Californium, a radioactive, metallic chemical element. Californium does not occur in nature; it is produced artificially. Its isotopes are usually produced in nuclear reactors and in particle accelerat/rs, but some isotopes are recovered also from the debris of thermonuclear explosions. The most important isotope is californium 252 (half-life about 2.6 years). It is used as a source of neutrons in radiography and in the treatment of cancer and to manufacture isotopes of heavier chemical elements (such as fermium and nobelium). The most stable isotope is californium 251, with a half-life of about 800 years.

Californium was first produced in 1950 by Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiorso, and Glenn T. Seaborg at the University of California, Berkeley. The element was prepared by bombarding curium 242 with helium ions in a cyclotron.

Symbol: Cf. Atomic number: 98. Atomic weight of most stable isotope: about 251. Californium has 12 known isotopes. It is a transuranium element belonging to the actinide series of the Periodic Table.