The loss of collectivity at high spins in ¹⁶⁴Yb and its neighboring even-even nuclei

Lifetimes of high-spin states in ¹⁶⁴Yb have been measured by the Doppler-broadened line shape (DBLS) method using the reaction ¹²⁴Sn(⁴⁴Ca, 4n)¹⁶⁴Yb at a bombarding energy of 189 MeV. The Q_t values from the data decrease sharply over the rotational frequency range 0.3 to 0.5 MeV (I = 20 to 32 ℏ). At the same rotational frequencies the kinematic moment of inertia is nearly constant, increasing slightly at the higher frequencies ℏω = 0.40 to 0.55 MeV. These observations can be understood qualitatively as a rotationally-induced deoccupation of strongly shape polarizing anti-aligned high-j, low-Ω orbitals i.e., those in which the nucleons are moving in the opposite direction to the rotation. A summary of the Q_t values extracted from the lifetime data for the yrast decay sequences of the even-A nuclei ^160-168Yb are compared with the deformations from self-consistent cranking TRS calculations. The TRS calculations show the expected neutron number dependence, i.e., Δβ₂/β₂(ℏω = 0) is smaller with increasing N and the corresponding loss of collectivity, Q_t vs ℏω, has a smaller slope at the larger ℏω with increasing N. For ^164,166,168Yb the loss of collectivity from the DBLS measurements occurs at higher ℏω than predicted and the slope of Q_t, vs ℏω is much larger than predicted. The enhanced E1/E2 branching ratios and the large transition dipole moments for decay of the states in the (-, 1) band to the ground band and the (+, 0) band in ¹⁶⁴Yb suggest an octupole instability for ¹⁶⁴Yb.