The four analyser diffractometers are based on the Cox [1] parallel beam geometry. This compact unit consists of a three-stage rack 480mm long and 350mm high. Its base plate is fixed with the 2 goniometer (880mm in diameter) of the Huber diffractometer. The middle stage carries the crystal analysers and detectors. The top plate holds the analyser shielding houses. A single stepping motor simultaneously adjusts all crystal analysers and a Heidenhain encoder is reading the angle.
Fig. 1 Three stage rack of the Multiple-Detector-System;
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In addition the relative positions of the angles are set by individual drives. Due to the identical design and the simultaneous control of the four analyser diffractometers data corrections are not needed. The output of the measured data is a standard ASCII file and the simultaneous profile refinement of the four data sets may be done by the programs GSAS and / or SIMREF. The later has been particularly designed for powder data from different sources.
Simple and efficient mechanical control: one stepping motor and encoder for four analyser axes (light grey parts in Fig. 2) and one stepping motor and encoder for four detector axes (dark grey parts in Fig. 2).
Fig. 2 Schematic diagram of the simultaneous adjustment of the MDS and the realisation (photo).
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Modular system of analyser units
The modular system of the analyser units consists in their exchangeable crystal analysers. The user can chose between two settings: four flat Si(111) crystal analysers (Fig. 3) or four Ge(111) channel-cut crystal analysers (Fig. 4). Using the multiple-detector system with the vacuum chamber the channel-cut crystal analysers enable measurements up to a wavelength of 2.5Å.
Fig. 3 The flat Si(111) crystal analyser with the adjustable lead shield in front of it.
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The four flat Si(111) crystal analysers are designed for a wavelength range from 0.5Å to 1.7Å. The dimensions of the crystals are 75mm in length, 25mm high and 6mm thick. Adjustable lead shields eliminated cross talk between the entrance and the exit of the analysers at short wavelengths.
Fig. 4 The Ge(111) channel-cut crystal analyser is mounted on the modular part of the analyser unit. To set the individual position of each crystal a small adjustable plate carries the crystal.
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The channel-cut crystal is made of a monolithic Ge crystal. The (+n,-n) arrangement is designed for the symmetrical reflection on the Bragg plane [111] in a wavelength range from 1.2Å to 2.5Å. The channel-cut crystal is 42mm long and 30mm high. The (+n) crystal is only 14mm long and the (-n) crystal is 35mm in length. The channel is 2.5mm wide. To avoid mechanical stress small channels are cut in the lower part of the crystal. The individual position of each crystal can be set by a small adjustable plate that carries the crystal. The main property of the channel-cut analyser in the (+n,-n) arrangement is the small shift between the incident and reflected beam. For this parallel beam path the position of the detector must be not changed for different wavelengths.
Adjustable analyser shielding houses and collimators
The multiple-detector system is constructed for measurements in a wavelength range of 0.5 < lambda < 2.5 Å. Cross talk between the entrance and the exit of the analysers at short wavelengths is eliminated by adjustable lead shields. They operate up to a distance to the crystal of 4mm and are adjustable with a precision of 0.01mm. Special analyser shielding houses adapted to the beam path enhance the typical peak / background ratio to 300 to 1.
Fig. 5 Detailed view of one analyser shielding house (transparent) and the adjustable lead shield (magenta). The flat Si(111) crystal is fixed with small copper springs (left). The Ge channel cut analyser crystal is mounted on the modular part of the analyser unit (right). Outside the analyser shielding house an additional lead shield covers the entrance opening. In front of it a symmetrical slit restricts the incident beam.
To reduce the air scattering between the sample and the entrance slit of the analyser units the MDS is equiped with collimators. These are used for experiments without the vacuum chamber.
Fig. 6 MDS with collimators mounted in front of the analyser units.
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In designing the MDS we have payed much attention to integrate existing sample environments e.g. the vacuum chamber and the cryostat. Experiments with wavelengths > 1.8Å are done with the vacuum chamber (Fig. 7). It is mounted on the omega 1 circle. The MDS operates in the usual way on the 2theta circle.
Fig. 7 MDS with vacuum chamber. The vacuum chamber is mounted on the left side of the diffractometer. The MDS is standing at approximately 110° (top of the photo). Under the MDS the sample holder can be seen.
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[1] D.E. Cox, J.B. Hastings. W. Thomlinson, C.T. Prewitt, Nuclear Instruments and Methods in Physics Research, 208:573-578, 1983
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