Prescission and postscission neutrons from the reactions p+235,236,238U with Ep≤25.6 MeV

Abstract
Prescission (νiso) and postscission (νpost) neutron multiplicities have been measured for the reactions p+235,236,238U at several projectile energies between 12.7 and 25.6 MeV in coincidence with binary fragments. Separation of νiso from νpost was achieved under the assumption of isotropic emission in the respective source frames. Both multiplicities increase with initial excitation energy ECN* with comparable rates ΔECN*/Δν≊17±3 MeV. νiso(ECN*) extends existing data of heavy-ion induced fusion-fission with ECN*≥50 MeV; for ECN*≥20 MeV it is incompatible with the transition-state model and cannot be reproduced without a delay time for the fission competition in the order of 3×10201019 s. The fragment mass dependence νpost(m) shows a clear sawtooth structure for all three targets at Ep=12.6 MeV that is gradually washed out with increasing Ep or decreasing total kinetic energy TKE, because the heavier fragment receives most of the additional excitation energy. The nuclear temperatures of the heavy fragments exceed those of the light ones, but seem to approach each other with increasing fragment excitation.