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Californium is a synthetic chemical element; it has Chemical symbol Cf and atomic number 98. It was first synthesized in 1950 at Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory) by bombarding curium with (helium-4 ions). It is an actinide element, the sixth transuranium element to be synthesized, and has the second-highest atomic mass of all elements that have been produced in amounts large enough to see with the naked eye (after einsteinium). It was named after the university and the U.S. state of California.
Two crystalline forms exist at normal pressure: one above and one below . A third form exists at high pressure. Californium slowly tarnishes in air at room temperature. Californium compounds are dominated by the +3 oxidation state. The most stable of californium's twenty known is californium-251, with a half-life of 898 years. This short half-life means the element is not found in significant quantities in the Earth's crust. Cf, with a half-life of about 2.645 years, is the most common isotope used and is produced at Oak Ridge National Laboratory (ORNL) in the United States and Research Institute of Atomic Reactors in Russia.
Californium is one of the few transuranium elements with practical uses. Most of these applications exploit the fact that certain isotopes of californium emit . For example, californium can be used to help start up , and it is used as a source of neutrons when studying materials using neutron diffraction and neutron spectroscopy. It can also be used in nuclear synthesis of higher mass elements; oganesson (element 118) was synthesized by bombarding californium-249 atoms with calcium-48 ions. Users of californium must take into account radiological concerns and the element's ability to disrupt the formation of red blood cells by bioaccumulation in skeletal tissue.
The element has two crystalline forms at standard atmospheric pressure: a double-hexagonal close-packed form dubbed alpha (α) and a face-centered cubic form designated beta (β). The α form exists below 600–800 °C with a density of 15.10 g/cm3 and the β form exists above 600–800 °C with a density of 8.74 g/cm. At 48 GPa of pressure the β form changes into an orthorhombic crystal system due to delocalization of the atom's Electron shell, which frees them to bond.
The bulk modulus of a material is a measure of its resistance to uniform pressure. Californium's bulk modulus is , which is similar to trivalent lanthanide metals but smaller than more familiar metals, such as aluminium (70 GPa).
| + Representative californium compounds ! state !! compound !! formula !! color !! | |||
| +3 | californium(III) polyborate | CfBO(OH) | |
The element slowly tarnishes in air at room temperature, with the rate increasing when moisture is added. Californium reacts when heated with hydrogen, nitrogen, or a chalcogen (oxygen family element); reactions with dry hydrogen and aqueous are rapid.
Californium is only aqueous solution as the californium(III) cation. Attempts to redox the +3 ion in solution have failed. The element forms a water-soluble chloride, nitrate, perchlorate, and sulfate and is precipitated as a fluoride, oxalate, or hydroxide. Californium is the heaviest actinide to exhibit covalent properties, as is observed in the californium borate.
Cf is formed by beta decay of berkelium-249, and heavier californium isotopes are made by subjecting berkelium to intense neutron radiation in a nuclear reactor. Though californium-251 has the longest half-life, its production yield is relatively low due to its rapid depletion by reaction with another neutron (high neutron cross section).
Cf is a very strong neutron emitter, which makes it an extremely hazardous radioactive isotope. Cf, 96.9% of the time, to curium-248; the other 3.1% of decays are spontaneous fission. One microgram of Cf emits 2.3 million neutrons per second (about 3.7 neutrons per fission). The other main isotopes of californium (248-251) also alpha decay to those of curium, with a much smaller fraction of fission.
To produce californium, a microgram-size target of curium-242 () was bombarded with 35 MeV () in the cyclotron at Berkeley, which produced californium-245 () plus one free neutron ().
The discoverers named the new element after the university and the state. This was a break from the convention used for elements 95 to 97, which drew inspiration from how the elements directly above them in the periodic table were named. However, the element directly above element 98 in the periodic table, dysprosium, has a name that 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 amounts of californium were first produced by the irradiation of plutonium targets at Materials Testing Reactor at National Reactor Testing Station, eastern Idaho; these findings were reported in 1954. The high spontaneous fission rate of californium-252 was observed in these samples. The first experiment with californium in concentrated form occurred in 1958. The isotopes Cf to Cf were isolated that same year from a sample of plutonium-239 that had been irradiated with neutrons in a nuclear reactor for five years. Two years later, in 1960, Burris Cunningham and James Wallman of Lawrence Radiation Laboratory of the University of California created the first californium compounds—californium trichloride, californium(III) oxychloride, and californium oxide—by treating californium with steam and hydrochloric acid.
The High Flux Isotope Reactor (HFIR) at ORNL in Oak Ridge, Tennessee, started producing small batches of californium in the 1960s. By 1995, HFIR nominally produced of californium annually. Plutonium supplied by the United Kingdom to the United States under the 1958 US–UK Mutual Defence Agreement was used for making californium.
The Atomic Energy Commission sold Cf to industrial and academic customers in the early 1970s for $10/microgram, and an average of of Cf were shipped each year from 1970 to 1990. Californium metal was first prepared in 1974 by Haire and Baybarz, who reduced californium(III) oxide with lanthanum metal to obtain microgram amounts of sub-micrometer thick films.
Nuclear fallout from atmospheric nuclear weapons testing prior to 1980 contributed a small amount of californium to the environment. Californium-249, -252, -253, and -254 have been observed in the radioactive dust collected from the air after a nuclear explosion. Californium is not a major radionuclide at United States Department of Energy legacy sites since it was not produced in large quantities.
Californium was once believed to be produced in , as their decay matches the 60-day half-life of Cf. However, subsequent studies failed to demonstrate any californium spectra, and supernova light curves are now thought to follow the decay of nickel-56.
The transuranic elements up to fermium, including californium, should have been present in the natural nuclear fission reactor at Oklo, but any quantities produced then would have long since decayed away.
Prolonged irradiation of americium, curium, and plutonium with neutrons produces milligram amounts of Cf and microgram amounts of Cf. As of 2006, curium isotopes 244 to 248 are irradiated by neutrons in special reactors to produce mainly californium-252 with lesser amounts of isotopes 249 to 255.
Microgram quantities of Cf are available for commercial use through the U.S. Nuclear Regulatory Commission. Only two sites produce Cf: Oak Ridge National Laboratory in the U.S., and the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. As of 2003, the two sites produce 0.25 grams and 0.025 grams of Cf per year, respectively.
Three californium isotopes with significant half-lives are produced, requiring a total of 15 neutron captures by uranium-238 without nuclear fission or alpha decay occurring during the process. Cf is at the end of a production chain that starts with uranium-238, and includes several isotopes of plutonium, americium, curium, and berkelium, and the californium isotopes 249 to 253 (see diagram).
has a number of specialized uses as a strong [[neutron emitter|Neutron source]]; it produces 139 million neutrons per microgram per minute. This property makes it useful as a startup neutron source for some nuclear reactors and as a portable (non-reactor based) neutron source for neutron activation analysis to detect trace amounts of elements in samples. Neutrons from californium are used as a treatment of certain [[cervical|Cervical cancer]] and [[brain cancers|brain tumor]] where other radiation therapy is ineffective. It has been used in educational applications since 1969 when [[Georgia Institute of Technology|Georgia Tech]] got a loan of 119 μg of Cf from the Savannah River Site. It is also used with online elemental [[coal analyzer]]s and bulk material analyzers in the coal and cement industries.
Neutron penetration into materials makes californium useful in detection instruments such as fuel rod scanners; neutron radiography of aircraft and weapons components to detect corrosion, bad welds, cracks and trapped moisture; and in portable metal detectors. Neutron moisture gauges use Cf 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. The main uses of Cf in 1982 were, reactor start-up (48.3%), fuel rod scanning (25.3%), and activation analysis (19.4%). By 1994, most Cf was used in neutron radiography (77.4%), with fuel rod scanning (12.1%) and reactor start-up (6.9%) as important but secondary uses. In 2021, fast neutrons from Cf were used for wireless data transmission.
has a very small calculated [[critical mass]] of about , high lethality, and a relatively short period of toxic environmental irradiation. The low critical mass of californium led to some exaggerated claims about possible uses for the element.
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 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. In addition, californium-249 and californium-251 can cause tissue damage externally, through gamma ray emission. Ionizing radiation emitted by californium on bone and in the liver can cause cancer.
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