Neutron capture

Neutron capture occurs when a nucleus absorbs a neutron, forming a compound nucleus in an excited state. The excitation energy equals the neutron's binding energy plus its kinetic energy. For thermal neutrons (energies around 0.025 eV), the capture cross-section often follows a 1/v law, where the probability decreases with increasing neutron speed. However, resonances—sharp peaks at specific energies—can dramatically enhance capture, as seen in isotopes like ¹¹³Cd and ¹⁵⁷Gd, which have enormous thermal cross-sections of thousands of barns. The resulting compound nucleus typically de-excites by emitting gamma rays (radiative capture) or, in fissile isotopes, by undergoing fission. The cross-section is measured in barns (10⁻²⁴ cm²) and varies by orders of magnitude across the periodic table.

Neutron capture is the primary mechanism for building elements heavier than iron in stars. The slow process (s-process) occurs in asymptotic giant branch (AGB) stars, where neutrons are produced by reactions like ¹³C(α,n)¹⁶O. The s-process operates on timescales of years, allowing beta decay to compete with further capture, and produces about half of the heavy elements up to bismuth. The rapid process (r-process) happens in explosive environments like supernovae and neutron star mergers, where neutron fluxes are so high that captures outpace beta decay, enabling the synthesis of the heaviest elements, including uranium and thorium. The r-process is responsible for the abundance peaks at neutron magic numbers, such as A=130 and A=195.

In nuclear reactors, neutron capture is essential for controlling the chain reaction. Control rods use materials like boron or cadmium, which have high capture cross-sections, to absorb neutrons and regulate power. Capture also produces activation products in structural materials, leading to radioactive waste. In medicine, neutron capture therapy (NCT) exploits the high cross-section of ¹⁰B or ¹⁵⁷Gd to destroy cancer cells: after a boron compound is delivered to a tumor, irradiation with thermal neutrons triggers a capture reaction that releases alpha particles and lithium nuclei, killing the cells. Additionally, neutron capture is used in neutron activation analysis (NAA), a sensitive technique for detecting trace elements in samples, and in the production of medical isotopes like ⁹⁹Mo, which decays to ^99mTc.

Beyond mainstream applications, neutron capture has niche roles. In archaeology, NAA has been used to fingerprint the provenance of obsidian tools and pottery. In geochemistry, it helps date meteorites via the production of ³⁶Cl. A curious edge case is the capture on ³He, which produces tritium and a proton—a reaction used in neutron detectors. The ¹⁴N(n,p)¹⁴C reaction is the basis of radiocarbon dating, though it is a capture that emits a proton rather than a gamma ray. Historically, the discovery of neutron capture by Enrico Fermi in 1934 led to the first transuranium elements, though he misinterpreted the results as new elements. The s-process was first proposed by Margaret Burbidge, Geoffrey Burbidge, William Fowler, and Fred Hoyle in their famous 1957 paper, which laid the foundation for modern nucleosynthesis theory.

Cross-section

A measure of the probability of a nuclear reaction, expressed in barns.

Thermal neutron

A neutron in thermal equilibrium with its surroundings, typically having energy of about 0.025 eV.

s-process

Slow neutron capture process in stars, occurring over long timescales.

r-process

Rapid neutron capture process in explosive environments, producing heavy elements.

Radiative capture

Neutron capture followed by emission of gamma rays.