Nuclei far from Stability  Nuclear Structure under Extreme  Nuclear Structure under Extreme

Introduction

The atomic nucleus is a finite quantal system made up of strongly interacting fermions. Its properties are shaped by the interplay of electromagnetic, weak and strong forces. The study of the nucleus presents many aspects which challenge our understanding, ranging from many-body manifestations of nuclear properties to the forces between nucleons in the nuclear medium and their relationship to the underlying fundamental interactions. Knowledge obtained in modelling the nucleus is useful for the description of other extended quantal systems such as metallic clusters, quantum dots, high-T_c superconductors and Bose-Einstein condensates. The three-body models developed for the description of weakly bound nuclei can be applied to similar problems in atoms, molecules and hypernuclei. In spite of significant progress during the last decade, we are still lacking a precise and complete knowledge of the behaviour of the nucleus. For example, it is currently impossible to predict even the exact limits of stability. In the course of recent studies, new questions have merged about the properties of the nucleus at the limits of excitation energy, angular momentum, isospin and mass. Penetration to unexplored extremes in these quantities is likely to reveal fundamentally new phenomena. The first observations along this way are, the new state of matter associated with the halo nuclei, the surprising breakdown of the established magic numbers and the existence of extremely deformed shapes in nuclei. One major reason for the study of exotic nuclei, i.e. nuclei with extreme values of the proton-to-neutron ratio Z/N, is to provide more basic data, for increasingly unstable systems, that will help to answer these open questions. One excellent example is our new understanding of shell structure based on how shell closures develop as proton and neutron numbers change. Another outstanding achievement, which generated head-lines in the press, is the synthesis of the new elements Z=109-112. There is now a real prospect of expanding the Periodic Table to even heavier elements. Investigations at the limits of existence, at and even beyond the drip-lines, have revealed new and completely unforeseen structures. Pushing the N/Z ratio to extreme values has resulted in the discovery of halo nuclei and other new exotic nuclei at or near the proton and neutron drip lines. They present interesting problems in themselves and lead to a deeper comprehension of the nucleus in general. The detailed study of halo nuclei and the mapping of much of the proton drip line, are among the first fruits of these developments. The doubly magic ⁷⁸Ni and ¹⁰⁰Sn nuclei have been observed and their detailed study brought within reach once we have higher intensity beams of unstable nuclei. Data on nuclei far from stability are prime ingredients in astrophysical network calculations, especially for high temperature explosive phenomena. Many unstable nuclei drive astrophysical stellar scenarios. Such nuclei, which do not exist on Earth, are now becoming accessible at radioactive nuclear beam facilities. At the extremes of excitation energy and angular momentum nuclear structure studies are probing nuclear shapes and their evolution, the influence of the thermal environment on low modes of excitation and giant modes of excitation. The most conspicuous findings have concerned superdeformed bands and the spectroscopy of strongly deformed shapes. Small systematic shifts in the energy levels in some of these bands have revealed new symmetries in the rapidly rotating quantal system. Equally surprising was the observation of superdeformed rotational bands with almost identical level spacings in neighbouring nuclei. The dynamics of the decay between states at large deformation and those closer to sphericity, and the possibility of hyperdeformed shapes are among the most intriguing questions in this research. Current theoretical models are stretched to their limits to encompass the wealth of observed new phenomena. One of the strengths of present nuclear theories is the ability to describe simultaneously single particle and collective modes of excitation, whose coexistence at the same excitation energy is one of the most striking and original features of nuclear dynamics. In the new regions of the nuclear chart, mean field theories, large scale shell model descriptions and cluster models are the necessary tools to achieve this goal. Our ability to cope with this new physics relies heavily on improving our knowledge of the effective interaction of the nucleons in the nuclear medium even at low densities. Already, significant progress has been made in developing mean field theories and theoretical approaches which use effective interactions or effective Lagrangians to describe low-energy nuclear states. The rapid growth in computational power has allowed us to calculate the complicated wavefunctions which are needed for a full shell model description of the ground state and low-lying collective states of medium mass nuclei. In even heavier nuclei, ingenious stochastic methods allow the sampling of the very large configuration spaces needed to describe the strength distribution of many kinds of collective modes. Physics research with exotic as well as stable nuclear beams is in an exciting period of evolution. Most of the striking new results have been obtained by scientists using the very attractive facilities presently available in Europe. Innovative developments in ISOL and fragmentation techniques have played an important role in allowing us firstly to observe, and secondly to study, nuclei much further from stability than ever before. Of vital importance has been the continuing innovation in instrumentation, with advances in highly efficient ion sources and accelerators, recoil-separators, traps and storage rings, ultra-sensitive detection of nuclear radiation with high resolving power, fast data acquisition and modern hardware and software for computing. These developments have opened up new frontiers. In studies of high spin excitations, powerful detector arrays, based on Compton suppressed Ge detectors have been in full operation. Coupled with recoil in-flight separators their selectivity and sensitivity will be further enhanced, allowing studies of excited structures of nuclei very far from stability. The more advanced spectrometer EUROBALL, constructed by a broad European collaboration, is poised to produce its first results at the time of writing. In future, the development of gamma-tracking detectors will lead to the construction of gamma-ray spectrometers with even better performance. Over the next decade we expect dramatic improvements in our knowledge and understanding of how nuclei respond as we vary their temperature, subject them to rotational stress, and alter both their mass and their isospin. In this report we wish to outline the main directions and goals of this research effort within the European context.