Fine Structure of the Isoscalar Giant Quadrupole Resonance from High-Resolution Inelastic Proton Scattering Experiments

• Main Author: Shevchenko, Artem
• Format: Others
• Language: English; en
• Published: 2005
• Online Access: http://tuprints.ulb.tu-darmstadt.de/607/1/diss_shevchenko.pdf; Shevchenko, Artem <http://tuprints.ulb.tu-darmstadt.de/view/person/Shevchenko=3AArtem=3A=3A.html> : Fine Structure of the Isoscalar Giant Quadrupole Resonance from High-Resolution Inelastic Proton Scattering Experiments. [Online-Edition] Technische Universität, Darmstadt [Ph.D. Thesis], (2005)

In the present work the phenomenon of fine structure in the region of the isoscalar giant quadrupole resonance in a number of heavy and medium-heavy nuclei is systematically investigated for the first time. High energy-resolution inelastic proton scattering experiments were carried out in September-October 2001 and in October 2003 at the iThemba LABS cyclotron facility in South Africa with an incident proton energy of 200 MeV. The obtained data with the energy resolution of 50 keV FWHM revealed the appearance of fine structure in all the nuclei studied (58Ni, 89Y, 90Zr, 120Sn, 142Nd, 166Er, 208Pb), thereby establishing the global character of this phenomenon. Fine structure can be described using characteristic energy scales, appearing as a result of the decay of collective modes towards the compound nucleus through a hierarchy of couplings to complex degrees of freedom. For the extraction of the characteristic energy scales from the spectra an entropy index method and a novel technique based on the wavelet analysis are utilized. The global analysis of available data shows the presence of three groups of scales, according to their values. To the first group belong the scales with the values around and below 100 keV, which were detected in all the nuclei studied. The second group contains intermediate scales in the range of 100 keV to 1 MeV. These scales show large variations depending on the nuclear structure of the nucleus. The largest scales above 1 MeV are classified to the third group, describing the global structure of the resonance (the width). The interpretation of the observed scales is realized via the comparison with microscopic model calculations including the coupling of the initial one-particle–one-hole excitations to more complex configurations. A qualitative agreement of the experimentally observed scales with those obtained from the theoretical predictions supports the suggestion of the origin of fine structure from the coupling to the two-particle–two-hole states. However, quantitatively, large deviations are observed for the values of scales given by different models. A more detailed study of the physical nature of extracted scales is provided with the help of the quasiparticle-phonon model and extended time-dependent Hartree-Fock model, which allow to separate the contributions from different damping mechanisms. The main source of the observed scales is identified to arise from the collective damping mechanism, which is the coupling to low-lying surface vibrations. This conforms with the doorway picture of the damping of giant resonances. At the same time, through the comparison with a model of stochastic coupling to many particle-hole states some generic features of the non-collective damping mechanism could be extracted.