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

Photoabsorption cross sections sigma_gamma up to the neutron-separation energy S_n 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 S_n a huge number of resolved transitions and in addition an even greater intensity portion in an unresolved continuum have been observed. Simulations of gamma-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 sigma_gamma. The combination of the present data with (gamma,n) data allows us to obtain sigma_gamma 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 sigma_gamma reproduce very well the experimental values around S_n.