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.[1]
The first neutron supermirror concept was proposed by,[2] 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 (e.g. nickel) exhibits total external reflection at small grazing angles up to a critical angle
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0.1 ⋅ λ
λ
A mirror with a larger effective critical angle can be made by exploiting diffraction (with non-zero losses) that occurs from stacked multilayers.[3] The critical angle of total reflection, in degrees, becomes approximately
0.1 ⋅ λ ⋅ m
m
m
Nickel has a positive scattering cross section, and titanium has a negative scattering cross section, and in both elements the absorption cross section is small, which makes Ni-Ti the most efficient technology with neutrons. The number of Ni-Ti layers needed increases rapidly as
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z