Feeding of the11/2−isomers in stable Ir and Au isotopes

Abstract

Excited states in $^{191}\mathrm{Ir}$, $^{193}\mathrm{Ir}$, and $^{197}\mathrm{Au}$ were studied and absolute partial $\ensuremath{\gamma}$-ray cross sections were measured using the ($n,{n}^{'}\ensuremath{\gamma}$) reaction. A Compton-suppressed germanium-detector array (GEANIE) for $\ensuremath{\gamma}$-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 $\ensuremath{\gamma}$-ray cross sections were measured up to incident neutron energy of 20 MeV for several transitions feeding directly the $11/{2}^{\ensuremath{-}}$ isomers and ground states in $^{191}\mathrm{Ir}$, $^{193}\mathrm{Ir}$, and $^{197}\mathrm{Au}$. The feeding of the $11/{2}^{\ensuremath{-}}$ isomers, which originate from the odd proton occupying the ${h}_{11/2}$ 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 ${E}_{n}~10$ 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.