Nuclear History · 081
A New Element in a Five-Cent Cigar Box — Glenn Seaborg and the Discovery of Plutonium (1941)
A stormy night in Room 307 of Gilman Hall, half a microgram kept in a cigar box, and a symbol chosen partly as a joke opened a second road to nuclear energy.
On the night of 23 February 1941, in Room 307 of Gilman Hall at Berkeley, Glenn Seaborg's small team identified element 94 for the first time. Five weeks later half a microgram of its isotope plutonium-239, later kept in a cigar box, proved to fission under slow neutrons, opening a second road to nuclear energy: a fuel made from common uranium-238 rather than dug out of the ground.
Half a microgram in a cigar box
The Smithsonian's National Museum of American History in Washington keeps an odd exhibit: a battered wooden cigar box with "5¢" printed inside the lid. Inside sits a metal disc smaller than a fingernail. What is on that disc weighs half a microgram, about one two-millionth of a gram. In the spring of 1941 that invisible smear split the road ahead for nuclear energy in two. [1]
Aiming for the square after Neptune
In 1940 Edwin McMillan and Philip Abelson at Berkeley identified element 93, neptunium. The next square, element 94, became the obvious target. When McMillan was called away to radar research that November, the baton passed to a 28-year-old chemistry instructor, Glenn T. Seaborg (1912–1999).
His team was small: Joseph W. Kennedy (1916–1957), who built the detectors, and the graduate student Arthur C. Wahl (1917–2006). On 14 December 1940 they used Berkeley's 60-inch cyclotron, a circular particle accelerator, to fire deuterons at uranium oxide. A deuteron is the nucleus of heavy hydrogen: one proton bound to one neutron. [1]
A stormy night in Room 307
On paper, uranium-238 hit by a deuteron should become neptunium-238, which beta-decays within a couple of days into element 94. A new alpha activity did appear. But radioactivity alone proves little; the team had to show that its source behaved chemically unlike anything known.
Seaborg later recalled the decisive moment: "On the stormy night of Feb. 23, 1941, in an experiment that ran well into the next morning." The place was Room 307 of Gilman Hall. When Wahl oxidised the sample strongly, the alpha activity, which until then had come down together with a carrier substance, separated from it. Element 94 had its first chemical identity card. [1] [4]
Because the work ran past midnight, some sources date the discovery to 24 February. And the isotope found that night was plutonium-238, not the one that would matter for fission. The team first guessed its half-life at about 50 years; the modern value is 87.7 years.
The question half a microgram answered
The real prize was a different isotope of the same element, plutonium-239. Theory suggested it might split under slow neutrons just as uranium-235 does; in Cambridge, England, the same prediction had been made in 1940.
This time the team irradiated uranium with cyclotron neutrons to make uranium-239, then waited for it to turn into neptunium-239 and finally plutonium-239. The total they collected was that half microgram. On 28 March 1941 Kennedy, Seaborg and Emilio Segrè (1905–1989) exposed the sample to slow neutrons and saw it fission. By May their measurements showed plutonium-239 was about 1.7 times more likely to fission than uranium-235. [2] [3]
Why did that matter so much? Uranium-235 makes up only 0.7% of natural uranium, and because it is chemically identical to uranium-238, separating it meant giant plants sorting atoms by a tiny difference in mass. Plutonium is a different element, so it can be pulled out with ordinary chemistry. Run a uranium reactor, let plentiful uranium-238 turn into plutonium-239, and extract it chemically: a second road, to bombs and to fuel, had opened.
Pluto, and "Pu"
Uranium was named after Uranus, neptunium after Neptune, so the next name belonged to Pluto, discovered in 1930. Seaborg said the team weighed "plutium" against "plutonium" and chose "the name that sounded better."
For the symbol they picked Pu rather than Pl. According to a story often told, this was a deliberate joke on "P-U", the noise English speakers make at a bad smell. Seaborg himself remembered that, once the work was declassified, the reaction was much quieter than they had expected. The name was first needed for a government report in March 1942, and the discovery papers appeared in a journal only in 1946, after the war. [1]
From picograms to kilograms
In April 1942 Seaborg moved to the Metallurgical Laboratory in Chicago to lead plutonium chemistry. Experiments that began with tracer amounts as small as a picogram, a trillionth of a gram, led on 20 August 1942 to the first visible plutonium compound, and on 10 September to the first weighing. In less than five years the United States was producing plutonium by the kilogram in the Hanford reactors. [1] [2]
That plutonium was used in the Trinity test in July 1945 and in the bomb dropped on Nagasaki on 9 August 1945. The same physics also works quietly in today's power reactors. Inside light-water reactor fuel, uranium-238 turns into plutonium-239, which then fissions; by the time fuel is ready to be replaced, plutonium is commonly said to supply about a third of the energy.
Seaborg shared the 1951 Nobel Prize in Chemistry with McMillan, and Room 307 of Gilman Hall became a U.S. National Historic Landmark in 1966. The half microgram in the cigar box was the first proof that nuclear energy could move from a resource you dig up to a fuel you make. [5] [4]