Evidence for Deformed Ground States in Light Kr Isotopes

The levels in $^{74,76}\mathrm{Kr}$ were studied with in-beam $\ensuremath{\gamma}$-spectroscopy techniques and the ${\ensuremath{\beta}}^{+}$ decay of $^{76}\mathrm{Rb}$. The energies of the ${2}_{1}^{+}$ states in $^{74,76}\mathrm{Kr}$ deviate from smooth behavior compared with the higher spin levels. The yrast cascade $B(E2)$'s are highly collective. The $^{74,76}\mathrm{Kr}$ ground states have unusually large deformation. The origin of this deformation and of shape coexistence in this region is described in terms of the protons driving the deformation.