This website is being migrated. In case you don't find the information you are looking for, the old website can be found here. Thank you for reporting any anomalies to communication@ill.eu.
Switch instrument:
Virtual tour

How it works

Simulated experiment

IN16B is a cold neutron backscattering spectrometer - "a variable incident energy, fixed final energy spectrometer". The simulated experiment corresponds to IN16B in guide-side-position ('low background' configuration'). For the guide-end-position ('high flux configuration') incident wavelength band is selected by a velocity selector instead of a graphite deflector:

  • The polychromatic neutron beam from the reactor is deflected towards the instrument. The Bragg reflection selects neutrons in a wavelength band around 6Å. (alternatively (not shown) the neutrons pass a velocity selector and continue straight on. Imagine the focusing nose to be at the guide end).
  • The neutron beam is pulsed by the "background" chopper.
  • A rotating deflector sends the beam to the monochromator.
  • A 90 degree Bragg relection from high quality silicon crystals gives a more precise selection of neutrons with wavelength 6Å. The selected wavelength is also determined by the speed of the monochromator, it is mounted on a Doppler drive.
  • Neutrons pass back through windows in the rotating deflector, the backgound chopper is closed during this time interval. A pulsed beam of neutrons with an energy range determined by the Doppler machine is incident on the sample.
  • Neutrons are scattered in all directions. Only those with a certain final energy, determined by the backscattering relection at the analyser crystals, are reflected to the detectors and counted.

Doppler monochromator

The movie shows the old mechanical Doppler drive of IN16.

On IN16B a linear motor Doppler drive (AEROLAS) is in operation. It follows the same principle but achieves higher monochromator speeds.

The focusing monochromator shown ('polished' IN16 monochromator) is a spherical assembly of perfect single crystals (small 4x4mm2 oriented silicon single crystals Si(111) ).

When the monochromator stands still it reflects in backscattering only the neutron wavelength λ = 2d, where d is the lattice spacing of the monochromator crystals parallel to the surface.

When the monochromator moves it reflects a slightly different wavelength due to the Doppler effect, the magnitude of which is directly related to the speed of the monochromator motion:

  • when it moves towards the sample it reflects a slightly shorter wavelength λ - Δλ
  • when it moves backwards it reflects a slightly longer wavelength λ + Δλ.