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 has been recently upgraded and rechristened FLEXX. Its new position at the end of the NL1B guide will allow a larger coverage in wavevector transfer, and 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. 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.

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.

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 cavity-type transmission polarizing analyzer may be used, whilst an S-bender polarizer maybe 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.

Before applying for an experiment utilizing the high resolution NRSE option, 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 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

Selected examples

Standard FLEX: Dynamics of the anisotropic Haldane antiferromagnet NDMAP

FLEX with NRSE: Phonon-lifetime in lead

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' available
Monochromator Pyrolythic graphite (002) with variable vertical and horizontal curvature
Take off angle of monochromator 30° < 2ΘM < 150°
Wave length range 2 Å < λ < 6.5 Å
Energy range 1.9meV < EM < 27 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
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
Instrument options • NRSE option (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 < 150°
Wave length range 2.5 Å < λ < 6.5 Å
Energy range 6 meV < EM < 27 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