A collision between a neutron and an atom is affected by the thermal motion of the atom, and in most cases the collision is also affected by the presence of other atoms nearby.

Depending on the compound (essentially, moderators) and on the neutron energy, two treatments are done. For slow neutrons (En < 4eV) and for carbon, water, etc., the crystalline structure as well as effects of chemical binding have to be taken into account via an S(a,0) treatment[31] (these S(a, 0) tables are special sets of ENDF files). For higher energies or other compounds, a free gas thermal treatment is applied: the medium is assumed to be a free gas, and, in the range of atomic weight and neutron energy where thermal effects are significant, the elastic scattering cross­section at zero temperature is assumed to be independent of neutron energy and the reaction cross-sections are independent of temperature; the elastic scattering cross-section at temperature T is then given by

where A is the atomic weight and En the neutron energy.^

Then the collision is sampled in the target-at-rest frame and the target laboratory velocity is chosen according to the Maxwell law P(V )/V exp(—V2/kT). However, if the elastic scattering cross-section has been processed (say, by NJOY) at the desired temperature, this treatment is not applied. The temperature entered in MCNP is used only to modify (if

Figure 5.2. Selection of nucleus k from N.

needed) elastic scattering, and thus total, cross-sections; no other Doppler effect is taken into account via this temperature.