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IN16B Versatile high flux backscattering spectrometer

How it works

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

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.