Studying prompt neutron energy spectra is of importance in many nuclear reactor applications (e.g. criticality calculations) and also constitute the most important component of the source term for nuclear reactor shielding calculations.
Basic features of prompt neutron energy spectra are summarized below:
- The neutrons produced by fission are high energy neutrons.
- Almost all fission neutrons have energies between 0.1 MeV and 10 MeV.
- The prompt neutron energy distribution, or spectrum, may be best described by dependence of the fraction of neutrons per MeV on neutron energy.
- The most probable neutron energy is about 0.7 MeV.The mean neutron energy is about 2 MeV.
- These values are for thermal fission of 235U, but these values vary only slightly for other nuclides.
Prompt neutron fission spectra evaluation is one of the most interesting aspects of evaluation of actinides. Many experimental and theoretical researches have been carried out for the determination of prompt neutron spectra. There are several representations of prompt fission neutron spectra. Two early models of the prompt fission neutron spectrum, which are still used today, are the Maxwellian and Watt spectrum.
The modern spectrum representation of the prompt fission neutron spectrum and average prompt neutron multiplicity is called the Madland-Nix Spectrum (Los Alamos Model). This model is based upon the standard nuclear evaporation theory and utilizes an isospin-dependent optical potential for the inverse process of compound nucleus formation in neutron-rich fission fragments.
Prompt Neutron Energy Spectra – Dependence on fissioning nucleus.
Source: Madland, David G., New Fission-Neutron-Spectrum Representation for ENDF, LA-9285-MS, April 1982.
Prompt Neutron Energy Spectra – Dependence on incident neutron energy.
Source: Madland, David G., New Fission-Neutron-Spectrum Representation for ENDF, LA-9285-MS, April 1982. http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/14/718/14718824.pdf