Wednesday, November 04, 2009

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.

7 comments:

DanM said...

The phrase "one neutron shy of a typical helium atom" would be a useful euphemism for some people I know, analogous to "a few cards shy of a full deck" or something.

Patrick said...

I shouldn't have inhaled all those helium balloons.

jonah said...

Dry fridges are the way to go. Once people learn en masse that those now exist, we'll never have to deal with bloody cryogens. Nevermind the carbon emissions...

Anonymous said...

Even a dry fridge needs mixture, unless someone has invented a pulse tube that goes < 100 mK.

BTW, I've heard single-shot charcoal sorbs can pump He-4 down to 0.7 K, though it's not very commonly done.

Doug Natelson said...

In the long term, dry tech is a good direction to go. Still, pulse tubes aren't the solution to everything. They're hideous power hogs. Needing an 11 kW constant electrical power to give 1 W of cooling power at 4.2 K is awful, when you remember that that same cooling power is available in the latent heat of 1 liter of liquid 4He. They also still have some vibrational issues. Finally, as anon. points out, to get really low temperatures without cryogens is hard. The best approach would be to do demag refrigeration, which can get you probably 12-24 hours at 100 mK starting from 4.2K, assuming no measurement heat load at all. If only the various all-electrical refrigeration schemes worked better....

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