How have they guessed the existence of neutrons?

Subtle in atomic nuclei, but penetrating when radiated

In the early thirties it became clear that (i) nuclei could not consist of protons and electrons as it was thought at the time, (ii) in beta decay, the electrons emitted from nuclei are born exactly at the moment of the decay, and (iii) in addition to protons, some neutral particles must be present in nuclei. The explanation of the fine structure of the emission spectra of atoms by the presence of nuclear magnetic moments due to their spin resulted in ‘nitrogen catastrophe’. Around the same time, it was found that if high-energy alpha particles strike light chemical elements, unknown radiation is formed. It has been shown that when the radiation falls on a hydrogen-rich compound, high energy protons are formed. At first it was thought that is gamma radiation, but it turned out that it has a much higher penetrating power than any known gamma rays. In 1932, the English physicist James Chadwick conducted a series of experiments, which showed that the gamma ray hypothesis is untenable. He suggested that the unknown radiation consists of uncharged particles with mass close to the mass of a proton, and made a series of experiments to confirm his hypothesis. Those uncharged particles were named neutrons.

 Tri Three quarks bound into a neutron

Whichever way you look at them, experiments showed the existence of heavy neutral particles.

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