Institute of research into the fundamental laws of the Universe

Neutronography, or neutron radiography, consists of producing a 2D image of an object crossed by a neutron flux by measuring the differences in absorption and scattering of these particles as they pass through the materials. It is a non-destructive diagnostic and these images have extremely interesting characteristics, very different from those obtained by X-ray. Indeed, neutrons, which are essentially sensitive to the nuclear interaction, are affected by light chemical elements (hydrogen), present in particular in organic materials, whereas most heavier elements, such as metals, are transparent to them. Neutronography thus finds unique applications in materials science, engineering, archaeology or the study of works of art.

Until now, nuclear research reactors have been producing neutrons, but these facilities are at the end of their lives, such as the Orphée reactor in Saclay, which was shut down in 2019. Alternative sources are developed, based on neutrons emitted during nuclear reactions produced by a beam of accelerated particles (e.g. protons), such as the SONATE project. These new facilities are cheaper and more flexible than nuclear reactors, but provide lower neutron fluxes. To avoid excessively long measurement times, it is necessary to use imaging technologies that are more sensitive than traditional silver films. The aim of this thesis is to qualify different modern imaging technologies and to optimise them for industrial neutronography. Different detector technologies are possible: detectors based on microchannel wafers, or detectors based on scintillating films coupled with CCD cameras. The analysis of the signals from these detectors can also be optimised, and the "event by event" mode can be studied to improve the selectivity and resolution of the images. Finally, post-processing of the image, based on noise reduction and super-resolution algorithms, can also be used, which can make use of advanced machine learning methods for image reconstruction, potentially allowing a gain in reconstruction quality as well as rapid analysis.

Pushing ab initio calculations of atomic nuclei to higher precision



The theoretical description of atomic nuclei from first principles, or in a so-called ab initio fashion, has become possible only recently thanks to crucial advances in many-body theory and the availability of increasingly powerful high-performance computers. Such ab initio techniques are being successfully applied to study the structure of nuclei starting from the lighter isotopes. Still, extensions to heavy elements and nuclear reactions are posing considerable difficulties. The objective of the thesis is to contribute to this on-going progress in nuclear many-body theory. The project will focus on a developing ab initio technique (the so-called Gorkov-Green function approach, devised at CEA Saclay) designed to describe open-shell or superfluid systems (the majority of atomic nuclei). After the first promising applications to light and medium-mass nuclei, the method faces crucial upgrades to reach the precision and competitiveness of state-of-the-art approaches. The proposed work will aim to put in place the necessary tools towards this direction.



In particular, the Gorkov-Green function approach will be extended to the next level of precision. After some formal work, this will require a careful numerical implementation on top of the existing code. Given the increased cost of the corresponding numerical calculations, expected to go from moderately (100 proc.) to massively parallel (1000 proc.), special attention will have to be paid to the code optimisation and the use of pre-processing techniques like importance truncation or tensor factorisation.



Overall, the thesis work will exploit the latest advances in nuclear theory, including the use of nuclear interactions from chiral effective field theory and renormalisation group techniques, as well as high-performance computing codes and resources. The work will consist in formal developments, computational tasks and application of the new technology to cases of experimental interest. International collaborations are envisaged.

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.

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.