|Name, Symbol, Number||californium, Cf, 98|
|Group, Period, Block||n/a, 7, f|
|Appearance||unknown, probably silvery
white or metallic gray
|Atomic mass||(251) g/mol|
|Electron configuration||[Rn] 5f10 7s2|
|Electrons per shell||2, 8, 18, 32, 28, 8, 2|
|Density (near r.t.)||15.1 g/cm³|
|Melting point||1173 K
(900 °C, 1652 °F)
|Oxidation states||2, 3, 4|
|Electronegativity||1.3 (Pauling scale)|
|Ionization energies||1st: 608 kJ/mol|
|CAS registry number||7440-71-3|
Californium (chemical symbol Cf, atomic number 98) is a chemical element in the periodic table. A radioactive transuranic element, it was discovered by bombarding curium with alpha particles (helium ions). Californium is the heaviest element that has been produced in weighable amounts.
Although difficult to produce in large quantities, californium is useful for several applications, particularly as a neutron source. For instance, it is a neutron startup source for some nuclear reactors, it can be used to treat certain cancers, and it may be used to detect metal fatigue in aircraft and explosives at airports. It is also used in portable detectors for landmines and precious metals, as well as gauges that detect petroleum and water in oil wells. Recently, it was used to produce element 118, the heaviest chemical element synthesized so far.
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.
Californium was first synthesized by University of California, Berkeley 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. It was named after the U.S. state of California and for the University of California system.
To produce element 98, the team bombarded a microgram-sized target of 242Cm with 35 MeV alpha particles in the 60-inch Berkeley cyclotron which produced atoms of 245Cf (half-life 44 minutes) and a free neutron.
Californium is an inner transition metal of the actinide series, located in period 7 of the periodic table, between berkelium and einsteinium. As weighable amounts of californium have been produced, it has become possible to study its properties using macroscopic quantities. The only californium ion that is stable in aqueous solution is the californium(III) cation.
Californium has no biological role.
Many radioisotopes of californium have been characterized, of which the most stable ones are 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 amu (237Cf) to 256.093 amu (256Cf).
252Cf (2.6 year half-life) is a very strong neutron emitter and is thus extremely radioactive and harmful—one microgram spontaneously emits 170 million neutrons per minute. The decay of 254Cf (55-day half-life) may have been detected through telescopes in supernovae remnants. 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.
Only a few californium compounds have been made and studied. They include:
Although californium is difficult to produce in large quantities, it has some specialty applications that take advantage of its radioactive characteristics. Some of its uses are listed below.
In October 2006, it was announced that on three occasions, californium-249 atoms had been bombarded with calcium-48 ions to produce ununoctium (element 118), the heaviest chemical element synthesized so far.
The isotope 251Cf is well-known for its small critical mass, high lethality, and short period of toxic environmental irradiation relative to other radioactive elements used for radiation explosive weaponry. These properties have led to speculation about possible use of californium in pocket nukes. This application, however, is unlikely, as it would be very difficult to make a 251Cf bomb weighing less than 2 kilograms (kg), and the cost of such a bomb would be prohibitive. Other weaponry uses, such as showering an area with californium, are not impossible but are considered inhumane and are subject to conditions such as inclement weather and porous terrain.
It is important to ensure that the concentration of curium in MOX nuclear fuel is kept low, because neutron irradiation of curium converts some of it to californium. Given that californium is a strong neutron emitter of neutrons, it will cause the used fuel to be more difficult to handle.
All links retrieved March 27, 2013.
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