Tracing Plutonium's Roots

IN THE ARTICLE "Antique Plutonium," the following statement was made: "The oldest sample of purified plutonium is housed at the Smithsonian Institution, in Washington, D.C. It was made in the early 1940s by Glenn Seaborg and Art Wahl at the University of California, Berkeley" (C&EN Online Latest News, Jan. 29).

The initial 1940 Berkeley, Calif., transuranium studies from bombarding uranium with deuterons indicated the probable existence of a new alpha-emitting element 94 (later named "plutonium"), produced from beta-decay of neptunium. The first successful chemical isolation and proven discovery of element 94 (as ²³⁸Pu from ²³⁸Np decay) was carried out Feb. 23–25, 1941, by Arthur C. Wahl, who was Seaborg's graduate student. However, this ²³⁸Pu material was in trace quantity.

The plutonium sample housed in the Smithsonian Institution was separated from 1.2 kg of uranyl nitrate hexahydrate (UO₂(NO₃)2·6H₂O, UNH), irradiated in late-February 1941, with slow neutrons produced from a deuteron beam on a beryllium target at the Berkeley 60-inch cyclotron. Neptunium-239 (t_1/2 = 2.4 days) was separated from that irradiated material in early March and allowed to beta-decay to produce (based on ²³⁹Np activity) 0.5 µg of plutonium-239. On May 12, 1941, Wahl repurified the ²³⁹Pu and coprecipitated it with 0.2 mg of a lanthanide carrier. This plutonium (called "Sample B") was later used to first determine that the thermal neutron fission cross-section of ²³⁹Pu was 1.7 times that of ²³⁵U (repositories.cdlib.org/lbnl/PUB-97).

After being used for various characterization measurements, the plutonium was stored in a cigar box originally belonging to Seaborg's cigar-smoking former laboratory supervisor, University of California chemistry professor G. N. Lewis. A note taped to the inside of the lid is dated "7-13-41" and includes some data on Sample B and a "Sample F," prepared by Wahl from another irradiation.

After 25 years, the cigar box was rediscovered during routine housecleaning operations, and on March 28, 1966, Seaborg and Emilio Segrè presented the Smithsonian Institution with the oldest known collection of ²³⁹Pu in the world—the cigar box containing the 0.5-μg plutonium Sample B, prepared on May 12, 1941, by Wahl.

Incidentally, the second oldest known collection of Pu-239 is the carrier-free 2.77-μg Pu oxide prepared on Sept. 9, 1942, by Burris Cunningham and Lewis Werner and weighed on their own constructed microbalance on Sept. 10 at the University of Chicago's wartime Metallurgical Laboratory. This preserved 1942 sample is in the Lawrence Hall of Science at UC Berkeley.

Richard G. Strickert
Austin, Texas