Californium (Cf)
Californium (symbol Cf) is the chemical element with atomic number 98: an actinide, solid at 0 °C, radioactive, made artificially, with an atomic mass of 251 u. Discovered in 1950 (Glenn T. Seaborg et al.).
Californium is a synthetic element named after the State and University of California. Its isotope californium-252 has a rare property: its nuclei split apart spontaneously, releasing neutrons. A single microgram emits about 2.3 million of them per second, making it a compact and highly sought-after neutron source. It is one of the most expensive materials in the world, with only a few tens of milligrams produced each year.
Key facts
- Atomic number
- 98
- Symbol
- Cf
- French name
- Californium
- Atomic mass
- 251 u
- Molar mass
- [251] g/mol
- Family
- Actinides
- Position
- period 7, f-block
- State at 0 °C
- Solid
- Melting point
- 899.9 °C
- Boiling point
- 1,469.9 °C
- Density
- 15.1 g/cm³
- Electronegativity (Pauling)
- 1.3
- First ionization energy
- 608 kJ/mol
- Atomic radius
- 186 pm
- Electron configuration
- [Rn] 5f¹⁰ 7s²
- Indicative price
- 60 billion $/kg (order of magnitude)
- Radioactive
- yes
- Discovery
- 1950, Glenn T. Seaborg et al.
Origin of the name
From California.
What does californium look like?
A soft, silvery metal, first obtained in pure form in 1974. Only minuscule grains of it have ever been seen.
Where is californium found?
Absent from nature on Earth. It is made in two specialized reactors in the world. It was also detected in the debris of the first H-bomb test, in 1952.
How is californium produced?
Curium, americium or plutonium is irradiated in a very high-flux neutron reactor for many months: the nuclei capture neutrons one by one until they become californium. It is then separated by painstaking chemistry. The HFIR reactor at Oak Ridge in the United States and the SM-3 reactor at Dimitrovgrad in Russia account for most of the world's output.
Top producers
- United States (Oak Ridge)
- Russia (Dimitrovgrad)
What is californium used for?
- Starting up nuclear reactors (startup neutron source)
- Oil and mineral prospecting: probing the rocks around a borehole
- Detecting explosives and nuclear materials in luggage or shipping containers
- Neutron radiography of aircraft parts and munitions
- Target for making oganesson (element 118)
In everyday life
- No everyday objects
- Indirectly: some industrial safety inspections and power plant startups
Role in living things
No biological role. It would settle in the bones and liver. Californium-252 has been used experimentally in brachytherapy against certain cancers.
Hazards of californium
Extremely dangerous because of its neutron radiation, which is highly penetrating and hard to stop: it calls for thick shielding of water, paraffin or concrete. A few micrograms are enough to pose a serious hazard to an unprotected handler.
Cosmic origin
- Human-made: Does not exist (or barely) in nature: synthesized in a reactor or accelerator.
History of californium
In February 1950, Stanley Thompson, Kenneth Street Jr., Albert Ghiorso and Glenn Seaborg bombarded a few micrograms of curium-242 with alpha particles in the Berkeley cyclotron and produced about 5,000 atoms of californium-245. It was the sixth transuranium element discovered in less than ten years.
Did you know?
- The New Yorker magazine joked that Berkeley should have named elements 97 and 98 “universitium” and “ofium,” saving “californium” for the next one. The researchers replied that they had thereby prevented some New York scientist from discovering elements 97 and 98 and naming them “newium” and “yorkium.”
- Its price is often estimated at several tens of millions of dollars per gram.
- Californium-252 has a half-life of 2.6 years; the most stable isotope, californium-251, lasts nearly 900 years.
Isotopes of californium
No stable isotopes; californium-252 is one of the most powerful neutron emitters there is: a single microgram releases more than two million of them every second.
- californium-245: radioactive, half-life 45 min. The element's first synthesized isotope, in 1950.
- californium-249: radioactive, half-life 351 years. The target that made it possible to create oganesson.
- californium-251: radioactive, half-life 898 years. The longest-lived.
- californium-252: radioactive, half-life 2.65 years. A neutron source for starting up reactors, detecting explosives and oil, or treating certain cancers.
Related elements
Same family (actinides): Actinium (Ac), Thorium (Th), Protactinium (Pa), Uranium (U), Neptunium (Np), Plutonium (Pu), Americium (Am), Curium (Cm), Berkelium (Bk), Einsteinium (Es), Fermium (Fm), Mendelevium (Md), Nobelium (No), Lawrencium (Lr).