Institute of research into the fundamental laws of the Universe

The objective of this PhD project is to work on the commissioning plan and the first experiment of the S3 Low Energy Branch (S3-LEB), currently under construction at S3 (Super Separator Spectrometer), as part of the SPIRAL2 facility at the GANIL (Grand Accélérateur National d’Ions Lourds) laboratory in Caen, France. The major attribute of the S3-LEB is to use atomic physics techniques - more specifically, high resolution spectral measurements of the atomic transitions – in order to provide fundamental and nuclear-model-independent data on the structure of ground and isomeric nuclear states. In this context, this setup will allow the measurements of static properties of exotic nuclei such as charge radii, electromagnetic moments, nuclear spins and atomic masses, giving information on the distribution of the nucleons inside the nucleus and providing information on structural changes throughout the chart of nuclei. This state-of-the-art technique will be used at S3 with rare beams never studied by low-energy measurements.

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.

The SPIRAL2 linear accelerator is optimized for light ions (protons, deuterons), but it will also deliver heavier ions (O, Ne, Ar,…Ni) for the S3 spectrometer. The first objective of the PhD, is to propose and study the methods allowing to tune a heavy ion beam in 26 independent accelerating RF cavities in a fast and reproducible way up.
The S3 electromagnetic separator will use the beams from the linac to create and purify radioactive ions with a high efficiency. The complexity of its superconducting magnets requires an optimization of many parameters. Thanks to numerous hexapolar and octupolar corrections, we will be able to reduce the beam optical aberrations. The commissioning of the separator will require numerous measurements with beams and the development of an algorithm to optimize the optics for the 2 different operating modes. The second objective is to provide the simulations tools to nuclear physicists allowing them to prepare their experiments on S3 and to adjust the parameters of the spectrometers during the experiments.
The thesis work will be based on beam dynamics simulations and experimental measurements with beams.

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.

Understanding the limits of existence of the nucleus, especially concerning its mass limit, is one of the major fields of research in contemporary nuclear physics. In the region of heavy nuclei, neutron-deficient actinides are of particular interest. Indeed, pronounced octupole (pear-shaped) deformations are predicted and have even been observed in some nuclei. The aim of this thesis is to study these octupole deformed nuclei using the new-generation detector SEASON, whose detection efficiency and energy resolution are unprecedented for this type of experiment. The thesis work will focus on the installation, testing, experimental data-taking and analysis from an experiment to be carried out in 2025 at the University of Jyväskylä. In this experiment, the proton-induced fusion-evaporation reaction 232Th(p,X)Y will be used to populate neutron-deficient actinide isotopes, whose decay products will be analyzed using SEASON. The thesis will be in cotutelle with the University of Jyväskylä and divided into two parts:
i) a 1-year period at the University of Jyväskylä, during which the experiment will take place
ii) the following two years at CEA Saclay will be devoted to data analysis and preparation of the experimental program with SEASON at the new facility S3-LEB at GANIL-SPIRAL2.

In current and future high-energy physics experiments (i.e. upgrades of large detectors at the LHC and experiments in future colliders), the granularity of particle detectors continues to increase, and the use of multi-channel submicron integrated circuits has become a standard.

This granularity was taken one step further in the field of "Monolithic Active Pixel Sensor" (MAPS) technology, where pixel sizes can be as small as 10 x 10 µm2. These small pixels make it possible to achieve record spatial resolutions or greatly improve the radiation resistance of the trace detector, at the cost of a large quantity of data produced. This large amount of data is acceptable where a maximum spatial resolution is required, but can be prohibitive when this is not necessary, or when space and consumption constraints put limits on the number of fast downstream links.

Each experiment therefore requires to redefine the combination of the pixel size and the architecture of the detector's readout electronics, in order to meet the occupancy rate requirements of each physics experiment, and the detector's readout capabilities.
A major innovation in the design of pixel sensors for particle physics is to decouple the pixel matrix from the data rate sent.
As part of a team that has been developing MAPS since 1999, the approach required for the thesis is in a first step to study the existing trace detector architecture in order to understand its limitations in terms of radiation resistance. In a second step, the thesis will focus on information grouping options, assessing the impact of these options on data reduction as well as on induced information loss.

This will be supported by the design of a system-on-chip architecture, including pixel array optimization and digital processing, validating the work carried out in an integrated circuit.

To this end, this thesis will focus specifically on one of the major experiments at the European Center for Nuclear Research (CERN): the Upstream Tracker detector for the LHC Beauty Quark Experiment (LHCb).

The anti-(p, n, d, t, 3He, 4He)-nucleus reactions are both instructive and complicated to study. In addition to knowledge of the products of the antinuclon-nucleon reaction, they require the- nuclear environment to be taken into account, in particular the interactions in the final state.
Antiproton-nucleon reactions are/will be used/studied in particular at Cern's antiproton decelerator (AD) ring and at the FAIR facility in Germany to understand the behaviour oft antimatter. Reactions with light anti-ions (dbar, 3He-bar, for example) are of more recent interest, in particular with the GAPS (General AntiParticle Spectrometer) experiment, which aims to measure the fluxes of these particles in cosmic rays. The idea is to identify dark maJer, of which these particles are decay products, and whose measured quantities could 'easily' emerge from the cosmic background noise.
Recently, antiproton-nucleus reactions have been added to the INCL (IntraNuclear Cascade Liège) nuclear reaction code developed at the CEA (Irfu/DPhN) and this code is currently being implemented in the Geant4 transport code. The aim of the proposed thesis is to now include the reactions anti-(d, t, 3He, 4He)-nucleus in the INCL code.

Recent developments have shown that coupling a Micromegas gaseous detector on a glass substrate with a transparent anode and a CCD camera enable the optical readout of Micromegas detectors with an impressive spatial resolution showing that the glass Micromegas detector is well-suited for imaging. This feasibility test has been effectuated with low-X-ray photons permitting energy resolved imaging. This test opens the way to different applications. Here we will focus, on one hand, on neutron imaging for non-destructive examination of highly gamma-ray emitting objects, such as fresh irradiated nuclear fuel or radioactive waste and on the other hand, we would like to develop a beta imager at the cell level in the field of anticancerous drug studies.
Both applications require gas simulations to optimize light yields, optimization of the camera operation mode and design of the detectors in view of the specific constraints of reactor dismantling and medical applications: spatial resolution and strong gamma suppression for neutron imaging and precise rate and energy spectrum measurements for the beta. The image acquisition will be optimized for each case and dedicated processing algorithms will be developed.

Nuclear reaction modeling has been continuously being improved for many decades now. That's especially the case for our nuclear cascade code INCL. An ANR project has been funded for the next four years (2024-2027) to work on the issue of uncertainty and error estimate. Since this code is implemented in the particle transport code Geant4, the next step is to propagate these uncertainties from INCL to Geant4. There was a recent study on uncertainty propagation, called Transport Monte Carlo (TMC). However, this study only addresses the propagation of uncertainties related to model parameters, there was no propagation of model biases (related to hypotheses) and their uncertainties, which are both outside the physical model. Therefore, the propagation of biases and their uncertainties, which are coming from Monte Carlo collision models, is unexplored territory. The aim of the proposed PhD project is then to develop methods for this kind of propagation and to study the functioning and features of the developed methods in schematic scenarios. The full implementation of the developed methods into a transport code, such as GEANT4, however, is not within the core scope of the thesis, but it might be possible if time permits.

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.

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.

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.