A global study of the electric dipole strength in heavy nuclei

The dipole strength function strongly influences neutron capture processes in heavy nuclei and it is of great importance for photon induced processes like electromagnetic disintegration and nuclear resonance fluorescence. Above 10 MeV the absolute value and energy dependence are well known from (g,n) experiments usually described by the Lorentzian shape of the Giant Dipole Resonance (GDR). Below the threshold photon scattering studies on nuclei above mass 80 indicate no decrease of the spreading width with falling energy allows to describe its shape by a continuation of the GDR Lorentzian with constant width. The constant width is in accordance to primary photons observed after neutron capture in deformed nuclei which show a clear double peak structure. Capture data taken for nearly spherical nuclei seem to show a one-component Lorentzian shape with width varying with the square of the gamma-energy Eg. This apparent disaccord between more or less deformed nuclei is reinvestigated with the aid of Average Resonance Capture (ARC) data measured for various transitional nuclei. Our suggestion, a 3-fold Lorentzian for all kinds of deformation, will be explained.