The pygmy dipole resonance in Ni-68 and the neutron skin

A search of the pygmy resonance in Ni-68 was made using the virtual photon technique. The experiment was carried out using the radioactive beam Ni-68 at 600 A MeV, produced with fragmentation of Kr-86 at 900 A MeV on a Be-9 target. The Ni-68 beam was separated by a fragment separator, and the gamma-rays produced at the interaction with the Au target were detected with the RISING and FRS set-up at the GSI laboratory in Germany, also including the HECTOR array. The measured gamma-ray spectra show a peak centered at approximately 11 MeV, whose intensity can be explained in term of an enhanced strength of the dipole response function (pygmy resonance). A pygmy structure of this type was also predicted by different models for this unstable neutron-rich nucleus. Correlations between the behavior of the nuclear symmetry energy, the neutron skins, and the percentage of energy-weighted sum rule (EWSR) exhausted by the pygmy dipole resonance (PDR) are investigated by using different random phase approximation (RPA) models.