Americium: the story on HearLore

Americium

In the autumn of 1944, a team of scientists at the University of Chicago worked in the shadows of the Manhattan Project to create a substance that would remain hidden from the public for over a year. Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso used a massive 60-inch cyclotron at Berkeley to bombard plutonium-239 with neutrons, creating a new element that they initially named pandemonium and delirium due to the sheer difficulty of separating it from its neighbors. This element, americium, was the fourth transuranic element to be discovered, yet it was the third in the series to be synthesized, appearing after curium. The discovery was so classified that Seaborg accidentally leaked the existence of elements 95 and 96 on a children's radio show called Quiz Kids just days before the official announcement. The initial samples were so small they weighed only a few micrograms and were barely visible to the naked eye, identified solely by their intense radioactivity. The name americium was chosen to honor the Americas, drawing an analogy to europium, which sits directly above it in the periodic table, placing this new element in the actinide series under the lanthanides.

The Painstaking Separation

The process of isolating americium from its parent elements was so arduous that the researchers at Berkeley referred to the separation of curium and americium as a descent into madness. They began with a solution of plutonium-239 nitrate coated onto a platinum foil, which was then evaporated and converted into plutonium dioxide through calcining. After irradiating the sample in the cyclotron, the residue was dissolved in nitric acid and precipitated as a hydroxide using concentrated aqueous ammonia. The subsequent steps involved dissolving the residue in perchloric acid and performing ion exchange to separate the isotopes. This multi-step procedure yielded four different isotopes of americium, including 241Am, 242Am, 239Am, and 238Am, but the separation of curium and americium remained a significant challenge. It was not until 1951 that the first substantial amounts of metallic americium, weighing between 40 and 200 micrograms, were successfully prepared by reducing americium(III) fluoride with barium metal in a high vacuum at 1100 degrees Celsius. The complexity of the separation meant that for decades, the element remained a laboratory curiosity rather than a commercial product.

The Silent Guardian

Today, the most common application of americium is found in the smoke detectors of millions of homes, where it serves as a silent guardian against fire. The isotope 241Am is used in the form of americium dioxide to ionize air within a small chamber, creating a constant current between two electrodes. When smoke particles enter the chamber, they absorb the alpha particles emitted by the americium, reducing the ionization and triggering the alarm. A typical smoke detector contains only 1 microcurie of americium, which is approximately 0.29 micrograms, yet this tiny amount is sufficient to detect smoke particles that are too small to produce significant light scattering in optical detectors. The half-life of 241Am is 432.2 years, meaning the source remains effective for decades, though it slowly decays into neptunium-237. This application was chosen over alternatives like radium-226 because americium emits five times more alpha particles while producing relatively little harmful gamma radiation. Despite the radioactivity, the amount used is so small that it poses no health risk to users, and the element has become an ubiquitous part of modern safety infrastructure.

Common questions

When was americium discovered and by whom?

Scientists Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso discovered americium in the autumn of 1944 at the University of Chicago and the University of California, Berkeley. They used a 60-inch cyclotron to bombard plutonium-239 with neutrons to create the element. The discovery was officially announced after the team accidentally leaked the existence of elements 95 and 96 on the children's radio show Quiz Kids.

How much americium is in a typical smoke detector?

A typical smoke detector contains 1 microcurie of americium, which equals approximately 0.29 micrograms. This tiny amount of americium-241 dioxide is sufficient to ionize air and detect smoke particles. The isotope has a half-life of 432.2 years, ensuring the source remains effective for decades.

What is the half-life of americium-242m and how is it used in space exploration?

The isotope americium-242m has a half-life of 141 years and possesses the largest cross-section for absorbing thermal neutrons of any known isotope. Scientists propose using this isotope as fuel for compact nuclear reactors and propulsion systems for space ships. The European Space Agency is considering americium for space probes to address waning stockpiles of plutonium-238.

Who was David Hahn and what did he do with americium?

David Hahn, known as the Radioactive Boy Scout, was a 17-year-old who attempted to build a breeder nuclear reactor in 1994. He extracted americium from approximately 100 smoke detectors to create his own nuclear device. His project attracted the attention of the FBI and the Environmental Protection Agency, highlighting the risks of amateur nuclear experimentation.

What are the chemical properties and oxidation states of americium?

Americium exhibits oxidation states ranging from +2 to +7, with +3 being the most stable state. The element forms insoluble salts such as fluoride, oxalate, and hydroxide in the +3 state. Americium metal has a density of 12 grams per cubic centimeter and changes crystal structure from hexagonal to face-centered cubic under extreme conditions.

What is metamictization and how does it affect americium-241?

Metamictization is a process where americium damages its own crystal structure by emitting alpha particles that strike surrounding atoms. This self-irradiation causes electrical resistivity to increase over time, rising from 2 microohm-centimeters to 10 microohm-centimeters after 40 hours. The effect saturates at about 16 microohm-centimeters after 140 hours and makes determining properties like the melting point uncertain.

Americium

The Painstaking Separation

The Silent Guardian

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Common questions

When was americium discovered and by whom?

How much americium is in a typical smoke detector?

What is the half-life of americium-242m and how is it used in space exploration?

Who was David Hahn and what did he do with americium?

What are the chemical properties and oxidation states of americium?

What is metamictization and how does it affect americium-241?

The Space Age Fuel

The Radioactive Boy Scout

The Chemical Chameleon

The Self-Damaging Crystal