Evidence for shape coexistence in $^{96}$Y

The energy levels of neutron rich $^{96}$Y have been studied by analyzing the high statistics $\gamma$-$\gamma$-$\gamma$ and $\gamma$-$\gamma$-$\gamma$-$\gamma$ coincidence data from the spontaneous fission of $^{252}$Cf at Gammasphere. Twelve new transitions and 9 new levels in $^{96}$Y have been identified. Spins and parities are tentatively assigned according to the systematics and energy spacing. A small deformation with near spherical shape is proposed for $^{96}$Y ground state. A deformed rotational band has been established at about 1 MeV. Such result indicates shape coexistence of the $^{96}$Y nuclei. The results are consistent with the onset of very strong deformation at N=59 among Y isotopes. Shell model calculations are performed to explain the energy levels and are found to be in good agreement with experimental data.