Study of the Pygmy Dipole Resonances Using High - Resolution Gamma Spectrometers

Abstract

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.