Collective dipole excitations in neutron-rich nuclei from Sn-132 mass region, the nuclear symmetry energy and neutron skins

Dipole strength distributions measured in unstable neutron-rich nuclei in the Sn-132 mass region by utilizing relativistic Coulomb excitation of radioactive beams reveal a sizable fraction of "pygmy" strength below the giant dipole resonance. On the grounds of the RQRPA model, a strong linear correlation between the pygmy strength, parameters of the nuclear symmetry energy and the neutron skin thickness has been found. From the experimentally observed low-lying strength, the symmetry energy pressure p(0) = 2.3 +/- 0.8 MeV/fm(3) and neutron skin thicknesses of 0.23 +/- 0.04 fm Sn-130 and 0.24 +/- 0.04 fm Sn-130 have been extracted.