Giant resonance in transitional nuclei: Photoneutron cross sections for osmium isotopes

Photoneutron cross sections, including σ[(γ,n)+(γ,pn)], σ[(γ,2n)+(γ,p2n)], and σ(γ,3n), were measured for ¹⁸⁸Os, ¹⁸⁹Os, ¹⁹⁰Os, and ¹⁹²Os from 7 to 30 MeV and for ¹⁸⁶Os from 11 to 20 MeV, with a photon energy resolution of about 300 keV. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial photoneutron cross sections were determined by neutron multiplicity counting, and the average neutron energies for each multiplicity were determined simultaneously with the cross-section data by the ring-ratio technique. Nuclear information extracted from the data includes parameters of the giant dipole and giant quadrupole resonances, integrated cross sections and their moments, nuclear symmetry energies, and nuclear deformation parameters and intrinsic quadrupole moments. No fewer than eight kinds of evidence point to a sudden change of behavior between ¹⁸⁹Os and ¹⁹⁰Os, which could be interpreted as a phase transition from a statically deformed prolate nucleus to a γ-unstable one, in general (but not detailed) agreement with the prediction of a dynamic-collective-model calculation.NUCLEAR REACTIONS 186,188,189,190,192Os(γ,n,$2n$,$3n$), E_γ=7-30 MeV; measured 4π neutron yield, multiplicities, average energies for monenergetic photons; σ(E_γ,1n), σ(E_γ,2n), σ(E_γ,3n), GDR parameters, nuclear shape parameters, integrated cross sections and moments, GQR parameters, nuclear phase transition.