Beyond the nuclear shell model

© 2007 American Institute of Physics, S-0031-9228-0711-030-9 Nuclei, the fuel that burns in stars, make up 99.9% of all baryonic matter in the universe.1 The complex nature of the nuclear forces among protons and neutrons generates a broad range and diversity of nuclear phenomena. Developing a comprehensive description of nuclei and their reactions represents one of the great intellectual opportunities for physics. As nuclear physicists have seen during the past 10 years, success will require theoretical and experimental investigations of isotopes with unusual neutron-to-proton ratios. Such nuclei, which are typically not found on Earth, are called exotic or rare. Exotic nuclei, particularly those that have extremely short lifetimes, are difficult to produce experimentally. National user facilities at Argonne and Oak Ridge national laboratories and at Michigan State University, along with other university and national laboratories, are paving the way to exciting initial discoveries. And a new generation of rareisotope facilities is now coming into service to meet the challenge. Notable among them are the Rare Isotope Beam Factory at Japan’s RIKEN research institute, which began operation in November 2006, and the Facility for Antiproton and Ion Research, which is under construction at GSI, the heavy-ion research facility in Darmstadt, Germany. Isotope separation techniques continue to be developed as part of the SPIRAL2 project at France’s GANIL laboratory and at TRIUMF in Canada. This new generation, along with the proposed Facility for Rare-Isotope Beams (FRIB) in the US,2 holds the key to unlocking the mysteries of nuclei and cosmic nuclear production. Several questions that have come into focus during the past few years illustrate the issues now being addressed in the field of nuclear physics: What is the nature of the nuclear force that binds protons and neutrons into stable nuclei and rare isotopes? What is the origin of the many simple patterns that emerge in studies of heavy nuclei? What is the nature of neutron stars and dense nuclear matter? What is the origin of the elements in the cosmos? What are the nuclear reactions that drive stars and stellar explosions? Primary aspects of answering the first two questions are to test the predictive power of models by extending experiments to new regions of mass and neutron-to-proton ratio and to identify new phenomena that challenge existing many-body theory. On the theoretical side, new and powerful conceptual, analytic, algorithmic, and computational tools are enabling scientists to peer into the inner workings of nuclei with far greater precision than was previously possible. Those tools engender a clear vision to move from a qualitative to a quantitative and comprehensive understanding of all nuclei.