On the trail of polonium – The history of polonium-210
Following the poisoning and death of the former Russian spy, Alexander Litvinenko, I have been assailed by kith and kin to give an account of the poisoning agent, polonium-210. The substance is one of the most fascinating elements in the periodic table and has been used by experimental physicists to help unravel the structure of the atom. This dangerous and mysterious element is also a vital component of atomic bombs.
Polonium (Po), a highly radioactive metalloid, was discovered in pitchblende, a uranium ore, by Marie and Pierre Curie in 1898. This was a considerable achievement as one tonne of pitchblende contains only 100µg of polonium. The husband and wife team subsequently discovered radium, another product of uranium decay, which has become well known since its use in the treatment of cancer. For these discoveries, the Curies shared with Henri Becquerel the 1903 Nobel Prize for Physics.
Producing polonium from radium
For cancer treatment, radium was employed, not as a direct source of radiation, but as a provider of one of its decay products - the inert radioactive gas, radon. Samples of radon would be extracted from a radium source and sealed in small glass tubes known as ‘seeds'. These could then be placed adjacent to a tumour that would be exposed to intense gamma irradiation. The main types of ‘natural' radioactive rays are
- Alpha - two protons and two neutrons bound together.
- Beta - accelerated electrons.
- Gamma - penetrating electromagnetic radiation.
The gamma rays generated in the tubes would come more from the decay products of radon, particularly bismuth-214, than from the gas itself. Because of short half-lives, the seeds soon became inactive so the treatment centre would rapidly accumulate numerous spent seeds. These were eagerly sought after by experimental physicists as polonium is also a decay product of radon and used seeds contain small but useful amounts of this element. Most of the experiments described below were in fact carried out with polonium from this source.
Polonium-induced beryllium radiation and discovery of the neutron
In 1928, physicists Bothe and Becker from the University of Giessen, Germany, discovered that if beryllium was irradiated with alpha particles from polonium, a very intense and penetrating irradiation beam was produced. In January 1932, the Joliot-Curies reported that polonium-induced beryllium radiation caused paraffin wax to emit protons. They attributed this to the emission of gamma rays from the beryllium.
A month later, James Chadwick at the Cavendish Laboratory in Cambridge, UK, demonstrated that the alpha bombardment of beryllium generates not gamma radiation but a stream of uncharged particles of mass close to that of the proton. In other words, Chadwick had confirmed the existence of the neutron. This was a most important observation - if the neutron had remained undiscovered for just a few more years, it is unlikely that uranium fission would have been discovered before the outbreak of World War II and hence the atom bomb would not have been created. It is not without significance that the trigger, the initiator which kick-started the chain reaction in the bombs, was a small piece of beryllium in contact with a tiny piece of polonium.
The Litvinenko murder is something of an enigma. To obtain polonium would seem to be the action of a sophisticated and well-connected person, but such an individual would also know that polonium leaves a radioactive trace whenever it is used or transported. Such a trail might surely lead the investigating authorities right back to the perpetrator of the crime.