About this blogDiffraction is a technique to determine the atomic structure of materials very accurately. Most scientists use X-ray diffraction (XRD) for this, but hydrogen and neighbours in the periodic table of Mendeleev are sometimes hard to find or distinguish. Neutron diffraction can do precisely that! Moreover, magnetic structures can be revealed and atoms can be highlighted or masked by isotope substitution.
Since neutrons are difficult to generate, such a diffractometer is not avaible within 500km, so the Reactor Institute Delft (TU Delft) has taken up the initiative to provide such an instrument to Dutch users.
This blog is about the progress of the design and building of PEARL: the Dutch neutron (powder) diffractometer.
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The ISIS detector group has visited our facility to perform some tests with us on their new scintillator detectors. Scintillator detectors are usually more gamma sensitive and less neutron sensitive than the standard 3-He detectors. But since the enormous increase in the price of 3-He, more efforts have been put into the development of scintillator based detectors. The ISIS team came to the RID for tests and demonstration of clear-fibre and wavelength-shifting fibre detectors and a whole set of electronics to evaluate the output of these detectors.
During a three-day test on our reflectometer, we have compared these detectors with the standard 3-He detector of the reflectometer. The preliminary results of these tests show that the neutron detection efficiency is some 70-80% of this 3-He detector and the gamma sensitivity (without optimization of the pulse analysis of the scintillator electronics) is between 20 and 50% higher than the 3-He tube. More involved tests and optimizations need to be done for the gamma-sensitivity. A more severe testing would be counting neutrons in a strong(er) gamma background.
The photo above shows the installation of one of the detectors (the aluminium box) inside the sample-chamber of our neutron-reflectometer at RID. The neutron beam impinges from the left on the scintillators inside the box (not visible) and the light generated by these scintillator crystals is amplified by the photon-multiplier-tubes (PMTs) which are read out by the electronics (not visible). The pulse-shape- and coincidence analysis of these electronics determine whether a neutron or a gamma is detected. The detectors were tested on the reflectometer, so that we could use the time-of-flight option and diafragms of this instrument.
The neutron powder diffractometer (in spe) is now properly announced on the web page of our group Neutron and Positron Methods in Materials (NPM2) under "Facilities" (although we’re of course in development still….).
The NPM2 group is part of the research teams "Radiation, Radionuclides & Reactors" (RRR) housed at the reactor facility Reactor Institute Delft (RID). The RRR is a section of the Faculty of Applied Sciences of the Delft University of Technology.
Now you have the complete overview….
The first components are in: perfect single crystals lent to us kindly by the ILL. The Copper ones are made in-house at the ILL and the Germanium ones are available commercially. But then, they are so perfect that they reflect too few neutrons, so the trick is to make them less perfect. At the ILL they are pressed at high temperature to introduce a controlled ‘micro-cracking’. Like that the whole block will consist of many small perfect crystallites that have slight misorientations with respect to the original orientation. This mosaic spread increases the intensity by allowing a larger wavelength band to be reflected. Unfortunately this also increases the divergence of the reflected beam. Typical mosaic spreads are less than a half a degree, although for instance with graphite crystals you can have 2 or 3 degrees.
The director of the institute prof. Bert Wolterbeek has formally initiated the ‘task force Diffractometer’ on Monday. Now that the main specs of the instrument are known (those that define the performance of the instrument), we foresee that we could for instance trade off some neutron beam intensity for a lower radiation background. Or the monochromator take-off angle could be reduced for easier accessibility:
Time for team work!
We need to iterate over several designs of the instrument as well as radiation shielding and technical solutions to choose the –over-all– best layout. This means the design engineers need to be involved in detail, together with shielding and radio-protection experts. The management board of the institute keeps an eye on progress.The first meeting of the task force will be coming Monday. A first rough Gantt chart of the planning is done and people need to have their say about it.
The constant wavelength (CW) neutron diffractometer can be placed on one of two locations that are available at the reactor. Although one has a 20% higher thermal flux, the instrument would be better accessible at the other beam line. This practicality makes the daily operation easier, but also the installation of the (heavy) shielding at the beginning.
In Ruud’s drawing above: The (yellow) beam travels from the core to the (red) focussing monochromator which reflects 1.7AA neutrons to the (blue) sample. The scattered neutrons from the sample are counted by the (green) banana-shaped detector that is centered on the sample. The environment around the instrument is shielded from the gamma and neutron radiation by thick (grey semi-transparent) walls.