Neutron inelastic scattering triple-axis spectrometers

In order to determine the dynamics of atoms we have to carry out an inelastic neutron scattering measurement. With a reactor source this can be done with a triple-axis spectrometer, which has an analyzer crystal. Tripleaxis refers to the three axes for the monochromator, sample, and analyzer, all moving independently and controlled by a computer. With a pulsed source we use a mechanical chopper, which is a rotating cylinder with a hole perpendicular to the rotating axis that allows neutrons with a chosen range of velocity to go through. The neutrons scattered by the sample are detected... 

Inelastic neutron scattering, on the other hand, usually employs a monochromatic neutron beam and records the intensity of the scattered neutron beam as a function of neutron kinetic energy. Such inelastic collision spectra are monitored as a function of the applied field and the (usually low) temperature. The observed peaks then represent the energy differences of thermally populated and excited unpopulated multiplet states. Inelastic neutron scattering experiments can be conducted using triple-axis, backscattering, or time-of-flight spectrometers. 

Measurements of dispersion curves provide information about the interatomic forces in solids. In fact, a dispersion curve is a function of the vibration frequency V on the wavelength X. The methods of neutron spectroscopy based on the phenomenon of diffraction of heat neutrons by crystals enables one to graph the dispersion curves of solids. The most accurate measurements of these curves are obtained by the inelastic neutron scattering using triple axis spectrometers. 

Neutrons in this energy range are known as thermal neutrons, because typical energies correspond to room temperature see Table 1. Thermal-neutron INS spectroscopy was the first type to be studied and the instrument invented by Bertram Brockhouse in 1952, the triple-axis spectrometer, is stUl a mainstay of inelastic instrumentation at reactor sources. In 1994, Brockhouse, together with the pioneer of elastic neutron scattering (diffraction) Clifford Shull, received the Nobel prize for physics for his invention and subsequent work with it. 

Figure 1. Schematic of a triple-axis inelastic neutron spectrometer. The thermal-ized beam from the reactor is monochromated by Bragg reflection from crystal M. Neutrons that scatter from the sample S are energy analyzed by Bragg reflection from crystal A and enter detector D.

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