Robert Hofstadter. Portrait of Robert Hofstadter (1915-90), American physicist. In 1948 Hofstadter invented an improved scintillation counter using sodium iodide activated by thallium. He used linearly accelerated electrons scattered by atomic nuclei to study nuclear structure. He found the nuclear charge density was constant but that it fell off rapidly at the suface of the nucleus. The charge distribution was also found to be related to the nuclear mass. Hofstadter also found that neutrons and protons have size and shape; he concluded that they were made of charged shells but that the charges cancelled out in the neutron. In 1961 he shared the Nobel prize for physics.