Sample of Plutonium-239

The discovery of nuclear fission in uranium, announced in 1939, allowed physicists to advance with confidence in the project of creating "trans-uranic" elements - artificial ones that would lie in the periodic table beyond uranium, the last and heaviest nucleus known in nature. The technique was simply to bombard uranium with neutrons. Some of the uranium nuclei would undergo fission, newly understood phenomenon, and split violently into two pieces. In other cases, however, a uranium-238 nucleus (atomic number 92) would quietly absorb a neutron, becoming a nucleus of uranium-239, which in turn would soon give off a beta-particle and become what is now called neptunium-239 (atomic number 93). After another beta decay it would become Element 94 (now plutonium-239)
By the end of 1940, theoretical physicists had predicted that this last substance, like uranium, would undergo fission, and therefore might be used to make a nuclear reactor or bomb. Enrico Fermi asked Emilio Segre to use the powerful new 60-inch cyclotron at the University of California at Berkeley to bombard uranium with slow neutrons and create enough plutonium-239 to test it for fission. Segre teamed up with Glenn T. Seaborg, Joseph W. Kennedy, and Arthur C. Wahl in January 1941 and set to work.
They carried out the initial bombardment on March 3-6, then, using careful chemical techniques, isolated the tiny amount (half a microgram) of plutonium generated. They put it on a platinum disc, called "Sample A," and on March 28 bombarded it with slow neutrons to test for fission. As expected, it proved to be fissionable - even more than U-235. To allow for more accurate measurements, they purified Sample A and deposited it on another platinum disc, forming the "Sample B" here preserved. Measurements taken with it were reported in a paper submitted to the Physical Review on May 29, 1941, but kept secret until 1946. (The card in the lid of the box bears notes from a couple of months later.)
After the summer of 1941, this particular sample was put away and almost forgotten, but the research that began with it took off in a big way. Crash programs for the production and purification of plutonium began at Berkeley and Chicago, reactors to make plutonium were built at Hanford, Washington, and by 1945 the Manhattan Project had designed and built a plutonium atomic bomb. The first one was tested on July 16, 1945 in the world's first nuclear explosion, and the next was used in earnest over Nagasaki. (The Hiroshima bomb used U-235.)
Why is our plutonium sample in a cigar box? G.N. Lewis, a Berkeley chemist, was a great cigar smoker, and Seaborg, his assistant, made it a habit to grab his boxes as they became empty, to use for storing things. In this case, it was no doubt important to keep the plutonium undisturbed and uncontaminated, on the one hand, but also, on the other hand, to make it possible for its weak radiations to pass directly into instruments - not through the wall of some closed container. Such considerations, combined probably with an awareness of the historic importance of the sample, brought about the storage arrangement we see.
Object Name
chemical sample, plutonium 239, first isolated
Date made
Associated Date
Segre, Emilio
Seaborg, Glenn T.
Kennedy, Joseph W.
Wahl, Arthur C.
Lewis, G. N.
University of California, Berkeley
Segre, Emilio
Seaborg, Glenn
Physical Description
metal (overall material)
platinum (plate material)
wood (cigar box material)
box (closed): 90 cm x 19 cm x 15.9 cm; 35 7/16 in x 7 1/2 in x 6 1/4 in
disc: 1.5 cm; 9/16 in
Place Made
United States: California, Berkeley, Lawrence Berkeley National Laboratory
United States: California, Berkeley
United States: California
United States: Illinois, Chicago
United States: Illinois
United States: Washington, Hanford
United States: Washington
ID Number
catalog number
accession number
See more items in
Medicine and Science: Modern Physics
Science & Mathematics
National Treasures exhibit
Energy & Power
American Stories
Exhibition Location
National Museum of American History
Data Source
National Museum of American History, Kenneth E. Behring Center


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