Evidence for Rotational Bands near theTh232(n, f)Fission Threshold

The ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}(n, f)$ cross section and the angular anisotropy of fission fragments have been measured up to 5 MeV. The broad vibrational levels located above 1 MeV are resolved into sharp structures which are interpreted as rotational states. The rotational constants $\frac{h^2}{2\mathcal{I}}$ of highly deformed ${}_{}{}^{233}{}_{}{}^{}\mathrm{Th}$ are found to be 2.46 and 2.73 keV at 1.5 and 1.6 MeV, respectively. Our results are interpreted in terms of a hypothetical third minimum in the fission barrier.