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.

Thermal beam tube H11

H11 is a thermal neutron beam tube feeding the single crystal diffractometer D19, the powder difractometers D2B and D20.


Capture flux




H11 D2O 2.6 1010 2.0 1010

The capture flux φc = ∫(∂φ(λ)/∂λ)(λ/1.8 Å)dλ is measured with the activation of a gold foil at the outlet of the reactor shielding (September 2003).