Abstract
Optical focusing using a long curved mirror at grazing incidence is a novelty in small angle neutron scattering (SANS) instrumentation. A good quality focused image is essential if such a device is to be used in a SANS spectrometer. On the one hand, a very smooth mirror surface is most important, since surface roughness produces a detrimental halo surrounding the focused image. This halo has been carefully investigated using X-ray and neutron measurements for different kinds of flat mirror samples. The best image quality is obtained with a highly polished ‘zerodur’ substrate coated with a 65Cu layer protected by a thin Al layer. On the other hand, the quality of the image strongly depends on parameters such as object size, mirror shape, wavelength distribution and gravity; the influence of these parameters has been investigated using ray-tracing Monte-Carlo simulations. These studies have encouraged us to build a 4 m long toroidal mirror to be installed late in 1995 at the new IN15 neutron spin-echo spectrometer at ILL.
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