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History

Californium was first synthesized by University of California, Berkeley 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.

Glenn T. Seaborg

To produce element 98, the team bombarded a microgram-sized target of ²⁴²Cm with 35 MeV alpha particles in the 60-inch Berkeley cyclotron which produced atoms of ²⁴⁵Cf (half-life 44 minutes) and a free neutron.

Characteristics

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

²⁵²Cf (2.6 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).  The decay of ²⁵⁴Cf (55-day half-life) may have been detected through telescopes in supernovae remnants. 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.

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.

Applications

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 barain 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

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.  A government need not consider these as prohibitive.

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 concentrations of curium and californium among the Minor actinides are important.

Compounds

In 1960 a few tenths of a microgram of compounds of californium were first produced 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.

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 amu (²³⁷Cf) to 256.093 amu (²⁵⁶Cf).

All californium isotopes are radioactive; the long-lived isotopes are produced from berkelium-249 or from californium-249. They are: californium-249 (360-year half-life); californium-250 (13-year half-life); californium-251 (800-year half-life); and californium-252 (2.65-year half-life). These isotopes have been used in tracer amounts for investigating the chemistry of californium (which exhibits an oxidation state of +3 in acidic aqueous solution) and for preparing microgram quantities of compounds such as the oxychloride CfOCl, the oxide Cf₂O₃, and the trichloride CfCl₃. There is some evidence for a +2 state also. Metallic californium has not yet been prepared.

Califomium (III) is the only ion stable in aqueous solutions, all attempts to reduce or oxidize californium (III) having failed. The isotope ²⁴⁹Cf results from the beta decay of ²⁴⁹Bk while the heavier isotopes are produced by intense neutron irradiation by the reactions:

²⁴⁹Bk(n,y) ²⁵⁰Bk --B--> ²⁵⁰Cf and 249Cf(n,y) ²⁵⁰Cf followed by ²⁵⁰Cf(n,y) ²⁵¹Cf(n,y) ²⁵²Cf

The existence of the isotopes ²⁴⁹Cf, ²⁵⁰Cf, ²⁵¹Cf, and ²⁵²Cf makes it feasible to isolate californium in weighable amounts so that its properties can be investigated with macroscopic quantities. Californium-252 is a very strong neutron emitter. One microgram releases 170 million neutrons per minute, which presents biological hazards. Proper safeguards should be used in handling californium. Eighteen isotopes of californium are now recognized. ²⁴⁹Cf and ²⁵²Cf have half-lives of 351 years and 900 years, respectively.

Californium-252, an isotope with a half-life of about 2.6 years, is a very strong neutron emitter.  Because 3 percent of its decay occurs by spontaneous fission, it is industrially and medically important as a very intense point source of neutrons.  One microgram (0.000001 grams) of californium-252 produces 170 million neutrons per minute, which presents biological hazards. Proper safeguards should be used in handling californium.

Californium's most stable isotope, californium-251, has a half-life of about 898 years. It decays into curium-247 through alpha decay or decays through spontaneous fission.

Precautions

²⁵¹Cf is famous for having a very small critical mass, high lethality, and short period of toxic environmental irradiation relative to radioactive elements.  Due to its extreme rarity in nature these concerns are exceedingly minimal.  There is currently no evidence that californium plays any biological role.