Robert Hofstadter

Physicist

• Born: February 5, 1915
• Birthplace: New York, New York
• Died: November 17, 1990
• Place of death: Stanford, California

American Physicist

American physicist Robert Hofstadter was awarded the 1961 Nobel Prize in Physics for his discoveries about the fundamental composition and structure of the atomic nucleus. His other contributions include a series of investigations into the hydrogen bond and the development of several instruments for measuring radiation and electron scattering.

Primary field: Physics

Specialty: Nuclear physics

Early Life

Robert Hofstadter was born on February 5, 1915, in New York City, to first-generation Jewish European immigrant parents of modest means. His father, Louis Hofstadter, who operated a cigar store, was born in Austria. His mother, Henrietta Koenigsberg, was born in Russia.

Although Hofstadter grew up in an urban environment, he had a great interest in nature and loved the outdoors. As a child, he attended public elementary and high schools, and displayed an early aptitude in the sciences. He was particularly absorbed by physics and mathematics.

In 1931, Hofstadter enrolled as an undergraduate at City College of New York. The institution accepted students with demonstrated academic merit and provided them with a tuition-free university education. Hofstadter flourished at City College, studying theoretical and experimental physics under well-known researchers like Mark Zemansky, Henry Semat, and Simon Sonkin. He also made several lifelong friendships with other young physicists, such as physicist Robert Herman, who would later become his colleagues. Hofstadter graduated with a bachelor’s degree in physics in 1935. He won the college’s Kenyon Prize for his outstanding performance in physics and mathematics and was elected to the Phi Beta Kappa academic honors society.

Between 1935 and 1938, Hofstadter earned his master’s and doctoral degrees in physics at Princeton University in New Jersey. His work was funded by the General Electric Company’s Charles A. Coffin Fellowship and by Princeton’s own Proctor Fellowship.

During these years, his research focused on the infrared spectra of organic molecules. Infrared radiation is a class of electromagnetic radiation with wavelengths longer than those of visible light but shorter than those of microwaves or radio waves. This class of radiation is absorbed by the chemical bonds that exist in organic molecules. Since the intensity and frequency of the radiation that a particular bond absorbs depends on its structure, infrared spectroscopy can be used to analyze a compound by examining the nature of the bonds it contains. Hofstadter’s most significant publication at Princeton was a project he worked on with theoretical physicist Edward Condon to describe the hydrogen bond: a weak force of attraction that attaches a (positively charged) proton in one molecule to a negatively charged atom in another molecule.

Life’s Work

After completing his PhD work, Hofstadter remained at Princeton for one year as a postdoctoral research fellow. His postdoctoral work focused on the question of how the presence of light changes the photoelectric conductivity of a particular type of zinc silicate crystal known as willemite. Hofstadter worked on the project with Herman, who was then a doctoral student but would later become widely known for his prediction (along with Ralph Alpher) of the existence of the cosmic microwave background radiation.

Photoelectric objects, such as willemite, emit a current of electrons when exposed to radiation. Hofstadter and Herman found that if willemite was cooled to extremely cold temperatures, suffused with a particular kind of ultraviolet light, and then allowed to warm back up, the crystal’s “dark current”—the residual electrical current that flows through a photoelectric object when it is not being exposed to radiation—would leap up to very high levels. The two eventually determined that this effect was due to electrons that had been trapped during irradiation being released later on. This was the first demonstration of a phenomenon that has since come to be known as “trapping states” in crystals.

In 1939, Hofstadter accepted a second postdoctoral fellowship, this time at the University of Pennsylvania. He began working on the construction of a Van de Graaff generator, a device that creates a large static electricity charge that can then be used experimentally in various ways. The Van de Graaff generator Hofstadter helped to build accelerated atomic particles to very high speeds so that they could be used for nuclear research.

During World War II, Hofstadter worked on technological projects for the US military. At the National Bureau of Standards, a government research body now known as the National Institute of Standards and Technology (NIST), he worked on developing proximity fuses, devices that detonate an explosive automatically when it comes within a certain distance from its target. Later, at the Norden Laboratories Corporation, Hofstadter shifted his attention to aircraft autopilot systems and altimeters.

In 1942, Hofstadter married Nancy Givan. The couple had three children together. Their only son is the Pulitzer Prize–winning cognitive scientist and writer Douglas Hofstadter.

When World War II ended, Hofstadter returned to the academic world as an assistant professor of physics at Princeton. He spent four years in this position, studying the behaviors and effects of atomic particles and developing a number of instruments used for measuring these behaviors and effects.

Hofstadter tested the use of Nal(Tl), a sodium iodide compound made reactive by a small quantity of thallium, as a scintillation counter. In physics, “scintillation” is a brief flash of either visible or ultraviolet light that fluoresces from some materials when they are exposed to ionizing radiation, a highly energetic type of radiation that creates ions by removing electrons from an atom or molecule. A scintillation counter is a device that takes advantage of this property to measure ionizing radiation. Hofstadter’s device connected a crystal of sodium iodide to a light-sensitive tube that sent an electrical signal to an amplifier. The strength of the signal was determined by the amount of light the crystal emitted—which was in turn dependent on the amount of radiation in its vicinity.

Hofstadter became an associate professor of physics at Stanford University in Stanford, California, in 1950—the institution remained his professional home for the next four decades. At Stanford, he focused his energies mainly on of the study of atomic physics. It was this line of research that won him the 1961 Nobel Prize in Physics. Hofstadter’s interest in physics measurements continued to drive his work, and he developed another, faster, scintillation counter using inorganic cesium fluoride, and a counter for a particular type of radiation known as Cerenkov radiation. He also studied cosmic rays, which are highly charged atomic particles that travel through outer space and flow into the Earth’s atmosphere—and cascade showers, which are streams of charged electrons created by passing a cosmic ray through matter.

Hofstadter served as the director of the High Energy Physics Laboratory at Stanford from 1967 to 1975. In 1985, he retired from full-time research and teaching and became a professor emeritus. He died in 1990 at the age of seventy-five.

Impact

Hofstadter is best remembered for his contributions to nuclear physics. His work expanded scientific understanding of the shape and size of both the proton and the neutron, and he played a key role in elucidating how the particles inside an atomic nucleus are arranged. Hofstadter was also responsible for naming the fermi, a unit of length used in nuclear physics. Hofstadter first used the term in a 1956 paper, in honor of the Italian physicist Enrico Fermi.

Hofstadter felt his most important contribution to the scientific world—as measured by the range of fields to which it was relevant—was the discovery he made with nuclear physicist J. A. McIntyre in 1950 that Nal(Tl) could be used as an effective spectrometer. Nal(Tl) was the same crystal that Hofstadter had previously showed could be used as a scintillation counter. He and McIntyre discovered that it also produced bright, linearly proportional scintillations in response to gamma rays. The sodium-iodide-based gamma ray spectrometer that arose from this discovery quickly became one of the most commonly used instruments of measurement for gamma rays in many fields of research and the applied sciences, including astrophysics and medicine.

Hofstadter received numerous scientific awards during his lifetime besides his 1961 Nobel Prize. In 1958, he became a fellow of the United States National Academy of Sciences, and in 1959 the state of California named him its scientist of the year. He also won a Guggenheim Fellowship in 1958.

Bibliography

Brown, Laurie M., Max Dresden, and Lillian Hoddeson, eds. Pions to Quarks: Particle Physics in the 1950s. New York: Cambridge UP, 2009. Print. Presents a collection of articles by scientists and historians of science that covers the discoveries, successes, failures, and political and social factors affecting particle physics during the era in which Hofstadter worked. Black and white photographs, list of notations and symbols.

Friedman, Jerome Isaac, and William A. Little. “Robert Hofstadter: February 5, 1915–November 17, 1990.” National Academy of Sciences (U.S.). Biographical Memoirs, Vol. 79. Ed. National Research Council.Washington, DC: National Academy P, 2001. 159–82. Print. Detailed biographical record that covers Hofstadter’s early life, career, and work.

Kidd, J. S., and Renee A. Kidd. Nuclear Power: The Study of Quarks and Sparks. New York: Chelsea House, 2006. Print. Offers a history-of-science perspective on the fundamental discoveries that paved the way for nuclear power. Chapter 11, “Advances in Nuclear Science,” covers Hofstadter’s contribution to the research about high-energy collisions between protons and electrons. Glossary, further reading, index, black and white photos.