Il "protone neutro". Ovvero della laboriosa esclusione degli elettroni dal nucleo

Abstract

The coming into light of the neutron is discussed. It is remarked that some experiments had already suggested that the penetrating radiation from beryllium had an electromagnetic component, before Joliot-Curies suggested the beryllium radiation consisted of high-energy gamma rays. Joliot-Curies' idea also relied on the many Compton electron tracks in the Wilson chamber. In 1920, both Harkins and Rutherford foresaw the existence of a particle of mass 1 and zero charge, consisting of one proton and one electron, though they did not give any name to it. In 1932, the neutron was finally observed by Chadwick. However, the question whether considering the neutron as a fundamental or a compound particle still bothered the physicists and Chadwick himself for more than one year. Instead, as soon as Majorana heard about Joliot-Curies' experiments he guessed the existence of a "neutral proton." In 1933 he published some "corrections" to Heisenberg's theory. A very simple analysis shows that Heisenberg's original version of exchange interactions and Majorana's version clearly reflect, respectively, that the nucleus contained "electrons" and that the nucleus was free from electrons. The emission of electrons from the nucleus still needed to be accounted for. Fermi's theory of beta-decay settled the question. Thanks to exchange interactions and beta-decay theory, quantum mechanics could now be applied to the nucleus, which could be considered free from electrons. Also the neutron could be considered free from electrons, and treated as a fundamental particle.