Neutron radiation

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Neutron radiation is a form of

free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new nuclides—which, in turn, may trigger further neutron radiation. Free neutrons are unstable, decaying into a proton, an electron, plus an electron antineutrino. Free neutrons have a mean lifetime of 887 seconds (14 minutes, 47 seconds).[1]

Neutron radiation is distinct from

gamma radiation.[2]

Sources

Main article: Neutron source
See also: Category:Neutron sources

Neutrons may be emitted from nuclear fusion or nuclear fission, or from other nuclear reactions such as radioactive decay or particle interactions with cosmic rays or within particle accelerators. Large neutron sources are rare, and usually limited to large-sized devices such as nuclear reactors or particle accelerators, including the Spallation Neutron Source.

Neutron radiation was discovered from observing an alpha particle colliding with a beryllium nucleus, which was transformed into a carbon nucleus while 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

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 nuclear 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, the

fast neutron reactors) and all nuclear weapons
rely on fast neutrons.

Cosmogenic neutrons

Main article:
Cosmogenic neutron

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 into carbon-14 (14C), which is extensively utilized in radiocarbon dating.[4]

Uses

neutron radiography when using film, neutron radioscopy when taking a digital image, such as through image plates, and neutron 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.

Ionization mechanisms and properties

Neutron radiation is often called indirectly

beta radiation. In some cases they are more penetrating than gamma radiation, which is impeded in materials of high atomic number. In materials of low atomic number such as hydrogen
, a low energy gamma ray may be more penetrating than a high energy neutron.

Health hazards and protection

In

radioactivity in most substances it encounters, including bodily tissues.[7] 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 such that eventually it must be replaced and disposed of as low-level radioactive waste
.

Neutron

radiation 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 than iron nuclei. 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 must be provided to absorb it. Care must be taken to avoid using materials whose nuclei undergo fission or neutron capture that causes radioactive decay
of nuclei, producing gamma rays.

Neutrons readily pass through most material, and hence the absorbed dose (measured in grays) from a given amount of radiation is low, but interact enough to cause biological damage. The most effective shielding materials are water, or hydrocarbons like polyethylene or paraffin 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.[8] Hydrogen-based materials are suitable for shielding as they are proper barriers against radiation.[9]

Plexiglas
have niche uses.

Because neutrons that strike the hydrogen nucleus (

deuteron) impart energy to that nucleus, they in turn break from their chemical bonds and travel a short distance before stopping. Such hydrogen nuclei are high linear energy transfer particles, and are in turn stopped by ionization of the material they travel through. Consequently, in living tissue, neutrons have a relatively high relative 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.[10] Neutrons are particularly damaging to soft tissues like the cornea
of the eye.

Effects on materials

High-energy neutrons damage and degrade materials over time; bombardment of materials with neutrons creates

point defects
and
annealing of the vessel, reducing the number of the built-up dislocations). Graphite neutron moderator blocks are especially susceptible to this effect, known as Wigner effect, and must be annealed periodically. The Windscale 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 the

MeV neutron creating a PKA in an iron lattice produces approximately 1,100 Frenkel pairs.[11] 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.[12]

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 of

dislocation loops and later, lattice voids.[11]

The collision cascade creates many more vacancies and interstitials in the material than equilibrium for a given temperature, and diffusivity in the material is dramatically increased as a result. This leads to an effect called radiation-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.[13]

  • Radiation-induced segregation results from the aforementioned flux of vacancies to sinks, implying a flux of lattice atoms away from sinks; but not necessarily in the same proportion to alloy composition in the case of an alloyed material. These fluxes may therefore lead to depletion of alloying elements in the vicinity of sinks. For the flux of interstitials introduced by the cascade, the effect is reversed: the interstitials diffuse toward sinks resulting in alloy enrichment near the sink.[11]
  • Zircaloys are subject to the creation of dislocation loops, but do not exhibit void formation. Instead, the loops form on particular lattice planes, and can lead to irradiation-induced growth, a phenomenon distinct from swelling, but that can also produce significant dimensional changes in an alloy.[15]
  • Irradiation of materials can also induce
    phase transformations in the material: in the case of a solid solution, the solute enrichment or depletion at sinks radiation-induced segregation can lead to the precipitation of new phases in the material.[16]

The mechanical effects of these mechanisms include irradiation hardening, embrittlement, creep, and environmentally-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 and embrittlement (loss of ductility) in the material.[17] 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 by hydrogen absorption at crack tips resulting from radiolysis of the water, leading to a reduction in the required energy to propagate the crack.[11]

See also

References

  1. S2CID 17006418
    .
  2. ^ "What Are The Different Types of Radiation?".
  3. ^ "Cosmogenic Nucleide Principle - CEREGE". www.cerege.fr. 2022-10-26. Retrieved 2024-07-16.
  4. ^ "What is Carbon Dating? | University of Chicago News". news.uchicago.edu. Retrieved 2024-09-19.
  5. ISSN 1875-3892
    .
  6. ^ www.ndt.net http://web.archive.org/web/20220603210640/https://www.ndt.net/article/wcndt2004/pdf/aerospace/264_bastuerk.pdf. Archived from the original (PDF) on 2022-06-03. Retrieved 2025-03-17. {{cite web}}: Missing or empty |title= (help)
  7. ^ "How Radiation Damages Tissue". Michigan State University. Retrieved 2017-12-21.
  8. ^ "Neutron Radiation Shielding". www.frontier-cf252.com. Frontier Technology Corporation. Retrieved 2017-12-21.
  9. ^ Carrillo, Héctor René Vega (2006-05-15). "Neutron Shielding Performance of Water-Extended Polyester" (PDF). TA-3 Dosimetry and Instrumentation. Retrieved 2017-12-21.
  10. ^ Specialist, WPI, Environmental Information Services -- Shawn Denny, Information Architect; Mike Pizzuti, Graphic Designer; Chelene Neal, Web Information Specialist; Kate Bessiere, Web Information. "Advisory Committee On Human Radiation Experiments Final Report". ehss.energy.gov. Retrieved 2017-12-21.{{cite web}}: CS1 maint: multiple names: authors list (link)
  11. ^ a b c d Dunand, David. "Materials in Nuclear Power Generation." Materials Science & Engineering 381: Materials for Energy Efficient Technology. Northwestern University, Evanston. 3 Feb. 2015. Lecture
  12. ^ A. Struchbery, E. Bezakova "Thermal-Spike Lifetime from Picosecond-Duration Preequilibrium Effects in Hyperfine Magnetic Fields Following Ion Implantation". 3 May. 1999.
  13. doi:10.1155/2012/905474
    .
  14. S2CID 4238714
    .
  15. ^ Adamson, R. "Effects of Neutron Radiation on Microstructure and the Properties of Zircaloy" 1977. 08 Feb. 2015.
  16. ^ Hyun Ju Jin, Tae Kyu Kim. "Neutron irradiation performance of Zircaloy-4 under research reactor operating conditions." Annals of Nuclear Energy. 13 Sept. 2014 Web. 08 Feb. 2015.
  17. ISBN 978-0-8031-0539-3. {{cite book}}: |website= ignored (help)CS1 maint: DOI inactive as of March 2025 (link
    )

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.111.222501

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