Dilution Refrigeration
Cooling far below 1 K is not possible using conventional refrigeration techniques; however, a 'dilution refrigerator' can cool below 2 mK in continuous operation. This section describes the basic operation of a dilution refrigerator.
Of the numerous unique features exhibited by helium, one of the most convenient for a low temperature physicist is that at sufficiently low temperatures (about 0.8 K), a mixture of helium-3 and helium-4 will spontaneously separate, with the lighter helium-3 rich fraction floating on top of the heavier helium-4 rich fraction.
All helium atoms (whether the lighter helium-3 atoms or the more abundant helium-4 atoms) are attracted to one another by van der Waals forces; however, as a helium-3 atom is much lighter than its helium-4 counterpart, it has a much more violent zero point motion. As a consequence, the helium-3 atoms are further apart from each other than the helium-4 atoms; as a comparison helium-3 is 26% less dense than helium-4. As the atoms in the helium-4 rich fraction can pack together more closely, the forces between the atoms are greater. As a result, all helium atoms prefer to be surrounded by helium-4 atoms.
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