Americium

Americium (pronounced /ˌæməˈrɪsiəm/) 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.

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Properties

Main article: Actinides in the environment

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.^[1]

Applications

This element can be produced in kilogram amounts and has some uses (mostly ²⁴¹Am since it is easier to produce relatively pure samples of this isotope). Americium has found its way into the household, where one type of smoke detector contains a tiny amount (about 0.2 microgram) of ²⁴¹Am as a source of ionizing radiation. 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. It has also been cited for use as an advanced nuclear rocket propulsion fuel.^[2] 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."^[3] The discovery of americium and curium was first announced informally on a children's quiz show in 1945.^[4]

Isotopes

Main article: isotopes of americium

 18 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 dioxide is used in smoke detectors.^[5]

Americium, unlike uranium, does not readily form a dioxide americyl core (AmO₂).^[6] 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.^{[7][8][9][10][11]}

References

• WebElements.com - Americium

1. ^ Fissile Materials & Nuclear Weapons: Introduction. International Panel on Fissile Materials. Retrieved on 2007-11-22.
2. ^ "Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two Weeks", ScienceDaily, 3 Jan 2001. Retrieved on 2007-11-22. 
3. ^ U.S. Patent 3,156,523 
4. ^ Rachel Sheremeta Pepling. It's Elemental: The Periodic Table: Americium. Chemical & Engineering News.
5. ^ [1]
6. ^ David L. Clark (2000). "The Chemical Complexities of Plutonium" (Reprinted at fas.org). Los Alamos Science (26).
7. ^ Michael J. Hudson, Michael G. B. Drew, Mark R. StJ. Foreman, Clément Hill, Nathalie Huet, Charles Madic and Tristan G. A. Youngs (2003). "The coordination chemistry of 1,2,4-triazinyl bipyridines with lanthanide(III) elements – implications for the partitioning of americium(III)". Dalton Trans.: 1675 - 1685. doi:10.1039/b301178j.
8. ^ Andreas Geist, Michael Weigl, Udo Müllich, Klaus Gompper (11-13 Dec 2000). [http://www.nea.fr/html/pt/docs/iem/madrid00/Paper14.pdf Actinide(III)/Lanthanide(III) Partitioning Using n-Pr-BTP as Extractant: Extraction Kinetics and Extraction Test in a Hollow Fiber Module]. 6th Information Exchange Meeting on Actinide and Fission Product Partitioning and Transmutation. OECD Nuclear Energy Agency.
9. ^ Sanex-BTP Process Development Studies. Atalante 2000: Scientific Research on the Back-end of the Fuel Cycle for the 21st Century. Commissariat à l'énergie atomique (24-26 Oct 2000 author = C. Hill, D. Guillaneux, X. Hérès, N. Boubals and L. Ramain).
10. ^ Andreas Geist, Michael Weigl and Klaus Gompper (14-16 Oct 2002). Effective Actinide(III)-Lanthanide(III) Separation in Miniature Hollow Fibre Modules. 7th Information Exchange Meeting on Actinide and Fission Product Partitioning and Transmutation. OECD Nuclear Energy Agency.
11. ^ D.D. Ensor. Separation Studies of f-Elements. Tennessee Tech University.

Further reading

• Nuclides and Isotopes - 14th Edition, GE Nuclear Energy, 1989.
• Patent US3,156,523 (PDF version) (1964-11-10) Glenn T. Seaborg Element 95 and Method of Producing Said Element 
• 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 (January 10, 1993). "An americium-fueled gas core nuclear rocket" in Tenth symposium on space nuclear power and propulsion. AIP Conf. Proc. 271: 585-589. DOI:10.1063/1.43073.