Section 4.4.2.4.2. Focusing mirrors

Optical imaging of neutrons can be achieved using ellipsoidal or torroidal mirrors, but, owing to the small critical angle of reflection, the dimensions of the mirrors themselves and the radii of curvature must be large. For example, a 4 m long toroidal mirror has been installed at the IN15 neutron spin echo spectrometer at the Institut Laue–Langevin, Grenoble (Hayes et al., 1996), to focus neutrons with wavelengths greater than 15 Å. The mirror has an in-plane radius of curvature of 408.75 m, and the sagittal radius is 280 mm. A coating of ⁶⁵Cu is used to obtain a high critical angle of reflection while maintaining a low surface roughness. Slope errors of less than 2.5 × 10⁻⁵ rad (r.m.s.) combined with a surface roughness of less than 3 Å allow a minimum resolvable scattering vector of about 5 × 10⁻⁴ Å⁻¹ to be reached.

For best results, the slope errors and the surface roughness must be low, in particular in small-angle scattering applications, since diffuse scattering from surface roughness gives rise to a halo around the image point. Owing to its low thermal expansion coefficient, highly polished Zerodur is often chosen as substrate.