Supermirrors are a special case of multi-layer neutron reflectors with varying layer thicknesses.
[1] The first neutron supermirror concept was proposed by Ferenc Mezei,[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
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
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
This has a strong bearing on the economic strategy of neutron instrument design.