STUDY OF OFF-SPECULAR NEUTRON REFLECTIVITY USING A MODEL SYSTEM

The specular reflection of neutrons gives information on the neutron r efractive-index profile normal to an interface. The refractive index i s simply related to the scattering-length density and hence the compos ition. However, in-plane inhomogeneities or corrugated surfaces can gi ve rise to strong off-specular scattering. This has been observed in s everal systems. The difficulty arises in the interpretation of off-spe cular data from systems having poorly defined or unknown structure. We have measured off-specular reflection from a gold replica optical dif fraction grating with 1 mu m line spacing. A series of data was obtain ed using the CRISP neutron reflectometer at the Rutherford Appleton La boratory in Oxfordshire, England. The analysis of specular and off-spe cular data, and the determination of the periodicity of the grating an d the thickness of the metal film will be discussed. The maximum perio dicity that could be observed was 42 mu m. This is in agreement with t he off-specular resolution which is determined by the horizontal diver gence of the ribbon-shaped beam, but very much larger than the longitu dinal coherence length of the neutron wave packet.