Our physical world is governed by four fundamental forces: gravitation, electromagnetism, the weak interaction responsible for radioactive decay, and the strong interaction holding together subatomic particles. The strong nuclear force, derived from the strong interaction, binds nucleons (protons and neutrons) together within atomic nuclei. It is responsible for complex quantum phenomena and the formation of atoms, from lightest to heaviest, in stars.
Certain nuclei with specific numbers of neutrons and protons are particularly stable. Nuclear physicists refer to them as 'magic number nuclei'. Understanding the mechanisms responsible for this relative stability and providing a universal description of atomic nuclei remains a challenge for modern theories.
EU framework program and one-of-a-kind Japanese accelerator facility
In order to address this challenge, a new device called MINOS has been developed to measure the spectroscopic properties (i.e. energy levels) of unstable nuclei. This new system is operational since 2014 at the RIKEN Nishina Center's Radioactive Isotope Beam Factory (RIBF), the most advanced facility in the world for producing neutron-rich nuclei and studying previously inaccessible nuclei.
Unraveling the mystery of magic numbers
After 5 years of technical development at CEA and analysis of the first experimental campaign results, the Franco-Japanese research team has just published a first set of results. The first experiment enabled the study of the most neutron-rich chromium and iron nuclei accessible to date. The initial results published in Physical Review Letters question the 'magic' character of N=50 (number of neutrons) for neutron-rich nuclei in this region. MINOS will continue to be used for other experiments, and a wealth of new results is expected. In particular, these experiments should contribute to solving the mystery of magic numbers for unstable nuclei by improving our understanding and modeling of the atomic nucleus.
French participation
Designed and developed at CEA's Institute for Research on the Fundamental Laws of the Universe (IRFU), MINOS is funded by the European Research Council (ERC) as part of the EU Framework Program for Research and Innovation. The European Research Council is the first pan-European funding agency for frontier research. Created in 2007, it is one of the cornerstones of the 'Horizon 2020' EU framework program.
About the nuclear shell model
The nuclear structure inside the nucleus is governed by the nuclear interaction between nucleons and is particularly dependent on the number of protons and neutrons. A nuclear 'shell' model was developed in the 1940s and 50s, based on the study of stable nuclei. In this model, nucleons are arranged according to well-defined energy levels (called 'shells'), in a manner similar to the electron shells in the atomic model.
For certain combinations of numbers of neutrons and protons, a nucleus is particularly stable (as compared to neighboring nuclei) when its shells are completely filled. These so-called 'magic' nuclei are harder to excite.
The conventional shell model based on stable nuclei is compromised when unstable (radioactive) nuclei are considered. This is because the magic numbers of protons and neutrons are not the same for stable and unstable nuclei.