Institute of research into the fundamental laws of the Universe

GANIL has a long tradition in the use and development of low-pressure off-balance ion sources based on the Electron Cyclotron Resonance (ECR) process feeding the GANIL accelerators with highly charged ions. One of the challenge of this type of sources used upstream of the accelerator is to deliver high charge state ions at high intensity specifically for the production of metal ions. An early simulation tool (PhD Thesis of Alexandre Leduc) has been developed to better understand how the ECR Phoenix V3 source operates. This work has already led to the understanding of some limitations of these sources, but a step forward must be done to obtain more realistic modelling of an ECR plasma: include electron dynamics and the creation of self-consistent electrostatic potentials essential for the production of multicharged ions. Before achieving that, an intermediate milestone will be done by developing the simulation tools on an axisymmetric ECR ion source before extending them to a standard ECR ion source.

The present project aims at better understanding the nuclear superfluidity of atomic nuclei, which manifests for instance in a smaller moment of inertia as compared to a rigid body, deduced from their rotational or vibrational spectra. Superfluidity is thought to be induced by the pairing of nucleons, but the size of the pairs, their evolution as a function of the atomic mass and of the binding energy, or with shell structure are not known. Moreover, superfluidity may be caused by a larger number of correlated nucleons, such as quartets. These properties of the atomic nucleus are almost impossible to study as nucleons are bound inside a nuclear potential. We have proposed an innovative route to suddenly promote neutron pairs or quartets out of the nuclear potential and study the correlations they had inside the nucleus from the observation of their many-body decay. The experimental strategy is to use the instrumentation of the R3B beam line at FAIR which offers the unique possibility to measure all relevant information, such as neutrons and residual nucleus momenta, to study nuclear superfluidity and its possible change of regime towards the neutron drip line.

The nuclear fission process is driven by a complex interplay between the dynamical evolution of a quantum system composed of a large number of nucleons and the intrinsic nuclear structure of the system at extreme deformations as well as heat flows. The balance between these various aspects decide the characteristics of the emerging fragments. Innovative experiments are conducted to widen

Recently, two theoretical physicists put forward a thrilling hypothesis: neutrons may undergo a dark matter decay mode. Such a decay could explain the existing discrepancy of 4 standard deviations between two different methods of neutron lifetime measurements. If such a neutron decay is possible, then it could also occur in nuclei with sufficiently low neutron binding energy, a quasifree neutron decay. In this work, we consider the case of 6He with a two-neutron separation energy lower than the one for a single neutron. The observation of a free neutron from 6He decay would, although difficult to do, be a unique signature for the dark neutron decay.

A few micro-seconds after the Big Bang, the Universe was in a quark gluon plasma (QGP) state. Such state is predicted by Quantum Chromodynamics, which is the theory of strong interactions, and should be reached at very high temperature or energy density. Such conditions are reproduced in ultra-relativistic heavy ion collisions at the LHC at CERN.

Among the various QGP observables, the study of hadrons with heavy-flavour quarks (charm c or beauty b) and quarkonia (c-cbar or b-bbar bound states) is particularly important to understand the properties of the QGP.

Quarkonia are rare and heavy particles which are produced in the initial stages of the collision even before the QGP is formed, mainly through gluon-fusion processes, and are therefore ideal probes of the QGP. As they traverse the QGP, the quark/anti-quarks pair will get screened by the many free quarks and gluons of the QGP. Quarkonia will then be suppressed by a colour screening mechanism in the QGP. Since the various quarkonium states have different binding energies, each state will have a different probability of being dissociated. This results in a sequential suppression pattern of the quarkonium states. Additionally, if the initial number of produced quark/anti-quark pairs is large and if heavy quarks do thermalise in the QGP, then new quarkonia could be produced in the QGP by recombination of heavy quarks. This mechanism is known as regeneration. At the LHC, Upsilon (b-bbar) and J/psi (c-cbar) are complementary. The former are thought to be more suited than to address the sequential suppression, while the latter should allow to study possible regeneration mechanisms. In addition, non-prompt J/psi, i.e. from weak decays hadrons containing one valance b quark, give access to the transport properties of b quarks in the QGP. More recently, photoproduction of J/psi has been observed in peripheral Pb-Pb collisions; J/psi are produced from the photon flux of the moving Pb ions mostly at very low transverse momenta. The characterization of these photoproduced quarkonia would allow to better constrain the initial state of the collisions as well as the properties of the QGP.

We propose to study the production of prompt and non-prompt quarkonia Pb-Pb collisions at a center-of-mass energy per nucleon pair (sqrt(sNN)) of 5 TeV at the LHC with the first data of Run 3 (2022-2024). An upgrade of the ALICE apparatus is ongoing with, in particular, the addition of silicon pixel tracker that will complement the ALICE forward spectrometer as well as new readout electronics for the latter. These upgrades will allow us to: Profit from the planned increase in luminosity of the LHC, thus tripling in one year the data collected in the full LHC Run 2 (2015-2018); Separate the prompt and non-prompt contributions thanks to the precise measurement of the quarkonium decay vertex into two muons.

The student will first develop the procedures to separate prompt and non-prompt quarkonia. In doing so, the student will thus contribute to the development of the new software for data reconstructions, simulation, calibration and analysis that the ALICE Collaboration is developing for Runs 3 and 4 of the LHC. Secondly, the student will study the production of prompt and non-prompt quarkonia in terms of production yields and azimuthal anisotropy. These studies could be performed as a function of the centrality of the collision and transverse momentum and rapidity of the quarkonia, for various types of quarkonia. Depending on the progress of the thesis work, these studies, which are a priority for quarkonia produced by the hadronic collision, could be extended to photoproduced quarkonia.

It is proposed to study the salient effects of coupling between discrete and continuous states near various particle emission thresholds using the shell model in the complex energy plane. This model provides the unitary formulation of a standard shell model within the framework of the open quantum system for the description of well bound, weakly bound and unbound nuclear states.

Recent studies have demonstrated the importance of the residual correlation energy of coupling to the states of the continuum for the understanding of eigenstates, their energy and decay modes, in the vicinity of the reaction channels. This residual energy has not yet been studied in detail. The studies of this thesis will deepen our understanding of the structural effects induced by coupling to the continuum and will support for experimental studies at GANIL and elsewhere. The student of this theoretical thesis will develop the numerical tools necessary for the evolution of the "Gamow Shell Model" (GSM), a tool par excellence for spectroscopic studies.

Most of the visible mass of the universe is contained in nucleons. However, the origin of this mass remains mysterious, with the portion from the Higgs mechanism in standard renormalization schemes corresponding to only a few percent of the total mass. The answer is to be found in the dynamics of strong interaction, described by the theory of quantum chromodynamics (QCD) in terms of quarks and gluons. Thus, the interaction between quarks and gluons is responsible for the emergence of known and measured properties of hadrons such as their masses or spins.

There is now a strong theoretical and experimental dynamic to determine the 3D structure of hadrons in terms of quarks and gluons. From a theoretical point of view, the classical tools of quantum field theory, namely perturbative expansion, do not allow the study of the emerging properties of hadrons. The latter are inherently non- disruptive.

The aim of this thesis is to develop and use a non-perturbative formalism based on the Dyson-Schwinger and Bethe-Salpeter equations to determine the 3D structure of hadrons, in particular the nucleon. Different dynamic assumptions will be used to obtain a 3D mapping of the charge, mass and orbital angular momentum effects. To do so, a significant part of the Ph.D. will be dedicated to numerical development and analysis, in order to tackle different inverse problems. A comparison of the results obtained with the experimental data will be carried out in collaboration with the other LSN members.

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. An experiment to search for the double-gamma decay in 72Ge and 70Se has already been accepted by the GSI Programme Committee and should be realised in 2021/22.

Radiotherapy is an important modality in treatment cancer. In this domain, proton beams have ballistic superiority against photon beams. Nevertheless, the use of protontherapy to treat small volume tumors (typically less than 27 cm3) is limited because of the lack of well adapted dosimetry tools for small irradiation fields quality assurance. To answer this issue, an innovative dosimetry system has been developed. It is based on a scintillating block of 10 × 10 × 10 cm3 and two ultra-fast cameras recording the scintillation from different points of view to reconstruct 3-dimensional dose maps. The current reconstruction method uses a library of preliminary beam measurements.

The objective of this PhD thesis will be to develop a new method directly converting scintillation maps into dose maps. This includes, in the first stage of the thesis, the study of the energy dependence of the scintillation yield with proton beams. The new reconstruction method will then be evaluated and compared to ionization chamber and dosimetry films measurements. Finally, the dosimetric system will be used to study dose uncertainties during treatment plan.

DESCRIPTION

Positron emission tomography (PET) is a nuclear imaging technique widely used in oncology and neurobiological research. Decay of the radioactive tracer emits positrons, which annihilate in the nearby tissue. Two gamma quanta of 511 keV energy are produced by positron annihilation and allow one to reconstruct the annihilation vertex and distribution of the tracer activity in the body of the patient.

The precise determination of the position of the positron annihilation vertex is important for an accurate image reconstruction with a good contrast. In particular, it is useful for neuroimaging studies of brain and for pre-clinical studies with animal models (rodents) as well as for full body PET imaging. In this thesis we propose to contribute to an ambitious detector based on Cherenkov/Scintillating crystals. We have selected technologies that are particularly effective for PET imaging. The principles of the detector are patented. They should allow one to produce PET scanner with highly improved performances. The device uses advanced particles detector technologies: a dense scintillator crystal, micro-channel plate photomultipliers, gigahertz bandwidth amplifiers and fast data acquisition modules (WaveCatcher, SAMPIC). Data processing will involve Monté-Carlo simulations and data analysis based on GATE/Geant4 and Root C++ software libraries.



SUPERVISION

The successful candidate will work in the Department of Particle Physics of IRFU in close collaboration with the Department of Electronics Detectors and Computer Science for Physics. The CaLIPSO group includes two physicists and two students. Two Postdoc will join the project next spring. We collaborate closely with CNRS- IJCLabs on fast readout electronic, with CPPM of Marseille and CEA-SHFJ, and CEA-DES for simulations of medical imaging devices and image reconstruction algorithms, and with the University of Munster (Germany).



THE PROPOSED WORK

You will calibrate and optimize the detector prototypes and analyze the measured data. Your work will be focussing on detector time and spatial resolution optimization. This will involve many skills of an instrument scientist : fast photo-detection, fast electronics read-out (analog and digital) with picosecond precision, hardware and detector simulations with GEANT4 and GATE software.



REQUIREMENTS

Knowledge in physics, particle interactions with matter, radioactivity and particle detector principles, a vocation for instrumental (hardware) work, data analysis are mandatory. Being comfortable in programming, having a background in Gate/Geant4 simulation and C++ will be an asset.



ACQUIRED SKILLS

You will acquire skills in particle detector instrumentation, simulation of ionizing radiation detectors, photo-detection, implementation, operation of fast digitizing electronics, and data analysis.

Hunting for super heavy elements one of the most exciting and active topics during the last few years and has already produced new elements such as 113, 115, 117 and 118 in accelerator experiments. All these nuclei can be produced through fusion-evaporation reactions. However their studies are greatly hampered by the extremely low production rates, hence experimental information in this region is very scarce. The high-intensity stable beams of the superconducting linear accelerator of the SPIRAL2 facility at GANIL coupled with the Super Separator Spectrometer (S3) and a high-performance focal-plane spectrometer (SIRIUS) will open new horizons for the research in the domains of such rare nuclei and low cross-section phenomena at the limit of nuclear stability. The student will take an active part in the tests of the whole SIRIUS detector.

Information on the heaviest elements have been obtained up to now via fusion evaporation reactions. It is however well known that the only nuclei one can reach using fusion-evaporation reactions are neutron deficient and moreover in a very limited number (because of the limited number of beam-target combinations). An alternative to fusion-evaporation could be a revolutionary method based on be deep-inelastic collisions. The student will take, therefore, an active part in the new scientific activities of the group having as primary aim the investigation of nuclear structure in the heavy elements employing the new alternative method using multi-nucleon transfer reactions.

The exploration of nuclei close to the limit of their existence (called dripline) offers the unique opportunity to observe and study many phenomena not - or insufficiently - predicted by theory such as the appearance of neutron "halos" as well as the emergence of new magic numbers and the disappearance of those observed in nuclei close to stability.

The proposed thesis topic revolves around the study of these emerging phenomena in exotic nuclei (see beyond dripline) via the analysis of data from experiments carried out in RIKEN (Japan) and using the state-of-the-art experimental devices SAMURAI and MINOS which are key for the study of these phenomena.

This thesis is devoted to the study of nuclear reactions induced by neutrons between 15 and 40 MeV at the NFS facility using the Medley detector. The double differential cross sections of light charged particles emitted during reactions on carbon and chromium will be measured in order to enrich databases and improve certain reaction codes. The fission cross sections of uranium 235 and 238, which are standards, will also be measured relatively to the elastic diffusion cross section on hydrogen.