Fission ofTh232Induced by Intermediate-EnergyHe4Ions

The cross sections for 34 nuclides covering the mass range 24 to 199 formed from the ${\mathrm{He}}^4$ ion-induced fission of ${\mathrm{Th}}^{232}$ at 20-41 MeV have been determined radiochemically. The resulting mass-yield curve clearly indicates contributions from at least three different types of fission: the symmetric binary, asymmetric binary, and asymmetric ternary modes. The identification of very light fragments and the low limits set on ${\mathrm{Au}}^{199}$ confirm the existence of ternary fission seen previously in this laboratory in plutonium compound nuclei. Certain similarities and differences in this fissioning system to other heavier elements are discussed.