THE FRIDGE

3He / 4He Dilution Refrigerators

The central part of any experiment is the dilution refrigerator, which should achieve a base temperature of around 10 mK. Cooling involves the continuous cycle of a 3He / 4He mixture, so a steady base temperature is maintained. This is more convenient than ``single shot'' methods such as adiabatic demagnetisation.

The cooling mechanism exploits the properties of 3He / 4He mixtures, shown in the diagram.

[ figure : phase diagram ]
Figure: Phase diagram for a mixture of 3He / 4He,
where x is the fractional content of the lighter isotope.

Below 0.87 K the mixture separates into two phases, a dilute (mainly 4He)and a concentrated (mainly 3 He). The concentrated phase sits on top of the dilute phase due to the relative densities. The vital property which allows cooling is that the 3 He concentration in the dilute phase is finite and remains finite (6.6%) at saturated vapour pressure even for T=0K. This finite solubility is of utmost importance for 3He / 4He Dilution Refrigerators. The 4He acts as an inert background since the 4 He atoms are Bosons in their quantum mechanical ground state at this temperature, but the 3 He Fermions still have a significant vapour pressure. Pumping on the lower dilute phase will therefore remove 3 He, causing 3He from the concentrated phase to cross the phase boundary to replace it. This crossing of the phase boundary can be thought of as a kind of evaporation, which removes heat from the system.

[ figure : dilution unit ]
Figure: A typical Oxford Instruments dilution unit
NUCLEAR ORIENTATION OLNO