Division of accelerators, cryogenics and magnetism

The hidden mysteries of exotic nuclei

In atomic nuclei, the interactions between nucleons are at the origin of the "nuclear structure" and phenomena that depend, in part, on the asymmetry of the numbers of protons and neutrons. The nuclear structure, as we know it today, has been established mainly by the study of stable nuclei. The "exotic" nuclei differ from stable nuclei by their neutron-proton asymmetry and / or their low binding energy, therefore they give us new study conditions. In a similar way to atomic physics, nucleons (protons or neutrons) in the nucleus occupy ordered orbitals. This arrangement of orbitals is not universal for all nuclei. Differences have already been observed between stable and exotic nuclei, which reflect different correlations between nucleons. These differences are enhanced approaching the driplines (the limits of existence for nuclei with very unusual proton / neutron asymmetries) this is the case of neutron skins and halos with "di-neutron" correlations.

Exotic nuclei raise fundamental questions:
• Can we come up with a universal description of the nuclear structure?
• What is the origin of the evolution of the shell structure of nuclei?
• How do the so-called "di-neutron" correlations appear in Borromeean nuclei?
• What is the role of the underlying structure of nucleons in nuclear structure?

Data campaigns 2014/2015 of the French-Japanese collaboration

An IRFU team of DACM, DEDIP, DIS and DPhN technicians, engineers and researchers built the MINOS program to answer these questions. A physics program has been established in collaboration with the RIKEN Nishina Center. Today RIKEN with the Radioactive Isotope Beam Factory (RIBF) is the world's most efficient accelerator for producing neutron-rich nuclei at intermediate energies of several hundred MeV.

The first physics issues that the collaboration tackled are:

• the evolution mechanisms of the nuclear shell structure for the neutron-rich nuclei (campaigns of experiments in May 2014 and in May 2015),
• the origin of the correlations of "di-neutron" type in the halo nuclei ¹¹Li, ¹⁴Be and ¹⁷B (December 2014)
• the role played by the three-nucleon interactions in the nuclear structrue of the oxygen isotopes in particular for the unbound ²⁸O (November 2015).

Minos, an instrument dedicated to the spectroscopy of the most unstable nuclei 

see the Minos page project for detailed information about the detection principle and performances.

Magic numbers of the shell model under question

The SEASTAR (Shell Evolution And Search for Two-plus energies at the RIBF) collaboration between Irfu and RIKEN Nishina Center first objectives were to perform the first gamma spectroscopy of the most exotic nuclei that can be reached today, based on on the operation of MINOS coupled to the DALI2 photon detector at the RIBF. The collaboration has gathered more than 100 physicists worldwide. The program of SEASTAR with MINOS is illustrated in the figure.

The first result obtained with MINOS was published in  Physical Review Letters [2].

It deals with the spectroscopy of ⁶⁶Cr, ^70,72Fe. The experiment has shown that the neutron-rich Cr and Fe isotopes have very low energy first excited levels whose excitation energy varies little from one isotope to another. This plateau is interpreted in the shell model as a combined effect of deformation and pairing correlations. It raises the question of the robustness of the nuclear magic number N = 50 for the isotopes of Cr and Fe, in the region of ⁷⁸Ni, expected as a doubly magic nucleus within the shell model.