| Popis: |
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 $\frac{{\ensuremath{\sigma}}_{F}(0\ifmmode^\circ\else\textdegree\fi{}, {E}_{n})}{{\ensuremath{\sigma}}_{F}(90\ifmmode^\circ\else\textdegree\fi{}, {E}_{n})}$ 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\ifmmode^\circ\else\textdegree\fi{} to the neutron direction. All previous measurements have shown maxima along the neutron direction. |
