Helium temperatures in a four-circle neutron diffractometer

The cooling of single-crystal samples to helium temperatures within a four-circle neutron diffractometer has been achieved by a novel continuous-flow system. The system relies solely on helium for its cooling power with two flow circuits being used. The first delivers liquid to a reservoir cryostat of capacity 1-4 l located close to the Eulerian cradle of the diffractometer and the second draws the refrigerant from this cryostat via a flexible transfer tube into the sample region of a compact cooling unit mounted off the cradle. This arrangement allows unhindered motion of the diffractometer circles for sample temperature operation in the range 3.9-300 K. The system requires a total helium flow of 1.0 l h^-1 to maintain a temperature of 20±0.1 K, decreasing to 0.5 l h^-1 at higher temperatures.