Curium

General DescriptionCurium was discovered by Seaborg, James, and Ghiorso in 1944 during chemical fractionation of plutonium irradiated with alpha particles (32 MeV). The element was isolated in hydroxide form by Werner and Perlman in 1947 in microgram amounts, and later in 1950 by Crane, Wallmann, and Cunningham in elemental form. Crane et al. also studied its magnetic susceptibility and assigned 5f7 electron configuration to this element, analogous to 4f7 configuration of the element gadolinium in the lanthanide series. This man-made element was named curium in honor of Marie and Pierre Curie. Curium does not occur in nature. Even if it had occured in the primordial age of earth, its longest lasting isoptope, Cm-247 (half-life of 17 million years),would almost have fully disintegrated during the more than three billion years of earth’s existence. The element does not have any important commercial applications. Its isotopes Cm-242 and Cm-244 have potential applications to generate thermoelectric power for operation of instruments in space ships.

OccurrenceThere is no natural curium on Earth. All of its isotopes are man-made and artificiallyproduced through nuclear reactions with other elements. The curium isotope Cm-242 wasfirst produced by bombarding plutonium-239 with helium nuclei (alpha particles), whichcontributed neutrons that changed 94Pu to 96Cm.

Although curium follows americium in the periodic system, it was actually known before americium and was the third transuranium element to be discovered. In 1944, Seaborg, James, and Giorso discovered curium by bombarding 239Pu with helium ions at the Chicago War Metallurgical Laboratory using the 60-inch cyclotron in Berkeley, California. Visible amounts (30 μg) of 242Cm, in the form of the hydroxide, were first isolated by Werner and Perlman of the University of California in 1947. In 1950, Crane, Wallmann, and Cunningham found that the magnetic susceptibility of microgram samples of CmF3 was of the same magnitude as that of GdF3. This provided direct experimental evidence for assigning an electronic configuration to Cm+3. In 1951, the same workers prepared curium in its elemental form for the first time. Sixteen isotopes of curium are now known. The most stable, 247Cm, with a half-life of 16 million years, is so short compared to the Earth’s age that any primordial curium must have disappeared long ago from the natural scene. Minute amounts of curium probably exist in natural deposits of uranium, as a result of a sequence of neutron captures and β decays sustained by the very low flux of neutrons naturally present in uranium ores. The presence of natural curium, however, has never been detected. 242Cm and 244Cm are available in multigram quantities. 248Cm has been produced only in milligram amounts. Curium is similar in some regards to gadolinium, its rare-earth homolog, but it has a more complex crystal structure. Curium is silver in color, is chemically reactive, and is more electropositive than aluminum. CmO2, Cm2O3, CmF3, CmF4, CmCl3, CmBr3, and CmI3 have been prepared. Most compounds of trivalent curium are faintly yellow in color. 242Cm generates about three watts of thermal energy per gram. This compares to one-half watt per gram of 238Pu. This suggests use for curium as a power source. 244Cm is now offered for sale by the O.R.N.L. at $185/mg plus packing charges. 248Cm is available at a cost of $160/μg, plus packing charges, from the O.R.N.L. Curium absorbed into the body accumulates in the bones, and is therefore very toxic as its radiation destroys the red-cell forming mechanism. The maximum permissible total body burden of 244Cm (soluble) in a human being is 0.3 μCi (microcurie).