| Science withneutrons |
|---|
 |
| Foundations |
| Neutron scattering |
| Other applications |
|
| Infrastructure |
| Neutron facilities |
Neutron radiation is a form ofionizing radiation that presents asfree neutrons. Typical phenomena arenuclear fission ornuclear fusion causing the release of free neutrons, which thenreact withnuclei of otheratoms to form newnuclides—which, in turn, may trigger further neutron radiation. Free neutrons are unstable,decaying into aproton, anelectron, plus anelectron antineutrino. Free neutrons have a mean lifetime of 887 seconds (14 minutes, 47 seconds).[1]
Neutron radiation is distinct fromalpha,beta andgamma radiation.[2]
Neutrons may be emitted fromnuclear fusion ornuclear fission, or from othernuclear reactions such asradioactive decay or particle interactions withcosmic rays or withinparticle accelerators. Large neutron sources are rare, and usually limited to large-sized devices such asnuclear reactors orparticle accelerators, including theSpallation Neutron Source.
Neutron radiation was discovered from observing analpha particle colliding with aberylliumnucleus, which was transformed into acarbonnucleus while emitting aneutron,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.
The neutrons in nuclear reactors are generally categorized asslow (thermal) neutrons orfast neutrons depending on their energy. Thermal neutrons are similar in energy distribution (theMaxwell–Boltzmann distribution) to a gas inthermodynamic equilibrium; but are easily captured by atomic nuclei and are the primary means by which elements undergonuclear transmutation.
To achieve an effective fission chain reaction, neutrons produced during fission must be captured by fissionable nuclei, which then split, releasing more neutrons. In most fission reactor designs, thenuclear fuel is not sufficiently refined to absorb enough fast neutrons to carry on the chain reaction, due to the lowercross section for higher-energy neutrons, so aneutron moderator must be introduced to slow the fast neutrons down to thermal velocities to permit sufficient absorption. Common neutron moderators includegraphite, ordinary (light)water andheavy water. A few reactors (fast neutron reactors) and allnuclear weapons rely on fast neutrons.
Cosmogenic neutrons are produced from cosmic radiation in the Earth's atmosphere or surface, as well as in particle accelerators. They often possess higher energy levels compared to neutrons found in reactors. Many of these neutrons activate atomic nuclei before reaching the Earth's surface, while a smaller fraction interact with nuclei in the atmospheric air.[3] When these neutrons interact with nitrogen-14 atoms, they can transform them intocarbon-14 (14C), which is extensively utilized inradiocarbon dating.[4]
Cold,thermal andhot neutron radiation is most commonly used inscattering anddiffraction experiments, to assess the properties and the structure of materials incrystallography,condensed matter physics,biology,solid state chemistry,materials science,geology,mineralogy, and related sciences. Neutron radiation is also used inBoron Neutron Capture Therapy to treat cancerous tumors due to its highly penetrating and damaging nature to cellular structure. Neutrons can also be used for imaging of industrial parts termedneutron radiography when using film, neutron radioscopy when taking a digital image, such as through image plates, andneutron tomography for three-dimensional images.Neutron imaging is commonly used in the nuclear industry,[5] the space and aerospace industry,[6] as well as the high reliability explosives industry.
Neutron radiation is often calledindirectlyionizing radiation. It does not ionize atoms in the same way that charged particles such asprotons andelectrons 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 thegamma ray (photon) 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.[7] Because neutrons are uncharged, they are more penetrating thanalpha radiation orbeta radiation. In some cases they are more penetrating than gamma radiation, which is impeded in materials of highatomic number. In materials of low atomic number such ashydrogen, a low energy gamma ray may be more penetrating than a high energy neutron.[8][9]
Inhealth physics, neutron radiation is a type of radiation hazard. Another, more severe hazard of neutron radiation, isneutron activation, the ability of neutron radiation to induceradioactivity in most substances it encounters, including bodily tissues.[10] This occurs through the capture of neutrons by atomic nuclei, which are transformed to anothernuclide, frequently aradionuclide. This process accounts for much of the radioactive material released by the detonation of anuclear weapon. It is also a problem in nuclear fission and nuclear fusion installations as it gradually renders the equipment radioactive such that eventually it must be replaced and disposed of as low-levelradioactive waste.
Neutronradiation protection relies onradiation shielding. Due to the high kinetic energy of neutrons, this radiation is considered the most severe and dangerous radiation to the whole body when it is exposed to external radiation sources. In comparison to conventional ionizing radiation based on photons or charged particles, neutrons are repeatedly bounced and slowed (absorbed) by light nuclei so hydrogen-rich material is more effective at shielding thaniron nuclei. The light atoms serve to slow down the neutrons byelastic scattering so they can then be absorbed bynuclear reactions. However, gamma radiation is often produced in such reactions, so additional shielding must be provided to absorb it. Care must be taken to avoid using materials whose nuclei undergo fission orneutron capture that causesradioactive decay of nuclei, producing gamma rays.
Neutrons readily pass through most material, and hence the absorbed dose (measured ingrays) from a given amount of radiation is low, but interact enough to cause biological damage. The most effective shielding materials arewater, orhydrocarbons likepolyethylene orparaffin wax. Water-extended polyester (WEP) is effective as a shielding wall in harsh environments due to its high hydrogen content and resistance to fire, allowing it to be used in a range of nuclear, health physics, and defense industries.[11] Hydrogen-based materials are suitable for shielding as they are proper barriers against radiation.[12]
Concrete (where a considerable number of water molecules chemically bind to the cement) andgravel provide a cheap solution due to their combined shielding of both gamma rays and neutrons.Boron is also an excellent neutron absorber (and also undergoes some neutron scattering). Boron decays into carbon or helium and produces virtually no gamma radiation withboron carbide, a shield commonly used where concrete would be cost prohibitive. Commercially, tanks of water or fuel oil, concrete, gravel, and B4C are common shields that surround areas of large amounts ofneutron flux, e.g., nuclear reactors. Boron-impregnated silica glass, standardborosilicate glass, high-boron steel, paraffin, andPlexiglas have niche uses.
Because neutrons that strike the hydrogen nucleus (proton, ordeuteron) impart energy to that nucleus, they in turn break from their chemical bonds and travel a short distance before stopping. Such hydrogen nuclei are highlinear energy transfer particles, and are in turn stopped by ionization of the material they travel through. Consequently, in living tissue, neutrons have a relatively highrelative biological effectiveness, and are roughly ten times more effective at causing biological damage compared to gamma or beta radiation of equivalent energy exposure. These neutrons can either cause cells to change in their functionality or to completely stop replicating, causing damage to the body over time.[13] Neutrons are particularly damaging to soft tissues like thecornea of the eye.
High-energy neutrons damage and degrade materials over time; bombardment of materials with neutrons createscollision cascades that can producepoint defects anddislocations in the material, the creation of which is the primary driver behind microstructural changes occurring over time in materials exposed to radiation. At high neutronfluences this can lead toembrittlement of metals and other materials, and toneutron-induced swelling in some of them. This poses a problem for nuclear reactor vessels and significantly limits their lifetime (which can be somewhat prolonged by controlledannealing of the vessel, reducing the number of the built-up dislocations). Graphiteneutron moderator blocks are especially susceptible to this effect, known asWigner effect, and must be annealed periodically. TheWindscale fire was caused by a mishap during such an annealing operation.
Radiation damage to materials occurs as a result of the interaction of an energetic incident particle (a neutron, or otherwise) with a lattice atom in the material. The collision causes a massive transfer of kinetic energy to the lattice atom, which is displaced from its lattice site, becoming what is known as theprimary knock-on atom (PKA). Because the PKA is surrounded by other lattice atoms, its displacement and passage through the lattice results in many subsequent collisions and the creations of additional knock-on atoms, producing what is known as the collision cascade or displacement cascade. The knock-on atoms lose energy with each collision, and terminate asinterstitials, effectively creating a series ofFrenkel defects in the lattice. Heat is also created as a result of the collisions (from electronic energy loss), as are possiblytransmuted atoms. The magnitude of the damage is such that a single 1MeV neutron creating a PKA in an iron lattice produces approximately 1,100 Frenkel pairs.[14] The entire cascade event occurs over a timescale of 1 × 10−13 seconds, and therefore, can only be "observed" in computer simulations of the event.[15]
The knock-on atoms terminate in non-equilibrium interstitial lattice positions, many of which annihilate themselves by diffusing back into neighboring vacant lattice sites and restore the ordered lattice. Those that do not or cannot leave vacancies, which causes a local rise in the vacancy concentration far above that of the equilibrium concentration. These vacancies tend to migrate as a result ofthermal diffusion towards vacancy sinks (i.e.,grain boundaries,dislocations) but exist for significant amounts of time, during which additional high-energy particles bombard the lattice, creating collision cascades and additional vacancies, which migrate towards sinks. The main effect of irradiation in a lattice is the significant and persistent flux of defects to sinks in what is known as thedefect wind. Vacancies can also annihilate by combining with one another to formdislocation loops and later,lattice voids.[14]
The collision cascade creates many more vacancies and interstitials in the material than equilibrium for a given temperature, anddiffusivity in the material is dramatically increased as a result. This leads to an effect calledradiation-enhanced diffusion, which leads to microstructural evolution of the material over time. The mechanisms leading to the evolution of the microstructure are many, may vary with temperature, flux, and fluence, and are a subject of extensive study.[16]
The mechanical effects of these mechanisms includeirradiation hardening,embrittlement,creep, andenvironmentally-assisted cracking. The defect clusters, dislocation loops, voids, bubbles, and precipitates produced as a result of radiation in a material all contribute to the strengthening andembrittlement (loss ofductility) in the material.[20] Embrittlement is of particular concern for the material comprising the reactor pressure vessel, where as a result the energy required to fracture the vessel decreases significantly. It is possible to restore ductility by annealing the defects out, and much of the life-extension of nuclear reactors depends on the ability to safely do so.Creep is also greatly accelerated in irradiated materials, though not as a result of the enhanced diffusivities, but rather as a result of the interaction between lattice stress and the developing microstructure. Environmentally-assisted cracking or, more specifically,irradiation-assisted stress corrosion cracking (IASCC) is observed especially in alloys subject to neutron radiation and in contact with water, caused byhydrogen absorption at crack tips resulting fromradiolysis of the water, leading to a reduction in the required energy to propagate the crack.[14]
{{cite web}}: CS1 maint: multiple names: authors list (link){{cite book}}:|website= ignored (help)CS1 maint: DOI inactive as of July 2025 (link)https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.111.222501
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||
