Nuclear Moment of Inertia at High Spin

The moment of inertial $\mathcal{I}$ of deformed nuclei increases with increasing angular momentum $I$ in the ground-state rotational band. Recent energy measurements of rare-earth nuclei show that while the increase is smooth at low angular momenta, at about spin 14-16 in a number of nuclei there is a sudden substantial rise in the moment of inertia. In some cases $\mathcal{I}$ rises faster than $I$ so that the rotational frequency $\omega =\frac{I}{\mathcal{I}}$ actually decreases with increasing $I$ producing a characteristic backbending $\mathcal{I}$ vs $\omega$ curve. A singular behavior at about these spin values was predicted over ten years ago by Mottelson and Valatin as a Coriolis antipairing effect. The present article will discuss the experimental data and describe calculations of the nuclear moment of inertia. Calculations made both before and after the recent experiments will be considered.