"Atomic element 94 was named "plutonium" after Pluto, the ninth planet from the Sun (now demoted to "minor planet" status.) By tradition, plutonium should have been assigned the symbol "Pl," but co-discoverer Glenn Seaborg gave it the symbol "Pu" as in "pee-yew," dark humor that reflected the element's potential use.
Since plutonium is not found in nature, it had to be created in the lab. This was accomplished in 1941 at the Berkeley Radiological Laboratory by Seaborg, Edwin McMillan, J.W. Kennedy, and Andrew Wahl, using the 60-inch cyclotron there.
Now the group hoped to find a particular isotope of plutonium, Pu-239. Like other forms of plutonium, it was expected to sustain fission, a process that splits atomic nuclei in a chain reaction that releases tremendous amounts of energy. And there was reason to think it could be isolated in the quantities needed to build a nuclear weapon.
They would begin by producing neptunium-239, another human-made element that had been discovered at the lab just a year before. The plan was to chemically purify it and allow it to decay into plutonium-239.
These pages from Seaborg's notebook contain instructions that the scientists checked off as they went along. It was a laborious, time-consuming process that required carrying samples from lab to lab on campus. To minimize radiation exposure, researchers wore lead gloves and carried the material in lead beakers inside wooden boxes.
On Thursday, March 6, 1941, a purified sample of neptunium-239 was placed in a lab dish and covered with a thin layer of Duco cement. "This is called sample A," Seaborg wrote, triumphantly, in red. The team documented its transformation, through natural radioactive decay, into Pu-239, and they determined that it underwent fission much faster that uranium does.
In May 1942, Seaborg, Kennedy, Wahl, and collaborator Emilio Segr� sent a letter describing the work on Pu-239 to Physical Review, where it was held for publication until after the war.
On Aug. 9, 1945, a nuclear bomb powered by Pu-239 exploded over Nagasaki."
Symmetry, Volume 4, Issue 6, August 2007