Californium · element 98
The heaviest element with a real job, spraying neutrons to start reactors and hunt hidden explosives
Key facts
Californium is where the useful part of the periodic table ends. Everything beyond it exists only in traces for research, but californium-252 is made deliberately, sold by the microgram, and put to work. It is one of the most powerful neutron sources available, so intense that a single gram would emit more than two million million neutrons every second.
It was the sixth transuranium element to be found at Berkeley, in 1950, from about five thousand atoms. Today it is produced in milligram batches in just two reactors in the world, and it has served as a target for creating the heaviest element of all.
Where it is found
Californium is not found in nature on Earth. Its longest-lived isotope, californium-251, lasts about 900 years, far too short to survive geological time. Its presence in the environment comes only from nuclear weapons tests and a few accidents, and even then in vanishing amounts.
Production takes place at the High Flux Isotope Reactor at Oak Ridge in the United States and the SM reactor at Dimitrovgrad in Russia. Curium targets are irradiated for long periods, capturing neutron after neutron until they climb up to californium. The material is then chemically separated. A year's world production is a few tens of milligrams, and californium-252 has been quoted at tens of millions of dollars per gram.
How it was discovered
Stanley Thompson, Kenneth Street, Albert Ghiorso and Glenn Seaborg made californium in February 1950 by bombarding a microgram-scale target of curium-242 with alpha particles in the Berkeley 60-inch cyclotron. The product was californium-245, with a half-life of about 45 minutes. The team identified it from its behaviour on an ion-exchange column and its alpha decay, working with only a few thousand atoms in total.
They named it after California and the University of California. In their paper they noted that the name did not follow the pattern of dysprosium, its lanthanide counterpart, whose name means hard to get at, though they felt the discovery had been just as difficult. The first weighable sample came in 1958, and the metal was produced in the 1970s.
What we use it for
Californium-252 decays by spontaneous fission about three per cent of the time, and each fission releases several neutrons. Sealed sources containing micrograms of it are used to start up nuclear reactors, providing an initial flow of neutrons so instruments can monitor the chain reaction as it builds. In industry, californium neutron sources sit inside analysers that measure the composition of coal, cement and ores as they move along conveyors.
The same neutrons can reveal hidden materials. Californium sources are used in portable systems that detect explosives and landmines, and in neutron radiography to inspect aircraft parts for flaws that X-rays miss. Oil companies lower them down boreholes to gauge the water and oil content of rock. In medicine, californium neutron sources have been tried for treating certain cancers. Californium-249 is also a key target for making superheavy elements: oganesson, element 118, was first produced by firing calcium-48 at it.
Reading the numbers
Californium melts at around 900 °C, well below the melting point of copper, and boils near 1,470 °C, a short liquid range for a metal. Its density of 15.1 grams per cubic centimetre is nearly twice iron and about a third more than lead. The configuration [Rn] 5f10 7s2 is the pattern of a late actinide, with the f shell more than half full and no 6d electron. The +3 oxidation state dominates, as for dysprosium above it, though +2 and +4 compounds have been made. Its Pauling electronegativity of 1.3 reflects a metal that oxidises readily. The first ionisation energy, about 608 kilojoules per mole, continues the gentle upward trend along the actinide row as the nuclear charge grows.
Safety and the environment
Californium is hazardous in two ways. Like other actinides it is an alpha emitter that concentrates in the skeleton if it enters the body. Unlike most of them it also emits neutrons, which pass through ordinary shielding and can irradiate people at a distance. Neutron sources are stored in thick containers of water or polythene and handled remotely. Because production is tiny and sources are sealed and tracked, environmental contamination is not a realistic concern, but lost or stolen sources are treated as serious incidents.
Did you know?
- A single microgram of californium-252 emits over two million neutrons every second.
- Californium-252 sources have been used to search for landmines and explosives, because neutrons make hidden nitrogen-rich compounds give off telltale gamma rays.
- Oganesson, the heaviest element ever made, was first produced by bombarding a californium-249 target with calcium ions at Dubna in 2002.
- World production of californium is measured in tens of milligrams per year, from just two reactors.
- Californium was identified from roughly five thousand atoms in 1950, and it took eight more years to produce a visible amount.
More in the actinides
The nearest neighbours of californium in the family, by atomic number.
See all 15 actinides on one page, or browse all 118 elements.
Frequently asked questions
What is californium used for?
Mainly as a neutron source. Californium-252 starts up nuclear reactors, analyses coal and cement in industry, scans for explosives and helps oil companies probe boreholes.
Why is californium so expensive?
Only two reactors in the world can make it, the process takes months of irradiation, and output is measured in milligrams. Prices of tens of millions of dollars per gram have been quoted.
Is californium found in nature?
Not on Earth. Its isotopes decay far too quickly to have survived since the planet formed, and no natural process makes it.
Is californium dangerous?
Yes. It emits alpha particles and, unusually, neutrons that can penetrate shielding. It is only handled in sealed sources or in specialised laboratories.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Californium reference article. Spotted an error? Tell us.