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

Abstract

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 ($\ensuremath{\alpha}, n$), ($\ensuremath{\alpha}, 2n$), ($\ensuremath{\alpha}, 3n$), ($\ensuremath{\alpha}, 4n$), ($\ensuremath{\alpha}5n$), ($\ensuremath{\alpha}, p$), ($\ensuremath{\alpha}, \mathrm{pn} or d$), ($\ensuremath{\alpha}, p2n or \mathrm{dn} or t$), and ($\ensuremath{\alpha}, p3n or d2n or \mathrm{tn}$) 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 ($\ensuremath{\alpha}, f$) 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.The small spallation and large fission cross sections observed indicate that fission competes successfully for most of the total reaction cross section in the energy range studied. Analysis from a compound nucleus viewpoint of the cross sections for the surviving ($\ensuremath{\alpha}, \mathrm{xn}$) products reveals mean $\frac{{\ensuremath{\Gamma}}_{f}}{{\ensuremath{\Gamma}}_{n}}$ values for compound and intermediate compound nuclei from 1 to 7, the value decreasing with increasing mass number and apparently not greatly dependent on excitation energy above fission and neutron-emission thresholds. The relatively high cross sections ("tails") evident in the spallation excitation functions beyond their maxima constitute evidence for processes other than compound nucleus formation, e.g., direct interaction. Even more convincing evidence for non-compound-nucleus processes is seen in the fact that the cross sections for ($\ensuremath{\alpha}, \mathrm{pxn}$) reactions are of the same order of magnitude as ($\ensuremath{\alpha}, \mathrm{xn}$) cross sections. Their explanation rests strongly on the supposition that the ejection of high-energy protons, deuterons, and tritons occurs leading to residual intermediate nuclei of low excitation energy, which then escape from fission. This unique description of the escape of charged particle emission reactions from fission competition is believed to have wide application for the explanation of spallation cross-section data in the heaviest element region.Fission yield curves for ${\mathrm{Pu}}^{238}$ and ${\mathrm{Pu}}^{239}$ have been constructed from the production cross sections (mass chain yield plus direct production) for the isotopes ${\mathrm{Br}}^{82,83}$, ${\mathrm{Sr}}^{89,91,92}$, ${\mathrm{Ru}}^{105}$, ${\mathrm{Cd}}^{115,115m,117}$, ${\mathrm{I}}^{131,133}$, ${\mathrm{Ba}}^{139,140}$, ${\mathrm{Ce}}^{143,145}$, ${\mathrm{Nd}}^{147}$, ${\mathrm{Eu}}^{156,157}$, and ${\mathrm{Tb}}^{161}$. The more complete curves for ${\mathrm{Pu}}^{239}$ show a change with increasing energy from asymmetric to symmetric fission for about 40-Mev helium ions accompanied by an increase in number of neutrons lost, as determined by the best fit of reflection points. Integration of the fission yield curves gives total fission cross sections for various energies which, when combined with the appropriate total spallation cross sections, define a total reaction cross-section function consistent with a nuclear radius parameter in the range of (1.3-1.6)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm.Further investigations in the present series should elucidate the effects of $Z$ and $A$ upon fission competition.