New analysis of levels in $^{96,100}$Y

Exited states of neutron rich $^{96,100}$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 and also from the prompt $\gamma$-rays in coincidence with isotopically-identified fission fragments using VAMOS++ and EXOGAM at GANIL by using $^{238}$U beams on a $^{9}$Be target at energies around the Coulomb barrier. Nine new transitions and 6 new levels in $^{96}$Y, and 25 new transitions and 17 new levels in $^{100}$Y have been identified. Spins and parities are tentatively assigned according to the systematics. A small deformation with near spherical shape is proposed for $^{96}$Y. Bands in $^{100}$Y are proposed to be pairing free ones with very large deformation parameter ($\beta_2$=0.35(4) [1]). Two of the new bands in $^{100}$Y can form a 4$^+$ $\pi$5/2[422]+$\nu$3/2[411] and 5$^+$ $\pi$5/2[422]+$\nu$5/2[413] neutron pseudo spin doublet. The results are consistent with the onset of very strong deformation at N=59 among Y isotopes.\\[4pt] [1] J. Phys. G: Nucl. Part. Phys. {\bf 37}, 105103 (2010).