californium

berkelium ← californium → einsteinium Dy ↑ Cf ↓ (Uqo) 98Cf Periodic table
Appearance
silvery
General properties
Name, symbol, number	californium, Cf, 98
Element category	actinide
Group, period, block	n/a, 7, f
Standard atomic weight	(251) g·mol−1
Electron configuration	[Rn] 5f10 7s2
Electrons per shell	2, 8, 18, 32, 28, 8, 2 (Image)
Physical properties
Phase	solid
Density (near r.t.)	15.1 g·cm−3
Melting point	1173 K, 900 °C, 1652 °F
Boiling point	1743 K, 1470 °C, 2678 °F
Atomic properties
Oxidation states	2, 3, 4
Electronegativity	1.3 (Pauling scale)
Ionization energies	1st: 608 kJ·mol−1
Miscellanea
CAS registry number	7440-71-3
Most stable isotopes
Main article: Isotopes of californium
iso NA half-life DM DE (MeV) DP 248Cf syn 333.5 d SF - - α 6.361 244Cm 249Cf syn 351 y SF - - α 6.295 245Cm 250Cf syn 13.08 y α 6.128 246Cm SF - - 251Cf syn 898 y α 6.176 247Cm 252Cf syn 2.645 y α 6.217 248Cm SF - - 253Cf syn 17.81 d β− 0.285 253Es α 6.124 249Cm 254Cf syn 60.5 d SF - - α 5.926 250Cm
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Californium (pronounced /ˌkælɨˈfɔrniəm/, KAL-ə-FOR-nee-əm) is a metallic chemical element with the symbol Cf and atomic number 98. A radioactive transuranic element, californium is used in starting nuclear reactors, optimizing coal-fired power plants and cement production facilities (via online analyzers), medical treatment of cancer, and oil exploration via down hole well logging. It was first produced by bombarding curium with alpha particles (helium ions).

History

60 Inch Cyclotron

Californium was first synthesized at the University of California, Berkeley by researchers Stanley G. Thompson, Kenneth Street, Jr., Albert Ghiorso and Glenn T. Seaborg in 1950. It was the sixth transuranium element to be discovered and the team announced their discovery on March 17, 1950.^[1][2][3] They named it after the U.S. state of California and the University of California, Berkeley. Unlike the names for the elements 95 to 97, this name did not reflect the chemical homology of element 98 to its corresponding sixth-period element, dysprosium (No. 66).^[2]

To produce element 98, the team bombarded a microgram-sized target of ²⁴²Cm with 35 MeV alpha particles in the 60-inch (1.52 m) Berkeley cyclotron, which produced atoms of ²⁴⁵Cf (half-life 44 minutes) and a free neutron.

24296Cm + 42He → 24598Cf + 10n

Elutioncurves:
chromatographic separation of Dy, Tb, Gd, Eu and Cf, Bk, Cm, Am.^[2]

Weighable quantities of califorinium were first produced by long-duration irradiation of plutonium targets at the Materials Testing Reactor.^[4] The high spontaneous fission rate of Cf-252 was observed in these samples.

In the 1960s, reactors at Savannah River and the High Flux Isotope Reactor(HFIR) started producing batches of californium regularly. The U.S. Atomic Energy Commission began selling and loaning californium sources to industrial and academic customers in 1970 for $10 per microgram. By the 1990s, 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 8 grams of californium, peaking at about 200 mg per year.^[5]

Plutonium supplied by the United Kingdom to the U.S. under the 1958 US-UK Mutual Defence Agreement was used for californium production.^[6]

Isotopes

Twenty radioisotopes of californium have been characterized, the most stable being ²⁵¹Cf with a half-life of 898 years, ²⁴⁹Cf with a half-life of 351 years, and ²⁵⁰Cf with a half-life of 13 years. All of the remaining radioactive isotopes have half-lives that are less than 2.7 years, and the majority of these have half-lives shorter than 20 minutes. The isotopes of californium range in atomic weight from 237.062 u (²³⁷Cf) to 256.093 u (²⁵⁶Cf).

Energy spectrum of neutrons emitted by ²⁵²Cf.^[7]

²⁵²Cf has a half life of 2.645 years. ²⁵²Cf 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. ²⁵⁴Cf decays nearly quantitatively by spontaneous fission with a half-life of 60.5 days. Both materials can be used as a neutron source.

Occurrence

Natural occurrence

Californium bromide CfBr₃

Although californium does not occur naturally on Earth, the element and its decay products occur elsewhere in the universe. Their electromagnetic emissions are regularly observed in the spectra of supernovae.^{[8][9][10][11]}

Fallout

On November 1, 1952, the fallout of the United States hydrogen bomb test Ivy Mike contained plutonium, californium, einsteinium, and other transuranium elements. The isotopes ²⁴⁹Cf, ²⁵²Cf, ²⁵³Cf, and ²⁵⁴Cf were observed for the first time in the debris from the explosion.^[12]

Characteristics

Weighable amounts of californium make it possible to determine some of its properties using macroscopic quantities.

²⁵²Cf (2.645-year half-life) is a very strong neutron emitter and is thus extremely radioactive and harmful.^{[13][14][15][16][17]} One microgram spontaneously emits 2,314 million neutrons per second^[18] and one gram emits 39 watts of heat^[19]. ²⁴⁹Cf is formed from the beta decay of ²⁴⁹Bk and most other californium isotopes are made by subjecting berkelium to intense neutron radiation in a nuclear reactor.

Californium has no biological role and only a few californium compounds have been made and studied. Included among these are californium oxide (Cf₂O₃), californium trichloride (CfCl₃) and californium oxychloride (CfOCl). The only californium ion that is stable in aqueous solution is the californium(III) cation.

Nuclear fuel cycle

Californium is produced by neutron capture on berkelium-249. Three californium isotopes with significant halflives are produced, requiring a total of 12 to 14 neutron captures on uranium-238 without nuclear fission or alpha decay. Their neutron cross sections are :

Thus ²⁵⁰Cf and ²⁵¹Cf will be transmuted fairly quickly, with the majority fissioning at mass 251, but with a large fraction surviving to become ²⁵²Cf. The ²⁵²Cf will not be transmuted or destroyed quickly in a well-thermalized reactor, but has a short decay halflife. These isotopes decay into long-lived isotopes of curium.

²⁵²Cf has a relatively high rate of spontaneous fission. Although still much less likely than alpha decay, this makes californium a significant neutron radiation emitter. MOX fuel containing enough curium would likely contain enough californium after use to preclude manual handling of the spent fuel or its nuclear reprocessing products with a glove box that protects against alpha and beta radiation but not against gamma radiation and especially neutron radiation.

Applications

General

²⁵²Cf has a number of specialized applications as a strong neutron emitter. Each milligram of fresh californium produces 2.3 x 10⁹ neutrons / second. Some of its uses are:^[5][20][21]

• neutron startup source for some nuclear reactors, calibrating instrumentation
• treatment of certain cervical and brain cancers where other radiation therapy is ineffective
• radiography of aircraft to detect metal fatigue
• airport neutron-activation detectors of explosives
• portable (non-reactor based) neutron sources for neutron activation analysis
• portable metal detectors^[22]
• neutron moisture gauges used to find water and petroleum layers in oil wells
• portable neutron source for gold and silver prospecting for on-the-spot analysis
• ²⁵²Cf is used to detect ground water movement.^[23]

In October 2006 it was 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 ,^[24][25][26] making this the heaviest element ever synthesized.

Military

²⁵¹Cf is famous for having a very small critical mass of 5 kg,^[27] high lethality, and short period of toxic environmental irradiation relative to radioactive elements commonly used for radiation explosive weaponry, creating speculation about possible use in pocket nukes. Other weaponry uses, such as showering an area with californium, are not impossible but are seen as inhumane and are subject to inclement weather conditions and porous terrain considerations.

References

Literature

• Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1

External links

Wikimedia Commons has media related to: Californium

Look up californium in Wiktionary, the free dictionary.

• WebElements.com - Californium
• NuclearWeaponArchive.org - Californium
• It's Elemental - Californium
• Hazardous Substances Databank – Californium, Radioactive