Shell Evolution and Structure of Two-Neutron Halo in Exotic Nuclei

Shell evolution in neutron-rich carbon isotopes is studied with repulsive contributions from three-nucleon forces among valence neutrons. Three-nucleon forces are shown to determine the dripline and improve the energy levels of the isotopes. A three-body model with low-energy neutron-neutron interaction is used to study structure of two-neutron halos of exotic nuclei. Observed halo radii in C-22 and F-29 are found to be well explained by the three-body model. A possible existence of an excited 2+ state in the halo of F-29 and its consequences are discussed. The halo in B-17 is found to be d-wave dominant, while the halo in B-19 has s-wave component with about 40% probability.