Angular Distribution of Fragments from Neutron-Induced Fission ofU238andTh232

The angular anisotropy of neutron-induced fission in ${\mathrm{Th}}^{232}$ and ${\mathrm{U}}^{238}$ has been investigated with a double fission chamber whose common center high-voltage electrode was also a collimator. Ratios of $\frac{{\sigma }_F(0\mathrm{}°, E_n)}{{\sigma }_F(90\mathrm{}°, E_n)}$ were measured at a number of neutron energies from the thresholds near 1 Mev to 20 Mev. Larger variations were found in the anisotropy of fission fragments for ${\mathrm{U}}^{238}$ and ${\mathrm{Th}}^{232}$ than in earlier measurements for other nuclei, particularly at neutron energies immediately above the fission thresholds. For ${\mathrm{Th}}^{232}$, there seems to be direct correlation between the variations in the anisotropy and those in the fission cross section. At 1.6 Mev, corresponding to a peak in the ${\mathrm{Th}}^{232}$ fission cross section, the angular distribution of fission fragments showed a maximum at 90° to the neutron direction. All previous measurements have shown maxima along the neutron direction.