Dipole-strength distributions up to the Giant-Dipole Resonance deduced from photon scattering

Dipole-strength distributions up to the neutron-separation energies of the even-mass Mo isotopes from {92}Mo to {100}Mo and of the N=50isotones {88}Sr, {89}Y, {90}Zr have been investigated in photon-scattering experiments using the bremsstrahlung facility at the superconducting electron accelerator ELBE of the Forschungszentrum Dresden-Rossendorf. A measurement using polarised bremsstrahlung impinging on {88}Sr revealed that all resolved transitions with energies greater than 6 MeV in this nuclide except for one are E1 transitions. The intensity distributions obtained from the measured spectra after a correction for detector response and a subtraction of atomic background in the target contain a continuum part in addition to the resolved peaks. It turns out that the dipole strength in the resolved peaks amounts to about 30\% of the total dipole strength while the continuum contains about 70\%. In order to estimate the distribution of inelastic transitions and to correct the ground-state transitions for their branching ratios simulations of gamma-ray cascades were performed. The photoabsorption cross sections obtained in this way connect smoothly to (\gamma,n) cross sections and give novel information about the strength on the low-energy tails of the Giant Dipole Resonances below the neutron-separation energies. The experimental cross sections are compared with predictions of a Quasiparticle-Random-Phase Approximation in a deformed basis. The calculations describe the experimentally observed increase of the dipole strengths with increasing neutron number of the Mo isotopes as a consequence of increasing nuclear deformation.