1994 Nobel Prize in Physics(2)
Reason for Award
for pioneering contributions to the development of neutron scattering techniques for studies of condensed matter (development of the neutron diffraction technique) Phys. Rev. 76(1949) 1256-1257; Phys. Rev. 81(1951) 527-535; Phys. Rev. 83(1951) 333-345; Rev. Mod. Phys. 25(1953) 100-107; Phys. Rev. 97(1955) 304-310
Laureates
United States of America
Explanation
Crystals are like building blocks where atoms are neatly lined up. Mr. Shull thought of a new way to find out this arrangement by throwing neutrons, tiny neutral particles, at the crystal. It is like throwing balls at a wall and learning its shape from the rebounds. Neutrons go deep inside, so the sample is not damaged. Thanks to his method we can uncover secrets of materials used in magnets and medicines.
Related Keywords
neutron diffraction
Neutron diffraction uses Bragg scattering of neutrons by crystals to determine atomic and magnetic structures. Because nuclear and magnetic scattering phases are independent, it reveals positions of light elements and magnetic moments invisible to X-ray diffraction. Shull developed instruments capable of measuring diffraction peaks in powder samples with high precision, greatly advancing materials science. Today the technique is indispensable for stress analysis and for evaluating hydrogen storage materials. The deep penetration of neutrons has also expanded applications to non-destructive testing of industrial products.
crystal structure analysis
Crystal structure analysis reconstructs the three-dimensional atomic arrangement from diffraction data. Using neutrons clarifies light-element positions and anisotropic thermal parameters in detail. Fourier synthesis and Rietveld refinement are representative methods, achieving sub-picometre accuracy. Structural knowledge is the first step to understanding physical and chemical properties of materials. It is indispensable for developing new catalysts and battery materials.
magnetic structure analysis
Magnetic structure analysis determines spin directions and periodic ordering. Neutrons interact directly with magnetic moments, and polarised-neutron techniques using He-3 cells provide high contrast. Shull’s instrumentation evolved into studies of heterogeneous magnetic materials and spin-density waves. Data are interpreted with group-symmetry and magnetic space-group theory. Recently the method has been employed to elucidate multiferroics and skyrmion lattices.
powder diffraction
Powder diffraction analyses the intensity distribution of diffraction rings from polycrystalline powders, powerful for materials lacking single crystals. Neutron powder diffraction collects data over all directions in short time while minimising absorption and radiation damage. Shull improved peak resolution through carefully designed collimation and detector arrangements. Today high-resolution powder diffractometers combined with Rietveld analysis are used for phase identification and fraction quantification. Operando measurements following battery charge–discharge processes in real time are active research fields.
single-crystal diffraction
Single-crystal diffraction obtains complete structure factors from the positions and intensities of discrete diffraction spots. Neutron single-crystal diffraction is particularly powerful for analysing low-symmetry structures and hydrogen-bond networks. Using cold neutrons lengthens the wavelength and improves low-Q data precision. Shull’s early experiments paved the way for multi-axis goniometer design. Today stroboscopic techniques allow time-resolved measurements with the crystal kept stationary.
Bragg's law
Bragg’s law, 2d sinθ = nλ, relates the lattice plane spacing d to wavelength λ and diffraction angle θ. It lets one directly determine lattice constants of unknown crystals. Neutrons can use longer wavelengths, so large d-spacings are measurable at small θ. In magnetic scattering additional Bragg peaks appear corresponding to spin ordering. The equation’s simplicity makes it a staple concept in crystallography textbooks.
scattering length
The scattering length quantifies the strength of interaction between a neutron and an atomic nucleus and is unique to each isotope. Unlike X-ray scattering factors it does not vary smoothly across the periodic table, enabling isotope-contrast techniques. By isotope substitution one can hide or highlight specific atoms. The magnetic scattering length is proportional to the Bohr magneton and helps analyse spin arrangements. Scattering-length data are also input to quantum-chemistry calculations and are indispensable in nuclear-reaction design.
structure factor
The structure factor is a complex number linking atomic arrangement in a crystal to the scattering amplitude; diffraction intensity I equals |F|². Nuclear and magnetic structure factors superpose, so analyses must separate them. Rietveld refinement determines F by minimising the difference between observed and calculated patterns. Hydrogen–deuterium substitution alters phase information and improves accuracy. In time-resolved experiments, temporal changes of structure factors track phase-transition dynamics.
low-temperature properties
Low-temperature properties refer to material characteristics close to absolute zero where quantum effects dominate. Neutron scattering works even under high magnetic fields and millikelvin temperatures, essential for studying superconductors and quantum spin liquids. Shull’s diffraction techniques combined with dilution refrigerators mapped magnetic ordering boundaries. Lattice contraction and zero-point vibrations can also be precisely analysed at low temperature. Applications include evaluating materials for cosmic background detectors and quantum-computing devices.
high-pressure experiment
High-pressure experiments investigate structural changes by applying pressures up to gigapascals. Neutrons penetrate diamond-anvil cells, allowing diffraction patterns from the sample inside. Shull’s diffractometers combined with high-pressure cells clarified phase transitions of ice and deep-Earth minerals. The technique is also applied to pressure-induced superconductivity and electronic anisotropy studies. Laser heating combined with pressure now simulates planetary interior conditions.