Systematics of electric dipole strength in the stable even-mass Mo isotopes

Photoabsorption cross sections σγ up to the neutron-separation energy Sn were measured for the stable even-mass isotopes 92-100Mo in photon-scattering experiments at the ELBE accelerator. As a consequence of the high level density at excitation energies close to Sn a huge number of resolved transitions and in addition an even greater intensity portion in an unresolved continuum have been observed. Simulations of γ-ray cascades were performed to estimate the intensity distribution of inelastic transitions to low-lying levels and, hence, to deduce intensities and branching ratios of the ground-state transitions needed for the determination of σγ. The combination of the present data with (γ,n) data allows us to obtain σγ in the energy range from about 4 MeV up to the giant dipole resonance. The experimental cross sections are compared with predictions of a new approach called Instantaneous Shape Sampling that calculates dipole strengths by means of QRPA for instantaneous shapes of the nucleus with probabilities obtained from IBA. The calculated σγ reproduce very well the experimental values around Sn.