Long-sought isomer turns out to be the ground state of ⁷⁶Cu

Isomers close to the doubly magic nucleus Ni-78 (Z = 28, N = 50) provide essential information on the shell evolution and shape coexistence far from stability. The existence of a long-lived isomeric state in Cu-76 has been debated for a long time. We have performed high-precision mass measurements of Cu-76 with the JYFLTRAP double Penning trap mass spectrometer at the Ion Guide Isotope Separator On-Line facility and confirm the existence of such an isomeric state with an excitation energy E-x = 64.8(25) keV. Based on the ratio of detected ground- and isomeric-state ions as a function of time, we show that the isomer is the shorter-living state previously considered as the ground state of Cu-76. The result can potentially change the conclusions made in previous works related to the spin-parity and charge radius of the Cu-76 ground state. Additionally, the new Cu-76(n, gamma) reaction Q-value has an impact on the astrophysical rapid neutron-capture process.