Discovery of the neutron

In 1932, James Chadwick identified the neutron as a neutral subatomic particle after showing that radiation from alpha-bombarded beryllium could eject protons from paraffin.

Beryllium alpha-particle experiment: An experiment in which alpha particles striking beryllium produced penetrating radiation that could eject protons from paraffin. Chadwick used this setup to infer that the radiation consisted of neutral particles.
Ernest Rutherford: A New Zealand-born physicist who established the nuclear model of the atom and proposed a neutral nuclear particle. His 1920 proposal framed the search for a constituent combining nuclear mass with no charge.
James Chadwick: An English physicist who discovered the neutron in 1932 and received the 1935 Nobel Prize in Physics. He combined recoil measurements with conservation laws to identify the neutral particle.
Nuclear physics: The branch of physics that studies atomic nuclei, their constituents, and their interactions. The neutron became a central constituent and experimental probe in the field.
Gamma ray: A high-energy photon emitted in nuclear or particle processes, with no electric charge and no rest mass. Gamma rays were the leading alternative explanation for beryllium’s penetrating radiation.
Alpha particle: A helium-4 nucleus consisting of two protons and two neutrons, emitted in some radioactive decays. Alpha particles supplied the bombardment that generated the puzzling beryllium radiation.
Rutherford model: An atomic model in which a small, positively charged nucleus is surrounded by electrons. The model left nuclear mass unexplained without additional neutral material.
Frédéric Joliot-Curie: A French physicist who, with Irene Joliot-Curie, investigated radiation from alpha-bombarded elements. Their proton-recoil observations pointed toward a massive neutral projectile.
Atomic nucleus: The compact central region of an atom, containing protons and usually neutrons. The discovery established neutrons as a second kind of nuclear constituent.
Proton: A positively charged subatomic particle found in atomic nuclei. Unlike the neutron inferred by Chadwick, a proton carries electric charge.

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