Inelastic neutron scattering

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Inelastic neutron scattering is an experimental technique commonly used in condensed matter research to study atomic and molecular motion as well as magnetic and crystal field excitations. It distinguishes itself from other neutron scattering techniques by resolving the change in kinetic energy that occurs when the collision between neutrons and the sample is an inelastic one. Results are generally communicated as the dynamic structure factor (also called inelastic scattering law) S(\mathbf{Q},\omega), sometimes also as the dynamic susceptibility  \chi^{\prime \prime}(\mathbf{Q},\omega) where the scattering vector \mathbf{Q} is the difference between incoming and outgoing wave vector, and \hbar \omega is the energy change experienced by the sample (negative that of the scattered neutron). When results are plotted as function of \omega, they can often be interpreted in the same way as spectra obtained by conventional spectroscopic techniques; insofar as inelastic neutron scattering can be seen as a special spectroscopy.

Generic layout of an inelastic neutron scattering experiment

Inelastic scattering experiments normally require a monochromatization of the incident or outgoing beam and an energy analysis of the scattered neutrons. This can be done either through time-of-flight techniques (neutron time-of-flight scattering) or through Bragg reflection from single crystals (neutron triple-axis spectroscopy, neutron backscattering). Monochromatization is not needed in echo techniques (neutron spin echo, neutron resonance spin echo), which use the quantum mechanical phase of the neutrons in addition to their amplitudes.

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Literature:

  • G L Squires Introduction to the Theory of Thermal Neutron Scattering Dover 1997 (reprint?)

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