Nuclear shapes and resonances

The atomic nucleus is a many-body quantum system that is bound together by the strong nuclear force. Many properties of nuclei can be understood by describing them as electrically charged droplets of nuclear matter. Other properties indicate that nuclei have a microscopic structure in which the protons and neutrons occupy shells that are characterized by specific quantum numbers. Both liquid-drop properties and the shell structure govern the shape of a nucleus: nuclei with closed proton and neutron shells are generally spherical, whereas nuclei with open, partially filled shells tend to deform and assume ellipsoidal shapes. It is found that different shapes can compete and coexist in some nuclei, a quantum mechanical phenomenon known as shape coexistence. If a nucleus is excited in a nuclear reaction, its response depends crucially on its shape, similar to how the shape of a bell determines its sound with characteristic frequencies and overtones. Such characteristic modes of excitation are called resonances. Their properties are important to understand the forces between the protons and neutrons in the nucleus, and to determine the likelihood of reactions between different nuclei. Obtaining information about nuclear shapes and the properties of nuclear resonances is therefore important to understand the processes in which atomic nuclei are produced and transformed in violent astrophysical processes, for example, when a star explodes in a supernova or when two neutron stars collide. Experiments at GANIL (France) have provided new information on how deformation evolves in the chain of neutron-rich zirconium isotopes from spherical to elongated shape as the neutron number increases, with shape coexistence occurring in some zirconium isotopes. The studies of nuclear deformation are continuing with measurements for neutron-rich ruthenium nuclei, which exhibit shapes without axial symmetry. Experiments at iThemba Labs (South Africa) resulted in new information on resonances in nickel isotopes and provided evidence for an enhanced probability to emit gamma rays of low energy. Similar experiments focusing on tin and holmium nuclei were performed at the Oslo Cyclotron Laboratory using the OSCAR detector infrastructure, with more experiments under way. Experimental results from these studies were used to benchmark theoretical models. Effort was also devoted to further develop experimental methods to extract information on nuclear shapes and resonances. The project has provided opportunities for young researchers to conduct cutting-edge research at an international level and continues to do so.

The research project addresses some of the key questions of modern nuclear physics: How does the structure of certain nuclei influence the synthesis of elements in the universe? How do resonances in nuclei affect reaction rates? How does the structure and shape of nuclei evolve across the nuclear chart toward more exotic nuclei? The research project addresses specific cases in different regions across the nuclear chart, ranging from the synthesis of 12C through the triple- process to the structure, neutron-capture cross sections, and fission properties of heavy actinides, and including nuclear structure studies for medium-heavy nuclei on both the neutron-deficient and neutron-rich side of the valley of stability. Experiments will be performed both locally with the Oslo Scintillator Array (OSCAR) at the Oslo Cyclotron Laboratory (OCL), and at international facilities abroad. The performance of OSCAR in terms of energy and time resolution, efficiency, and data throughput is far superior compared to the previous detector setup at OCL. The nuclear level density and photon strength function will be measured for a wide range of nuclei. These quantities are key inputs in cross section calculations for nuclear reactions. New data will constrain reaction rates for nucleosynthesis in stellar environments and nuclear reactors. Similarly, information on how nuclear shapes evolve across the nuclear chart and new results on shape coexistence will be used to benchmark, and ultimately refine, models for nuclear structure calculations. Experiments measuring prompt fission gamma rays have the potential to improve our understanding of fission and the competition between gamma and neutron emission within this process. The project will allow recruiting young doctoral and postdoctoral researchers, who will be given the opportunity to conduct research at the forefront of nuclear science within international collaborations.