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History

Americium was first synthesized by Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso in lat 1944 at the wartime Metallurgical Laboratory at the University of Chicago (now known as the 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. 

The result of successive neutron capture reactions by plutonium isotopes in a nuclear reactor: 

²³⁹Pu(n,y) ²⁴⁰Pu(n,y) ²⁴¹Pu ---B---> ²⁴¹Am

Since the isotope ²⁴¹Am can be prepared in relatively pure form by extraction as a decay product over a period of years from strongly neutron- bombarded plutonium, ²⁴¹Pu, this isotope is used for much of the chemical investigation of this element. Better suited is the isotope ²⁴³Am due to its longer half-life (7370 years as compared to 432.2 years for ²⁴¹Am).  A mixture of the isotopes ²⁴¹Am, ²⁴²Am, and ²⁴³Am can be prepared by intense neutron irradiation of ²⁴¹Am according to the reactions:

²⁴¹Am (n,y) ²⁴²Am (n,y) ²⁴³Am

Nearly isotopically pure ²⁴³Am can be prepared by a sequence of neutron bombardments and chemical separations.  Neutron bombardment of ²⁴¹Am yields ²⁴²Pu by the reactions:

²⁴¹Am(n,y) ²⁴²Am ²⁴²Pu

after chemical separation the ²⁴²Pu can be transformed to ²⁴³Am via the reactions:

²⁴²Pu(n,y) ²⁴³Pu ²⁴³Am

and the ²⁴³Am can be chemically separated.  Fairly pure ²⁴²Pu can be prepared more simply by very intense neutron irradiation of ²³⁹Pu as the result of successive neutron-capture reactions.  Americium metal has been prepared by reducing the trifluoride with barium vapor at 1000^oC to 1200^oC or the dioxide by lanthanum metal. 

Seaborg was granted patent 3,156,523 for "Element 95 and Method of Producing Said Element".   The discovery of americium and curium was first announced informally on a children's quiz show in 1945.

Characteristics

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 thought to exist in two forms: an alpha form which has a double hexagonal close-packed structure and a loose-packed cubic beta form.  Americium must be handled with great care to avoid personal contamination.  As little as 0.03 uCi of ²⁴¹Am is the maximum permissible total body burden.  The alpha activity from ²⁴¹Am is about three times that of radium.  When gram quantities of ²⁴¹Am are handled, the intense gamma activity makes exposure a serious problem.  Americium dioxide, AmO₂, is the most important oxide.  AmF₃, AmF₄, AmCl₃, AmBr₃, AmI₃, and other compounds have been prepared.  The isotope ²⁴¹Am has been used as a portable source for gamma radiography.  It has also been used as a radioactive glass thickness gage for the flat glass industry, and as a source of ionization for smoke detectors.  Americum-243 is available from the Oak Ridge National Laboratory at a cost of $160/mg plus packing charges.

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

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.  This isotope is, however, extremely expensive to produce in usable quantities.

Compounds

In aqueous systems the most common oxidation state is +3. It is very much harder to oxidise 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.

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 aqeuous 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.

Isotopes

19 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 amu (²³¹Am) to 249.078 amu (²⁴⁹Am).