Principle

In classic dilution cryostats the liquid helium component (4.2 K) and the dilution component are combined in the same vessel. The film below [1] shows a dilution insert with the two functions separated. The insert is fitted below the liquid helium cryostat and the cold is produced in a mixing box.

In either case, gravity plays an essential role. These devices must not be overturned or even inclined.

Satellites function in zero gravity, and the sample in a four-circle diffractometer like D10 rotates in every direction. This means that, if we want to cool using ³He/⁴He dilution, we need to find a way to obtain the mix without gravity, and that is what we see in the movie below [2].

The film shows a series of neutron images taken on Neutrograph at ILL in 2005. We can see the progressive dilution of the helium 3 droplets in helium 4 inside a capillary tube. The in situ observation of cold being produced in the D10 cryostat is possible because helium-4 is almost transparent to neutrons, whilst helium-3 is very opaque (isotopic effect).