ON 23 FEBRUARY 1941, Glenn Theodore Seaborg and two other researchers at the University of California, Berkeley, produced plutonium for the first time. This was the culmination of an experiment that had begun three months earlier, in which uranium was bombarded by deuterons—the nucleus of a hydrogen isotope with one proton and one neutron—to create a new element. Seaborg named the element after what was then considered the ninth planet in the solar system and gave it the chemical symbol Pu as a joke, because the symbol resembled a child’s reaction to a bad odour.
Seaborg had joined Berkeley as a graduate student in 1934, around the time that Enrico Fermi was leading efforts at the University of Rome to create elements heavier than uranium by bombarding it with neutrons. Fermi was unable to do so, but his work led to the discovery of nuclear fission by the German physicists Otto Hahn and Friedrich Straßmann, in 1939. A year later, Seaborg’s Berkeley colleagues Edwin McMillan and Philip Abelson produced the first transuranium element, which they called neptunium. Seaborg and his fellow instructor Joseph Kennedy, as well as a graduate student, Arthur Wahl, took over the experiment when McMillan left to work on radar technology. They used deuterons instead of neutrons and created an element that was much more stable than neptunium, with a half-life of over twenty-four thousand years. In March 1941, Seaborg found that a plutonium isotope underwent fission when struck with neutrons, releasing a large amount of energy.
Nine months later, the United States entered the Second World War. Seaborg was soon drafted into the Manhattan Project and led the team seeking to create enough fissionable plutonium to make a nuclear weapon. The bomb they created, called Fat Man, was dropped on Nagasaki, on 9 August 1945, killing between forty thousand and seventy thousand people.