californium

[kal-uh-fawr-nee-uhm]

californium [from California], artificially produced, radioactive metallic chemical element; symbol Cf; at. no. 98; mass no. of most stable isotope 251; m.p. about 900°C;; b.p. about 1,470°C;; density unknown; valence +3. Californium is a member of the actinide series of chemical elements, found in Group 3 of the periodic table. Its chemical properties are similar to those of lanthanum. Eighteen isotopes of californium are known, with half-lives ranging from about 40 sec for californium-239 to about 900 years for californium-251, the most stable isotope. Californium-249 (half-life 351 years) is most useful for chemical investigations; it is obtained by the decay of berkelium-249. Four solid compounds of californium have been prepared; they are the trichloride, oxychloride, oxyfluoride, and oxide. Californium-252 (half-life 2.6 years) is produced in nuclear reactors for use as a source of neutrons for counters and electronic systems in industrial and medical applications. The sixth transuranium element to be synthesized, californium has yet to be found in the earth's crust. Californium was first produced in 1950 by Glenn T. Seaborg, Stanley G. Thompson, Albert Ghiorso, and Kenneth Street, Jr., in a cyclotron at the Univ. of California at Berkeley by bombarding curium-242 with alpha particles, resulting in californium-245 (half-life 45 min).

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

Californium

Californium is a metallic chemical element with the symbol Cf and atomic number 98. A radioactive transuranic element, californium has very few uses. Uses include starting nuclear reactors (civilian and military); optimizing coal-fired power plants and concrete production facilities (via online analyzers); medical treatment of cancer; and oil exploration via down hole well logging. It was first produced by bombarding curium with alpha particles (helium ions).

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.

Applications

General

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

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.

Nuclear fuel cycle

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:

	Capture		Fission		HL
	Th	RI	Th	RI	(a)
²⁵⁰Cf	2000	12000			13.1
²⁵¹Cf	2900	1600	4800	5500	900
²⁵²Cf	20	44	32	1100	2.646

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.

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, Berkeley, being that California is one of a few nicknames for the university.

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

Twenty 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.