californium

californium (Cf), synthetic chemical element of the actinoid series of the periodic table, atomic number 98. Not occurring in nature, californium (as the isotope californium-245) was discovered (1950) by American chemists Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiorso, and Glenn T. Seaborg at the University of California, Berkeley, as a product resulting from the helium-ion bombardment of curium-242 (atomic number 96) in the 152-cm (60-inch) cyclotron, followed by chemical separation from other elements by chromatography. The element was named after the state of California, where it was discovered.

All californium isotopes are radioactive; the long-lived isotopes are produced from berkelium-249 or from californium-249. They are californium-249 (351-year half-life), californium-250 (13-year half-life), californium-251 (898-year half-life), and californium-252 (2.645-year half-life). The isotope californium-249 has been used in tracer levels and microgram amounts to investigate the chemistry of californium (which exhibits an oxidation state of +3 in aqueous solution) and for preparing microgram quantities of compounds such as the oxychloride CfOCl, the oxides Cf₂O₃ and CfO₂, and the trichloride CfCl₃. The dihalides CfCl₂, CfBr₂, and CfI₂ have been made. There is also some evidence from electrochemical studies in solutions for a +2 state. Metallic californium has been prepared and is structurally similar to metallic americium, curium, berkelium, and most lanthanoid metals.

Californium-252, because 3 percent of its decay occurs by spontaneous fission, is industrially and medically important as a very intense source of neutrons. One microgram releases 170,000,000 neutrons per minute.

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