The low-lying dipole response of medium-mass nuclei - Study of 64Ni using complementary real-photon scattering experiments

This thesis deals with the analyses of real-photon scattering experiments to investigate the dipole response of the proton-magic Z=28 nucleus 64Ni. Studies based on the (g,g') reaction are commonly performed up to the particle-separation energies of the nuclide of interest. Up to these energies, E1 decays are mainly associated with the Pygmy Dipole Resonance (PDR) and M1 transitions above 5 MeV with spin-flip resonances, i.e., transitions between spin-orbit partners, in the A approximately 60 mass region. It is of utmost importance to characterize the observed decays, i.e., differentiate between E1 and M1 transitions, for the study of the various dipole-excitation modes. Furthermore, the absolute cross sections have to be determined. Therefore, two complementary (g,g') experiments were performed on 64Ni. On the one hand, an energetically-continuous and mainly-unpolarized photon beam was used at the gELBE facility in Dresden-Rossendorf, Germany, to extract absolute cross sections of observed transitions. On the other hand, quasimonenergetic and linearly-polarized g rays were utilized at the HIgS facility in Durham, US, for the distinction between E1 and M1 transitions. Both complementary measurements are necessary to obtain complete information about the dipole response of atomic nuclei. In all, 87 1− states and 23 1+ levels of 64Ni were firmly identified between 4.3 MeV and the neutron-separation threshold S = 9.7 MeV. For transitions up to 9.3 MeV, absolute energy-integrated cross sections were determined. Besides, absolute photoabsorption cross sections were calculated between 5.86 MeV and 9.05 MeV. The results corresponding to the E1 decay channel were compared to theoretical calculations within the equation of motion (EOM) framework. Furthermore, M1 ground-state decays of 64Ni were interpreted using two shell-model calculations. Besides these comparisons between experiment and theory, systematic investigation of the dipole response in the A approximately 60 mass region were performed. Results of real-photon scattering experiments on 54,56Fe, 58,60Ni, and 66Zn were compiled together with the obtained results of 64Ni and compared. Based on this compilation, it was concluded that the nuclear shell structure has not only an impact on the spin-flip resonance but also on the PDR in this region of the nuclear chart

Investigation of the Pygmy Dipole Resonance in photon scattering experiments

The Pygmy Dipole Resonance (PDR) is the dominating electric dipole excitation below and around the particle separation threshold and exhausts only a few percent of the energy-weighted sum rule. Nevertheless, it may have some impact on reaction rates in nucleosynthesis processes. Therefore, investigations to get more insights in this excitation mode are crucial. A common approach to study the PDR of atomic nuclei is the Nuclear Resonance Fluorescence method (NRF) which bases on real-photon scattering. Absolute cross sections, spin and parity quantum numbers are determined in a model-independent way if suited experimental setups are used. In general, there are two complementary NRF experiments which are presented in this paper

Low-energy excitations and γ -decay branchings in 124Sn via(p,p’γ ) at E p =15 MeV

The dipole response of the proton-magic nucleus was previously investigated with electromagnetic and hadronic probes. Different responses were observed revealing the so-called isospin splitting of the Pygmy Dipole Resonance (PDR). Here we present the results of a new study of using inelastic proton scattering at low energies to test an additional probe possibly exciting states of the PDR. The response to the new probe as well as the -decay behavior of excited states were studied. The (p,p’) experiment was performed at using the combined spectroscopy setup SONIC@HORUS at the Tandem accelerator of the University of Cologne. Proton- coincidences were recorded, enabling a state-to-state analysis due to the excellent energy resolution for both particles and rays. states in the PDR region were populated in the present inelastic proton scattering experiment. Many -decay branching ratios could be determined