Enhanced electric dipole strength below particle-threshold as a consequence of nuclear deformation

Abstract

Photoabsorption cross sections ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ up to the neutron-separation energy ${S}_{n}$ were measured for the stable even-mass isotopes $^{92\ensuremath{-}100}\mathrm{Mo}$ in photon-scattering experiments. The photon-scattering data were analyzed in a novel way by taking into account the intensity of unresolved levels at high excitation energy and high level density. Simulations of $\ensuremath{\gamma}$-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 ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$. The present $(\ensuremath{\gamma},{\ensuremath{\gamma}}^{'})$ data can be combined for the first time with $(\ensuremath{\gamma},n)$ data, which allows us to obtain ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ in the energy range from about 4 MeV up to the giant dipole resonance for a series of isotopes. The ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ values below ${S}_{n}$ increase with the number of neutrons above the neutron shell closure at $N=50$. Calculations using a quasiparticle random-phase approximation in a deformed Woods-Saxon potential describe this effect as a consequence of the increasing nuclear deformation.