Английская Википедия:Californium

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Шаблон:About Шаблон:Use mdy dates Шаблон:Featured article Шаблон:Infobox californium Californium is a synthetic chemical element; it has symbol Cf and atomic number 98. The element was first synthesized in 1950 at Lawrence Berkeley National Laboratory (then the University of California Radiation Laboratory), by bombarding curium with alpha particles (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). The element was named after the university and the U.S. state of California.

Two crystalline forms exist for californium at normal pressure: one above and one below Шаблон:Convert. 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 isotopes 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.Шаблон:Efn ²⁵²Cf, with a half-life of about 2.645 years, is the most common isotope used and is produced at Oak Ridge National Laboratory in the United States and Research Institute of Atomic Reactors in Russia.

Californium is one of the few transuranium elements with practical applications. Most of these applications exploit the property of certain isotopes of californium to emit neutrons. For example, californium can be used to help start up nuclear reactors, and it is employed as a source of neutrons when studying materials using neutron diffraction and neutron spectroscopy. Californium 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 bioaccumulating in skeletal tissue.

Characteristics

Physical properties

Californium is a silvery-white actinide metalШаблон:Sfn with a melting point of Шаблон:Convert and an estimated boiling point of Шаблон:Convert.Шаблон:Sfn The pure metal is malleable and is easily cut with a razor blade. Californium metal starts to vaporize above Шаблон:Convert when exposed to a vacuum.Шаблон:Sfn Below Шаблон:Convert californium metal is either ferromagnetic or ferrimagnetic (it acts like a magnet), between 48 and 66 K it is antiferromagnetic (an intermediate state), and above Шаблон:Convert it is paramagnetic (external magnetic fields can make it magnetic).Шаблон:Sfn It forms alloys with lanthanide metals but little is known about the resulting materials.Шаблон:Sfn

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).Шаблон:Efn 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/cm³.Шаблон:Sfn At 48 GPa of pressure the β form changes into an orthorhombic crystal system due to delocalization of the atom's 5f electrons, which frees them to bond.Шаблон:SfnШаблон:Efn

The bulk modulus of a material is a measure of its resistance to uniform pressure. Californium's bulk modulus is Шаблон:Val, which is similar to trivalent lanthanide metals but smaller than more familiar metals, such as aluminium (70 GPa).Шаблон:Sfn

Chemical properties and compounds

Representative californium compoundsШаблон:SfnШаблон:Efn

state	compound	formula	color
+2	californium(II) bromide	CfBr2	yellow
+2	californium(II) iodide	CfI2	dark violet
+3	californium(III) oxide	Cf2O3	yellow-green
+3	californium(III) fluoride	CfF3	bright green
+3	californium(III) chloride	CfCl3	emerald green
+3	californium(III) bromide	CfBr3	yellowish green
+3	californium(III) iodide	CfI3	lemon yellow
+3	californium(III) polyborate	Cf[B6O8(OH)5]	pale green
+4	californium(IV) oxide	CfO2	black brown
+4	californium(IV) fluoride	CfF4	green

Californium exhibits oxidation states of 4, 3, or 2. It typically forms eight or nine bonds to surrounding atoms or ions. Its chemical properties are predicted to be similar to other primarily 3+ valence actinide elementsШаблон:Sfn and the element dysprosium, which is the lanthanide above californium in the periodic table.Шаблон:Sfn Compounds in the +4 oxidation state are strong oxidizing agents and those in the +2 state are strong reducing agents.Шаблон:Sfn

The element slowly tarnishes in air at room temperature, with the rate increasing when moisture is added.Шаблон:Sfn Californium reacts when heated with hydrogen, nitrogen, or a chalcogen (oxygen family element); reactions with dry hydrogen and aqueous mineral acids are rapid.Шаблон:Sfn

Californium is only water-soluble as the californium(III) cation. Attempts to reduce or oxidize the +3 ion in solution have failed.Шаблон:Sfn The element forms a water-soluble chloride, nitrate, perchlorate, and sulfate and is precipitated as a fluoride, oxalate, or hydroxide.Шаблон:Sfn Californium is the heaviest actinide to exhibit covalent properties, as is observed in the californium borate.

Isotopes

Шаблон:Main Twenty isotopes of californium are known (mass number ranging from 237 to 256); the most stable are ²⁵¹Cf with half-life 898 years, ²⁴⁹Cf with half-life 351 years, ²⁵⁰Cf with half-life 13.08 years, and ²⁵²Cf with half-life 2.645 years. All other isotopes have half-life shorter than a year, and most of these have half-lives less than 20 minutes.

²⁴⁹Cf is formed from beta decay of berkelium-249, and most other californium isotopes are made by subjecting berkelium to intense neutron radiation in a nuclear reactor.Шаблон:Sfn Though californium-251 has the longest half-life, its production yield is only 10% due to its tendency to collect neutrons (high neutron capture) and its tendency to interact with other particles (high neutron cross section).Шаблон:Sfn

Californium-252 is a very strong neutron emitter, which makes it extremely radioactive and harmful. ²⁵²Cf, 96.9% of the time, alpha decays to curium-248; the other 3.1% of decays are spontaneous fission. One microgram (μg) of ²⁵²Cf emits 2.3 million neutrons per second, an average of 3.7 neutrons per spontaneous fission. Most other isotopes of californium, alpha decay to curium (atomic number 96).

History

Large pieces of equipment with a man standing nearby.

The Шаблон:Convert cyclotron used to first synthesize californium

Californium was first made at University of California Radiation Laboratory, Berkeley, by physics researchers Stanley Gerald Thompson, Kenneth Street Jr., Albert Ghiorso, and Glenn T. Seaborg, about February 9, 1950.Шаблон:Sfn It was the sixth transuranium element to be discovered; the team announced its discovery on March 17, 1950.

To produce californium, a microgram-size target of curium-242 (Шаблон:Nuclide) was bombarded with 35 MeV alpha particles (Шаблон:Nuclide) in the Шаблон:Convert cyclotron at Berkeley, which produced californium-245 (Шаблон:Nuclide) plus one free neutron (Шаблон:SubatomicParticle).Шаблон:Sfn

Шаблон:Nuclide + Шаблон:Nuclide → Шаблон:Nuclide + Шаблон:SuШаблон:SubatomicParticle

To identify and separate out the element, ion exchange and adsorsion methods were undertaken. Only about 5,000 atoms of californium were produced in this experiment,Шаблон:Sfn and these atoms had a half-life of 44 minutes.Шаблон:Sfn

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.Шаблон:SfnШаблон:Efn 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.Шаблон:Sfn They added that "the best we can do is to point out [that] ... searchers a century ago found it difficult to get to California".Шаблон:Sfn

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.Шаблон:Sfn The isotopes ²⁴⁹Cf to ²⁵²Cf were isolated that same year from a sample of plutonium-239 that had been irradiated with neutrons in a nuclear reactor for five years.Шаблон:Sfn 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 Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee, started producing small batches of californium in the 1960s. By 1995, HFIR nominally produced Шаблон:Convert of californium annually.Шаблон:Sfn 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 ²⁵²Cf to industrial and academic customers in the early 1970s for $10 per microgram, and an average of Шаблон:Convert of ²⁵²Cf were shipped each year from 1970 to 1990.Шаблон:SfnШаблон:Efn 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.Шаблон:SfnШаблон:Efn

Occurrence

Traces of californium can be found near facilities that use the element in mineral prospecting and in medical treatments.Шаблон:Sfn The element is fairly insoluble in water, but it adheres well to ordinary soil; and concentrations of it in the soil can be 500 times higher than in the water surrounding the soil particles.

Nuclear fallout from atmospheric nuclear weapons testing prior to 1980 contributed a small amount of californium to the environment. Californium isotopes with mass numbers 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 supernovas, as their decay matches the 60-day half-life of ²⁵⁴Cf. However, subsequent studies failed to demonstrate any californium spectra, and supernova light curves are now thought to follow the decay of nickel-56.Шаблон:Sfn

The transuranium elements from americium to fermium, including californium, occurred naturally in the natural nuclear fission reactor at Oklo, but no longer do so.

Spectral lines of californium, along with those of several other non-primordial elements, were detected in Przybylski's Star in 2008.

Production

Шаблон:See also Californium is produced in nuclear reactors and particle accelerators.Шаблон:Sfn Californium-250 is made by bombarding berkelium-249 (Шаблон:Nuclide) with neutrons, forming berkelium-250 (Шаблон:Nuclide) via neutron capture (n,γ) which, in turn, quickly beta decays (β⁻) to californium-250 (Шаблон:Nuclide) in the following reaction:Шаблон:Sfn

Шаблон:Nuclide(n,γ)Шаблон:Nuclide → Шаблон:Nuclide + β⁻

Bombardment of californium-250 with neutrons produces californium-251 and californium-252.Шаблон:Sfn

Prolonged irradiation of americium, curium, and plutonium with neutrons produces milligram amounts of californium-252 and microgram amounts of californium-249.Шаблон:Sfn As of 2006, curium isotopes 244 to 248 are irradiated by neutrons in special reactors to produce primarily californium-252 with lesser amounts of isotopes 249 to 255.Шаблон:Sfn

Microgram quantities of californium-252 are available for commercial use through the U.S. Nuclear Regulatory Commission.Шаблон:Sfn Only two sites produce californium-252: the Oak Ridge National Laboratory in the United States, 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 californium-252 per year, respectively.Шаблон:Sfn

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.Шаблон:Sfn Californium-253 is at the end of a production chain that starts with uranium-238, includes several isotopes of plutonium, americium, curium, berkelium, and the californium isotopes 249 to 253 (see diagram).

A complex flow diagram showing various isotopes.

Scheme of the production of californium-252 from uranium-238 by neutron irradiation

Applications

Large conical structure on a pulley with a man on top and two near the base.

Fifty-ton shipping cask built at Oak Ridge National Laboratory which can transport up to 1 gram of ²⁵²Cf.Шаблон:Sfn Large and heavily shielded transport containers are needed to prevent the release of highly radioactive material in case of normal and hypothetical accidents.

Californium-252 has a number of specialized uses as a strong neutron emitter; it produces 139 million neutrons per microgram per minute. This property makes it useful as a startup neutron source for some nuclear reactorsШаблон:Sfn and as a portable (non-reactor based) neutron source for neutron activation analysis to detect trace amounts of elements in samples.Шаблон:Efn Neutrons from californium are used as a treatment of certain cervical and brain cancers where other radiation therapy is ineffective.Шаблон:Sfn It has been used in educational applications since 1969 when Georgia Institute of Technology got a loan of 119 μg of ²⁵²Cf from the Savannah River Site.Шаблон:Sfn It is also used with online elemental coal analyzers 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;Шаблон:Sfn neutron radiography of aircraft and weapons components to detect corrosion, bad welds, cracks and trapped moisture;Шаблон:Sfn and in portable metal detectors. Neutron moisture gauges use ²⁵²Cf to find water and petroleum layers in oil wells, as a portable neutron source for gold and silver prospecting for on-the-spot analysis,Шаблон:Sfn and to detect ground water movement. The main uses of ²⁵²Cf in 1982 were, reactor start-up (48.3%), fuel rod scanning (25.3%), and activation analysis (19.4%).Шаблон:Sfn By 1994, most ²⁵²Cf was used in neutron radiography (77.4%), with fuel rod scanning (12.1%) and reactor start-up (6.9%) as important but secondary uses.Шаблон:Sfn In 2021, fast neutrons from ²⁵²Cf were used for wireless data transmission.

²⁵¹Cf has a very small calculated critical mass of about Шаблон:Convert, 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.Шаблон:Efn

In October 2006, researchers announced that three atoms of oganesson (element 118) had been identified at Joint Institute for Nuclear Research in Dubna, Russia, from bombarding ²⁴⁹Cf with calcium-48, making it the heaviest element ever made. The target contained about 10 mg of ²⁴⁹Cf deposited on a titanium foil of 32 cm² area. Californium has also been used to produce other transuranium elements; for example, lawrencium was first synthesized in 1961 by bombarding californium with boron nuclei.

Precautions

Californium that bioaccumulates in skeletal tissue releases radiation that disrupts the body's ability to form red blood cells.Шаблон:Sfn The element plays no natural biological role in any organism due to its intense radioactivity and low concentration in the environment.Шаблон:Sfn

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.

Bibliography

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External links

• Californium at The Periodic Table of Videos (University of Nottingham)
• NuclearWeaponArchive.org – Californium
• Hazardous Substances Databank – Californium, Radioactive