Investigation of prescission neutron emission in 235U (nth,f) through fragment-neutron angular correlation studies

Measurements of prompt neutron energy spectra and angular distributions from mass and kinetic energy selected fission fragments were carried out in the thermal neutron fission of 235U. Neutron energy was determined by the time of flight technique and fission fragment energy and angle were measured using a back-to-back gridded ionization chamber. The measured angular distributions of neutrons emitted from fragment pairs of various mass and kinetic energy were compared with results of Monte Carlo calculations assuming neutron emission from fully accelerated fragments to determine the component of neutrons which may be emitted in the prescission stage. The calculations were carried out using inputs the measured center of mass neutron energy spectra and multiplicities and assuming isotropic emission of neutrons in the centre of mass frame of both the fission fragments, and a three source fitting of the angular distributions was done to deduce the component of prescission neutrons. The value of the prescission neutron multiplicity γ pre averaged over all fragment masses is found to be 0.25 ± 0.05 (about 10% of the total neutron multiplicity). The value of γ pre is found to be nearly the same for all fragment masses except in the region of doubly closed heavy fragment shell region, where it is somewhat larger. It is also seen that γ pre shows an increase with fragment total kinetic energy. Calculations carried out under the assumptions of the statistical model give an estimate of the time scale from saddle-to-scission transition which is compatible with the values obtained from heavy ion fusion-fission experiments. The present results have been discussed on the basis of the energy damping and timescale of the saddle to scission transition in the thermal neutron induced fission process. (author). Abstract only