Fission neutrons are neutrons produced in nuclear fission. They have a typical spectrum, and it is known that fission neutrons are of importance in any chain-reacting system. Neutrons trigger the nuclear fission of some nuclei (235U, 238U, or even 232Th). What is crucial is that the fission of such nuclei produces 2, 3, or more free neutrons.
But not all neutrons are released at the same time following fission, and even the nature of the creation of these neutrons is different. From this point of view, we usually divide the fission neutrons into two following groups:
- Prompt Neutrons. Prompt neutrons are emitted directly from fission and are emitted within a very short time of about 10-14 seconds.
- Delayed Neutrons. Delayed neutrons are emitted by neutron-rich fission fragments that are called delayed neutron precursors. These precursors usually undergo beta decay, but a small fraction is excited enough to undergo neutron emission. The fact that the neutron is produced via this type of decay, which happens orders of magnitude later compared to the emission of the prompt neutrons, plays an extremely important role in the reactor’s control.
A spectrum of Fission Neutrons
Region of Fast Neutrons
The first part of the neutron flux spectrum in thermal reactors is the region of fast neutrons. All neutrons produced by fission are born as fast neutrons with high kinetic energy.
At first, we have to distinguish between fast neutrons and prompt neutrons. The prompt neutrons can sometimes be incorrectly confused with the fast neutrons. But there is an essential difference between them. Fast neutrons are neutrons categorized according to kinetic energy, while prompt neutrons are categorized according to the time of their release.
Most of the neutrons produced in fission are prompt neutrons. Usually, more than 99 percent of the fission neutrons are prompt neutrons. Still, the exact fraction depends on the nuclide to be fissioned and on an incident neutron energy (usually increases with energy). For example, fission of 235U by thermal neutron yields 2.43 neutrons, of which 2.42 neutrons are the prompt neutrons, and 0.01585 neutrons (0.01585/2.43=0.0065=ß) are the delayed neutrons.
The vast of the prompt neutrons and even the delayed neutrons are born as fast neutrons (i.e., with kinetic energy higher than > 1 keV). But these two groups of fission neutrons have different energy spectra, contributing to the fission spectrum differently. Since more than 99 percent of the fission neutrons are prompt neutrons, it is obvious that they will dominate the entire spectrum.
Therefore the fast neutron spectrum can be described by the following points:
- Almost all fission neutrons have energies between 0.1 MeV and 10 MeV.
- The mean neutron energy is about 2 MeV
- The most probable neutron energy is about 0.7 MeV.
The fast neutron spectrum can be approximated by the following (normalized to one) distribution:
On average, the neutrons released during fission with an average energy of 2 MeV in a reactor undergo many collisions (elastic or inelastic) before they are absorbed. As a result of these collisions, they lose energy, so the reactor spectrum is not identical to the fission spectrum, and it is always ‘softer’ than the fission spectrum. The fact is that the fission spectrum is part of the reactor spectrum.