Discovery of the neutron
James Chadwick publishes the first report identifying the neutron
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Original prompt & settings (JSON) ↗The decisive target was ordinary paraffin wax: its hydrogen nuclei recoiled too violently for the proposed gamma rays to make sense.
27 February 1932: “Possible Existence of a Neutron”
Nature published James Chadwick's short communication “Possible Existence of a Neutron” on 27 February 1932, reporting experiments conducted at the Cavendish Laboratory in Cambridge. Chadwick had dated the letter 17 February.
He argued that the penetrating radiation produced when beryllium was bombarded with alpha particles was better explained as neutral particles of approximately proton mass than as extremely energetic gamma rays, and explicitly proposed particles with mass 1 and charge 0.
Two weeks from puzzle to publication
The Joliot-Curies' observation that beryllium radiation could eject very fast protons from paraffin made the prevailing gamma-ray explanation physically implausible to Chadwick, prompting rapid repeat experiments with several target materials.
Chadwick did not observe a neutral particle directly. He inferred its mass and lack of charge from the recoil energies of nuclei struck by the beryllium radiation, moving from the Paris results to a published report in about two weeks.
Who supplied what
The report converted earlier anomalous results by Bothe and Becker and by Irène and Frédéric Joliot-Curie into evidence for a new neutral nuclear particle. The Joliot-Curies supplied the crucial proton-ejection result but interpreted the radiation through a gamma-ray model; Chadwick supplied the neutron interpretation.
Rutherford's earlier “neutron” had generally been conceived as a tightly bound proton-electron combination rather than the independent particle Chadwick's evidence established. Chadwick immediately continued the work for a fuller Royal Society paper, “The Existence of a Neutron,” received on 10 May.
Sources
Researched 23 Aug 2026 6 sources 3 audit passes Date corrected
How this was checked
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Date corrected. The research places this at 1932-02-27.
What the sources leave uncertain
- The discovery has several defensible milestone dates. Chadwick's Nature letter was dated 17 February and published 27 February; his fuller paper, “The Existence of a Neutron,” was received on 10 May and published on 1 June. The listed event gives only 1932, so 27 February is used as the first public report.
- Some timelines use 10 May because that is the receipt date printed on the fuller Royal Society paper, not the first announcement.