Editing Neutron moderator (section)

===Distribution of neutron velocities===
After sufficient impacts, the speed of the neutron will be comparable to the speed of the nuclei given by thermal motion; this neutron is then called a [[thermal neutron]], and the process may also be termed ''thermalization''. Once at equilibrium at a given temperature the distribution of speeds (energies) expected of rigid spheres scattering elastically is given by the [[Maxwell–Boltzmann distribution]]. This is only slightly modified in a real moderator due to the speed (energy) dependence of the absorption cross-section of most materials, so that low-speed neutrons are preferentially absorbed,<ref name="DB" /><ref>[http://www.ncnr.nist.gov/resources/n-lengths/ Neutron scattering lengths and cross sections] V.F. Sears, ''Neutron News'' 3, No. 3, 26-37 (1992)</ref> so that the true neutron velocity distribution in the core would be slightly hotter than predicted.