Californium (Cf)

Most radioactive isotopes decay by spitting out a small particle. Californium-252 does something rarer: about three times in every hundred decays, the nucleus simply comes apart in the middle, without being hit by anything. Spontaneous fission releases several neutrons each time, and the arithmetic that follows is what makes element 98 a working tool rather than a curiosity. A single microgram of californium-252 — an invisible speck — emits over two million neutrons every second, continuously, for years. No other substance available in a laboratory comes close as a compact neutron source, and that one number is the whole of californium's commercial existence.

The state, not the analogy

Element 98 was made in February 1950 at Berkeley by Stanley Thompson, Kenneth Street, Albert Ghiorso and Glenn Seaborg, from a target of curium under an alpha beam. The first identification rested on something like five thousand atoms.

The name broke the pattern the group had been following. Americium and curium had been named by transposing the names of the lanthanides directly above them; berkelium had done the same thing geographically. Element 98 sits below dysprosium, whose name comes from the Greek for "hard to get at" — an unusually apt description that the discoverers passed over. They named it for the state and the university instead, and made the point explicitly that traversing to California had itself been difficult enough to deserve the reference.

The supernova hypothesis that californium lost

For a few years in the 1950s, californium looked as though it might explain the death of stars.

Type Ia supernovae fade along a remarkably regular exponential curve, with a characteristic time of around 55 to 60 days. In 1956 Walter Baade, Geoffrey Burbidge, William Fowler and Fred Hoyle noticed that californium-254, which had just been identified in the debris of thermonuclear tests, decays by spontaneous fission with a half-life of about 60 days. The match was close enough to build a theory on: perhaps a supernova synthesises a large quantity of the heaviest nuclei, and the light we see afterwards is the energy of californium tearing itself apart.

It was wrong, and the way it was shown to be wrong is one of the neater results in nuclear astrophysics. The real clock is a decay chain of much lighter nuclei — nickel-56 to cobalt-56 to iron-56 — and cobalt-56's 77-day half-life sets the fading rate. The californium hypothesis is now a footnote, but it was a serious and testable idea, and it is the only occasion on which a synthetic element was proposed as the engine of an astronomical phenomenon.

Two buildings on Earth

Californium-252 is produced in exactly two places. One is the High Flux Isotope Reactor and its associated processing complex at Oak Ridge in Tennessee; the other is the Research Institute of Atomic Reactors at Dimitrovgrad in Russia. Between them they make something under a gram a year, which is the entire world supply.

The reason for the bottleneck is the route. Californium is reached by piling neutrons onto plutonium one at a time and letting the resulting nuclei beta-decay upward through americium, curium and berkelium. Each step is inefficient, the target has to sit in an intense flux for months to years, and most of what emerges is not californium. The isotope is sold by the microgram, and the commonly quoted price of tens of millions of dollars a gram reflects a production line, not a market.

Where the neutrons actually go

Californium-252's uses all come down to the same thing: putting neutrons somewhere that no reactor or accelerator can be taken.

• Starting reactors. A subcritical reactor core needs a reliable neutron population before startup so that the instrumentation has something to count. Californium sources provide it.
• Looking inside bulk material. Neutrons pass through metal and are scattered by hydrogen, the reverse of X-ray behaviour. That makes neutron radiography the standard way to find corrosion, trapped moisture or adhesive voids inside aircraft components, and to inspect ordnance.
• Analysing material on a conveyor belt. In prompt gamma neutron activation analysis, neutrons excite the nuclei in coal or cement raw meal as it passes, and the gamma rays that come back give a continuous elemental assay of a moving stream. Cement plants use it to control kiln feed in real time.
• Logging boreholes. Neutron moisture and porosity gauges lowered into a well measure how much hydrogen — meaning water or hydrocarbon — surrounds them.
• Medicine. Californium has been used as a source for neutron brachytherapy, chiefly in treating cervical tumours, where neutrons damage oxygen-poor tumour cells that respond poorly to conventional photon radiation.
• Calibration. Neutron detectors of every kind, including those in underground dark-matter and neutrino experiments, are calibrated against a known californium source.

The isotopes that are not the useful one

Twenty isotopes of californium are known and the longest-lived is not the working one. Californium-251 lasts about 898 years and californium-249 about 351, but neither undergoes spontaneous fission at a rate that would make it a neutron source, so neither is what anyone buys. Californium-249 does have a quiet role in chemistry: it grows in from decaying berkelium, and because it can be obtained isotopically pure that way, most of what is known about californium's solid-state chemistry — including the dioxide and the +4 state — was measured on it rather than on the isotope that pays for the facility.