Neutron decay is a type of radioactive decay of nuclei containing excess neutrons (especially fission products), in which a neutron is simply ejected from the nucleus. This type of radiation plays key role in nuclear reactor control, because these neutrons are delayed neutrons. At this place we must distinguish between:
- Spontaneous neutron emission. Spontaneous neutron emission is a mode of radioactive decay in which one or more neutrons are ejected from a nucleus.
- Decay of free neutron. The free neutron is, unlike a bounded neutron, subject to radioactive beta decay (with a half-life of about 611 seconds). It decays into a proton, an electron, and an antineutrino (the antimatter counterpart of the neutrino, a particle with no charge and little or no mass).
- Induced neutron emission. Among nuclear reactions are also reactions, in which a neutron is ejected from nucleus and they may be referred to as neutron emission reactions. These nuclear reactions are, for example:
- Scattering Reactions
- Nuclear Fission
- Photoneutrons Emission
- Other nuclear reactions (e.g. (alpha,n) reactions)
This article describes mainly spontaneous neutron emission (prompt neutron decay). This mode of decay occurs only in the most neutron-rich/proton-deficient nuclides (prompt neutron decay), and also from excited states of other nuclides as in photoneutron emission and beta-delayed neutron emission. As can be seen, if a nucleus decays via neutron emission, atomic number remains the same, but daughter become a different isotope of the same element. Nuclei which can decay by this mode are described as lying beyond the neutron drip line. Two examples of isotopes that emit neutrons are beryllium-13 (decaying to beryllium-12 with a mean life 2.7×10−21 s) and helium-5 (helium-4, 7×10−22 s).
Beta-delayed Neutron Emission – Delayed Neutrons
Neutron emission usually happens from nuclei that are in an excited state, such as the excited 87Kr* produced from the beta decay of 87Br. This isotope has half-life of 55.6 seconds. It undergoes negative beta decay through its two main branches with emission of 2.6 MeV and 8 MeV beta particles. This decay leads to the formation of 87Kr* and the 87Kr* nucleus subsequently decays via two successive beta decays into the stable isotope 87Sr. But there is also one possible way for the 87Br nucleus to beta decay. The 87Br nucleus can beta decay into an excited state of the 87Kr* nucleus at an energy of 5.5 MeV, which is larger than the binding energy of a neutron in the 87Kr nucleus. In this case, the 87Kr* nucleus can undergo (with probability of 2.5%) a neutron emission leading to the formation of stable 86Kr isotope. The neutron emission process itself is controlled by the nuclear force and therefore is extremely fast, sometimes referred to as “nearly instantaneous”. The ejection of the neutron may be as a product of the movement of many nucleons, but it is ultimately mediated by the repulsive action of the nuclear force that exists at extremely short-range distances between nucleons. The life time of an ejected neutron inside the nucleus before it is emitted is usually comparable to the flight time of a typical neutron before it leaves the small nuclear “potential well”, or about 10−23 seconds. As can be seen, the rate of emission of these neutrons is governed primarily by beta decay, therefore this emission is known as beta-delayed neutron emission and is responsible for production of delayed neutrons in nuclear reactors.
While the most of the neutrons produced in fission are prompt neutrons, the delayed neutrons are of importance in the reactor control. In fact the presence of delayed neutrons is perhaps most important aspect of the fission process from the viewpoint of reactor control.
See also: Precursors of Delayed Neutrons