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.
First observations of the TeV gamma-ray sky with the NectarCAM camera for the CTA observatory
Very high energy gamma-ray astronomy is a relatively young part of astronomy (30 years), looking at the sky above 50 GeV. After the success of the H.E.S.S. array in the 2000s, an international observatory, the Cherenkov Telescope Array (CTA), should start operating by 2026. This observatory will include a total of 50 telescopes, distributed on two sites. IRFU is involved in the construction of the NectarCAM, a camera intended to equip the "medium" telescopes (MST) of CTA. The first NectarCAM (of the nine planned) is being integrated at IRFU and will be shipped on site in 2025. Once the camera is installed, the first astronomical observations will take place, allowing to fully validate the functioning of the camera. The thesis aims at finalizing the darkroom tests at IRFU, preparing the installation and validating the operation of the camera on the CTA site with the first astronomical observations. It is also planned for the student to participate in H.E.S.S. data analysis on astroparticle topics (search for primordial black holes, constraints on Lorentz Invariance using distant AGN).
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.
ARTIFICIAL INTELLIGENCE TO SIMULATE BIG DATA AND SEARCH FOR THE HIGGS BOSON DECAY TO A PAIR OF MUONS WITH THE ATLAS EXPERIMENT AT THE LARGE HADRON COLLIDER
There is growing interest in new artificial intelligence techniques to manage the massive volume of data collected by particle physics experiments, particularly at the LHC collider. This thesis proposes to study these new techniques for simulating the rare-event background from the two-muon decay of the Higgs boson, as well as to implement a new artificial intelligence method for simulating the response of the muon spectrometer detector resolution, which is crucial for this analysis.
SEARCH FOR DIFFUSE EMISSIONS AND SEARCHES IN VERY-HIGH-ENERGY GAMMA RAYS AND FUNDAMENTAL PHYSICS WITH H.E.S.S. AND CTAO
Observations in very-high-energy (VHE, E>100 GeV) gamma rays are crucial for understanding the most violent non-thermal phenomena at work in the Universe. The central region of the Milky Way is a complex region active in VHE gamma rays. Among the VHE gamma sources are the supermassive black hole Sagittarius A* at the heart of the Galaxy, supernova remnants and even star formation regions. The Galactic Center (GC) houses a cosmic ray accelerator up to energies of PeV, diffuse emissions from GeV to TeV including the “Galactic Center Excess” (GCE) whose origin is still unknown, potential variable sources at TeV, as well as possible populations of sources not yet resolved (millisecond pulsars, intermediate mass black holes). The GC should be the brightest source of annihilations of massive dark matter particles of the WIMPs type. Lighter dark matter candidates, axion-like particles (ALP), could convert into photons, and vice versa, in magnetic fields leaving an oscillation imprint in the gamma-ray spectra of active galactic nuclei (AGN).
The H.E.S.S. observatory located in Namibia is composed of five atmospheric Cherenkov effect imaging telescopes. It is designed to detect gamma rays from a few tens of GeV to several tens of TeV. The Galactic Center region is observed by H.E.S.S. for twenty years. These observations made it possible to detect the first Galactic Pevatron and place the strongest constraints to date on the annihilation cross section of dark matter particles in the TeV mass range. The future CTA observatory will be deployed on two sites, one in La Palma and the other in Chile. The latter composed of more than 50 telescopes will provide an unprecedented scan of the region on the Galactic Center.
The proposed work will focus on the analysis and interpretation of H.E.S.S observations. carried out in the Galactic Center region for the search for diffuse emissions (populations of unresolved sources, massive dark matter) as well as observations carried out towards a selection of active galactic nuclei for the search for ALPs constituting dark matter. These new analysis frameworks will be implemented for the future CTA analyses. Involvement in taking H.E.S.S. data. is expected.
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.
STUDY OF THE MULTI-SCALE VARIABILITY OF THE VERY HIGH ENERGY GAMMA-RAY SKY
Very high energy gamma ray astronomy observes the sky above a few tens of GeV. This emerging field of astronomy has been in constant expansion since the early 1990s, in particular since the commissioning of the H.E.S.S. array in 2004 in Namibia. IRFU/CEA-Paris Saclay is a particularly active member of this collaboration from the start. It is also involved in the preparation of the future CTAO observatory (Cherenkov Telescope Array Observatory), which is now being installed. The detection of gamma rays above a few tens of GeV makes it possible to study the processes of charged particles acceleration within objects as diverse as supernova remnants or active galactic nuclei. Through this, H.E.S.S. aims in particular at answering the century-old question of the origin of cosmic rays.
H.E.S.S. allows measuring the direction, the energy and the arrival time of each detected photon. The time measurement makes it possible to highlight sources which present significant temporal or periodic flux variations. The study of these variable
Direction de la Recherche Fondamentale
Institut de recherche
sur les lois fondamentales de l’univers
emissions (transient or periodic), either towards the Galactic Center or active nuclei of galaxies (AGN) at cosmological distance allows for a better understanding of the emission processes at work in these sources. It also helps characterizing the medium in which the photons propagate and testing the validity of some fundamental physical laws such as Lorentz invariance. It is possible to probe a wide range of time scales variations in the flux of astrophysical sources. These time scales range from a few seconds (gamma ray bursts, primordial black holes) to a few years (binary systems of high mass, active galaxy nuclei).
One of the major successes of H.E.S.S.'s two decades of data-taking. was to conduct surveys of the galactic and extragalactic skies in the very-high energy range. These surveys combine observations dedicated to certain sources, such as the Galactic Center or certain remains of supernovae, as well as blind observations for the discovery of new sources. The thesis subject proposed here concerns an aspect of the study of sources which remains to be explored: the research and study of the variability of very-high energy sources. For variable sources, it is also interesting to correlate the variability in other wavelength ranges. Finally, the source model can help predict its behavior, for example its “high states” or its bursts.