We describe the next generation general purpose Evaluated Nuclear Data File, ENDF/B-VII.0, of recommended nuclear data for advanced nuclear science and technology applications. Up to date publication reference details. In addition, peculiarities connected with pre-fission neutron emission are also discussed. The main attention is paid to the discussion of the evaluation procedure for two very important PFNS, 252Cf(sf) and 235U(th), and to the uncertainties for evaluated spectra and average energy of fission neutrons for these isotopes. However, the spectrum shape, predicted with these approaches and adjusted parameters, allows us to normalize experimental data, extrapolate the calculated PFNS to different input energies, and prepare the data library. All these approaches do not have any strong physical basis. Therefore, prompt fission neutron spectra (PFNS) are analyzed with semi-empirical formulas: Maxwellian distribution, single-Watt or two-Watt spectra approaches, “scale method,” and so on.
All these peculiarities and possible methods of their investigations are discussed in this chapter.Ī rather realistic theoretical model has been in the hand of researchers since the 1950s, the so named “traditional assumptions” (see Introduction) however, the spectrum shape calculated on the basis of this assumption contradicts the experimental data, and this “physical” approach cannot help with the main request-to provide any useful function for data analysis.
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Some of them are as follows: different types of experiments (microscopic and macroscopic, differential and total FF-integrated experiments, selection of fission events according to neutron energy) and their possible systematical influence on the final result FF selection and fission fragment counting efficiency timing properties of the FF detector which is applied as a “stop” signal for the time of flight experiments neutron detector properties and how to estimate the efficiency of these detectors different components of correlated and uncorrelated backgrounds multiple neutron scattering on the detector’s environment and additional corrections for time resolution and bin width. Many factors or parameters of the experimental setup should be investigated in detail to reach high accuracy of PFNS. Each experiment has particular details which should be known in order to make proper corrections or avoid negative influence on the final results.
A different experimental setup may be used for experimental investigation of PFNS.
Due to the complicated nature of neutron emission in fission (several mechanisms of neutron emission are practically unknown, broad-mass and kinetic energy distribution, possible selection of fission fragment (FF) parameters, background neutrons), the prompt fission neutron spectrum (PFNS) measured in a particular experiment may be destroyed very much.
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