Americium- (the Americas), Am; at. wt. 243; at. no. 95, m.p. 1176 deg C; b.p. 2011 deg C; sp.gr. 13.67 (20 deg C); valence 2, 3, 4, 5, or 6. Americium was the fourth transuranium element to be discovered; the isotope 241Am was identified by Seaborg, James, Morgan, and Ghiorso late in 1944 at the wartime Metallurgical Laboratory of the University of Chicago as the result of successive neutron capture reactions by plutonium isotopes in a nuclear reactor:
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(7.37x10cu 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 as follows: 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. Sixteen radioactive isotopes and isomers are now recognized. Americium metal has been prepared by reducing the trifluoride with barium vapor at 1000 to 1200 deg C or the dioxide by lanthanum metal. The luster of freshly prepared americium metal is white and more silvery than plutonium or neptunium prepared in the same manner. It appears to be more malleable than uranium or neptunium and tarnishes slowly in dry air at room temperature. 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, AM02, 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.