Photon data shed new light upon the GDR spreading width in heavy nuclei

A global study of the electric dipole strength in and below the isovector giant dipole resonance (GDR) is presented for mass numbers A > 80. It relies on the recently established remarkably good match between data for the nuclear photo effect to novel photon scattering data covering the region below the neutron emission threshold as well as by average resonance neutron capture (ARC). From the wide energy coverage of these data the correlation of the GDR spreading width with energy can be studied with remarkable accuracy. A clear sensitivity to details of the nuclear shape, i.e. the beta- and gamma-deformations, is demonstrated. Based hereon a new parameterization of the energy dependence of the nuclear electric-dipole strength is proposed which - with only two new parameters - allows to describe the dipole strength in all heavy nuclei with A > 80. Although it differs significantly from previous parameterizations it holds for spherical, transitional, triaxial and well deformed nuclei. The GDR spreading width depends in a regular way on the respective resonance energy, but it is independent of the photon energy. (c) 2008 Elsevier B.V. All rights reserved.