Ternary Fission ofU238Induced by Intermediate-Energy Helium Ions

Evidence is presented for the existence of ternary fission of a heavy excited nucleus into three fragments of comparable size from radiochemical studies on ${\mathrm{U}}^{238}$ bombarded with 20-120-MeV helium ions. The absolute formation cross sections for ${\mathrm{Na}}^{24}$, ${\mathrm{Mg}}^{28}$, ${\mathrm{Si}}^{31}$, ${\mathrm{S}}^{38}$, ${\mathrm{Ca}}^{47}$, ${\mathrm{Mn}}^{56}$, and ${\mathrm{Ni}}^{66}$ in the (39±1)-MeV ${\mathrm{He}}^4$-induced fission of ${\mathrm{U}}^{238}$ clearly establish the transition region between binary and ternary processes, the crossover occurring around $A=47\mathrm{}$. The corresponding data for ${\mathrm{Ta}}^{183}$ and ${\mathrm{Ta}}^{184}$ and upper limits for ${\mathrm{Au}}^{199}$, ${\mathrm{Pb}}^{209}$, and ${\mathrm{Pb}}^{212}$ also indicate the absence of any possible complimentary heavy fragments. The fission-product nature of the light ternary fragments was confirmed by the forward-to-backward recoil properties. The excitation function for the formation of ${\mathrm{Mg}}^{28}$ and the ratio $\frac{{\sigma }_{{\mathrm{Mg}}^{28}}}{{\sigma }_{\mathrm{total}}}$ drop very rapidly below an excitation energy of 20 MeV, making it highly unlikely that such light fragments can be observed at lower excitation energies. These conclusions are consistent with the previously reported lack of radiochemical evidence for the possible thermal-neutron-induced ternary fission of ${\mathrm{U}}^{235}$, but are inconsistent with the claims to the contrary based on purely instrumental measurements. The mass-yield curve appears to be smooth in both the ternary and low-mass binary regions, and gives no indication of any "fine structure" in the ternary region.