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Dear Augustin,
<div class="">(Cc: Erik)
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<div class="">Also thanks for your mail and considerations from here.</div>
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<div class="">Yesterday evening I took a look at the Test_Monochromators instrument where in total 6 models can be used to look at a single monochromator slab, the test models Pyrolytic Graphite (PG). </div>
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<div class="">In that model it turns out that the Single_crystal gives a higher intensity than that of the more phenomenological monochromators, and that several parameters are important to consider:</div>
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<li class="">The “flat” monochromator models have no material “thickness”, everything is described by setting R0</li><li class="">The scattering from the Single_crystal (of course) heavily depends on the geometry and size of the used slab of material</li><li class="">Mosaicity - and therefore divergence - plays an important role wrt. what “peak intensity” one can achieve</li><li class="">In the PG case, the Single_crystal ought to be modelled using the PG flag which makes a random orientation of the crystallites “in-plane”, which is yet another type of mosaicity</li></ul>
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<div class="">It is hence clear that monochromator performance is a non-trivial matter to describe, and to achieve full correspondence between e.g. Monochromator_flat and Single_crystal one should probably</div>
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<li class="">Start from an existing monochromator size / geometry on an existing, well characterised source</li><li class="">Perform some benchmarking experiments, e.g. on a Ge wafer (we did some in the past in connection with the PhD work of Linda Udby, will send you a copy outside of this list)</li><li class="">Allow some “tuning” of mosaicity in the case of Single_crystal</li><li class="">Allow some “tuning” of both mosaicity and reflectivity in the case of the other monochromators</li></ul>
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<div class="">All in all, monochromators is not a trivial topic and have been the subject of much research, development and modeling in the past - see e.g. the papers from Scherm, Kruger, Freund.</div>
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<div class="">If you start by sending Erik and I a copy of your instrument file and also your documents used to describe Stress-Spec we might be able to help further. :-)</div>
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<div class="">Best,</div>
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<div class="">Peter</div>
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<div class="">On 5 Nov 2018, at 22.11, Erik B Knudsen <<a href="mailto:erkn@fysik.dtu.dk" class="">erkn@fysik.dtu.dk</a>> wrote:</div>
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<div class="">Dear Agustin,<br class="">
<br class="">
Gear to hear that you've made comparisons between Monochromator_curved<br class="">
and Single_crystal. Obviously not so great that they don't match up.<br class="">
This should not be.<br class="">
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We'd like to help to get to the bottom of this. I assume you have set<br class="">
the mosaicity of the crystals to be identical, but what did you set the<br class="">
mosaic to?<br class="">
Could you also supply an instrument file with a set of example<br class="">
parameters so that we can reproduce the mismatch easily.<br class="">
<br class="">
In particular - which reflection file did you use for Ge?<br class="">
One thing springs to mind - There were some indications some years ago<br class="">
that there was an oddity in the McStas supplied Ge data file, but I<br class="">
don't know what the conclusion was.<br class="">
Perhaps we could retry the experiment with a Si crystal, to see if the<br class="">
factor is still 0.23 (or thereabout).<br class="">
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kind regards<br class="">
Erik B Knudsen<br class="">
<br class="">
On 05/11/18 19:23, Agustin Beceyro wrote:<br class="">
<blockquote type="cite" class="">Hello to everyone!<br class="">
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I'm writing this email because I'm finding some trouble while using the<br class="">
Single_crystal component and comparing it with the Monochromator_curved<br class="">
component.<br class="">
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My input is extremely simple: a source, a Ge monochromator, and a PSD<br class="">
monitor. The monochromator has vertical focusing, so it is formed by 7<br class="">
crystals (for Monochromator_curved Nv=7, for Single_crystal I've grouped<br class="">
7 components).<br class="">
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I've been playing around a little bit with this configuration, change<br class="">
the distance source-monochromator, monochromator-detector, the<br class="">
monochromator take-off angle, add collimators, even changed the number<br class="">
of crystals used. For each configuration, I made a scan of Rv (vertical<br class="">
focusing radio). While plotting the Rv vs. normalized intensity I get<br class="">
the same behavior in all cases, and maximum intensity is achieved with<br class="">
the theoretically calculated value of Rv.<br class="">
<br class="">
My problem arose when evaluating the PSD intensity. With Single_crystal<br class="">
I get between 0.22 and 0.24 of the intensity I get with the other<br class="">
component (in every single case). I believed it was a reflectivity<br class="">
issue, but changing the take-off angle affects lambda and the<br class="">
reflectivity. Besides, I compared these values with different<br class="">
bibliography and it seems OK.<br class="">
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Besides, I've done another test with a geometry very similar to<br class="">
Stress-Spec and compared it to some published data. With<br class="">
Monochromator_curved I obtained about 30% more intensity (but I don't ha<br class="">
the exact geometry or source), with Single_crystal I obtained 70% less<br class="">
intensity. Still, the ratio between both cases remains 0.23.<br class="">
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I'm sure I'm missing something, but I can't figure out what and I can't<br class="">
find anything related to the validation of the component. Someone, any<br class="">
clues?<br class="">
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With best regards,<br class="">
<br class="">
<br class="">
Ing. Agustín Beceyro Ferrán<br class="">
Departamento de Física de Neutrones<br class="">
Centro Atómico Bariloche - CNEA<br class="">
San Carlos de Bariloche<br class="">
Rio Negro, Argentina<br class="">
<br class="">
<br class="">
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<br class="">
</blockquote>
<br class="">
-- <br class="">
Erik Bergbäck Knudsen, Research Engineer | DTU | morituri<br class="">
NEXMAP, DTU Fysik, DK-2800 Kgs. Lyngby, Denmark |<>-<>| te<br class="">
phone: (+45) 2132 6655 |<>-<>| salutant<br class="">
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