Study of nuclear structure in the region of 78Ni via fusion-fission and Coulomb-excitation experiments

This thesis explores the nuclear structure of neutron-rich nuclei near doubly-magic 78Ni, where the effect of different components of the nuclear interaction can modify the energy of the single-particle levels. Such shell evolution may weaken traditional magic numbers and favor the appearance of nuclear deformation at low energy competing with spherical closed-shell configurations. This work aims at studying nuclear structure in the region of 78Ni using two different techniques. In the first experiment described in the thesis, a 208Pb+9Be fusion-fission reaction in inverse kinematics at the AGATA+PRISMA setup at LNL (Italy) was used to perform in-beam γ-ray spectroscopy on the neutron-rich fission fragments close to 78Ni, in order to study the evolution of the N = 50 shell gap. A study on fission dynamics was also performed by reconstructing the isotopic fission yields from the 217Rn compound nucleus, in order to study the effect of shell structure on the fission process. The fragments were identified and measured by the PRISMA magnetic spectrometer, and the γ rays emitted in the de-excitation of the fragments were detected by the AGATA γ-ray tracking array. The measured fission fragment distributions were compared with phenomenological calculations from the GEF code and the predicted isotopic yield matched well with experimental data. However, the experimental data seemed to suggest a higher pre-scission neutron evaporation than predicted, as well as some possible structure effects that determined a reduction of the average neutron excess at Z ∼ 35. The Doppler- corrected γ-ray spectra from AGATA were studied for N = 50 isotones down to 82Ge and for Zn isotopes as neutron-rich as 78Zn. Several unreported transitions were observed in odd Zn isotopes. The second experiment performed to study nuclear structure close to 78Ni was the measurement of the Coulomb excitation of 79Zn, in order to investigate the collectivity of its intruder state band, built on a long-lived isomer. The radioactive 79Zn beam, populated partially in the isomeric and partially in the ground state, was produced at the ISOLDE facility at CERN (Switzerland) and directed onto a 208Pb and a 196Pt target. The Coulomb excitation scattering partners were measured with the CD DSSSD detector and the de-excitation γ rays were detected with the Miniball HPGe array. The population of several excited states from the ground and isomeric state was observed, and the lifetime of the intruder 5/2+1 state was measured with a kind of RDDS technique. The electromagnetic matrix elements of the involved transitions were extracted by fitting the γ-ray yields with the Coulomb excitation code GOSIA. The results were compared with shell model calculations with a modified GWB interaction and seemed to match quite well with the measured matrix elements. Assuming the rigid rotor model, the β deformation extracted for the intruder states seems to be incompatible with the value that was estimated from the charge radius obtained in a previous laser spectroscopy measurement. This might suggest that the quadrupole deformation in this intruder structure is not as large as it was previously suggested, but the enhanced mean squared charge radius from laser spectroscopy could be related to larger spherical orbits in the wavefunctions of the intruder states.