Long-sought isomer turns out to be the ground state of $^{76}$Cu

L. Canete, S. Giraud, A. Kankainen, B. Bastin, F. Nowacki, P. Ascher, T. Eronen, V. Girard Alcindor, A. Jokinen, A. Khanam, I. D. Moore, D. Nesterenko, F. De Oliveira, H. Penttilä, C. Petrone, I. Pohjalainen, A. De Roubin, V. Rubchenyaa, M. Vilen, J. Äystö
Nuclear Experiment, Nuclear Experiment (nucl-ex)
2024-01-25 00:00:00
Isomers close to the doubly magic nucleus $^{78}$Ni ($Z=28$, $N=50$) provide essential information on the shell evolution and shape coexistence far from stability. The existence of a long-lived isomeric state in $^{76}$Cu has been debated for a long time. We have performed high-precision mass measurements of $^{76}$Cu with the JYFLTRAP double Penning trap mass spectrometer at the Ion Guide Isotope Separator On-Line facility and confirm the existence of such a 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 $^{76}$Cu. The result can potentially change the conclusions made in previous works related to the spin-parity and charge radius of the $^{76}$Cu ground state. Additionally, the new $^{76}$Cu$(n,\gamma)$ reaction $Q$-value has an impact on the astrophysical rapid neutron-capture process.
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