Neutron absorption

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

Neutron capture plays an important role in the cosmic nucleosynthesis of heavy elements. In stars it can proceed in two ways: as a rapid (r-process) or a slow process (s-process). Nuclei of masses greater than 56 cannot be formed by thermonuclear reactions (i.e. by nuclear fusion), but can be formed by neutron capture.

At small neutron flux, as in a nuclear reactor, a single neutron is captured by a nucleus. For example, when natural gold (¹⁹⁷Au) is irradiated by neutrons, the isotope ¹⁹⁸Au is formed in a highly excited state, and quickly decays to the ground state of ¹⁹⁸Au by the emission of γ rays. In this process, the mass number increases by one. This is written as a formula in the form ¹⁹⁷Au+n → ¹⁹⁸Au+γ, or in short form ¹⁹⁷Au(n,γ)¹⁹⁸Au. If thermal neutrons are used, the process is called thermal capture.

The isotope ¹⁹⁸Au is a beta emitter that decays into the mercury isotope ¹⁹⁸Hg. In this process the atomic number rises by one.

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 in between neutron captures. The mass number therefore rises by a large amount while the atomic number (i.e., the element) stays the same. Only afterwards, the highly unstable nuclei decay via many β⁻ decays to stable or unstable nuclei of high atomic number.

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