252Cf (2.645-year half-life) is a very strong neutron emitter and is thus extremely radioactive and harmful (one microgram spontaneously emits 170 million neutrons per minute). 249Cf is formed from the beta decay of 249Bk and most other californium isotopes are made by subjecting berkelium to intense neutron radiation in a nuclear reactor.
Californium has no biological role and only a few californium compounds have been made and studied. Included among these are californium oxide (Cf2O3), californium trichloride (CfCl3) and californium oxychloride (CfOCl). The only californium ion that is stable in aqueous solution is the californium(III) cation.
In October 2006 it was announced that on three occasions californium-249 atoms had been bombarded with calcium-48 ions to produce ununoctium (element 118), making this the heaviest element ever synthesized.
251Cf is famous for having a very small critical mass of 5 kg (), high lethality, and short period of toxic environmental irradiation relative to radioactive elements commonly used for radiation explosive weaponry, creating speculation about possible use in pocket nukes. However, the costs of such a bomb would be extremely high (around US $100 billion ). Other weaponry uses, such as showering an area with californium, are not impossible but are seen as inhumane and are subject to inclement weather conditions and porous terrain considerations.
Californium is produced by neutron capture on berkelium-249. Three californium isotopes with significant halflives are produced, requiring a total of 12 to 14 neutron captures on uranium-238 without nuclear fission or alpha decay. Their neutron cross sections are:
Thus 250Cf and 251Cf will be transmuted fairly quickly, with the majority fissioning at mass 251, but with a large fraction surviving to become 252Cf; the 252Cf however will not be transmuted or destroyed quickly in a well-thermalized reactor.
252Cf has a relatively high rate of spontaneous fission. Although still much less likely than alpha decay, this makes californium a significant neutron radiation emitter. MOX fuel containing enough curium would likely contain enough californium after use to preclude manual handling of the spent fuel or its nuclear reprocessing products with a glove box that protects against alpha and beta radiation but not against gamma radiation and especially neutron radiation.
To produce element 98, the team bombarded a microgram-sized target of 242Cm with 35 MeV alpha particles in the 5-foot (1.52 m) Berkeley cyclotron, which produced atoms of 245Cf (half-life 44 minutes) and a free neutron.
Due to its $27 million per gram price tag, only 8 grams of 252Cf have been made in the western world since its discovery by Seaborg in 1950. Plutonium supplied by the United Kingdom to the U.S. under the 1958 US-UK Mutual Defence Agreement was used for californium production.