Unit:
Department of PhysicsLibrary of the School of Science
Dissertation committee:
Theo J. Mertzimekis, Associate Professor, Physics Department, National and Kapodistrian University of Athens
Efstathios Styliaris, Associate Professor, Physics Department, National and Kapodistrian University of Athens
Michael Kokkoris, Professor, School of Applied Mathematics and Physical Sciences, National Technical University of Athens
Tassos Lagoyiannis, Senior Researcher, Institute of Nuclear and Particle Physics, NCSR Demokritos
Michael Axiotis, Researcher, Institute of Nuclear and Particle Physics, NCSR Demokritos
Dennis Bonatsos, Director of Research,Institute of Nuclear and Particle Physics, NCSR Demokritos
Nicu Marginean, Senior Researcher I, Horia Hulubei National Institute for Physics and Nuclear Engineering (NIPNE)
Original Title:
Nuclear structure of the neutron-rich nuclei 140Ba and 180Hf
Translated title:
Nuclear structure of the neutron-rich nuclei 140Ba and 180Hf
Summary:
The study of neutron-rich nuclei is one of the most active fields in contemporary nuclear physics. Special interest is focusing on even-even nuclides far from closed shells, some of which are characterized by a deformed shape. This deformation arises from the reordering of the single-particle states in equilibrium, which can eventually lead to new closed shells, different from those predicted by the shell model. The development of large multi-detector arrays along with the availability of radioactive beams produced via fragmentation or spallation reactions has greatly increased our knowledge for nuclei located in the neutron-rich area of the isotopic chart. High-effιcient γ-arrays enhance the amount of information extracted from a nucleus resulting in more precise measurements in a significantly reduced laboratory time. Furthermore, the development of radioactive beam facilities help in probing exotic isotopes in the neutron-rich side of the valley of stability. Instead of radioactive beams, there is a more "traditional" way of populating excited states of a neutron-rich nucleus, especially the low-lying ones. Multi-nucleon transfer reactions are a widely used tool in nuclear structure, and it has been proven successful in populating excited states in neutron-rich isotopes. In this thesis, the two neutron-rich nuclei 180Hf and 140Ba have been studied experimentally. Two PAC-approved experiments were carried out at the 9MV Tandem accelerator laboratory of IFIN-HH, Bucharest, Romania. Both nuclei were formed using multi-nucleon transfer reactions, and the subsequent γ-decay was detected by the ROSPHERE array. The stable neutron-rich nucleus 180Hf is located in a region where both protons and neutrons typically occupy high Ω orbitals located near the Fermi surface. This results in the presence of high K-isomers, which along with the associated rotational bands can provide important information about the underlying single-particle orbitals. In this work, the nucleus 180Hf was populated using the proton pick-up reaction 181Ta(11B,12C)180Hf for the first time, aiming at the observance of possible new K-isomers. An additional aim of the experiment was the measurement of unknown values of spins-parities of the populated levels, as well as the mixing ratios of the γ-transitions depopulating them. Mixing ratio measurements are highly important, as this quantity expresses the degree of partitioning of the multipole components of a mixed transition and consequently serve as a sensitive test for nuclear model calculations. In this work, measurements for spins-parities for the nucleus 180Hf are presented for the ground-state band up to the 1084 keV state, the side-band states of 1370 keV(band 6), 1374 keV (band 2) and the non-band levels of 1821 keV and 1608 keV. Especially for the 1374 keV level, the value measured in this work favors a spin-parity assignment of 4-, which settles the disagreement in the bibliography between the spin-parity values of 3- and 4-. Furthermore, the mixing ratio value for the transition 1374 -> 309 keV, presented in this work, agrees with the previously reported measurement but is more precise by about 60%. In addition, mixing ratio values for the transitions 1370 -> 309, 1821 -> 1200 keV and 1608 -> 1200 keV are reported for the first time. For the cases where the statistics is not sufficient, all possible values for spins-parities and mixing ratios are presented. The second nucleus studied in this work is the neutron-rich 140Ba. There are strong hints that this nucleus is characterized by enhanced octupole correlations. Such effects have been previously observed in the Ra and Ba isotopic chains, leading to pear-shaped nuclei. The two neighboring neutron-rich isotopes 144Ba and 146Ba have been studied recently using the Coulomb excitation technique along with radioactive beams, yielding a high value for the corresponding reduced transition probabilities, higher than any theoretical calculation. Furthermore, the spectroscopic information regarding the nucleus 140Ba is still poor, with only the lifetime of the 2^+_1 state measured. The specific nucleus was populated using the 2n-transfer reaction 138Ba(18O,16O)140Ba. Despite the high technical diffιculties, e.g. in target preparation, the ground-state band states up to the 8+ level have been populated. The degree of population was sufficient to extract lower-limits for the lifetimes of the ground-state band. In addition, the relative cross sections of the reaction 138Ba(18O,16O)140Ba with respect to the fusion-evaporation reaction 138Ba(18O,4n)152Gd and to the total inelastic channel have been measured. By using theoretical calculations for the reaction 138Ba(18O,4n)152Gd, it was possible to extract the unknown cross sections of the 2n-transfer reaction 138Ba(18O,16O)140Ba in the 61-67 MeV beam energy range, near the Coulomb barrier. Such measurements are important for nuclear structure, as well as for the understanding of the mechanisms that such reactions proceed. Even though the 3- state (whose lifetime can lead to the evaluation of the degree of octupole correlations) was not populated, the results presented in this work are sufficient to guide future studies of 140Ba to the right direction, either regarding the estimation of octupole correlations or the choice of an alternative experimental method (radioactive beams with inverse kinematics).
Main subject category:
Science
Keywords:
Nuclear structure, angular correlations of gamma-rays, octupole correlations, mixing ratio, 140Ba, 180Hf, Cross section, K-isomers, nuclear lifetimes, ROSPHERE array, IFIN-HH, NIPNE, Magurele, multi-nucleon transfer reactions, fusion-evaporation reactions, PACE4, Grazing model, HPGe detectors, gamma-gamma coincidence
