Direct processes and collective effects in high energy proton induced fission

Abstract

Nuclear emulsions loaded with thorium have been exposed to the external 600 MeV proton beam of the CERN synchro-cyclotron. For each fission event both ranges, a characteristic angle relative to the proton beam and the angle between the two fragments was measured. From these data one computes, under certain assumptions, the longitudinal momentum imparted to the fissioning nucleus. The distribution of momentum transfers obtained falls off exponentially towards higher values; its maximum is estimated to lie around 170 MeV/ c. At momentum transfers above 300 MeV/ c the light fragment is emitted preferentially in the forward direction. This indicates the existence of a direct, fast fission mode. The fore-aft asymmetry is largest for fissions almost parallel to the proton beam; it increases with increasing momentum transfer. The range distribution is symmetric. Qualitatively the fast fission may be described as a deformation of a viscous liquid drop by the fast cascade. At momentum transfers below 300 MeV/ c there is no significant fore-aft asymmetry. The angular distribution, instead, is peaked around 90° to the primary direction. It can be represented by 1 + α sin 2 θ with α = 0.64±0.10. The range distribution has an asymmetric component. Angular anisotropy and mass asymmetry are consistent with the model of I. Halpern who assumes a collective rotation initiated by the capture of slow secondary nucleons.