Neutron Guides and Mirrors
Neutrons as particle waves follow the same law for the total reflection as light waves. Neutrons can move freely in vacuum. To construct a neutron mirror or neutron guide the first question to solve was to find a medium for neutrons, which is optically "thinner" than vacuum. Scientists found such a medium, which is a multi-layer system containing layers with alternating scattering characteristics (scattering length) and decreasing thickness.
Nowadays one can produce neutron mirrors made of several hundred double layers of nickel-titan. The limiting angle for total reflection of a neutron mirror is much smaller than for light in the optical glass fiber; it also depends on the neutron wavelength's. A rough formula for the limiting angle for neutrons is 1° times the wavelength of neutrons in nano meter, which results for the wavelength of 0.5 nm to approximately 0.5°. This small angular range for the total reflection reduces the efficiency in comparison to an optical glass fiber, but it still allows transporting neutrons to a remarkable distance up to 10 - 50 meter.
The transmission of neutrons is improved by using so-called super-mirrors, with limiting angles larger by factors 2 - 3 as compared to normal neutron mirrors. Super-mirrors are using for example layer systems with alternating layers of pure isotopes 58Ni and 62Ni, which differ even more in their neutron scattering properties (scattering length). As such layer systems are rather expensive (material costs) there are limits of possible applications to just a few cases.
Texts: Th. Wilpert , Ch. Schulz
