It is usually measured in barns.Ībsorption cross section is often highly dependent on neutron energy. The absorption neutron cross section of an isotope of a chemical 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. When further neutron capture is no longer possible, the highly unstable nuclei decay via many β − decays to beta-stable isotopes of higher-numbered elements. The mass number therefore rises by a large amount while the atomic number (i.e., the element) stays the same. The r-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. In this process, the atomic number rises by one. The isotope 198Au is a beta emitter that decays into the mercury isotope 198Hg. If thermal neutrons are used, the process is called thermal capture. This is written as a formula in the form 197Au + n → 198Au + γ, or in short form 197Au(n,γ) 198Au. In this process, the mass number increases by one. For example, when natural gold ( 197Au) is irradiated by neutrons (n), the isotope 198Au is formed in a highly excited state, and quickly decays to the ground state of 198Au by the emission of gamma rays (𝛾). Neutron capture at small neutron flux Decay scheme of 198AuĪt small neutron flux, as in a nuclear reactor, a single neutron is captured by a nucleus. Neutron capture on protons yields a line at 2.223 MeV predicted and commonly observed in solar flares. Nuclei of masses greater than 56 cannot be formed by thermonuclear reactions (i.e., by nuclear fusion) but can be formed by neutron capture. In stars it can proceed in two ways: as a rapid process ( r-process) or a slow process ( s-process).

Neutron capture plays a significant role in the cosmic nucleosynthesis of heavy elements. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, which are repelled electrostatically. Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus.

Beta decay vs electron capture