Cosmogenic neutrons are produced from cosmic radiation in the Earth's atmosphere or surface, as well as in particle accelerators.
[4] Cold, thermal and hot neutron radiation is most commonly used in scattering and diffraction experiments, to assess the properties and the structure of materials in crystallography, condensed matter physics, biology, solid state chemistry, materials science, geology, mineralogy, and related sciences.
[5] This occurs through the capture of neutrons by atomic nuclei, which are transformed to another nuclide, frequently a radionuclide.
The light atoms serve to slow down the neutrons by elastic scattering so they can then be absorbed by nuclear reactions.
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.
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.
[7] Concrete (where a considerable number of water molecules chemically bind to the cement) and gravel provide a cheap solution due to their combined shielding of both gamma rays and neutrons.
Commercially, tanks of water or fuel oil, concrete, gravel, and B4C are common shields that surround areas of large amounts of neutron flux, e.g., nuclear reactors.
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.
At high neutron fluences this can lead to embrittlement of metals and other materials, and to neutron-induced swelling in some of them.
The knock-on atoms lose energy with each collision, and terminate as interstitials, effectively creating a series of Frenkel defects in the lattice.
Heat is also created as a result of the collisions (from electronic energy loss), as are possibly transmuted atoms.
The magnitude of the damage is such that a single 1 MeV neutron creating a PKA in an iron lattice produces approximately 1,100 Frenkel pairs.
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.
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.