The neutron is an uncharged, subatomic particle \((\sim 10^{-15}\;\textrm{m})\) consisting of an up quark and two down quarks, with a net electric charge equal to zero, located in the nucleus of an atom.

Quark model of the neutron.

As it consists of quarks, it belongs to the family of hadrons and in particular to the group of baryons. Having half-integer spin is a fermion. As regards the strong interactions, the neutron and the proton constitute the two different states of charge of the same particle: the nucleon.

It has a resting mass of \(939.57\;\textrm{MeV/c}^2\), slightly higher than that of the proton, and with the exception of the more common hydrogen isotope (whose atomic nucleus consists of a single proton) the neutron composes the nuclei together with the proton, with which is continually transformed by the emission and absorption of pions.

Outside the nucleus the neutrons are unstable and have a half-life of about 15 minutes. They decay into a proton emitting an electron and an antineutron (beta decay or β-decay).

History and discovery

The discovery of the neutron originated from some experiences of Walther Bothe and Herbert Becker, who in Berlin-Charlottenburg in 1930 observed a penetrating secondary radiation emitted by various light elements (Li, Be, B, F, etc.) bombarded with alpha particles emitted from a sample of polonium, and interpreted as consisting of hard gamma rays, i.e., very penetrating.

Shortly after that, John Chadwick, in Cambridge (Great Britain), experimentally demonstrated that the penetrating radiation in question was capable of transferring energies of the same order of magnitude even to nitrogen nuclei, which are endowed with a mass about 14 times that of the proton. Chadwick immediately realized that all the phenomena observed up to then could be fully explained if one assumed that the penetrating radiation was constituted, at least in part, by a new type of electrically neutral corpuscles with a mass very close to that of the proton.

Chadwick published the results of his experiences and their interpretation in a letter to the scientific journal Nature February 17, 1932, letter that is universally considered as the birth certificate of the neutron.

The fact that the neutrons were emitted by nuclei of light elements under the action of alpha particles suggested a new model of the nucleus. Before the discovery of the neutron, it was thought that the nuclei of all atoms consisted of aggregates of the two elementary corpuscles then known, protons and electrons, in such quantities as to give rise to a system endowed with the right value of electric charge and mass.

However, with the advent of quantum mechanics, a few years earlier, this model had become unacceptable. As a consequence of the uncertainty principle, an electron confined within the dimensions of a nucleus was necessarily equipped with a quantity of motion, and therefore also with kinetic energy, so high as to be incompatible with what was already known then about the energies involved in the nuclei. Thus was born a new model of nucleus consisting only of protons and neutrons (F. Perrin, W. Heisenberg, D. Ivanenko, 1932), widely confirmed by all subsequent experiences.

Two discoveries made by E. Fermi and collaborators, closely linked, gave an extraordinary impulse to the study of nuclear reactions and neutron properties. In January 1934, I. Curie and F. Joliot had discovered that some light elements, in particular, boron and aluminum, bombarded with alpha particles, gave rise to radioactive substances (artificial radioactivity due to alpha particle bombardment). Immediately after the disclosure of these results, Fermi in Rome thought of causing artificial radioactivity using neutrons as projectiles, although these, being corpuscles produced in nuclear reactions initiated by alpha particles, were then only available with very small intensities, of the order of 104-105 times lower than the intensities of the alpha particle sources used by the Joliot-Curie spouses.

Fermi started from the idea that the lack of electric charge of the neutron, thanks to which it is not rejected by a nucleus when it passes in its proximity, could more than compensate for their low number. The discovery of artificial radioactivity by neutron bombardment, made by Fermi in March 1934, was particularly interesting also for the variety and importance of the phenomena discovered in the systematic study, performed in various research centers and in particular in Rome by Fermi and collaborators (E. Amaldi, O. D’Agostino, F. Rasetti, E. Segré). Already during 1934, it was shown that neutrons could produce processes of different types, indicated with the symbols (n, α), (n, p) and (n, γ), where the first and second letters in brackets represent the incident particle and the emitted particle respectively. It was also demonstrated, in Rome, that the last of the processes as mentioned above, often referred to as the radiative neutron capture, can be produced in all chemical elements, from the lightest to the heaviest, such as thorium and uranium.

In later years, many other types of neutron-induced nuclear reactions were discovered in several laboratories. Particularly noteworthy was the discovery made in 1939 in Berlin, by O. Hahn and F. Strassmann, who showed that, under the action of neutrons, heavy elements such as thorium and uranium undergo the phenomenon of fission.