Enhanced dipole strength below particle threshold

Dipole-strength functions up to the neutron-separation energies Sn of the N=50 isotones 88Sr, 89Y, 90Zr, and the even-mass Mo isotopes from 92Mo to 100Mo have been studied in photon-scattering experiments using the bremsstrahlung facility at the superconducting electron accelerator ELBE of the Forschungszentrum Dresden-Rossendorf.
To estimate the distribution of inelastic transitions from high-lying levels at high level density to low-lying levels, simulations of gamma-ray cascades were performed. On the basis of these simulations intensities of inelastic transitions were subtracted from the experimental intensity distributions, including the resolved peaks as well as a continuous part formed by unresolved transitions, and the intensities of elastic transitions to the ground state were corrected for their branching ratios. The photoabsorption cross sections obtained in this novel way are combined with (gamma,n) and (gamma,p) data and give detailed information about the dipole-strength functions in the energy range from about 4 MeV up to the giant dipole resonance (GDR). In all nuclides extra strength in excess to simple Lorentzian-like approximations of the tail of the GDR is found in the energy range from about 5 MeV up to about the respective particle thresholds. Calculations in the framework of the quasiparticle-random-phase approximation (QRPA) underestimate the dipole strength at low energy because they do not take into account the coupling of two-quasiparticle to multi-quasiparticle excitations. A new approach is presented that calculates the dipole strength for nuclei with shape fluctuations by combining the interacting boson model (IBA) with QRPA. Based on the slow shape dynamics and the fast dipole vibrations an Instantaneous Shape Sampling (ISS) is performed that describes the photoabsorption at a fixed shape with QRPA with probabilities given by IBA. The ISS-QRPA improves the description of the experimental photoabsorption cross sections.