Medium-spin excited levels in ${}^{90}\mathrm{Rb}$, populated in the fission of ${}^{235}\mathrm{U}$ induced by neutrons, have been observed for the first time. $\gamma$ radiation from fission has been measured by using the EXILL array of Ge detectors at the cold-neutron-beam facility PF1B of the Institut Laue–Langevin, Grenoble. Low-energy levels are interpreted as members of the $\pi p_{3/2}^{-1}\nu {\left(d_{5/2}\right)}^3$, $\pi f_{5/2}^{-1}\nu {\left(d_{5/2}\right)}^3$, and $\pi g_{9/2}\nu {\left(d_{5/2}\right)}^3$ multiplets with the $0^{-}$ ground state due to the seniority-3 coupling in the $\nu d_{5/2}$ shell. Analogous anomalous coupling within the $\pi g_{9/2}\nu {\left(d_{5/2}\right)}^3$ configuration explains the $5^{+}$, $6^{+}$, and $7^{+}$ triplet of states, observed at medium spins, similar to the triplet seen in the $N=53$ isotone, ${}^{88}\mathrm{Br}$. Shell-model calculations reproduce well the proposed structures in ${}^{88,90}\mathrm{Rb}$ and support the seniority-3 coupling in $N=53$ isotones and its absence in $N=51$ isotones. The structure of the odd-odd ${}^{88}\mathrm{Rb}$ and ${}^{90}\mathrm{Rb}$ nuclei provides an argument in favor of the collectivity building up at the neutron number $N=53$.

• Received 20 July 2015
• Revised 16 February 2016

DOI:https://doi.org/10.1103/PhysRevC.93.034318