Feeding of the 11/2- isomers in stable Ir and Au isotopes

Excited states in 191Ir, 193Ir, and 197Au were studied and absolute partial γ-ray cross sections were measured using the (n,n'γ) reaction. A Compton-suppressed germanium-detector array (GEANIE) for γ-ray spectroscopy was used for the measurement and the broad-spectrum pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility provided energetic neutrons. The energy of the incident neutrons was determined using the time-of-flight technique. Absolute partial γ-ray cross sections were measured up to incident neutron energy of 20 MeV for several transitions feeding directly the 11/2- isomers and ground states in 191Ir, 193Ir, and 197Au. The feeding of the 11/2- isomers, which originate from the odd proton occupying the h11/2 orbital, was found for the three targets to be very similar and increasing relative to the feeding of the corresponding ground state with increasing neutron energy up to En∼10 MeV. Above this neutron energy the opening of the (n,2n) reaction channel strongly affects the population of the isomers and leads to a decrease of their relative population compared to the population of the ground states. The experimental results are compared with theoretical predictions from the GNASH reaction model calculation implementing a version of the spin distribution for the pre-equilibrium reaction piece with either a compound nucleus spin distribution (CN-GNASH) or a Feshbach-Kerman-Koonin (FKK-GNASH) quantum mechanical spin distribution. The effects of the spin cutoff parameter values on the population of states are examined. Evidence is presented that FKK-GNASH provides a description of the experimental data that mitigates the need for adjustment of the level density parameter to fit the data.