Californium a nuclear explosion. Californium is not a major radionuclide at United States Department of Energy legacy sites since it was not produced in large quantities. Californium was once believed to be produced in supernovas, as their decay matches the 60-day half-life of Cf. However, subsequent studies failed to demonstrate any californium spectra, and supernova light curves are now thought to follow the decay of nickel-56. The transuranic elements from americium to fermium, including californium, occurred naturally in the natural nuclear fission reactor at Oklo, but no longer do so. Californium is produced in nuclear reactors and particle accelerators. Californium-250 is
Californium made by bombarding berkelium-249 () with neutrons, forming berkelium-250 () via neutron capture (n,γ) which, in turn, quickly beta decays (β) to californium-250 () in the following reaction: Bombardment of californium-250 with neutrons produces californium-251 and californium-252. Prolonged irradiation of americium, curium, and plutonium with neutrons produces milligram amounts of californium-252 and microgram amounts of californium-249. As of 2006, curium isotopes 244 to 248 are irradiated by neutrons in special reactors to produce primarily californium-252 with lesser amounts of isotopes 249 to 255. Microgram quantities of californium-252 are available for commercial use through the U.S. Nuclear Regulatory Commission. Only two