Experimental demonstration of phase determination in neutron reflectometry by variation of the surrounding media

We have recently shown that it is possible to unambiguously determine the real part of the complex reflection coefficient for a thin film structure by measuring the neutron specular reflectivities for the him in contact with two different backing or two different fronting media, thus simplifying the reference methodology of phase determination and making it more practical. Here we demonstrate the technique in two different experiments. In the first, one backing medium is air, the other, heavy water. In the second, sapphire and silicon serve as two different fronting (incident) media. In addition, we show how the two possible branches of the imaginary part of the reflection coefficient inferred by the real part, while not required for inverting the data to find the film structure, can be a sensitive diagnostic for indicating whether the him of interest is inhomogeneous on a transverse scale comparable to the neutron coherence length. This is especially important in avoiding misinterpretation of data which result from the incoherent average of reflectivities from large-scale heterogeneities. (C) 2000 Elsevier Science B.V. All rights reserved.