Shape coexistence in neutron-rich nuclei near N=40

Recent data show that both the 2(+) and 4(+) levels in the even neutron-rich Cr and Fe isotopes decrease in excitation energy toward N = 40. This observation, along with Coulomb excitation and lifetime data, strongly indicates an increase in collectivity near N = 40 in contradiction with expectations based on first principles. A straightforward two-band mixing model is used to investigate the structure of these neutron-rich Cr and Fe nuclei. The approach takes advantage of the extensive data available for Fe-60 to provide the parameter values with which to reproduce the experimental observations in the Cr58-64 and Fe60-68 isotopic chains. Comparisons between the model and the data suggest marked structural differences for the ground-state configurations of N = 40 Cr and Fe.