Change in triaxiality in ¹⁵⁹Tm

The lifetimes of levels up to high spin in ^{158, 159}Tm 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 47 2^-) and the lowest-lying positive-parity side band (up to spin 39 2⁺) 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 i _{13 2} 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.