So far we have been discussing the properties of neu­trons that are released within 10“14 s of fission occur­ring. These are known as the prompt neutrons. There are a few more neutrons, the delayed neutrons (see Fig 1.7), that arise as part of the radioactive decay of some of the fission products; these neutrons do not appear immediately but at a rate characteristic of the decay of the precursor fission product.

For example, bromine 87 is a fission product. This decays by beta emission to krypton 87 with a half life of 55 s. Figure 1.10 shows that there are two modes of decay. About 30% of the Br-87 decays directly to the ground state of Kr-87, emitting a beta particle with maximum energy of 8 MeV. However, about 70% of the Br-87 emits a beta particle with maximum energy of only 2.6 MeV, resulting in an excited Kr-87 nucleus. The excited Kr-87 may then

either drop to the ground state emitting a 5.4 MeV gamma ray or, rarely — 3%, transform from Kr-87 to Kr-86 by the emission of a neutron. This is a delayed neutron and the original bromine 87 is called a delayed neutron precursor.

A number of delayed neutron precursors have been identified. In analytical work the precursors are di­vided into six groups, each group covering a defined range of half life values. For our purposes we can regard the precursors as leading to an overall mean delay time; it will be seen later that the delayed neutrons are absorbed in the reactor relatively quickly and hence the mean delay time is effectively the mean lifetime of the delayed neutron precursors.

The value of the delayed neutron lifetime т depends on the fissile material as does the delayed neutron yield that is, the fraction of the total neutrons released by fission which are delayed. Table 1.4 gives the thermal fission values of (3 and r for the fissile materials U-235 and Pu-239. They are also given for U-238 but of course for fission by fast neutrons, which makes an important contribution to the overall fission rate in fast reactor systems.

It may be seen that although the fraction of de­layed neutrons is small — 99.32<7o prompt, О. бв^о delayed for U-235 — their lifetime of several seconds is very long compared to the 10 “3 s or less that is the typical lifetime of prompt neutrons. Delayed neu­trons have a profound effect on the kinetics of re­actor behaviour and indeed without delayed neutrons the control of nuclear reactors would not be possible.