Californium

History

Californium was created for the first time around February 9, 1950, by a team of physics researchers at the University of California Radiation Laboratory in Berkeley: Stanley Gerald Thompson, Kenneth Street Jr., Albert Ghiorso, and Glenn T. Seaborg. It became the sixth transuranium element discovered, with the announcement made on March 17, 1950. The synthesis involved firing 35 MeV alpha particles at a microgram-sized target of curium-242 inside the lab's 60-inch cyclotron, yielding californium-245 and a single free neutron.

Only about 5,000 atoms were produced, each with a 44-minute half-life. The element’s name came from the university and the state, a departure from the naming pattern used for elements 95 through 97; the researchers explained that a century earlier, searchers had found it hard to reach California. Weighable amounts first appeared in 1954, after plutonium targets were irradiated at the Materials Testing Reactor in eastern Idaho. The first work with concentrated californium happened in 1958, and that same year isotopes 249 through 252 were isolated from plutonium-239 that had spent five years under neutron irradiation in a reactor.

In 1960, Burris Cunningham and James Wallman at the Lawrence Radiation Laboratory produced the first californium compounds—trichloride, oxychloride, and oxide—by reacting the element with steam and hydrochloric acid. The High Flux Isotope Reactor at Oak Ridge National Laboratory in Tennessee began turning out small californium batches in the 1960s, and by 1995 its nominal output reached 500 milligrams per year. Plutonium from the United Kingdom, provided under the 1958 US–UK Mutual Defence Agreement, was used in this production.

Characteristics

Californium is a silvery-white actinide metal that melts at 900 K and has an estimated boiling point of 1743 K. The pure metal is malleable and can be cut with a knife. In a vacuum, it begins to vaporize above 300 °C. Its magnetic behavior changes with temperature: below 51 K it is ferromagnetic or ferrimagnetic, between 48 and 66 K it is antiferromagnetic, and above 160 K it is paramagnetic. The element forms alloys with lanthanide metals, though little is known about them. At standard pressure, californium has two crystalline forms: a double-hexagonal close-packed alpha form (density 15.10 g/cm³) below 600–800 °C, and a face-centered cubic beta form (density 8.74 g/cm³) above that range.

Under 48 GPa of pressure, the beta form changes to an orthorhombic crystal system. Its bulk modulus is similar to trivalent lanthanide metals but smaller than that of aluminium. Californium exhibits oxidation states of +4, +3, and +2, typically forming eight or nine bonds.

Compounds in the +4 state are strong oxidizing agents; those in the +2 state are strong reducing agents. The metal tarnishes slowly in air, faster with moisture, and reacts with hydrogen, nitrogen, or chalcogens when heated. It is water-soluble only as the californium(III) cation, and attempts to alter that ion in solution have failed. Californium is the heaviest actinide to show covalent properties, as seen in californium borate.

Production

Californium is produced in nuclear reactors and particle accelerators. Californium-250 is made by bombarding berkelium-249 with neutrons to form berkelium-250, which then beta decays to californium-250. Bombarding californium with neutrons yields californium-252 and californium-253. Prolonged neutron irradiation of americium, curium, and plutonium produces milligram amounts of californium-252 and microgram amounts of californium-249.

As of 2006, curium isotopes 244 to 248 are irradiated in special reactors to produce mainly californium-252. Only two sites produce californium: Oak Ridge National Laboratory in the United States and the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. As of 2003, they produce 0.25 grams and 0.025 grams per year, respectively. Three californium isotopes with significant half-lives require 15 neutron captures from uranium-238 without fission or alpha decay.

Applications

Californium-252 serves as a strong neutron emitter, producing 139 million neutrons per microgram per minute. It is used as a startup neutron source for nuclear reactors and as a portable source for neutron activation analysis to detect trace elements. Neutrons from californium are employed in treating certain cervical and brain cancers. Since 1969, it has been used in education, beginning with a loan of 119 μg to Georgia Institute of Technology.

It is also used in online elemental coal analyzers and bulk material analyzers in the coal and cement industries. Neutron penetration allows californium to be used in fuel rod scanners, neutron radiography of aircraft and weapons components, and portable metal detectors. Neutron moisture gauges with californium find water and petroleum layers in oil wells and are used for gold and silver prospecting. In 1982, main uses were reactor start-up (48.3%), fuel rod scanning (25.3%), and activation analysis (19.4%); by 1994, neutron radiography dominated (77.4%).

In 2021, fast neutrons from californium were used for wireless data transmission. In 2006, californium-249 was bombarded with calcium-48 to produce oganesson (element 118). Californium has also been used to synthesize other transuranic elements, such as lawrencium. Its calculated critical mass is about 5 kg.

Did You Know?

• Traces of californium can be found near facilities that use it in mineral prospecting and medical treatments, and it adheres to soil at concentrations up to 500 times higher than in surrounding water.
• Californium was once believed to be produced in supernovas because its decay matched the 60-day half-life of californium-254, but subsequent studies failed to show californium spectra.
• Only about 0.05% of ingested or inhaled californium reaches the bloodstream; of that, about 65% deposits in the skeleton and 25% in the liver.