EXED – the new Extreme Environment Diffractometer at the Hahn-Meitner-Institute Berlin

J. Peters¹ and F. Mezei^1,2

¹Hahn-Meitner-Institute Berlin, Glienicker Str. 100, 14109 Berlin, Germany.

²LANSCE, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.

The EXED instrument is a very high resolution time-of-flight powder diffractometer, which has been optimised for diffraction in extreme environments. A special focus is on high magnetic fields and thus the instrument will be equipped with a dedicated 25 T cryomagnet. The instrument is being built at the steady state reactor BERII of the Hahn-Meitner-Institut Berlin. However, its sophisticated chopper system allows the application of the time-of-flight (TOF) principle and, compared to a common crystal monochromator instrument, EXED offers a number of advantages on a continuous source: a) it can provide higher resolution, comparable to what is now achieved at synchrotron radiation sources; b) it makes small d-spacing readily accessible; c) it is more efficient in terms of neutron intensity for conventionally high resolution neutron diffraction work and d) it facilitates the use of extreme sample environment equipment by providing a full coverage of the relevant Q domain at very limited angular access in scattering angles, for instance due to the magnet geometry (see Fig. 1). The physical reason for these advantages is that at high scattering angles good resolution can be achieved without collimators.

Fig. 1: Schematic view of the EXED instrument. The grey tubes correspond to a ballistic neutron guide (straight guide and the compressor [1]). The neutron pulses are produced alternatively by a Fermi chopper (in green) or by a counter rotating double disk chopper (in red), which are exchangeable. The other disk choppers (in red) are frame overlap choppers and a wavelength band chopper. The position sensitive gas detectors are presented in yellow and can be moved around the sample and the magnet.

The chopper system allows for a very flexible use of the instrument: As the repetition rate is not defined by the source, almost any d-spacing can be achieved in the forward as well as the backward scattering direction. In addition, a slewing of the chopper phases permits a continuous variation of the wavelength band. In the backward direction it can be combined with a very high resolution. The EXED beam line is unique in the sense that it has access to both the cold and the thermal moderator and takes advantage of a large wavelength band ranging from 0.7 to 20 å with local maxima at 1.4 and 3.8 å.

The detector banks are planned to be equipped with tubes with a diameter of 1 cm and an effective length of 90 cm, which are filled with ³He gas. They shall be position sensitive with a resolution of 1 cm and will cover an effective surface of 50 x 90 cm². It will be possible to move them around the sample position or to translate them. In the 2 m position, the two detector banks will cover a 28° scattering angle range, while in the 6 m position only 10° are covered. In the latter configuration, a movable vacuum or gas tank between the sample and the detectors will be used to reduce air scattering.

Different collimation modules can be applied to further optimise the resolution: the final interchangeable guide section either homogenises the beam whilst maintaining the divergences or further focuses it to the sample position. Or a pine hole collimator permits the fine tuning of the horizontal and vertical divergences. The d-spacings and momentum transfers Q accessible with the various detector bank configurations are listed in table 1. The instrument is designed for both narrow-bandwidth and broad-bandwidth operations, the latter achieved by repetition-rate reduction and/or chopper slewing.

Table 1: Various detector bank configurations. For the two first columns the symmetrical arrangement of the detector is supposed with the corresponding angular coverage of 15° on each side of the neutron beam axis, the two last columns correspond to an asymmetrical arrangement with an angular coverage of 30° on one side of the neutron beam axis.

Bank	(171 ± 7)°	(9 ± 7)°	(163 ± 15)°	(17 ± 15)°
d-spacing	0.35 – 10.1 Ǻ	2.52 - 571 Ǻ	0.35 – 10.4 Ǻ	1.27 - 571 Ǻ
Q	0.62 – 17.9 Ǻ^-1	0.011 – 2.49 Ǻ^-1	0.6 – 17.9 Ǻ^-1	0.011 – 4.95 Ǻ^-1

First Monte Carlo simulation results show typical powder pattern diagrams for an Al sample and confirm the theoretically calculated resolution in backscattering direction of Dd/d ≈ 3 x 10^-4 (see fig. 2) for the wavelength range of 0.7 Ǻ < l < 1.8 Ǻ and at scattering angles 156 ° < 2q < 179 °.

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Fig. 2: Powder pattern MC simulation results for 0.7 Ǻ < l < 1.8 Ǻ and 156 ° < 2q < 179 ° as a function of the d-spacing. At 2q ≈ 176 ° and l = 1.34 Ǻ (rhs), nearly the highest total resolution of dd/d ~ 3´10^-4 is obtained.

Concerning the extreme environments, the most important feature is the high field steady state magnet which can create a magnetic field of up to 25 T (later on possibly 40 T). Depending on the range of scattering angle required the magnet can either be used in a symmetric or asymmetric neutron beam configuration or it can be removed from the experiment to allow for usual elastic diffraction.

Additionally to the magnet or separately further sample environment elements like cryostats creating low or high temperatures ranging from 1.5 – 700 K and pressure cells of up to 20 kbar can be employed.

In a medium-term perspective possible extensions are a small angle scattering (SANS) option with a collimator length of up to 6 m and inelastic scattering at high magnetic fields.

[1] F. Mezei, J. Neutron Res. 6 (1997) 3,

F. Mezei, M. Russina and S. Schorr, Physica B 276 – 278 (2000) 128.