Americium

Americium is a synthetic element that has the symbol Am and atomic number 95. A radioactive metallic element, americium is an actinide that was obtained in 1944 by bombarding plutonium with neutrons and was the fourth transuranic element to be discovered. It was named for the Americas, by analogy with europium. Americium is widely used in commercial ionization-chamber smoke detectors.

Properties

Pure americium has a silvery and white lustre. At room temperatures it slowly tarnishes in dry air. It is more silvery than plutonium or neptunium and apparently more malleable than neptunium or uranium. Alpha emission from ²⁴¹Am is approximately three times that of radium. Gram quantities of ²⁴¹Am emit intense gamma rays which creates a serious exposure problem for anyone handling the element.

Americium is also fissile; the critical mass for an unreflected sphere of ²⁴¹Am is approximately 60 kilograms. It is unlikely that Americium would be used as a weapons material, as its minimum critical mass is considerably larger than more readily obtained plutonium or uranium isotopes.

Applications

Americium can be produced in kilogram amounts and has some uses, mostly involving ²⁴¹Am since it is easiest to produce relatively pure samples of this isotope. Americium is the only synthetic element to have found its way into the household, where one common type of smoke detector contains a tiny amount (about 0.2 microgram) of ²⁴¹Am as a source of ionizing radiation. This amount emits about 1 microcurie of nuclear radiation when new, with the amount declining slowly as the americium decays into neptunium, a different transuranic element, with a much longer half-life (about 2.14 million years). With its half-life of 432 years, the americium-241 in a smoke detector includes about 5% neptunium after 22 years, and about 10% after 43 years. After the 432-year americium-241 half-life, a smoke detector's original americium would, by definition, be more than half neptunium.

²⁴¹Am has been used as a portable gamma ray source for use in radiography. The element has also been employed to gauge glass thickness to help create flat glass. ²⁴²Am is a neutron emitter and has found uses in neutron radiography as well as a neutron emitting radioactive source used in well logging applications (²⁴¹AmBe). It has also been cited for use as an advanced nuclear rocket propulsion fuel. This isotope is, however, extremely expensive to produce in usable quantities.

History

Americium was first isolated by Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso in late 1944 at the wartime Metallurgical Laboratory at the University of Chicago (now known as Argonne National Laboratory). The team created the isotope ²⁴¹Am by subjecting ²³⁹Pu to successive neutron capture reactions in a nuclear reactor. This created ²⁴⁰Pu and then ²⁴¹Pu which in turn decayed into ²⁴¹Am via beta decay. Seaborg was granted a patent for "Element 95 and Method of Producing Said Element," whose unusually terse claim number 1 reads simply, "Element 95. The discovery of americium and curium was first announced informally on a children's quiz show in 1945.

Isotopes

Eighteen radioisotopes of americium have been characterized, with the most stable being ²⁴³Am with a half-life of 7370 years, and ²⁴¹Am with a half-life of 432.2 years. All of the remaining radioactive isotopes have half-lives that are less than 51 hours, and the majority of these have half-lives that are less than 100 minutes. This element also has 8 meta states, with the most stable being ^242mAm (t_½ 141 years). The isotopes of americium range in atomic weight from 231.046 u (²³¹Am) to 249.078 u (²⁴⁹Am).

Chemistry

In aqueous systems the most common oxidation state is +3. It is very much harder to oxidize Am(III) to Am(IV) than it is to oxidise Pu(III) to Pu(IV).

Currently the solvent extraction chemistry of americium is important as in several areas of the world scientists are working on reducing the medium term radiotoxicity of the waste from the reprocessing of used nuclear fuel.

See liquid-liquid extraction for some examples of the solvent extraction of americium.

Americium, unlike uranium, does not readily form a dioxide americyl core (AmO₂). This is because americium is very hard to oxidise above the +3 oxidation state when it is in an aqueous solution. In the environment, this americyl core could complex with carbonate as well as other oxygen moieties (OH^-, NO₂^-, NO₃^-, and SO₄^-2) to form charged complexes which tend to be readily mobile with low affinities to soil.

• AmO₂(OH)⁺¹
• AmO₂(OH)₂⁺²
• AmO₂CO₃⁺¹
• AmO₂(CO₃)₂^-1
• AmO₂(CO₃)₃^-3

A large amount of work has been done on the solvent extraction of americium, as it is the case that americium and the other transplutonium elements are responsible for the majority of the long lived radiotoxicity of spent nuclear fuel. It is thought that by removal of the americium and curium that the used fuel will only need to be isolated from man and his environment for a shorter time than that required for the isolation of untreated used fuel. One recent EU funded project on this topic was known by the codename "EUROPART". Within this project triazines and other compounds were studied as potential extraction agents.

References

• WebElements.com - Americium

Further reading

• Nuclides and Isotopes - 14th Edition, GE Nuclear Energy, 1989.
• Patent
  • Gabriele Fioni, Michel Cribier and Frédéric Marie Can the minor actinide, americium-241, be transmuted by thermal neutrons?. Commissariat à l'énergie atomique. .
  • Terry Kammash, David L. Galbraith, and Ta-Rong Jan "An americium-fueled gas core nuclear rocket". AIP Conf. Proc., 585-589. .

  External links

  • WebElements.com - Americium
  • Los Alamos National Laboratory - Americium
  • Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1
  • It's Elemental - Americium
  • ATSDR - Public Health Statement: Americium