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For optical mirrors, see Perfect mirror.

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 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 . For nickel with natural isotopic abundances, in degrees is approximately where is the neutron wavelength in angstroms.

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 , where is the “m-value” relative to natural nickel. Values of 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), 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 , with in the range 2–4, which affects 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” (PDF). 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”. Journal of Applied Crystallography. 22 (1): 35–41. Bibcode:1989JApCr..22…35H. doi:10.1107/S0021889888010003. S2CID 94163755.
  4. ^ Bentley, P. M. (2020). “Instrument suite cost optimisation in a science megaproject”. Journal of Physics Communications. 4 (4): 045014. Bibcode:2020JPhCo…4d5014B. doi:10.1088/2399-6528/ab8a06.