Shape Evolution in Neutron-Rich Krypton Isotopes beyond N=60: First Spectroscopy of Kr 98,100

Abstract

We report on the first γ-ray spectroscopy of low-lying states in neutron-rich Kr98,100 isotopes obtained from Rb99,101(p,2p) 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 Kr98 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.