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

Abstract

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$(n,{\hat{I}}^3)$ reaction $Q$-value has an impact on the astrophysical rapid neutron-capture process.

8 pages, 4 figures