Nucleon Alignment and Shape Competition at High Spin in $^{180}$Hf

In light even-N Hf isotopes (N = 96-106), the first i$_{13/2}$ neutron alignment occurs at h$\omega \quad <$ 0.3 MeV. In contrast, no alignment was observed up to $\sim $ 0.4 MeV in $^{180,182}$Hf (N = 108,110) [1]. Theoretical calculations predict that oblate collective rotation becomes yrast at high spins in $^{180}$Hf [2, 3]. In the present work, the yrast band of $^{180}$Hf has been extended to high spins, via inelastic excitation, using a 1300 MeV $^{180}$Hf beam incident on a thin $^{232}$Th target. The $\gamma $ rays were detected by Gammasphere, with event by event Doppler correction and Q-value selectivity provided by CHICO. The data reveal onset of the first nucleon alignment in $^{180}$Hf at h$\omega $ $\sim $ 0.43 MeV, which is significantly higher than predictions ($\sim $ 0.35 MeV). Interestingly, the $\gamma $-vibrational band is crossed by a band with apparent high moment-of-inertia at $\sim $ 0.25 MeV. This structure, which becomes near yrast at the highest observed spins will be discussed in the context of nucleon alignment and shape competition at high spin in $^{180}$Hf. [1] E. Ngijoi-Yogo, Ph.D. thesis, U.Mass. Lowell (2004) [2] R.R. Hilton and H.J. Mang, Phys. Rev. Lett. 43, 1979 (1979). [3] F.R. Xu et al., Phys. Rev. C62, 014301 (2000).