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Time of flight spectrometer

A set of pedagogical interactive animations demonstrating the behavior of some neutron spectrometers.

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Films and animations

Neutron Time-of-flight spectrometer IN5

IN5 is a typical direct geometry neutron Time-of-flight (ToF) spectrometer. This high precision instrument is used to study low-energy transfer processes as a function of momentum transfer (e.g. quasi-elastic scattering in solids, liquids, molecular crystals and inelastic scattering, etc).
The Flash animation below shows how measurements are performed :

  • the continuous polychromatic neutron flux from the neutron guide H16 is turned into a pulsed monochromatic beam by a set of 6 choppers.
  • the monochromatic neutron bursts are inelastically scattered by the sample, either a liquid, a powder or a single crystal.
  • the scattered neutrons are collected by a huge detectors array  covering 30 m2.
  • the time-of-flight analysis of incoming neutrons from each monochromatic burst results in a three dimensional spectrum x, y (detector position), t (time).

 This gives access to the so-called dynamic structure factor, S(Q,ω), describing the structure and dynamics of the sample at atomic level.

Click HERE to launch the animation.
File download (14.4 Mb) : in5_ill.swf  or in5_ill.swf.zip

Step-by-step comments about the animation

Neutron beam: The neutron beam from the cold neutron guide H16 is continuous and polychromatic (with a nearly Maxwellian distribution).

Pulsing chopper (P): Two counter-rotating disks creating polychromatic neutrons bursts.

  • Contaminant order chopper (CO): A single chopper. It removes harmonics due to the large bandwidth after the P choppers from the incident beam. This is equivalent to reducing the bandwidth before the monochromator choppers. 
  • Frame overlap chopper (FO) : A single chopper. It avoids overlapping of scattered neutrons from successive bursts after sample by suppressing some of the produced pulses.

Monochromator choppers (M): A pair of counter-rotating disks chops a single wavelength (a narrow band of nearly monochromatic neutrons) from the bursts that have spread over the distance between P and M choppers.


  • Intensity: Shows the angular or Q-dependence of the scattered intensity.
  • Time-of-flight : Shows that for each detector pixel the time of arrival to the detector is directly related to the energy exchange between the sample and neutrons.
  • Complete analysis: Example of 3D spectrum recorded in time-of-flight. Here the spectrum from helium in the polycrystaline hexagonal phase.

Page by A. Filhol <filhol(at)ill.eu> and J. Ollivier <ollivier(at)ill.eu>: 20 March 2011