The Spectroscopy group of the ILL operates four different classes of instruments that measure the atomic, molecular and crystal motions of samples in powder, glass or liquid.
- Time-of-flight spectrometers which analyse the change of velocity of neutrons on interaction with the sample by measuring their time of arrival; I
- Backscattering spectrometers, a hybrid between three-axis and inverted TOF spectrometers. Very high energy resolution is achieved by reflecting the scattered neutrons under normal incidence from perfect crystals: IN10, IN13 and IN16B. The incident energy is varied by temperature (IN13) or Doppler drives (IN10, IN16B) and for the latter the neutron flight time is needed to determine the incident neutron energy. BATS uses exclusively the neutron flight time for determination of the energy of incident neutrons emerging from a sharp chopper pulse.
- Spin-echo spectrometers which measure small changes in neutron velocities very precisely by analising how their spins precess - like the axis of a spinning top - before and after interaction with the sample.
- Three-axis spectrometers, instruments in which the monochromator, sample and analyser can each be aligned independently:
IN1, IN8, IN12 (CRG), IN20, IN22 (CRG), ThALES
Two types exist, depending how the monochromatization of the beam is performed. In both types, a series of bursts of monochromatized neutrons is send upon the sample. The energy of the scattered neutrons is then analysed by measuring their time of arrival at the detectors which cover a wide angular range. In IN5 (cold neutrons) the monochromatization is performed by a set of six disc choppers turning at high speed: at a wavelength of 10 Å the resolution is 10 µeV, the resolution function is perfectly gaussian. In IN4 and IN6, the monochromatization is realised by a set of crystals and the beam is then pulsed by a Fermi chopper (vertical axis of rotation). IN6 is a cold neutron instrument (4.1 Å < <5.9 Å). IN4 is a thermal neutron spectrometer, permitting to cover a wide range of incident energies (up to 100 meV). The resolution function has a nearly Gaussian shape, but with slightly more wings.
IN5 and IN6 are used mainly to study stochastic motions of atoms, molecules, ions and chemical entities on a microscopic level. They are also useful to study low frequency vibrations, in glasses for example. IN4 is designed to study excitations (vibrational or magnetic) at high energies (<100 meV).
They exploit the fact that the wavelength spread of a monochromator is minimal for a scattering angle 2Ѳ =180° to reach very high energy resolutions. Two instruments (IN10 and IN16B) are situated on a cold guide, while IN13 (CRG instrument) is on a thermal guide. The resolution of IN10 and IN16B is better than 1 µeV, while the one of IN13 is of the order of 10 µeV. These instruments are used for studying the dynamics of local motions, diffusion in solids, liquids, polymers or proteins and of low lying inelastic excitations. In particular, IN13 is specialised to study the onset of diffusive motions in proteins as a function of temperature and hydration by using the "fixed window" method (pure elastic scattering). Combinations of such elastic (efws) and inelastic fixed window (ifws) measurements are highly requested on IN16B for parametric studies.
They are based on the determination of neutron velocities by using the Larmor precession of their spin in a constant magnetic field. This is a trick which decouples intensity and resolution: one can use a coarse monochromatization (5 to 15 %). The resolution goes as λ3, so it becomes excellent at long wavelength. IN15 which can use 24 Å neutrons (wavelength twice the one of IN11) can reach resolution around 1 neV (240 kHz). IN11 is equipped with an optional extension for using a detector bank covering 30° of scattering angle: this gives an intensity gain of 20, at the expense of a slight deterioration in energy resolution. The spin-echo spectrometers have made a big contribution in the understanding of the dynamics of polymer melts (concept of reptation), magnetic materials (spin glass, superparamagnetism, frustrated magnets), glass transition and membrane and protein dynamics .
The monochromator, sample and analyser can each be aligned independently. While TOF spectrometers can map out a large q-omega space in one measurement albeit slowly, triple axis spectrometers can focus on a more restricted area in q-omega but collect more precise data with more adaptable resolution.
- Read a general introduction to three-axis spectrometry. Get an impression of inelastic neutron scattering and how it can contribute to your scientific research.
- Read a review of the TAS technique at the ILL.