In a system without leakage or absorption, fission neutrons are slowed down by successive collisions with moderator atoms until their energy reaches an equilibrium with the thermal motion of the moderator. This means that in such an ideal system the low-energy part of the neutron spectrum is a Maxwellian distribution corresponding to the moderator temperature. In real systems a certain number of neutrons is absorbed or leaks out of the reactor before reaching this equilibrium.

While the slowing-down of fast neutrons is not influenced by the thermal motion of the moderator, below a certain neutron energy this thermal motion is no longer negligible in comparison with the neutron energy. In this lower energy range (thermal energy range) neutrons can gain energy in a collision (up-scattering).

As a Maxwellian distribution goes only asymptotically to zero toward higher energies, the upper limit of the thermal range cannot be exactly defined but it must be set at an energy above which up-scattering becomes negligible.

Usually energies of 1 eV or lower are used, but in HTRs, because of the high temperature, limits between 2 and 4 eV are more appropriate. In case of negligible neutron losses the scattering properties of the moderator are unimportant because the neutron spectrum is always a Maxwellian distribution. The higher the losses, the more the neutron spectrum depends on the competition between loss and moderation, and hence on the scattering properties of the moderator. At high neutron energies all moderators can be treated as monoatomic gases, but at low neutron energy the molecular and crystal forces binding the moderator atoms start playing a role in the moderation process.