Californium

• 1. Introduction

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  Californium is a silvery-white metal that can be cut with an ordinary kitchen knife, yet a single microgram of its most common isotope releases 2.3 million neutrons every second. It was conjured into existence in a Berkeley laboratory in 1950 by bombarding a target of curium with alpha particles, and only about 5,000 atoms were produced in that first experiment. Those atoms had a half-life of just 44 minutes, meaning they were gone almost as quickly as they appeared. Yet today californium is produced at exactly two places on Earth and shipped commercially to customers who use it to scan jet aircraft for hidden cracks, probe oil wells for water and petroleum, and treat cancers that resist other forms of radiation. How does a synthetic element so difficult to make, and so dangerous to handle, end up with so many practical uses? And why did the scientists who created it name it after a state known more for gold rushes than for nuclear physics?

• 2. The Berkeley Experiment

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  On about the 9th of February 1950, four physicists at the University of California Radiation Laboratory in Berkeley achieved something that had never been done before. Stanley Gerald Thompson, Kenneth Street Jr., Albert Ghiorso, and Glenn T. Seaborg fired 35 MeV alpha particles from a 60-inch cyclotron at a microgram-sized target of curium-242. The collision produced californium-245 and one free neutron. To separate those roughly 5,000 newly created atoms from everything else in the experiment, the team used ion exchange and adsorption methods. They announced the discovery on the 17th of March 1950.

  Naming the new element posed an unexpected problem. Elements 95 through 97 had been named by analogy with the elements directly above them on the periodic table. The element sitting above element 98 is dysprosium, whose name means "hard to get at." The researchers decided that naming their discovery after that meaning would be a poor fit. Instead they named it after the university and the state where it was made, and noted with some humor that searchers a century earlier had also found it difficult to get to California.

• 3. From Atoms to Milligrams

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  Those first 5,000 atoms were not enough to weigh, see, or study in any meaningful chemical way. The path from a handful of atoms to a usable quantity took years and required a different approach entirely. Plutonium targets irradiated at the Materials Testing Reactor at National Reactor Testing Station in eastern Idaho produced the first weighable amounts of californium, and those findings were reported in 1954. In those samples, researchers first observed the high spontaneous fission rate of californium-252.

  The first experiment using californium in concentrated form came in 1958. That same year, isotopes of californium ranging from Cf-249 through Cf-252 were isolated from a sample of plutonium-239 that had been exposed to neutron irradiation in a nuclear reactor for five years. Two years later, in 1960, Burris Cunningham and James Wallman of the Lawrence Radiation Laboratory of the University of California created the first californium compounds by treating the element with steam and hydrochloric acid. They produced californium trichloride, californium(III) oxychloride, and californium oxide.

  Californium metal itself was not prepared until 1974, when Haire and Baybarz reduced californium(III) oxide with lanthanum metal. The result was microgram amounts of films thinner than a micrometer.

• 4. Two Factories on Earth

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  Californium-252 is the isotope at the heart of nearly every practical application, and it is produced at only two locations in the world. Oak Ridge National Laboratory in Oak Ridge, Tennessee, and the Research Institute of Atomic Reactors in Dimitrovgrad, Russia, are the sole sources. As of 2003, Oak Ridge produces 0.25 grams of californium-252 per year, while the Russian facility produces 0.025 grams. The High Flux Isotope Reactor at Oak Ridge began producing small batches in the 1960s, and by 1995 it nominally turned out 500 milligrams annually.

  The production chain begins with uranium-238 and requires a total of 15 neutron captures without any fission or alpha decay interrupting the sequence. Along the way the chain passes through several isotopes of plutonium, americium, curium, and berkelium. As of 2006, curium isotopes 244 through 248 are irradiated by neutrons in specialized reactors to yield mainly californium-252, with smaller amounts of isotopes 249 through 255.

  The Atomic Energy Commission began selling californium-252 to industrial and academic customers in the early 1970s at $10 per microgram. Between 1970 and 1990, an average of 150 milligrams were shipped each year. Plutonium supplied by the United Kingdom to the United States under the 1958 US-UK Mutual Defence Agreement was among the material used to make californium during this period.

• 5. A Metal Unlike Others

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  Californium melts at 900 degrees Celsius, plus or minus 30 degrees, and its estimated boiling point is 1,743 Kelvin. At room temperature it is malleable and can be sliced with a knife, but expose it to a vacuum and it begins to vaporize above 300 degrees Celsius. Its behavior changes dramatically with temperature in another way: below 51 Kelvin it acts like a magnet, either ferromagnetic or ferrimagnetic; between 48 and 66 Kelvin it enters an antiferromagnetic intermediate state; and above 160 Kelvin it becomes paramagnetic, meaning an external field is needed to make it magnetic.

  Two crystalline forms exist at standard atmospheric pressure. The alpha form, a double-hexagonal close-packed structure, has a density of 15.10 grams per cubic centimeter and exists below 600-800 degrees Celsius. The beta form, face-centered cubic, has a density of 8.74 grams per cubic centimeter and takes over above that temperature range. Apply 48 gigapascals of pressure and the beta form shifts again, into an orthorhombic crystal system, as 5f electrons become delocalized and free to bond.

  Californium's bulk modulus is 50 gigapascals, meaning it resists compression about as readily as trivalent lanthanide metals do, but considerably less than aluminium, whose bulk modulus is 70 gigapascals.

• 6. The Neutron at Work

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  Californium-252 produces 139 million neutrons per microgram per minute, and that single property underlies most of its practical value. In 1969, Georgia Institute of Technology received a loan of 119 micrograms of californium-252 from the Savannah River Site, marking the element's entry into educational use. By 1982, the three largest uses were reactor start-up at 48.3 percent, fuel rod scanning at 25.3 percent, and activation analysis at 19.4 percent. By 1994, neutron radiography had risen to dominate at 77.4 percent, with fuel rod scanning at 12.1 percent and reactor start-up at 6.9 percent.

  Neutrons travel through solid materials in ways that X-rays cannot, making californium-based instruments useful for detecting corrosion, bad welds, cracks, and trapped moisture in aircraft and weapons components. Neutron moisture gauges use the element to locate water and petroleum layers in oil wells and to detect the movement of groundwater. In the coal and cement industries, californium is used in bulk material analyzers and online elemental coal analyzers.

  The element also has a role in medicine. Neutrons from californium are used to treat certain cervical and brain cancers where other forms of radiation therapy have not worked. In 2021, fast neutrons from californium-252 were applied to a new purpose: wireless data transmission.

• 7. Building Heavier Elements

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  Californium's densely packed nucleus makes it a useful target for creating elements heavier than itself. In 1961, lawrencium was first synthesized by bombarding californium with boron nuclei. That synthesis was only the beginning.

  In October 2006, researchers at the Joint Institute for Nuclear Research in Dubna, Russia, announced that three atoms of oganesson, element 118, had been identified after bombarding californium-249 with calcium-48 ions. The target held about 10 milligrams of californium deposited on a titanium foil with an area of 32 square centimeters. At the time, oganesson was the heaviest element ever made.

  Californium-249 and californium-251 can also cause tissue damage through external exposure, via gamma ray emission, which meant the experimenters in Dubna worked under strict handling protocols. The same isotope that helped create the heaviest known element also ranks among the most hazardous radioactive materials researchers handle.

• 8. Hazards and the Body

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  Once californium enters a living body, it follows a specific and damaging path. Only 0.05 percent of ingested californium reaches the bloodstream. Of that fraction, about 65 percent deposits in the skeleton, 25 percent in the liver, and the remainder distributes to other organs or is excreted, mainly in urine. The californium lodged in the skeleton adheres to bone surfaces before slowly migrating deeper into the bone. Half of the californium deposited in the skeleton is gone within 50 years; half of what settles in the liver clears within 20 years.

  The radiation released by californium sitting in bone and liver tissue can cause cancer. Ionizing radiation also disrupts the body's production of red blood cells. Californium plays no natural biological role in any organism, and its concentration in the environment is low enough that environmental exposure is not a primary concern for the general population.

  Near facilities that use the element in mineral prospecting or medical treatment, traces of californium can be detected in local soil. The element adheres well to soil particles, and concentrations in the soil can run 500 times higher than in the surrounding water. Its half-life of 2.645 years for the californium-252 isotope means that environmental contamination from a spill or from nuclear weapons test fallout does not persist indefinitely, though fallout from atmospheric nuclear tests conducted before 1980 did introduce californium-249, -252, -253, and -254 into the environment in measurable quantities.