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.