Continuous SANS Instrument Components
A brief description of the main components of reactor-based SANS instruments follows: -- Cold neutrons are transported through total internal reflection at glancing angles inside neutron guides. These transmit neutrons from the cold source to the entrance of scattering instruments with little loss. -- Beam filters (for example, Be for neutrons and Bi for gammas) are used to clean up the beam from unwanted epithermal neutrons (Be transmits neutrons with wavelengths > 4 A) and gamma radiation (stopped by high-Z materials as Bi). Note that if a curved guide is used, no filter is needed because there is no direct line-of-sight from the reactor core (no gammas in the beam) and curved guides transmit only wavelengths above a cutoff wavelength (no epithermal neutrons in the beam). Typical filter thickness is between 15 cm and 20 cm. For better effectiveness, filters are cooled down to liquid nitrogen temperature.