Neutron radiation

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Neutron radiation consists of free neutrons. Neutrons may be emitted during nuclear fission (either spontaneous or triggered), nuclear fusion, very high energy reactions such as in a Spallation Neutron Source, or from certain other reactions, most famously the (α,n) reaction, for example when a beryllium nucleus absorbs an alpha particle and emits a neutron.

Cold, thermal and hot neutron radiation is most commonly used for scattering and diffraction experiments in order to access the properties and the structure of materials in crystallography, condensed matter physics, biology, solid state chemistry, materials science, geology, mineralogy and related sciences.

Neutron radiation is often called indirectly ionizing radiation. It does not ionize atoms in the same way protons, photons, and electrons do (exciting an electron) because neutrons have no charge. However, neutron interactions are largely ionizing, for example when neutron absorption results in gamma emission and the gamma subsequently removes an electron from an atom, or a nucleus recoiling from a neutron interaction is ionized and causes more traditional subsequent ionization in other atoms. Because neutrons are uncharged, they are more penetrating than alpha radiation or beta radiation. In some cases they are more penetrating than gamma radiation, which is impeded in materials of high atomic number. In hydrogen, a low energy neutron may not be as penetrating as a high energy gamma. In health physics neutron radiation is considered a fourth radiation hazard alongside these other types of radiation. Another, sometimes more severe, hazard of neutron radiation is neutron activation, the ability of neutron radiation to induce radioactivity in most substances it encounters, including the body tissues of the workers themselves. This occurs through the capture of neutrons by atomic nuclei, which are transformed to another nuclide, frequently a radionuclide. This process accounts for much of the radioactive material released by the detonation of a nuclear weapon. It is also a problem in nuclear fission and nuclear fusion installations, as it gradually renders the equipment radioactive; eventually the hardware must be replaced and disposed of as low-level radioactive waste.

Neutrons also degrade materials; intense bombardment with neutrons creates dislocations in the materials, leading to embrittlement of metals and other materials, and to swelling of some of them. This poses a problem for nuclear reactor vessels, and significantly limits their lifetime (which can be somewhat prolonged by controlled annealing of the vessel, reducing the number of the built-up dislocations). Graphite moderator blocks are especially susceptible to this effect, known as Wigner effect, and have to be annealed periodically; the well-known Windscale fire was caused by a mishap during such an annealing operation.

The neutrons in reactors are generally categorized as slow (thermal) neutrons or fast neutrons depending on their energy. Thermal neutrons are similar to a gas in thermodynamic equilibrium but are easily captured by atomic nuclei and are the primary means by which elements undergo atomic transmutation.

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, 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, 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.

Neutron radiation protection relies on radiation shielding. In comparison with conventional ionizing radiation based on photons or charged particles, neutrons are absorbed in light nuclei, so a large mass of hydrogen-rich material is needed. The most effective materials are eg. water, polyethylene, paraffin wax, or concrete, where a considerable amount of water molecules is chemically bound to the cement. The light atoms serve to slow down the neutrons by elastic scattering, so they can then be absorbed by nuclear reactions. However, gamma radiation is often produced in such reactions, so additional shielding has to be provided to absorb it.