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Neutron radiation is a kind of ionizing radiation which consists of free neutrons.
A result of nuclear fission or nuclear fusion, it consists of the release of free
neutrons from atoms, and these free neutrons react with nuclei of other atoms to
form new isotopes, which, in turn, may produce radiation.
Sources
Neutrons may be emitted from nuclear fusion or nuclear fission, or from any
number of different nuclear reactions such as from radioactive decay or
reactions from particle interactions (such as from cosmic rays or particle
accelerators). Large neutron sources are rare, and are usually limited to large-
sized devices like nuclear reactors or particle accelerators (such as the
Spallation Neutron Source).
Neutron radiation was discovered as a result of observing a beryllium nucleus
reacting with an alpha particle thus transforming into a carbon nucleus and
emitting a neutron, Be(α, n)C. The combination of an alpha particle emitter and
an isotope with a large (α, n) nuclear reaction probability is still a common
neutron source.
Neutron radiation from fission
The neutrons in nuclear reactors are generally categorized as slow (thermal)
neutrons or fast neutrons depending on their energy. Thermal neutrons are
similar in energy distribution (the Maxwell–Boltzmann distribution) to a gas in
thermodynamic equilibrium but are easily captured by atomic nuclei and are the
primary means by which elements undergo atomic transmutation.
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In order to achieve an effective fission chain reaction, the neutrons produced
during fission must be captured by fissionable nuclei, which then split, releasing
more neutrons. In most fission reactor designs, the nuclear fuel is not
sufficiently refined to be able to absorb enough fast neutrons to carry on the
fission chain reaction, due to the lower cross section for higher-energy neutrons,
so a neutron moderator must be introduced to slow the fast neutrons down to
thermal velocities to permit sufficient absorption. Common neutron moderators
include graphite, ordinary (light) water and heavy water. A few reactors (fast
neutron reactors) and all nuclear weapons rely on fast neutrons. This requires
certain changes in the design and in the required nuclear fuel. The element
beryllium is particularly useful due to its ability to act as a neutron reflector or
lens. This allows smaller quantities of fissile material to be used and is a
primary technical development that led to the creation of neutron bombs.
Cosmogenic neutrons
Cosmogenic neutrons, neutrons produced from cosmic radiation in the Earth's
atmosphere or surface, and those produced in particle accelerators can be
significantly higher energy than those encountered in reactors. Most of them
activate a nucleus before reaching the ground; a few react with nuclei in the air.
The reactions with nitrogen-14 lead to the formation of carbon-14, widely used
in radiocarbon dating.
Uses
Cold, thermal and hot neutron radiation is most commonly used for scattering
and diffraction experiments in order to assess the properties and the structure of