MLZ is a cooperation between:

Technische Universität München> Technische Universität MünchenHelmholtz-Zentrum Geesthacht> Helmholtz-Zentrum GeesthachtForschungszentrum Jülich> Forschungszentrum Jülich
Logo

MLZ (eng)

Lichtenbergstr.1
85748 Garching

POWTEX (under construction)

High-intensity time-of-flight diffractometer

POWTEX scheme POWTEX scheme


The high-intensity TOF diffractometer POWTEX is designed and built by RWTH Aachen University and Forschungszentrum Jülich. The University of Göttingen cooperates by developing the texture analysis and by providing additional sample environments. Both projects have been granted by the German Federal Ministry of Education and Research (BMBF). POWTEX will be part of the JCNS instrumentation pool.

POWTEX is an acronym for POWder and TEXture because the instrument will fulfill the needs of the solid-state chemistry, the geoscience as well as the materials science communities with regard to powder and texture diffraction. The instrument design combines several new concepts. The neutron guide system consists of two double-elliptic parts with a common focal point of 1×1 cm2 at the position of the pulse chopper. Hence, the focal point is an “eye of a needle” in time and space, optimizing the time resolution by the counter-rotating disks of the pulse chopper and reducing the background from the source. The second guide focuses on the small sample spot of 1×1 cm2. The guide system was optimized using the Monte Carlo simulation package VITESS. Herein several new developments were implemented:1 The second guide will correspond to an octagonal cross section to achieve a Gaussian intensity and divergence distribution at the sample. The last part of the second guide is replaceable by an absorbing guide to reduce the beam divergence by a factor of two for the vertical and horizontal dimension.

In order to produce sharp pulses of 10 µs, the pulse chopper uses two counter-rotating discs at 200 Hz and at a short distance of only 0.5 cm. A variation of the chopper settings allows a flexible time structure to change the wavelength band or the pulse frequency (pulse-overlap mode like at POLDI, PSI yields a ten times higher intensity).

Because of the 3He shortage, two new detector developments were initiated (6LiF-WSF and 10B-Jalousie). While both prototypes successfully passed a test beam time, the 10B-Jalousie detector was chosen as the future POWTEX detector concept. The detector design will cover a huge solid angle (10 sr) and its dimensions will allow large sample environments (e.g. in situ experiments). The high detector coverage is of particular importance for texture measurements because it avoids the need for tilting the sample. Furthermore, it allows texture measurements without sample rotation (without sample environment) and it reduces the number of sample rotations with sample environment. It allows simultaneous strain/stress/texture measurements and recrystallization analysis. For powder diffraction the covered solid angle relates directly to the efficiency/measurement time. With the three-dimensional (2θ, TOF, intensity) diffractograms and an adapted data treatment, POWTEX will simultaneously benefit from the higher resolution in back-scattering and the high intensity at lower angles.

[1] A. Houben, W. Schweika, Th. Brückel, R. Dronskowski, Nucl. Instr. and Meth. A, 2012, 680, 124–133.

Typical Applications

The high intensity will allow comparatively short measurement times and a high sample throughput.

  • crystallographic and magnetic structure determination
  • multiphase analysis using the Rietveld method
  • in situ experiments, e.g., on chemical reactions
  • phase transitions as a function of T, p, B0, etc.
  • parametric studies
  • simultaneous stress-/texture measurements
  • in situ deformation (also long-time experiments)
  • recrystallization/annealing experiments
Sample Environment

A tailored set of sample environment is inevitable to avoid shadowing the detector. To benefit from the short measurement times a cryo-furnace with a temperature range from 10 K to 700 K including a sample changer is planned. Our colleagues at the University of Göttingen and the FRM II design a mirror oven for temperatures up to 2000 °C which can be operated with inert or reactive gases. Göttingen University also designs sample environments for specific geoscience applications, e.g., a unique uniaxial and triaxial deformation apparatus.

Technical Data

Beam layout*

  • Thermal neutrons on SR5a
  • NL1: double elliptic with m = 1.75–5.25; quadratic cross section
  • NL2: double elliptic with m = 2.00–5.00; octagonal cross section
  • Divergence ≈ 0.33° – 0.66°

Chopper system*

  • Double-disk pulse-chopper as needle eye; sharp 10 µs pulses; background reduction
  • CFK disks with 75 cm diameter on magnetic bearings
  • Standard range of λ = 1.0–2.4 Å
  • Wavelength bandwidth: Δλ = 1.4 Å (flexible range selection)
  • Pulse duration ≈ 10 µs
  • Pulse frequency 200 Hz (standard) to 2000 Hz (pulse overlap mode)

Detector*

  • Coverage: 2θ ≈ 10–170° (10 sr)
  • d-Range ≈ 0.5–12.5 Å
  • Sample detector distance = 80 cm
  • Spatial resolution ≈ 5×5 mm2

*Expected design values

Instrument Scientist

Dr. Andreas Houben
Phone: +49 (0)241 80-90061
E-Mail:

Further contact

Dr. Werner Schweika (JCNS)
Phone: +49 (0)2461 61-6650
E-Mail:

Dr. Jens Walter (GZ Göttingen)
Phone: +49 (0)551 39-33196
E-Mail:

POWTEX

Operated by

RWTH Aachen
Uni Göttingen
JCNS

Publications

Find the latest publications regarding POWTEX in our publication database iMPULSE:

impulse.mlz-garching.de

Gallery

POWTEX
POWTEX
Beamline
Beamline

The main components of POWTEX are shown from the in-pile part at the source (right) to the detector (left).

The cylindrical detector arrangement of the 10B-Jalousie detector almost avoids blind spots. Every module in the center part is 160 cm in length and tilted by 10°; the azimuthal coverage is 270°. The end cap modules are coiled and tilted at the same time.

Between detector and closely connected to the band chopper a neutron-guide changer is used to change super-mirror elements against absorbing elements in three steps. This allows to reduce the divergence by a factor of two in horizontal and vertical direction without affecting the shape of the divergence profile.

2D-Rietveld
2D-Rietveld

In analogy to typical plots as resulted from refinements using the Rietveld method the picture shows “experimental” diffractograms produced by the Monte-Carlo instrument simulation on the left. The analytical fit to these data is shown in the middle. In the right, a difference pattern of both is shown. To use the full potential of POWTEX the simultaneous refinement as function of the scattering angle and the wavelength is a necessity, while the data presentation itself is versatile.

The two double-elliptic neutron-guides possess a common focal point at the pulse-chopper (blue dots) and with respect to the neutron guide system of the high-pressure station SAPHiR also at the sample position. The double-disk pulse-chopper operates at 200 Hz to produce pulses of 10 µs. By adjusting the chopper settings, the wavelength-band can be changed (1.0 Å ≤ λ ≤ 2.4 Å in the picture) while the overlap- and band-chopper shape it to a constant width of Δλ = 1.4 Å.

MLZ is a cooperation between:

Technische Universität München> Technische Universität MünchenHelmholtz-Zentrum Geesthacht> Helmholtz-Zentrum GeesthachtForschungszentrum Jülich> Forschungszentrum Jülich