Study of Off-Specular Neutron Reflectivity Using a Model System

The specular reflection of neutrons gives information on the neutron refractive-index profile normal to an interface. The refractive index is simply related to the scattering-length density and hence the composition. However, in-plane inhomogeneities or corrugated surfaces can give rise to strong off-specular scattering. This has been observed in several systems. The difficulty arises in the interpretation of off-specular data from systems having poorly defined or unknown structure. We have measured off-specular reflection from a gold replica optical diffraction grating with 1 μm line spacing. A series of data was obtained using the CRISP neutron reflectometer at the Rutherford Appleton Laboratory in Oxfordshire, England. The analysis of specular and off-specular data, and the determination of the periodicity of the grating and the thickness of the metal film will be discussed. The maximum periodicity that could be observed was 42 μm. This is in agreement with the off-specular resolution which is determined by the horizontal divergence of the ribbon-shaped beam, but very much larger than the longitudinal coherence length of the neutron wave packet.