Is seniority a partial dynamic symmetry in the first νg9/2 shell?

The low-lying structures of the midshell ν g 9 / 2 Ni isotopes 72 Ni and 74 Ni have been investigated at the RIBF facility in RIKEN within the EURICA collaboration. Previously unobserved low-lying states were accessed for the first time following β decay of the mother nuclei 72 Co and 74 Co. As a result, we provide a complete picture in terms of the seniority scheme up to the first ( 8 + ) levels for both nuclei. The experimental results are compared to shell-model calculations in order to define to what extent the seniority quantum number is preserved in the first neutron g 9 / 2 shell. We find that the disappearance of the seniority isomerism in the ( 8 1 + ) states can be explained by a lowering of the seniority-four ( 6 + ) levels as predicted years ago. For 74 Ni, the internal de-excitation pattern of the newly observed ( 6 2 + ) state supports a restoration of the normal seniority ordering up to spin J = 4 . This property, unexplained by the shell-model calculations, is in agreement with a dominance of the single-particle spherical regime near 78 Ni. [ABSTRACT FROM AUTHOR]