X-ray and neutron scattering from macroscopically rough surfaces and interfaces is considered and a new method of analysis based on the variation of the shape of the total reflection edge in the reflectivity profile is proposed. It was shown that in the limit that the correlation length and the height of the surface roughness are larger than the wavelength (at least 100 times bigger) of the incoming beam, the total reflection edge in the reflection profile becomes rounded. This technique allows direct analysis of the variation of the reflectivity pro le in terms of the structure of the surface pro le in the particular case where the height variation of the surface texture is smooth and small compare to the surface correlation length (surface profile with slow variation).
Goldar, A., Roser, S., Hughes, A., Edler, K., & Gerstenberg, M. C. (2002). The effect of surface texture on total reflection of neutrons and X-rays from modified interfaces. Physical Chemistry Chemical Physics, 4(11), 2379-2386. https://doi.org/10.1039/b200410k