Description of collective phenomena in atomic nuclei beyond Time-Dependent Density Functional
Context :
Predicting the organization and dynamics of neutrons and protons within atomic nuclei is a significant
scientific challenge, crucial for designing future nuclear technologies and addressing fundamental questions
such as the origin of heavy atoms in our universe. In this context, CEA, DAM, DIF develops theoretical
approaches to simulate the dynamics of the elementary constituents of atomic nuclei. The equations of
motion, derived within the framework of quantum mechanics, are solved on our supercomputers. The 2010s
saw the rise of the time-dependent density functional theory (TDDFT) approach for tackling this problem.
While TDDFT has provided groundbreaking insights into phenomena such as giant resonances observed in
atomic nuclei and nuclear fission, this approximation has intrinsic limitations.
Objectives :
This PhD project aims to develop and explore a novel theoretical approach to describe the collective motion
of protons and neutrons within the atomic nucleus. The goal is to generalize the TDDFT framework to
improve the prediction of certain nuclear reaction properties, such as the energy distribution among the
fragments resulting from nuclear fission. Building on initial work in this direction, the PhD candidate will
derive the equations of motion for this new approach and implement them as an optimized C++ library
designed to leverage the computational power of CEA's supercomputers. The final objective will be to assess
how this new framework enhances predictions of phenomena such as the damping of giant resonances in
atomic nuclei and the formation of fragments during nuclear fission.
Microscopic description of fission fragment properties at scission
Fission is one of the most difficult nuclear reactions to describe, reflecting the diversity of dynamic aspects of the N-body problem. During this process, the nucleus explores extreme deformation states leading to the formation of two fragments. While the number of degrees of freedom (DOF) involved is extremely large, the mean-field approximation is a good starting point that drastically reduces the DOF, with elongation and asymmetry being unavoidable. This reduction introduces discontinuities in the successive generation of states through which the nucleus transits, since continuity in energy does not ensure the continuity of states resulting from a variational principle. Recently, a new method based on constraints associated with wave function overlaps has been implemented to ensure this continuity up to and beyond the scission (Coulomb valley). This continuity is crucial for describing the dynamics of the process.
The objective of the proposed thesis is to carry out for the first time a two-dimensional implementation of this new approach in order to take into account the whole collectivity generated by elongation and asymmetry DOF. The theoretical and numerical developments will be done within the framework of the time-dependent generator coordinate method. This type of approach contains a first static step, which consists of generating potential energy surfaces (PES) obtained by constrained Hartree-Fock-Bogoliubov calculations, and a second dynamic step, which describes the dynamic propagation of a wave packet on these surfaces by solving the time-dependent Schrödinger equation. It is from this second step that the observables are generally extracted.
As part of this thesis, the PhD student will:
- as a first step, construct continuous two-dimensional PESs for the adiabatic and excited states. This will involve the three algorithms Link, drop and Deflation
- secondly, extract observables that are accessible using this type of approach: yields, the energy balance at scission, fragment deformation and the average number of emitted neutrons. In particular, we want to study the impact of intrinsic excitations on the fission observables, which are essentially manifested in the descent from the saddle point to the scission.
Finally, these results will be compared with experimental data, in actinides and pre-actinides of interest. In particular, the recent very precise measurements obtained by the SOFIA experiments for moderate to very exotic nuclei should help to test the precision and predictivity of our approaches, and guide future developments of N-body approaches and nuclear interaction in fission.
Towards a multimodal photon irradiation platform: foundations and conceptualization
Photonic irradiation techniques exploit the interactions between a beam of high-energy photons and matter to carry out non-destructive measurements. By inducing photonuclear reactions such as photonic activation, nuclear resonance fluorescence (NRF) and photofission, these irradiation techniques enable deep probing of matter. Combining these different nuclear measurement techniques within a single irradiation platform would enable precise, quantitative identification of a wide variety of elements, probing the volume of the materials or objects under study. The high-energy photon beam is generally produced by the Bremsstrahlung phenomenon within a conversion target of a linear electron accelerator. An innovative alternative is to exploit the high-energy electrons delivered by a laser-plasma source, converted by Bremsstrahlung radiation or inverse Compton scattering. A platform based on such a source would open up new possibilities, as laser-plasma sources can reach significantly higher energies, enabling access to new advanced imaging techniques and applications. The aim of this thesis is to establish the foundations and conceptualize a multimodal photonic irradiation platform. Such a device would aim to be based on a laser-plasma source and would enable the combination of photonic activation, nuclear resonance fluorescence (NRF) and photofission techniques. By pushing back the limits of non-destructive nuclear measurements, this platform would offer innovative solutions to major challenges in strategic sectors such as security and border control, radioactive waste package management, and the recycling industry.
Nuclear reactions induced by light anti-ions - contribution of the INCL model
The interaction of an antiparticle with an atomic nucleus is a type of reaction that needs to be simulated in order to answer fundamental questions. Examples include the PANDA (FAIR) collaboration with antiproton beams of the order of GeV, which plans to study nucleon-hyperon interactions, as well as the neutron skin by producing hyperons and antihyperons. This same neutron skin is also studied with antiprotons at rest in the PUMA experiment (AD - Cern). At the same site, we are collaborating with the ASACUSA experiment to study the production of charged particles. To respond to those studies, our INCL nuclear reaction code has been extended to antiprotons (thesis by D. Zharenov, defended at the end of 2023). Beyond the antiproton there are antideuterons and antiHe-3. These antiparticles are of more recent interest, notably with the GAPS (General AntiParticle Spectrometer) experiment, which aims to measure the fluxes of these particles in cosmic rays. The idea is to highlight dark matter, of which these particles are thought to be decay products, and whose measured quantity should emerge more easily from the astrophysical background noise than in the case of antiprotons. The proposed subject is therefore the implementation of light anti-nuclei in INCL with comparisons to experimental data.
Measurement of charm elliptic flow in semi-central Pb-Pb collisions at 5 TeV at CERN with LHCb.
Heavy-ion collisions provide a unique opportunity to study the quark-gluon plasma (QGP), an exotic state of matter where quarks and gluons are no longer confined within hadrons and believed to have existed just a few microseconds after the Big Bang. Charm quarks are among the key probes for investigating the QGP. Indeed, they retain information about their interactions with the QGP, making them essential for understanding the properties of the plasma. The production of charm quarks and their interactions with the QGP is studied through the measurements of hadrons, mesons and baryons, containing at least one charm quark or antiquark, like D0 mesons or Lambda_c baryons. However, the hadronization process—how charm quarks become confined within colorless baryons or mesons—remains poorly understood.
A promising approach to gaining deeper insights into charm hadronization is to measure the elliptic flow of charm hadrons, which refers to long-range angular correlations and is a signature of collective effects due to thermalization. By comparing the elliptic flow of D0 mesons and Lambda_c baryons, researchers can better understand the charm hadronization mechanism, which is sensitive to the properties of the created medium.
To measure elliptic flow, the selected student will develop an innovative method that leverages the full capabilities of the detector. This method, which has never been applied before, provides a more intuitive and theoretically sound interpretation of the results. The candidate will adapt this technique for use with the LHCb detector to measure, compare, and interpret the elliptic flow of Lambda_c charm baryons and D0 mesons with the PbPb samples collected by LHCb in 2024.
Study of reaction mechanisms for the synthesis of super-heavy elements
One of the main activities in nuclear physics is the study of the properties of the exotic nuclei up to the limits of the nuclear chart, in regions with extreme proton-neutron ratios (proton/neutron driplines) and at the highest masses A and atomic numbers Z. The so-called super-heavy nuclei (SHN) are expected to exist beyond the liquid drop limit of existence defined by a vanishing fission barrier, thanks to the quantum mechanical shell effects. These nuclei are particularly interesting because they are at the limit between few-body and large n-body physics: the magic proton and neutron numbers, Z and N, are replaced by a magic region or island extended in Z and N.
The synthesis of these very and super-heavy nuclei by fusion-evaporation reactions is an experimental challenge due to the extremely low cross-sections. Modelling the complete reaction in order to guide the experiments is also a difficult challenge, as models developed for lighter nuclei cannot simply be extrapolated. Fusion reactions are hindered compared to what is observed with light nuclei, because the very strong Coulomb interaction is enhanced by the strong repulsion caused by the large number of positive charges (protons) in the system in competition with the attractive strong (nuclear) force in a highly dynamic regime. The predictive power of the models needs to be improved, although the origin of the hindrance phenomenon is qualitatively well understood. The quantitative ambiguities are large enough to observe a few orders of magnitude differences in the fusion probabilities calculated by different models. A small change in the cross-section could result in many months being required to perform successful experiments.
At GANIL, in collaboration with other institutes, we have developed a model that describes all the three steps of the reaction to synthesise super-heavy nuclei. Future developments will focus on finding ways to assess the models in order to improve their predictive power, including the design of dedicated experiments to constrain the so-called fusion hindrance. Of course, a careful uncertainty analysis, which is new in theoretical nuclear physics, will be necessary to assess the different ideas. Standard methods as well as state-of-the-art data analysis methods such as Bayesian analysis may be used.
This PhD work will be done in collaboration with the experimental group at GANIL and a research team in Warsaw (Poland). Depending on the skills of the student, the thesis will be more oriented towards formal developments or towards the experiments at the new S3 facility at Spiral2. Participation in experiments is possible.
Experimental and theoretical studies of the fission fragment excitation energy and angular momentum generation
The discovery of nuclear fission in 1939 profoundly changed our understanding of nuclear physics. The fission reaction is the splitting of heavy nuclei, such as uranium 235, into two lighter nuclei, together with the release of a large amount of energy. Many years of research have led to the development of nuclear fission models, from which evaluated nuclear data files are derived. These files are essential inputs to reactor simulations; yet, their quality needs to be improved.
This PhD thesis aims to study the generation of angular momentum and the excitation energy of fission fragments from both experimental and theoretical standpoints. These studies will not only improve our understanding of the underlying process and our models, but also enhance the predictive power of simulation tools, particularly those used to predict gamma heating in reactors. Part of the work will involve finalizing the analysis of data acquired as part of a recent thesis. The student will take part in complementary experimental campaigns at the nuclear reactor of the Institut Laue-Langevin (ILL), using the LOHENGRIN spectrometer to measure isomeric ratios and the kinetic energy distributions of fission fragments.
The doctoral student will be based in a nuclear and reactor physics unit. He/she will develop skills in nuclear physics, data analysis, and computer programming. The programming languages used will be C++ and Python. Professional perspectives include academic research, R&D organisations, nuclear industry, and possibly also data scientist positions.
Time reversal invariance test in nuclear beta decay: Analysis of the data of MORA at JYFL
The Matter’s Origin from RadioActivity (MORA) experiment searches for a sign of CP violation in nuclear beta decay, via the precise measurement of the so-called D correlation. An innovative technique of in-trap ion polarization for such a measurement enables attaining unprecedented sensitivity to New Physics, which could explain the matter-antimatter asymmetry observed in the universe. With a goal in sensitivity on a non-zero D of a few 10-4, the measurement that MORA is undertaking at Jyväskylä will be competitive with the best limit obtained so far on a non-zero D correlation in neutron decay [5]. To attain such precision regime several weeks of data taking are required along the coming years (2025-2027) at Jyväskylä, both for 23Mg+ and 39Ca+. The data analysis has to be undertaken in parallel. Crosschecks and adaptation of existing simulations of individual detectors of MORA, performed with GEANT4 and PENELOPE Monte Carlo codes, are required to pursue the investigation of systematics effects potentially affecting the final sensitivity on D. Dissemination of the results of the data analysis at national and international conferences will be asked to the PhD student.
Development of a dosimetry system to track alpha particles in in vitro assays for Targeted Alpha Therapy
Targeted Alpha Therapy (TAT) is a promising new method of treating cancer. It uses radioactive substances called alpha-emitting radioisotopes that are injected into the patient's body. These substances specifically target cancer cells, allowing the radiation to be concentrated where it is needed most, close to the tumors. Alpha particles are particularly effective because of their short range and ability to target and destroy cancer cells.
As with any new treatment, TAT must undergo preclinical studies to test its effectiveness and compare it to other existing treatments. Much of this research is done in laboratory, where cancer cells are exposed to these radioactive substances to observe their effects, such as cell survival. However, assessing the effects of alpha particles requires special methods because they behave differently than other types of radiation.
Recently, a method for measuring the radiation dose received by cells in laboratory experiments has been successfully tested. This method uses detecto
Study of radiative decay of the nucleus using a technic like Oslo-method
Radiative neutron capture is a nuclear reaction forming a compound nucleus which decays by emitting gamma-rays at excitation energy around the neutron binding energy. This well-known reaction which we known how to accurately measure its cross section at low incident neutron energies for most stable and few unstable nuclei close the stability valley, remains difficult to measure for exotic nuclei like fission fragments. Nuclear reaction models based essentially on stable nuclei, also struggle to provide reliable predictions of cross sections for these exotic nuclei. However, in the recent years, progress made related to the models and the measurements for the radiative capture show that significant improvements in including microscopic ingredients studies. These micoscopic ingredients: gamma strength function and nuclear level density, remain accesible to the experiment. These ingredients which respectively manage the way of how the gamma cascade occurs and the nuclear structure at high excitation energy can also be measured and calculated to be compared and suggest ways to improve the predictability of models. This kind of improvements have a direct impact for instance on the cross sections for these exotic nuclei which are produced in the stellar nucleosynthesis. The subject of thie thesis is to measure these quantities for a nucleus involved in the nucleosythesis using a new setup called SFyNCS.