← nuclearjs fission scattering
Neutron resonances
A slow neutron that enters ²³⁸U is not scattered by a hard ball: it is caught, and its energy is shared among all 239 nucleons, like a billiard ball shot into a full rack. The compound nucleus ²³⁹U then lives far longer than the time to cross it, and forgets how it was made: it decays by a gamma ray or by sending a neutron back out, in proportions set by its widths alone. It can only form at the energies of its own levels, which sit just above the neutron separation energy, packed about 22 eV apart for the s-wave levels alone (about 6 eV counting the p-wave levels as well): the forest of resonances.