Shape coexistence in the odd-odd nucleus $^{98}\mathrm{Y}$: The role of the ${g}_{9/2}$ neutron extruder

Excited states in $^{98}$Y, populated in neutron-induced fission of U$^{235}$ and in spontaneous fission of $^{248}$Cm and $^{252}$Cf, have been studied by means of $\gamma$ spectroscopy using the Lohengrin fission-fragment separator at ILL Grenoble and the EXILL, Eurogam2, and Gammasphere Ge arrays. Two new isomers have been found in $^{98}$Y: a deformed one with T$_{1/2}$=180(7) ns and a rotational band on top of it, and a spherical one with T$_{1/2}$=0.45(15)μs, analogous to the 8$^+$ isomer in $^{96}$Y, corresponding to the (νg7/2,πg$_{9/2}$)8$^+$ spherical configuration. Using the JYFLTRAP Penning trap, an accurate excitation energy of 465.7(7) keV has been determined for the 2.36-s isomer in $^{98}$Y. This result and the studies of excited levels in $^{98}$Zr, populated in β$^−$ decay of the isomer, indicate a new spin-parity, Iπ=(7)$^+$ for the isomer. The high spin and the decay properties of this isomer suggest the presence of the 9/2$^+$[404] neutron extruder orbital in its structure. This is consistent with the large deformation of the isomer, reported recently. The present work does not provide arguments to support the special role of the νg7/2-πg$_{9/2}$ interaction (the spin-orbit-partner, or SOP, mechanism).