Study of shape evolution in the neutron-rich osmium isotopes with the advanced gamma-tracking array AGATA

This thesis describes the major results of a γ-ray spectroscopy experiment aiming at the investigation of the shape evolution in the neutron-rich even-even osmium isotopes. The first in-beam γ-ray spectroscopy measurement of 196 Os has been performed and the results are compared to state-of-the-art beyond mean-field calculations, that have revealed its presumably γ-soft character. Finite many-body systems, such as molecules, many man-made nano materials and atomic nuclei can be understood as a drop of viscous liquids confined by an elastic membrane, where its equilibrium shape (ground state) has a unique feature: the existence of deformed (non-spherical) shapes. Non-spherical shapes represent spontaneous symmetry breaking. However, the shape of the nucleus is not a direct observable. By comparing its low-lying excited states to predictions of nuclear models, the shape of the nucleus can anyway be deduced, as it will be shown in this work. Nuclear deformation is caused by the subtle interplay between single-particle and collective degrees-of-freedom. Hence, studies of nuclear deformation open a window on the underlying NN interaction and its role in the microscopic origin of deformation. In particular, regions of the nuclear chart where oblate deformation is observed are of special interest due the paucity of oblate deformed nuclei in nature. For the neutron-rich osmium isotopes, a shape transition from prolate to oblate deformation with increasing neutron number is predicted. Experimentally, the yrast band of 194Os was interpreted to be prolate (rugby-ball) deformed, instead the low-lying excitations of 198Os were suggested to be characteristic of an oblate (pumpkin) shape. The even-even nucleus in between, 196Os, was investigated in the eighties and two excited levels were proposed. The uncertainties in the energy and in the spin assignment of the 2nd level prevented a clear comparison with nuclear models. With the current stable beam and target combinations the exotic nucleus 196 Os can not be populated in fusion-evaporation reactions. It is reachable via fragmentation reactions or multi-nucleon transfer reactions. 196Os was produced via fragmentation reactions in the large-scale radioactive ion beam facility GSI, Germany. However, in the isomeric decay spectroscopy no γ-rays were observed. Multi-nucleon transfer reactions are complementary to fragmentation reactions and were performed using the most powerful γ-detector array, Gammasphere, in Lawrence Berkeley and Argonne National laboratories, USA. However, the experiment setup was not sensitive enough to identify 96 Os among the reaction products. In order to study the structure of 196Os by means of γ-ray spectroscopy, a multi-nucleon transfer experiment was performed at the Laboratori Nazionali di Legnaro (Italy) using the new-generation Advanced Gamma Tracking Array (AGATA) demonstrator in coincidence with the magnetic spectrometer PRISMA using a 82Se beam with an energy of 426 MeV on a 198Pt target. The very selective binary partner method was applied, i.e. the lighter beam-like recoil was identified in the magnetic spectrometer and the γ rays of the corresponding heavier target-like recoil were detected in coincidence with it. The AGATA demonstrator is a γ-tracking array which relies on Pulse Shape Analysis (PSA) for the identification of the interaction points of the γ ray, which is crucial for the reconstruction of its interaction path through the detector. Being one of the first experiments performed with the AGATA+PRISMA setup, the procedure for presorting, calibration and analysis of the data will be reported in detail. The information of the target-like recoil (such as 196Os) is reconstructed via the reaction kinematics. Due to the evaporation of neutrons a tight gate on the reconstructed Q value has to be applied in order to select the true binary partner. In the γ-ray spectrum in coincidence with 196Os, three γ rays are identified (324.4 keV, 467.0 keV and 639.2 keV) that are assigned to originate from the decay of the three lowest-lying 2+ , 4+ and 6+ states.The E(4+))/E(2+) ratio extracted from the new data is 2.44, deviating less than 3% from the expectation value of a γ-soft/triaxial rotor (2.5). The new excited states of 196 Os are compared, as well as the known levels in the even-even osmium isotopes 188−198Os, to state-of-the-art beyond mean field (BMF) calculations. The quality of the results allows a detailed analysis of the shape evolution from a predominantly prolate rotational in 188Os to a more vibrational one in 198Os. The new experimental data obtained for 196Os, reveals an almost perfect γ-soft/triaxial rotor yrast-band, which is in perfect agreement with the calculations. A great variety of nuclei were produced in the 82Se + 198Pt reaction. For example, the yrast band of 200Pt is extended to higher spin and the shape evolution of the even-even platinum isotopes 190−200Pt is compared to calculations based on the same nuclear model.The shape evolution through the platinum isotopes is less rapid than the one of the osmium isotopes, with a potential which remains γ-soft for all these nuclei. In particular, the yrast band of 200Pt has the most vibrational character among the investigated isotopes. For the beam like isotopes new data were also obtained and the yrast band of the even-even zinc isotopes 72−76Zn, populated in the 4 proton transfer channel, is reported. Previous to the AGATA spectrometer, large-scale γ-ray arrays have collimators and anti-Compton shields, in order to reach a good peak-to-total ratio. The conceptual design of the AGATA is substantially different and no such passive material exists in the setup. Hence, the sensitivity for the measurement of the decay of isomeric states belonging to the target-like recoils implanted in the target chamber or in the DANTE array, is higher than that of traditional γ-ray arrays. Through the measurement of the beam-like recoils in the PRISMA spectrometer it is possible to observe in coincidence the γ-rays belonging to the implanted binary partner and to discover new isomeric states. In this thesis the first lifetime measurement of isomeric states with AGATA will be reported. In total 44 isomeric states are identified, including 3 previously unknown