Americium Am

Discovery

Americium was identified in late 1944 at the University of Chicago's Metallurgical Laboratory by Glenn Seaborg, Ralph James, Leon Morgan and Albert Ghiorso. When plutonium-239 is irradiated in a reactor for long periods it captures neutrons in succession to become plutonium-241, which then beta-decays into americium-241.

Separating the new element proved maddening. Americium and element 96, curium, discovered at almost the same time, behave so similarly that the team half-jokingly nicknamed them 'pandemonium' and 'delirium'.

The discovery was classified during the war. In November 1945, days before the formal announcement, Seaborg appeared as a guest on a children's radio quiz programme. When a young contestant asked whether any new elements had been found, he revealed elements 95 and 96, so the news reached a radio audience of children before it reached scientific meetings. Americium metal was first prepared in 1951.

Origin of the name

Directly above americium in the periodic table sits europium, a lanthanide named after Europe. Seaborg's team mirrored that pairing by naming element 95 after the Americas, where it was discovered. The symbol is Am. The name thus reflects Seaborg's actinide concept, which placed the actinides as a row parallel to the lanthanides.

Main uses

• Smoke detectors: In an ionisation detector, alpha particles from americium-241 ionise the air inside a small chamber so that a tiny current flows. Smoke particles disrupt the current and trigger the alarm. Each detector contains well under a microgram.
• Neutron sources: Mixed with beryllium, americium-241 emits neutrons. Such sources are used to log oil and gas wells and in soil-moisture gauges.
• Thickness and density gauges: The low-energy gamma rays from americium-241 are used in industrial gauges for sheet material and glass, and in X-ray fluorescence analysis.
• Superheavy synthesis: Bombarding an americium-243 target with calcium-48 produced element 115, moscovium.

Isotopes

Americium has no stable isotopes. Americium-241, used in smoke detectors, has a half-life of about 432 years, so its activity barely changes over a detector's working life. The longest-lived isotope is americium-243, with a half-life of about 7,370 years. Because americium-241 steadily builds up from the decay of plutonium-241 in spent fuel and stored plutonium, it is an important consideration in managing nuclear waste.

In everyday life

The smoke alarm on a ceiling is the closest most people come to americium. The source is sealed in a metal foil, and its alpha particles travel only a few centimetres in air, so an intact, properly mounted detector poses no health concern. Detectors should not be taken apart, however, and should be disposed of according to local guidance. Inside the body americium concentrates in bone and liver, which is why it is tightly controlled in the facilities that handle larger amounts. Photoelectric detectors that contain no radioactive material are also widely available.