It glows in the dark in more ways than one
Americium is a man-made, highly radioactive element. Yet, as a vital component of smoke detectors, it also saves lives. John Emsley reports
Tuesday 21 November 1995
Americium is highly radioactive, emitting alpha particles and gamma rays as it transmutes to neptunium. It has a half-life of 430 years; in other words, half of the sample will undergo radioactive decay during this time. It is potentially dangerous because of its radiation, and if it enters the body, it tends to concentrate in the skeleton.
Despite its radioactivity, americium actually saves lives, and several kilograms are made each year. Most of this ends up in our homes in smoke detectors. These rely on americium's alpha radiation to ionise the air in a gap between two electrodes, causing a tiny current to flow between them. When smoke gets between the electrodes, particles of soot absorb the ions and the current falls, and an electric circuit monitoring the current sounds the alarm. Waft away the smoke, the current rises again and the alarm stops.
Jeff Cutler is senior development engineer at Apollo Fire Detectors in Havant, Hampshire, where almost 800,000 industrial detectors are made each year for use in factories, offices, shops and hotels. More than half the output is exported.
"We buy the americium sources from the US," Mr Cutler says. "They arrive in gold-covered sealed foils, each containing about 150 micrograms [millionths of a gram] of americium oxide. They are reliable and cheap, which is why ionisation-type detectors are the most popular."
The US Atomic Energy Commission first offered the oxide, AmO2, for sale in March 1962 at $1,500 per gram, which is still the price today. A gram will supply enough americium for more than 5,000 detectors. Smoke detectors must conform to standards set out by the National Radiological Protection Board, but it admits that disposal is not supervised.
However, the radiation hazard this poses is insignificant compared to the benefits. Although 33,000 americium atoms per second undergo radioactive decay in a smoke detector, no alpha particles escape because these cannot penetrate solid matter - a thin sheet of paper will stop them. Even in air they do not get more than a few inches before they collide with oxygen or nitrogen molecules and, as they do, the alpha particles steal electrons and become non-radioactive atoms of helium gas.
Americium can also be made to produce neutrons, and these can be used in non-destructive testing of machinery and equipment. Dr Ron Smith, industrial business manager at the National Non-Destructive Testing Centre at AEA Technology in Harwell, Oxfordshire, says: "An alpha particle transmutes a beryllium atom into carbon and, as it does, it emits a neutron. We use the stream of neutrons to ensure flasks designed to hold radioactive materials are radiation proof."
Gamma rays from americium are more penetrating than X-rays because they have shorter wavelengths. They were once used in radiography to determine mineral content of bones and fat content of soft tissue, but are now used only to determine the thickness of plate glass and metal sheeting.
There is no natural source of americium, and it is unlikely that any has existed on Earth. If it had, it would not have lasted long. The most stable isotope is americium-243, which has a half-life of 7,300 years. This is short compared with the age of the planet - about 4.5 billion years. A billion tonnes of americium would be reduced to a single atom in less than a million years.
In November 1945, americium had no name and was merely element number 95. The following year, Seaborg named it americium after the continent where it was discovered. By then, more of its properties had been measured. We now know americium is a silvery, shiny metal that is attacked by air, steam and acids. It is denser than lead and melts at 994C. Several americium compounds have been made.
The discovery of americium required the periodic table of elements to be redesigned. As Seaborg did this, he predicted that many more man- made elements were waiting to be discovered. He and his co-workers later made some of these: berkelium in 1949, californium (1950), mendelevium (1955) and nobelium (1958). In 1994, Seaborg was immortalised when the American Chemical Society named element 106 as seaborgium.
The writer is science writer in residence at Imperial College, London
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