1911 John Ratcliffe and Ernest Rutherford (smoking) at the Cavendish Laboratory. In 1911, Rutherford discovered the atomic nucleus, using a "scattering" experiment, diagramed at right, that would become a classic technique of particle physics. Using a lump of radium as a source, Rutherford sent alpha particles (helium atoms minus their electrons) through a thin sheet of gold foil and observed how the particles scattered off the gold atoms. Most passed through the foil; but, amazingly, a few particles (those that struck the dense, charged gold nuclei) bounced back. Rutherford said he was as surprised as if he had fired a gun at tissue paper and a few of the bullets had come back at him. The open door at left in the photo leads to the laboratory in which James Chadwick discovered the neutron.