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D7

Diffuse Scattering Spectrometer

ILL7 building, Cold neutron guide H15

(Vertical cold source)

Key Parameters

Wavelength 
λ [Å]

Takeoff angle
2θ [°]		Polarized Flux 
Φ [n cm2 s-1] 		Incident Energy
Ei [meV]		δE/E

Qmin,Qmax [Å-1]

3.1

55.11.2 × 1068.516.3 %

0.4, 4

4.8

92.31.8 × 1063.554.4 %

0.3, 2.5

5.7

116.30.8 × 1062.513.7 %

0.2, 2.1

Maximum beam size at sample: 55 × 25 mm

Technical Specifications

Monochromator

3 × HOPG (002), each with vertical focusing (5 crystals, 7.5 x 1.7 cm2)

Polarizer

Focusing bender supermirror (Co/Ti, m=2.8)

Flipper

Adiabatic spin rotation (Mezei-type)

Chopper

Gd-coated Si Fermi chopper
Available for XYZ

XYZ Coils

Quadrupole coils for orientation in the XY plane
Helmholtz pair for orientation along Z 

Analysers

66 Schärpf-type bender supermirrors (Co/Ti, m=2.8)

Detectors

3 Banks × 44 detectors (Ø:25 mm (1°), Height: 255mm)
Total angular range: ~10° < 2θ < ~155°

Sample Environment

Dedicated

Orange cryostat: 1.5 - 300 K

Dedicated

Furnace: 300 - 800 K 

Pool

Dilution refrigerator: 0.05 - 300 K

Pool

Cryofurnace: 2 - 550 K

Pool

Cryomagnet: vertical field up to 5.5 T at sample position, compatible with dilution

Pool

High pressure: ≤ 4 GPa (40 kbar), 10 < temperature < 300 K

Pool

Gas handling

 D7 has been dismantled.  The new permanently-polarized diffuse scattering spectrometer D007 will deliver its first neutrons in 2024.

Instrument Description

Diffuse scattering, found beneath nuclear and magnetic Bragg peaks in a neutron diffraction pattern, yields detailed information on magnetic configurational disorder, and of the interplay between such disorder and local atomic defect structures. However, magnetic disorder scattering is generally small in amplitude and often coexists with nuclear disorder scattering and spin-incoherent scattering. In order to separate each of these components it is necessary to use polarized neutron techniques. D7 [1] features a unique combination of 3-directional (XYZ) polarization analysis in a multidetector setup with the option of time-of-flight energy analysis, in order to examine this diffuse scattering.

A vertically and horizontally focusing pyrolytic graphite monochromator situated in the cold neutron guide H15, monochromates the neutrons with wavelengths of 3.1 Å, 4.8, or 5.7 Å.  The beam is polarized using a focusing, m = 2.8 Schärpf bender-type supermirror polarizer. The neutron spins are manipulated using a precession coil flipper, followed by a set of guide-field coils situated around the sample position. There are currently 66 Schärpf bender-type, m = 2.8 supermirror analysers in the secondary spectrometer in front of 132 3He detectors, situated at 1.5 m from the sample position.

There are two modes of operation of D7:

Diffraction:

The most common use of the instrument is as a diffractometer, measuring the scattering from powders and single crystals. The data are analysed in the static approximation [2]. Depending on the experiment, the polarization direction at the sample position is usually manipulated in one of the following manners:

  • Uniaxial Polarization Analysis:
    The polarization direction is fixed normal to the scattering plane and the spin-flip and non-spin-flip scattering is measured. Used for separation of coherent and spin-incoherent scattering in non-magnetic samples; the separation of aligned and transverse spins in non-collinear ferromagnets and single crystals of complex antiferromagnets.
  • XYZ Polarization Analysis:
    The polarization direction alternates between three orthogonal directions and the spin-flip and non-spin-flip scattering is measured at each. Used for the separation of the magnetic, incoherent, and coherent nuclear contributions to the total scattering from disordered/antiferromagnetic/paramagnetic samples [1,3]. This is referred to as the 6-point method because a total of 6 polarization-dependent measurements are made at each instrument position.
  • 10-point measurement:
    The polarization direction alternates between 5 directions: the normal to the scattering plane (Z); two orthogonal directions in the scattering plane (X and Y); and two other orthogononal directions rotated 45 degrees to the previous directions (X+Y and X-Y). The spin-flip and non-spin-flip scattering is measured at each instrument position, giving a total of 10 points. The measurement is an extension of the 6-point method to account for the vertical divergence in the scattered beam [4].

Spectroscopy:

D7 has a Fermi chopper and can be operated as a time-of-flight spectrometer with permanent polarization analysis.
It is trivial to switch between the two modes, however each mode needs separate calibrations. Furthermore, the Fermi chopper cuts the incident beam flux by a factor ~150.  Thus, spectroscopy measurements need substantially more measurement time.

Finally, the resolution of the instrument has been charcterized and details can be found in Ref. [5].

    References:

    1. J. R. Stewart, et al.J. Appl. Cryst. 42, 69 (2009) Disordered materials studied using neutron polarization analysis on the multi-detector spectrometer D7

    2. G. L. Squires, Introduction to the theory of thermal neutron scattering (2012) Cambridge University Press

    3. O. Schärpf and H. Capellmann, Phys. Stat. Sol. A 135, 359 (1993) The XYZ-difference method with polarized neutrons and the separation of coherent, spin incoherent, and magnetic scattering cross sections in a multidetector

    4. G. Ehlers, et al. Rev. Sci. Instrum. 84, 093901 (2013) Generalization of the classical xyz-polarization analysis technique to out-of-plane and inelastic scattering

    5. T. Fennell, et al. Nuclear Instrum. Methods Phys. Res. A 857, 24 (2017) Wavevector and energy resolution of the polarized diffuse scattering spectromenter D7