The Discovery of the Neutron
In the course of Rutherford’s effort to develop a plausible model of the atomic nucleus, he postulated the existence of a particle having the same mass as the hydrogen nucleus but having no electrical charge whatsoever. He conceived of this neutral particle—essentially a bound-state of a proton and an electron—in order to account for isotopes having identical atomic charges but different atomic weights (e.g. helium-3 and helium-4). Just a few years later, Rutherford’s hypothetical particle was discovered by one of his students.
In the reading selection that follows, Chadwick describes the 1932 discovery for which he is most famous, and for which he was awarded the Hughes Medal from the Royal Society in 1932 and the Nobel Prize for physics in 1935. He begins by describing the work of Bothe and Becker, who had recently detected very penetrating radiation emitted when certain light elements, such as beryllium and boron, were bombarded with α-rays from a polonium source. This radiation was initially thought to be gamma rays.