Who discovered the neutron?

James Chadwick discovered the neutron.

Chadwick announced the discovery in 1932 after showing that a penetrating radiation produced by bombarding beryllium with alpha particles consisted of neutral particles with roughly the mass of protons. Because neutrons carry no electric charge, they could pass through matter and enter atomic nuclei without being repelled by positive nuclear charge.

The work built on experiments by Walther Bothe and Herbert Becker, who had observed the unusual beryllium radiation. Frédéric and Irène Joliot-Curie also found that the radiation could knock protons from paraffin wax, but initially interpreted it as high-energy gamma radiation. Chadwick recognized that neutral particles provided the better explanation and performed decisive measurements at the Cavendish Laboratory.

Chadwick published his first report in Nature in February 1932 and received the Nobel Prize in Physics in 1935. Ernest Rutherford had earlier proposed the idea of a neutral nuclear component, but Chadwick is credited with experimentally discovering the neutron. The discovery transformed nuclear physics and helped make nuclear fission, neutron activation and the production of heavier elements possible.