It emits deadly radiation – but every home should have some
Extremely stringent precautions have to be taken when producing or working with americium because it is a so-called α-emitter. In other words its atoms give off α-particles (helium nuclei) which are highly destructive. Outside the human body they are relatively harmless and can even be stopped by a sheet of paper; nor do they penetrate skin. However, when released inside a cell they can cause irreparable damage, and even if the cell survives it may become cancerous.
Americium was first made in 1944 at the University of Chicago, Illinois, by a team which included 33-year-old Glenn T. Seaborg. It was he who revealed its existence when he appeared as a guest scientist on a popular children's radio show called Quiz Kids, broadcast on 11 November 1945. The discovery of americium was officially announced a few days later, and the following year Seaborg suggested naming it americium.
Seaborg had worked as a nuclear scientist for the project to make the atomic weapons, two of which destroyed the Japanese cities of Hiroshima and Nagasaki in August 1995 so ending World War II. Seaborg himself was clearly not affected by americium – or any of the radioactive elements he worked with – because he lived to be 86 and died in 1999, not long after element 106 had been named after him.
Americium is produced by bombarding plutonium, atomic number 94, with neutrons in a nuclear reactor. This forms plutonium-241 which undergoes radioactive decay by losing a beta particle (a nuclear electron) thereby forming element atomic number 95, i.e. americium-241. This has a half life of 430 years and emits an α-particle to form element 93, neptunium, as its isotope 239.
Americium-241 is used in smoke detectors, but its radiation poses little threat. This isotope costs around $1500 per gram but this is enough americium for more than five thousand detectors. (The other isotope, americium-243, is a hundred times more expensive.)
A smoke detector contains 150 micrograms of AmO2, and it relies on the α-rays to ionise the air in a gap between two electrodes, thereby allowing a tiny current to flow. When smoke gets between the electrodes its particles of soot absorb the ions causing the current to fall and an alarm sounds. When the smoke is wafted away the current is restored, and the alarm stops. Although 33,000 americium atoms per second undergo radioactive decay in a smoke detector, none of the α-particles escapes.
Americium also emits gamma rays. These have shorter wavelengths than X-rays, and so are more penetrating. They were once used in radiography to determine the mineral content of bones and the fat content of soft tissue, but are now only used to determine the thickness of plate glass and metal sheeting. The transmission of the rays shows how thick the material is.
Americium can also be made to produce neutrons, and these too can be used in analytical probes. When an α-particle hits a beryllium atom it transmutes this element into carbon, and as it does so it emits a neutron. The stream of neutrons from an americium-beryllium source is used to test flasks designed to hold radioactive materials, to ensure they are completely radiation proof.
Americium has a longer lived isotope, americium-243, which is also produced in kilogram quantities from plutonium-239. This is the most stable isotope with a half-life of 7,370 years, yet this is far too short for any americium to occur naturally on Earth. Even if there had been a billion tonnes of americium-243 when the Earth was formed, this would have reduced to a single atom in less than a million years. All that which has been available for study has been manufactured.
Although it is dangerous, several compound of americium have been prepared and most of them are coloured, such as the more stable americium (IV) oxide AmO2 which is black and the less stable americium (III) oxide Am2O3 which is red-brown. The halides are also coloured: AmF3 and AmCl3 are pink, while AmF4 is light brown.