Americium

Americium was synthesised in 1944 at the wartime Metallurgical Laboratory in Chicago by Glenn Seaborg, Ralph James, Leon Morgan and Albert Ghiorso, by bombarding plutonium with neutrons. It was named after the Americas, the continent of its discovery. Americium is a silvery, radioactive metal, and, remarkably for so exotic an element, it is put to everyday use: a tiny amount of americium-241 provides the ionising source in most household smoke detectors. The steady alpha particles from a speck of the isotope ionise the air in the detector so that drifting smoke disturbs the current and sounds the alarm, and americium also serves in the industrial gauges that measure the thickness and density of materials.

Americium's existence was first revealed to the public not in a scientific journal but on the radio: in November 1945, Glenn Seaborg announced the discovery of both americium and curium live on the popular children's quiz programme Quiz Kids, after one young panellist asked whether any new elements had been found, a disclosure that reportedly startled his own laboratory colleagues, who had expected the formal scientific announcement to come first. Wartime secrecy had kept both elements classified for over a year before that broadcast, so the news reached the general public before it reached most of the scientific community in print.

Americium's name was chosen by direct analogy with the element sitting immediately above it in the periodic table. In laying out the actinides beneath the lanthanides, Seaborg placed americium below europium, the element named for Europe, and so named the new element for the corresponding New World continent, the Americas. Its neighbour curium was named, in the same act, by analogy with gadolinium above it. Americium has no ancient, mythological or alchemical tradition, being a synthetic creation of the Manhattan Project era, but it holds the distinction of being perhaps the only transuranium element most people own, sealed in milligram traces inside the smoke alarms of ordinary homes, where its steady alpha radiation quietly stands guard against fire.

A typical household smoke detector contains only about a quarter of a microgram of americium-241, sealed behind a foil that blocks its alpha particles entirely except for the small window that ionises air inside the sensor chamber, a quantity regulators consider safe for ordinary use and disposal, though most jurisdictions still recommend returning old detectors to designated collection points rather than simply discarding them with household rubbish. That tiny quantity is nonetheless enough to have made smoke detectors, quietly, the single largest consumer application of any transuranium element.