© Top: Wikimedia Commons, Public Domain. Author: Joachim Wuttke, 22 February 2007. Bottom: Courtesy Joe Wong, Lawrence Livermore National Laboratory.
In an INS experiment, the neutron beam from a reactor is sent through a monochromator to fix its initial energy before it is scattered by the crystal. By applying energy and momentum conservation laws, one can then determine the spectrum of the phonons responsible for the scattering events. Part of the phonon spectrum for lead determined in this way is shown in the bottom figure; there one sees how INS scattering enables one to obtain the entire spectrum of both longitudinal (L) and transverse (T) phonons, not just the long wavelength linear portion defined by the straight line that is the sound velocity determined from measurements of elastic constants. (Unit: 8)