Structure and evolution of electric dipole strength in Pb-204,Pb-206,Pb-208 below the neutron emission threshold

The electric dipole strength distributions below threshold in stable even-m ass Pb isotopes have been extracted from photon scattering experiments at t he superconducting Darmstadt electron linear accelerator S-DALINAC. Between 4 and 6.5 MeV excitation energy a resonance-like clustering of strength is observed. A strong evolution of a fragmentation of the dipole strength wit h opening the neutron shell is found. The fine structure of the strength is compared to microscopic quasiparticle-phonon model calculations that quant itatively reproduce the data quite well and explain the fragmentation of th e dipole strength. Models suggesting the oscillation of excess neutrons wit h respect to an N similar or equal to Z core as a possible origin of low-ly ing El strength are critically examined. (C) 2000 Elsevier Science B.V. All rights reserved.