Discovery of nuclear fission
Hahn and Strassmann perform the decisive barium-identification experiment
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Original prompt & settings (JSON) ↗The notebook for the decisive test records 15.5 grams of uranium. It is still preserved in the Deutsches Museum archive.
17 December 1938: an indicator experiment in Berlin-Dahlem
At the Kaiser Wilhelm Institute for Chemistry in Berlin-Dahlem, Otto Hahn and Fritz Strassmann ran an indicator experiment on neutron-irradiated uranium on 17 December 1938. Earlier separations had produced an activity resembling radium; this more discriminating test asked whether the substance followed radium or its chemical homologue barium.
Hahn's surviving laboratory notebook carries an entry dated that day, recording 15.5 grams of uranium.
Trusting the chemistry against the physics
The result contradicted expectation: the radioactive product behaved like barium — atomic number 56 — after starting from uranium, atomic number 92. Hahn and Strassmann could not supply the nuclear-physical explanation for how that was possible.
They trusted the separations anyway, and prepared a paper, completed and submitted on 22 December.
A process, not a moment
The commemorative date is defensible because of the dated notebook, but the discovery ran across a week: irradiation began on 16 December, measurements continued for days afterward, and the manuscript went out on the 22nd. Nor was it one man's work — Strassmann performed the decisive chemistry alongside Hahn.
Hahn communicated the puzzling result to Lise Meitner, who had fled Nazi Germany that July, setting up the physical interpretation she and Otto Frisch produced over Christmas.
Sources
Researched 23 Aug 2026 5 sources 3 audit passes Traditional date
How this was checked
Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →
Traditional date. This date is the conventional one, not one fixed by a contemporary record.
What the sources leave uncertain
- The discovery was a process, not a single instantaneous observation. The uranium irradiation began on 16 December, Hahn’s notebook entry is dated 17 December, measurements continued over following days, and the manuscript was submitted on 22 December.
- Calling 17 December the discovery date is defensible because of the dated notebook and commemorative tradition, but it should not imply that Hahn alone or one isolated moment completed the chemical and physical discovery.