Neutron guides allow the transmission of neutrons from the reactor away to the monochromators, or velocity selectors on the instruments. They use the principle of total reflection on glass or metallic surfaces, with nickel or multilayer supermirror coatings. The glancing angle depends on the coating and the wavelength of the neutrons. It is 0.1 times for natural nickel surfaces, e.g. 1 degree for 10 Å neutrons from a cold source. With supermirror coatings this angle becomes greater by a factor m=2 or m=3. Neutrons propagate by multiple reflection over lengths greater than 50 m.

The ILL guides are curved with radii between 10 and 27000 m. This allows instruments to be installed outside of the direct view of the radiation from the reactor core, giving a better signal to noise ratio. Also it cuts down the transmitted spectrum, in function of the guide geometry at the lower end of wavelengths.

Cold neutron guide H15

The neutron guides labeled H1x refer to its neutrons originating from the vertical cold source (VCS).

H15 is split into three guides H15, H151 and H152 which feed instruments IN6, D7 and D11 respectively.