The scene during the approach to criticality of the first self-sustaining nuclear reactor, CP-1, at Stagg Field, University of Chicago, December 2, 1942. The painting shows many of the leading lights of the nuclear physics community who were present at that event including Enrico Fermi, Eugene Wigner, and Emilio Segré. The startup of CP-1 marked the beginning of the development of neutron sources strong enough to support materials science studies using neutrons.Neutron scattering is a technique for studying materials at the atomic, molecular, and supramolecular levels. We shine beams of radiation, neutrons, onto an object and observe how they bounce off. From the details of the way they bounce off, or, scatter, we infer properties of the object. The materials are those that we use in everyday life, as well as not-so-common substances that future technologies depend upon. From these studies, we reason how that fundamental information, both spatial structure and energetic excitations, relates to the performance of materials and how to improve them. Slow neutrons, not so slow as speeds in common experience—hundreds of meters per second up to tens of kilometers per second—have wave properties as well as particle properties. Neutrons have no charge and interact primarily with atomic nuclei through the “strong” force although are influenced also by magnetic and gravitational fields.
Slow neutrons have wavelengths comparable to the scales of lengths in the materials being studied. In addition, neutrons have magnetic properties, tiny magnetic dipoles, which interact with intrinsic and imposed magnetic fields, revealing magnetic structure and excitations. Moreover, slow-neutron energies are comparable to the energies of excitation of the materials of study, therefore enabling probing those that affect materials’ behavior at commonly accessible temperatures.
The theory underpinning the understanding of slow-neutron scattering is well understood and is highly mathematical, both in the macroscopic sense of neutron production, transport, and detection and in the interactions with materials in the microscopic sense. These existing theories facilitate interpretation of measurement. Treatment of neutrons as particles is appropriate for macroscopic purposes. Treatment of neutrons as quantum-mechanical entities, with wave properties and intrinsic spins, is necessary for understanding neutron-sample interactions at the microscopic level. The particle-transport theory can be derived from the quantum theory but not vice versa. The particle theory is more closely related to everyday experience and is much simpler and intuitive in applications to macroscopic systems than a quantum treatment and, therefore, is the usual basis for treating those situations.
This duality of nature is part of Nature itself, or, of our easy way of understanding Nature.