Fission Fragment Angular Distributions in Charged-Particle-Induced Fission ofRa226

The angular anisotropy of fission fragments from the charged-particle-induced fission of radium has been measured for protons of 10.5-MeV energy and for deuterons and helium ions in the energy ranges from 14 to 21 MeV and 21 to 43 MeV, respectively. A preliminary calculation of the ${K_0}^2$ dependence on excitation energy has been made for thorium nuclei of mass 226-230 stretched to the saddle point. (${K_0}^2$ is the average square of the Gaussian distribution in $K$, the projection of the total angular momentum on the nuclear symmetry axis.) It is shown that the ${K_0}^2\propto E^{\frac{1}{2}}$ relation expected on the basis of the Fermi gas model does not hold for excitation energies below some very approximate energy of 16 MeV. Below this energy a ${K_0}^2\propto E$ dependence is consistent with the experimental data. The fission cross sections for radium with the projectiles mentioned previously have been measured. These cross sections have been compared with total reaction cross sections calculated on the basis of an optical model with volume absorption. A stepwise increase of the helium-ion-induced fission cross section with projectile energy has been interpreted on the basis of multiple-chance fission. Values of $\frac{{\Gamma }_n}{{\Gamma }_f}$ have been determined for various thorium nuclei. The magnitude of the so-called "radium anomaly" of fission is shown to be dependent upon the projectile energy at which measurements are made.