Active absorber spectrometer for Bremsstrahlung emitted during Laser-Plasma interaction
The SAABRE project involves the development of a new diagnostic tool to measure the continuous Bremsstrahlung X-ray radiation produced when a high-power laser interacts with matter.
Currently, the instrument known as the Bremsstrahlung Cannon (BC) uses a stack of X-ray-sensitive films separated by filters. Each film absorbs a portion of the incident radiation based on its energy, allowing the energy spectrum of the incident X-rays to be reconstructed. However, these films must be removed from the experimental chamber and scanned after each shot, which severely limits the use of the BC on the new generation of high-repetition-rate lasers (such as APOLLON, VEGA3, or ELI) and complicates its deployment on single-shot facilities like the LMJ.
The project therefore proposes replacing the films with millimeter-sized semiconductor crystals (Si, CdTe, CZT, GaAs, and perovskites). These sensors will enable instantaneous measurement of the radiation dose, conversion of the signal into digitized electrical data analyzed in real time, and adaptation of the instrument to different types of experiments by changing the configuration of the filters used (inertial confinement fusion or laser-plasma acceleration).
The postdoctoral fellow hired will lead the entire development of this new detector, from the study of the proposed semiconductors to the creation of a prototype simulated with GEANT4, which will then be calibrated and experimentally tested on facilities producing high-energy gamma rays. The device will be used in particular for the EUROPA project, dedicated to the production of innovative medical radioisotopes, and will lead to scientific publications and conference presentations.
Development of a neutron imaging simulation software applied to NDT
CEA-List is developing CIVA, a reference platform for non-destructive testing (NDT) simulations, particularly X-ray radiography using the Monte Carlo method.
The project aims to extend these capabilities to neutron imaging, which complements X-ray radiography due to its different sensitivity to chemical elements. The goal is to develop the necessary digital tools for industrial neutron imaging simulation, with an eventual link to an experimental platform.
The post-doctoral researcher will first work on implementing a simplified neutron imaging model for thermal and fast neutrons. Next, they will account for scattering effects using the Monte Carlo method, in collaboration with CEA DES. Finally, experimental validation will assess the model's ability to reproduce experimental observations, using existing data and data to be acquired on large instruments.
The project is cross-cutting across CEA-List/DRT, DES, and DRF, with exchanges focusing on the comparison of different neutron simulation codes.
Exploratory study of actinium-225 production capabilities in research reactors and particle accelerators
This postdoctoral research project will examine various methods of producing the medical radioisotope 225Ac. These methods will involve irradiating 226Ra using either a material testing reactor (such as the Jules Horowitz Reactor) or a particle accelerator (such as a cyclotron or a rhodotron). The study will determine the optimal conditions for producing 225Ac, including the type and energy of the incident particles, the target material used, and the corresponding production yields, as well as the quantity of contaminants generated.
The first part of the work will involve modelling the irradiation characteristics in research reactors and particle accelerators. This modelling will be carried out at the Department of Reactor Studies at the IRESNE Institute, at the CEA Cadarache site. Monte Carlo simulations using TRIPOLI4®, GEANT4 and PHITS – developed by the CEA, CERN and JAEA, respectively – will be employed to model the JHR and the particle accelerators.
The second part of the project will be carried out in close collaboration with several CEA laboratories. These laboratories will host the postdoctoral researcher on an ad hoc basis to support them through the subsequent stages of the project.
The CHICADE facility in Cadarache is responsible for designing the targets. The proposed work involves identifying the main steps for manufacturing the targets by electrodeposition, starting with a stock of 226Ra-containing material. If possible, a feasibility study will also be conducted on non-irradiated targets with the same properties (cerium/barium model material).
In collaboration with the DRMP units responsible for modelling the thermomechanical aspects of the targets, the proposed work will also involve conducting a feasibility study on the ion irradiation of a model target at a facility such as JANNuS-Saclay or GANIL.
Decomposition of Fission Fragment Energy from Microscopic Approaches to Provide Input Data for the FIFRELIN Code
The FIFRELIN code (FIssion FRagment Evaporation modeLINg), developed since 2009 at the CEA, simulates the formation and decay of nuclear fission fragments. It contributes to the enrichment of the European nuclear data library JEFF, which is used for reactor simulations. The calculation proceeds in two steps: the generation of fission fragments (with their physical properties), followed by their decay using a Monte Carlo Hauser-Feshbach approach. At the moment of scission into two fragments, the total energy is split between kinetic energy (TKE) and excitation energy (TXE). The TXE is further divided into deformation energy and intrinsic excitation energy, which govern the emission of neutrons and photons. Accurate knowledge of both TXE and TKE is essential to improve FIFRELIN’s performance. Microscopic theoretical approaches (such as Hartree-Fock-Bogoliubov and the Generator Coordinate Method) are used and developed within DES to provide theoretical input supporting evaluated nuclear data. This postdoctoral position aims to use and enhance these models to gain a more detailed understanding of nuclear properties at scission. The desired candidate has several years of experience (3 years or more) in nuclear mean-field theory (such as Hartree-Fock-Bogoliubov, relativistic mean-field, etc.) or in the generator coordinate method.
VALERIAN: caracterizing electron transport for the ITkPix modules of ATLAS
A precise description of the transport of electrons and photons in matter is crucial in several of the CEA's flagship fields, notably radiation protection and nuclear
instrumentation. Their validation requires dedicated parametric studies and measurements.Given the scarcity of public experimental data, comparisons between calculation codes are also used. The challenge for the coming years is to qualify these codes in a broad energy domain, as certain discrepancies between their results have been identified during preliminary SERMA studies involving the coupled transport of neutrons, photons and electrons. The VALERIAN project involves seizing the opportunity created by a unique data collection Campaign planned for 2025-2026 at the IRFU (DRF) to better characterise these discrepancies. The IRFU has undertaken to check at least 750 pixel modules for the new trajectograph of the ATLAS experiment, as part of the rejuvenation of the large detectors at CERN. Numerous measurements with beta sources will be carried out in 2025-2026 for the qualification of these modules.
Influence of laser bandwidth and wavelength on laser plasma instabilities
As part of the Taranis project initiated by Thales and supported by BPI France and in collaboration with numerous scientific partners such as CEA/DAM, CELIA and LULI, work on target design and definition of the laser intended to energy production in direct drive will take place. A prerequisite for this work is to understand the laser-plasma interaction mechanisms that will occur when the laser is coupled with the target. These deleterious mechanisms for the success of fusion experiments can be regulated by the use of so-called “broadband” lasers. In addition, the choice of the laser wavelength used for the target design and the laser architecture must be defined. The objective of the postdoctoral position is to study the growth and evolution of these instabilities (Brillouin, Raman) in the presence of “broadband” lasers both from an experimental and simulation point of view, and thus to be able to define the laser conditions making it possible to reduce these parametric instabilities.
Evolution of ISAAC and Xpn codes for an extension of the QRPA method to the complete processing of odd nuclei; towards a database without interpolation for odd nuclei
The treatment of odd-isospin nuclei in microscopic approaches is currently limited to the so-called «blocking» approximation. In the Hartree-Fock Bogolyubov (HFB) approach, the ground state of an odd-mass nucleus is described as a one-particle excitation (qp) on its reference vacuum. Thus, in the QRPA approach, where the basic excitations are states «with 2 quasi-particles», the blocked qp is excluded from the valence space under the Pauli exclusion principle. As a result, the chosen qp is a spectator and is not involved in the QRPA collective states. If the single nucleon should have a significant contribution some levels will not be reproduced. The development in the QRPA codes (ISAAC and Xpn) of a procedure that allows all nucleons to participate in collective states is mandatory for a microscopic description of odd nuclei. Moreover, recent Xpn developments have allowed the description of forbidden ß- first decays improving the estimation of half-life time of fission fragments. This could be extended to address ß+ and electronic captures and could be adapted to large-scale calculations useful for nuclear astrophysics.
Development of a new spectrometer for the characterization of the radionuclide-based neutron sources
Since few years, the LNHB is developing a new instrument dedicated to the neutron spectrometry, called AQUASPEC. The experimental device consists of a polyethylene container that is equipped with a central channel accommodating the source and 12-measurement channels (in a spiral formation) around the source, into which detectors can be placed. The container is filled with water in order to moderate neutrons emitted from the source. Measurements have performed with 6Li-doped plastic scintillators, optimized for the simultaneous detection of fast neutrons, thermal neutrons and gamma rays through the signal processing based on pulse shape discrimination (PSD). The spectrum reconstruction is performed with an iterative ML-EM or MAP-EM algorithm, by unfolding experimental data through the detector's responses matrix calculated with MCNP6 code. The candidate will work in the general way on issues related to the neutron spectrometry in the laboratory: Contribution to the development and validation of the new spectrometer AQUASPEC; Participation to the sources measurements and working on aspects of neutron detection and signal processing, in particular issue of the discrimination of neutron/gamma based on the pulse shape discrimination technique (PSD); Usage of Monte Carlo simulation codes and algorithms to reconstruct initial neutron energy distribution; Investigation and integration of information related to neutron/gamma coincidence specific to the XBe type sources.
Numerical studies of laser plasma interaction in intermediate field on Laser Megajoule
In the Inertial Confinement Fusion experiments (ICF), intense laser beams cross a gas filled hohlraum. The gas is fully ionized and laser beams then propagate into a sub-critical plasma where laser plasma instabilites can develop. Optical smoothing techniques enable to break both spatial and temporal coherences so that both spatial and temporal scales of the beam become smaller than those required for the development of the instabilites. The breaking of spatial coherence is done thanks to the use of a phase plate which spreads the laser energy in a multitude of light grains called speckles. The breaking of temporal coherence is done by using a phase modulator which widens the spectrum and by dispersing each frequency with a grating. It is essential to know the statistical properties of speckles (width, lenght, contrast, coherence time, velocities ...) to be able to predict the instabilities levels which can depend on time and on the distance of propagation of the beam. .
For the sake of simplicity, the laser plasma instabilities are very often studied at the best focus of the beam. However, in the FCI experiments, laser beams are focused near the laser entrance hole of the hohlraum whose length is about 1 cm. The development of instabilities can then occur before the best focus (outside the hohlraum) and mainly beyond the best focus (far inside the hohlraum). The goal of this post-doctoral contract is to study the development of instabilities when it occurs in the intermediate field (far from the best focus of the beam) and to assess the efficiency of different smoothing options on Lase MagaJoule (LMJ) to limit these instabilities. We will especially study propagation instabilities (self-focusing, forward stimulated Brillouin scattering) and stimulated Brillouin backscattering. This work will be done thanks to numerous existing numerical codes and diagnostic tolls.
Minimizing the laser imprint through machine learning within the frameword of inertial confinement fusion
The postdoc will be based at the CELIA laboratory which develops studies on different patterns of inertial fusion by laser. In order to optimize the implosion of the target, the laser pulse is shaped spatially and temporally, in particular by a pre-pulse of a hundred picoseconds and intensity of a few hundred TW /cm2. However, the latter introduces spatial inhomogeneities to the surface and volume of the target, amplified by the initial solid behavior of matter. These fingerprints generated by the pre-pulse will degrade the symmetry of the target during its implosion, and therefore decrease the effectiveness of inertial confinement. At present, most models assume a plasma state from the beginning of the interaction, and are thus unable to account for certain experimental observations. To overcome this lack, we have just developed an original multi-physics simulation tool that includes the phase transition of a homogeneous material induced by the laser. In order to mitigate the laser imprint effect, a polystyrene foam (heterogeneous material) can be deposited on the surface of the target. The multiple optical reflections in the foam smooth the spatial profile of laser intensity, thus reducing absorption inhomogeneities. In order to reduce the influence of the laser fingerprint, the post-doctoral fellowship will aim to develop a microscopic model describing the evolution of the optical response of a foam during the solid-to-plasma transition. The first step of the work will be to couple the Helmholtz equation (describing laser propagation) to a solid transition model-plasma, and to study the influence of parameters. The second step will be to use an artificial intelligence algorithm (neural network) to optimize the optical response of the foam.