Multianalyser & multidetector at the FRM2 in Munich Garching
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The thermal neutron three-axis spectrometer PUMA at the Heinz Maier-Leibnitz Neutron Research Source (FRM II) in Garching near Munich was developed and built as part of a project funded by the Federal Ministry of Research and Technology at the Institute of Physical Chemistry at the University of Göttingen and is operated in cooperation with the Technical University of Munich.Using neutrons as a probe, this large-scale research device can be used to investigate the co-operative movements of atoms in crystalline matter and draw conclusions about their bonding relationships and thus about technically relevant material properties.
The detected signals are generally very weak, i.e. relatively long measurement times are required to detect optimally analysable signals. This fact and the need to investigate the materials under realistic conditions of use (temperature change, pressure change or temporal progression of these changes) led to the development of an innovative component for detecting the signals, a so-called multidetector in combination with a multianalyser.
This system can simultaneously cover an angle of approx. 16° (previous coverage of the standard detector approx. 1°) and simultaneously realise any measurement settings.
The time saved by reducing the repositioning of the device and efficient background reduction of the system now not only allow faster measurements, but also access to time-dependent measurement techniques that were previously not possible.
The multi-detector unit offers two measurement modes, single aperture and vane aperture mode. These stand for the optimum utilisation of the neutron beam for two beam guidance geometries. Both are mounted on a base unit that can be rotated by 90° around the centre of the analyser with a precision of 0.01° using air feet. For vane operation, 11 cylindrical detectors with a diameter of 25.4 mm are moved on a rail system (R= 761 mm) with an angular resolution of 0.01°. This movement option together with the central rotation enables a wide range of measurement settings. For effective background reduction, vane diaphragms are used which can be individually aligned to the respective analyser crystals with regard to their rotation and thus only allow the desired neutrons to pass towards the detector. Measurements utilising this geometry prove the high efficiency of the detector, especially when detecting very weak signals.
An additional measuring geometry using a so-called position-sensitive detector (PSD) covers those beam guides for which the angular resolution of the individual detectors is not sufficient to set the measuring range due to their expansion. The PSD is mounted behind the individual detectors at a distance of 900 or 920 mm and can be swivelled up to 90° via the common base plate. Due to its horizontal extension of 220 mm, it covers an angular range of 14° simultaneously. To give the neutron beam a clear view of the PSD, the vane diaphragms of the individual detectors can be moved out of the beam path. The use of a linear diaphragm and additional shielding material also enables the detection of very weak signals for this configuration. The aperture unit has a fixed vertical aperture of 130 mm and a variable horizontal aperture of up to 40 mm. To ensure adaptation to the respective beam guidance, it can also be translated by 100 mm.
All drives for carrying out the rotary and translational movements are realised by geared stepper motors and their absolute position is partly sensed by absolute encoders. The high positioning accuracy of these units has already been demonstrated in real measuring operations and thus enables new types of measuring set-ups.
Poster download as PDF file