The $\frac{7}{2}\mathrm{}\left[523\right]\mathrm{}$ and $\frac{7}{2}\mathrm{}\left[404\right]\mathrm{}$ bands are observed in ${}_{}{}^{159}{}_{}{}^{}\mathrm{Tm}$ to $I=\frac{61}{2}$ and $\frac{43}{2}$, respectively, well beyond the $i_{\frac{13}{2}}$ neutron backbend. Before the backbend, the $\frac{7}{2}\mathrm{}\left[523\right]\mathrm{}$ band exhibits large signature splitting, the $\frac{7}{2}\mathrm{}\left[404\right]\mathrm{}$ none, which is interpreted as resulting from the very different driving influences of the $K=\frac{7}{2}$, $h_{\frac{11}{2}}$, and $g_{\frac{7}{2}}$ orbits, respectively, on the core triaxiality. The $i_{\frac{13}{2}}$ neutron alignment changes the signaure splitting in each band as a result of its dominant driving force to positive values of $\gamma$. Measured $\frac{B(M1;I\to I-1)}{B(E2;I\to I-2)}$ values increase dramatically after the backbend, and this is explained in terms of calculated increases in the $M1\mathrm{}$ transition rates and decreases in the $E2\mathrm{}$ rates.

• Received 27 February 1984

DOI:https://doi.org/10.1103/PhysRevC.29.1934