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# Neutron supermirror

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 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 ${\displaystyle \theta _{c}}$. For nickel with natural isotopic abundances, ${\displaystyle \theta _{c}}$ in degrees is approximately ${\displaystyle 0.1\cdot \lambda }$ where ${\displaystyle \lambda }$ is the neutron wavelength in Angstrom units.

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 ${\displaystyle 0.1\cdot \lambda \cdot m}$, where ${\displaystyle m}$ is the "m-value" relative to natural nickel. ${\displaystyle m}$ values in the range of 1-3 are common, in specific areas for high-divergence (e.g. using focussing optics near the source, choppers, or experimental areas) m=6 is readily available.

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 ${\displaystyle \propto m^{z}}$, with ${\displaystyle z}$ in the range 2-4, which affects the cost. This has a strong bearing on the economic strategy of neutron instrument design [4].

## References

1. ^ Chupp, T. "Neutron Optics and Polarization" (PDF). Retrieved 16 April 2019.
2. ^ Mezei, F (1976). "Novel polarized neutron devices: supermirror and spin component amplifier". Communications on Physics (London). 1 (3): 81–85.
3. ^ Hayter, J. B.; Mook, H. A. (1989). "Discrete Thin-Film Multilayer Design for X-ray and Neutron Supermirrors". J. Appl. Cryst. 22: 35–41.
4. ^ Bentley, PM. "Instrument suite cost optimisation in a science megaproject". Journal of Physics Communications. 4 (4): 045014. doi:10.1088/2399-6528/ab8a06.