Fission ofRa226by Deuterons and Helium Ions

Fission induced in ${\mathrm{Ra}}^{226}$ by 14.5- and 21.5-Mev deuterons, and by 23.5-, 31-, and 43-Mev He ions has been studied using radiochemical techniques. The mass distributions of fission products for deuteron-induced fission is triple-humped, corresponding to separate symmetric and asymmetric fission modes. The symmetric mode dominates at the higher bombarding energy. The mass distributions observed for fission products from He-ion induced fission look more "normal": asymmetric at the lowest bombarding energy, becoming a single broad peak at the highest bombarding energy. These results are interpreted in terms of a symmetric fission mode which increases strongly with increasing excitation energy, and an asymmetric fission mode which occurs mainly at low excitation energies following neutron evaporation from highly excited compound nuclei. Asymmetric fission is interpreted to be disappearing as a fission mode for nuclei of lower atomic number than thorium.