HFM/EXED

High Magnetic Field Facility for Neutron Scattering

!!! HFM/EXED accepts PROPOSALS !!! (for details, there is a short presentation available in the download area on the right)

Next soft deadline (after which the proposals are sent to the referees) will be announced soon. Note, no proposal will be considered if it has not been discussed with the local contact. It is strongly recommended to do it well in advance.

The changes to the instrument made it necessary to make the current development version of EXEQ available for download. This is the version 0.15-RC3, recommended for any new proposals. More information can be found below, in the Experiment Planning section.

Note, since no sample rotation is available at the moment, the sample has to be properly oriented and fixed on a sample holder prior to the experiment. Information about the sample space geometry and dimensions are available in the download area.

 

Facility description

High Magnetic Field Facility for Neutron Scattering consists of two main components: the High Field Magnet (HFM) and the Extreme Environment Diffractometer (EXED). The former is a  dedicated 26 T hybrid magnet, built by the HZB in collaboration with the National High Magnetic Field Laboratory (Florida, US). The latter is a time-of-flight instrument optimized for neutron scattering in restricted angular geometry of the magnet.

The High Field Magnet

The HFM is a "first of its kind" hybrid magnet system reaching fields as high as 26 T, making it by far the strongest continuous field available for neutron scattering experiments worldwide (see Fig. 1 below) [1]. 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. 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 horizontal cryostat allows high-field experiments to be combined with temperatures down to 0.6 K for samples with cross section <1.5x1.5 cm2.

The instrument

EXED (see Fig. 2 below) is equipped with a multispectral extraction system followed by about 70 m long supermirror guide [2]. 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 are positioned in forward and backward scattering reflecting the geometry of the magnet (Fig. 2). While the smallest sample-detector distance will be about 2.5 m, a large He-filled detector chamber allows positioning of two detector panels at 6 m away from the sample.

Operation Modes of HFM/EXED

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 [2].

Single crystal and powder diffraction will be performed in the same manner as before the HFM installation [2]. 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 [3]. 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 [3].

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

Experiment planning

We provide a web-based interface for calculating the instrument range and finding the optimal sample orientation. It is available at:

www.helmholtz-berlin.de/pubbin/exeq.pl

An interactive version of EXEQ software is available for download (link on the right). The code is written in Python 2.7.x, and installation notes are included in the archive. Please keep in mind that this is a beta-release and correct operation of the software may not always be guaranteed. Also, this is free software, and therefore all the code is available for reading. The current version available for download is 0.15-RC3, which will be shortly replaced by 0.16, as soon as is becomes available. The web-interface is at the moment still at version 0.13.1.

Please note that the code has not been optimised for speed as yet. If the web-based version of the code (which performs the calculations on the server side) is not performing up to your expectations, you may want to try downloading the interactive version and running it on your own computer. The downloadable version writes processed detector definitions to hard disk, therefore its start-up speed should improved after the first time it has been used.

Fig. 1: HFM and its technical characteristics

Fig. 1: HFM and its technical characteristics

Fig. 2: Schematic layout of HFM/EXED

Fig. 2: Schematic layout of HFM/EXED

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
Wave length 0.7 < λ < 15 A
Flux ~1.5·109 n/cm2/s - continuous flux
Range of scattering angles Elastic      0–30°, 150–170°
Inelastic    0–30°
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 <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
Instrument options Elastic: Powder and Single Crystal Diffraction;             Low Q
Inelastic: under development (direct TOF spectrometer)
Sample environment T > 0.5 K .. RT, B=26 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 2.5, 4.5 m
Sample rotation under construction

Publications