Monday 26 May 2008

Meteor pistols! Sweet!

I can only apologise for my once-again-ruined update schedule. I've been preoccupied with research and personal matters. This story was too cool to overlook:

Science probe for 'space pistols'

Given pride of place in an unassuming museum on the East Coast of America is a pair of 200-year-old duelling pistols shrouded in mystery. The intricately decorated guns were said to have been forged from the iron of a fallen meteorite.

SILVER-CLAD SPACE PISTOLS! In order to figure out whether their metal really is distinctly meteoritey (honestly I'm not all that good with the solid state), they're subjecting the pistols to a tour-de-force of non-destructive chemical analysis. That's where it gets fun for me.

The big technique which the BBC is excited about is neutron diffraction at the ISIS facility. Imagine light shining through a glass crystal - it's refracted and scattered by the material. If you use finer and finer wavelengths, eventually the light (by now, X-rays) is scattered in a very organised manner by the atoms in the crystal. The pattern of atoms in that crystal can be described by a "repeating unit" which just repeats and stacks up through space, so light interacts with the whole crystal in a very well-defined way. Through some rather irksome maths, you can figure out where the atoms are in the repeating unit, and their size and therefore identity. Now you know the chemical structure of your crystal! This is "single-crystal X-ray diffraction".

For various technical reasons, X-ray diffraction doesn't provide the most in-depth information in the world. X-rays let you see the atoms by scattering through the electrons, so the clarity drops off as you move to smaller and smaller atoms, and at wide angles. Neutrons are big, heavy particles, but if you can scatter those through the sample (and that's where the big cool stuff from the BBC article comes in) then you get much better results. In fact, you can even see hydrogen atoms - the smallest atoms of all - and their nuclei.

This is all a gross simplification so I'm sure someone in Central Facilities work will pop along to point out how hilariously inaccurate this is, especially that I've not discussed powder X-ray diffraction instead of single crystal and so on, but I digress.

The other technique they used was X-ray fluorescence. The everyday sort of fluorescence that I'm used to - like you'll see if hold a banknote under UV light, for example - happens by moving electrons about. The electrons in an atom or a molecule fill into "levels", like steps on a staircase. The incoming light gives energy to an electron in the molecule, kicking it up to an unusually high level, like moving it up several steps. It then loses some of that energy, basically as heat, until the only way for it to drop in any energy is by a really big step. That big step is achieved by giving out some light again, but because we've already lost some energy, the colour of light is different (lower-energy light has a longer wavelength). That's why tonic water, for example, can take in high energy, invisible UV light and give out a low energy, visible green glow. The specific structure of the steps determines what wavelength of light the molecule absorbs, and what wavelength of light it gives back out, so by knowing the fluorescences, we can (if we're lucky) tell what molecule we're looking at.

That's everyday fluorescence. X-rays are still electromagnetic radiation, like light, but each packet of X-ray "light" has a really stupendous amount of energy. If you drop that energy onto an electron, it's not just going up a few steps, it's going to be thrown right up the staircase and out the window! That's what we call ionising radiation - it can rip charges right out of atoms and molecules. However the particles in the nucleus of the atom have a step layout too, with much bigger energy gaps between the steps. They can take the energy from a packet of X-rays, and go through this whole dissipate-some-energy-then-re-emit thing. Again, the wavelength of energy it takes in, and the different wavelength it gives out, is pretty specific to the atom. So we shoot different wavelengths of X-rays at something, and watch for fluorescences, and we can identify the materials in it.

These techniques are both really useful because they don't damage the guns! Usually to tell what's in something, chemically, you have to take a bit of it off, and dissolve it, and react it with stuff, or burn it, or shine some lasers through it. That's not really something you want to do to potential METEOR PISTOLS.

Anyway, getting back to the job in hand... it turns out they're not space pistols. In fact, the handles aren't even silver, they're an unusual kind of brass. Well, at least the analytical chemists had fun.

No comments: