International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. C. ch. 2.9, pp. 129130

One of the simplest, yet powerful, examples of the use of neutron reflectivity is in the study of selfdiffusion. Most techniques to measure diffusion coefficients rely on chemical and mechanical methods to measure density profiles after a sample has been annealed. Then a model for the diffusion is assumed, and the coefficients are calculated. Using standard techniques, researchers are unable to detect the movement of an atom through a sample of like atoms. However, using single bilayers of amorphous ^{10}B and ^{11}B, it was shown (Smith, Hamilton, Fitzsimmons, Baker, Hubbard, Nastasi, Hirvonen & Zocco, 1992) through neutronreflectivity measurements that the diffusion of boron in boron could be measured by studying the density profile (see Figs. 2.9.7.1 and 2.9.7.2 ) of one isotope in the other as a function of annealing time. Also, because of the sensitivity of the technique to the interfacial density profile, it was found that standard Fickian diffusion models could not explain the measured density profiles.
References
Smith, G. S., Hamilton, W., Fitzsimmons, M., Baker, S. M., Hubbard, K. M., Nastasi, M., Hirvonen, J.P. & Zocco, T. G. (1992). Neutron reflectivity study of thermallyinduced boron diffusion in amorphous elemental boron. SPIE Proc. Ser. 1738, 246–253.Google Scholar