What Is Berkelium?

Berkelium is a synthetic radioactive element with atomic number 97 and the chemical symbol Bk, belonging to the actinide series of the periodic table. It does not occur in nature and must be produced artificially through nuclear reactions in dedicated high-flux reactors or particle accelerators. Berkelium sits between curium and californium in the actinide row and shares with its neighbors a propensity for forming +3 and +4 oxidation states in solution. It is studied primarily within nuclear physics and radiochemistry, with research applications that extend to the synthesis of heavier transactinide elements.

The element is named after the city of Berkeley, California, following the convention established for earlier actinides: curium was named for Marie and Pierre Curie, californium for the state and university where it was first produced. The naming reflects the historical concentration of transuranium element discoveries at the Lawrence Berkeley National Laboratory in the 1940s and 1950s.

Discovery and Synthesis

Berkelium was first synthesized in December 1949 by a team led by Glenn T. Seaborg, Albert Ghiorso, and Stanley G. Thompson at the Lawrence Berkeley National Laboratory. The original synthesis bombarded microgram quantities of americium-241 with helium-4 ions (alpha particles) in a cyclotron, producing berkelium-243. Today the primary route to usable quantities is neutron capture irradiation of curium targets in high-flux reactors. The Oak Ridge National Laboratory High Flux Isotope Reactor is one of the few facilities in the world capable of producing berkelium-249, the most practical isotope, in milligram quantities over irradiation periods of many months. After irradiation, the target material is processed through radiochemical separation to isolate the berkelium fraction from other actinide byproducts.

Nuclear and Chemical Properties

Twenty isotopes of berkelium have been characterized, spanning mass numbers from 233 to 253, all radioactive. Berkelium-247 has the longest half-life at approximately 1,380 years, but berkelium-249, with a half-life of 327 days, is the isotope most accessible for chemistry experiments because it is produced in practical quantities. Berkelium metal is silvery in appearance, chemically reactive, and forms an oxide layer on exposure to air. In aqueous solution it displays two main oxidation states: Bk3+, which is stable and dominates most chemistry, and Bk4+, which is a strong oxidizer accessible under specific conditions. The Department of Energy's Office of Science overview of berkelium documents the element's role in the broader actinide chemistry program and the experimental challenges posed by working with intensely radioactive, submilligram quantities.

Role in Transactinide Research

Berkelium-249 serves a critical enabling function in the synthesis of the heaviest known elements, the so-called superheavy elements beyond atomic number 103. Because berkelium targets can be prepared in sufficient quantity at dedicated production facilities, they are used as target material in heavy-ion bombardment experiments. In 2009, a collaboration at the Joint Institute for Nuclear Research in Dubna, Russia, bombarded a berkelium-249 target with calcium-48 ions over a period of 150 days, producing five atoms of tennessine (element 117), which was subsequently confirmed and added to the periodic table. This experiment, described in research published through the Joint Institute for Nuclear Research, demonstrated that berkelium functions as a stepping stone to elements that cannot otherwise be accessed, with value extending well beyond its own chemical characterization.

Applications

Berkelium has applications in a narrow range of specialized scientific disciplines, including:

• Superheavy element synthesis as a target material in accelerator experiments
• Actinide chemistry research to understand bonding in heavy elements
• Nuclear data measurement for reactor physics modeling
• Fundamental nuclear structure studies using radioactive decay spectroscopy