Sunday, 14 August 2011

Skeptics Guide to helium

Listening, as I do every week without fail, to the excellent and entertaining Skeptics Guide to the Universe podcast, I was reminded of my opinion that they really need to recruit someone who has a better familiarity with the physical sciences and engineering.  Whenever they talk about medicine, evolution and the mind, their words seem to make sense to me.  Of course I speak as a non-specialist in those topics, but still I have read enough on to be able to make a judgment that they are talking with authority.

They are also quite expert on the topic of logical fallacies and can spot one a mile away.  In honour of that I created my one page of Delusional Logic a few months ago.  (I have no doubt that the SGU panel could find gaps in my logic on that if they saw it.)

But this week's brief mention (episode 317) on the topic of helium was somewhat jumbled and confused.  First they concluded (2:09) that we must make helium, and that we "pretty much have to, as there is not a lot of natural helium around".  Later (3:42) they said that "we don't have a way to make it chemically.  We separate it from the air" - which was immediately corrected to "from the natural gas - right?"  Then they mentioned that the price was kept low for strategic reasons because the US government wanted to reduce its stockpile.

So - OK nobody can be a specialist at everything and they can be forgiven quite easily.  Besides that, their longer article about anti-matter in the Van Allen belts was fascinating and well researched.  However, for those who might be interested, let's set a few facts straight about helium.

Let's address the last item first.  The (US) Helium Privatization Act of 1996 was aimed to reduce the US government stockpile in a planned way without disrupting the market too much.  You can read about it in this 1996 CRS Report - The Federal Helium Program: The Reaction Over An Inert Gas.  The purpose of the act was actually  to offer the stockpile for sale at a price slightly above market value in order to release it slowly onto the market.  Helium is not especially cheap.  At wholesale prices, weight for weight liquid helium is worth about as much as a good malt whisky in the shops.  In smaller quantities you would expect to pay 5 times as much.  However you buy it, the storage vessels for helium are much more expensive than whisky bottles.  We use vacuum-insulated dewar flasks for liquid and high pressure metal bottles for gas.

Before describing where helium is actually sourced, let's examine a bit about its properties.

Helium is the second lightest of all the elements.  It has atomic number 2 (meaning that its nuclei contain 2 protons each).  Helium has two isotopes, namely helium-4 (the common form who's nuclei include 2 neutrons in addition to the protons) and helium-3 (the rarer form which occurs at about one part per million in common sources of helium-4, containing a single neutron).  Being from the family of inert gases (along with Neon, Argon and others) helium does not react with any other elements to form compounds. 

The nucleus of a helium-4 atom is created either by fusing hydrogen atoms together (as in the process that powers the sun and stars) or in the radioactive decay of larger atomic nuclei.  In radioactive decay a helium ion is ejected from the decaying nucleus as an 'alpha particle'.  It was the confirmation of the nature of alpha particles by Ernest Rutherford in 1908 that triggered the topic on the SGU, and they got this aspect quite correct.

Alpha particles get neutralised as they interact with other matter around them.  They 'steal' two spare electrons from the surroundings, and the alpha and its two captive electrons become very stable in their new form as a helium atom.  These atoms are so stable that it is very difficult to break them up at all.  It is the extremely high stability of the configuration of their electrons that prevents them from forming chemical compounds. 

Now we can see why we can't make helium chemically.  It is a physical process involving no chemistry whatever.  We do have processes that can be used to make helium (nuclear fusion being one) but we can't do it economically.  Therefore we have to 'find' helium and to 'mine' it.  Separating it from the air is an option, but its far too expensive to do that because it is only present at a few parts per million.

The place we find it is indeed in natural gas which comes from geological structures that have been successful at capturing the hydrocarbons that we use as fuel.  By accident these domed structures capture any gas that is created underneath them, and that includes the helium gas that is created by radioactive (alpha) decay in the rocks.  Helium can make up 20% or more of the natural gas that is extracted from the ground, and since it is totally chemically inert it is actually a nuisance to the companies who want to sell fuel.  They have to separate it out before they sell the gas.

As it happens, it is quite easy to do this because helium is the liquid with the lowest boiling point of all.  It is a liquid only at a temperature 4.2 degrees celsius above absolute zero.  Natural gas (methane) is easily liquefied at a much higher temperature and it is then readily shipped in liquid form.  This means that it is not a great inconvenience to have to carry out this process, but it is an inconvenience to have to capture the unliquefied helium and sell it.  It is much easier to vent it into the air where it is wasted.

Why does this matter?  What do we use it for?  In our everyday lives we see it used mainly to inflate party balloons.  We also know that deep sea divers use it with oxygen as the gas that they breath.  They do this because it is so inert.  Nitrogen becomes very toxic to humans if the pressure is very high, and divers suffer from nitrogen narcosis if they breath normal air.  But a mixture of oxygen and helium has no such effect. 

Surely we wouldn't care if there was no helium.  We just have to do without balloons and deep sea divers.

Well, not so fast!  Next time you hear about someone having an MRI scan in a hospital, think helium.  The superconducting magnet at the heart of the scanner is cooled by liquid helium.  Nothing else is cold enough to make the magnet work.  Without helium, cost-effective high-resolution MRI would be nearly impossible.  Modern MRI scanner magnets recycle their helium and lose very little of it, but they do need to be topped up in some cases.

Moving to more esoteric devices, we only understand the physics of semi-conductors so well because we can study their behaviour at ultra low temperatures.  Low temperatures simplify the picture by removing most of the thermal 'noise', making it possible to carry out sensitive measurements of the behaviour of electrons in semi-conductors.  To get to those low temperatures (perhaps just a few thousandths of a degree above absolute zero) we need helium - in fact both types of liquid helium together.

Looking to the future, we are slowly developing machines to harness the power of nuclear fusion.  The current large international project, ITER, is being built in the south of France and it will come into operation in about 10 years.  It will use huge superconducting magnets to control the hot plasma which is at over 100 million degrees.  Many experts are actually worried that we will finally learn how to use nuclear fusion as a commercially viable energy source just at the time that we run out of helium.  Although the fusion reaction actually produces helium, a commercial sized reactor would only make about 500kg per year - barely enough to replace losses from the cryogenic systems that cool the magnets.

Enough about helium for now.  Don't let my blog post put you off listening to a podcast that is one of the best around.  Try the Skeptics Guide to the Universe for yourself.

Coming soon: Helium-3. The precious little sister of helium-4.

p.s.  The Skeptics' Guide to the Universe Podcast #318 - Aug 17 2011 did a pretty good job of clearing up some of the points above. 

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