Shape Coexistence in Transitional Nuclei

The ``transitional'' nuclei near $N=90$ have long been a focus of experimental and theoretical investigations. We report on a program of study of the $N=90$ and $N=88$ nuclei with a focus on the structure of $^{150}$Sm elucidated through new high-statistics, precision $\gamma$-ray coincidence spectroscopy and $\gamma-\gamma$ angular correlation data from the radioactive decay of $^{150}$Pm ($T_{1/2} = 2.68$ h, $Q^- = 3454$ keV, $J^\pi = 1^-$) and $^{150m,g}$Eu ($T_{1/2} = 12.8$ h, $J^\pi = 0^-$ and $T_{1/2} = 36.9$ y, $J^\pi = 5^{(-)}$, respectively, $Q^+\mathrm{(g.s.)} = 2261$ keV). In particular, very weak key collective transitions (e.g., the $2^+_2 (1046) \rightarrow 4^+_1 (773)$ 272~keV $\gamma$ ray) are observed and precision $\delta(E2/M1)$ mixing ratios are extracted (determining $\Delta J = 0$ transitions). This data, when combined with published results from conversion electron measurements, two-neutron transfer studies, and Coulomb excitation supports the results from detailed multiple-spectroscopy studies of $^{152}$Sm [1] indicating that shape coexistence underlies the structure at $N=88, 90$. [1] W. D. Kulp, et al., arXiv:0706.4129 [nucl-ex].