The discovery of the neutron
The neutron, which we have already described as one of the nucleons or constituent particles of which the nuclei of atoms are composed, was discovered by James Chadwick in 1933. The story behind it really started in 1930, when two German physicists, W. Bothe and H. Becker, found that when beryllium and certain other light elements were bombarded with ex-particles some very penetrating radiation was produced which could easily pass through a good many centimetres of lead. Irene Curie (daughter of the famous Marie) and her husband Jean Joliot also experimented with the new radiation and found that it caused protons of very high energy to be knocked out of compounds containing hydrogen. They formed the opinion that this radiation was simply gamma radiation of unusually high energy. However, when they made calculations to measure the energies of the supposed gamma photons the results did not agree with the laws of conservation of momentum and energy.
The problem was finally solved by Chadwick, who pointed out that all these
difficulties disappeared if the radiation was regarded as being composed of
uncharged particles instead of gamma photons. The apparatus he used is shown in Fig. 47.13. A plate made of the metal beryllium was bombarded by ex-particles and the new radiation from it allowed to impinge on a plate of paraffin wax. Protons were ejected from the paraffin and detected by means of an ionization chamber. Chadwick measured the energies of the protons and was then able to show that if the process was treated as a case of simple collisions between protons and uncharged particles of the same mass, then all the calculations agreed with the laws of momentum and energy. This newly discovered uncharged particle was given the name of neutron.