Californium

Californium is a radioactive synthetic element that emits neutrons prolifically and is one of the few elements sold commercially in gram quantities for a specific application: starting up nuclear reactors and detecting oil deposits. Named after the state of California and the University of California.

• Atomic Number9898 protons, 98 electrons
• Atomic Mass251.07959 u98× heavier than hydrogen
• State at Room TempSolidSolid
• DensityNot measured
• Melting / Boiling899.9°C
• Discovered1950

What is Californium?

Californium has 98 protons. Cf-252 (half-life 2.65 years) spontaneously fissions and emits approx. 2.4 neutrons per fission, making it an intense portable neutron source. Used to start nuclear reactors that have been shut down, in neutron activation analysis for geological exploration and in cancer radiotherapy (Cf-252 brachytherapy). Produced in 1950 by Thompson, Street, Ghiorso and Seaborg at Berkeley.

Fact Californium is element 98, symbol Cf. As an actinide, it is part of the f-block of the periodic table: one of 15 radioactive elements from actinium (89) to lawrencium (103). All actinides are radioactive; none has a stable isotope. They are produced in nuclear reactors and by decay of heavier elements, and their chemistry has been studied using only microgram or nanogram quantities.

Where you find Californium

On Earth

Californium does not occur in significant natural abundance. It is produced only artificially, by bombarding heavier actinide targets with neutrons or lighter ions in nuclear reactors or particle accelerators. World production is measured in micrograms or milligrams per year.

How we use Californium

At element 98, practical applications are limited by the extreme difficulty of production and the intense radioactivity. Californium has 98 protons Future applications in targeted cancer radiotherapy are being investigated using alpha-emitting actinide isotopes bound to tumour-targeting molecules.

How it was discovered

Californium has 98 protons. Cf-252 (half-life 2.65 years) spontaneously fissions and emits approx. 2.4 neutrons per fission, making it an intense portable neutron source. Used to start nuclear reactors that have been shut down, in neutron activation analysis for geological exploration and in cancer radiotherapy (Cf-252 brachytherapy). Produced in 1950 by Thompson, Street, Ghiorso and Seaborg at Berkeley.

Deeper dive: californium and the actinide series

The actinides (elements 89-103) form the lower of the two rows below the main body of the periodic table. They represent the filling of the 5f electron subshell. Unlike the lanthanides (the upper row), the actinides show greater variety in their chemistry because the 5f, 6d and 7s orbitals are close in energy. The early actinides, thorium through neptunium, can show many different oxidation states (e.g. uranium from +3 to +6). The heavier actinides increasingly resemble the lanthanides in preferring the +3 state.

All actinides beyond bismuth (83) are radioactive. The lightest, thorium, protactinium and uranium, have long enough half-lives to survive from the formation of the solar system. Neptunium and beyond are almost entirely synthetic, produced in nuclear reactors or accelerators. The transuranic elements were created at remarkable facilities including Oak Ridge National Laboratory, the Berkeley Cyclotron, the GSI in Darmstadt and JINR in Dubna.

Moving to 99 protons brings us to the next element on the periodic table.