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Browsing by Author "Kulessa, Reinhard"

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    Badanie wysokospinowych stanów z seniority ν > 2,3 w neutrononadmiarowych izotopach Sn produkowanych w reakcjach fuzja‒rozszczepienie
    (Institute of Nuclear Physics Polish Academy of Sciences, 2015) Iskra, Łukasz; Broda, Rafał; Jastrzębski, Jerzy; Kulessa, Reinhard
    Excited states above the seniority ν = 2 isomers have been investigated in even neutron‒rich 118‒128Sn isotopes produced by fusion‒fission of 6.9 MeV/A 48Ca beams with 208Pb and 238U targets and by fission of a 238U target initiated by 6.7 MeV/A 64Ni beam. Level schemes up to excitation energies in excess of 8 MeV have been established based on multifold γ‒ray coincidence relationships measured with the Gammasphere array. Isotopic identification of crucial transitions was achieved through a number of techniques. In the data analysis, the delayed gamma coincidence technique was used to establish high‒spin state structures in the Sn isotopes with isomeric half‒lives below few sec. For cases with long‒lived isomeric states, the gamma cross‒coincidence method was employed to identify such structures. This required a more detailed knowledge of the fusion–fission process in order to establish which of the complementary fragments accompanying a specific Sn isotope should be expected in the reaction exit channel. As a result of employed analysis methods the seniority ν = 4, 15−, and 13− isomers were observed and their half‒lives determined. These long‒lived states in turn served as steppingstones to delineate the isomeric decays and to locate higher‒lying states with good sensitivity. As the observed isomeric decays feed down to 10+ and 7− isomers, firm spin parity assignments could be proposed for most of the seniority ν = 4 states. Higher‒lying, seniority ν = 6 levels were assigned tentatively on the basis of the observed deexcitation paths as well as of general yrast population arguments. Shell‒model calculations were carried out down to 122Sn in the g7/2, d5/2, d3/2, s1/2, and h11/2 model space of neutron holes with respect to a 132Sn core. Effective two‒body interactions were adjusted such that satisfactory agreement with data was achieved for 130Sn. The results reproduce the experimental level energies and spin‒parity assignments rather well. The intrinsic structure of the states is discussed on the basis of the calculated wave functions which, in many instances, point to complex configurations. In a few cases, the proposed assignments lead to unresolved issues. The smooth, systematic decrease of the level energies with mass A is accompanied by the similarly regular behavior with A of the reduced transition probabilities extracted from the isomeric half‒lives. This A dependence is discussed for the E1 and E2 transitions in the decay of the seniority ν = 4 isomers and is compared to that determined in earlier work for the E2 transition rates from the ν = 2,3 isomers.
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    Study of the Pygmy Dipole Resonances Using High - Resolution Gamma Spectrometers
    (Institute of Nuclear Physics Polish Academy of Sciences, 2016) Krzysiek, Mateusz; Bednarczyk, Piotr; Kulessa, Reinhard; Rusek, Krzysztof
    The excitation spectrum of atomic nucleus is, at high energies, dominated by the Giant Dipole Resonance (GDR), which is a collective oscillation of almost all neutrons vs.protons. Around neutron separation energy, well below the GDR, the presence of low-lying dipole strength have been reported in many nuclei and associated to so called Pygmy Dipole Resonance (PDR). In most simple picture it is interpreted as an oscillation of the excess neutrons forming the neutron-skin versus isospin saturated core of the nucleus. Many microscopic calculations have shown the relationship between the PDR strength and the neutron-skin thickness, which is determined by the symmetry energy of the equation of state (EOS). Furthermore, it was found that the PDR contribution to the cross-section of radiative neutron capture process is significant. Therefore, studies on PDR nature are attracting a lot of attention in recent years. So far the experimental evidence of PDR was based on scattering of real photons (γ,γ’), inelastic scattering of electrons, protons and alpha particles, and on relativistic Coulomb excitations. In the present thesis, for the first time the inelastic scattering of heavy ions, combined with high-resolution gamma spectroscopy, was conducted.General aim of the study, which is a subject of this thesis, was to investigate the properties of the dipole states in 140Ce nucleus around particle threshold where the pygmy dipole states are expected to appear. Of particular interest was the possible isospin mixing observed in previous (γ,γ’) and (α,α’γ) experiments. The more specific goal was to confront the experimental data with theoretical calculations using Distorted Wave Born Approximation (DWBA) based on a form factor obtained by folding the microscopically calculated transition densities which was done for the first time in this type of experiments. The experiment using the inelastic scattering of 17O at 20 MeV/u was performed in LNL-Legnaro in Italy. The scattered ions were detected by the two ΔE-E silicon telescopes of TRACE array in coincidence with the γ-rays emitted by the target nucleus. The gamma detection was done with a high energy resolutionusing the state-of-art AGATA array which is based on high-purity germanium (HPGe) detectors. To increase the detection efficiency in high energy region, nine large volume LaBr3:Ce scintillator detectors of HECTOR+ array were additionally applied.The results of the study allowed to confirm predominantly isoscalar character of the low-energy PDR states. For the first time, the fraction of isoscalar energy-weighted sum rule exhausted by the PDR and its strength was extracted for the 140Ce. Comparison with the predictions of the theoretical models allowed shedding some new light into the structure of the pygmy dipole states. Furthermore, obtained results will enrich still very limited database of experimental results, whatis necessary to gain complete knowledge of the PDR properties.