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

Abstract

Abstract Lifetimes of high-spin states in 164 Yb have been measured by the Doppler-broadened line shape (DBLS) method using the reaction 124 Sn( 44 Ca, 4n) 164 Yb 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 h ω = 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 160–168 Yb are compared with the deformations from self-consistent cranking TRS calculations. The TRS calculations show the expected neutron number dependence, i.e., Δβ 2 β 2 ( h ω = 0) is smaller with increasing N and the corresponding loss of collectivity, Q t vs ħ ω, has a smaller slope at the larger ħ ω with increasing N . For 164,166,168 Yb the loss of collectivity from the DBLS measurements occurs at higher ħ ω than predicted and the slope of Q t vs ħ ω 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 164 Yb suggest an octupole instability for 164 Yb.