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Enrico Fermi ’ s impact on science

E N R I C O FE R M I I S the father of modern nuclear physics and a physicist’s physicist, whose legacy was one of style as well of substance—a style so attractive and so productive for science that it became substantive in itself. In his 1938 Nobel Prize speech, Fermi acknowledged the grandfather of this field, Lord Rutherford, who a generation earlier had begun bombarding substances with alpha particles, the positively charged nuclei of helium atoms that shoot out from uranium, radium, and other heavy elements. Those early experiments bore much fruit for physics, including today’s atomic model of a tiny massive nucleus orbited by electrically bound lightweight electrons. But while they elucidated the overall structure of the atom, Rutherford’s experiments did not penetrate the mysteries of the nucleus itself. Fermi realized that the electrostatic repulsion on the positive charge of Rutherford’s bombarding alpha particles prevented them from entering the positively charged nucleus. He perceived that the neutron, discovered by James Chadwick in 1932, having no charge at all, would be the ideal probe for nuclear studies. Starting in 1933, Fermi and his students conducted systematic studies of the effect of neutron irradiation of the chemical elements. This was the beginning of our knowledge of nuclear matter. Neutrons were produced for these studies by the collisions between alpha particles from a radioactive source (an alpha emitter) and a light element such as beryllium. The two elements were mixed together in pellets to form compact “neutron guns.” Later beryllium targets were bombarded by deuterium ions energized in an accelerator. The most modern sources produce neutrons in a similar way, by directing a high-energy proton beam onto a target, dislodging a spray of neutrons in what is called a spallation reaction. The Department of Energy’s huge Spallation Neutron Source, now under construction at Oak Ridge, Tenn., is designed to provide neutrons for thousands of scientists whose work follows in a direct line from that of Fermi and his many students.