3He

The lighter helium isotope, ³He, is not something that most people have ever heard of.  ³He is one neutron shy of the typical helium atom, and is present at a level of around 13 atoms per 10 million atoms of regular helium.  Every now and then there is some discussion out there in the sci-fi/futurist part of the world that we should mine the moon for ³He as a potential fuel for fusion reactors.  However, it turns out that ³He has uses that are much more down to earth.  

For example, in its pure form it can be used as the working fluid in an evaporative refrigerator.  Just as you cool off your tea by blowing across the top and allowing the most energetic water molecules to be carried away, it is possible to cool liquid helium by pumping away the gas above it.  In the case of regular ⁴He, the lowest temperature that you can reach this way ends up being about 1.1 K.  (Remember, helium is special in that at low pressures in bulk it remains a liquid all the way down as far as you care to go.)  This limit happens because the vapor pressure of ⁴He drops exponentially at very low temperatures - it doesn't matter how big a vacuum pump you have; you simply can't pull any more gas molecules away.  In contrast, ³He is lighter, as well as being a fermion (and thus obeying different quantum statistics than its heavier sibling).  This difference in properties means that it can get down to more like 0.26 K before its vapor pressure is so low that further pumping is useless.  (You don't throw away the pumped ³He.  You recycle it.)  This is the principle behind the ³He refrigerator.


You can do even better than that.  If you cool a mixture of ³He and ⁴He down well below 1 K, it will spontaneously separate into a ³He-rich phase (the concentrated phase, nearly pure), and a dilute phase of 6% ³He dissolved in 94% ⁴He.  At these temperatures the ⁴He is a superfluid, meaning that in many ways it acts like vacuum as far as the ³He atoms are concerned.  If you pump away the (nearly pure ³He) gas above the dilute phase, more ³He atoms are pulled out of the concentrated phase and into the dilute phase to maintain the 6% solubility.  This lets you evaporatively cool the concentrated phase much further, all the way down to milliKelvin temperatures.  (The trick is to run this in closed-cycle, so that the ³He atoms eventually end up back in the concentrated phase.)  This is the principle behind the dilution refrigerator, or "dil fridge".

Unfortunately, right now there is a major shortage of ³He.  Its price has shot up by something like a factor of 20 in the last year, and it's hard to get any at all.  This is a huge problem for a large number of (mostly) condensed matter physicists, as reported in the October issue of Physics Today (reprinted here (pdf)).  The reasons are complicated, but the proximate causes are an increase in demand (it's great for neutron detectors, which are handy if you're looking for nuclear weapons) and a decrease in supply (it comes from decay of tritium, mostly from triggers for nuclear warheads).  There are ways to fix this issue, but it will take time and cost money.  In the meantime, my sympathies go out to experimentalists who have spent their startups on fridges that they can't get running.