The nucleus can capture a neutron only if the kinetic energy of the neutron is close to the energy of one of the energy levels of the new nucleus formed as a result of capture. The capture cross section of such a neutron by the nucleus increases sharply. The energy at which the neutron-nucleus interaction cross section reaches a maximum is called the resonance energy. The resonance energy range is divided into two parts, the region of resolved and unresolved resonances. The first region occupies the energy interval from 1 eV to E. In this region, the energy resolution of the instruments is sufficient enough to distinguish any resonance peak. Starting from the energy E, the distance between resonance peaks becomes smaller than the energy resolution. Subsequently, the resonance peaks are not separated. For heavy elements, the boundary energy E≈1 keV.
In thermal neutron reactors, the main resonant neutron absorber is Uranium-238. In the table for U, several resonance neutron energies E, the maximum absorption cross sections σ in the peak, and the width G of these resonances are given.