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Gravitational spectrometer

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

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Films and animations

GRANIT - neutron quantum states in a gravitational potential

by Dominique Rebreyend, Guillaume Pignol and Alain Filhol. ©2013-21024 ILL & LPSC-IN2P3.

GRANIT is an interactive 3D animation of the instrument GRANIT lets you discover how the instrument works. It is based on the plug-in VirtualGRIP (formerly SPOK), produced by the firm IPTER. It shows the whole instrument from the initial neutron source (the ILL's high-flux reactor) to the monochromator, the Ultra Cold Neutron (UCN) source and the gravitational spectrometer itself. You can rotate, zoom and pan the scene, have a closer look at the main parts of the instrument, and see how neutrons behave on their journey from the reactor core to the detector of GRANIT.

GRANIT is part of the application Neutrons4Science which is available for iOS, Android and the web.



or download a standalone version (8.4 Mb)

French texts : Alain Filhol, Translations: English by Susan Tinniswood, German by Roland May, Italian by Lucia Capogna, Spanish by Miguel-Angel Gonzalez y Leidy Hoyos, Russian by Valery Nesvizhevsky, 中文版本翻译由柳金彦完成 (Jinyan LIU).

GRANIT in a nutshell

The animation demonstrates the following:

  • The reactor core produces a high flux of fast neutrons.
  • These are slowed down either to thermal neutrons by the heavy water of the moderator or, as shown here, to cold neutrons by a secondary moderator, namely a volume of liquid deuterium.
  • A neutron guide transports these cold neutrons from the cold source to the monochromator.
  • The monochromator is a single crystal which selects one wavelength from the spectrum of the incoming neutrons.
  • A second guide then transports these monochromatic neutrons to the UCN source, a volume of superfluid helium which dramatically slows down (cools down) the incoming cold neutrons to ultra cold neutrons.
  • On reaching the buffer container, these UCNs fill the transfer volume like a gas. A slit then feeds the spectrometer with UCNs, which rebound on the surface of a mirror until they eventually reach the detector.
  • An absorber located above the mirror absorbs (or scatters) neutrons with too high an energy (neutrons jumping too high), thus acting as a low-pass filter. At this point we can no longer speak of rebounding and neutron trajectories in conventional terms. As already demonstrated by an earlier version of GRANIT, neutrons which pass through the spectrometer are distributed on discrete energy levels. The instrument is therefore able to differentiate between neutron quantum states. The absorber height is adjusted so that only the first and second quantum orders pass through.
  • Then a small step in the mirror adds a "pinch" of energy to the neutrons, which converts most of the first-order neutrons to second-order neutrons. In other words, the neutrons reaching the detector are almost exclusively of the second quantum order.

The spectrometer is now ready to start measurements. Any perturbation, such as accelerations or magnetic fields, no matter how weak, will alter the quantum state of the neutrons, and the detector will observe an incoming neutron distribution that differs from the distribution expected for second-order neutron quantum states.


Page by A. Filhol: 10 Feb 2011, 20 Feb 2011, 16 March 2011, 1 June 2012, 3 July 2013.