Testing new nuclear structure concepts. A quantitative understanding of nuclear structure is important to problems ranging from the origin of the elements to the use of nuclei as laboratories for probing new interactions. The nuclear many-body problem—strongly interacting, with two kinds of particles (protons and neutrons), and with competing effects due to short-range multiple scattering and long-range collectivity—is also of broad intrinsic interest. The phenomena that arise—shell structure, pairing, superfluidity, collective motion and its connections with many-body symmetries, and spectral transitions from order to chaos—and the methods that nuclear physicists employ are also fundamental to fields such as atomic and condensed-matter physics and quantum chemistry. Nuclear structure theory has made significant progress in recent years by adapting numerical techniques for high-performance computing and through conceptual advances such as effective field theory and improved density functionals. However, the reexamination of old paradigms and subsequent development and validation of new nuclear models require data. This is a role for a FRIB: to test the predictive power of models by extending experiments to new regions of mass and proton-to-neutron ratio and to identify new phenomena that will challenge existing many-body theory. A FRIB’s rare-isotope beams of unprecedented intensity and its sophisticated detector arrays would allow experimentalists to explore the limits of nuclear stability. A FRIB’s technological developments would allow nuclear physicists, for the first time, to study nuclei that previously could be found only in the billion-degree explosions of distant supernovae.
Production and properties of superheavy nuclei. Theory predicts that super-heavy nuclei that do not exist anywhere else in the universe can be assembled. The nuclei would contain in excess of 120 protons; hence their stored Coulomb energy would be huge. However, with a large number of excess neutrons and an appropriate geometry, the attractive nuclear force could allow such a unique system to exist for times exceeding a day. The synthesis of such nuclei and their proper identification constitute an experimental challenge, but an advanced exotic-beam facility such as a FRIB is required if any meaningful search is to be carried out. These superheavy systems will provide great insight into the nuclear reactions and structure and, if they possess sufficient lifetimes, may reveal unusual chemical properties.
Probing neutron skins. Very-neutron-rich nuclei that can be reached by a FRIB offer the only laboratory access to matter made of pure neutrons. The