Angular Distributions in Fission Induced by Alpha Particles, Deuterons, and Protons

Abstract

Protons of 11 Mev, deuterons of energies up to 22 Mev, and alpha particles of energies up to 43 Mev were used to study fission fragment angular distributions in the following targets: ${\mathrm{Bi}}^{209}$, ${\mathrm{Ra}}^{226}$, ${\mathrm{Th}}^{232}$, ${\mathrm{U}}^{238}$, ${\mathrm{U}}^{235}$, ${\mathrm{Np}}^{237}$, and ${\mathrm{Pu}}^{239}$. All of the measured distributions were qualitatively similar in that more fragments were emitted forward and backward along the beam direction than sideways. The largest ratios of 0° to 90° differential cross sections were slightly greater than 2 and were obtained in the alpha-particle bombardments. The smallest ratios occurred in the proton bombardments. If one decomposes the fissions observed at a given bombarding energy into symmetric and asymmetric mass components, the anisotropy for each component decreases smoothly as the value of $\frac{Z^2}{A}$ of the compound nucleus increases. The asymmetric anisotropies are larger than the symmetric ones. There is no observable effect of the value of the target spin on the observed anisotropies. It is pointed out that some of the observed features of the anisotropies may be accounted for in terms of the fact that some of the fissions occur only after the evaporation of neutrons.