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Isomers are intriguing excited nuclear states with long half-lives, sometimes comparable or even longer than that of the ground state of the nuclide. The reasons for these long half-lives are diverse, such as large angular momentum differences between states or shape coexistence [1,2]. There is a strong ongoing experimental effort to measure and understand the underlying nuclear structure effects that explain their existence, since they have a significant impact on other research fields and practical uses. These applications range from medicine to potential nuclear batteries or clocks. Isomers also play a key role in stellar nucleosynthesis and it has been suggested that some superheavy elements could be more stable in isomeric states than in their ground states, similar to $^{180m}$Ta.
Near the doubly-magic nucleus $^{78}$Ni ($Z=28$, $N=50$), there has been a decades-long debate on the existence of a long-lived isomer in $^{76}$Cu [3]. A recent mass measurement claimed to have settled the debate, by measuring the excitation energy of the isomer and shedding light on the structure of the nuclide [4].
In this work, performed at the Isolde Decay Station at CERN, we present more accurate and precise values of the half-lives of the isomeric and ground states in $^{76}$Cu. Our findings suggest that both states have very similar half-lives, in disagreement with the literature values, implying that they cannot be differentiated by their decay curves. These results raise more questions than they answer, reopening the debate and showing that the structure in $^{76}$Cu is still not fully understood.
[1] Walker, P., Dracoulis, G. Energy traps in atomic nuclei. Nature 399, 35–40 (1999).
[2] Philip Walker and Zsolt Podolyák 2020 Phys. Scr. 95 044004
[3] J. A. Winger et al, Structure of 76Zn from 76Cu decay and systematics of neutron-rich Zn nuclei, Phys. Rev. C 42 (1990) 954–960.
[4] L. Canete et al, Long-sought isomer turns out to be the ground state of 76Cu, Phys. Lett. B 853 (2024) 138663.
