A suggested explanation for the anomalous anisotropies in high energy fission

Abstract

Recent experiments of fission induced by high energy protons have shown that the angular distributions of fragments are peaked at right angles to the beam. The fission fragments thus come off mainly with orbital angular momenta oriented along the beam axis despite the fact that the angular momenta of the incident protons with respect to the nuclear centre are oriented perpendicular to this direction. This “anomaly” is attributed to the cooperation of two effects: (1) There are relatively many collisions of an incident proton with individual nucleons in a nucleus which result in angular momentum transfers to the nucleus along the beam axis. This is due to the nature of the nucleon-nucleon differential cross section at high energy. (2) The collisions just described generally leave relatively small amounts of excitation energy in the nucleus. It has been shown that small excitation energies are particularly favourable for the production of large anisotropies.