[mcstas-users] Help with Single_crystal

[Peter Kjær Willendrup]

Dear Augustin,
(Cc: Erik)


Also thanks for your mail and considerations from here.


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).

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:


  *   The “flat” monochromator models have no material “thickness”, everything is described by setting R0
  *   The scattering from the Single_crystal (of course) heavily depends on the geometry and size of the used slab of material
  *   Mosaicity - and therefore divergence - plays an important role wrt. what “peak intensity” one can achieve
  *   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

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


  *   Start from an existing monochromator size / geometry on an existing, well characterised source
  *   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)
  *   Allow some “tuning” of mosaicity in the case of Single_crystal
  *   Allow some “tuning” of both mosaicity and reflectivity in the case of the other monochromators


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.


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. :-)

Best,

Peter


On 5 Nov 2018, at 22.11, Erik B Knudsen <erkn at fysik.dtu.dk<mailto:erkn at fysik.dtu.dk>> wrote:

Dear Agustin,

Gear to hear that you've made comparisons between Monochromator_curved
and Single_crystal. Obviously not so great that they don't match up.
This should not be.

We'd like to help to get to the bottom of this. I assume you have set
the mosaicity of the crystals to be identical, but what did you set the
mosaic to?
Could you also supply an instrument file with a set of example
parameters so that we can reproduce the mismatch easily.

In particular - which reflection file did you use for Ge?
One thing springs to mind - There were some indications some years ago
that there was an oddity in the McStas supplied Ge data file, but I
don't know what the conclusion was.
Perhaps we could retry the experiment with a Si crystal, to see if the
factor is still 0.23 (or thereabout).

kind regards
Erik B Knudsen

On 05/11/18 19:23, Agustin Beceyro wrote:
Hello to everyone!

I'm writing this email because I'm finding some trouble while using the
Single_crystal component and comparing it with the Monochromator_curved
component.

My input is extremely simple: a source, a Ge monochromator, and a PSD
monitor. The monochromator has vertical focusing, so it is formed by 7
crystals (for Monochromator_curved Nv=7, for Single_crystal I've grouped
7 components).

I've been playing around a little bit with this configuration, change
the distance source-monochromator, monochromator-detector, the
monochromator take-off angle, add collimators, even changed the number
of crystals used. For each configuration, I made a scan of Rv (vertical
focusing radio). While plotting the Rv vs. normalized intensity I get
the same behavior in all cases, and maximum intensity is achieved with
the theoretically calculated value of Rv.

My problem arose when evaluating the PSD intensity. With Single_crystal
I get between 0.22 and 0.24 of the intensity I get with the other
component (in every single case). I believed it was a reflectivity
issue, but changing the take-off angle affects lambda and the
reflectivity. Besides, I compared these values with different
bibliography and it seems OK.

Besides, I've done another test with a geometry very similar to
Stress-Spec and compared it to some published data. With
Monochromator_curved I obtained about 30% more intensity (but I don't ha
the exact geometry or source), with Single_crystal I obtained 70% less
intensity. Still, the ratio between both cases remains 0.23.

I'm sure I'm missing something, but I can't figure out what and I can't
find anything related to the validation of the component. Someone, any
clues?

With best regards,


Ing. Agustín Beceyro Ferrán
Departamento de Física de Neutrones
Centro Atómico Bariloche - CNEA
San Carlos de Bariloche
Rio Negro, Argentina


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--
Erik Bergbäck Knudsen, Research Engineer         | DTU | morituri
NEXMAP, DTU Fysik, DK-2800 Kgs. Lyngby, Denmark  |<>-<>|    te
phone: (+45) 2132 6655                           |<>-<>| salutant
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