Eur. Phys. J. A (2006) 27: 171-176
https://doi.org/10.1140/epja/i2006-08-025-6

Nuclear Physics in Astrophysics II

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

¹ Institut für Kern- und Hadronenphysik, Forschungszentrum Rossendorf, Postfach 510119, 01314, Dresden, Germany
² Institut für Kern- und Teilchenphysik, Technische Universität Dresden, 01062, Dresden, Germany

^* e-mail: e.grosse@fz-rossendorf.de

Received: 4 July 2005
Accepted: 20 December 2005
Published online: 13 March 2006

Abstract

The distribution of electromagnetic dipole strength in ^{92, 98, 100}Mo 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 6.5MeV and at ∼ 9MeV are linked to the pygmy dipole resonances postulated to occur in heavy nuclei.