Direct-Interaction Effects in Medium-Energy Fission of Uranium

The fraction of the fission events which occur after direct interactions rather than after compound nucleus formation has been determined in a number of bombardments of uranium. The projectiles used were 10.5-Mev protons, 21-Mev deuterons, and 42-Mev alpha particles. The fraction of post-direct-interaction fissions was obtained from measurements of the angular correlation of coincident pairs of fission fragments which emerged at approximately 90^∘{} to the incident particle beam. The results indicate that (2±{}3)% of fission events in 42-Mev alpha particle bombardments and (5±{}5)% in 21-Mev deuteron bombardments follow some type of direct interaction.