The ${}^{87}$Se nucleus has been studied via prompt $\gamma$-ray spectroscopy using the Eurogam2 Ge array to measure $\gamma$ rays following fission of ${}^{248}$Cm. Excited levels in ${}^{87}$Se have been observed for the first time. The yrast excitation scheme in this nucleus is similar to the excitations schemes of its $N=53$ neighbors and fits the energy systematics, indicating $j-1$ anomaly below $Z=38$. Large-scale shell-model calculations reproduce in detail yrast excitations in ${}^{87}$Se and other $N=53$ isotones. The $j-1$ anomaly is explained as due to the enhancement of collectivity towards the proton midshell. The coexistence of collective and single-particle excitations is predicted in ${}^{87}$Se.

• Received 21 April 2013

DOI:https://doi.org/10.1103/PhysRevC.88.034302