Versatile high flux backscattering spectrometer

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.