Excitation Functions ofU235andU238Bombarded with Helium and Deuterium Ions

Abstract

Cross sections were determined for the reactions ${\mathrm{U}}^{238}({\mathrm{He}}^{4}, n){\mathrm{Pu}}^{241}$ [including ${\mathrm{U}}^{238}({\mathrm{He}}^{4}, p){\mathrm{Np}}^{241}\ensuremath{-}{\ensuremath{\beta}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\mathrm{Pu}}^{241}$], ${\mathrm{U}}^{238}({\mathrm{He}}^{4}, 2n){\mathrm{Pu}}^{240}$, ${\mathrm{U}}^{238}({\mathrm{He}}^{4}, 3n){\mathrm{Pu}}^{239}$, ${\mathrm{U}}^{238}({\mathrm{He}}^{4}, 4n){\mathrm{Pu}}^{238}$, ${\mathrm{U}}^{238}$(${\mathrm{He}}^{4}$, fission), ${\mathrm{U}}^{235}({\mathrm{He}}^{4}, n){\mathrm{Pu}}^{238}$, and ${\mathrm{U}}^{235}({\mathrm{He}}^{4}, 3n){\mathrm{Pu}}^{236}$ induced by helium ions of energies from 18.0 to 43.0 Mev. The yields of ${\mathrm{Pu}}^{239}$, ${\mathrm{Pu}}^{240}$, and ${\mathrm{Pu}}^{241}$ were determined by a mass spectrometric technique. Cross sections were measured for the reactions ${\mathrm{U}}^{238}(d, p){\mathrm{U}}^{239}$, ${\mathrm{U}}^{238}(d, 2n){\mathrm{Np}}^{238}$, ${\mathrm{U}}^{238}(d, 4n){\mathrm{Np}}^{236}(22 \mathrm{hr})$, ${\mathrm{U}}^{238}(d, t; d, p2n){\mathrm{U}}^{237}$, ${\mathrm{U}}^{235}(d, n){\mathrm{Np}}^{236}(22 \mathrm{hr})$, ${\mathrm{U}}^{235}(d, 2n){\mathrm{Np}}^{235}$, and ${\mathrm{U}}^{235}(d, 3n){\mathrm{Np}}^{234}$ induced by deuterons of energies from 5.8 to 21.5 Mev. The above excitation functions are compared with similar excitation functions of nonfissile nuclei to show the effect of fissionability on spallation cross sections. Mean values of $\frac{{\ensuremath{\Gamma}}_{n}}{{\ensuremath{\Gamma}}_{f}}$ are derived for several heavy nuclides by fitting the experimental excitation functions with theoretical cross sections calculated from the Jackson model modified for fission. In addition to the discussion on the compound-nucleus formation mechanism, some discussion of the experimental evidence for direct interactions is given.