Angular Distribution of Fragments in Fission Induced by Mev Neutrons

A multiangle gas-filled counter has been used to measure the fragment angular distribution in fission induced by neutrons in the energy range $0.5\ensuremath{\le}{E}_{N}\ensuremath{\le}9$ Mev. The target nuclei used were: ${\mathrm{Th}}^{230}$, ${\mathrm{U}}^{233}$, ${\mathrm{U}}^{234}$, ${\mathrm{U}}^{235}$, ${\mathrm{U}}^{236}$, ${\mathrm{U}}^{238}$, ${\mathrm{Np}}^{237}$, and ${\mathrm{Pu}}^{239}$. In the cases of ${\mathrm{U}}^{233}$ and ${\mathrm{U}}^{235}$ the neutron energy range was extended to include energies between 14.8 and 23 Mev. The general features of these data are the following: The anisotropy ---($\frac{0\ifmmode^\circ\else\textdegree\fi{}}{90\ifmmode^\circ\else\textdegree\fi{}}$) intensity ratio---has values between 1.1 and 1.2 depending on the target and is roughly independent of energy for ${E}_{N}$ between 2 and 5.5 Mev. At higher energies a rise is observed such that at 7 Mev even-odd targets give values of anisotropy in the range 1.2 to 1.3 while even-even targets show greater values in the range 1.6 to 2.2. The anisotropy decreases somewhat by 9 Mev. Near thresholds for the even-even target nuclides considerable fluctuations of anisotropy are observed. The example of ${\mathrm{U}}^{236}$ at 0.85 Mev shows a new case of minimum intensity at 0\ifmmode^\circ\else\textdegree\fi{}, the anisotropy being 0.64. In the energy region 2-4 Mev, the anisotropy of ${\mathrm{Pu}}^{239}$, ${\mathrm{U}}^{233}$, and ${\mathrm{U}}^{235}$ increases by a few percent from one to the next as the spin increases. This is contrary to simple theoretical expectations. These data have been compared to recent theoretical developments of the Bohr model as given by Griffin and by Halpern and Strutinski. The theory provides a satisfactory account of many features of the data.