Evolution of deformation in the neutron-rich Zr region from excited intruder state to the ground state

Strong evidence for the existence of previously postulated deformed excited states in ${}_{}{}^{98}{}_{}{}^{}\mathrm{Zr}_{58}^{}{}_{}{}^{}$ has been obtained by an accurate measurement of the ${\mathrm{\rho }}^2$($0_3^{+}$→$0_2^{+}$) value for the decay of the second excited $0^{+}$ state at 1436 keV. In the neighboring isotope ${}_{}{}^{99}{}_{}{}^{}\mathrm{Zr}_{59}^{}{}_{}{}^{}$, at the edge of the region of strong ground-state deformation, evidence is found for a rotational band built on the state at 614 keV. Systematics of the excitation energies of the deformed states in Zr nuclei and their Sr neighbors show that the apparent sudden onset of deformation is explained by the steady lowering of a strongly deformed potential minimum. Therefore, an extra strong interaction between proton-neutron spin-orbit partners, acting in the $0_2^{+}$ states, need not be invoked to explain the origin of the sudden onset of deformation.