Clifford Glenwood Shull

Clifford Glenwood Shull, a 1994 Nobel Prize winner in physics for his pioneering work in neutron scattering, died of kidney failure on 31 March 2001 in Lexington, Massachusetts.

Born in Pittsburgh, Pennsylvania, on 23 September 1915, Cliff enrolled at Carnegie Tech (now Carnegie Mellon University) in Pittsburgh to study aeronautical engineering. Influenced by Emerson Pugh, he switched to physics after his freshman year, earning a BS in physics in 1937. He then went on to graduate school at New York University, where he received a PhD in nuclear physics in 1941. His thesis at NYU, under Frank Myers, involved the construction of a Van de Graaf accelerator and the scattering of polarized electrons.

During the time Cliff was working on his thesis, exciting things were happening at NYU involving the theory of neutron scattering from magnetic materials and the initial experimental efforts in the magnetic scattering of neutrons. In 1939, Otto Halpern and Montgomery Johnson of NYU published their definitive paper on the magnetic scattering of neutrons. At the same time, Martin Whitaker of NYU was using this theory in experimental attempts to measure the neutron magnetic moment using polychromatic neutrons from a radium–beryllium source. Cliff would later play a critical role in establishing the correctness of the Halpern and Johnson theory and in exploiting this theory in a series of brilliant experiments that opened the entire field of magnetic structures to experimental study using various neutron scattering techniques. Whitaker would later play a role in bringing Cliff to Oak Ridge National Laboratory and to his destiny with neutrons.

After he finished his thesis in June 1941, Cliff accepted a job with the Texas Co (later to become Texaco) in Beacon, New York, where he studied and characterized catalysts used in making high-octane aviation fuel. He used x-ray diffraction and small-angle scattering techniques to study powder samples; those techniques gave him valuable experience for his later work with neutrons.

After the Manhattan Project was initiated during World War II, Cliff’s friends and mentors from NYU urged him to join the University of Chicago’s Metallurgical Laboratory, being organized by Arthur Compton. Cliff wanted to do this, but the Texas Co convinced the War Manpower Board that Cliff’s work was critical to the war effort, and so he stayed at Beacon for the duration of the war.

After the war ended, Cliff was released from Texaco and was free to seek employment in the new fields created by the wartime work on fission. He contacted Whitaker, the first director of the Clinton Laboratories, which was then a part of the Manhattan Project and would soon become the Oak Ridge National Laboratory. On a visit to Oak Ridge in 1946, Cliff met Ernie Wollan and learned of Wollan’s exploratory work on the diffraction of monochromatic neutrons by powder samples of sodium chloride and scattering of neutrons by water and heavy water. Cliff was immediately excited by this work and came to Oak Ridge in June 1946.

Wollan had been a student of Compton’s at the University of Chicago and was well versed in the theory and practice of x-ray diffraction. When Shull arrived, Wollan had already assembled a two-axis neutron diffractometer, using a large NaCl crystal as a monochromator along with the Compton-designed sample table and counter arm that Wollan had used in his thesis work at the University of Chicago. At first, they concentrated on developing a complete understanding of the scattering of neutrons by powder samples. They then used this powder diffraction technique to measure the neutron coherent scattering amplitudes of almost all the elements and many isotopes. For years, this work served as reference data for interpreting all neutron scattering experiments.

Early in this work of building a library of neutron scattering amplitudes, Shull and Wollan determined the hydrogen and deuterium amplitudes (both fairly large and opposite in sign) by measuring the diffraction patterns of sodium hydride and sodium deuteride. By combining the hydrogen result with the known total cross section, they deduced the nuclear singlet and triplet scattering amplitudes and explained the large total cross section in terms of nuclear spin incoherent scattering. They used the deuterium result in a study of the structure of ice that gave the first direct evidence in support of Linus Pauling’s double minimum potential model of hydrogen bonding. Varying the coherent neutron scattering amplitude of hydrogen by controlled deuteration is now one of the most powerful techniques available in the study of polymers and biological materials.

It was the study of magnetic materials that most excited Shull and Wollan. They had the theory of Halpern and Johnson, the first neutron source capable of producing sufficient flux, and the right experimental technique—powder diffraction. They quickly exploited these factors in a series of experiments that established neutron scattering as the key to understanding magnetic materials. The first direct evidence of antiferromagnetism came from determining the magnetic structure of manganese oxide. In addition, the Néel model of ferrimagnetism was confirmed for magnetite (Fe₃O₄); the first magnetic form factor data were obtained by measuring the paramagnetic scattering by Mn compounds; the production of polarized neutrons by Bragg reflection from ferromagnets was demonstrated; the magnetic diffuse scattering was used to determine the distribution of magnetic moments in 3d alloys; and the magnetic critical scattering at the Curie point of iron was measured. For his work on magnetic materials, Cliff was awarded the Buckley Prize by the American Physical Society in 1956.

Attracted by a new research reactor being constructed at MIT and by the opportunity to teach, Cliff joined MIT in 1955 as a professor of physics; he remained there until his retirement in 1986. His first experimental work at MIT involved constructing and using a highly sensitive polarized neutron diffractometer to make extensive and precise measurements of the magnetic form factor in Fe. That work revealed the spatial variation of the spin density and the existence of negative spin density in regions of the unit cell far removed from the Fe sites.

In the 1960s, Cliff studied dynamical diffraction and the propagation of neutron waves in perfect crystals. That work, which required imaginative experimental concepts, careful attention to experimental details, and thorough understanding of theory, was ideally suited to Cliff’s character; he remained active in this field until his retirement.

Cliff loved experimental work and typically had an experiment in progress, usually in collaboration with a postdoc student. His success as an educator can be measured by the distinguished careers of many of his graduate students. The MIT physics community held Cliff in high regard as an inspiring colleague, always kind and generous, who maintained high academic standards and high scientific productivity. Early in his career at MIT he developed several neutron experiments for undergraduate physics students using his equipment at the reactor. He took great pleasure in this program, which continued until his retirement. He was elected to the National Academy of Sciences in 1975.

Cliff was modest and unpretentious with regard to his accomplishments, gentle in his interaction with others (including students), always willing to help by sharing his knowledge and experience, and careful to give credit to others. These qualities resulted in universal admiration, respect, and affection for Cliff within the neutron scattering community. In honor of his accomplishments, Cliff’s sons endowed the Cliff Shull Scholarship Fund for undergraduate physics students at Carnegie Mellon University. Cliff was a much-loved father of his field.