The monochromator and its Doppler drive

IN16 is a backscattering spectrometer using neutron optical focusing in the primary spectrometer. A special graphite (002) double deflector device selects roughly the desired wavelength and transmits a divergent beam to a spherically shaped monochromator mounted on a Doppler drive. Highly monochromatic neutrons are selected by near perfect monochromator crystals in exact backscattering geometry (e.g. Si (111)) glued onto a spherical support. As the plate moves the exact wavelength (energy) selected is effectively scanned by the Doppler effect, shifted in phase with the cyclic mirror motion. These neutrons are focussed onto the sample and the scattered neutrons analysed.

The movies below show the Doppler drive of the monochromator in operation. The monochromator is made of a set of carefully cut silicon crystals, aligned on a spherical support. The optical quality of its surface is demonstrated by the reflected image of observers.