Delayed nucleon alignment and search for prolate-to-oblate shape transition in $^{180}$Hf at high spins*

Early calculations predict a giant backbend in $^{180}$Hf at I$\approx $26$\hbar $ due to a crossing of two bands with different intrinsic shapes [1]. More recent cranking calculations [2,3] predict that oblate collective rotational states coexist with prolate ones in neutron-rich Hf nuclei, with the oblate becoming yrast at higher spins. The first nucleon alignment in $^{180}$Hf, predicted at $\hbar $$\omega $$\approx $0.35 MeV, was not observed in recent studies up to the $\hbar $$\omega$$\approx $0.43 MeV [3]. In the present work, high spin states in $^{180}$Hf were populated by inelastic excitation, with a 1300 MeV $^{180}$Hf beam from the ATLAS accelerator at Argonne incident on a thin $^{232}$Th target. The recoiling beam- and target-like nuclei were identified using the position-sensitive detector CHICO, which allowed event-by-event Doppler correction for the $\gamma $-rays emitted in flight by the recoiling nuclei and detected with the Gammasphere array. The extended level scheme of $^{180}$Hf will be discussed in the context of the predicted alignments and shape changes. [1] R.R. Hilton and H.J. Mang, Phys. Rev. Lett. 43, 1979 (1979). [2] F.R. Xu et al., Phys. Rev. C62, 014301 (2000). [3] E. Ngijoi-Yogo, Ph.D. thesis, U.Mass. Lowell (2004)\\ $*$Supported by US Department of Energy Grants DE-FG02-94ER40848, W-31-109-ENG-38 and the National Science Foundation.