Rotation around the longest principle axis in $^{142}$Gd

It is difficult to verify triaxial shape in nuclei and no case has been clearly established where a triaxial nucleus rotates around the longer axis ($\gamma < -60^{\circ}$). In view of this fact, the yrast band of $^{142}$Gd (Z=64, N=78), which has been observed to I=34, displays some interesting features. The general properties of the band show that it is built with no particles excited across the N=82 gap, but with four protons excited across the Z=64 gap, i.e. it has a $\pi$(h$_{11/2})^4\nu$(h$_{11/2})^{-2}$ high-$j$ configuration. This configuration leads to triaxial shape with well-developed potential energy minima calculated for rotation around all three principal axes but with rotation around the intermediate axis energetically favoured at $I \approx 20$. However, a change of rotation axis takes place, because it is only for rotation around the longer axis that it is possible to build $I>30$ states. This conclusion is based on general rules how high-$j$ particles and holes align their spin vectors, and is supported by our recent lifetime measurements indicating a small collectivity at high spins. Another interesting feature is that the yrast states around I=40 are predicted to be dominated by favoured terminations at prolate shape. Cranking calculations and particle-rotor calculations will be presented to support our general conclusions.