Development of an uncertainty propagation method of function-typed input data applied to the decay heat calculation
Characterising the energy released by the disintegration of the radionuclides present in spent nuclear fuel is essential for the design, safety and analysis of storage, transport and disposal systems. Few measurements of this decay heat are available today. In addition, the available experimental values do not cover the wide spectrum of possible combinations between parameters such as discharge burn-up rate, 235U enrichment, cooling time, fuel design parameters, or operating conditions. The estimation of decay heat is therefore mainly based on calculation codes.
The evaluation of the uncertainty associated with the estimation of decay heat is important to achieve reliable predictions. Many efforts have been made to properly evaluate biases and uncertainties coming from nuclear data such as cross sections. The number of studies concerning uncertainties of an epistemic nature (uncertainty in the manufacture of some components, error in reading or adjusting mobile structures, etc…) is comparatively small. Among the latter, while the treatment of complex dependencies of scalar input parameters is well taken into account today, functional-type dependencies, i.e., expressed in the form of a function, are very little explored.
While uncertainties arising from the processing of fixed input parameters, such as fuel manufacturing parameters, independent of time, are quite well covered, the uncertainties coming from the processing of variable (or functional) parameters, such as operating history, evolving during reactor operations, are not. Irradiation history actually brings together several inter-correlated quantities (operating power, absorber movements, core evolution …), subject to modifications over time and influencing the value of numerous observables of interest, including decay heat. The models used today in industrial simulation tools do not make it possible to estimate this impact and to infer a validated uncertainty.
This research work will investigate the impact on decay heat of the uncertainties associated with input parameters having functional dependencies. We will particularly focus on the irradiation history of the reactors (PWR type). A first part of the work will be dedicated to the development of a substitution model for decay heat estimation and quantification of uncertainties of a functional nature. The second part will be devoted to the development of a sensitivity analysis method. Finally, a third part will concern the development of an inverse method for quantifying the uncertainties coming from irradiation modelling.
The doctoral student will be hosted in a reactor physics research unit of the CEA IRESNE located in Cadarache where he will collaborate with other doctoral students and specialists in the field.
Artificial Intelligence for Mass Measurement of Exotic Isotopes
Artificial intelligence opens new perspectives for basic science. It is no exception for nuclear structure studied at the extreme of the nuclear chart by the Super Separator Spectrometer (S3) under construction at GANIL-SPIRAL2. The Piège à Ions Linéaire du Ganil pour la Résolution des Isotopes en Masse (PILGRIM) is a Multi-Reflection time-of-flight Mass Spectrometer (MR-ToF-MS), with state-of-the-art performances that can only be exploited fully thanks to a joint development with the FASTER (http://faster.in2p3.fr/) data acquisition at LPC Caen. The PhD thesis will consist in carrying out this development with the FASTER developers and the physicist in charge of PILGRIM. Machine learning techniques will have to be employed to recognize patterns in the time-of-flight of ions extracted as bunches from the S3 Low Energy Branch. For each individual ion, the time of flight will have to be determined with sub-nanosecond precision, correcting for effects due to pile-up, gain and baseline fluctuations. This development should lead to the determination of masses of exotic nuclei with exquisite precision, enabling tests of nuclear physics models in previously uncharted territories.
INVESTIGATION OF THE NUCLEAR TWO-PHOTON DECAY IN SWIFT FULLY STRIPPED HEAVY IONS
The nuclear two-photon, or double-gamma decay is a rare decay mode in atomic nuclei whereby a nucleus in an excited state emits two gamma rays simultaneously. Even-even nuclei with a first excited 0+ state are favorable cases to search for a double-gamma decay branch, since the emission of a single gamma ray is strictly forbidden for 0+ ? 0+ transitions by angular momentum conservation. The double-gamma decay still remains a very small decay branch (<1E-4) competing with the dominant (first-order) decay modes of atomic internal-conversion electrons (ICE) or internal positron-electron (e+-e-) pair creation (IPC). Therefore we will make use of a new technique to search for the double-gamma decay in bare (fully-stripped) ions, which are available at the GSI facility in Darmstadt, Germany. The basic idea of our experiment is to produce, select and store exotic nuclei in their excited 0+ state in the GSI storage ring (ESR). For neutral atoms the excited 0+ state is a rather short-lived isomeric state with a lifetime of the order of a few tens to hundreds of nanoseconds. At relativistic energies available at GSI, however, all ions are fully stripped of their atomic electrons and decay by ICE emission is hence not possible. If the state of interest is located below the pair creation threshold the IPC process is not possible either. Consequently, bare nuclei are trapped in a long-lived isomeric state, which can only decay by double-gamma emission to the ground state. The decay of the isomers would be identified by so-called time-resolved Schottky Mass Spectroscopy. This method allows to distinguish the isomer and the ground state state by their (very slightly) different revolution time in the ESR, and to observe the disappearance of the isomer peak in the mass spectrum with a characteristic decay time. After a first successful experiment establishing the double-gamma decay in 72Ge a new experiment has been accepted by the GSI Programme Committee and its realization is planned for 2024.
Exploring magicity and nuclear forces in 68Ni
We propose to study the magicity of 68Ni by means of neutron adding and neutron removal transfer reactions (d,p) and (p,d), respectively. This way, we get unique access to the occupancy of the normally occupied orbits and the vacancy of the valence ones. If a sharp transition in occupancy is found, the nucleus is considered as magic, otherwise rather superfluid. Furthermore, this study also allows to study the spin-orbit force, essential to the modeling of atomic nuclei, in a unique manner. 68Ni is produced by means of the LISE spectrometer at GANIL, protons and deuterons produced arising from transfer reactions are detected in the highly-segmented Si array MUST2, gamma-rays with EXOGAM2 and incoming/outgoing nuclei tracks, energy losses and time-of-flights with sets of gas-filled detectors.
Target development and new radioactive beams for SPIRAL1 - GANIL
The Target-Ion-source group at GANIL is seeking to extend the range of re-accelerated radioactive ion beams supplied by the SPIRAL1 facility, in order to increase the attractiveness of existing and future installations. This thesis proposes to produce new beam:
- By producing Fe, Ni, Co beams, by increasing the operating temperature of current TISS (target/ion-source system).
- By producing more intense beams using new targets that need to be designed, produced and tested.
Neutron-star crusts at finite temperature
Neutron stars are among the densest objects in the universe. Born from the explosion of core-collapse supernovae, they are initially very hot and consequently their outer layers (the crust) are made up of a dense liquid composed of various nuclear species immersed in a background “gas” of electrons (and possibly neutrons/protons).
During the doctoral thesis, a theoretical study of the neutron-star crust at finite temperature will be carried out, in particular with regard to the treatment of nuclei in the dense medium characterising the crust. The new model will be employed to calculate the equation of state and the composition of the crust, and applied to predict properties that are important for neutron-star (global) modelling.
Unified theory of nuclear structure and reactions in the open quantum system framework
Light weakly bound or resonant nuclei play an important role in various stellar processes of nucleosynthesis. The comprehensive understanding of these nuclei requires a correct description of the multi-particle continuum. It is proposed to study complex reactions of astrophysical interest and near-threshold narrow resonances which play crucial role in the nucleosynthesis of heavier elements, using Gamow Shell Model in the representation of coupled channels.
Variety of nuclear shapes in 96Zr studied with AGATA and GRIFFIN gamma-ray spectrometers
The thesis will focus on the experimental study of the nuulear properties of the heaviest stable zirconium isotope (96Zr).
Recently, observation of a low-lying deformed state in this magic nucleus has been explained by a reorganization of nuclear shells in function of their occupation by protons and neutrons. These sophisticated nuclear-structure calculations predict a variety of shapes, both ellipsoidal and pear-like, to appear at low excitation energy in the 96Zr nucleus. We will investigate them using the powerful Coulomb-excitation technique, which is the most direct method to determine the shapes of nuclei in their excited states. The experiment will be performed using AGATA, a new-generation gamma-ray spectrometer, consisting of a large number of finely segmented germanium crystals, which allows us to identify each point where a gamma ray interacts with the detector material and then, using the so-called “gamma-ray tracking” concept, to reconstruct the energies of all emitted gamma rays and their angles of emission with highest precision. A complementary measurement will be performed at TRIUMF (Vancouver, Canada) using the world’s leading setup for beta-decay measurements called GRIFFIN. This project is a part of an extensive experimental program on shape coexistence and evolution of nuclear shapes undertaken by our group.
High-precision measurements of nuclear recoil on the 100 eV scale for cryogenic detectors
The CRAB method aims to provide an absolute calibration of cryogenic detectors used in dark matter and coherent neutrino scattering experiments. These experiments have in common the fact that the signal they are looking for is a very low-energy nuclear recoil (around 100 eV), requiring detectors with a resolution of a few eV and a threshold of O(10eV). Until now, however, it has been very difficult to produce nuclear recoils of known energy to characterize the response of these detectors. The main idea of the CRAB method, detailed here [1, 2], is to induce a nuclear capture reaction with thermal neutrons (25 meV energy) on the nuclei constituent the cryogenic detector. The resulting compound nucleus has a well-known excitation energy, the neutron separation energy, being between 5 and 8 MeV, depending on the isotope. If it de-excites by emitting a single gamma ray, the nucleus will recoil with an energy that is perfectly known, given by the two-body kinematics. A calibration peak, in the desired range of some 100 eV, then appears in the energy spectrum of the cryogenic detector. A first measurement performed in 2022 with a CaWO4 cryogenic detector from the NUCLEUS experiment (a coherent neutrino scattering experiment supported by TU-Munich, in which CEA is heavily involved) has validated the method .
This thesis comes within the scope of the second phase of the project, which involves high precision measurements using a thermal neutron beam from the TRIGA-Mark-II reactor in Vienna (TU-Wien, Austria). Two complementary approaches will be used simultaneously to achieve a high precision: 1/ the configuration of the cryogenic detector will be optimized for very good energy resolution, 2/ large crystals of BaF2 and BGO will be placed around the cryostat for a coincident detection of the nuclear recoil in the cryogenic detector and the gamma ray that induced this recoil. This coincidence method will significantly reduce the background noise and will enable the CRAB method to be extended to a wider energy range and to the constituent materials of most cryogenic detectors. We expect these measurements to provide a unique characterization of the response of cryogenic detectors, in an energy range of interest for the search for light dark matter and coherent neutrino scattering. High precision will also open up a window of sensitivity to fine effects coupling nuclear physics (nucleus de-excitation time) and solid-state physics (nucleus recoil time in matter, creation of crystal defects during nucleus recoil) .
The PhD student will be heavily involved in all aspects of the experiment: simulation, on-site installation, analysis and interpretation of the results.
Drell-Yan production measurement in proton-proton collisions and preequilibrium dilepton production in heavy-ion collisions with the LHCb experiment at the LHC
At the Large Hadron Collider (LHC) at Geneva, collisions of lead nuclei are used to create a thermodynamic system described by fluid dynamics under extreme conditions. The temperature of the short-lived system is sufficiently large in order to release the building blocks of matter at a subnucleonic scale, quarks and gluons. This state of matter is commonly called Quark Gluon Plasma (QGP). The space-time evolution of heavy-ion collisions at the LHC is described by close-to-ideal hydrodynamics after a short lapse of time. However, key features of the early stages of these collisions are largely unknown. These characteristics are crucial to understand the applicability limits of hydrodynamics and to understand thermalisation of a strongly interacting system.
In recent publications, it was pointed out that the dilepton production in the intermediate mass scale between 1.5 and 5 GeV/c² is highly sensitive to the ´thermalisation´ time scale towards the equilibrium QGP.
In addition, the LHC provides highly energetic proton and heavy-ion beams. They allow us to access the hadronic structure of the projectiles at very small fractional longitudinal momenta and at the same time still relatively large four momentum transfers. This configuration enables hence for perturbative calculations allowing the extraction of hadron structure information at very small fractional longitudinal momenta.
The theoretically best understood process in hadronic collisions is the production of dilepton pairs, the so-called Drell-Yan process. However, so far, no measurement down to 3 GeV/c² at a hadron collider has been published despite its theoretical motivation to test the lowest fractional momenta. In fact, at masses below around 30 GeV/c², semileptonic decays from heavy-flavour hadron decays start to dominate the dilepton production. This process has obscured any attempt to extract dilepton production in this kinematic domain.
The first goal of the thesis is the first measurement of Drell-Yan dimuons at low invariant masses at the LHC in proton-proton collisions that will be taken in 2024. This measurement will be based on novel background rejection techniques exploiting the forward geometry of LHCb. In a second part, the feasibility of the measurement in heavy-ion collisions will be investigated in the present and the future LHCb set-up. Depending on the outcome of the studies, a measurement in heavy-ion collisions will be conducted.