Hot neutron instruments are installed at the outer end of neutron beam tubes pointing at the Hot Source, a graphite cylinder heated to 2000 K. This secondary moderator locally enhance the reactor flux in hot neutrons.

Hot neutrons are neutrons which are fast, energetic and with short wavelengths (100 to 1000 meV, i.e. about 1 to 0.2 Å or 4000 to 14000 m/s). They are highly valuable for the study of liquids, amorphous and small cell crystalline materials, high energy dynamics and for nuclear and particle physics.

Hot beam tubes: H3, H4, H8, H9

Instruments:
D3, D4, D9, IN1-Lagrange, PN1

Views of a typical neutron beam tube

Hot neutrons 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.

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.