Shape coexistence in the odd-odd nucleus 98Y : the role of the g9/2 neutron extruder

Abstract

Excited states in 98Y, populated in neutron-induced fission of 235U and in spontaneous fission of 248Cm and 252Cf, have been studied by means of γ 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 98Y: a deformed one with T1/2 = 180(7) ns and a rotational band on top of it, and a spherical one with T1/2 = 0.45(15)μs, analogous to the 8+ isomer in 96Y, corresponding to the (νg7/2,πg9/2)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 98Y. This result and the studies of excited levels in 98Zr, populated in β− decay of the isomer, indicate a new spin-parity, I π = (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 - πg9/2 interaction (the spin-orbit-partner, or SOP, mechanism).