Two prompt $\gamma$ rays of energies 2020 and 554 keV were observed in coincidence with delayed transitions depopulating the ${19/2}^{-}$ isomer in the $Z=29$, $N=42$ ${}^{71}\mathrm{Cu}$ nucleus. The newly identified transitions are proposed to deexcite the 4776- and 5330-keV levels above the ${19/2}^{-}$ isomer. Based on the comparison with the low-lying positive-parity states observed in the $Z=42$, $N=50$ ${}^{92}\mathrm{Mo}$ nucleus, spin and parity ${23/2}^{-}$ are proposed for the 4776-keV level in ${}^{71}\mathrm{Cu}$. The high-energy, 2020-keV transition is interpreted as arising from the breaking of the $N=40$ neutron core. Shell-model calculations with a ${}^{56}\mathrm{Ni}$ core reproduce the $(23/2^{-})\to (19/2^{-})$ gap well, suggesting that the ${23/2}^{-}$ state is dominated by $\pi p_{3/2}\nu ((\mathit{fp}){}^{10}(g_{9/2}){}^4)$ configurations. The present result constitutes further evidence supporting the view that the $N=40$ subshell closure persists in ${}^{71}\mathrm{Cu}$, herewith challenging recent suggestions that the coupling of two or more proton or neutron quasiparticles induces a large polarization of the ${}^{68}\mathrm{Ni}$ core.

• Received 4 December 2008

DOI:https://doi.org/10.1103/PhysRevC.79.034319