Are there signatures of harmonic oscillator shells far from stability?; First spectroscopy of Zr

Paul, N.*; Corsi, A.*; Obertelli, A.*; Doornenbal, P.*; Authelet, G.*; Baba, Hidetada*; Bally, B.*; Bender, M.*; Calvet, D.*; Chteau, F.*; Chen, S.*; Delaroche, J.-P.*; Delbart, A.*; Gheller, J.-M.*; Giganon, A.*; Gillibert, A.*; Girod, M.*; Heenen, P.-H.*; Lapoux, V.*; Libert, J.*; Motobayashi, Toru*; Niikura, Megumi*; Otsuka, T.*; Rodrguez, T. R.*; Rouss, J.-Y.*; Sakurai, Hiroyoshi*; Santamaria, C.*; Shimizu, Noritaka*; Steppenbeck, D.*; Taniuchi, R.*; Togashi, Tomoaki*; Tsunoda, Yusuke*; Uesaka, T.*; Ando, T.*; Arici, T.*; Blazhev, A.*; Browne, F.*; Bruce, A.*; Carroll, R.*; Chung, L. X.*; Corts, M. L.*; Dewald, M.*; Ding, B.*; Flavigny, F.*; Franchoo, S.*; Grska, M.*; Gottardo, A.*; Jungclaus, A.*; Lee, J.*; Lettmann, M.*; Linh, B. D.*; Liu, J.*; Liu, Z.*; Lizarazo, C.*; Momiyama, S.*; Moschner, K.*; Nagamine, S.*; Nakatsuka, Noritsugu*; Nita, C.*; Nobs, C.*; Olivier, L.*; Patel, Z.*; Podolyk, Zs.*; Rudigier, M.*; Saito, T.*; Shand, C.*; Sderstrm, P. A.*; Stefan, I.*; Orlandi, R.  ; Vaquero, V.*; Werner, V.*; Wimmer, K.*; Xu, Z.*

The first measurement of rays emitted by the very neutron rich nuclei Mo (Z=42) and Zr (Z=40) was carried out at the RIBF facility at RIKEN. These data shine light on the postulated N=70 harmonic oscillator shell. If the spin-orbit splitting which gives rise to the N=82 shell gap is reduced in very neutron-rich nuclei, the harmonic oscillator gap at N=70 should open instead. However, both the measured energy of the first excited state Mo and Zr, respectively 235(7) keV and 185(11) keV, and their E(4)/E(2) ratio, indicate that these nuclei are good rotors, and exclude the shell stabilization scenario at Z=40 and N=70. These results were compared to state-of-the-art density functional theoretical calculations using Gogny and Skyrme effective interactions.