Studio delle distribuzioni isotopiche dei frammenti prodotti da reazioni di fissione indotta da fusione e di quasi-fissione da U-238 in cinematica inversa

Since its discovery, fission appeared as a complex process where different nuclear properties interplay and shape the characteristics of the emerging fission fragment distributions. This process involves an extreme deformation of the fissioning system due to the collective motions of the nucleons and, at the same time, the identity of the fragments is strongly affected by the shell closures. In general, the influence of the shell effects is expected to decrease with increasing excitation energy of the fissioning nucleus. However, in recent experiments some features associated with shell effects were observed to survive at high excitation energy. Therefore, fission fragment distributions are crucial observables for modeling the fission process. However, the access to such observables was, until recently, limited to the mass information, where the contribution of protons and neutrons are mixed. Fusion-induced fission in inverse kinematics has proved to be a powerful tool to investigate nuclear fission, giving access to a full isotopic identification of the fission fragments and bypassing the limitations of the direct kinematics approach which, instead, only provides a partial information. This thesis presents a study of the fusion-induced fission of the high-excited 265Db system (E∗ = 62 MeV) and the competition with the quasi-fission mechanism. The experiment was performed at GANIL accelerating a 238U beam at 5.9 MeV/u onto a 27Al target. The inverse kinematics technique and the capabilities of the VAMOS spectrometer allowed the complete isotopic identification of the fission fragments. The information on both atomic and mass numbers is crucial to obtain observables sensitive to the structure effects, such as nuclear-charge-yield distribution, mass-yield distribution, the neutron excess, the total neutron evaporation. In addition, important correlation matrices, such as mass-angle and nuclear-charge-angle istributions, are derived. The latter give hints on the contribution of dynamical processes as quasi-fission mechanism. The nuclear-charge and mass-yield distributions of the studied system display a Gaussian-like shape, typical of a fission process only driven by liquid-drop properties. An overproduction in the heavy side, which is not compensated in the light-partner side, is observed in both distributions. This effect is ascribed to the quasi-fission process. A comparison of the experimental neutron excess and total neutron evaporation with GEF code predictions was also performed.