1940

Glenn T. Seaborg

Deuterons on uranium produce element 94 at Berkeley

On 14 December 1940 a deuteron beam struck a uranium target in Berkeley's 60-inch cyclotron, and what grew in it afterwards was an alpha activity nobody could yet name.

December 1940: 16 MeV deuterons on uranium

Glenn Seaborg, Joseph Kennedy and Arthur Wahl bombarded uranium with 16 MeV deuterons in Berkeley's 60-inch cyclotron on 14 December 1940. The neptunium-238 it produced beta-decayed into plutonium-238, and the alpha activity growing in the target was what the three suspected was element 94.

Edwin McMillan's discovery of neptunium with Philip Abelson had implied that an element with atomic number 94 should exist. McMillan left Berkeley for radar work in November 1940, having given Seaborg his assent to continue the search, and stated that he did not participate in the final chemical proof.

A suspicion was all it was. Producing the material settled nothing about whether it was a new element, and the question stayed open for more than two months.

The confirmation, on the night of 23–24 February 1941

During the night of 23–24 February 1941, Wahl carried out an oxidation experiment in Room 307 of Gilman Hall at the University of California, Berkeley. It separated the alpha-emitting material from neptunium and supplied chemical proof that it was a new element.

February 23 is the conventional anniversary, the one the American Chemical Society marks, while the Department of Energy sometimes lists February 24 for the same experiment. Sources also use "synthesized," "produced," "identified" and "discovered" interchangeably. The two dates are not rivals: the material was produced in December 1940 and chemically identified on the night of 23–24 February 1941.

An element studied without being seen

What the Berkeley group had was tracer quantities, followed through their radioactivity and chemical behaviour rather than looked at. Even half a microgram mixed with carrier material counted as a huge sample.

The element was named plutonium after Pluto, extending the sequence uranium–Uranus and neptunium–Neptune.

A visible amount was not isolated until 20 August 1942, and plutonium was first weighed on 10 September 1942.

Sources

Researched 23 Aug 2026 7 sources 2 audit passes, 2 issues open Traditional date

How this was checked

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Traditional date. This date is the conventional one, not one fixed by a contemporary record.

What the sources leave uncertain

December 14, 1940 is the date of the team’s first uranium bombardment, not the generally accepted date of plutonium’s conclusive chemical identification.
The decisive identification occurred during the night of February 23–24, 1941. February 23 is the conventional anniversary used by the American Chemical Society, while the Department of Energy sometimes lists February 24.
Some sources use “synthesized,” “produced,” “identified” and “discovered” interchangeably. The archival Berkeley description calls the February experiment the first synthesis and identification; Seaborg’s oral history distinguishes it from the December 1940 bombardment that began the production sequence.