First measurement of the isoscalar excitation above the neutron emission threshold of the Pygmy Dipole Resonance in Ni-68

The excitation of the Pygmy Dipole Resonance (PDR) in the Ni-68 nucleus, above the neutron emission threshold, via an isoscalar probe has been observed for the first time. The excitation has been produced in reactions where a Ni-68 beam, obtained by the fragmentation of a Zn-70 primary beam at INFN-LNS, impinged on a C-12 target. The.-ray decay was detected using the CsI(Tl) detectors of the CHIMERA multidetector sphere. The Ni-68 isotope as well as other heavy ion fragments were detected using the FARCOS array. The population of the PDR was evidenced by comparing the detected.-ray energy spectra with statistical code calculations. The isotopic resolution of the detection system allows also to directly compare neutron decay channels with the Ni-68 channel, better evidencing the PDR decay response function. This comparison allows also the extraction of the PDR cross section and the relative gamma-ray angular distribution. The measured gamma-ray angular distribution confirms the E1 character of the transition. The lambda decay cross section for the excitation of the PDR was measured to be 0.32 mb with a 18% of statistical error. (C) 2018 The Authors. Published by Elsevier B.V.