Americium

Americium is a chemical element classified as an Actinide and known by the chemical symbol Am. It is a synthetic (man-made) metal that only occurs in radioactive isotopes. Discovered in 1944 by scientists in the Manhattan project, americium was a bi-product of experiments with plutonium. The name americium comes from the element above it in the lanthanide series, europium. Just as europium takes its name from the continent Europe, americium is named after America.

During nuclear decay Americium 241 (²⁴¹Am) emits both alpha particles as well as gamma rays, giving it several uses. For example, traces of americium are used in household smoke detectors. Americium can also occasionally be used as a portable source of gamma radiation.

Properties

Americium, a member of the actinides, is a silver-white metal.^[2] With an atomic number of ninety-five and a mass number of two hundred forty-three, this element only occurs in unstable, radioactive isotopes. As there are no stable forms of Americium, each form has a half life between five nanoseconds and 7370 years.^[1] With a melting point of 1176 ^oC (1449.15 K) and a boiling point of 2607 ^oC (2880 K), americium is found in a solid state in room temperature.^[3] Similar to many other metals, Americium tarnishes very slowly when exposed to air. When placed in acids, this element dissolves.^[4] All isotopes of this element are radioactive, but the most common forms emit alpha particles. Because of the intense radiation, americium can be extremely dangerous to anyone without proper protective equipment and adequate knowledge of how to handle it.^[1]

Synthesis

Americium, like other transuranium elements does not occur naturally. All forms of this element are synthetically produced.^[5] Americium 241 (²⁴¹Am) was first produced through a process of bombarding plutonium 239 (²³⁹Pu) with neutrons, resulting in plutonium 241 (²⁴¹Pu). This new, unstable isotope decays through beta decay into americium 241 (²⁴¹Am). ^[6]

Uses

Americium is one of the few radioactive elements that one should find in a common household object- the smoke detector. Most smoke detectors contain a small amount of americium 241 (²⁴¹Am) in the compound americium dioxide. This form of americium undergoes radioactive decay primarily through alpha decay (α), but does emit small amounts of gamma rays as well.^[4] The amount of radioactive material within smoke detectors is minimal. In fact, the amount of radioactive material found in a single smoke detector is so minuscule that about three million detectors could be made with a single gram of americium dioxide.^[2] Even though such a small amount of radioactive material is found within smoke detectors, and most of the gamma rays emitted harmlessly escape, tampering with the device is not advisable and could be harmful.^[4]

Inside the smoke detector, the americium is located within an ionization chamber where the emitted alpha particles collide with air particles to produce ions. A battery produces a small electric current that runs across these ions. When smoke enters the ionization chamber, its particles neutralize the charge of the ions and interfere with the flow of electricity, and therefore setting off the alarm.^[4]

As well as its use in smoke detectors, americium has also previously used as a source of radiation in the field of radiography and a portable source of gamma rays.^{[1] [6]}

History

In 1944, scientists in the Manhattan Project, Glenn Seaborg, Ralph James, Leon Morgan, and Albert Ghiorso successfully created the first samples of americium in the University of Chicago's metallurgical laboratory. As stated before, the first samples of the ninety-fifth element were created with neutron capture reactions of plutonium 239. This process involves the bombardment of the isotope with neutrons. Initially, the discovery of americium was considered classified as a discovery of the Manhattan project, but was later declassified. The public announcement of this new element by Glenn Seaborg occurred five days before the expected presentation at the American Chemistry Society. On a radio show, a child asked if there were any newly discovered transuranium elements other than plutonium and neptunium. Seaborg responded with his discovery of americium. ^[7]

Following the discovery of americium, Seaborg found that it and another recently discovered element, curium, both chemically behaved like the lanthanide series. This discovery brought issues with the periodic table, and lead Seaborg to revise it. This new revision included the new actinide series.^[7]

Video

This video explains some of the basic properties of americium and describes how it is used in household smoke detectors. [フレーム]

References

• Element
• Americium
• Actinide