Alpha-Particle-Induced Fission of ${\mathrm{Th}}^{230}$, ${\mathrm{Th}}^{232}$, and ${\mathrm{U}}^{233}$

Measurements have been made of the kinetic energies of coincident-fission fragment pairs emitted in the 22.1-, 25.7-, and 29.5-MeV ${\mathrm{He}}^4$-induced fission of ${\mathrm{Th}}^{230}$, ${\mathrm{Th}}^{232}$, and ${\mathrm{U}}^{233}$ to determine the mass and total kinetic-energy distributions of the fragments. The results are found to be consistent with the hypothesis of two distinct fission modes, corresponding to predominantly symmetric and predominantly asymmetric mass divisions. From a comparison of the mass distributions obtained from double-energy measurements with the mass distributions obtained from double-velocity time-of-flight measurements, it is possible to deduce the average number of prompt neutrons emitted as a function of the fragment mass. These results are also found to be qualitatively consistent with the two-mode hypothesis.