Deformed Structures and Shape Coexistence in Zr-98

The nuclear structure of the zirconium isotopes evolves from a mid-open neutron shell deformed region ($^{80}$Zr), through a closed shell ($^{90}$Zr), to a closed subshell ($^{96}$Zr), and then to a sudden reappearance of deformation ($^{100}$Zr). This rapid onset of deformation across the Zr isotopes is unprecedented, and the issue of how collectivity appears and disappears in these isotopes is of special interest. Until recently, only $^{98}$Zr (and maybe $^{100}$Zr) had indirect and weak evidence for shape coexistence, with only speculative interpretation of the experiments. Recent results from high precision B(E2) measurements provided direct evidence of shape coexistence in $^{94}$Zr and suggested that it may happen in many other nuclei in this region. In order to provide direct evidence of shape coexistence in $^{98}$Zr a high-statistical-quality $\gamma\gamma$ experiment was carried out with the $8\pi$ spectrometer at ISAC-TRIUMF. The array consists of 20 Compton-suppressed hyper-pure germanium detectors plus $\beta$ particle and conversion electron detectors. Excited states up to $\sim5$ MeV in $^{98}$Zr were populated in the $\beta^-$ decay of $^{98}$Y J$^\pi=(0^-)$ and $^{98m}$Y J=(4,5). Preliminary results on key branching ratios will be presented.