Friday, 1 December 2006

Polonium Puzzles

Alexander Litvinenko was apparently killed with Polonium 210 and now another poor individual, Mario Scaramella, has been found to have the chemical in his system. While this is clearly tragic, it does raise some questions.

Firstly where would you get Polonium 210? It is really rare. The best natural source of Polonium is Uranium ore, where it occurs at a density of roughly 100 micrograms per metric ton. So you have to make it, typically by exposing Bismuth to a high neutron flux: you will need a reactor or conceivably a particle accelerator for this. There aren't too many facilities that are up to this job, and so global production is less than 100g a year. It would be cheaper and easier to beat someone to death with a baseball bat made of solid platinum than to use Polonium.

Once you have made your Polonium you have to transport it. At first sight, this isn't going to be difficult: Polonium is a dull grey metal. The problem is that it is extremely radioactive: a milligram emits as many alpha particles as 5 grams of radium. Now here is where it gets funky: because Po 210 is such an energetic emitter, it heats itself up with its own radioactivity and slowly boils away. A gram of Po 210 generates 140 watts of power: an extraordinary amount. So your lump of metal will slowly vapourise.

The answer is probably to dissolve it in something. Chemically Polonium is similar to Thallium and not too different from Lead, so you could use dilute sulphuric acid for instance to create a solution of Polonium sulphide. But don't go spilling it. The toxic dose of Polonium is around a picogram: it is 250 billion times as toxic as hydrogen cyanide. Even if you dissolved a microgram in ten litres of water, each drop would be lethal. So is it really credible that someone transported this stuff by plane, then poisoned Litvinenko with it in a busy restaurant while managing not to ingest any of this extremely toxic chemical themselves?

Finally, how on earth did they discover Polonium in the restaurants, planes and so on? It's an alpha emitter, not gamma, so it isn't easy to locate. Something like Thorium could be easily spotted using a gamma ray spectrometer, but alphas get stopped so quickly in air, you would have to be really close to find potential contamination, then you would need a mass spectrometer to confirm what caused it. So what's going on?

Update. They have not found Polonium in the planes, in the restaurant, etc. They have found radioactivity. Presumably this is consistent with Polonium spillage or excretion -- the small amounts Litvinenko would have been sweating out after the initial poisoning are probably detectable -- but it would interesting to know exactly what has been found.

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