Anisotropic Photofission

The angular distribution of photofission fragments with respect to the direction of a high-energy x-ray beam has been found to be anisotropic. The distribution seems to be of the same form, $a+b{sin}^2\theta$, in a large variety of situations. The ratio of $b$ to $a$ depends on the energy of the photons producing the fission, on the particular fissionable target being irradiated, and on the particular fission fragments being observed. The photofission excitation curve is compared with the ($\gamma , n$) excitation curve in the energy region near the fission threshold where the fission anisotropy seems to be largest. Recent attempts to develop a theory of anisotropic fission are briefly discussed.