Americium (Am)

Of all the elements made by people rather than found in rock, americium is the one that ended up in the most houses. There is a fraction of a microgram of it in most ionisation smoke alarms, and enough alarms have been fitted that element 95 sits in more separate buildings than any other synthetic element — not by weight, where the world's plutonium dwarfs it, but by the number of places it can be found in. It is also the only element whose existence was first made public in answer to a question from a child on the radio.

Announced on a quiz show

The wartime work was done at the Metallurgical Laboratory in Chicago in late 1944, where Glenn Seaborg, Ralph James, Leon Morgan and Albert Ghiorso were separating the products of heavily irradiated plutonium. The results were classified for the duration.

On 11 November 1945, Seaborg appeared as a guest on The Quiz Kids, an American radio programme in which a panel of precocious children fielded questions. One of them turned the format around and asked the guest whether any new elements had been discovered besides neptunium and plutonium. Seaborg said yes, there were two — 95 and 96 — and described them. His formal announcement to the American Chemical Society was five days later. A significant piece of Manhattan Project chemistry was therefore released to the world by a scientist being caught off guard by an eleven-year-old.

From pandemonium to a continent

Before they were named, the pair had unofficial labels reflecting how the work had gone. Element 95 was pandemonium and element 96 delirium around the laboratory, a comment on how excruciatingly difficult the two were to prise apart from each other and from the rare earths they resembled.

The formal names came from Seaborg's structural argument about where these elements belong. If the actinides are a second f-block echoing the lanthanides, then element 95 sits directly beneath europium — and europium is named for a continent. Americium follows the same rule one row down. The parallel is not decoration: the naming was an assertion that the actinide concept was correct, at a time when the placement of these elements in the table was still contested.

Why one alarm needs so little

The isotope in domestic use is americium-241, which alpha-decays with a half-life of 432 years. An ionisation smoke detector uses those alpha particles to keep a small volume of air weakly conducting between two electrodes; smoke particles entering the chamber capture ions and drop the current, and the drop is what trips the alarm.

The quantity required is around a third of a microgram — roughly one microcurie. Two features of the isotope make such a small amount workable. Alpha particles are ionising out of all proportion to their energy, so very few decays per second are needed to sustain a measurable current. And 432 years is long enough that the source does not measurably weaken across the life of the device, so the alarm's failure mode is a dead battery or a dirty chamber, never a spent isotope.

Americium-241 also emits a gamma photon at 59.5 keV, low enough in energy to be shielded easily and stable enough in intensity to serve as a reference. That line is the basis of a second family of uses that nobody sees: thickness gauges that measure sheet glass and paper as it runs off the machine, and bone-densitometry and portable X-ray fluorescence instruments that rely on a known-energy photon source. Mixed with beryllium, americium is also a standard neutron source for logging oil wells.

The element that accumulates by itself

Nobody has to set out to make americium-241. It arrives.

Plutonium separated from reactor fuel always contains some plutonium-241, which beta-decays with a half-life of 14.3 years. Its daughter is americium-241. A stock of separated plutonium therefore grows americium continuously, and the older the stock, the more of it there is — which is a persistent complication for anyone storing civil plutonium, because the americium brings a gamma emission the plutonium did not have.

The United Kingdom's civil plutonium stockpile, the largest in the world, has been ageing for decades, and the ingrowth turns out to be an asset rather than only a nuisance.

A European answer to a plutonium shortage

Deep-space missions have run on plutonium-238 for sixty years, and Europe has never had a source of it. Rather than buy in, the European Space Agency and the UK National Nuclear Laboratory chose to build radioisotope heaters and generators around americium-241 instead, extracted from that ageing plutonium.

The trade is honest and unflattering on paper. Americium-241 releases only about a fifth as much heat per gram as plutonium-238, so an americium generator is bulkier and heavier for the same power, and the gamma emission means more shielding. Against that, the fuel already exists in quantity, requires no reactor campaign to produce, and has a half-life four times longer — so an americium-fuelled probe would still be delivering usable power on missions long enough to outlast a plutonium one. The first americium-heated demonstration units were operated at Sellafield in the late 2010s.

What the metal is actually like

Very little americium chemistry is done on anything but a small scale, but the element has been prepared in metallic form and characterised. It is silvery when fresh and tarnishes slowly in air, takes a double-hexagonal close-packed structure like the lighter actinides, and is noticeably less dense than the plutonium it is made from — part of a general loosening of the 5f bonding that begins at americium and continues up the series.

Its dominant oxidation state is +3, which is exactly what the lanthanide analogy predicts and exactly what made the wartime separations so miserable: an element that behaves like a rare earth has to be separated from rare earths by very small differences indeed.