The particle-hole excitations through the $N=40\mathrm{}$ subshell around ${}^{68}\mathrm{Ni}$ have been studied by the $\beta$ decay of ${}^{69}\mathrm{Co}$ and ${}^{69}\mathrm{Ni}$. The half-life of ${}^{69}\mathrm{Co}$ was measured to be 0.22(2) s, and a new $\beta$-decaying isomer with a half-life of 3.5(5) s was identified in ${}^{69}\mathrm{Ni}$. From the decay of the ${}^{69}\mathrm{Ni}$ isomer a 9(4)% mixing of the $\pi p_{3/2}^{+1\mathrm{}}\nu p_{1/2}^{-2\mathrm{}}\nu g_{9/2}^{+2\mathrm{}}$ configuration into the ground state of ${}^{69}\mathrm{Cu}$ can be deduced. Significant polarization of the ${}^{68}\mathrm{Ni}$ core nucleus is observed with the coupling of a single nucleon, which implies a rapid decrease in the stabilizing effect of the $N=40\mathrm{}$ semimagic shell gap.

• Received 10 June 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.3613