First observation of the Pygmy Dipole Resonance in the nucleus 68Ni using an isoscalar probe above the neutron emission threshold

Abstract

The thesis concerns the investigation of the Pygmy Dipole Resonance (PDR) in the nucleus 68Ni. Understanding the properties of the PDR is a current subject in the nuclear structure. From the study of the PDR properties, indeed it is possible to extract information regarding the neutron-skin of the neutron-rich nuclei and the symmetry energy of the Equation of State of the nuclear matter. Moreover, the PDR might influence also the r-process, responsible of the nucleosynthesis of elements heavier than Fe. Many theoretical approaches are able to describe the PDR and they predict a mixing between the isoscalar and isovector character of PDR. For this reason, various experiments have been carried out below the neutron emission threshold using both probes. The comparison between such experiments shows that only a group of states at low energies is excited by both probes, whereas another group, at higher energies, is populated just with the isovector ones. This observation, called isospin splitting, needs to be better understood. The investigation of the PDR in unstable nuclei has been carried out mainly using isovector probes. We performed at INFN-LNS an experiment aimed to study for the first time the PDR in the 68Ni at 28A MeV using an isoscalar target of 12C. Semiclassical calculations indeed showed that the cross section of the PDR in the 68Ni + 12C at 28A MeV is mainly due to the nuclear contribution. A 70Zn primary beam, accelerated to an energy of 40A MeV using the Superconducting Cyclotron, impinged on a 9Be target to produce the exotic beam delivered via the FRIBs@LNS facility. An appropriated tagging system was used to detect the 68Ni beam. The main goal of the experiment was to study the $\gamma$-decay channel of the PDR, performing coincidence measurements between $\gamma$-rays, the 68Ni beam and 68Ni fragments. The CsI(Tl) scintillators of the CHIMERA multidetector were used to detect the $\gamma$-rays. While, the FARCOS array was used to detect the scattered 68Ni as well as other ions produced in the reaction. The comparison between the $\gamma$-rays energy spectrum in coincidence with the 68Ni channel and the one in coincidence with the 66,67Ni allowed to evaluate the PDR $\gamma$-decay cross section. Moreover, we performed statistical calculations to extract the PDR strength. Such calculations were also useful to verify that the influence of other states, such as the Giant Dipole Resonance and Giant Quadrupole Resonance, in the region of the PDR, is negligible. We evaluated also the $\gamma$-rays angular distribution that shows the typical character of an E1 transition. Finally, we performed a comparison with previous experiments realized with an isovector probe. Such comparison did not show important differences in the PDR energy distribution. Therefore, the isospin splitting seems not to be present for unstable nuclei above the neutron emission threshold. Moreover, the configuration of the experimental set up has provided also preliminary information on the PDR neutron decay channel.