The neutron emission is one of the radioactive decays, by which unstable nuclei may reach the 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 the neutron emission is usually associated with nuclear decay, it must be also mentioned in connection with neutron nuclear reactions. Some neutrons interacts with a target nucleus via a compound nucleus. Among these compound nucleus reactions are also reactions, in which a neutron is ejected from nucleus and they may be referred to as neutron emission reactions. The point is that compound nuclei lose its excitation energy in a way, which is identical to the radioactive decay. Very important feature is the fact the mode of decay of compound nucleus does not depend on the way 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 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 formation of the compound nucleus, initial and final neutron are not the same and this reactions may be also 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 to the recoiling nucleus and the nucleus remains in the excited state. The neutron is emitted then with a lower kinetic energy. If the kinetic energy of an incident neutron is sufficient the double, triple, or more, neutron emission may take place. 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 reaction 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 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 very short time of about 10-14 second.
- Delayed Neutrons. Delayed neutrons are emitted by neutron rich fission fragments that are called the delayed neutron precursors. These precursors usually undergo beta decay but a small fraction of them are excited enough to undergo neutron emission. The fact the neutron is produced via this type of decay and this happens orders of magnitude later compared to the emission of the prompt neutrons, plays an extremely important role in the control of the reactor.