To calculate the magnitude of the correction of measured neutron structure factors for thermal diffuse scattering, it is necessary to know the ratio of the sound velocity in the crystal, cs, to the neutron velocity, vn. For β(= cs/vn) < 1, the correction is independent of vn and is evaluated using exactly the same procedure as for X-ray structure factors. For β > 1, the correction depends on vn and requires a more complicated analysis than for X-rays, although under certain conditions (discussed in the text) there is no correction at all to the measured intensities. Accurate neutron structure factor measurements are conveniently made with neutrons which are faster than the maximum sound velocity in the crystal; for a `soft' material such as lead this condition implies that the neutron wavelength does not exceed 1.70 Å, whereas for tungsten, a `hard' material, the upper limit is 0.77 Å.
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Β. Τ. M. Willis (1970) studied this question.