Angular Distributions of Fragments from Neutron-Induced Fission ofU233andPu239

Abstract

Angular distributions of fragment activities from neutron-induced fission of ${\mathrm{U}}^{233}$ and ${\mathrm{Pu}}^{239}$ have been measured and show in detail the energy dependence of these distributions. The theoretically expected difference in low-energy anisotropy for these nuclides, which have similar fission thresholds but significantly different target spins, was not observed. A statistical model of fission anisotropy is applied to the data from the low-spin target ${\mathrm{Pu}}^{239}$ to determine the energy dependence of $K_0$, the standard deviation of the distribution in the angular-momentum projection on the nuclear symmetry axis. The anisotropies indicate an increase of $K_0$ with energy in excess of the fission barrier, with $K_0$ values for even-even fissioning nuclei at excitation energies approximately 1 Mev higher than those for odd-$A$ nuclei. The effective moment of inertia about the symmetry axis, similarly obtained from application of a statistical model to the data, is found to be ∼0.1 that of a rigid spherical nucleus.