On the quadrupole collectivity in the yrast band of 168Yb

The lifetimes of eight lower spin levels of the yrast band in 168Yb, populated via the 154Sm(18O,4n) and 124Sn(48Ca,4n) reactions, were measured, six of them for the first time, by means of the recoil-distance Doppler-shift method. Two versions of the differential decay-curve method have been applied for the data analysis resulting in a very good agreement. The reduced transition probabilities indicate some reduction of the collectivity for states just above the 61+ level, i.e. well below the band-crossing. The reasons of this effect are discussed in terms of an interaction between the ground-state band and yet unknown part of the two-quasiparticle (ν13/2) spin-aligned S-band by invoking the shell-structure or alternatively/additionally, as changes in the internal structure/shape induced by the rotation.