The neutron emission is one of the radioactive decays by which unstable nuclei reach stability. In general, this type of radioactive decay may occur when nuclei contain significant excess of neutrons or excitation energy. In this type of decay, a neutron is simply ejected from the nucleus.
Although neutron emission is usually associated with nuclear decay, it must also be mentioned with neutron nuclear reactions. Some neutrons interact with a target nucleus via a compound nucleus. Among these compound nucleus reactions are reactions in which a neutron is ejected from the nucleus, and they may be referred to as neutron emission reactions. The point is that compound nuclei lose their excitation energy in a way, which is identical to radioactive decay. A very important feature is that the mode of decay of the compound nucleus does not depend on how the compound nucleus was formed.
The compound nucleus reactions in which neutron emission occurs are:
- Elastic Scattering Reaction. In some cases, if the kinetic energy of an incident neutron is just right to form a resonance, the neutron may be absorbed, and a compound nucleus may be formed. This interaction is more unusual (in comparison with potential scattering) and is also known as resonance elastic scattering. Due to the formation of the compound nucleus, initial and final neutrons are not the same, and these reactions may also be referred to as one type of neutron emission reaction.
- Inelastic Scattering Reaction. In this case, the connection with neutron emission is more obvious. In an inelastic scattering reaction between a neutron and a target nucleus, some energy of the incident neutron is absorbed into the recoiling nucleus and the nucleus remains in the excited state. The neutron is emitted with lower kinetic energy. If the kinetic energy of an incident neutron is sufficient, double, triple, or more neutron emissions may occur. These events are referred to as (n, 2n), (n, 3n), or (n, …n) reactions. The probability of such reactions increases with increasing incident neutron energies.
- Nuclear Fission. The fission reaction is very specific and is of importance in many fields of nuclear engineering. It is known the fission reaction produces fission neutrons that are of importance in any chain-reacting system. But not all neutrons are released at the same time following fission. 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 they 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 of them are excited enough to undergo neutron emission. The neutron is produced via this type of decay, which happens orders of magnitude later than the emission of the prompt neutrons, which plays an extremely important role in controlling the reactor.