In 1956, Hofstadter did a scattering experiment to explore the range of the nucleon (the proton or the neutron). He used an electron beam with a momentum of 188 MeV/c. He could then explore distances down to 10^-14 m. Hofstadter showed that the proton had a radius of (0.7 ± 0.3)x10^-15 m, it was not pointlike nor elementary.

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