Pygmy Dipole Strength and Neutron Skins in Exotic Nuclei

Dipole strength distributions were determined for the neutron-rich nuclei Sn129-132 and Sb-133,Sb-134 from electromagnetic excitation in an experiment using the FRS-LAND setup. For all nuclei, a sizeable fraction of "pygmy" dipole strength at excitation energies well below the giant dipole resonance was observed. The integrated low-lying dipole strength of the nuclei with low neutron separation energies can be compared to results for stable nuclei (e.g. N=82 isotopes) determined for the energy regime of 5-9 MeV. A clear increase of the dipole strength with increasing asymmetry of the nuclei is observed. Comparing the ratio of the low-lying dipole over the giant dipole strength to recent relativistic mean field calculations, values for the parameters a(4) and p(o) of the symmetry energy and for the neutron skin thickness are derived. Averaged over Sn-130 and Sn-132 we extract (a) over bar (4) = 31.8 +/- 1.3 MeV and (p) over bar (0) = 2.2 +/- 0.5 MeV/fm(3). The neutron skin sizes are determined to R-n-R-p = 0.23 +/- 10.03 fm, and 0.24 +/- 0.03 fm for Sn-130 and Sn-132, respectively. For Pb-208 a neutron skin thickness of R-n-R-p = 0.18 +/- 0.035 fm follows, when applying the same method and using earlier published experimental findings on the dipole strength.