
Approach to Absolute Zero 285
Hence the cooling power of dilution refrigerators goes down only quadratically
with temperature. In the cryostat, the latent heat was practically insensitive to tem-
perature, whereas in the dilution case it is ˙n
3
that is insensitive. A comparison of the
cooling rates for the same, but otherwise arbitrary, He3 pumping rate is shown in
figure 14.3.
As can easily be gathered from this, dilution refrigeration is far superrior to even
the He3 cryostat. The latter, as already remarked, becomes totally inefficient below
about 0.2 K. The dilution refrigeration, on the other hand, can cool objects to a few
mK. In the next section, ...