Berkelium (Bk, atomic number 97) is a synthetic element and a member of the actinide series. It does not occur naturally on Earth and is produced only in specialized nuclear facilities. Berkelium is intensely radioactive, making it exceedingly rare and challenging to study. It was first created in 1949 at the University of California, Berkeley, and samples exist only in minute, man-made quantities for scientific research.
The Physical Look of Berkelium
In its pure metallic form, berkelium is a soft, silvery-white metal with a distinct metallic luster. At room temperature, berkelium metal assumes a double hexagonal close-packed (dhcp) crystal structure.
When berkelium is dissolved or forms a chemical compound, its appearance changes dramatically based on its oxidation state. The most stable ionic form, berkelium(III) (\(\text{Bk}^{3+}\)), typically appears green in aqueous solutions. In contrast, the tetravalent ion, berkelium(IV) (\(\text{Bk}^{4+}\)), exhibits a yellow or orange-yellow color when dissolved in certain acids. Berkelium compounds display a variety of colors, such as the yellow-green of berkelium(III) oxide (\(\text{Bk}_{2}\text{O}_{3}\)) or the brown color of berkelium(IV) oxide (\(\text{BkO}_{2}\)).
Why Berkelium is Hard to See
The short lifespan and intense radioactivity of berkelium create significant barriers to its observation outside of controlled environments. The most readily available isotope, Berkelium-249, has a half-life of only about 330 days. Half of any sample transforms in less than a year into Californium-249, a much stronger alpha-particle emitter, making the original sample progressively more hazardous.
The continuous stream of high-energy particles emitted by the decaying atoms causes “self-irradiation.” This internal bombardment damages the element’s crystal lattice, causing its physical structure to swell and fatigue over time. This complicates precise measurements of its solid-state properties. Handling berkelium requires highly specialized facilities, such as shielded hot cells and inert atmosphere gloveboxes, to protect researchers from the intense radiation and prevent the material from reacting with air and moisture.
Creating the Element: Synthesis and Scarcity
Berkelium is produced in only two high-flux nuclear reactors globally, primarily at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Synthesis begins by subjecting targets of Curium-248 to a prolonged, intense flux of neutrons. This process involves a series of neutron capture events and subsequent beta decays, gradually building up the atomic mass until Berkelium-249 is formed.
After irradiation, the target material requires a cooling period lasting several months before chemical separation begins. The challenge is isolating the minute quantity of berkelium from the curium target and dozens of other radioactive byproducts. Scientists utilize solvent extraction and ion-exchange chromatography, often exploiting berkelium’s unique ability to be oxidized to the tetravalent \(\text{Bk}^{4+}\) state. This separation technique isolates it from the trivalent state of most other actinides. The entire process, from irradiation to final purification, takes approximately one year and yields only milligram quantities.
Primary Scientific Uses
Berkelium plays a distinct role in heavy element research. Its primary use is as a target material for synthesizing even heavier, artificially created elements, rather than for any practical application. The most notable example is the discovery of Tennessine (element 117), achieved by bombarding a Berkelium-249 target with a beam of Calcium-48 ions.
A batch of just 22 milligrams of Berkelium-249 was prepared at ORNL and shipped to Russia to complete this experiment. Beyond superheavy element synthesis, researchers study berkelium to understand the chemical behavior of the actinide series. Examining berkelium’s properties helps scientists refine theoretical models for the entire group, which is crucial for managing nuclear waste and developing new nuclear technologies.