A simulated experiment

• The incident polychromatic neutron beam coming from the guide H144 passes in the standard set-up a velocity selector and only a selected wavelength band continues through the guide afterwards. It arrives on the monochromator crystals which select an initial wavelength out of it, directing this beam onto the sample.
• The monochromatic beam arrives on the sample - for example a single crystal - and is scattered in many directions, sometimes with a change in energy (wavelength). This change in energy is the signature of the excitations within the sample and is what we want to study.
• The measurement of the energy of scattered beams is devoted to the secondary spectrometer, comprising the analyser and the detector. The secondary spectrometer is moved to choose the direction of measuring the final intensity, and the analyser and detector are then set to measure neutrons at the final wavelength.
• The intensity of the scattered neutron bean is measured while scanning the initial or final wavelengths. Thus changing the angle of the secondary spectrometer and the sample orientation maps out the spectra of excitations propagating in different directions in the sample.

The animation shows:

• the names of the parts of the spectrometer
• the initial positioning of the monochromator, sample, analyser and detector
• several steps of a scan at constant final energy.