Neptunium

By 1940, element 93's discovery was not a matter of if but when. Every component needed had converged at Berkeley: Ernest Lawrence's cyclotron provided the particle beam, the 1938 fission discovery cleared away confusion, and Edwin McMillan possessed both the machine and the moment.

The foundation began in 1932 when James Chadwick discovered the neutron, giving physicists a projectile penetrating atomic nuclei without being repelled by positive charge. Within two years, Enrico Fermi's Rome group was bombarding uranium with neutrons, convinced they had created elements 93 and 94. Fermi won the 1938 Nobel Prize, but he was wrong—the products were fission fragments.

The confusion persisted until December 1938 when Otto Hahn and Fritz Strassmann found barium among uranium's bombardment products. Lise Meitner and Otto Frisch realized the nucleus was splitting, naming it "fission." This clarified the path forward: now physicists knew what to look for—products that weren't fission fragments, with half-lives consistent with beta decay, chemically resembling uranium's periodic table neighbors.

Berkeley's Radiation Laboratory was nuclear physics' epicenter by 1939. Lawrence had invented the cyclotron in 1930; by 1939, the 60-inch cyclotron began operations with its 220-ton magnet producing intense neutron beams. McMillan subjected uranium to bombardment in early 1940, detecting two radioactive products: one with 23-minute half-life (uranium-239), another with 2.3 days. In spring 1940, Philip Abelson joined him. Using chemical separation, they demonstrated the 2.3-day product resulted from U-239's beta decay—element 93.

They announced the discovery June 8, 1940, naming it neptunium. Its significance was the cascade: Glenn Seaborg correctly deduced neptunium must decay into element 94, plutonium. By December 1940, Seaborg's team had synthesized plutonium-238. This pathway—uranium to neptunium to plutonium—became the Manhattan Project's route to the bomb.