Pygmy dipole strength close to particle-separation energies -The case of the Mo isotopes

The distribution of electromagnetic dipole strength in 92,98,100Mo has been investigated by photon scattering using bremsstrahlung from the new ELBE facility. The experimental data for well-separated nuclear resonances indicate a transition from a regular to a chaotic behaviour above 4MeV of excitation energy. As the strength distributions follow a Porter-Thomas distribution much of the dipole strength is found in weak and in unresolved resonances appearing as fluctuating cross section. An analysis of this quasi-continuum -here applied to nuclear resonance fluorescence in a novel way- delivers dipole strength functions, which are combining smoothly to those obtained from (γ,n) data. Enhancements at 6.5 MeV and at ∝9 MeV are linked to the pygmy dipole resonances postulated to occur in heavy nuclei. (orig.)