Change in triaxiality in 159Tm

The lifetimes of levels up to high spin in 158, 159Tm have been measured with the recoil distance technique. The extracted reduced transition probabilities indicate that in 159Tm, the states of the yrast band (up to spin 472−) and the lowest-lying positive-parity side band (up to spin 392+) have a nearly constant deformation of β ∼ 0.21, while in 158Tm, β ∼ 0.19 up to spin 17. At the backbend of both 159Tm bands, the B(M1) probabilities experience a sharp increase which can be attributed to the alignment of a i132 neutron pair. The observed trends in the B(M1), B(E2, I → I-1) and B(E2, I → I-2) values suggest that, along the yrast line, the 159Tm nucleus undergoes a shape change from negative triaxiality values (γ ∼ -15°) below the backbend to zero or slightly positive ones above it. The results for the side band of 159Tm are consistent with a similar, but smaller, increase in γ.