Page 71 - Neutrons for Sciences and Society

Chapter 3 - Why invest so much money in a source of neutrons?
angle. A monochromatic beam may also be produced by a set of rotating disks with slits which only allow neutrons with a certain speed to pass through.
Fig. 3.3: Layout of the inelastic scattering instrument IN5, a typical time-of-flight spectrometer (updated layout)
Let us take the ILL instrument IN5 as a prototype of a time-of-flight spectrometer (Fig. 3.3).
The incident polychromatic neutron beam comes from a 20
cm high cold neutron guide (peak flux between 4 and 5 Å). It impinges three pairs of contra-rotating discs (choppers) coated with neutron absorbing material, except for two opposite windows to allow the neutrons to pass. The choppers can rotate at up to 12000 rpm with phasing accuracy within 0.5 milliradians. This assembly “chops” the beam into short quasi-monochromatic pulses as follows:
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