Slow 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, D2O) or towards a secondary moderator (hot source, cold source). The outer end is beyond the reactor shield.

Thru-going thermal beam tube H6-H7

H6 is one side of the thru-going thermal neutron beam tube H6-H7. It feeds the gamma spectrometer PN3-GAMS4.

Tube filled with helium.
Neutron flux: 5 x 1014 cm-2 s-1