Fission Fragment Anisotropy and Pairing Effects on Nuclear Structure

The dependence of the angular distributions on projectile energy for helium-ion-induced fission of ${\mathrm{Th}}^{232}$, ${\mathrm{U}}^{233}$, and ${\mathrm{U}}^{238}$ has been investigated using solid-state fission fragment detectors. The measurements cover the range of projectile energies between 20 and 43 Mev in steps of 1 Mev or less. The energy dependence of the anisotropy has a structure which is in qualitative agreement with theoretical predictions. The most fissionable target studied, ${\mathrm{U}}^{233}$, shows the least structure while the least fissionable target, ${\mathrm{Th}}^{232}$, shows the most structure, demonstrating that large anisotropies result from fission occurring at low excitation energy following neutron emission from the compound nucleus. The energy dependence of $K_0$, the standard deviation of the distribution in the angular-momentum projection on the nuclear symmetry axis at the saddlepoint, has been deduced from these and other data on angular distributions from neutron-induced fission. At high excitation energies $K_0$ is consistent with predictions of statistical theory with a moment of inertia close to that of a rigid rotator. At excitation energies in excess of the fission threshold of 10 Mev or less, $K_0$ is much reduced due to nuclear pairing effects.