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 gr. In this region, the energy resolution of the instruments is sufficient enough to distinguish any resonance peak. Starting from the energy E gr, 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 gr ≈1 keV. In thermal neutron reactors, the main resonant neutron absorber is Uranium-238. In the table for U, several resonance neutron energies E r, the maximum absorption cross sections σ a, r in the peak, and the width G of these resonances are given.