Roughness replication in neutron supermirrors

• Published in: Journal of applied crystallography Vol. 50; no. 1; pp. 184 - 191
• Main Authors: Veres, Tamás, Sajti, Szilárd, Cser, László, Bálint, Szabolcs, Bottyán, László
• Format: Journal Article
• Language: English
• Published: 5 Abbey Square, Chester, Cheshire CH1 2HU, England International Union of Crystallography 01.02.2017; Blackwell Publishing Ltd
• Subjects: Correlation; Correlation analysis; Crystallography; Detectors; Interface roughness; neutron supermirrors; Neutrons; Parameters; Reflectance; resonant diffuse scattering; Roughness; roughness replication; Scattering; Surface roughness; X-rays
• ISSN: 1600-5767; 0021-8898; 1600-5767
• DOI: 10.1107/S1600576716019385

Neutron supermirrors (SMs), the major components of neutron optical devices, are depth‐graded d‐spacing multilayers of several hundreds to several thousands of bilayers. The interface roughness is a major factor in the reflectivity of multilayers. This influence is especially significant if the number of bilayers is large. In this work, the interface roughness and its correlations were studied in DC‐sputtered Ni–Ti neutron supermirrors. Detector scans were carried out to observe off‐specular neutron scattering in selected regions of the q space from (increasing bilayer thickness) normal‐ and (decreasing bilayer thickness) reverse‐layer‐sequence SMs. In‐plane and out‐of‐plane roughness correlations are manifested in diffuse scatter plateaus and peaks which are interpreted in terms of resonant diffuse scattering. Distorted wave Born approximation simulations quantitatively reproduce the characteristic features of the measured detector scans with reasonable roughness correlation parameters, i.e. in‐plane and out‐of‐plane correlation lengths, common interface roughness, and Hurst parameters. The different character of resonant diffuse scattering from normal‐ and reverse‐layer‐sequence SMs is qualitatively explained and systematized using quasi‐kinematical considerations in terms of material and SM parameters. The total off‐specular intensity of the supermirrors was found to be non‐monotonic with respect to the specular reflectivity at the corresponding angle of incidence. Long‐range interface roughness correlation in Ni–Ti neutron supermirrors is found to manifest in plateaus and peaks in the diffuse neutron scatter. The features are of resonant character and are strongly dependent on the type of the supermirror structure (slowly increasing or decreasing bilayer thickness). Distorted wave Born approximation calculations yield the statistical parameters of the interface roughness correlations.