Effect of interfacial roughness correlation on diffuse scattering intensity in a neutron supermirror

Abstract

Neutron supermirrors are increasingly important devices for transporting, bending, and focusing neutron beams. Reflected neutrons from a supermirror are divided into specular and off-specular (diffuse) components. Suppression of the diffuse component is important since it reduces the signal-to-noise ratio, a serious problem when a supermirror is used in a focusing system for purposes such as small angle scattering measurements. The diffuse intensity can be decreased by more than one order of magnitude by adopting NiC/Ti multilayers instead of conventional Ni/Ti multilayers. In order to obtain insight into the mechanism that controls the diffuse intensity from a supermirror, the crystal structure of Ni and NiC monolayers and the interface structure of Ni/Ti and NiC/Ti multilayers were investigated. The crystallite size in the NiC monolayer was found to be smaller than that in the Ni monolayer by a factor of 4.1 by x-ray diffraction measurement. The interface structure of the Ni/Ti and NiC/Ti multilayers was observed by neutron reflectivity and diffuse intensity measurements. For the NiC/Ti multilayer, the lateral correlation length was smaller by a factor of 3.6 than for the Ni/Ti multilayer, whereas the vertical correlation length was greater by a factor exceeding 25. Diffuse intensity calculations based on the distorted wave Born approximation revealed that these differences explain the difference between the diffuse intensity of the Ni/Ti and NiC/Ti supermirrors. This study demonstrated that a multilayer with a large vertical correlation length and a small lateral correlation length effectively suppresses the diffuse intensity from a supermirror. © 2009 American Institute of Physics.