Schematic illustration of the low-energy structure of . Energy is increasing along the vertical axis, and the horizontal axis represents the extent of nuclear deformation (represented by a parameter ). Positive values correspond to prolate (football) shapes, negative values to oblate (pancake) shapes, and represents a spherical shape. The deformed band structure is taken from . The linewidths of the transitions from the two levels to the ground state qualitatively represent the difference in transition rates for these decays. The existence of both a spherical state and a deformed state at low energy indicates the collapse of the shell gap at neutron number 28, as confirmed by the measurements of Gaudefroy et al. .