Discovery of Neutrons
Walther Bothe and Herbert Becker described in 1930 an unusual type of a "gamma radiation", which occurred, when they irradiated metallic beryllium with alpha-particles (helium nuclei). James Chadwick recognized that the properties of this type of radiation were similar to a radiation of neutral particles, which were predicted 12 years before by Ernest Rutherford. This radiation was assumed to correspond to an uncharged nuclear constituent. When finally Irène Joliot-Curie (the daughter of Marie and Pierre Curie) and her husband Frédéric Joliot-Curie claimed that the "gamma radiation" of Bothe was able to knock out protons with a high energy out of paraffin, it became clear to James Chadwick that only "neutrons" with a mass comparable to protons would be able to do so.
In the following experiments in the year 1932, Chadwick succeeded to show that the radiation from the irradiated beryllium was indeed the assumed uncharged nuclear particle, which was finally called the neutron. This discovery was essential for completing the description of the atom and its nucleus: the atomic nucleus consists of protons and neutrons and is surrounded by an electron cloud. In an electrically neutral atom the number of the negatively charged electrons in the electron cloud is exactly equal to the number of positively charged protons in the atomic nucleus. The number of neutrons in the nucleus, in contrast, can vary freely within certain ranges, giving rise to different isotopes of the same element (with given charge z, the number of protons).
For the discovery of neutrons James Chadwick was honored in 1935 with the Nobel Prize in physics. Thus, the development of nuclear physics was pushed forward and led in the year 1938 to the discovery of neutron induced fission of an atomic nucleus by Otto Hahn and Fritz Strassmann. In 1945 Chadwick was knighted by The British Queen.
Texts: Th. Wilpert , Ch. Schulz
