Twocrystalline forms exist at normal pressure: one above and one below 900 °C (1,650 °F). A third form exists at high pressure. Californium slowly tarnishes in air at room temperature.Californium compounds are dominated by the +3oxidation state. The most stable of californium's twenty knownisotopes is californium-251, with ahalf-life of 898 years. This short half-life means the element is not found in significant quantities in the Earth's crust.[a]252Cf, with a half-life of about 2.645 years, is the most common isotope used and is produced atOak Ridge National Laboratory (ORNL) in the United States andResearch 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 certainisotopes of californium emitneutrons. For example, californium can be used to help start upnuclear reactors, and it is used as a source of neutrons when studying materials usingneutron diffraction andneutron spectroscopy. It can also be used in nuclear synthesis of higher mass elements;oganesson (element 118) was synthesized by bombarding californium-249 atoms withcalcium-48 ions. Users of californium must take into account radiological concerns and the element's ability to disrupt the formation ofred blood cells bybioaccumulating in skeletal tissue.
Californium is a silvery-whiteactinide metal[11] with amelting point of 900 ± 30 °C (1,650 ± 50 °F) and an estimatedboiling point of 1,743 K (1,470 °C; 2,680 °F).[12] The pure metal is malleable and is easily cut with a knife. Californium metal starts to vaporize above 300 °C (570 °F) when exposed to a vacuum.[13] Below 51 K (−222 °C; −368 °F) californium metal is eitherferromagnetic orferrimagnetic (it acts like a magnet), between 48 and 66 K it isantiferromagnetic (an intermediate state), and above 160 K (−113 °C; −172 °F) it isparamagnetic (external magnetic fields can make it magnetic).[14] It formsalloys withlanthanide metals but little is known about the resulting materials.[13]
Thebulk modulus of a material is a measure of its resistance to uniform pressure. Californium's bulk modulus is50±5 GPa, which is similar to trivalent lanthanide metals but smaller than more familiar metals, such as aluminium (70 GPa).[17]
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 elements[19] and the elementdysprosium, which is the lanthanide above californium in the periodic table.[20] Compounds in the +4 oxidation state are strongoxidizing agents and those in the +2 state are strongreducing agents.[11]
The element slowly tarnishes in air at room temperature, with the rate increasing when moisture is added.[16] Californium reacts when heated withhydrogen,nitrogen, or achalcogen (oxygen family element); reactions with dry hydrogen and aqueousmineral acids are rapid.[16]
All nuclear data not otherwise stated is from the standard source:[22]
Twentyisotopes of californium are known withmass number ranging from 237 to 256; the most stable are251Cf withhalf-life 898 years,249Cf with half-life 351 years,250Cf at 13.08 years, and252Cf at 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 bybeta decay of berkelium-249, and heavier californium isotopes are made by subjecting berkelium to intense neutron radiation in anuclear 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 (highneutron cross section).[23]
252Cf is a very strongneutron emitter, which makes it an extremely hazardousradioactive isotope.[24][25][26]252Cf, 96.9% of the time,alpha decays tocurium-248; the other 3.1% of decays arespontaneous fission. Onemicrogram of252Cf emits 2.3 million neutrons per second (about 3.7 neutrons per fission).[27] The other main isotopes of californium (248-251) also alpha decay to those ofcurium, with a much smaller fraction of fission.
To produce californium, a microgram-size target of curium-242 (242 96Cm) was bombarded with 35 MeValpha particles (4 2He) in the 60-inch-diameter (1.52 m)cyclotron at Berkeley, which produced californium-245 (245 98Cf) plus onefree neutron (n).[28][29]
242 96Cm +4 2He →245 98Cf +1 0n
To identify and separate out the element,ion exchange and adsorsion methods were undertaken.[29][31] Only about 5,000 atoms of californium were produced in this experiment,[32] and these atoms had a half-life of 44 minutes.[28]
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.[33][e] 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.[35] They added that "the best we can do is to point out [that] ... searchers a century ago found it difficult to get to California".[34]
Weighable amounts of californium were first produced by the irradiation of plutonium targets atMaterials Testing Reactor atNational Reactor Testing Station,eastern Idaho; these findings were reported in 1954.[36] The high spontaneous fission rate of californium-252 was observed in these samples. The first experiment with californium in concentrated form occurred in 1958.[28] The isotopes249Cf to252Cf were isolated that same year from a sample ofplutonium-239 that had been irradiated with neutrons in a nuclear reactor for five years.[11] 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 andhydrochloric acid.[37]
TheAtomic Energy Commission sold252Cf to industrial and academic customers in the early 1970s for $10/microgram,[27] and an average of 150 mg (0.0053 oz) of252Cf were shipped each year from 1970 to 1990.[41][f] 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.[42][43][g]
Traces of californium can be found near facilities that use the element in mineral prospecting and in medical treatments.[45] 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.[46]
Nuclear fallout from atmosphericnuclear weapons testing prior to 1980 contributed a small amount of californium to the environment.[46] Californium-249, -252, -253, and -254 have been observed in the radioactive dust collected from the air after a nuclear explosion.[47] Californium is not a major radionuclide atUnited States Department of Energy legacy sites since it was not produced in large quantities.[46]
Californium was once believed to be produced insupernovas, as their decay matches the 60-day half-life of254Cf.[48] However, subsequent studies failed to demonstrate any californium spectra,[49] and supernova light curves are now thought to follow the decay ofnickel-56.[50]
The transuranic elements up tofermium, including californium, should have been present in thenatural nuclear fission reactor atOklo, but any quantities produced then would have long since decayed away.[51]
Californium is produced innuclear reactors andparticle accelerators.[52] Californium-250 is made by bombarding berkelium-249 (249Bk) with neutrons, forming berkelium-250 (250Bk) vianeutron capture (n,γ) which, in turn, quicklybeta decays (β−) to californium-250 (250Cf) in the following reaction:[53]
249 97Bk(n,γ)250 97Bk →250 98Cf + β−
Bombardment of250Cf with neutrons produces251Cf and252Cf.[53]
Prolonged irradiation ofamericium, curium, and plutonium with neutrons produces milligram amounts of252Cf and microgram amounts of249Cf.[54] 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.[55]
Three californium isotopes with significant half-lives are produced, requiring a total of 15 neutron captures byuranium-238 withoutnuclear fission or alpha decay occurring during the process.[56]253Cf is at the end of a production chain that starts with uranium-238, and includes severalisotopes of plutonium,americium,curium, andberkelium, and the californium isotopes 249 to 253 (see diagram).
Scheme of the production of californium-252 from uranium-238 by neutron irradiation
Fifty-ton shipping cask built at ORNL which can transport up to 1 gram of252Cf.[57] Large and heavily shielded transport containers are needed to prevent the release of highly radioactive material in case of normal and hypothetical accidents.[58]
Neutron penetration into materials makes californium useful in detection instruments such asfuel rod scanners;[16]neutron radiography of aircraft and weapons components to detectcorrosion, bad welds, cracks and trapped moisture;[62] and in portable metal detectors.[63]Neutron moisture gauges use252Cf to find water and petroleum layers in oil wells, as a portableneutron source for gold and silver prospecting for on-the-spot analysis,[20] and to detect ground water movement.[64] The main uses of252Cf in 1982 were, reactor start-up (48.3%), fuel rod scanning (25.3%), and activation analysis (19.4%).[65] By 1994, most252Cf was used in neutron radiography (77.4%), with fuel rod scanning (12.1%) and reactor start-up (6.9%) as important but secondary uses.[65] In 2021, fast neutrons from252Cf were used for wireless data transmission.[66]
In October 2006, researchers announced that three atoms ofoganesson (element 118) had been identified atJoint Institute for Nuclear Research inDubna,Russia, from bombarding249Cf withcalcium-48, making it the heaviest element ever made. The target contained about 10 mg of249Cf deposited on a titanium foil of 32 cm2 area.[67][68][69] Californium has also been used to produce other transuranic elements; for example,lawrencium was first synthesized in 1961 by bombarding californium withboron nuclei.[70]
251 Cf has a very small calculatedcritical mass of about 5 kg (11 lb),[71] 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.[i]
Californium thatbioaccumulates in skeletal tissue releases radiation that disrupts the body's ability to formred blood cells.[73] The element plays no natural biological role in any organism due to its intense radioactivity and low concentration in the environment.[45]
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.[46]
The element is most dangerous if taken into the body. In addition, californium-249 and californium-251 can cause tissue damage externally, throughgamma ray emission.Ionizing radiation emitted by californium on bone and in the liver can cause cancer.[46]
^ The Earthformed 4.5 billion years ago, and the extent of natural neutron emission within it that could produce californium from more stable elements is extremely limited.
^A double hexagonal close-packed (dhcp)unit cell consists of two hexagonal close-packed structures that share a common hexagonal plane, giving dhcp an ABACABAC sequence.[15]
^The three lower-mass transplutonium elements—americium,curium, andberkelium—require much less pressure to delocalize their 5f electrons.[17]
^Other +3 oxidation states include the sulfide andmetallocene.[18]
^Europium, in the sixth period directly above element 95, was named for the continent it was discovered on, so element 95 was namedamericium. Element 96 was namedcurium forMarie Curie andPierre Curie as an analog to the naming ofgadolinium, which was named for the scientist and engineerJohan Gadolin.Terbium was named for the village it was discovered in, so element 97 was namedberkelium.[34]
^TheNuclear Regulatory Commission replaced the Atomic Energy Commission when theEnergy Reorganization Act of 1974 was implemented. The price of californium-252 was increased by the NRC several times and was $60 per microgram by 1999; this price does not include the cost of encapsulation and transportation.[27]
^In 1975, another paper stated that the californium metal prepared the year before was the hexagonal compound Cf2O2S and face-centered cubic compound CfS.[44] The 1974 work was confirmed in 1976 and work on californium metal continued.[42]
^By 1990, californium-252 had replaced plutonium-beryllium neutron sources due to its smaller size and lower heat and gas generation.[60]
^An article entitled "Facts and Fallacies of World War III" in the July 1961 edition ofPopular Science magazine read "A californium atomic bomb need be no bigger than a pistol bullet. You could build a hand-held six-shooter to fire bullets that would explode on contact with the force of 10 tons of TNT."[72]
^Haire, R. G.; Baybarz, R. D. (1974). "Crystal Structure and Melting Point of Californium Metal".Journal of Inorganic and Nuclear Chemistry.36 (6): 1295.doi:10.1016/0022-1902(74)80067-9.
^Zachariasen, W. (1975). "On Californium Metal".Journal of Inorganic and Nuclear Chemistry.37 (6):1441–1442.doi:10.1016/0022-1902(75)80787-1.
^Fields, P. R.; Studier, M.; Diamond, H.; Mech, J.; Inghram, M.; Pyle, G.; Stevens, C.; Fried, S.; et al. (1956). "Transplutonium Elements in Thermonuclear Test Debris".Physical Review.102 (1):180–182.Bibcode:1956PhRv..102..180F.doi:10.1103/PhysRev.102.180.
^Baade, W.; Burbidge, G. R.; Hoyle, F.; Burbidge, E. M.; Christy, R. F.; Fowler, W. A. (August 1956)."Supernovae and Californium 254"(PDF).Publications of the Astronomical Society of the Pacific.68 (403):296–300.Bibcode:1956PASP...68..296B.doi:10.1086/126941.Archived(PDF) from the original on October 10, 2022. RetrievedSeptember 26, 2012.
^Davis, S. N.; Thompson, Glenn M.; Bentley, Harold W.; Stiles, Gary (2006). "Ground-Water Tracers – A Short Review".Ground Water.18 (1):14–23.doi:10.1111/j.1745-6584.1980.tb03366.x.
^Schewe, P.; Stein, B. (October 17, 2006)."Elements 116 and 118 Are Discovered".Physics News Update. American Institute of Physics. Archived fromthe original on October 26, 2006. RetrievedOctober 19, 2006.
^<Please add first missing authors to populate metadata.> (April 1961). "Element 103 Synthesized".Science News-Letter.79 (17): 259.doi:10.2307/3943043.JSTOR3943043.
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Haire, Richard G. (2006). "Californium". In Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean (eds.).The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Springer Science+Business Media.ISBN978-1-4020-3555-5.
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