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

Abstract The excitation of the Pygmy Dipole Resonance (PDR) in the 68 Ni 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 68 Ni beam, obtained by the fragmentation of a 70 Zn primary beam at INFN-LNS, impinged on a 12 C target. The γ -ray decay was detected using the CsI(Tl) detectors of the CHIMERA multidetector sphere. The 68 Ni 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 γ -ray energy spectra with statistical code calculations. The isotopic resolution of the detection system allows also to directly compare neutron decay channels with the 68 Ni channel, better evidencing the PDR decay response function. This comparison allows also the extraction of the PDR cross section and the relative γ -ray angular distribution. The measured γ -ray angular distribution confirms the E1 character of the transition. The γ decay cross section for the excitation of the PDR was measured to be 0.32 mb with a 18% of statistical error.