Discovery of the Neutron (Chadwick)
In February 1932 James Chadwick showed that a penetrating radiation from beryllium is a neutral particle about as heavy as the proton: the neutron, the missing building block of the nucleus.
Open in the interactive tree →Bothe and Becker (1930) and Irène and Frédéric Joliot-Curie (1932) had hit beryllium with alpha particles from polonium and found a radiation that knocked protons out of paraffin wax; they took it for gamma rays. Chadwick's letter to Nature (dated 27 February 1932) showed that only a neutral particle of about the proton's mass balances energy and momentum. With no electric charge, neutrons slip into nuclei: Fermi's slowed neutrons made many elements radioactive from 1934, and uranium hit by neutrons led to the discovery of fission in December 1938 and to chain reactions. Chadwick received the 1935 Nobel Prize in Physics.
Prerequisites
- Radioactivity1896Alpha particles from radioactive polonium were the projectiles
- Nucleus & Bohr Atom1911The neutron completed Rutherford's picture of the nucleus
Unlocks
- Nuclear Physics1938The neutron made the discovery of fission and chain reactions possible
- First Nuclear Reactor (CP-1)1942A chain reaction is carried by neutrons