Under the Microscope/Prof William Reville:The recent poisoning of the former Russian spy Alexander Litvinenko by a lethal dose of polonium (Po-210) grabbed headlines worldwide.
The TV pictures of Litvinenko lying in his hospital bed, bald and aged looking, contrasted eerily with recent pictures of a fit, good-looking man with a fine head of hair. The poison used occurs naturally in rocks. We routinely ingest small amounts of Po-210, and it's a significant fraction of the risk posed by cigarettes.
There are 92 elements in nature. The smallest part of an element that can exist is the atom. The central nucleus of an atom contains almost all its mass. The nucleus contains two types of subatomic particles - protons and neutrons, which both have a mass of one atomic mass unit. The proton has a positive charge, neutrons have no charge. The nucleus is very small and almost all the volume of the atom is occupied by electrons that revolve around the nucleus. The electron has a very tiny mass and each electron has a negative charge. The number of electrons equals the number of protons.
The number of protons in the nucleus (atomic number) determines what element the atom represents. The number of protons plus the number of neutrons is the atomic mass. The atomic number of polonium is 84 and the nucleus also has 126 neutrons, so the atomic mass is 210. The nuclei of some elements are unstable. Stability is determined by the ratio of protons to neutrons. If the ratio is unstable the nucleus spits out bits of itself, thereby changing the ratio of protons to neutrons in an attempt to achieve stability.
Two types of particles can be emitted by the nucleus - alpha particles and beta particles. An alpha particle is composed of 2 protons and 2 neutrons. A beta particle is an electron. How can the nucleus emit an electron? Well, we can visualise the neutron as being a very close combination of a proton and an electron. This will have no charge and its mass will essentially be the same as a proton because the electron is so tiny. The origin of the beta particle is the electron embedded in the neutron. And of course when a neutron emits a beta particle it changes into a proton.
When a radioactive atom emits an alpha or a beta particle it changes its elemental identity (because the atomic number changes), transforming into a "daughter" element. If the nucleus of the daughter is still unstable it will in turn emit an alpha or a beta particle to form a granddaughter, and the process will continue until eventually a stable atom is reached. The whole sequence is called a radioactive decay scheme. After a particle is emitted from the nucleus, a burst of radiant energy is also released called a gamma ray.
There are several radioactive decay schemes in nature. The most important is the uranium decay scheme. Uranium (U-238) is naturally present in igneous rocks. It is radioactive and decays through 14 daughters, eventually reaching stable lead-94. One of the radioactive daughters is Po-210, which is naturally present in soil. It is taken up by growing plants, animals eat the plants, we eat the animals and the plants and so Po-210 enters our bodies in small amounts. Po-210 is an alpha emitter.
Alpha particles, beta particles and gamma rays differ greatly in their ability to penetrate matter. Alpha particles have very poor penetrating power and will not even pierce tissue paper. Beta particles are more penetrating but still will not penetrate more than a couple of millimetres into skin. Gamma rays are very penetrating.
Alpha, beta particles and gamma rays are collectively called ionising radiation because when they interact with matter they pull some electrons off the atoms they encounter, producing positively charged atoms - ions. This is damaging to the orderly structure of matter, eg it breaks covalent bonds between atoms. Much of the matter in the world is present as compounds or combinations of elements. The smallest part of a compound that can exist is the molecule. Water is a familiar compound, made of hydrogen and oxygen. In most compounds the atoms bond together by sharing electrons - covalent bonds.
When ionising radiation interacts with biological tissue it damages the biological molecules, potentially causing ill-health. If the dose of radiation is intense it can cause death within weeks. If the dose is small it may initiate a malignant tumour which will not express itself for many years, or else cause a hereditary defect to be expressed in a future generation.
Po-210 naturally concentrates on leaves of tobacco plants. When you smoke a cigarette it enters the lung where it irradiates the tissue at close quarters. Most smoking-related lung cancers are caused by radioactivity, according to the US surgeon general. Alpha particles are not a significant external radiation hazard as they cannot penetrate the skin. However if they get inside the body they can intensely irradiate tissue at close quarters. Po-210 is soluble and it can distribute itself widely around the tissues.
A small quantity of Po-210, even one milligram, would have been sufficient to kill Litvinenko. This could have been sprinkled onto his food or into a drink. Ar dheis Dé go raibh a anam.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC. http://understandingscience.ucc.ie