The spectroscopy of neutron-rich ${}^{109,110,111,112}\mathrm{Ru}$ nuclei was studied by measuring the prompt $\gamma$ rays that originate from fission fragments, produced by the ${}^{238}\mathrm{U}$($\alpha ,f)$ fusion-fission reaction, in coincidence with the detection of both fragments. For ${}^{109,111}\mathrm{Ru}$, both the negative-parity ($h_{11/2}$ orbitals) and $K=5/2$ positive-parity (mainly $g_{7/2}$ and $d_{5/2}$ orbitals) bands were extended to substantially higher spin and excitation energy than known previously. The ground-state and $\gamma$-vibrational bands of ${}^{110,112}\mathrm{Ru}$ also were extended to higher spin, allowing observation of the second band crossing at the rotational frequency of $\approx 450$ keV in ${}^{112}\mathrm{Ru}$, which is $\approx 50$ keV above the first band crossing. At a similar rotational frequency, the first band crossing for the $h_{11/2}$ band in ${}^{111}\mathrm{Ru}$ was observed, which is absent in ${}^{109}\mathrm{Ru}$. These band crossings most likely are caused by the alignment of the $g_{9/2}$ proton pair. This early onset of the band crossing for the aligned $\pi g_{9/2}$ orbitals may be evidence of a triaxial shape transition from prolate to oblate occurring in ${}^{111}\mathrm{Ru}$. The data together with a comparison of cranked shell‐model predictions are presented.

• Received 22 July 2005

DOI:https://doi.org/10.1103/PhysRevC.73.034312