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A neutron supermirror is a highly polished, layered material used to reflect neutron beams. Supermirrors are a special case of multi-layer neutron reflectors with varying layer thicknesses.
The first neutron supermirror concept was proposed by Mezei, inspired by earlier work with x-rays.
Supermirrors are produced by depositing alternating layers of strongly contrasting substances, such as nickel and titanium, on a smooth substrate. A single layer of high refractive index material exhibits total external reflection at small grazing angles up to a critical angle θ c {\displaystyle \theta _{c}}. For nickel with natural isotopic abundances, θ c {\displaystyle \theta _{c}} in degrees is approximately 0.1 ⋅ λ {\displaystyle 0.1\cdot \lambda } where λ {\displaystyle \lambda } is the neutron wavelength in Angstrom units.
A mirror with a larger effective critical angle can be made by exploiting diffraction that occurs from stacked multilayers. The critical angle of total reflection, in degrees, becomes approximately 0.1 ⋅ λ ⋅ m {\displaystyle 0.1\cdot \lambda \cdot m} , where m {\displaystyle m} is the "m-value" relative to natural nickel. m {\displaystyle m} values in the range of 1-3 are common, in specific areas for high-divergence m=6 is readily available.