The neutron before the neutron: Pontremoli' s compound models

Rutherford’s 1920 Bakerian Lecture was the first attempt to advance the existence of the neutron as a proton-electron compound structure, similar to but much smaller than a hydrogen atom. The first attempts to detect the formation of compound neutrons in hydrogen-filled discharge tubes were due to Glasson in 1921. The negative result was not considered definitive because of the general lack of knowledge about the neutron’s properties. Aldo Pontremoli, at that time at the Cavendish Laboratory, was aware of both theoretical and experimental problems concerning the existence of the neutron. Once back to Rome he faced the theoretical analysis of the problem of the neutron’s compound structure in the framework of Bohr’s atomic theory. The smallest permitted stable orbital radius prevented the existence of a compound state with an electron in orbit around a positive nucleus at a much smaller distance. Pontremoli advanced two models. According to the first one, the neutron was a compound system with the electron tangent to the nucleus. Using Silberstein’s relativistic formula of the electromagnetic mass of compound systems, Pontremoli calculated the difference in mass between the neutron and the hydrogen atom. The second model advanced by Pontremoli considered the neutron as an extremely contracted hydrogen molecule with the nuclei in orbit around the two electrons. The consequent modification of the dynamical formulae of the hydrogen molecule made Pontremoli able to confirm the neutron’s nuclear dimensions. This result appeared promising of an experimental study of the spectral lines of the two nuclei’s transitions, for a spectroscopic confirmation of the model. Furthermore, a comparison of the mass defects with the electromagnetic mass due to the close charges proximity would have been a confirmation of the electromagnetic origin of matter.