Abstract
Prompt fission neutrons (PFN) angular and energy distributions for the reaction 235 U( n th , f ) are calculated as a function of the mass asymmetry of the fission fragments using two extreme assumptions: 1) PFN are released during the neck rupture due to the diabatic coupling between the neutron degree of freedom and the rapidly changing neutron-nucleus potential. These unbound neutrons are faster than the separation of the nascent fragments and most of them leave the fissioning system in few 10 −21 sec. i.e., at the begining of the acceleration phase. Surrounding the fissioning nucleus by a sphere one can calculate the radial component of the neutron current density. Its time integral gives the angular distribution with respect to the fission axis. The average energy of each emitted neutron is also calculated using the unbound part of each neutron wave packet. The distribution of these average energies gives the general trends of the PFN spectrum: the slope, the range and the average value. 2) PFN are evaporated from fully accelerated, fully equilibrated fission fragments. To follow the de-excitation of these fragments via neutron and γ-ray sequential emissions, a Monte Carlo sampling of the initial conditions and a Hauser-Feshbach statistical approach is used. Recording at each step the emission probability, the energy and the angle of each evaporated neutron one can construct the PFN energy and the PFN angular distribution in the laboratory system. The predictions of these two methods are finally compared with recent experimental results obtained for a given fragment mass ratio.
Highlights
The main properties of the fission neutrons have been initially measured in the frame of the Manhattan project [1, 2] and remeasured in the 50’s [3, 4]
The simplest explanation of these two features was immediately embraced: the fission neutrons are evaporated from moving fission fragments
An alternative explanation of the main properties of the promptfission neutrons. It was recently shown [8] that the calculated properties of the neutrons released during a non-adiabatic transition at scission for 236U are in agreement with data on prompt fission neutrons in the reaction 235U(nth, f )
Summary
The main properties of the fission neutrons (their multiplicity and their angular and energy distributions) have been initially measured in the frame of the Manhattan project [1, 2] and remeasured in the 50’s [3, 4]. To see the quality of the agreement, the calculated angular distribution, with respect to the light fragment direction, of the scission neutrons (SN) is represented c The Authors, published by EDP Sciences. One can say that the agreement is surprising but it is better than expected from a model that uses little-known scission shapes and scission times If this alternative explanation had existed in the 50’s, the field of fission neutrons would have probably known a different development. The function Peim(Eni ) gives the distribution of the SN average energies
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