Nuclear symmetry energy and neutron skins derived from pygmy dipole resonances

By exploiting Coulomb dissociation of high-energy radioactive beams of the neutron-rich nuclei ^129-132Sn and ^133,134Sb, their dipole-strength distributions have been measured. A sizable fraction of “pygmy” dipole strength, energetically located below the giant dipole resonance, is observed in all of these nuclei. A comparison with available pygmy resonance data in stable nuclei (²⁰⁸Pb and N=82 isotones) indicates a trend of strength increasing with the proton-to-neutron asymmetry. On theoretical grounds, employing the RQRPA approach, a one-to-one correlation is found between the pygmy strength and parameters describing the density dependence of the nuclear symmetry energy, and in turn with the thicknesses of the neutron skins. On this basis, by using the experimental pygmy strength, parameters of the nuclear symmetry energy (a₄=32.0±1.8 MeV and p_o=2.3±0.8 MeV/fm³) are deduced as well as neutron-skin thicknesses R_n-R_p of 0.24±0.04 fm for ¹³²Sn and of 0.18±0.035 fm for ²⁰⁸Pb, both doubly magic nuclei. Astrophysical implications with regard to neutron stars are briefly addressed.