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— CH. 1 · INTRODUCTION —

Berkelium

10 min listen · Ch. 1 of 7
7 sections

  • Berkelium sits at atomic number 97 on the periodic table, a soft, silvery-white metal that glows faintly with radioactivity and exists nowhere in nature. It was first deliberately created in December 1949 at the University of California, Berkeley, by a team of four scientists working with a 60-inch cyclotron. In the decades since, just over one gram of it has been produced in the entire United States. That single gram represents the sum total of human effort to bring this element into being, and yet it played a direct role in creating a brand-new element, tennessine, in 2009. How does a substance too rare and too short-lived to have a single commercial use become indispensable to the frontier of nuclear science? And what does it take to make, handle, and study something that begins destroying itself the moment it is formed?

  • Glenn T. Seaborg, Albert Ghiorso, Stanley Gerald Thompson, and Kenneth Street Jr. created berkelium by bombarding americium-241 with alpha particles accelerated to 35 MeV inside the 60-inch cyclotron at the Lawrence Radiation Laboratory. The target was a platinum foil coated with americium nitrate solution, which was then converted to americium dioxide by heating before irradiation. The whole bombardment lasted only six hours, producing the isotope berkelium-243 and releasing two free neutrons in the process.

    Separating the result from everything else in the reaction vessel was the harder problem. The team did not yet know that berkelium could be coaxed into a +4 oxidation state, so they used a laborious multi-step procedure: dissolving, precipitating, centrifuging, and re-dissolving the material through various acid and ammonia solutions. A citric acid and ammonium buffer at a slightly acidic pH of roughly 3.5 was used for the final ion-exchange separation step, a technique borrowed by analogy from terbium, the lanthanide element that sits directly above berkelium in the periodic table.

    The first results were discouraging. No alpha-particle emission appeared in the expected elution product. The team shifted to searching for characteristic X-rays and conversion electron signals, and only then did a berkelium isotope surface in the data. Even the mass number of that isotope was initially uncertain, listed in the original report as either 243 or 244 before being settled at 243.

  • Element 97 was named berkelium after the city of Berkeley, California, following a deliberate pattern the Californian group had established with earlier discoveries. The logic ran like this: each new actinide they discovered should be named in a way that echoed the lanthanide element sitting directly above it in the periodic table. Americium took its name from a continent, mirroring europium. Curium honored scientists Marie and Pierre Curie, reflecting how gadolinium had honored the explorer of rare-earth elements Johan Gadolin.

    Berkelium's analogue was terbium, atomic number 65, whose name derived from the Swedish town of Ytterby, where rare-earth minerals were first found. So berkelium took its name from a city, matching terbium's city-derived name. The discovery report from the Berkeley group stated explicitly that element 97 should be named "after the city of Berkeley in a manner similar to that used in naming its chemical homologue terbium."

    Berkelium was the fifth transuranium element discovered, following neptunium, plutonium, curium, and americium. And it was the last element for which this lanthanide-mirroring naming convention held. The very next element discovered, californium, was named after the state and university rather than after its lanthanide analogue, dysprosium, marking the end of the tradition.

  • Producing berkelium today relies on the 85-megawatt High Flux Isotope Reactor at Oak Ridge National Laboratory in Tennessee, and the SM-2 loop reactor at the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. These two facilities operate at similar power and flux levels and are the world's primary sources for elements with atomic numbers above 96.

    The process starts with uranium or plutonium in the reactor and works through a chain of neutron captures and beta decays. Plutonium-239 absorbs neutrons under a flux several times higher than a conventional reactor, eventually converting to curium-249. Curium-249 has a short half-life of 64 minutes, so rather than capturing another neutron, it almost always undergoes beta decay into berkelium-249. That berkelium-249 half-life of 330 days is long enough to allow collection and purification.

    At Oak Ridge, a typical processing campaign irradiates tens of grams of curium to yield californium in decigram quantities, berkelium-249 in milligram quantities, einsteinium in milligram quantities, and fermium only in picogram quantities. The price of berkelium-249 has reached the order of 185 US dollars per microgram, and only 0.66 grams were produced in the United States during the period 1967 to 1983. The first preparation of any macroscopic quantity of the element was achieved in 1958 by Burris B. Cunningham and Stanley Gerald Thompson, who produced 0.6 micrograms after six continuous years of reactor irradiation of an eight-gram plutonium-239 target at the Material Testing Reactor in Arco, Idaho.

  • Berkelium-249 carries within it a slow-motion chemical contamination problem. With a half-life of 330 days, it steadily beta-decays into californium-249, a strong emitter of ionizing alpha particles. By the time a researcher has finished purifying and studying a sample, a growing fraction of it has already become a different element.

    This contamination rate amounts to 0.22 percent per day for berkelium-249 in a bromide crystal sample. Researchers have tracked this process by probing fresh and aged samples of berkelium-249 bromide with X-ray diffraction over periods exceeding three years. The crystal structure does not change as berkelium transforms to californium within the same lattice. But the chemistry changes in measurable ways. Californium-249 bromide, for instance, can be reduced with hydrogen to californium-249 dibromide, while its berkelium-249 counterpart cannot undergo the same reaction.

    The practical consequence is that alpha particles from accumulated californium heat the sample from within and damage the crystal lattice through self-irradiation. Researchers have developed a workaround: measuring properties as a function of time and then extrapolating the results back to a pure berkelium starting point. In the skeleton, berkelium's radiation can damage red blood cells, and the maximum permissible amount of berkelium-249 in the human skeleton is 0.4 nanograms.

  • In 2009, a 22-milligram batch of berkelium-249 was prepared during a 250-day irradiation period at Oak Ridge, then spent a further 90 days in purification. The resulting target was then shipped to the Joint Institute for Nuclear Research in Dubna, Russia, where it was bombarded with calcium-48 ions inside the U400 cyclotron for 150 days. The bombardment yielded the first six atoms of element 117, now named tennessine.

    This synthesis represented the culmination of a collaboration between the Joint Institute for Nuclear Research and Lawrence Livermore National Laboratory that had begun in 1989 and aimed at synthesizing elements 113 through 118. Berkelium's role was as the raw material; californium, lawrencium, rutherfordium, and bohrium are also accessible starting from berkelium-249 targets. Without berkelium, the upper reaches of the periodic table would remain out of reach.

    Berkelium-249 is also a practical indirect source of californium-249, which scientists prefer over californium-252 for studying californium chemistry. Californium-252 is the more radioactive of the two and is produced in neutron bombardment facilities including the same High Flux Isotope Reactor at Oak Ridge, making the milder californium-249 the more manageable research material.

  • At ordinary room conditions, berkelium settles into its alpha form, a hexagonal structure with a double-hexagonal close packing arrangement that it shares with alpha-lanthanum and with actinide elements beyond curium. Squeezing it to 7 GPa at room temperature converts it to the beta form, a face-centered cubic structure. That transition happens without any change in volume, though the enthalpy rises by 3.66 kJ per mole. Compressing further to 25 GPa drives a transition to an orthorhombic gamma form similar to alpha-uranium, and that transition is accompanied by a 12 percent volume decrease. No further structural changes appear up to 57 GPa.

    In solution, berkelium ions take on a green color in most acids, yellow in hydrochloric acid, and orange-yellow in sulfuric acid. The trivalent state, +3, is the most stable in water, but berkelium is unusual among actinides in its relative ease of reaching the +4 state in solids and moderately in liquids. That chemical property has become the primary tool for separating berkelium from other actinides produced alongside it in the reactor, since most neighboring elements prefer to stay at +3. Oxidizing agents including bromates, bismuthates, chromates, and even ozone can push berkelium to the +4 state, and more recently the chelator 3,4,3-LI(1,2-HOPO) has been found to accomplish the same shift under mild conditions.

    In 2025, researchers synthesized an organometallic compound from just 0.3 milligrams of berkelium and named it berkelocene, extending the element's known chemistry into the realm of metal-organic compounds.

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Common questions

When and where was berkelium first discovered?

Berkelium was first intentionally synthesized, isolated, and identified in December 1949 at the University of California, Berkeley. Glenn T. Seaborg, Albert Ghiorso, Stanley Gerald Thompson, and Kenneth Street Jr. created it using the 60-inch cyclotron at the Lawrence Radiation Laboratory.

Why is berkelium named after Berkeley?

Berkelium is named after the city of Berkeley, California, following the tradition of the Berkeley research group of naming new actinide elements in analogy with the lanthanide element positioned above them in the periodic table. Berkelium's lanthanide analogue is terbium, whose name derived from the town of Ytterby, Sweden, so element 97 was similarly named after a city.

How much berkelium has been produced in the United States?

Just over one gram of berkelium has been produced at Oak Ridge National Laboratory since 1967. During the period 1967 to 1983 alone, only 0.66 grams were made. Berkelium-249 costs on the order of 185 US dollars per microgram.

What was berkelium used for in the discovery of tennessine?

A 22-milligram batch of berkelium-249 was prepared during a 250-day irradiation at Oak Ridge in 2009, then purified for 90 days and shipped to the Joint Institute for Nuclear Research in Dubna, Russia. There it was bombarded with calcium-48 ions for 150 days, yielding the first six atoms of element 117, tennessine.

What are the main isotopes of berkelium and how long do they last?

The most important isotope for production and research is berkelium-249, which has a half-life of 330 days. The longest-lived isotope is berkelium-247, with a half-life of 1,380 years. Berkelium-248 is estimated to have a half-life in excess of 300 years, though its exact value is uncertain.

Where is berkelium produced today?

Berkelium is produced primarily at two facilities: the 85-megawatt High Flux Isotope Reactor at Oak Ridge National Laboratory in Tennessee, USA, and the SM-2 loop reactor at the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. Both are dedicated to producing elements with atomic numbers above 96.

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