Crystals are defined by a regular spatial arrangement of atoms and molecules that can act as a three-dimensional grating that can scatter photons or particles into specific patterns which can be used to determine the precise locations of the atoms in the crystal, and in turn, the structure of the molecules comprising the crystal. Although many species can be scattered, the entities that are most commonly scattered are x-ray photons and neutrons. The two techniques are complementary: heavy atoms (those with lots of electrons) scatter x-rays more efficiently than light atoms with few electrons. However, light atoms have large neutron scattering cross sections, i.e. they scatter neutrons efficiently. Therefore, molecules that contain many light atoms, such as biomolecules that contain many hydrogen atoms are better probed with neutrons than with x-rays.
Neutrons for scattering are produced by nuclear reactors or in particle accelerators. The Atomic Energy Commission built several dedicated nuclear reactors for neutron scattering, such as those at Brookhaven and at Oak Ridge, and, later, accelerators for neutron scattering at Los Alamos and Argonne, and currently by the Department of Energy, at Oak Ridge. It was natural, therefore, for the AEC to support neutron scattering in its research programs.
Last Modified: June 28, 2012