Shape Evolution in Neutron-Rich Krypton Isotopes Beyond N =60 : First Spectroscopy of Kr,10098

We report on the first γ -ray spectroscopy of low-lying states in neutron-rich ^Kr,100₉₈ isotopes obtained from ^Rb,101₉₉(p ,2 p ) reactions at ∼220 MeV /nucleon . A reduction of the 2₁⁺ state energies beyond N =60 demonstrates a significant increase of deformation, shifted in neutron number compared to the sharper transition observed in strontium and zirconium isotopes. State-of-the-art beyond-mean-field calculations using the Gogny D1S interaction predict level energies in good agreement with experimental results. The identification of a low-lying (0₂⁺, 2₂⁺) state in <mml:mmultiscripts>Kr 98 </mml:mmultiscripts> provides the first experimental evidence of a competing configuration at low energy in neutron-rich krypton isotopes consistent with the oblate-prolate shape coexistence picture suggested by theory.