Low-lying isomeric levels inCu75

Isomeric low-lying states were identified and investigated in the ${}^{75}\mathrm{Cu}$ nucleus. Two states at $61.8(5)$- and $128.3(7)$-keV excitation energies with half-lives of $370(40)$- and $170(15)$-ns were assigned as ${}^{75m1}\mathrm{Cu}$ and ${}^{75m2}\mathrm{Cu}$, respectively. The measured half-lives combined with the recent spin assignment of the ground state allow one to deduce tentatively spin and parity of the two isomers and the dominant multipolarities of the isomeric transitions with respect to the systematics of the Cu isotopes. Shell-model calculations using an up-to-date effective interaction reproduce the evolution of the $1/2^{-}$, $3/2^{-}$, and $5/2^{-}$ states for the neutron-rich odd-mass Cu isotopes when filling the $\nu g_{9/2}$. The results indicate a significant change in the nuclear structure in this region, where a single-particle $5/2^{-}$ state coexists with more and more collective $3/2^{-}$ and $1/2^{-}$ levels at low excitation energies.