Sunday, 3 December 2006

Polonium 210 as a murder weapon

The death of Alexandr Litvenenko and the expected death of his contact the academic Mario Scaramella from the effects of Polonium 210 have caused a storm of comment within the blogsphere and the conventional media most of it, typical of topics with a technial background, long on opinion and short on facts. Rather than rely on the uneven authority of Wikipedia, here is an accessible background paper on this evil substance, but the physical properties of polonium 210 raise a number of questions. Your comments would be appreciated.

Polonium 210 (Po210) was originally called Radium F when Marie and Pierre Curie discovered it in 1898. It is a very rare mineral and extremely radioactive. Po210 is so radioactive that less than a milligram (one thousandth of a gram) will glow in the dark with blue Cherenkov radiation. Po210 is so radioactive that the metal containers used to hold it become hot to the touch and can become hot enough to burn and blister skin. If the container is placed on a laboratory bench it could cause the wood to char and smoke. Po210 has been used as a heat source for the thermoelectric generators in satellites. How do you secretly poison someone with this? It's not as though you could sprinkle it on their food like salt.

Po210 is usually produced as a very fine dust, finer than talcum powder - so fine in fact that the particles can be as small as a few hundred atoms - too small to see. As Po210 decays, it releases alpha radiation which gives the dust particles a negative electric charge. Each particle then repels the other, and the dust moves towards the top of its container as though it was actively trying to escape. You had better hope that the container has a good tight seal because the dust is so fine that it will creep out of its container, even through a screw thread, and then disperse on air currents. I have no doubt that if the Po210 was delivered in a container, then whoever handled it has spread it all over themselves and all over wherever they have been and whoever they have been in contact with. Perhaps the courier and the poisoner were considered expendable but that seems more like fiction. Intelligence services do not throw away the lives of their officers. This does not, of course, eliminate ignorance or stupidity.

The alpha radiation emitted by Po210 is not normally dangerous as even paper is thick enough to block it, but if the dust particles enter the system then Po210 will immediately start to kill any cells it comes into contact with: blood cells, immune system cells, kidney and liver cells, and body cells. As the cells die they allow the entry of opportunistic bacteria, so the sufferer presents with a constellation of symptoms that are impossible to diagnose until the classic effects of radiation poisoning appear. If you are poisoned with Po210, you shed radioactive material in tears, sweat, saliva, urine and faces, so you contaminate whatever you touch and whoever you touch.

Po210 has a half life of 138 days, long enough for even a fraction of a gram to do its work. Po210 decays into Lead 206 (Pb206) which is stable and would be undetectable because each of us who live in an industrial society has accumulated lead from petrol, paint and other sources.

Where do you get this stuff? It is too rare to extract from pitchblende, unless, like the Curies, you dedicate years to the task. Most Po210 is the product of nuclear reactors where Bismuth (Bi209) is bombarded with neutrons to give Bi210, the precursor to Po210. A few grams of Bi209 are needed to make a milligram of Po210, so a lengthy chemical extraction process is required. It might be possible for someone with access to a research reactor – say a post-doc student – to make a small amount but usually management of a reactor is so tight that whoever runs the reactor would be aware of this happening. This suggests a government agency.

The media is reporting that Litvenenko's urine contained Po210 and Thallium (some mention Ruthenium) which is puzzling. You would expect some bismuth contamination, but nothing else, unless something other than straight Po210 was used. There have also been reports in various US blogs that X-rays of Litvenenko's stomach show three unidentified objects of 2cm diameter. Do these people know how big 2cm is? That's the size of an Australian dollar coin. It takes a bit of effort to swallow something that big. There are individuals who swallow coins. It causes great pain and when they present at the emergency ward, they are immediately scheduled as involuntary psychiatric patients because swallowing coins is considered a type of self-harm. It seems out of character for both Litvenenko and Scaramella to do this.

Po210 is part of the Radon 222 (Rn222) decay series. Rn222 is is a radioactive gas and is surprisingly common in small concentrations, being found in most houses in granite areas. Georgius Agricola's De Re Metallica, a mediaeval text on geology, mentions that the mines of Bohemia and Ruthenia were considered to be guarded by the earth spirits, the gnomes, whose spells caused the miners to die young. These mines produced uranium which was widely used as a lead substitute to stain glass and to replace the lead-lighting in church windows. Rn222 is present in large concentrations in these mines and it was the Rn222 progeny, including Po210, that killed the miners. Perhaps the Po210 was delivered by inhalation.

The decay series for Rn222 progeny is:
. Rn222 (half-life 3.8 days) -> alpha particle gives
. Rn218 (half-life 3.05 minutes) -> alpha particle gives
. Pb214 (half-life 27 minutes) -> beta particle gives
. Bi214 (half-life 19.8 minutes) -> beta particle gives
. Po214 (half-life 180 microseconds) -> alpha particle gives
. Pb210 (half-life 22 years) -> alpha particle gives
. Bi210 (half-life 5 days) -> beta particle gives
. Po210 (half-life 138 days) -> alpha particle gives
. Pb206 stable

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