Nuclear Structure far from Stability

P. Ring

Physics Department, Technical University Munich, D-85748 Garching, Germany

Nuclei far from the valley of stability play an important role in our understanding of astrophysical processes for the formation of heavy elements. It is therefore a challenge for theoretical investigations to provide models with a high predictive power. In the region of very light nuclei, essential progress has been made in solving the many-body problem ab initio using forces derived from the nucleon-nucleon scattering data. For the large majority of nuclei, however, in particular those interesting for astrophysical processes, the most successful theoretical techniques are still microscopic models based on the mean field approximation. These models are in principle density functional theories, where the functional is parameterised by a phenomenological ansatz. They are very successful, because being effective theories they include in a global way a large number of important effects going beyond the mean field, such as Brueckner correlations, ground state correlations, vacuum polarization, exchange terms etc.

After thirty years of considerable progress with non-relativistic models connected with the names of Skyrme or Gogny relativistic models gain more and more interest in recent years. It is not relativistic kinematics, which plays an important role at such low energies, but it is the relativistic structure of the mean field equation, which contains large attractive and repulsive fields, which cancel for the large components, but which add up for the small components, leading in this way to a large spin-orbit term. Relativistic invariance lead to nuclear saturation, a large spin-orbit interaction, time-reversal breaking mean fields, properties which have to be adjusted in the non-relativistic models separately. Very essential is a density dependence in form of non-linear couplings between the mesons. A quantitative description of properties of ground-states and excited states in nuclei far from stability, which are characterized by the closeness of the Fermi surface to the particle continuum, necessitates a unified description of mean-field and pairing correlations, as for example in the framework of the Hartree-Fock-Bogoliubov (HFB) theory.

We discuss recent applications of Relativistic Mean Field (RMF) and Relativistic Hartree-Bogoliubov (RHB) theory for structure phenomena at the limits of stability, such as the quenching of the spin-orbit splitting far from the valley of stability, halo phenomena at the neutron drip-line, proton-radioactivity, rotational bands at superdeformed shapes and collective vibrations with low-lying dipole strength for nuclei with large neutron excess.