Thermal neutron beams are either beam tubes or guide tubes looking at the D2O moderator. They extract thermal neutrons from the reactor and feed the instruments listed below. Thermal neutrons have energies in the range 10 to 100 meV (i.e. about 3 to 1 Å or 1400 to 4000 m/s). Their wavelength is commensurate with inter-atomic and inter-molecular distances and their energy is in the range of that of molecular motions in materials.
Thermal beam tubes: IH3, IH4, H6, H7, H10, H11, H12, H13
Instruments: BRISP, D15, D19, D2B, D20, IN4, IN8, IN20, PN3
Thermal neutron guides: H22, H23, H24, H25
Instruments: Cyclops, D1B, D10, D20, D23, IN3, IN13, IN22, LADI-III, OrientExpress, S18, SALSA, VIVALDI
Thermal neutrons from the D2O moderator are extracted from the reactor through neutron beam tubes. These are often called thimbles.
A thimble is a sealed tube, under vacuum or filled with helium. One of its ends is very close to the reactor core, where the neutron flux reaches a maximum. It is directed either towards the primary moderator (heavy water) or towards a secondary moderator (hot source, cold source). The outer end is beyond the reactor shield.
Scientific instruments are installed at the outer end of the tubes and their first task is to select the desired portion of the neutron spectrum.
The photos shown here were taken during a thimble replacement. The lifetime of a thimble is generally shorter than that of the reactor vessel itself.