Directional Correlation of Fission Fragments and Prompt Gamma Rays Associated With Thermal Neutron Fission

Prompt fission $\gamma$-ray intensity as a function of the angle from the fragment direction has been measured for the thermal neutron fission of ${\mathrm{U}}^{233}$, ${\mathrm{U}}^{235}$, and ${\mathrm{Pu}}^{239}$. For each of these three nuclides, the fission $\gamma$ rays were found to be emitted preferentially in the direction of fragment motion by as much as 15%. It was found that the experimental data were best fitted by assuming that $\gamma$ rays emitted by fission fragments have two components with different angular distributions. One component, accounting for about 85% of the $\gamma$ rays, was isotropic about the fission fragment direction. The remaining 15% had the angular distribution of $\gamma$ rays from quadrupole transitions of nuclei having approximately 7 units of angular momentum distributed in, or near, a plane perpendicular to the direction of fragment motion. A calculation of the torque resulting from electrostatic forces between the fission fragments is discussed. Results of this calculation indicate that these electrostatic forces could give rise to the fragment rotations indicated by the $\gamma$-ray measurements.