Levels above the 19/2^- isomer in ⁷¹Cu: Persistence of the N=40 neutron shell gap

Two prompt γ 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 ⁷¹Cu 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 ⁹²Mo nucleus, spin and parity 23/2^- are proposed for the 4776-keV level in ⁷¹Cu. The high-energy, 2020-keV transition is interpreted as arising from the breaking of the N=40 neutron core. Shell-model calculations with a ⁵⁶Ni core reproduce the (23/2^-)→(19/2^-) gap well, suggesting that the 23/2^- state is dominated by πp_3/2ν((fp)¹⁰(g_9/2)⁴) configurations. The present result constitutes further evidence supporting the view that the N=40 subshell closure persists in ⁷¹Cu, herewith challenging recent suggestions that the coupling of two or more proton or neutron quasiparticles induces a large polarization of the ⁶⁸Ni core.