Evolution of shapes and collectivity in exotic nuclei

New experimental techniques and progress in the production of radioactive ion beams have greatly contributed to our understanding of short-lived, exotic nuclei and their fundamental properties. Experimental data showed that shell gaps and magic numbers disappear in exotic nuclei, while new ones appear. This so-called shell evolution is related to certain properties of the strong nuclear force, which affect the energies of proton and neutron orbitals with increasing neutron-to-proton ratios. While our knowledge on how the shell structure develops for exotic nuclei has been studied for the last two decades, the consequences of the shell evolution on all the features of their nuclear structure such as collectivity, nuclear shape changes, and coexistence are unknown to a great extent. Near shell closures particle-hole excitations of nucleons across the major shell gaps drive the nucleus to either prolate (like an American football) or oblate (like a pumpkin) from its spherical shape. If these deformed structures are similar in energy to spherical ones, different shapes will coexist. For exotic nuclei, the particle-hole excitations will be enhanced since major shell gaps are substantially reduced due to shell evolution. An onset of deformation and shape coexistence is expected in the 78Ni mass region as a result of a reduction of the proton Z=28 shell gap but such information is almost non-existing at present. The research project will provide valuable experimental information through gamma-ray spectroscopy by measuring lifetimes of excited states ranging from few picoseconds to several microseconds via recoil distance Doppler shift and fast timing methods. Experiments will be performed within international collaborations at major international research facilities RIKEN (Japan) and LNL (Italy). The described experimental study will combine measurements of both very short and long lifetimes of the most exotic nuclei one can reach today.

The present project will look for consequences of the shell evolution on the properties of neutron-rich nuclei, with a focus on the development of deformation and shape coexistence. EVOLUTION will focus on the 78Ni mass region where the experimental information on nuclear shapes and deformation is almost non-existing at present. The project will provide valuable experimental information through gamma-ray spectroscopy by measuring lifetimes of excited states ranging from few picoseconds to several microseconds via recoil distance Doppler shift and fast timing methods, respectively. Multi-nucleon transfer reactions and beta-decay processes will be exploited in combination with high resolution fragment separators coupled to an array of high efficienct gamma-ray detectors. The described experimental study will combine measurements of both very short and long lifetimes of the most exotic nuclei one can reach today. One of the major challenges for nuclear theory is to predict the properties of all nuclei within a single framework. With newly obtained spectroscopic data in the exotic region of 78Ni, EVOLUTION will provide a good testing ground for the predictive power of nuclear theory. Furthermore, knowledge of the shell structure in this mass region will help to improve mass models, which is important since the pathway of the astrophysical rapid neutron capture process, essential to understanding the origin of the elements in the Universe, passes through the 78Ni region. Two experiments will be performed in two large-scale international beam factories: The radioactive ion beam laboratory RIKEN (Japan) and LNL-Legnaro National Laboratories (Italy). They will target to cover the physics of exotic nuclei from lighter to heavier exotic nuclei in the 78Ni region.