Californium (pronounced /ˌkælɨˈfɔrniəm/, KAL-ə-FOR-nee-əm) is a radioactive metallic chemical element with the symbol Cf and atomic number 98. The element was first produced in 1950 by bombarding curium with alpha particles (helium ions) at the University of California, Berkeley. It was the sixth transuranic element to be synthesized. Californium is one of the highest atomic mass elements to have been produced in weighable amounts. It is named for the state and university of California.
Californium is primarily used in applications that take advantage of its strong neutron-emitting properties. For example, in starting nuclear reactors, medical treatment of cancer, and detecting explosives and metal fatigue. It is also used in oil exploration via down hole well logging. optimizing coal-fired power plants and cement production facilities (via online analyzers). Element 118 was synthesized by bombarding californium-249 atoms with calcium-48 ions.
Californium was first synthesized at the University of California, Berkeley by the physics researchers Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiorso, and Glenn T. Seaborg on February 9, 1950. It was the sixth transuranium element to be discovered, and this team announced its discovery on March 17, 1950.
To produce element 98, the University of California team bombarded a microgram-sized target of curium-242 with 35 MeV alpha particles in the 60-inch (1.52 m) Berkeley cyclotron, which produced atoms of californium-245 (half-life 44 minutes) and a free neutron. <-- Weeks 1968 p. 849 gives product as 98-244 -->
24296Cm + 42He → 24598Cf + 10n
The discoverers named the new element after the U.S. state of California and the University of California. This name was a break from the convention used for elements 95 to 97, which drew their inspiration from how the elements directly above them in the periodic table were named. Europium, in the sixth period directly above element 95, was named for the continent it was discovered on, so element 95 was named americium. Element 96 was named for Marie and Pierre Curie as an analog to the naming of gadolinium; named for scientist Johan Gadolin. Terbium was named after the city it was discovered in so element 97 was named berkelium. However, the element directly above element 98 in the periodic table, dysprosium, simply means "hard to get at" so the researchers decided to set-aside the informal naming convention. They added that "the best we can do is to point out [that] ... searchers a century ago found it difficult to get to California."
Weighable quantities of califorinium were first produced by long-duration irradiation of plutonium targets at the Materials Testing Reactor. The high spontaneous fission rate of Cf-252 was observed in these samples. The first experiment with californium in concentrated form occurred in 1958. Californium-249 to 252 were isolated that same year from a sample of plutonium-239 that had been irradiated with neutrons in a nuclear reactor for 5 years.
In the 1960s, reactors at the Savannah River Site and the High Flux Isotope Reactor(HFIR) started producing batches of californium regularly. The U.S. Atomic Energy Commission began selling and lending small amounts of californium to industrial and academic customers in 1970 for about ten dollars per microgram. By the 1990s, the reactor at Oak Ridge was producing 300–400 mg of Cf, mostly Cf-252, every two years.
Milligram-quantities of californium can only be made in specialized high-flux reactors; there are only two reactors operating that can efficiently produce it, the High Flux Isotope Reactor in the U.S. and Research Institute of Atomic Reactors in Dimitrovgrad, Russia, with HFIR filling about 2/3 of the world market of about 90 mg / year. Between 1960 and 1995, the HFIR produced only eight grams of californium, peaking at about 200 mg per year. The difficulty of obtaining bulk quantities of californium led, in 1974, to the misidentification of hexagonal Cf2O2S and face-centered cubic CfS as two forms of californium metal. The crystal structure needed to be determined by using microgram amounts of the element.
Plutonium supplied by the United Kingdom to the United States under the 1958 US-UK Mutual Defence Agreement was used for californium production.
Californium is a silvery white actinide metal that exists in three modifications. The element slowly tarnishes in air at room temperature, with the rate increasing with added moisture. Its chemical properties are predicted to be similar to dysprosium and it has valance of 4, 3, 2. Californium reacts when heated in the presence of hydrogen, nitrogen, or a chalcogen and oxidizes when warmed in air; reactions with dry hydrogen hydrides and aqueous mineral acids are rapid. Weighable amounts of californium make it possible to determine some of its properties. Californium-252 (2.645-year half-life) is a very strong neutron emitter and is thus extremely radioactive and harmful. One microgram spontaneously emits 2.314 million neutrons per second and one gram emits 39 watts of heat. Californium-249 is formed from the beta decay of berkelium-249 and most other californium isotopes are made by subjecting berkelium to intense neutron radiation in a nuclear reactor.
Californium disrupts the body's ability to form red blood cells by bio-accumulating in skeletal tissue. The element plays no natural biological role in any organism due to its intense radioactivity and low concentration in the environment.
The element has two crystalline forms: a double-hexagonal close-packed α form that exists below 900°C with a density of 15.10 g/cc and a face-centered cubic β form with a density of 8.74 g/cc. Metallic californium has not been produced yet.
Californium is not known to occur naturally on the Earth but very minute amounts might exist in some uranium ores. The element is produced in nuclear reactors and particle accelerators. Its use in mineral prospecting and in medical treatments and research means it can be found near facilities that use californium. Californium is not a major radionuclide at United States Department of Energy legacy sites since it was not produced in large quantities.
Fallout from atmospheric nuclear testing prior to 1980 contributed a small amount of californium to the environment. The element is fairly insoluble in water but adheres well to soil. Concentrations in soil are therefore 500 times higher than in interstitial water.
On November 1, 1952, the nuclear fallout of the world's first large hydrogen bomb test, the Ivy Mike explosion at Eniwetok Atoll, contained plutonium, californium, einsteinium, fermium, and traces of other transuranium elements. Besides the first observations of einsteinium and fermium being recorded after this bomb test, the californium isotopes with mass numbers 249, 252, 253, and 254 were observed for the first time in the radioactive dust from this explosion.
The element and its decay products may occur elsewhere in the universe. Electromagnetic emissions possibly caused by the decay of californium-254 are observed in the spectra of supernovae.
Californium is usually produced by bombarding berkelium-249 with neutrons. This forms berkelium-250 which quickly beta decays to californium-250 in the following reaction:
24997Bk(n,y)25097Bk → 25098Cf + β−
Bombardment of californium-250 with neutrons produces californium-251 and 252.
Prolonged irradiation of americium, curium, and plutonium with neutrons produces milligram amounts of californium-252 and microgram amounts of californium-249. Three californium isotopes with significant half-lives are produced, requiring a total of 12 to 14 neutron captures by uranium-238 without nuclear fission or alpha decay occurring during the process. Their neutron capture cross-sections are :
Thus californium-250 and californium-251 will be transmuted fairly quickly, with the majority undergoing nuclear fission at the mass 251, but with a large fraction surviving to become californium-252. The californium-252 will not be transmuted or destroyed quickly in a well-thermalized reactor, but it has a short decay half-life. These isotopes decay into long-lived isotopes of curium.
Californium-252 has a relatively high rate of spontaneous fission. Although still much less likely than alpha decay, this makes californium a significant neutron radiation emitter. Manufactured MOX fuel that contains enough curium would likely also contain enough californium after its use to preclude manual handling of the spent fuel, or its nuclear reprocessing products, with mere glove boxes -- that protects against alpha and beta radiation, but not against energetic gamma radiation and especially not against neutron radiation.
Californium-249 metal is produced by the reduction of californium(III) oxide with lanthanum metal.
Few californium compounds have been made and studied. The only californium ion that is stable in aqueous solution is the californium(III) cation. The other two oxidation states are IV (strong oxidizing agents) and II (strong reducing agents). If problems of availability of the element could be overcome, then CfBr2 and CfI2 would likely be stable.
The III oxidation state is represented by californium(III) oxide (yellow-green, Cf2O3), californium(III) fluoride (bright green, CfF3) and californium(III) iodide (lemon yellow, CfI3). Other +3 oxidation states include the sulfide and Cp3Cf. Californium(IV) oxide (black brown, CfO2), californium(IV) fluoride (green, CfF4), californium(IV) bromide (yellow, CfBr2) and californium(IV) iodide (dark violet, CfI2) represent the VI oxidation state. The II state is represented by californium(II) bromide (yellow, CfBr2) and californium(II) iodide (dark violet, CfI2).
Californium(III) chloride (CfCl3) is an emerald green compound with a hexagonal structure that can be prepared by combining Cf2O3 with hydrochloric acid at 500°C. CfCl3 is then used as a feeder stock to form the yellow-orange tri-iodide CfI3, which in turn can be reduced to the lavender-violet di-iodide CfI2.
Heating the sulfate in air at about 1200°C and then reducing with hydrogen at 500°C produces the sesquioxide (Cf2O3). The hydroxide Cf(OH)3 and the trifluoride CfF3 are slightly soluble.
Twenty radioisotopes of californium have been characterized, the most stable being californium-251 with a half-life of 898 years, californium-249 with a half-life of 351 years, and californium-250 with a half-life of 13.08 years. All of the remaining radioactive isotopes have half-lives that are shorter than 2.7 years, and the majority of these have half-lives shorter than 20 minutes. The isotopes of californium range in mass number from 237 to 256.
Californium-252 has a half life of 2.645 years. Californium-252 undergoes α-decay 96.9% of the time while the remaining 3.1% of decays are spontaneous fission. Each spontaneous fission decay emits an average of 3.77 neutrons per fission. Californium-254 decays nearly quantitatively by spontaneous fission with a half-life of 60.5 days. Both materials can be used as a neutron source.
Californium-252 has a number of specialized applications as a strong neutron emitter. Each microgram of fresh californium produces 170 million neutrons per minute. This makes it useful as a neutron startup source for some nuclear reactor and as a portable (non-reactor based) neutron source for neutron activation analysis to detect trace amounts of elements in samples. Neutrons from californium is employed as a treatment of certain cervical and brain cancers where other radiation therapy is ineffective.
Neutron penetration into materials makes it useful in detection instruments. This property makes it useful in fuel rod scanners, radiography of aircraft to detect metal fatigue, in airport neutron-activation detectors of explosives, and in portable metal detectors. Neutron moisture gauges use californium-252 to find water and petroleum layers in oil wells, as a portable neutron source for gold and silver prospecting for on-the-spot analysis, and to detect ground water movement.
In October 2006 researchers announced that three atoms ununoctium (element 118) had been identified at the Joint Institute for Nuclear Research in Dubna as the product of bombardment of californium-249 with calcium-48 , making this the heaviest element ever synthesized.
Californium-251 is notable among nuclear scientists and technologists for having a very small critical mass of about 5 kg, high lethality, and short period of toxic environmental irradiation relative to radioactive elements commonly used for radiation explosive weaponry, creating former speculation about possible use in pocket nukes.
Californium can enter the body from ingesting contaminated food or drinks or by breathing air with suspended particles of the element. Once in the body, only 0.05% of the californium will reach the bloodstream. About 65% of that californium will be deposited in the skeleton, 25% in the liver, and the rest in other organs or is excreted, mainly in urine. Half of the californium deposited in the skeleton and liver are gone in 50 and 20 years, respectively. Californium in the skeleton adheres to bone surfaces before slowly migrating throughout the bone.
The element is most dangerous if taken into the body but gamma rays emitted by californium-249 and californium-251 cause external tissue damage. Ionizing radiation emitted by californium on bone and in the liver can cause cancer. Two to six out of 100,000 people are estimated to die of a fatal cancer if they were continuously exposed to soil with an initial average concentration of 1 pCi/g of californium-251 and californium-249, respectively.
1. ^ a b c Cunningham 1968, p. 103.
* Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999). Advanced Inorganic Chemistry (6th ed.). New York: John Wiley & Sons, Inc.. ISBN 0-471-199957-5.
* Stwertka, Albert (1998), Guide to the Elements (Revised ed.), Oxford University Press, ISBN 0195080831 .