View source for Californium

{{About|the chemical element}}
{{use mdy dates|date=March 2018}}
{{featured article}}
{{infobox californium}}
'''Californium''' is a [[synthetic element|synthetic chemical element]]; it has [[Chemical symbol|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 [[alpha particle]]s ([[helium-4]] ions). It is an [[actinide]] element, the sixth [[transuranium element]] to be [[synthetic element|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 [[crystal structure|crystalline forms]] exist at normal pressure: one above and one below {{convert|900|C|-1}}. 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 [[isotope]]s 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.{{efn|name=age of earth}} {{sup|252}}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 [[neutron]]s. For example, californium can be used to help start up [[nuclear reactor]]s, 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 cell]]s by [[bioaccumulation|bioaccumulating]] in skeletal tissue.

== Characteristics ==

=== Physical properties ===
Californium is a silvery-white [[actinide]] metal{{sfn|Jakubke|1994|p=166}} with a [[melting point]] of {{convert|900|±|30|C|-1}} and an estimated [[boiling point]] of {{convert|1743|K|-1}}.{{sfn|Haire|2006|pp=1522–1523}} The pure metal is malleable and is easily cut with a knife. Californium metal starts to vaporize above {{convert|300|C|-1}} when exposed to a vacuum.{{sfn|Haire|2006|p=1526}} Below {{convert|51|K|C F|0|abbr=on}}<!-- KEEP KELVIN --> californium metal is either [[ferromagnetism|ferromagnetic]] or [[ferrimagnetism|ferrimagnetic]] (it acts like a magnet), between 48 and 66&nbsp;K it is [[antiferromagnetism|antiferromagnetic]] (an intermediate state), and above {{convert|160|K|C F}}<!-- KEEP IN KELVIN (−110&nbsp;°C) --> it is [[paramagnetic]] (external magnetic fields can make it magnetic).{{sfn|Haire|2006|p=1525}} It forms [[alloy]]s with [[lanthanide]] metals but little is known about the resulting materials.{{sfn|Haire|2006|p=1526}}

The element has two [[Crystal structure|crystalline forms]] at [[Standard atmosphere (unit)|standard atmospheric pressure]]: a double-[[Close-packing of equal spheres|hexagonal close-packed]] form dubbed alpha (α) and a [[Cubic crystal system|face-centered cubic]] form designated beta (β).{{efn|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.{{sfn|Szwacki|2010|p=80}} }} The α form exists below 600–800&nbsp;°C with a density of 15.10&nbsp;g/cm<sup>3</sup> and the β form exists above 600–800&nbsp;°C with a density of 8.74&nbsp;g/cm{{sup|3}}.{{sfn|O'Neil|2006|p=276}} At 48&nbsp;[[pascal (unit)|GPa]] of pressure the β form changes into an [[orthorhombic crystal system]] due to delocalization of the atom's [[Electron shell|5f electrons]], which frees them to bond.{{sfn|Haire|2006|p=1522}}{{efn|The three lower-mass transplutonium elements—[[americium]], [[curium]], and [[berkelium]]—require much less pressure to delocalize their 5f electrons.{{sfn|Haire|2006|p=1522}} }}

The [[bulk modulus]] of a material is a measure of its resistance to uniform pressure. Californium's bulk modulus is {{val|50|5|u=GPa}}, which is similar to trivalent lanthanide metals but smaller than more familiar metals, such as aluminium (70&nbsp;GPa).{{sfn|Haire|2006|p=1522}}

=== Chemical properties and compounds ===
{{Further|Californium compounds}}
{| class="wikitable"
|+ Representative californium compounds{{sfn|Jakubke|1994|p=166}}{{efn|Other +3 oxidation states include the sulfide and [[metallocene]].{{sfn|Cotton|Wilkinson|Murillo|Bochmann|1999|p=1163}}}}
! state !! compound !! formula !! color !!
|-
| +2 || [[californium(II) bromide]] || CfBr{{sub|2}} || yellow || style="background:#FFDA5C;" |
|-
| +2 || [[californium(II) iodide]] || CfI{{sub|2}} || dark violet || style="background:#702A97;" |
|-
| +3 || [[californium(III) oxide]] || Cf{{sub|2}}O{{sub|3}} || yellow-green || style="background:#B4E532;" |
|-
| +3 || [[californium(III) fluoride]] || CfF{{sub|3}} || bright green || style="background:#59FF00;" |
|-
| +3 || [[californium(III) chloride]] || CfCl{{sub|3}} || emerald green || style="background:#26B645;" |
|-
| +3 || [[californium(III) bromide]] || CfBr{{sub|3}} || yellowish green || style="background:#81C340;" |
|-
| +3 || [[californium(III) iodide]] || CfI{{sub|3}} || lemon yellow || style="background:#FFE826;" |
|-
| +3
|[[californium(III) polyborate]]
|Cf[B{{sub|6}}O{{sub|8}}(OH){{sub|5}}]
|pale green || style="background:#96CD91;" |
|-
| +4 || [[californium(IV) oxide]] || CfO{{sub|2}} || black brown || style="background:#3D3016;" |
|-
| +4 || [[californium(IV) fluoride]] || CfF{{sub|4}} || green || style="background:#2B9E22;" |
|}
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{{sfn|Seaborg|2004}} and the element [[dysprosium]], which is the lanthanide above californium in the periodic table.{{sfn|CRC|2006|p=4.8}} Compounds in the +4 oxidation state are strong [[oxidizing agent]]s and those in the +2 state are strong [[reducing agent]]s.{{sfn|Jakubke|1994|p=166}}

The element slowly tarnishes in air at room temperature, with the rate increasing when moisture is added.{{sfn|O'Neil|2006|p=276}} Californium reacts when heated with [[hydrogen]], [[nitrogen]], or a [[chalcogen]] (oxygen family element); reactions with dry hydrogen and aqueous [[mineral acid]]s are rapid.{{sfn|O'Neil|2006|p=276}} <!-- EXPLAIN Only californium-249 is suitable for chemical study.<ref name="Emeleus">{{cite book|last=Emeleus|first=H. J.|title=Advances in Inorganic Chemistry|page=33|year=1987|publisher=Academic Press | isbn = 978-0-12-023631-2 }}</ref> /EXPLAIN -->

Californium is only [[aqueous solution|water-soluble]] as the californium(III) [[cation]]. Attempts to [[redox|reduce or oxidize]] the +3 ion in solution have failed.{{sfn|CRC|2006|p=4.8}} The element forms a water-soluble [[chloride]], [[nitrate]], [[perchlorate]], and [[sulfate]] and is precipitated as a [[fluoride]], [[oxalate]], or [[hydroxide]].{{sfn|Seaborg|2004}} Californium is the heaviest actinide to exhibit covalent properties, as is observed in the californium borate.<ref>{{Cite journal|title = Unusual structure, bonding and properties in a californium borate|journal = Nature Chemistry|date = May 1, 2014|issn = 1755-4330|pages = 387–392|volume = 6|issue = 5|doi = 10.1038/nchem.1896|pmid = 24755589|language = en|first1 = Matthew J.|last1 = Polinski|first2 = Edward B. Garner|last2 = Iii|first3 = Rémi|last3 = Maurice|first4 = Nora|last4 = Planas|first5 = Jared T.|last5 = Stritzinger|first6 = T. Gannon|last6 = Parker|first7 = Justin N.|last7 = Cross|first8 = Thomas D.|last8 = Green|first9 = Evgeny V.|last9 = Alekseev|url = http://hal.in2p3.fr/in2p3-00966875|bibcode = 2014NatCh...6..387P|citeseerx = 10.1.1.646.749| s2cid=104331283 }}</ref>

=== Isotopes ===
{{main|Isotopes of californium}}
Twenty [[isotope]]s of californium are known ([[mass number]] ranging from 237 to 256<ref name="NNDC2008" />); the most stable are {{sup|251}}Cf with [[half-life]] 898 years, {{sup|249}}Cf with half-life 351 years, {{sup|250}}Cf at 13.08 years, and {{sup|252}}Cf at 2.645 years.<ref name="NNDC2008" /> All other isotopes have half-life shorter than a year, and most of these have half-lives less than 20 minutes.<ref name="NNDC2008" />

{{sup|249}}Cf is formed by [[beta decay]] of berkelium-249, and most other californium isotopes are made by subjecting berkelium to intense neutron radiation in a [[nuclear reactor]].{{sfn|CRC|2006|p=4.8}} 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]]).{{sfn|Haire|2006|p=1504}}

{{sup|252}}Cf is a very strong [[neutron]] emitter, which makes it extremely [[radioactive]] and harmful.<ref>{{cite journal|author = Hicks, D. A. |title = Multiplicity of Neutrons from the Spontaneous Fission of Californium-252|journal = Physical Review|date = 1955|volume = 97|issue = 2|pages = 564–565|doi = 10.1103/PhysRev.97.564|last2 = Ise|first2 = John|last3 = Pyle|first3 = Robert V.|bibcode = 1955PhRv...97..564H |url = http://www.escholarship.org/uc/item/6031k6m2}}</ref><ref>{{cite journal|author = Hicks, D. A. |title = Spontaneous-Fission Neutrons of Californium-252 and Curium-244|journal = Physical Review |date = 1955|volume = 98|issue = 5|pages = 1521–1523|doi = 10.1103/PhysRev.98.1521|last2 = Ise|first2 = John|last3 = Pyle|first3 = Robert V.|bibcode = 1955PhRv...98.1521H }}</ref><ref>{{cite journal|author =Hjalmar, E.|author2 =Slätis, H.|author3 =Thompson, S.G. |title = Energy Spectrum of Neutrons from Spontaneous Fission of Californium-252| journal = Physical Review| date = 1955| volume = 100|issue =5|pages = 1542–1543| doi = 10.1103/PhysRev.100.1542|bibcode = 1955PhRv..100.1542H }}</ref> {{sup|252}}Cf, 96.9% of the time, [[alpha decay]]s to [[curium]]-248; the other 3.1% of decays are [[spontaneous fission]].<ref name="NNDC2008" /> One [[microgram]] (μg) of {{sup|252}}Cf emits 2.3&nbsp;million neutrons per second, an average of 3.7 neutrons per spontaneous fission.<ref name="osti">{{cite journal|author = Martin, R. C.|author2 = Knauer, J. B.|author3 = Balo, P. A.| title = Production, Distribution, and Applications of Californium-252 Neutron Sources| date = 1999|url = http://www.osti.gov/bridge/purl.cover.jsp?purl=/15053-AE6cnN/native/ |doi = 10.1016/S0969-8043(00)00214-1|journal = Applied Radiation and Isotopes |volume = 53|issue = 4–5|pages = 785–92|pmid = 11003521 }}</ref> Most other isotopes of californium, alpha decay to curium ([[atomic number]] 96).<ref name="NNDC2008" />

== History ==
[[File:Berkeley 60-inch cyclotron.jpg|thumb|right|The {{convert|60|in|m|2|adj=mid|-diameter}} [[cyclotron]] used to first synthesize californium|alt=Large pieces of equipment with a man standing nearby.]]
Californium was [[timeline of chemical element discoveries|first made]] at [[University of California, Berkeley|University of California]] [[Lawrence Berkeley National Laboratory|Radiation Laboratory]], [[Berkeley, California|Berkeley]], by physics researchers [[Stanley Gerald Thompson]], [[Kenneth Street Jr.]], [[Albert Ghiorso]], and [[Glenn T. Seaborg]], about February 9, 1950.{{sfn|Cunningham|1968|p=103}} It was the sixth [[transuranium element]] to be discovered; the team announced its discovery on March 17, 1950.<ref name="CPoC1950">{{cite journal |last1 = Street | first1 = K. Jr. |last2 = Thompson |first2 = S. G. |last3 = Seaborg |first3 = Glenn T. |title = Chemical Properties of Californium |journal = Journal of the American Chemical Society |date = 1950 |volume = 72 |issue = 10 |page = 4832 |doi = 10.1021/ja01166a528 | bibcode = 1950JAChS..72R4832S |url = https://apps.dtic.mil/sti/pdfs/ADA319899.pdf |hdl = 2027/mdp.39015086449173 |access-date = February 20, 2011 |archive-date = January 19, 2012 |archive-url = https://web.archive.org/web/20120119092943/http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA319899&Location=U2&doc=GetTRDoc.pdf |url-status = live }}</ref><ref>{{cite book |author=Glenn Theodore Seaborg |author-link=Glenn T. Seaborg bibliography |title=Journal of Glenn T. Seaborg, 1946–1958: January 1, 1950{{snd}} December 31, 1950 |url=https://books.google.com/books?id=pvpDAQAAIAAJ |year=1990 |publisher=Lawrence Berkeley Laboratory, University of California |page=80}}</ref>

To produce californium, a microgram-size target of curium-242 ({{nuclide|Cm|242}}) was bombarded with 35&nbsp;MeV [[alpha particle]]s ({{nuclide|He|4}}) in the {{convert|60|in|m|2|adj=mid|-diameter}} [[cyclotron]] at Berkeley, which produced californium-245 ({{nuclide|californium|245}}) plus one [[free neutron]] ({{SubatomicParticle|neutron}}).{{sfn|Cunningham|1968|p=103}}<ref name="CPoC1950" />
<!-- Weeks & Leichester 1968 p. 849 gives product as 98-244 -->
: {{nuclide|curium|242}} + {{nuclide|helium|4}} → {{nuclide|californium|245}} + {{su|b=0|p=1}}{{SubatomicParticle|neutron}}
To identify and separate out the element, [[ion exchange]] and adsorsion methods were undertaken.<ref name="CPoC1950" /><ref>{{cite journal |last1=Thompson |first1=S. G. | last2=Street | first2=K. Jr. |first3=Ghiorso |last3=A. |last4=Seaborg |first4=Glenn T.|title = Element 98 |journal = Physical Review |date=1950 |volume = 78 |issue = 3 |page = 298 |doi = 10.1103/PhysRev.78.298.2 |url=http://escholarship.org/uc/item/44g7z6hk |bibcode = 1950PhRv...78..298T|doi-access=free }}</ref> Only about 5,000 atoms of californium were produced in this experiment,{{sfn|Seaborg|1996|p=82}} and these atoms had a half-life of 44&nbsp;minutes.{{sfn|Cunningham|1968|p=103}}

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.{{sfn|Weeks|Leichester|1968|p=849}}{{efn|[[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 [[curium]] for [[Marie Curie]] and [[Pierre Curie]] as an analog to the naming of [[gadolinium]], which was named for the scientist and engineer [[Johan Gadolin]]. [[Terbium]] was named for the village it was discovered in, so element 97 was named [[berkelium]].{{sfn|Weeks|Leichester|1968|p=848}} }} 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.{{sfn|Heiserman|1992|p=347}} They added that "the best we can do is to point out [that] ... searchers a century ago found it difficult to get to California".{{sfn|Weeks|Leichester|1968|p=848}}

Weighable amounts of californium were first produced by the<!-- long duration -HOW LONG? --> irradiation of plutonium targets at [[Materials Testing Reactor]] at [[Idaho National Laboratory|National Reactor Testing Station]], [[eastern Idaho]]; these findings were reported in 1954.<ref>{{cite journal |journal=[[Physical Review]] |volume=94 |issue=4 |pages=1083 |date=1954 |author=Diamond, H. |title=Identification of Californium Isotopes 249, 250, 251, and 252 from Pile-Irradiated Plutonium |doi = 10.1103/PhysRev.94.1083 |bibcode = 1954PhRv...94.1083D |last2=Magnusson |first2=L. |last3=Mech |first3=J. |last4=Stevens |first4=C. |last5=Friedman |first5=A. |last6=Studier |first6=M. |last7=Fields |first7=P. |last8=Huizenga |first8=J. }}</ref> The high spontaneous fission rate of californium-252 was observed in these samples. The first experiment with californium in concentrated form occurred in 1958.{{sfn|Cunningham|1968|p=103}} The isotopes {{sup|249}}Cf to {{sup|252}}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.{{sfn|Jakubke|1994|p=166}} 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]].<ref>{{cite journal |journal = Science News Letter |volume = 78 |issue = 26 |date=December 1960 |title = Element 98 Prepared }}</ref>

The [[High Flux Isotope Reactor]] (HFIR) at ORNL in [[Oak Ridge, Tennessee]], started producing small batches of californium in the 1960s.<ref>{{cite web |url=http://web.ornl.gov/sci/rrd/pages/hfir.html |title=The High Flux Isotope Reactor |publisher=Oak Ridge National Laboratory |access-date=August 22, 2010 |archive-url=https://web.archive.org/web/20100527164346/http://web.ornl.gov/sci/rrd/pages/hfir.html <!--Added by H3llBot--> |archive-date=May 27, 2010 }}</ref> By 1995, HFIR nominally produced {{convert|500|mg|oz}} of californium annually.{{sfn|Osborne-Lee|1995|p=11}} Plutonium supplied by the United Kingdom to the United States under the [[1958 US–UK Mutual Defence Agreement]] was used for making californium.<ref>{{cite web |archive-url=https://web.archive.org/web/20061213032416/http://www.mod.uk/NR/rdonlyres/B31B4EF0-A584-4CC6-9B14-B5E89E6848F8/0/plutoniumandaldermaston.pdf |archive-date=December 13, 2006 |url=http://www.mod.uk/NR/rdonlyres/B31B4EF0-A584-4CC6-9B14-B5E89E6848F8/0/plutoniumandaldermaston.pdf |title=Plutonium and Aldermaston – an Historical Account |publisher=UK Ministry of Defence |date=September 4, 2001 |access-date=March 15, 2007|page=30 }}</ref>

The [[United States Atomic Energy Commission|Atomic Energy Commission]] sold {{sup|252}}Cf to industrial and academic customers in the early 1970s for $10/microgram,<ref name="osti" /> and an average of {{convert|150|mg|oz|abbr=on}} of {{sup|252}}Cf were shipped each year from 1970 to 1990.{{sfn|Osborne-Lee|1995|p=6}}{{efn|The [[Nuclear Regulatory Commission]] replaced the Atomic Energy Commission when the [[Energy 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.<ref name="osti" /> }} 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.{{sfn|Haire|2006|p=1519}}<ref>{{cite journal |last1=Haire |first1=R. G. |last2=Baybarz |first2=R. D. |title=Crystal Structure and Melting Point of Californium Metal |journal=Journal of Inorganic and Nuclear Chemistry |volume=36 |issue=6 |pages=1295 |date=1974 |doi=10.1016/0022-1902(74)80067-9 }}</ref>{{efn|In 1975, another paper stated that the californium metal prepared the year before was the hexagonal compound Cf{{sub|2}}O{{sub|2}}S and face-centered cubic compound CfS.<ref>{{cite journal |doi=10.1016/0022-1902(75)80787-1 |journal=Journal of Inorganic and Nuclear Chemistry |date=1975 |pages=1441–1442 |volume=37 |issue=6 |title=On Californium Metal |last=Zachariasen |first=W. }}</ref> The 1974 work was confirmed in 1976 and work on californium metal continued.{{sfn|Haire|2006|p=1519}} }}

== Occurrence ==
Traces of californium can be found near facilities that use the element in mineral prospecting and in medical treatments.{{sfn|Emsley|2001|p=90}} 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.<ref name="ANL2005">{{cite web|url=http://www.evs.anl.gov/pub/doc/Californium.pdf |title=Human Health Fact Sheet: Californium |date=August 2005 |publisher=Argonne National Laboratory |url-status=dead |archive-url=https://web.archive.org/web/20110721032736/http://www.evs.anl.gov/pub/doc/Californium.pdf |archive-date=July 21, 2011 }}</ref>

[[Nuclear fallout]] from atmospheric [[nuclear weapons testing]] prior to 1980 contributed a small amount of californium to the environment.<ref name="ANL2005" /> Californium-249, -252, -253, and -254 have been observed in the radioactive dust collected from the air after a nuclear explosion.<ref>{{cite journal|title = Transplutonium Elements in Thermonuclear Test Debris|journal = Physical Review|date = 1956|volume = 102|issue = 1|pages = 180–182|doi = 10.1103/PhysRev.102.180|bibcode = 1956PhRv..102..180F|last1=Fields|first1=P. R.|last2 = Studier|first2 = M.|last3 = Diamond|first3 = H.|last4 = Mech|first4 = J.|last5 = Inghram|first5 = M.|last6 = Pyle|first6 = G.|last7 = Stevens|first7 = C.|last8 = Fried|first8 = S.|last9 = Manning|first9 = W.|display-authors=8}}</ref> Californium is not a major radionuclide at [[United States Department of Energy]] legacy sites since it was not produced in large quantities.<ref name="ANL2005" />

Californium was once believed to be produced in [[supernova]]s, as their decay matches the 60-day half-life of {{sup|254}}Cf.<ref name="super1">{{cite journal|last=Baade|first=W.|author2=Burbidge, G. R.|author3=Hoyle, F.|author4=Burbidge, E. M.|author5=Christy, R. F.|author6=Fowler, W. A.|title=Supernovae and Californium 254|journal=Publications of the Astronomical Society of the Pacific|date=August 1956|volume=68|issue=403|pages=296–300|doi=10.1086/126941|url=http://authors.library.caltech.edu/6553/1/BURpr56.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://authors.library.caltech.edu/6553/1/BURpr56.pdf |archive-date=2022-10-10 |url-status=live|access-date=September 26, 2012|bibcode = 1956PASP...68..296B |doi-access=free}}</ref> However, subsequent studies failed to demonstrate any californium spectra,<ref name="super2">{{cite journal|last=Conway|first=J. G.|author2=Hulet, E.K. |author3=Morrow, R.J. |title=Emission Spectrum of Californium|journal=Journal of the Optical Society of America|date=February 1, 1962|volume=52|issue=2|pages=222|doi=10.1364/josa.52.000222 |pmid=13881026|bibcode=1962JOSA...52..222C |osti=4806792|url=http://www.escholarship.org/uc/item/9c3297wf}}</ref> and supernova light curves are now thought to follow the decay of [[Isotopes of nickel|nickel-56]].{{sfn|Ruiz-Lapuente1996|p=274}}

The [[transuranic element]]s [[americium]] to [[fermium]], including californium, occurred naturally in the [[natural nuclear fission reactor]] at [[Oklo]], but no longer do so.<ref name="emsley">{{cite book|last=Emsley|first=John|title=Nature's Building Blocks: An A-Z Guide to the Elements|edition=New|date=2011|publisher=Oxford University Press|location=New York, NY|isbn=978-0-19-960563-7}}</ref>

[[Spectral line]]s of californium, along with those of several other non-primordial elements, were detected in [[Przybylski's Star]] in 2008.<ref name=gopka08>{{cite journal |last1=Gopka |first1=V. F. |last2=Yushchenko |first2=A. V. |last3=Yushchenko |first3=V. A. |last4=Panov |first4=I. V. |last5=Kim |first5=Ch. |title=Identification of absorption lines of short half-life actinides in the spectrum of Przybylski's star (HD 101065) |journal=Kinematics and Physics of Celestial Bodies |date=15 May 2008 |volume=24 |issue=2 |pages=89–98 |doi=10.3103/S0884591308020049 |bibcode = 2008KPCB...24...89G |s2cid=120526363 }}</ref>

== Production ==
{{see also|Nuclear fuel cycle}}
Californium is produced in [[nuclear reactor]]s and [[particle accelerator]]s.{{sfn|Krebs|2006|pp=327–328}} Californium-250 is made by bombarding berkelium-249 ({{sup|249}}Bk) with neutrons, forming berkelium-250 ({{sup|250}}Bk) via [[neutron capture]] (n,γ) which, in turn, quickly [[beta decay]]s (β{{sup|−}}) to californium-250 ({{sup|250}}Cf) in the following reaction:{{sfn|Heiserman|1992|p=348}}
:{{nuclide|Bk|249}}(n,γ){{nuclide|Bk|250}} → {{nuclide|Cf|250}} + β{{sup|−}}
Bombardment of {{sup|250}}Cf with neutrons produces {{sup|251}}Cf and {{sup|252}}Cf.{{sfn|Heiserman|1992|p=348}}

Prolonged irradiation of [[americium]], curium, and plutonium with neutrons produces milligram amounts of {{sup|252}}Cf and microgram amounts of {{sup|249}}Cf.{{sfn|Cunningham|1968|p=105}} 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.{{sfn|Haire|2006|p=1503}}

Microgram quantities of {{sup|252}}Cf are available for commercial use through the U.S. [[Nuclear Regulatory Commission]].{{sfn|Krebs|2006|pp=327–328}} Only two sites produce {{sup|252}}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 {{sup|252}}Cf per year, respectively.{{sfn|NRC|2008|p=33}}

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.{{sfn|NRC|2008|p=33}} {{sup|253}}Cf is at the end of a production chain that starts with uranium-238, and includes several [[isotopes of plutonium]], [[isotopes of americium|americium]], [[isotopes of curium|curium]],  and [[isotopes of berkelium|berkelium]], and the californium isotopes 249 to 253 (see diagram).
{{Clear}}
[[File:Cf 252 Produktion.png|center|upright=2.8|thumb|Scheme of the production of californium-252 from uranium-238 by neutron irradiation|alt=A complex flow diagram showing various isotopes.]]

== Applications ==
[[File:CfShield.JPG|thumb|Fifty-ton shipping cask built at ORNL which can transport up to 1 gram of {{sup|252}}Cf.{{sfn|Seaborg|1994|p=245}} Large and heavily shielded transport containers are needed to prevent the release of highly radioactive material in case of normal and hypothetical accidents.<ref>{{cite web|url=http://rampac.energy.gov/PCN/EM-PCP-certified-pkgs-8808.pdf|title=DOE Certified Radioactive Materials Transportation Packagings|last=Shuler|first=James|date=2008|page=1|publisher=United States Department of Energy|access-date=April 7, 2011|archive-date=October 15, 2011|archive-url=https://web.archive.org/web/20111015040627/http://rampac.energy.gov/PCN/EM-PCP-certified-pkgs-8808.pdf|url-status=dead}}</ref>|alt= Large conical structure on a pulley with a man on top and two near the base.]]

<!-- NEEDS CITE Californium is the heaviest metal known at this time that has a practical use outside of research laboratories; [[einsteinium]] and all other elements above it have sufficiently short half-lives that they have no use except the production of heavier elements.-->=== Neutron source ===
{{SimpleNuclide|Californium|252|link=yes}} has a number of specialized uses as a strong [[Neutron source|neutron emitter]]; it produces 139&nbsp;million neutrons per microgram per minute.<ref name="osti" /> This property makes it useful as a [[startup neutron source]] for some nuclear reactors{{sfn|O'Neil|2006|p=276}} and as a portable (non-reactor based) neutron source for [[neutron activation analysis]] to detect trace amounts of elements in samples.<ref name="Martin2000">{{cite conference|last=Martin |first=R. C. |title=Applications and Availability of Californium-252 Neutron Sources for Waste Characterization |date=September 24, 2000 |url=http://www.ornl.gov/~webworks/cpr/pres/107270_.pdf |access-date=May 2, 2010 |conference=Spectrum 2000 International Conference on Nuclear and Hazardous Waste Management |location=Chattanooga, Tennessee |url-status=dead |archive-url=https://web.archive.org/web/20100601160926/http://www.ornl.gov/~webworks/cpr/pres/107270_.pdf |archive-date=June 1, 2010 }}</ref>{{efn|By 1990, californium-252 had replaced plutonium-[[beryllium]] neutron sources due to its smaller size and lower heat and gas generation.{{sfn|Seaborg|1990|p=318}} }} Neutrons from californium are used as a treatment of certain [[Cervical cancer|cervical]] and [[brain tumor|brain cancers]] where other [[radiation therapy]] is ineffective.{{sfn|O'Neil|2006|p=276}} It has been used in educational applications since 1969 when [[Georgia Tech|Georgia Institute of Technology]] got a loan of 119&nbsp;μg of {{sup|252}}Cf from the [[Savannah River Site]].{{sfn|Osborne-Lee|1995|p=33}} It is also used with online elemental [[coal analyzer]]s and [[bulk material analyzer]]s in the coal and cement industries.

Neutron penetration into materials makes californium useful in detection instruments such as [[fuel rod]] scanners;{{sfn|O'Neil|2006|p=276}} [[Neutron imaging#Neutron radiography (film)|neutron radiography]] of aircraft and weapons components to detect [[corrosion]], bad welds, cracks and trapped moisture;{{sfn|Osborne-Lee|1995|pp=26–27}}<!-- NEEDS CITE in airport [[prompt gamma neutron activation analysis|neutron-activation]] detectors of explosives, --> and in portable metal detectors.<ref>{{cite web|url=http://www.pnl.gov/news/2000/00-43.htm|title=Will You be 'Mine'? Physics Key to Detection|date=October 25, 2000|publisher = Pacific Northwest National Laboratory|access-date = March 21, 2007 |archive-url = https://web.archive.org/web/20070218125029/http://www.pnl.gov/news/2000/00-43.htm <!-- Bot retrieved archive --> |archive-date = February 18, 2007 }}</ref> [[Neutron moisture gauge]]s use {{sup|252}}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,{{sfn|CRC|2006|p=4.8}} and to detect ground water movement.<ref>{{cite journal|journal = Ground Water|volume = 18|issue = 1|pages =14–23|date = 2006|title =Ground-Water Tracers – A Short Review|author = Davis, S. N. |doi = 10.1111/j.1745-6584.1980.tb03366.x|last2 = Thompson|first2 = Glenn M.|last3 = Bentley|first3 = Harold W.|last4 = Stiles|first4 = Gary }}</ref> The main uses of {{sup|252}}Cf in 1982 were, reactor start-up (48.3%), fuel rod scanning (25.3%), and activation analysis (19.4%).{{sfn|Osborne-Lee|1995|p=12}} By 1994, most {{sup|252}}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.{{sfn|Osborne-Lee|1995|p=12}} In 2021, fast neutrons from {{sup|252}}Cf were used for wireless data transmission.<ref>{{cite journal|journal = Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment|volume = 1021|issue = 1|pages = 165946|date = 2022|title = Wireless information transfer with fast neutrons|author = Joyce, Malcolm J.|last2 = Aspinall|first2 = Michael D.|last3 = Clark|first3 = Mackenzie|last4 = Dale|first4 = Edward|last5 = Nye|first5 = Hamish|last6 = Parker|first6 = Andrew|last7 = Snoj|first7 = Luka|last8 = Spires|first8 = Joe|doi = 10.1016/j.nima.2021.165946| bibcode=2022NIMPA102165946J | s2cid=240341300 |issn=0168-9002 |doi-access = free}}</ref>

=== Superheavy element production ===
{{See also|Superheavy element#Synthesis of superheavy nuclei}}
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 {{sup|249}}Cf with [[calcium-48]], making it the heaviest element ever made. The target contained about 10&nbsp;mg of {{sup|249}}Cf deposited on a titanium foil of 32&nbsp;cm{{sup|2}} area.<ref>{{cite journal |title = Synthesis of the isotopes of elements 118 and 116 in the californium-249 and <sup>245</sup>Cm+<sup>48</sup>Ca fusion reactions |journal = Physical Review C |date = 2006 |volume = 74 |issue =4 |pages = 044602–044611 |doi = 10.1103/PhysRevC.74.044602 |bibcode=2006PhRvC..74d4602O |last1=Oganessian |first1=Yu. Ts. |last2=Utyonkov |first2=V. |last3=Lobanov |first3=Yu. |last4=Abdullin |first4=F. |last5=Polyakov |first5=A. |last6=Sagaidak |first6=R. |last7=Shirokovsky |first7=I. |last8=Tsyganov |first8=Yu. |last9=Voinov |first9=A. |display-authors=8 |doi-access=free}}</ref><ref>{{cite journal |author = Sanderson, K. |title = Heaviest element made – again |journal = Nature News |publisher =Nature |date = October 17, 2006 |doi=10.1038/news061016-4 |s2cid = 121148847}}</ref><ref>{{cite web|author=Schewe, P. |author2=Stein, B. |title=Elements 116 and 118 Are Discovered |work=Physics News Update |publisher=American Institute of Physics |date=October 17, 2006 |url=http://www.aip.org/pnu/2006/797.html |access-date=October 19, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20061026072537/http://www.aip.org/pnu/2006/797.html |archive-date=October 26, 2006 }}</ref> <!-- EXPLAIN Calibration, [[dosimetry]], and fission fragment and half-life studies are other applications of californium.{{sfn|Osborne-Lee|1995|p=34}} --> Californium has also been used to produce other transuranic elements; for example, [[lawrencium]] was first synthesized in 1961 by bombarding californium with [[boron]] nuclei.<ref>{{cite journal|title = Element 103 Synthesized|journal = Science News-Letter|volume = 79|issue = 17|date=April 1961|page = 259|doi = 10.2307/3943043|author1 = <Please add first missing authors to populate metadata.> |jstor = 3943043}}</ref>

=== Hypothetical nuclear weapons ===
{{See also|Nuclear weapon design#Minor actinide fission weapons}}
{{SimpleNuclide|Californium|251|link=yes}} has a very small calculated [[critical mass]] of about {{convert|5|kg|0|abbr=on}},<ref>{{cite web |title=Evaluation of nuclear criticality safety data and limits for actinides in transport |url=http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110519171204/http://ec.europa.eu/energy/nuclear/transport/doc/irsn_sect03_146.pdf |archive-date=May 19, 2011 |access-date=December 20, 2010 |publisher=Institut de Radioprotection et de Sûreté Nucléaire |page=16}}</ref> 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.{{efn|An article entitled "Facts and Fallacies of World War III" in the July 1961 edition of ''[[Popular 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."<ref>{{cite journal|journal=[[Popular Science]]|pages= 92–95, 178–181|date=July 1961|volume=179|issue=1|issn=0161-7370|title=Facts and Fallacies of World War III|url=https://books.google.com/books?id=OiEDAAAAMBAJ&pg=PA180|author1=Mann, Martin}}"force of 10 tons of TNT" on page 180.</ref>}}

== Precautions ==
Californium that [[bioaccumulation|bioaccumulates]] in skeletal tissue releases radiation that disrupts the body's ability to form [[red blood cell]]s.{{sfn|Cunningham|1968|p=106}} The element plays no natural biological role in any organism due to its intense radioactivity and low concentration in the environment.{{sfn|Emsley|2001|p=90}}

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.<ref name="ANL2005" /><!-- for the whole paragraph -->

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.<ref name="ANL2005" /><!-- for the whole paragraph -->

== Notes ==
{{notes
| 30em
| notes =
{{efn
| name = age of earth
| The Earth [[Age of the Earth|formed 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.
}}
}}

== References ==
{{reflist|22em}}

== Bibliography ==
<!--NOTE only add URLs to places where at least some preview of the book is available -->
* {{cite book
  | last1 = Cotton | first1 = F. Albert
  | last2 = Wilkinson | first2 = Geoffrey
  | last3 = Murillo | first3 = Carlos A.
  | last4 = Bochmann | first4 = Manfred
  | title = Advanced Inorganic Chemistry
  | edition = 6th
  | date = 1999
  | publisher = John Wiley & Sons
  | isbn = 978-0-471-19957-1
  }}
* {{cite book
  | title = The Encyclopedia of the Chemical Elements
  | chapter-url = https://archive.org/details/encyclopediaofch00hamp
  | chapter-url-access = registration
  | publisher = Reinhold Book Corporation
  | date = 1968
  | editor-last = Hampel |editor-first = Clifford A.
  | last = Cunningham
  | first = B. B.
  | chapter = Californium
  | lccn = 68029938
  }}
* {{cite book
  | title = The Elements
  | last = Emsley
  | first = John
  | publisher = Oxford University Press
  | date = 1998
  | isbn = 978-0-19-855818-7
  | url=https://books.google.com/books?id=qgYpAAAAYAAJ
  }}
* {{cite book
  | title = Nature's Building Blocks: An A-Z Guide to the Elements
  | last = Emsley
  | first = John
  | publisher = Oxford University Press
  | date = 2001
  | isbn = 978-0-19-850340-8
  | chapter = Californium
  | url=https://archive.org/details/naturesbuildingb0000emsl
  | url-access = registration
  }}
* {{cite book
  | last1 = Greenwood
  | first1 = N. N.
  | last2 = Earnshaw
  | first2 = A.
  | title = Chemistry of the Elements
  | edition = 2nd
  | publisher = Butterworth-Heinemann
  | date = 1997
  | isbn = 978-0-7506-3365-9
  }}
* {{cite book
  | title = The Chemistry of the Actinide and Transactinide Elements
  | editor1-first = Lester R.
  | editor1-last = Morss|editor2-first = Norman M.
  | editor2-last = Edelstein
  | editor3-last = Fuger|editor3-first = Jean
  | last = Haire|first = Richard G.
  | chapter = Californium
  | publisher = Springer Science+Business Media
  | date = 2006
  | isbn = 978-1-4020-3555-5
  | edition = 3rd
  }}
* {{cite book
  | last = Heiserman
  | first = David L.
  | date = 1992
  | title = Exploring Chemical Elements and their Compounds
  | publisher = TAB Books
  | isbn = 978-0-8306-3018-9
  | chapter = Element 98: Californium
  | url=https://archive.org/details/exploringchemica01heis
  | url-access = registration
  }}
* {{cite book
  | others = trans. rev. Eagleson, Mary
  | editor1-first = Hans-Dieter | editor1-last = Jakubke
  | editor2-first = Hans | editor2-last = Jeschkeit
  | title = Concise Encyclopedia Chemistry
  | publisher = Walter de Gruyter
  | date = 1994
  | url = https://books.google.com/books?id=Owuv-c9L_IMC
  | isbn = 978-3-11-011451-5
  | ref = {{sfnRef|Jakubke|1994}}
  }}
* {{cite book
  | last = Krebs
  | first = Robert
  | title = The History and Use of our Earth's Chemical Elements: A Reference Guide
  | date = 2006
  | publisher = Greenwood Publishing Group
  | isbn = 978-0-313-33438-2
  }}
* {{cite book
  | title = Handbook of Chemistry and Physics
  | editor-first = David R.|editor-last = Lide
  | edition = 87th
  | date = 2006
  | publisher = CRC Press, Taylor & Francis Group
  | isbn = 978-0-8493-0487-3
  | ref = {{sfnRef|CRC|2006}}
  }}
* {{cite book
  | title = Radiation Source Use and Replacement: Abbreviated Version
  | author = National Research Council (U.S.). Committee on Radiation Source Use and Replacement
  | date = 2008
  | publisher = National Academies Press
  | ref = {{sfnRef|NRC|2008}}
  | url = https://books.google.com/books?id=3cT2REdXJ98C
  | isbn = 978-0-309-11014-3
  }}
* {{cite book
  | editor1-last = O'Neil|editor1-first = Marydale J.
  | editor2-last = Heckelman|editor2-first = Patricia E.
  | editor3-last = Roman|editor3-first = Cherie B.
  | title = The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals
  | edition = 14th
  | publisher = Merck Research Laboratories, Merck & Co.
  | date = 2006
  | isbn = 978-0-911910-00-1
  | url = https://books.google.com/books?id=kEYfRAAACAAJ
  | ref = {{sfnRef|O'Neil|2006}}
  }}
* {{cite journal
  | title = Californium-252: A Remarkable Versatile Radioisotope
  | last1 = Osborne-Lee|first1 = I. W.
  | last2 = Alexander|first2 = C. W.
  | journal = Oak Ridge Technical Report ORNL/TM-12706
  | date = 1995
  | url = http://www.osti.gov/bridge/product.biblio.jsp?query_id=1&page=0&osti_id=205871 |doi=10.2172/205871
  | ref = {{sfnRef|Osborne-Lee|1995}}
  | doi-access = free
  | osti = 205871| url-access = subscription
  }}
* {{cite book
  | last1 = Ruiz-Lapuente
  | first1 = P.
  | last2 = Canal
  | first2 = R.
  | last3 = Isern
  | first3 = J.
  | date = 1996
  | title = Thermonuclear Supernovae
  | publisher = [[Springer Science+Business Media]]
  | isbn = 978-0-7923-4359-2
  | url = https://books.google.com/books?id=I6Rl1VAAX3QC
  | ref = {{sfnRef|Ruiz-Lapuente|1996}}
  }}
* {{cite book
  | last1 = Seaborg|first1 = Glenn T.
  | author-link1 = Glenn T. Seaborg
  | last2 = Loveland|first2 = Walter D.
  | title = The Elements Beyond Uranium
  | publisher = John Wiley & Sons, Inc.
  | date = 1990
  | isbn = 978-0-471-89062-1
  | ref = {{sfnRef|Seaborg|1990}}
  | url = https://books.google.com/books?id=QFhRAAAAMAAJ
  }}
* {{cite book
  | last = Seaborg
  | first = Glenn T.
  | date = 1994
  | url = https://books.google.com/books?id=e53sNAOXrdMC&pg=PA245
  | title = Modern alchemy: selected papers of Glenn T. Seaborg
  | publisher = World Scientific
  | isbn = 978-981-02-1440-1
  }}
* {{cite book
  | last = Seaborg
  | first = Glenn T.
  | date = 1996
  | editor = Adloff, J. P.
  | title = One Hundred Years after the Discovery of Radioactivity
  | publisher = Oldenbourg Wissenschaftsverlag
  | isbn = 978-3-486-64252-0
  | url = https://books.google.com/books?id=bJ4rvgAACAAJ
  }}
* {{cite encyclopedia
  | last = Seaborg | first = Glenn T.
  | encyclopedia = Concise Encyclopedia of Chemistry
  | title = Californium
  | editor = Geller, Elizabeth
  | publisher = McGraw-Hill
  | date = 2004
  | isbn = 978-0-07-143953-4
  | page = 94
  | url = https://books.google.com/books?id=Owuv-c9L_IMC
  }}
* {{cite book
  | last1 = Szwacki | first1 = Nevill Gonzalez
  | last2 = Szwacka | first2 = Teresa
  | title = Basic Elements of Crystallography
  | publisher = Pan Stanford
  | date = 2010
  | isbn = 978-981-4241-59-5
  | url = https://www.scribd.com/doc/39588027/Basic-Elements-of-Crystallography
  | ref= {{sfnRef|Szwacki|2010}}
  }}
* {{cite book
  | title = Handbook of Metal Etchants
  | editor1-first = Perrin|editor1-last = Walker
  | editor2-first = William H.| editor2-last = Tarn
  | date = 1991
  | publisher = CRC Press
  | isbn = 978-0-8493-3623-2
  | ref = {{sfnRef|CRC|1991}}
  }}
* {{cite book
  | last1 = Weeks
  | first1 = Mary Elvira
  | author-link=Mary Elvira Weeks| last2 = Leichester |first2=Henry M.
  | date = 1968
  | title = Discovery of the Elements
  | chapter-url = https://archive.org/details/discoveryofeleme0000week
  | chapter-url-access = registration
  | publisher = Journal of Chemical Education
  | chapter = 21: Modern Alchemy
  | pages = [https://archive.org/details/discoveryofeleme0000week/page/848 848–850]
  | isbn = 978-0-7661-3872-8
  | lccn = 68015217
 }}

== External links ==
{{Commons}}
{{wiktionary|californium}}
* [http://www.periodicvideos.com/videos/098.htm Californium] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [http://www.nuclearweaponarchive.org/Nwfaq/Nfaq6.html#nfaq6.2 NuclearWeaponArchive.org – Californium]
* [http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@rel+@na+californium,radioactive Hazardous Substances Databank – Californium, Radioactive]

{{Periodic table (navbox)}}

{{Authority control}}
{{Californium compounds}}

{{Commons category-inline}}

[[Category:Californium| ]]
[[Category:Chemical elements]]
[[Category:Chemical elements with double hexagonal close-packed structure]]
[[Category:Actinides]]
[[Category:Synthetic elements]]
[[Category:Neutron sources]]
[[Category:Ferromagnetic materials]]