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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 Glenn T. Seaborg who was 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, as well as in neutron sources and industrial gauges.

Properties

Physical

Pure americium has a silvery and white luster. At room temperature 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. 241Am emits intense gamma rays, creating a serious exposure problem for anyone handling gram quantities of 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 that of more readily obtained plutonium or uranium isotopes.

Chemical

World's first sample of americium (as the hydroxide)
Americium oxidizes to AmO in air. Similarly, reaction with hydrogen results in AmH2 where Am is divalent. However, the most common oxidation state of Am is +3, especially in solutions which are colored red. It is much harder to oxidize Am(III) to Am(IV) than it is to oxidize Pu(III) to Pu(IV).

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 , NO , NO , and SO ) to form charged complexes which tend to be readily mobile with low affinities to soil: AmO2(OH)+, AmO2(OH) , AmO2CO , AmO2(CO3) and AmO2(CO3) .

Examples of americium +4 compounds are Am(OH)4 and AmF4. All pentavalent and hexavalent americium compounds are complex salts such as KAmO2F2, Li3AmO4 and Li6AmO6, Ba3AmO6, AmO2F2. Hexavalent americium is a strong oxidizing agent and is reduced to AmO2+ in oxidation-reduction reactions.

Extraction

A large amount of work has been done on the solvent extraction of americium, as americium and other transuranic elements are responsible for much 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 people and the 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.

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 (t½ 141 years). The isotopes of americium range in atomic weight from 231.046 u (231Am) to 249.078 u (249Am).

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 Chicagomarker (now known as Argonne National Laboratorymarker). 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.

\mathrm{^{239}_{\ 94}Pu\ \xrightarrow {(n,\gamma)} \ ^{240}_{\ 94}Pu\ \xrightarrow {(n,\gamma)} \ ^{241}_{\ 94}Pu\ \xrightarrow [14,35 \ a]{\beta^-} \ ^{241}_{\ 95}Am\ (\ \xrightarrow [432,2 \ a]{\alpha} \ ^{237}_{\ 93}Np)}


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.

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 uses 241Am in the form of americium dioxide as its source of ionizing radiation. The amount of americium in a typical smoke detector when new is 1 microcurie or 0.28 microgram. This amount declines slowly as the americium decays into neptunium-237, a different transuranic element with a much longer half-life (about 2.14 million years). With its half-life of 432.2 years, the americium in a smoke detector includes about 3% neptunium after 19 years, and about 5% after 32 years.

241Am has been used as a portable source of both gamma rays and alpha particles for a number of medical and industrial uses. Gamma ray emissions from 241Am can be used for indirect analysis of materials radiography and for quality control in manufacturing fixed gauges. For example, the element has been employed to gauge glass thickness to help create flat glass. 241Am gamma rays were also used to provide passive diagnosis of thyroid function. This medical application is obsolete. 241Am can be combined with lighter elements (e.g., beryllium or lithium) to become a neutron emitter. This application has found uses in neutron radiography as well as a neutron emitting radioactive source. The most widespread use of 241AmBe neutron sources is found in moisture/density gauges used for quality control in highway construction. 241Am neutron sources are also critical for well logging applications. 242mAm has been cited for use as an advanced nuclear rocket propulsion fuel. This isotope is, however, extremely expensive to produce in usable quantities.

241Am has recently been suggested for use as a denaturing agent in plutonium reactor fuel rods to render the fuel unusable for conversion to nuclear weapons.

Safety

Americium emits alpha and gamma radiation. The alpha decay of 241Am is three times as active as that of radium. It is associated with 5.48 MeV alpha particles and 59 keV gamma emission, which is a serious health hazard.

References

  1. G. T. Seaborg, R. A. James, L. O. Morgan: "The New Element Americium (Atomic Number 95)", NNES PPR (National Nuclear Energy Series, Plutonium Project Record), Vol. 14 B The Transuranium Elements: Research Papers, Paper No. 22.1, McGraw-Hill Book Co., Inc., New York, 1949; Abstract; Typoskript (Januar 1948).
  2. Patent
  3. Americium dioxide is used in smoke detectors. (Internet Archive)


See also



Further reading

  • Nuclides and Isotopes - 14th Edition, GE Nuclear Energy, 1989.
  • Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1
  • Gmelins Handbuch der anorganischen Chemie, System Nr. 71, Band 7 a, Transurane: Teil A 1 I, S. 30–34; Teil A 1 II, S. 18, 315–326, 343–344; Teil A 2, S. 42–44, 164–175, 185–188; Teil B 1, S. 57–67.


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