One of the most important aspects of the fission process from the viewpoint of reactor control, is the presence of delayed neutrons. A delayed neutron is a neutron emitted by an excited fission product nucleus during beta disintegration some appreciable time after the fission. How long afterward, is dependent on the half-life of the delayed neutron precursor, since the neutron emission itself occurs in a very short time. The symbol β is used to denote the total fraction of delayed neutrons.

There are many decay chains which are of significance in the emission of delayed neutrons. (Not all of these chains have been positively identified.) Correspondingly, delayed neutrons are commonly discussed as being in six groups. Each of these groups (i) are characterized by a fractional yield β_i and a decay constant λ_i.

Table 1 lists the properties of the six known groups of delayed neutrons emitted during the fission of U-235. The fractional yield β_i is the number of delayed neutrons in a reactor operating at steady state, which are due to neutron emission from decay of fission products (precursors) in group i. The total yield of delayed neutrons is the sum of the fractional values β_i over all groups i. In general, delayed neutrons are more effective than prompt neutrons because they are born at somewhat lower energy compared to prompt (fission) neutrons. Thus they have a better chance to survive leakage and resonance absorption. This is accounted for by giving the delayed neutrons a higher "weight", which is realized by an upward adjustment of the yield values. The effective total delayed neutron fraction is designated β_eff. The value of β_eff, for a given fuel, will vary with the average energy of the neutrons producing fission. [β_eff for the TRIGA using U-235 = 0.007.]