[mcstas-users] Help with Single_crystal

[Erik B Knudsen]

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