Neutron guide

Neutron guides are evacuated glass tubes with a mostly rectangular cross-section (e.g. 2 cm × 10 cm), which are able to guide low-energy neutrons over long distances (several 10 m) out of a neutron source with little loss of intensity. They are mostly used in research centers in which as many experimental arrangements as possible have to be arranged around a complex neutron source (thermal reactor or spallation source ), e.g. B. at the Institut Laue-Langevin in Grenoble, at the FRM II in Munich and at the research reactor of the Helmholtz-Zentrum Berlin, the BER II .

The functional principle of the neutron guide is based on the fact that neutrons can also be described as waves of matter . Such a wave experiences refraction when passing through matter , as is known from visible light in water or glass. If the radiation hits the surface at a very flat angle, total reflection can occur, similar to light on the inside of a pane of glass. In the case of neutrons, this effect is very weak; H. the refractive index is only very slightly different from that in a vacuum, and only neutrons with a very long wavelength of the matter wave (i.e. low-energy, “cold” neutrons ) are reflected at a very small angle of incidence. The reflection coefficient of the neutron guide is therefore usually by coating the glass surfaces with a special nickel - isotope ( ⁵⁸ Ni) is increased. A mirror whose reflection coefficient is the same as that of a simple coating of natural nickel is referred to as an m = 1 mirror. Even greater reflection coefficients can be achieved with neutron super mirrors . They are then referred to as m = 2, m = 2.78, etc., based on the standardization m = 1 for nickel.