Excited states in ${}^{166}\mathrm{Hf}$ were populated in the ${\beta }^{+}/\epsilon$ decay of ${}^{166}\mathrm{Ta}$ and studied through off-beam $\gamma$-ray spectroscopy at the Yale moving tape collector. New coincidence data found no support for two previously reported excited $0^{+}$ states and led to a substantially revised level scheme. Similarities between the revised level scheme of ${}^{166}\mathrm{Hf}$ and the X(5) critical point symmetry are discussed, and the extent of X(5) behavior in this mass region is explored through the W and Os isotopes. Among X(5) candidates with $N>90$, good agreement is observed for most energies and interband $B(E2)$ strengths, while all exhibit similar disagreements with other key observables, in particular, yrast $B(E2)$ values and spacing in the excited $K\hspace{1em}=\hspace{1em}{0}^{+}$ sequence.

• Received 26 October 2004

DOI:https://doi.org/10.1103/PhysRevC.71.024309