Californium, the sixth transuranium element to be discovered, was produced by Thompson, Street, Ghioirso, and Seaborg in 1950 by bombarding microgram quantities of 242Cm with 35 MeV helium ions in the Berkeley 60-inch cyclotronproducing 244Cf. Since the lanthanide homologue of californium (dysprosium) has a stable trivalent state in aqueous solution it was anticipated that californium would exhibit a stable trivalent state as well. This accurate prediction allowed for the successful chromatographic separation of californium from other actinides and for its unequivocal identification.
Twenty isotopes ranging in atomic mass from 237 to 256 have been reported for californium however the existence of the isotopes with mass of 237 and 238 has not yet been confirmed. The isotope 249Cf results from the beta decay of 249Bk while the heavier isotopes are produced by intense neutron irradiation by nuclear reactors or in thermonuclear explosions. The existence of the isotopes 249Cf, 250Cf, 251Cf, and 252Cf makes it feasible to isolate californium in weighable amounts so that its physicochemical properties can be investigated with macroscopic quantities. The first well-defined structure of a californium compound was the oxychloride by Cunningham and Wallmann a decade after discovery of the element. Microgram quantities of californium have been produced in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) in Tennessee and in Dimitrovgrad high-flux reactors in Russia. Californium-252 is a very strong neutron emitter. One microgram releases 170 million neutrons per minute, which presents biological hazards. Cf-252 also decays by energetic alpha emission (half-life 2.65 years, 6.1 MeV). Proper safeguards should be used when handling californium isotopes.