V15

Extreme environment diffractometer

 

Instrument description

Extreme Environment Diffractometer (EXED) is a time-of-flight instrument optimized for diffraction in restricted angular geometries typical for extreme sample environment. A special focus is on neutron scattering in high magnetic fields. In the near future the instrument will be furnished by a dedicated 25+ T High Field hybrid Magnet (HFM), currently being built by the HZB in collaboration with the National High Magnetic Field Laboratory (Florida, US).

The instrument

EXED is equipped with a multispectral extraction system followed by about 70 m long supermirror guide. As a result, neutrons from both thermal and cold moderators with wavelength range from 0.7 to 15 Å are available for experiments. The lower wavelength limit is given by the kink in the guide that blocks the direct view of the source and provides a sharp cut-off. Apart from the kink, the guide is essentially straight (100x60 mm2 (HxW)) and ends with an elliptic focusing section that compresses the beam spatially in both directions by a factor of 2. For applications requiring low beam divergence, the focusing end can be replaced by a 6 m long pin-hole collimation section with variable apertures. Flexibility of the instrument is ensured by three alternative systems that are available to create neutron pulses: a curved Fermi chopper for very high resolution (Δt ~ 6 μs), a straight Fermi chopper for high resolution (Δt ~ 15 μs) and a counter- or parallel-rotating double disk chopper for medium to low resolution (from 115 μs up to >5000 μs). A number of single disk choppers located downstream prevents frame overlap and defines the bandwidth of interest. Due to the chopper system one can operate the instrument from narrow (~ 0.6 Å) to wide (~ 14.4 Å) wavelength band mode centered at the region of interest, and easily trade resolution for intensity. The secondary instrument is equipped with position-sensitive 3He detector tubes. They are combined in 4 movable detector banks that can be positioned at different angles around the sample (Fig. 1). While the typical sample-detector distance is 1.2 m, a large He-filled detector chamber allows positioning of two detector panels at 6 m away from the sample.

In its current configuration (i.e. w/o HFM) EXED has no angular restrictions and can be used with all types of standard sample environment available at HZB, as well as user own equipment. It complements HZB’s diffraction instrument suite by providing characteristics typical for pulsed instruments e.g. high resolution in backscattering (Δd/d > 10-3) and large dynamic range (0.5 - 1000 Å) (Figs. 1-3). Event-recording data makes time-resolved measurements readily available.

The High Field Magnet

The HFM is a "first of its kind" hybrid magnet system which is capable to reach fields between 25 T and 32 T, making it by far the strongest continuous field available for neutron scattering experiments worldwide (see HFM data Table below). The HFM utilizes Series Connected Hybrid System Technology where water cooled resistive insert coils are mounted in the room temperature bore of a superconducting cable-in-conduit solenoid. Operation of the magnet system requires a dedicated technical infrastructure consisting of high-pressure water cooling for the resistive coil, 4 K Helium refrigerator for cooling of the superconducting coil and 20 kA DC power supply. For the moment all the magnet infrastructure components have been successfully tested and commissioning of the magnet system will start in 2014. More information on the magnet can be found under the following link: http://www.helmholtz-berlin.de/quellen/ber/hfm/hfm/index_en.html

As the field is horizontal, a special feature of the HFM is 30° degrees conical openings at both ends of the resistive insert envisaged for neutron-scattering access. Rotating the magnet by 15° with respect to the incoming beam will result in 2θmax=30° in forward scattering. A dedicated 3He cryostat will allow high-field experiments to be combined with temperatures down to 0.5 K on samples <1.5x1.5 cm2.

Operation Modes of EXED+HFM

In order to enable a broad range of scientific applications using unique combination of neutron scattering and high magnetic fields, EXED is being currently upgraded. A novel concept will combine several scattering techniques in one instrument. The operation modes of multi-purpose EXED are described below.

Elastic (available from the “day one” of combined HFM/EXED operation): Primarily built as a diffractometer, EXED will maintain this option while significantly expanding the low Q-range accessible for the experiments. In its low-Q mode momentum transfer down to 10-2 Å will be accessible using a 6m-long collimation combined with 6m-long He-filled detector chamber. The latter will enable studies of matter on nanoscales in high magnetic fields such as e.g. vortex state in type-two superconductors as shown in Fig. 3.

Single crystal and powder diffraction will be performed in the same manner as at present (Figs. 2-3). Upgrade for the inelastic mode (details are given below) will result in further improvement of the elastic performance (signal-to-noise ratio and full angular coverage in forward scattering).

Inelastic (under development): A major development is taking place to complement the instrument portfolio by inelastic capabilities in the form of a direct TOF spectrometer. The upgrade includes four main components: i) a detector chamber for forward scattering with a built-in ii) 3He detector array covering 30° in- and out- of plane and positioned 4.5 m away from the sample (Fig. 4), and iii) a new focusing guide section that accommodates iv) a monochromating chopper assembly. The chopper produces short monochromatic neutron pulses and TOF is used to analyze the change in the energy of the scattered neutrons. Limited sample size inside the HFM and weak inelastic scattering cross sections imply the need for optimization for signal strength and low-background conditions. The former is achieved by enhancing the flux at the sample using a novel focusing guide, while the latter is provided by means of a shielded and evacuated detector chamber. After completion, the upgraded EXED will enable energy-resolved measurements over a limited Q-range < 3.25/λ-1) in addition to the existing elastic capabilities (Fig. 4).

Applications

  • Quantum magnets and quantum phase transitions
  • Superconductivity
  • Correlated electrons in 3d, 4f and 5f metal compounds
  • Spin, charge and lattice degrees of freedom in transition metal
    oxides
  • Frustrated magnets
  • Novel states of matter

 



Instrument Data
Beam tube NL 4A, 75 m long ballistic multispectral guide
60 x 100 mm2 (straight section)
elliptically tapered down to 30 x 50 mm2 (7.5 m long focusing section)
Collimation i) None;
ii) 6 m long pin-hole collimation section can replace focusing section
Monochromator -
Take off angle of monochromator -
Wave length 0.7 < λ < 15 A
Flux ~1.5·109 n/cm2/s - continuous flux
Range of scattering angles i) Without HFM   +/- 0-170°
ii) With HFM
Elastic      0–30°, 150–170°
Inelastic    0–30°
Angle resolution
Range of lattice spacing Forward scattering: 2 < d < 1000 A
Backward scattering: 0.5 A < d < 7 A
d resolution Forward scattering: Δd/d>2·10-2
Backscattering: Δd/d>1·10-3
       (using full beam divergence)
Sample size i) Without HFM    2x4 cm2
ii) With HFM     <1.5x1.5 cm2
Detector 192 3He linear position sensitive detectors combined in 4 sections, each containing 48 detector tubes of 90 cm effective length and 0.5" diameter
Polarized neutrons -
Instrument options Elastic: Powder and Single Crystal Diffraction; Low Q
Inelastic: under development (direct TOF spectrometer)
Sample environment i) Without HFM    T = 0.05 - 600 K, B=17 T
ii) With HFM    T = 0.5 - 300 K, B=25+ T
Software egraph (event recording data reduction), Mantid (data reduction)
Chopper speed range 5 – 600 Hz (Fermi chopper)
5 – 215 Hz (double disc choppers)
5 – 120 Hz (single disc choppers)
Sample-detector distance i) Without HFM 1.2, 6 m
ii) With HFM 2.5, 4.5 m