Corecipient of the 1961 Nobel Prize in physics, Robert Hofstadter discovered that protons and neutrons are not indivisible particles, as was previously believed, but are complex components of the atom. He also developed the sodium iodide-thallium scintillator, a device for measuring radiation still used in particle accelerators.
Hofstadter was born in New York City on February 5, 1915, the third of four children born to Louis (a salesman) and Henrietta Koenigsberg Hofstadter. He went to public schools, then enrolled at the City College of New York (now the City University of New York). Hofstadter graduated magna cum laude with a B.S. in physics in 1935, and earned the Kenyon Prize for exceptional achievement in mathematics and physics.
He later earned M.A. and Ph.D. degrees from Princeton University, completing his work in 1938 but remaining at Princeton for postdoctoral studies under a Proctor fellowship. During this time, Hofstadter concerned himself with photoconductivity in crystals. In 1939 he took a position as instructor in physics at the University of Pennsylvania, then became Harrison research fellow before returning to the City College of New York, this time as an instructor in physics.
After the beginning of World War II, Hofstadter went to work as a research assistant at the National Bureau of Standards (NBS) in Washington, D.C. There he assisted American physicist James Van Allen in creating a proximity fuse, used to detonate a bomb just before it reached its target. He remained at NBS for a year, then went to Norden Laboratories, home of the Norden bombsight, in New York City. In 1942 he married Nancy Givan, and they had three children, including son Douglas, author of several science-related books, including Gödel, Escher, and Bach.)
After the war was over, Hofstadter went back to Princeton as an assistant professor of physics, and began work on problems in radiation detection. In 1948 he developed the sodium iodide-thallium scintillator, which emitted light whenever radiation passed through it. The intensity of the light varied with the degree of radiation, and the device proved so effective that it remains in use in particle accelerators.
In 1950 Hofstadter became associate professor of physics at Stanford University, where he would remain for the rest of his career. There he began work on the questions that would lead to his Nobel Prize a decade later. The first issue he approached was simply how to look at atoms. Because they are smaller than light rays, atoms cannot be seen even under the most high-powered microscopes. X rays, gamma rays, and high-speed electrons, however, all have wavelengths smaller than that of light. Hofstadter soon discovered that the high-energy electrons in Stanford's linear accelerator (linac) made it possible to view atomic particles.
Thus equipped, he made a number of significant discoveries. Although the nuclei of atoms varied in size, their density was nearly uniform. As he began to look deeper into the structure of the atom, Hofstadter found that protons and neutrons were not solid and indivisible like little ball bearings, but detailed in structure—more like cells. Each proton or neutron, he discovered, contained a dense, positively charged core, surrounded by two shells of mesonic material—one of which was negatively charged in the neutron.
Promoted to a full professorship in 1954, Hofstadter became Max H. Stein professor of physics at Stanford in 1971. He retired in 1985, after which he was made emeritus professor of physics. In addition to the Nobel Prize, which he shared with Rudolf Mössbauer (1929- ), Hofstadter earned the Roentgen Medal (1985), the National Science Medal (1986), and numerous other awards. He died of heart disease on November 17, 1990.