Californium – facts and properties

Position in the periodic table

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Occurrence and extraction

You will not find californium in nature. All its isotopes are radioactive, and the longest-lived one, californium-251, has a half-life of around nine hundred years. More useful in practice is californium-252, with a half-life of a little over two and a half years.

Stanley Thompson, Kenneth Street, Albert Ghiorso and Glenn Seaborg made the first californium in Berkeley in 1950. They bombarded a tiny amount of curium-242 with alpha particles, which produced californium-245 with a half-life of about 45 minutes. Only a few thousand atoms were formed, recognised by their radiation and their chemical behaviour. The discoverers named the element after the state and the university where they worked.

Today californium is made in high-flux reactors, chiefly at Oak Ridge in the United States and at Dimitrovgrad in Russia. Curium is irradiated with neutrons for weeks and months, and neutron capture and beta decay turn its nuclei into ever heavier ones until californium is among them and can be separated chemically. World production of californium-252 is, by common estimates, well under a gram per year.

Properties and reactions

Californium is in period 7 and is the tenth actinoid, in the actinoid row between berkelium on its left and einsteinium on its right. Dysprosium sits above it in the lanthanoid row. The electrons are arranged as [Rn] 5f¹⁰ 7s². The electronegativity of 1.3 is an estimate, and the melting point of 900 °C was measured on tiny metal samples. Its atomic mass is given as 251 u, the mass number of the longest-lived isotope, californium-251.

In compounds californium is usually trivalent, as Cf³⁺. Tetravalent californium is also known, for instance in the oxide CfO₂, and so is divalent californium. From californium onwards the divalent state gains importance, and at nobelium it is the most stable.

What sets californium-252 apart is its radiation. It decays mainly by emitting alpha particles, but in roughly one decay in thirty the nucleus splits spontaneously and releases neutrons. A single microgram therefore emits more than two million neutrons every second. Neutrons penetrate the body far more easily than alpha particles and are shielded with hydrogen-rich materials such as water or polyethylene. If californium enters the body it settles in the skeleton. Only trained specialists with the proper safeguards may handle it.

Use as a neutron source

Californium-252 is a compact, powerful neutron source.

When a nuclear reactor is started up, fresh fuel produces hardly any neutrons of its own. A californium source makes sure the instruments count neutrons from the very beginning, so the rise of the chain reaction can be followed.

In materials testing, components are imaged with neutrons, much as with X-rays. Neutrons pass through metal easily but are strongly scattered by hydrogen-containing substances such as water or plastic, which is how moisture and corrosion are found inside aircraft parts. In neutron activation analysis an irradiated sample briefly emits radiation of its own, which reveals the elements it contains, and ores, coal and cement are checked this way. A californium probe lowered into an oil or gas borehole measures how strongly the rock slows neutrons, which hints at layers holding water or oil.

In medicine, californium-252 was formerly used on certain tumours: a small source placed near the tumour irradiated it with neutrons from within. Today the method plays only a minor role.

Because a source loses half its neutrons in a little over two and a half years, it has to be replaced regularly. The price is quoted in millions of dollars per gram, among the highest for any material on sale. In research, californium-249 serves as target material: bombarded with calcium-48 ions, it yielded single atoms of oganesson.

Quick question

What is the atomic number of californium (Cf)?

FAQ