Capture of a neutron increases mass number without immediately changing atomic number. It avoids the charged-particle Coulomb barrier. Subsequent beta-minus decay can increase atomic number. Whether capture is slow or fast compared with relevant beta decays distinguishes the slow neutron-capture process from the rapid neutron-capture process.
After neutron capture on , the compound nucleus is . With negligible incoming kinetic energy, conservation of energy gives in natural units. This is the neutron separation energy of the compound nucleus. At finite incident energy, add the kinetic energy in the initial centre-of-momentum frame; in a nonrelativistic heavy-target approximation it is times the laboratory neutron kinetic energy. Nuclear pairing can significantly change capture excitation between neighboring even- and odd-mass targets.
