Neutron capture is anuclear reaction in which anatomic nucleus and one or moreneutrons collide and merge to form a heavier nucleus.^[1] Since neutrons have no electric charge, they can enter a nucleus more easily than positively chargedprotons, which are repelledelectrostatically.^[1]

Neutron capture plays a significant role in the cosmicnucleosynthesis of heavy elements. In stars it can proceed in two ways: as a rapid process (r-process) or a slow process (s-process).^[1] Nuclei ofmasses greater than 56cannot be formed by exothermicthermonuclear reactions (i.e., bynuclear fusion) but can be formed by neutron capture.^[1]Neutron capture on protons yields a line at 2.223 MeV predicted^[2] and commonly observed^[3] insolar flares.

At smallneutron flux, as in anuclear reactor, a single neutron is captured by a nucleus. For example, when naturalgold (¹⁹⁷Au) is irradiated by neutrons (n), theisotope¹⁹⁸Au is formed in a highly excited state, and quickly decays to the ground state of¹⁹⁸Au by the emission ofgamma rays (𝛾). In this process, themass number increases by one. This is written as a formula in the form¹⁹⁷Au + n →¹⁹⁸Au + γ, or in short form¹⁹⁷Au(n,γ)¹⁹⁸Au. Ifthermal neutrons are used, the process is called thermal capture.

The isotope¹⁹⁸Au is abeta emitter that decays into the mercury isotope¹⁹⁸Hg. In this process, theatomic number rises by one.

Ther-process happens inside stars if the neutron flux density is so high that the atomic nucleus has no time to decay via beta emission between neutron captures. The mass number therefore rises by a large amount while the atomic number (i.e., the element) stays the same. When further neutron capture is no longer possible, the highly unstable nuclei decay via many β⁻ decays tobeta-stable isotopes of higher-numbered elements.

The absorptionneutron cross section of an isotope of achemical element is the effective cross-sectional area that an atom of that isotope presents to absorption and is a measure of the probability of neutron capture. It is usually measured inbarns.

Absorption cross section is often highly dependent onneutron energy. In general, the likelihood of absorption is proportional to the time the neutron is in the vicinity of the nucleus. The time spent in the vicinity of the nucleus is inversely proportional to the relative velocity between the neutron and nucleus. Other more specific issues modify this general principle. Two of the most specified measures are the cross section forthermal neutron absorption and the resonance integral, which considers the contribution of absorption peaks at certain neutron energies specific to a particularnuclide, usually above the thermal range, but encountered asneutron moderation slows the neutron from an original high energy.

The thermal energy of the nucleus also has an effect; as temperatures rise,Doppler broadening increases the chance of catching a resonance peak. In particular, the increase inuranium-238's ability to absorb neutrons at higher temperatures (and to do so without fissioning) is a negativefeedback mechanism that helps keep nuclear reactors under control.

Neutron capture is involved in the formation of isotopes of chemical elements. The energy of neutron capture thus intervenes^{[clarification needed]} in thestandard enthalpy of formation of isotopes.

Neutron activation analysis can be used to remotely detect the chemical composition of materials. This is because different elements release different characteristic radiation when they absorb neutrons. This makes it useful in many fields related to mineral exploration and security.

In engineering, the most important neutron absorber is¹⁰B, used asboron carbide in nuclear reactorcontrol rods or asboric acid as acoolant water additive inpressurized water reactors. Other neutron absorbers used in nuclear reactors arexenon,cadmium,hafnium,gadolinium,cobalt,samarium,titanium,dysprosium,erbium,europium,molybdenum andytterbium.^[4] All of these occur in nature as mixtures of various isotopes, some of which are excellent neutron absorbers. They may occur in compounds such as molybdenum boride,hafnium diboride,titanium diboride,dysprosium titanate andgadolinium titanate.

Hafnium absorbs neutrons avidly and it can be used in reactorcontrol rods. However, it is found in the same ores aszirconium, which shares the same outerelectron shell configuration and thus has similar chemical properties. Their nuclear properties are profoundly different: hafnium absorbs neutrons 600 times better than zirconium. The latter, being essentially transparent to neutrons, is prized for internal reactor parts, including the metallic cladding of thefuel rods. To use these elements in their respective applications, the zirconium must be separated from the naturally co-occurring hafnium. This can be accomplished economically withion-exchange resins.^[5]

• Beta decay
• Induced radioactivity
• List of particles
• Neutron emission
• Radioactive decay
• Rays:α —β —γ —δ
• p-process (proton capture)

• XSPlot an online neutron cross section plotter
• Thermal Neutron Capture Data
• Thermal Neutron Cross Sections at the International Atomic Energy Agency