neutronabsorption

Neutron absorption is the process by which a neutron is captured by a nucleus, forming a heavier isotope and often releasing energy, typically in the form of gamma radiation. It is a major mechanism by which neutrons are removed from a flux, alongside scattering and, in reactors, fission.

The probability of absorption is described by the microscopic cross section, sigma_a, with units of barns. The macroscopic absorption cross section is Sigma_a = N sigma_a, where N is the number density of target nuclei. The mean free path for absorption is lambda = 1/Sigma_a. Absorption depends strongly on neutron energy: thermal neutrons (around 0.025 eV) often have larger capture cross sections for certain isotopes, such as boron-10, cadmium-113, and gadolinium-157, while fast neutrons generally have smaller cross sections except at resonances.

Resonance absorption occurs when neutron energy aligns with specific nuclear energy levels, producing sharp increases in cross section at particular energies. The capture process can lead to activation, where the formed radionuclide undergoes beta decay or other transmutations, producing radioactive products.

Common neutron absorbers include boron-10 used in borated polymers, cadmium-113 and hafnium in control materials, and gadolinium-157 and europium in shielding. In shielding, composite materials such as borated polyethylene or gadolinium-loaded substances are employed to attenuate neutron flux effectively.

In reactor physics, absorption competes with scattering and fission and is central to reactor control, safety, and material performance. The choice of absorbers depends on the neutron energy spectrum and desired reactivity effects, with attention to activation and compatibility with surrounding materials.