V2
Cold Neutron 3-Axis Spectrometer (FLEXX)
Instrument description
The instrument name is derived from the flexibility of the spectrometer parameters which permit non-standard experiments and its use in the development of novel methods. The instrument was upgraded in 2011 and rechristened FLEXX. Its new position at the end of the NL1B guide allows a large coverage in wavevector transfer. The guide itself has been upgraded with m=3 neutron supermirrors to allow more neutrons to be transported from the source to the instrument. In addition, there is an elliptical guide which focuses neutrons onto a virtual source which is subsequently imaged on the monochromator. Finally, a velocity selector is used to select a wavelength band, and so obviate the need for a second order filter and associated loss of neutrons.
The flexibility of the instrument comes from the ability to choose the distances between the monochromator and sample, between the sample and analyzer and between the analyzer and detector in order to obtain an optimal scattering geometry at the required angular and energy resolutions. For the same purpose a tunable curved (double focussing) monochromator and (horizontally focussing) analyzer are used. With an optimally curved monochromator, the increase in flux at the sample position as compared to the old FLEX is a factor of 5 to 10, at the expense of Q-resolution while retaining the good energy resolution of a cold TAS.
Where possible the spectrometer was made of non-magnetic materials to allow for the use of polarized neutrons in the same set-up as for unpolarized neutrons. For the polarized beam option Helmholtz coils, Mezei flippers and a Heusler analyzer may be used, whilst an S-bender polarizer may be translated into the guide before the elliptical section to minimize neutron losses.
FLEXX provides the opportunity to combine triple axis spectroscopy and high resolution spin-echo spectroscopy. The neutron resonance spin echo method (NRSE) option at FLEXX is particularly suited for the phonon focussing technique with tilt spin echo fields. By this method the line widths of dispersive elementary excitations can be measured with an energy resolution in the μeV range.
From September 2016, users can submit proposals to use the new multianalyzer option: MultiFLEXX. It consist of a multiplexing back-end, offering a simultaneous measurement of 5 different final energies and 31 different 2θ channels. With a range of fixed vertically scattering analyzers, this optional secondary spectrometer provides the opportunity to do overview measurements with very little effort, using less than half an hour to mechanically switch between back ends.
Before applying for an experiment utilizing the high resolution NRSE option or MultiFLEXX, please contact an instrument responsible directly for a feasibility dialogue.
Applications
- Low-energy phonon dispersion
- Soft modes
- Low-energy spin wave dispersion
- Low-energy crystal field exitations
- Quasielastic scattering
with MultiFLEXX
- Parametric overview studies of low energy excitations as a function of temperature or field
- Mapping of diffuse magnetic inelastic scattering
with NRSE
- Line widths of low-energy dispersive excitations in non-magnetic
and antiferromagnetic samples - Quasi-elastic linewidths
- High-resolution elastic scattering (Larmor diffraction) with single
crystal and powder samples
Instrument Data | |
---|---|
Neutron guide | NL 1B |
Collimation | Gd coated Soller collimators. Before monochromator: 40', 60' available on a motorised translation table. Before sample, analyzer and detector: 20', 40', 60', and radial collimator available |
Monochromator | Pyrolythic graphite (002) with variable vertical and horizontal curvature |
Take off angle of monochromator | 36° < 2ΘM < 136° |
Wave length range | 2 Å < λ < 6.2 Å (in first order of monochromator) |
Energy range | 2 meV < EM < 19 meV (ΔE range with Φ > 2·107 n/cm2s: 0 - 18 meV) |
Energy resolution | 0.05 to 0.5 meV (FWHM) |
Range of scattering angles | -130° < 2ΘS < 130° (for most configurations) |
Detector | vertical 3He single detector |
Analyzer | Pyrolythic graphite (002) with variable horizontal curvature |
Range of final energies | 2.6 < Ef < 15 meV |
Scattering angle at analyzer | -120° < 2ΘA < 120° |
Flux | • 6·107 n/cm2s at 2.5 Å • 3·107 n/cm2s at 4 Å (with focused monochromator) |
Filter | Velocity selector and possibility of Be-filter if needed |
Polarized neutrons | yes |
Polarizer | Compact S-bender for the incoming beam and Heusler Analyzer |
Instrument options | • NRSE option (see specification below) • MultiFLEXX (see specification below) |
Sample environment | Can accept all magnet cryostats and dilution sticks at HZB |
Software | TASMAD |
with NRSE | |
Neutron guide | NL 1B |
Collimation | Gd coated Soller collimators. Before monochromator: 40', 60' available on a motorised translation table. Before sample, analyzer and detector: 20', 40', 60' available |
Monochromator | Pyrolythic graphite (002) with variable vertical and horizontal curvature |
Take off angle of monochromator | 67° < 2ΘM < 136° |
Wave length range | 2.5 Å < λ < 6.2 Å |
Energy range | 6 meV < EM < 19 meV |
Energy resolution | in the order of µeV |
Range of scattering angles | • inelastic mode: -110° < 2ΘS < 110° (with configurational restrictions) • elastic (Larmor diffraction) mode: -110° < 2ΘS < 90° or 90° < 2ΘS < 110° |
Detector | vertical 3He single detector |
Analyzer | Heusler |
Range of final energies | |
Scattering angle at analyzer | -155° < 2ΘA < 155° |
Flux | 3·105 n/cm2s at 3.7 Å |
Filter | Velocity selector |
Polarized neutrons | yes |
Polarizer | compact S-bender |
Instrument options | • NRSE option (this specification) |
Sample environment | Orange cryostats and closed cycle refridgerators. |
Software | SEMAD |
with MultiFLEXX | |
Neutron guide | NL 1B |
Collimation | Radial collimator (oscillating) |
Monochromator | Pyrolythic graphite (002) with variable vertical and horizontal curvature |
Take off angle of monochromator | 55° < 2ΘM < 117° |
Wave length range | 3.1 Å < λ < 5.7 Å |
Energy range | 2.5 meV < EM < 9 meV |
Energy resolution | |
Range of scattering angles | 20.5 < 2ΘS < 124.5° |
Detector | 155 horizontal 3He single detectors |
Analyzer | 155 Pyrolythic graphite (002) pieces set to give 5 fixed final neutron energies |
Range of final energies | Ef = 2.5; 3; 3.5; 4; 4.5 meV |
Scattering angle at analyzer | |
Flux | • 6·107 n/cm2s at 2.5 Å • 3·107 n/cm2s at 4 Å (with focused monochromator) |
Filter | Velocity Selector |
Polarized neutrons | No |
Polarizer | |
Instrument options | |
Sample environment | Can accept all magnet cryostats and dilution sticks at HZB |
Software | TASMAD |