NUCLEAR DEFORMATION IN FISSION FRAGMENTS STUDIED WITH NOVEL IMPLEMENTATIONS OF DOPPLER SHIFT LIFETIME MEASUREMENT TECHNIQUES

The experimental investigation of the structure of atomic nuclei reveals the presence of different shapes as, for example, spherical or ellipsoidal. The latter can have sizable de- viation (i.e., deformation) with respect to the spherical shape. Nuclear deformation is found especially far from the magic numbers of nuclear stability. The evolution of nuclear shapes in different regions of the nuclear chart is the subject of extensive studies, by means of different experimental and theoretical techniques. In this Thesis, the deformation of neutron-rich nuclei with mass A≈100 has been in- vestigated at medium-high spin (8-10ħ) by measuring the lifetime of excited states to determine transition strengths, from which the size of the deformation can be inferred. Particular focus has been devoted to a novel implementation of the Doppler Shift At- tenuation Method (DSAM) for the measurement of lifetimes of excited states in fission fragments. This method has been applied to the first set of data taken with an active fis- sion target coupled to an array of germanium detectors. The nuclei have been populated via neutron-induced fission on 235 U, dissolved in a liquid scintillator (fission tag via active target). This reaction, combined with a high-resolution gamma detection system, has al- lowed for high-statistics studies, complementary to the those performed, for example, at radioactive ion beam facilities. Thermal neutrons have been delivered by the ILL (Insti- tut Laue-Langevin) nuclear reactor and the FIPPS (FIssion Product Prompt gamma-ray Spectrometer ) instrument has been used for gamma-ray spectroscopy. The active target has allowed to “tag” the fission events, suppressing the gamma rays produced by the fission fragments beta decay. The experimental data have been compared to simulations obtained with a Geant4 Monte Carlo code developed for FIPPS. Different event gener- ators, in particular the one for fission fragments, have been included in the simulation code, as well as the full geometry of the detection system and the gamma decay through complete level schemes. New results have been obtained for the lifetimes of excited states in 97,101 Zr and 100,102 Nb nuclei, together with re-evaluated values for 99,100,101,102 Zr. Those have been compared with the previously reported measurements in the literature, via an accurate evaluation of all the systematic errors. These results have been presented after a detailed evaluation of the performance of the instrument, including a study of angular correlations using a hybrid detector system. The presented technique can be applied to study the structure of other nuclei populated in fission reactions. A new development for a plunger device at a neutron beam has been included in the The- sis work. The aim is to measure lifetimes of the order of picoseconds in fission fragments, exploiting the high statistics at a neutron beam. The conceptual design includes a fission fragment identification setup, which exploits the v − E method, to distinguish among the nuclei produced in the fission reaction. Geant4 simulations have been performed to evaluate the setup characteristics and to validate the design. The system will be used in a campaign with a 252 Cf spontaneous fission source before applying it in neutron-induced fission reactions. The plunger device, coupled to a fission fragment identification system, will be a fundamental tool for future studies of the structure of neutron-rich nuclei via lifetime measurements.