The element was polonium, named after the double Nobel prizewinner’s native Poland. It is a bizarre choice of weapon for a poisoner — its use in a murder is thought to be unprecedented — but one that has proved brutally effective, both at causing death and at escaping detection by toxicologists.

Now that the source of the radiation that killed Alexander Litvinenko has been identified, however, the metal’s rarity could help to lead police to his killers.

Polonium-210, the isotope identified in Mr Litvinenko’s body, is known to be highly toxic and radioactive, and it is not easy to come by. It occurs naturally only at trace levels, and larger amounts that would be needed to kill are manufactured in nuclear reactors or particle accelerators by bombarding bismuth-209, a similar but inert metal, with neutrons.

Unlike certain other radioactive isotopes, such as caesium-137 and cobalt-60, it is not used in medical radiology, and therefore cannot be stolen from hospital waste. Its main use is in anti-static devices in industry, though in a form that could not be easily adapted for use as a poison. It is also sometimes used as a power source for satellites.

The difficulty of getting hold of polonium suggests that the killer was well resourced and possibly state-sponsored, scientists said.

Andrea Sella, lecturer in chemistry at University College London, said: “This is not the sort of thing that amateurs could have cooked up in a bathtub. You would have to go to a nuclear lab such as Oak Ridge, Los Alamos or Harwell — or to one of the Russian ones.”

Polonium-210 has a half life of 138 days, and emits alpha particles — helium atoms that are produced by radioactive decay. While these are easily stopped by the skin they become hugely damaging if they get inside the human body by swallowing, inhalation or through a wound. They cause extensive damage to tissues and to DNA, and could explain Mr Litvinenko’s symptoms.

The low hazards posed by alpha particles in the environment explains why the Health Protection Agency said that those who came into contact with Mr Litvinenko ought not to be at major risk. The poisoner would also have been little affected by the polonium.

Once inside the body the alpha radiation would have been difficult to detect and few toxicologists would have thought to look for polonium.

Samples from Mr Litvinenko’s organs will now be analysed to determine the dose of polonium-210 used to poison him. By measuring the extent of damage within his body, the pathology team hope to be able to give police a more precise estimate of the time at which he was poisoned.

Particles of polonium-210 are likely to have concentrated in the organs, particularly the liver but also in the colon, kidneys and brain. Cuttings taken from Mr Litvinenko’s hair and nails will be the focus of much of the laboratory analysis.

The three weeks it took to kill him is fast for a substance with a half-life of 138 days, which makes it likely he took a large dose, either in liquid or solid form.

Animal tests have been carried out using polonium-210 and the substance was shown to cause death quickly because of a rapid breakdown of organ functions.

Alistair Hay, professor of Environmental Toxicology at Leeds University, said: “Polonium-201 is very easy to transport. It could be carried around in a glass bottle and not penetrate through the glass.

“Polonium-201 is an incredibly radioactive substance, and a lot of damage can be done with small quantities.”

Alpha particles

The radiation emitted by polonium-210. They travel only a few centimetres and are stopped by a sheet of paper or a person's skin, but cause immense damage inside the body.

Source: Health Protection Agency

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