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

Abstract

The low-lying structures of the midshell νg 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.

Segmented YSO scintillation detectors as a new β-implant detection tool for decay spectroscopy in fragmentation facilities

“…All the γ-rays in our spectrum are consistent with the reported values in Ref. [22,23]. We also observed γ-rays at 2588 and 3611 keV which are reported as decays from the isomeric state in 74 Co [22,23].…”

“…[22,23]. We also observed γ-rays at 2588 and 3611 keV which are reported as decays from the isomeric state in 74 Co [22,23].…”

Effects of one valence proton on seniority and angular momentum of neutrons in neutron-rich Sb51122–131 isotopes

“…It is known that seniority is a good quantum number for l j f 7/2 . This becomes a partial symmetry in the case of l j = g 9/2 in 72,74 Ni [5]. In these isotopes, the (6 + ) υ ν =4 state has a lower excitation energy than the (6 + ) υ ν =2 , leading to a disappearance of the seniority (8 + ) υ ν =2 isomers.…”

Transition strengths in the neutron-rich Ni73,74,75 isotopes

“…Seniority remains a good quantum number for any two-body interaction acting within a single j shell when j 7/2, but it needs not be conserved for j 9/2 [6]. Partial conservation of the seniority quantum number-most eigenstates are mixed in seniority but some remain pure-has been recently predicted in nuclei, like the neutron-rich Ni isotopes, filling the g 9/2 shell [7][8][9]. Indeed, the seniority classification is a good approximation for n-identical nucleons in the j = 9/2 shell.…”