Evidence for a smooth onset of deformation in the neutron-rich Kr isotopes*

The aim of this work was to investigate the behaviour of the even-even Z=36 (Kr) isotopes in the phase transition region around A=100 by determining the energies of the 2$^{+}_{1}$ states and their E2 decay transition strengths to the ground state in $^{94}$Kr (N=58) and $^{96}$Kr (N=60). Information on the energies of the first excited 2$^{+}$ states exist only for the Kr isotopes up to N=58. For N=60, contradictory results on this observable were published recently. To clarify this contradiction several experimental runs were performed at the REX-ISOLDE facility at CERN, utilizing the high-efficiency MINIBALL $\gamma $-ray spectrometer and analyzing the emitted $\gamma $ -rays and scattered particles after the Coulomb-excitation reactions. The results of these experiments will be presented and discussed in the framework of the proton-neutron interacting boson model based on the constrained Hartree-Fock-Bogoliubov approach using the microscopic Gogny-D1M energy density functional.