Köln 2004 – scientific programme
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HK: Physik der Hadronen und Kerne
HK 19: Nuclear Structure/Spectroscopy III
HK 19.2: Group Report
Tuesday, March 9, 2004, 16:00–16:30, D
Dipole strength distributions in neutron-rich nuclei — •Nadezhda Tsoneva and Horst Lenske — Institut für Theoretische Physik, Universität Giessen, 35392 Giessen, Germany
Dipole excitations below the neutron threshold in neutron-rich nuclei are studied microscopically by a method combining self-consistent mean-field results from Hartree-Fock-Bogoliubov (HFB) theory and the Quasiparticle-Phonon Model (QPM). Including up to three-phonon components in the model configuration space, a realistic description of nuclear spectra and their fragmentation pattern up to the particle threshold is obtained. Of special interest are the low-lying two-phonon 1− states and the Pygmy Dipole Resonance (PDR). The evolution of the PDR strength in relation to the neutron skin thickness is investigated over the Z=50 isotopic and N=82 isotonic chains. For Sn isotopes a close correlation of the PDR excitation energy and transition strength with the neutron excess was found. The PDR strength increases with the neutron number while the centroid energy decreases. By comparison to the radii of HFB ground state densities a correlation with the size of the neutron skin could be identified. Hence, the PDR strength establishes a clear signature for the neutron skin size. The good agreement of our results with experiments confirms the success of such a microscopically inspired description. For the N=81 isotope 137Ba with one neutron-hole in the closed N=82 shell the distribution of the low-lying dipole strength is studied microscopically within the framework of the QPM including quasiparticle+two-phonon coupling. The results are compared with recent experimental data from Darmstadt and Stuttgart. Work supported by DFG.