Quadrupole Collectivity Beyond N=28: Intermediate-Energy Coulomb Excitation Of Ar-47,Ar-48

We report on the first experimental study of quadrupole collectivity in the very neutron-rich nuclei Ar-47,Ar-48 using intermediate-energy Coulomb excitation. These nuclei are located along the path from doubly magic Ca to collective S and Si isotopes, a critical region of shell evolution and structural change. The deduced B(E2) transition strengths are confronted with large-scale shell-model calculations in the sdpf shell using the state-of-the-art SDPF-Uand EPQQM effective interactions. The comparison between experiment and theory indicates that a shell-model description of Ar isotopes around N = 28 remains a challenge.