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.
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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.
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Tagged components, crystals, cu, diffraction, ge, ill, neutron, npm2, rrr, scattering, tests, tnw, tudelft
With the detailed planning of the development project of the neutron diffractometer, we also need to know more precisely what the components will cost.
The two most demanding items on the list of instrument components are the neutron detector that needs a high spatial resolution and a monochromator with the right crystalite mosaic.
In a short visit to the ILL we have discussed the possibilities to develop a detector and a monochromator for our needs. On our side, we now need to see what part of that development can be done in-house at the university. This should then clarify how much budget we have to reserve for these components. The collaboration on the development has to be formally signed by both institutes.
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Tagged components, crystals, cu, diffraction, ge, ill, neutron, npm2, rrr, scattering, shopping, tests, tudelft