Californium Cf

Discovery

Californium was first produced in early 1950 at the University of California, Berkeley, by Stanley Thompson, Kenneth Street Jr., Albert Ghiorso and Glenn Seaborg. They bombarded a few micrograms of curium-242 with alpha particles and obtained californium-245, with a half-life of about 44 minutes. The first sample amounted to only a few thousand atoms.

The discovery followed that of berkelium within weeks, by the same team, with the same cyclotron and similar separation techniques. Larger quantities later came from irradiating plutonium or curium with neutrons in reactors for long periods, and the first weighable amount of a californium compound was obtained around 1958. Today most of the world's supply comes from the High Flux Isotope Reactor at Oak Ridge, Tennessee, and from a research reactor in Russia.

Origin of the name

The team named the element after the state of California and the University of California. The symbol is Cf. After berkelium and californium appeared in quick succession, The New Yorker joked that the Berkeley group should have named elements 97 and 98 'universitium' and 'ofium', saving californium for 99 to spell out the university's name. Seaborg reportedly replied that, had they done so, New Yorkers might have claimed the next two elements.

Main uses

• Reactor start-up: Californium-252 sources supply the initial neutrons that kick-start the chain reaction in new reactors or ones restarting after a long shutdown.
• Exploration and process control: It is used in well logging for oil and gas, in prospecting for gold and silver, and in neutron activation analysers that monitor the composition of coal and cement in real time.
• Security screening: Neutron-based inspection systems use it to reveal the elemental make-up of hidden explosives or drugs.
• Medicine: Some countries have used californium-252 sources in brachytherapy for cancers such as cervical cancer.
• Research: Californium-249 was the target used to create element 118, oganesson.

Isotopes

Californium has no stable isotopes. The longest-lived, californium-251, has a half-life of about 900 years. Californium-252, the workhorse isotope, has a half-life of about 2.6 years; roughly 3 per cent of its decays are spontaneous fissions, which is why a single microgram emits about 2.3 million neutrons per second. Californium-249 (about 351 years) is obtained from the decay of berkelium-249 and has been used as a target in superheavy-element research.

In everyday life

Californium is invisible in daily life, but it works behind the scenes of oil production and electricity generation through well logging and reactor start-up. Californium-252 sources emit intense neutron and gamma radiation, so they are transported in heavily shielded casks and handled only by licensed specialists. If taken into the body, californium tends to deposit in bone.