Abstract

The advanced time-delayed $\mathrm{\gamma \gamma }\left(t\right)$ method has been applied to determine half-lives of low-lying states in the $N=50$ isotones $94\mathrm{Ru}$ and $96\mathrm{Pd}$. The inferred experimental $\mathrm{E2}$ strengths for the $4^{+}\to 2{}^{}$ transitions in the two nuclei show a dramatic deviation with respect to the shell model predictions in the $({f5}_5,p,{g9}_9)$ proton hole space in $100\mathrm{Sn}$. The anomalous behavior can be ascribed to a breakdown of the seniority quantum number in the $\mathrm{\pi gn9}{}_{/2}$ configuration due to particle-hole excitations across the $N=Z=50$ shell as confirmed by large-scale shell model calculations.