Spallation-Fission Competition in Heaviest Elements; Helium-Ion-Induced Reactions in Plutonium Isotopes

Excitation functions have been determined for the spallation and fission reactions induced in plutonium isotopes by 20- to 50-Mev helium ions. The method employed consists of cyclotron bombardments of plutonium oxide followed by the chemical isolation and alpha or beta counting of radioactive reaction products. Formation cross sections are given where possible for the curium and americium spallation products corresponding to ($\alpha , n$), ($\alpha , 2n$), ($\alpha , 3n$), ($\alpha , 4n$), ($\alpha 5n$), ($\alpha , p$), ($\alpha , \mathrm{pn} or d$), ($\alpha , p2n or \mathrm{dn} or t$), and ($\alpha , p3n or d2n or \mathrm{tn}$) reactions in ${\mathrm{Pu}}^{238}$, ${\mathrm{Pu}}^{239}$, and ${\mathrm{Pu}}^{242}$. Fission yield curves and fission cross sections for ${\mathrm{Pu}}^{238}$ and ${\mathrm{Pu}}^{239}$ serve to define the characteristics of the ($\alpha , f$) reaction for plutonium isotopes. Chemical procedures are outlined for the separation of both spallation and fission-product elements in a sequence of operations performed on the entire dissolved target.