Reducing edge enhancement effects in neutron imaging for metal surface studies

Imaging metal surfaces is essential for understanding material degradation and quantifying the associated chemical processes. Neutron imaging offers a variety of benefits, including the ability to image heavy metals, conduct attenuation-based studies, and its sensitivity to light elements such as hydrogen. However, the presence of neutron-optical artifacts is a main challenge in neutron imaging analysis of interfaces between different refractive index materials. A spread of the associated edge enhancement effect of tens and hundreds of micrometres makes surface imaging studies of corrosion layers a particular challenge. These effects cannot be eliminated completely but they can be mitigated by optimising instrument-dependent and sample-geometry parameters. In this work, we investigated edge enhancement effect as a function of neutron wavelength, neutron divergence, sample-to-detector distance, sample composition and sample geometry. Wavelength dependencies of edge enhancement effects indicate that their occurrence and strength is related to the wavelength range of Bragg diffraction. For strong, coherent neutron scatterers, such as steel, the strengths of wavelength-dependent edge enhancement effects deviate from the expected square dependence of the refractive index. Instead, they are significantly influenced by Bragg edge transmission, resulting in diminished edge enhancement below the Bragg cut-off. We present and demonstrate methods to reduce edge enhancement effects in neutron imaging at air-metal interfaces, thereby improving 2D and 3D surface analysis of metals, particularly through the targeted selection of a neutron wavelength range.