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 or from certain other reactions, most famously the (α,n) reaction, where a beryllium nucleus absorbs an alpha particle and emits a neutron.
Neutron radiation is a form of ionising radiation though there is active debate because it is not directly ionizing like protons, photons, and electrons. Because neutrons have no charge, they do not interact with charged electrons and nuclei from a distance. Therefore neutron radiation is more penetrating than alpha radiation, beta radiation, or gamma radiation. In health physics it 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).
The neutrons in reactors are generally categorized as slow (thermal) neutrons or fast neutrons depending on their energy. Thermal neutrons are easily captured by atomic nuclei and are the primary means by which elements undergo atomic transmutation. Fast neutrons are produced by fission and fusion reactions and have a much higher kinetic energy.
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 an particle accelerators can be significantly higher energy than those encountered in reactors.
References
- Radiological hazards and their control (http://www.usace.army.mil/publications/armytm/tm5-801-10/app-b.pdf)
- EPA definitions of various terms (http://www.epa.gov/radiation/terms/termnop.htm#neutronradiation)
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