While the assumptions of this model are naive, it explains at least qualitatively the typical measured energy dependence of the neutron absorption cross section. For neutrons of wavelength much larger than typical radius of atomic nuclei (1–10 fm, E = 10–1000 keV) R can be neglected. For these low energy neutrons (such as thermal neutrons) the cross section σ σ ( E ) is inversely proportional to neutron velocity.