What is the correct description of the slow neutron scattering in a gas?

The description of thermal neutron scattering in a noble gas is discussed. It is shown that the commonly used Placzek's approach is not consistent with Turchin's formalism, which is nicely confirmed by Monte Carlo calculations. A proposal is made concerning a more adequate description of the thermal motion of gas atoms in diffraction effect under slow neutron scattering by gas atoms