PYGMY STRUCTURES IN 208PB AND 90ZR STUDIED WITH THE INELASTIC SCATTERING OF 17O AT 20 MEV/U

Over the last years, the evolution of nuclear structure as a function of the N/Z ratio has become a key question in nuclear physics. Special attention has been given to the vibrational modes of dipole nature: nuclei with a neutron excess show an increase of dipole strength close to the neutron separation energy, commonly associated to a new type of collective motion called the Pygmy Dipole Resonance (PDR). Besides being intrinsically interesting as new structure phenomena, the pygmy resonance is also expected to play an important role in nuclear astrophysics, for the effect it has on neutron capture rates along the r-process path. From the experimental point of view, the PDR has been investigated systematically in a large number of stable nuclei with the photon scattering technique, but to gain more insight into its structure different experiments, with different probes, are needed. For this reason, an experiment was performed in June 2010 at Legnaro National Laboratories, aimed at the study of highly excited states in the target nuclei, including the pygmy region and up to the giant resonance region. The resonance states were populated by the inelastic scattering of a 17O beam at the energy of 20 MeV/u, and their subsequent gamma decay was measured with the AGATA (Advanced GAmma-ray Tracking Array) Demonstrator, the new generation HPGe array based on the techniques of pulse shape analysis and gamma-ray tracking. The Demonstrator was coupled to an array of 3 large volume LaBr3:Ce scintillators to increase the total efficiency; the remaining solid angle was covered by 20 BaF2 scintillators. Two segmented ΔE-E Si telescopes were used to identify of the scattered beam ions and to measure the excitation energy transferred to the target nucleus. The experiment measured the gamma decay from two targets: the doubly magic 208Pb, which had been previously measured with the same technique but a worse energy resolution, and the semimagic 90Zr. This thesis work details the analysis performed on this experiment and presents the first preliminary results on the measurements of the PDR in 208Pb and 90Zr with the inelastic scattering of 17O at 20 MeV/u.