Low-lying dipole response in the unstable Ni 70 nucleus

The Coulomb excitation of the neutron-rich nucleus Ni70 was measured in inverse kinematics at 260AMeV bombarding energy and with a Au197 target. The beam energy allowed us to study the dipole response around the neutron separation energy (up to about 12 MeV). The experiment was performed at the RIKEN Radioactive Isotope Beam Factory (RIBF). The γ decay from the scattered Ni70 ions at around zero degree was measured with a scintillator detection system composed of large volume LaBr3:Ce detectors (HECTOR+ array) and NaI(Tl) detectors (DALI2 array). Results were obtained for the E1 strength in the region where the dipole response is characterized by the presence of the pygmy dipole resonance. The measured E1 strength is found to be larger as compared with that of Ni68 below the neutron binding energy. The measured E1 strength as a function of energy is compared with available predictions based on relativistic and nonrelativistic approaches, and only in some cases does theory give a reasonable account of the data.