As a free particle, a neutron will lose its identity in about 12 minutes. Its predominant decay products are a proton, an electron, and an antineutrino of the electron. Its rest mass is slightly greater than that of the proton. Neutrons are found in all atoms except normal hydrogen. The two relatively abundant abnormal isotopes of hydrogen are deuterium and tritium. Two atoms of deuterium plus one atom of oxygen made up a molecule of heavy water. The nucleus of a deuterium atom is called a deuteron, composed of one proton and one neutron. The idea of a neutron was proposed by Rutherford in his Bakerian lecture of 1920. One of his student research assistants, the 1935 physics Nobel laureate, Chadwick was somewhat obsessed in finding a physical proof of neutron. Chadwick kept looking for a period of 12 years while his actual discovery took only a few days of hard labor.

The turning point was a key paper communicated by the husband and wife research team Joliot-Curies in January 28, 1932. After Chadwick and Rutherford read it, both were shocked by its serendipitous implication. Chadwick immediately went to work and on February 17, 1932 submitted a paper entitled ‘possible existence of neutron’ in which he proposed that the alpha-beryllium reaction is ��+����⁹→��¹²+�� and supported it by two distinct arguments: (1) From a comparison of recoil velocities produced by the radiation when it traverses a vessel filled either with hydrogen or with nitrogen he found that the radiation consists of particles with close to protonic mass. (2) Suppose nevertheless that the radiation consists of photons. Then the process would be ��+����⁹→��¹³+��. If so, then from known mass defects Chadwick estimated that the ��’s could have energy of at most 14 MeV – not enough. So he concluded, ‘all the evidence is in favor of the neutron [unless] the conservation law of energy and momentum were relinquished at some point’. At any other time this final caveat might have seemed excessive. But this was February, 1932, and the debate about the validity of the conservation laws in certain nuclear processes was still very much alive. Reference: Abraham Pais, Inward Bound, Chapter 17, Oxford University Press, Oxford, 1986.