corresponding fluorescent species will be carried out using the multi-channel (16-channel) TCSPC (Time Corelated Single Photon Counting) method. Ion beams or alpha particles from sealed sources will be used for proof-of-concept. Ion beams or alpha particles from sealed sources will be used for proof-of-concept in the CEA clean-up/dismantling program.
Reactive metals corrosion in innovative binders – Experimental study and hydro-chemo-mechanical modelling
Nuclear waste management requires the packaging of several kinds of metal wastes for long-term storage. These wastes, which can be very reactive metals, are prone to corrosion and commonly immobilised into containers with hydraulic binders as embedding matrices. Innovative binders (low carbon cements, alkali activated materials) are thus developed to increase the packaging performances. The main objective of the European project STREAM (in the frame of the Eurad-2 program) is to evaluate the interactions between these metal wastes and the selected cement matrices. The PhD thesis purpose is to investigate the reactive metal corrosion in the selected binder with electrochemical techniques. A generic experimental protocol will be developed in order to determine the impact of the corrosion products growth at the metal/binder interface on the global mechanical behaviour of the binder-waste composite and possible micro-cracks occurrence. A post-mortem characterisation will be performed on the metal/binder microstructure with mechanical properties measurements of the materials at the interface, especially the corrosion products. Afterwards, these results will feed a simplified Hydro-Chemo-Mechanical (HCM) model aiming the simulation of corrosion consequences on the composite material behaviour. Subsequently, this model will be used for long-term simulation at the waste package scale.
This research project is aimed at a PhD student wishing to improve his/her skills in materials science both in the experimental field and in the modelling/simulation of coupled physicochemical phenomena.
Impact of a sodium nitrate saline plume on the radionuclide confinement properties of cementitious matrices
Using modelling to predict the migration of radioactive species through a well-known porous matrix, such as concrete, is a major challenge for society, particularly in the context of studies linked to the radioactive waste management. Demonstrating that the proposed model is robust through targeted laboratory experiments under extreme chemico-physical conditions is one of the scientific challenges proposed by the CEA as part of this PhD research project.
The young reseacher will be responsible for designing, carrying out and modelling experimental lab-tests on the retention and diffusion of radionuclides of interest in controlled cementitious conditions or under perturbation due to the nitrate plume leading to very high concentrations in the pore solution. The main expected result is to propose a predictive model coupling chemistry under extreme ionic strenght conditions and transport through complex cementitious matrices, validated by experimental data acquired on simple systems.
Surrounded by a team of experts in the field of measuring and modelling radionuclides migration in porous media, the PhD student will be able to develop or extend his/her skills in the following areas: chemistry, analytical chemistry, physico-chemistry, radiochemistry and modelling.
Covalent 2D organic nanostructures by optically controlled cross-linking of molecular self-assemblies
The self-assembly of molecules on crystalline substrates leads to non-covalent 2D structures with interesting properties for various fields such as optoelectronics and sensors. The stabilization of these 2D networks into covalent networks, while preserving these properties, is a major challenge and a topical issue. Various demonstrations show that crosslinking can be triggered by thermal processes. Photocrosslinking, on the other hand, is poorly described and the few examples that have been found involve ultra-high vacuum conditions.
Building on our previously developed know-how and the additional expertise of chemist collaborators, we therefore propose to carry out photocrosslinking of 2D networks at atmospheric pressure. We will use a model oligophenyl system that will be functionalized to allow photocrosslinking towards the production of a covalent 2D network. The resulting networks will be characterized through the correlation of optical spectroscopy and local probe microscopy to monitor and highlight photo-induced cross-linking processes at wavelength scale.