Pygmy dipole resonance in 130,132Sn isotopes and the neutron skin thickness

The pygmy dipole resonance (PDR) manifests itself as a concentration of dipole strength near the neutron separation threshold below the giant dipole resonance (GDR) domain. It is related to structural changes in nuclei with a large neutron excess giving rise to a neutron skin. According to theoretical calculations, a very precise measurement of the neutron skin thickness in heavy doubly magic nuclei like Pb or Sn would help in constraining the neutron matter equation of state [1, 2]. At present, the neutron matter radius in nuclei as a fundamental ground-state property cannot be approached experimentally in a straightforward way and its extraction from experimental data involves a certain model dependence [3]. Existing results on the neutron skin thickness are limited to stable nuclei. As suggested in [4], the pygmy dipole strength modes provides insight into the skin thickness as both quantities are strongly correlated with the symmetry energy.