A Manhattan childhood
Robert Hofstadter was born in Manhattan on February 5, 1915, to Louis Hofstadter, who ran a cigar store and worked as a salesman, and Henrietta Koenigsberg. Both of his parents were Jewish immigrants from Poland. He went to public elementary and high schools in New York City, at a time when the city’s free schools and colleges were carrying a generation of immigrant children into the professions.
Like many of them, he went on to the tuition-free City College of New York. There he was inspired by the physicists Irving Lowen and Mark Zemansky, and he graduated in 1935, aged 20, magna cum laude, winning the college’s Kenyon Prize in mathematics and physics.
Princeton and the war years
A Coffin fellowship from the General Electric Company took him to graduate school at Princeton University. He studied quantum mechanics with Eugene Wigner and worked under Edward Condon, and by the age of 23 he had both a master’s degree and a doctorate. His thesis used infrared light to probe simple organic molecules and helped clarify the nature of the hydrogen bond, work that Linus Pauling later cited.
As a postdoctoral fellow he studied the electrical behaviour of crystals that conduct when light falls on them, discovering, with Robert Herman, effects that revealed trapped electrons inside the crystals. That interest in how crystals respond to radiation would prove decisive. In 1939 he went to the University of Pennsylvania, where he helped build a Van de Graaff accelerator for nuclear research and met the theorist Leonard Schiff, who became a lifelong friend and colleague.
During the Second World War he worked first at the National Bureau of Standards, on the optical proximity fuse, and then at the Norden Laboratory Corporation, on a radar altimeter. In 1946 he returned to Princeton as an assistant professor.
A crystal that sees gamma rays
Back at Princeton, Hofstadter set out to build better detectors for gamma rays, the high-energy light given off by radioactive atoms. Physicists already knew that certain organic crystals flash when struck by radiation, and that a light-sensitive tube can count the flashes; Hartmut Kallmann in Germany had shown it with naphthalene. Hofstadter, drawing on his knowledge of solid crystals, tried inorganic salts instead.
In 1948 he found that sodium iodide, with a trace of thallium added, made an excellent scintillation counter, and he filed a patent on it that year. The crystal is transparent to its own light, it is dense, and iodine is a heavy atom that stops gamma rays efficiently. In 1950, with J. A. McIntyre, he showed that well-formed crystals could do more than count: the brightness of each flash measured the energy of the gamma ray, so the crystal could serve as a spectrometer, telling one radioactive substance from another.
Thallium-activated sodium iodide became, and remains, one of the most widely used radiation detectors in the world. It is the heart of the gamma camera used in nuclear medicine, in which a large flat crystal of the material records the gamma rays from a radioactive tracer given to a patient. Hospitals use these cameras for heart stress tests, lung scans and SPECT imaging, which builds three-dimensional pictures of organs. When Hofstadter received the National Medal of Science in 1986, his citation singled out the sodium iodide counter and its use in medical imaging.
Measuring the proton
In 1950 Hofstadter moved to Stanford University as an associate professor. Stanford was building a new linear electron accelerator, the Mark III, designed by W. W. Hansen, and a colleague, Eugene Feenberg, asked him a simple question: why not use electrons to study nuclei, the way earlier physicists had used them to study atoms? Electrons are ideal probes. They are not affected by the strong nuclear force, so the way they bounce 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, and from 1953 electron scattering became his main work. With his students and colleagues he measured the size and the soft, fuzzy surface of many atomic nuclei. Then, between 1954 and 1957, he turned the beam on the simplest nuclei of all, the proton and the neutron.
The results were startling. The proton, long treated as a point, turned out to have a definite size, with a radius of a little less than a millionth of a billionth of a metre, and an internal structure of charge and magnetism. The neutron, though electrically neutral overall, also had a structure. In a 1956 paper he named 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 nuclei were themselves made of something, and it pointed the way to the discovery of quarks.
From the proton to the quarks
In 1954 Hofstadter was among those who proposed a much larger linear accelerator at Stanford, of several billion electron volts, an idea that grew into the Stanford Linear Accelerator Center, SLAC. There, in the late 1960s, experiments using the same electron-scattering method revealed point-like particles inside the proton, the quarks. Hofstadter was delighted when the physicists who led that work, Jerome Friedman, Henry Kendall and Richard Taylor, received the Nobel Prize in 1990, a few weeks before his own death.
His own Nobel Prize in Physics came in 1961, shared with Rudolf Mössbauer, for his pioneering studies of electron scattering in atomic nuclei and the structure of the nucleons that it revealed.
Detectors for medicine and the sky
Hofstadter never lost his love of detectors. In later years he developed large total-absorption detectors made of many sodium iodide crystals, including the Crystal Ball, used at Stanford’s SPEAR collider to study new particles. With Edward Rubenstein and Barrie Hughes he worked on a method of imaging blood vessels with intense X-rays from a synchrotron, aiming at safer angiography. And he was a principal investigator for EGRET, a gamma-ray telescope that NASA launched on the Compton Gamma Ray Observatory on April 5, 1991, a few months after he died. It used scintillator technology to map gamma rays from across the universe.
He was also a dedicated teacher who gave large freshman classes vivid demonstrations, and he held strong views: he valued small research groups and academic freedom, and he was wary of the growth of big, heavily funded science. He was elected to the National Academy of Sciences in 1958 and to the Institute of Medicine in 1984.
Jewish roots
Hofstadter’s Jewish heritage came from both parents, Polish Jewish immigrants who settled in New York. His path, from immigrant home through the city’s public schools and City College to the leading laboratories of the country, was shared by many Jewish scientists of his generation, at a time when private universities still limited the number of Jewish students they admitted. He is listed among Jewish Nobel laureates in physics by the Jewish Virtual Library.
The record says little about his religious practice, and he did not write publicly about it. His identity, as far as the sources show, was rooted in family and in the New York Jewish immigrant world in which he grew up.
Family and later years
On May 9, 1942, Hofstadter married Nancy Givan of Baltimore; they had met through a shared love of jazz. They had three children: Douglas, who became a cognitive scientist and won the Pulitzer Prize for his book Gödel, Escher, Bach; Laura, who wrote on medicine; and Mary, known as Molly. In the early 1960s the couple bought a large ranch near Red Bluff in northern California, where they raised horses, cattle and olives. He was a devoted fan of Stanford’s basketball and football teams.
He taught at Stanford from 1950 until 1985, and the university still holds an annual Robert Hofstadter Memorial Lecture in his honour. Robert Hofstadter died at Stanford on November 17, 1990, at the age of 75.
Legacy
Hofstadter’s legacy runs in two directions. In fundamental science he showed that the proton and neutron are not points but objects with size and structure, opening the road that led to the quarks. In practical terms, the humble crystal of sodium iodide and thallium that he found in 1948 has become one of the workhorses of radiation detection, used every day in nuclear medicine departments, laboratories and radiation monitors. Patients having a heart scan or a lung scan today are, quite literally, being seen through his invention.
Sources & further reading
Robert Hofstadter, Biographical, NobelPrize.org · Robert Hofstadter, Biographical Memoirs vol. 79, National Academy of Sciences (J. Friedman and W. Little) · Robert Hofstadter, National Medal of Science, NSF · Gamma camera, Wikipedia · Robert Hofstadter, Jewish Virtual Library · Robert Hofstadter, Wikipedia · Researched from the sources above and checked against them. Spotted an error? Tell us.
A life of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.