Americium

• 1. Introduction

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  Americium sits inside roughly 900 million smoke detectors worldwide, keeping watch in kitchens and hallways with a barely visible speck of radioactive metal. The element has symbol Am and atomic number 95. It is synthetic, meaning it does not occur in nature in any practical quantity, and it belongs to the actinide series of the periodic table. Its name was chosen by analogy: it sits directly below the lanthanide element europium, so its discoverers named it after the Americas just as europium was named for Europe. How a wartime secret became a fixture of everyday safety, and what makes this metal useful far beyond the home, are the questions this documentary sets out to answer.

• 2. Wartime Discovery

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  Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso first isolated and identified americium in late autumn 1944 at the University of California, Berkeley. They used a 60-inch cyclotron to drive the reaction, and the subsequent chemical identification took place at the Metallurgical Laboratory of the University of Chicago, the institution now known as Argonne National Laboratory. The work was part of the Manhattan Project, which meant the results were immediately classified.

  The separation process those researchers faced was genuinely punishing. Plutonium-239 nitrate solution was coated onto a platinum foil of about 0.5 cm2, evaporated, and converted to plutonium dioxide before cyclotron irradiation. After the irradiation, the coating was dissolved in nitric acid, precipitated as a hydroxide, and then dissolved again in perchloric acid before ion-exchange separation could even begin. The separation of the resulting curium from americium was so difficult that the Berkeley group nicknamed the two elements pandemonium, from the Greek for all demons, and delirium, from the Latin for madness.

  Seaborg announced the discovery not through an official channel but on a radio show for children called Quiz Kids, five days before the formal presentation at an American Chemical Society meeting on the 11th of November 1945. A listener asked whether any new transuranium elements beyond plutonium and neptunium had been found during the war, and Seaborg confirmed that two had. The first substantial metallic samples, weighing 40-200 micrograms, were not prepared until 1951, by reducing americium fluoride with barium metal in high vacuum at 1100 degrees Celsius.

• 3. Where Americium Comes From

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  A tonne of spent nuclear fuel contains about 100 grams of various americium isotopes, mostly 241Am and 243Am. Americium is not made directly from uranium; the path runs through plutonium-239, which first forms from uranium-238 by neutron capture and two successive beta decays. Two further neutron captures by that plutonium-239, followed by a beta decay, yield americium-241. Plutonium present in spent fuel contains about 12% of the isotope 241Pu, which beta-decays to 241Am over time: half the original 241Pu converts after about 15 years, and the americium reaches its maximum yield after roughly 70 years.

  Since americium was first offered for sale in 1962, the price of 241Am has remained around 1,500 dollars per gram, held steady by a very complex separation procedure. The heavier isotope 243Am costs considerably more because it is produced in far smaller amounts. Separation from the chemically similar curium is particularly demanding; a practical trick involves treating a mixture of their hydroxides in aqueous sodium bicarbonate with ozone at elevated temperatures, causing americium to oxidize to a soluble form while curium stays in the +3 state and can be removed separately.

• 4. A Metal Unlike Its Neighbors

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  Freshly prepared americium has a silvery-white metallic lustre, but it slowly tarnishes in air. Its density of 12 g/cm3 places it well below curium at 13.52 g/cm3 and far below plutonium at 19.8 g/cm3, though it is denser than europium at 5.264 g/cm3 mostly because of its greater atomic mass. Its melting point of 1173 degrees Celsius is notably higher than plutonium at 639 degrees Celsius and europium at 826 degrees Celsius.

  Under ambient conditions, americium takes its most stable alpha crystal form with hexagonal symmetry. Compress it to 5 GPa at room temperature and it shifts to a face-centered cubic beta form. Push to 23 GPa and it transitions again to an orthorhombic gamma structure similar to alpha-uranium. The element is also paramagnetic across a wide temperature range, a behavior that sets it apart from its neighbor curium, which undergoes an antiferromagnetic transition at 52 K.

  One consequence of the element's own radioactivity is that alpha particles continuously damage the crystal lattice from within, a process called metamictization. In americium-241, the electrical resistivity at 4.2 K increases with time from about 2 microohm-centimeters to around 10 after 40 hours, and saturates near 16 after 140 hours. Heating a sample back to room temperature can restore its resistivity, which means older samples may behave differently from freshly prepared ones in measurable ways.

• 5. Inside the Smoke Detector

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  A typical new ionization smoke detector contains just 1 microcurie, or 37 kBq, of 241Am, equivalent to 0.29 micrograms of the metal. That speck sits inside an ionization chamber between two electrodes, allowing a small constant current to flow through the air gap. When smoke particles enter, they absorb alpha particles, reducing ionization and disrupting the current enough to trigger the alarm.

  The choice of 241Am over radium-226 was deliberate: americium emits five times more alpha particles and produces relatively little harmful gamma radiation. Compared with optical smoke detectors, the ionization type is cheaper and can detect particles too small to scatter light significantly, though it is more prone to false alarms.

  Over time, the americium in a detector decays into neptunium-237, a transuranic element with a half-life of about 2.14 million years. After 19 years, about 3% of the americium has become neptunium; after 32 years, about 5%. Americium frequently enters landfills from discarded detectors, and disposal rules are relaxed in most jurisdictions. In 1994, a 17-year-old named David Hahn extracted americium from roughly 100 smoke detectors in an attempt to build a breeder reactor at home.

• 6. Space, Power, and Unusual Proposals

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  The isotope 242mAm, a nuclear isomer with a half-life of 141 years, carries properties that have attracted attention far beyond the laboratory. It holds the largest cross-section for absorption of thermal neutrons among known isotopes, at 5,700 barns, which gives it a critical mass of roughly 9-14 kg for a bare sphere, dropping to 3-5 kg with a metal reflector. Proposals exist for compact 10-kilowatt high-flux reactors using as little as 20 grams of this isomer as neutron sources for radiation therapy in hospitals.

  For spacecraft, 241Am offers a different angle. Its power yield is 114.7 mW/g, lower than the 390 mW/g of plutonium-238 but still sufficient to interest the European Space Agency, which has been considering americium for its space probes. In 2019, researchers at the UK National Nuclear Laboratory and the University of Leicester demonstrated that heat from americium could illuminate a small light bulb, pointing toward power systems capable of supporting missions lasting up to 400 years into interstellar space.

  A nuclear battery design proposed for 242mAm relies not on the heat from alpha decay but on the charge carried by those alpha particles, with the americium acting as a self-sustaining cathode. A single 3.2 kg charge of 242mAm in such a battery could theoretically supply about 140 kW of power for 80 days. The obstacle in every case is the same: 242mAm is scarce and expensive, and that constraint has not yet been resolved.

• 7. Building New Elements

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  Americium has served as a starting material for producing heavier elements on several occasions. In 1949, the same Berkeley group that discovered americium used the same 60-inch cyclotron to bombard 241Am with alpha particles, yielding berkelium, specifically the isotope 243Bk, making it the first intentional production of that element. Irradiation of 241Am with carbon-12 ions produces einsteinium-247, and with neon-22 ions it yields dubnium-260.

  At the Joint Institute for Nuclear Research in Dubna, Russia, in 1965, nobelium was produced through several reactions, one of which involved irradiating 243Am with nitrogen-15 ions. Lawrencium, discovered by scientists at Berkeley and Dubna, was also synthesized in part by bombarding 243Am with oxygen-18. Each of these heavier elements exists for only brief periods, but their creation maps the boundary of what nuclear physics can construct.

• 8. Health and Environmental Footprint

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  Harold McCluskey, a chemical operations technician, was exposed to 500 times the occupational standard for americium-241 after an explosion in his laboratory when he was 64 years old. He died at 75 from unrelated pre-existing disease, a case that remains the most severe documented americium exposure on record.

  When americium enters the human body, most is excreted within a few days. Only about 0.05% reaches the blood, of which roughly 45% goes to the liver and 45% to the bones, with the remaining 10% excreted. In bones, it first deposits on cortical and trabecular surfaces before slowly redistributing. The biological half-life of 241Am is 50 years in bone and 20 years in the liver; in the gonads it remains permanently. In all these tissues it promotes cancer-cell formation through its radioactivity.

  Environmentally, americium is concentrated at atmospheric nuclear test sites used between 1945 and 1980 and at accident locations such as Chernobyl. Trinitite, the glassy residue left on the desert floor near Alamogordo, New Mexico, from the Trinity test on the 16th of July 1945, contains traces of americium-241. Elevated levels were also detected at the 1968 crash site of a US B-52 bomber in Greenland that carried four hydrogen bombs. Away from such sites, average surface-soil radioactivity from residual americium is only about 0.01 picocuries per gram, though soil analysis has found americium concentration inside sandy soil particles roughly 1,900 times higher than in the water present in the same soil's pores.