Corrections of Prompt-neutron Emission in Fission-fragment Experiments

The number of emitted prompt neutrons in the fission process is strongly dependent on the fragment mass. The deformation of the fragments as well as shell effects give the characteristic “sawtooth” shape. It is well known that the total average neutron emission (ν̄tot) increases as a function of incident-neutron energy. But do these extra emitted neutrons also show a particular dependence on the fragment mass? Some experiments have shown that the additional neutrons are emitted from the heavy fragments only. Recent theoretical studies provided an explanation for this, thus emphasising the validity of these observations. Despite this, in various experiments and calculations an average increase of ν̄ (A) was and is still assumed. Experimental data on 234U (n, f), have been used in this work to study the influence of different neutron-multiplicity shapes. Based on the double-energy technique one cannot validate the one or the other method, because no prompt neutrons are detected in coincidence with the fission-fragment kinetic energies. However one may investigate the impact of the ν̄ (A) choice on the fission observables. Two methods were used in the analysis of the experimental data. In one case a higher ν̄ (A) was assumed for all fragment masses and in the other case a higher ν̄ (A) was only assumed for the heavy fragments. By comparing the two methods, the choice of ν̄ (A) was found to be important in the analysis of fission-fragments with relatively strong implications on the mass- and energy distributions. The results stress the need to determine ν̄ (A) by measuring fission fragments in coincidence with prompt-fission neutrons.