February 27, 1940

Carbon-14

Kamen and Ruben identify long-lived carbon-14 at Berkeley

The sample's faint radioactivity persisted instead of vanishing in minutes — the signal that carbon had a usable long-lived isotope at last.

27 February 1940: the count that did not fade

At the University of California Radiation Laboratory in Berkeley on 27 February 1940, Martin Kamen and Samuel Ruben concluded through repeated chemical separation and radioactivity measurements that irradiated graphite contained the long-lived isotope carbon-14.

Kamen had bombarded a graphite target with deuterons in Berkeley's cyclotrons; the intended reaction converted carbon-13 into carbon-14. Ruben performed the chemical separations and measurements with him.

Their first approximate half-life for the isotope was about 4,000 years.

Why 21 minutes was not enough

The two men needed a carbon tracer that survived far longer than carbon-11's 21-minute half-life, so that they could follow carbon through photosynthesis and other biological reactions.

Their result established that carbon possessed a radioisotope sufficiently long-lived to serve as a practical tracer.

The immediate application was biochemical tracing, not archaeological dating.

Submitted within two days

Ruben and Kamen submitted their short report, "Radioactive Carbon of Long Half-Life," on 29 February; Physical Review published it on 15 March 1940.

The discovery was announced two days after the identification, during Ernest Lawrence's Berkeley Nobel Prize ceremony.

Willard Libby, whose name is attached to radiocarbon dating, took no part in the 27 February work; he developed the dating method at the University of Chicago after the war.

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