The DPhN, in collaboration with the Department of Reactor Studies of Cadarache (DES) and the Institute of Particle and Nuclear Physics of Charles University in Prague (Czech Republic), studied the properties of gamma rays emitted by uranium isotopes during neutron capture reactions. Gamma-ray spectra measured at CERN's n_TOF facility were used as a test bed for nuclear reaction models and their ingredients, including the photon strength function that characterises the ability of a nucleus to emit or absorb photons. This work has enabled consistent modelling of the photon strength functions of the uranium isotopic chain (234U, 236U, 238U) and confirmed the presence of a particular oscillation mode of the nucleus shape at low excitation energy. This study was carried out as part of the PhD thesis of Javier Moreno-Soto [1] and the full results are published in Physical Review C [2].
A new window into the deformation of nuclei has been recently opened with the realisation that nuclear collision experiments performed at high-energy colliders, such as the BNL Relativistic Heavy Ion Collider (RHIC) or the CERN Large Hadron Collider (LHC), give access to the shape of the colliding isotopes. A collaboration between high- and low-energy theorists, including researchers from IRFU, has demonstrated that quantitative information on nuclear deformations can be obtained and has shown that the isotope 129Xe appears as triaxial, i.e. a spheroid with three unequal axes. This result represents the first evidence of triaxiality in nuclear ground states attained in high-energy experiments. Moreover, it opens the way to exciting future investigations at the interface of low- and high-energy nuclear physics.
The pygmy dipole resonance (PDR) is a vibrational mode of the nucleus that occurs in neutron-rich nuclei. It is described as the oscillation of a neutron skin against a core symmetric in number of protons and neutrons (Figure 1). The PDR has been the subject of numerous studies, both experimental and theoretical. Indeed, the study of the PDR has been and still is of great interest since it allows to constrain the symmetry energy, an important ingredient of the equation of state of nuclear matter that describes the matter within neutron stars. Moreover, the PDR is predicted to play a key role in the r-process (a process that could explain the synthesis of heavy nuclei) via the increase of the neutron capture rate. However, despite numerous experiments dedicated to the study of the PDR, using charged particle or gamma-ray beams, a consistent description could not be extracted. Thus, new experimental approaches are needed to better characterize this vibrational mode of the nucleus.
Nucleons (protons or neutrons) are the constituents of the nucleus of atoms. The exploration of their internal structure is traditionally done by determining "form factors". These quantities are accessible through the study of elastic electron-proton scattering and electron-positron annihilation reactions into proton-antiproton (or the time reverse reaction of proton-antiproton annihilation into electron-positron). They define the distribution of charge and magnetic moments inside the nucleon, induced by the quarks and gluons that are present in the nucleon. Ten years ago, a theoretical model giving an original vision of these distributions was proposed by a collaboration involving a physicist from Irfu [1]. Since then, the accumulation of experimental data has reinforced the validity of this model, which is able to give a picture of the proton and a scenario for the formation of hadronic matter with a spatio-temporal resolution never achieved before. We have access to phenomena on a spatial scale of a hundredth of a femtometer and a temporal scale of the order of 10-25 s, which is 100 times less than the time the light takes to cross a proton. These results are the subject of a recent publication in the journal Physical Review C [2].
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Dernière mise à jour : Feb 19th, 2024
