RELATIVISTIC DESCRIPTION OF EXOTIC COLLECTIVE EXCITATION PHENOMENA IN ATOMIC NUCLEI

The low-lying dipole and quadrupole states in neutron rich nuclei are studied within the fully self-consistent relativistic quasiparticle random-phase approximation (RQRPA), formulated in the canonical basis of the Relativistic Hartree{Bogoliubov model (RHB), which is extended to include the density dependent interactions. In heavier nuclei, the low-lying E1 excited state is identied as a pygmy dipole resonance (PDR), i.e. as a collective mode of excess neutrons oscillating against a proton{neutron core. Isotopic dependence of the PDR is characterized by a crossing between the PDR and one-neutron separation energies. Already at moderate proton{neutron asymmetry the PDR peak is calculated above the neutron emission threshold, indicating important implications for the observation of the PDR in ( , 0 ) scattering, and on the theoretical predictions of the radiative neutron capture rates in neutron-rich nuclei. In addition, a novel method is suggested for determining the neutron skin of nuclei, based on measurement of excitation energies of the Gamow{Teller resonance relative to the isobaric analog state.