Robert Hofstadter measured the size of the proton

Person: Robert Hofstadter
Lived: 1915–1990
Field: Physics
From: American

The story

Robert Hofstadter, the son of Polish Jewish immigrants in New York, moved to Stanford University in 1950. Stanford was building a new linear electron accelerator, and a colleague, Eugene Feenberg, asked him why not use electrons to study atomic nuclei, as earlier physicists had used them to study atoms. Electrons make ideal probes because they do not feel the strong nuclear force, so the way they scatter off a nucleus depends only on how its electric charge is spread out.

Hofstadter built a series of huge magnetic spectrometers to catch and measure the scattered electrons. From 1953 electron scattering was his main work, and with students and colleagues he measured the size and the soft surface of many nuclei. Between 1954 and 1957 he turned the beam on the proton and the neutron themselves. The proton, long treated as a point, turned out to have a definite size, a radius of a little under a millionth of a billionth of a metre, and an internal distribution of charge and magnetism; the neutron, though neutral overall, also had structure. In a 1956 paper he proposed naming the natural unit for such sizes, ten to the minus fifteen metres, the fermi, after Enrico Fermi.

The work showed for the first time that the building blocks of atomic nuclei are themselves made of something. Hofstadter also helped push for a much larger electron accelerator at Stanford, which became SLAC, and there, in the late 1960s, experiments using the same electron-scattering method found point-like quarks inside the proton. He was delighted when the leaders of that work won the Nobel Prize in 1990, weeks before his death.

Knowing the size and structure of the proton is part of the foundation of modern nuclear and particle physics, the science behind nuclear energy, medical isotopes and our understanding of how matter is built. Hofstadter shared the 1961 Nobel Prize in Physics with Rudolf Mössbauer for his electron-scattering studies of atomic nuclei and the structure of the nucleons.

Why it matters

Hofstadter’s measurements revealed that protons and neutrons have size and structure, a key step toward the quark picture of matter taught in every physics course today.

About Robert Hofstadter

Stanford physicist who invented the sodium iodide gamma-ray detector still used in hospital gamma cameras, and first measured the size and shape of the proton and neutron.

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Robert Hofstadter

1915–1990 · Physics

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