Neutron supermirror

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.

The first neutron supermirror concept was proposed by Ferenc 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 (e.g. nickel) exhibits total external reflection at small grazing angles up to a critical angle ${\theta }_c$. For nickel with natural isotopic abundances, ${\theta }_c$ in degrees is approximately $0.1\cdot \lambda$ where $\lambda$ 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. The critical angle of total reflection, in degrees, becomes approximately $0.1\cdot \lambda \cdot m$, where $m$ is the "m-value" relative to natural nickel. Values of $m$ 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 $\propto m^z$, with $z$ in the range 2–4, which affects cost. This has a strong bearing on the economic strategy of neutron instrument design.