Californium

Californium is a chemical element with the symbol Cf and atomic number 98. A radioactive transuranic element, californium has very few uses and was discovered by bombarding curium with alpha particles (helium ions).

Notable characteristics
Weighable amounts of californium make it possible to determine some of its properties using macroscopic quantities.

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.

General uses
The element does have some specialist applications dealing with its radioactivity but otherwise is largely too difficult to produce to have widespread useful significance as a material. Some of its uses are:
 * neutron startup source for some nuclear reactors, calibrating instrumentation
 * treatment of certain cervical and brain cancers where other radiation therapy is ineffective
 * radiography of aircraft to detect metal fatigue
 * airport neutron-activation detectors of explosives
 * portable metal detectors
 * neutron moisture gauges used to find water and petroleum layers in oil wells
 * portable neutron source in gold and silver prospecting for on-the-spot analysis

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.

Military use
251Cf is famous for having a very small critical mass, 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. This urban legend is unfounded since it would be very difficult to make a 251Cf bomb weighing less than 2 kg, and the costs of such a bomb would be prohibitive. 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. Often cited as a consideration is the cost of producing californium in quantity, but the cost citations are usually due to extra fees that laboratory materials companies insert for sake of caution and market needs.

Nuclear fuel cycle
Pertaining to californium's nuclear fuel cycle, it is important to make sure that the curium concentration in MOX nuclear fuel is kept low, as neutron irradiation of curium will convert some of it to californium. The californium will then cause the used fuel to be more difficult to handle as the californium is a strong neutron emitter (through spontaneous fission). Hence the concentration of curium and californium among the Minor actinides are important.

History
Californium was first synthesized at the University of California, Berkeley by researchers Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiorso and Glenn T. Seaborg in 1950. It was the sixth transuranium element to be discovered and the team announced their discovery on March 17, 1950. It was named after the U.S. state of California and for the University of California system.

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.

Isotopes
Nineteen radioisotopes of californium have been characterized, the most stable being 251Cf with a half-life of 898 years, 249Cf with a half-life of 351 years, and 250Cf with a half-life of 13 years. All of the remaining radioactive isotopes have half-lives that are less than 2.7 years, and the majority of these have half-lives shorter than 20 minutes. The isotopes of californium range in atomic weight from 237.062 u (237Cf) to 256.093 u (256Cf).

Natural occurrence
Although californium does not occur naturally on Earth, the element and its decay products occur elsewhere in the universe. Their electromagnetic emissions are regularly observed in the spectra of supernovae.