Since neutrons have no charge, detecting them is difficult. For this reason, neutrons were discovered relatively late, although Rutherford had postulated their existence in 1920.

In 1930, during the study of the energy levels of nuclei, Bothe and Becker irra­diated beryllium with alpha particles and observed the emission of radiation with a very long range and high energy (5 MeV). Since the energy levels of nuclei were studied, the emitted radiation was supposed to be gamma radiation of beryllium. This experiment was repeated in 1932 by Irene Curie and Frederic Joliot-Curie; however, they detected the emitted radiation by other techniques and found that the energy of the radiation was much higher than what was given by Bothe and Becker. Similarly, Chadwick measured energy readings that were as high as 50 MeV. As a consequence, Chadwick stated that if the energy of the “gamma” radiation of the beryllium depends on the detection method, it cannot be “gamma” radiation, or at least another particle must be emitted besides gamma radiation. In addition, Chadwick postulated that the long-range radiation should consist of neu­tral particles that transfer energy only by colliding with nuclei. These particles were called “neutrons.”

Chadwick measured the rest masses of the neutrons by elastic collision with hydrogen and nitrogen nuclei and found that the ratio of the neutron to the hydro­gen nucleus (proton) is about 1.1:1.