Measurements of angular and energy distributions of prompt neutrons from thermal neutron-induced fission of 235U(nth, f)

By now, a large number of theoretical and experimental works have been performed to study the prompt neutrons emission mechanism in fission. In spite of a significant advance in the understanding of this process a long standing question remains without answer: there are or not the neutrons in low energy fission which are emitted from not fully accelerated fragments. Up to now the prompt fission neutron spectra are evaluated on the base of semi-empirical systematic where a deficiency of knowledge about neutron emission mechanism is compensated by adjustment of model parameters. Therefore the predictive power of this model is very limited and constrained for nuclides and energy region for which no data exist. From the experiments performed earlier for 235U it should be expected to obtain the highest relative yield of neutrons emitted not from fully accelerated fragments. Due to the fact that it was suggested to use 235U as a neutron standard, the clarification of neutron emission mechanism becomes a very important problem. Only the investigations of the fission neutron angular and energy distribution relative to the fragment direction depending on mass split and fragment kinetic energy gives possibility to estimate the yield of neutrons with the formation nature other than evaporation from fully accelerated fragments. The measurements at 11 fixed angles between the neutron and light fragment directions in the range from 0 deg to 180 deg in 18 deg intervals were performed recently at the PNPI RAS. In the present paper we present first results. The energy spectra and angular distributions of fission neutrons have been measured for thermal neutron-induced fission of 235U. A comparison of the measured angular distribution of the 235U(nth , f ) fission neutrons with calculation using the model of isotropic emission of neutrons from fully accelerated fragments shows that in the integral spectra a difference in the neutron yield, which could be assigned to a contribution of 'scission' neutrons, does not exceed 5%, while a maximum effect is ∼10% at the angles near 90 deg. Introduction of anisotropy of the fission neutron angular distribution in the center-of-mass system of fission fragments into the model calculation leads to an increase discrepancy with obtained experimental data. The shape of the neutron spectrum and the number of neutrons obtained in the center-of-mass system both depend on the fragment velocities. As an evaluation, one can use a spectrum written in the center-of-mass system treating the case of two fragments characterized by the average parameters. Taking the example of 252Cf, it was shown by Madland that a transition from the velocity distributions of fragments to the model of two fragments with average parameters has only a minor influence (∼ 2%) on the total neutron energy distribution. It is necessary to perform a complete self-consistent analysis of neutron energy and angular distributions for each fixed pair of fission fragments which should be analogous to that carried out above under approximation of two fragments with average parameters. Only in that case one has a chance to determine for sure a degree of difference between calculated and measured distributions and by this way to clarify a mechanism of neutron emission