[mcstas-users] Component Monochromator-reflect

[Peter Kjær Willendrup]

Dear Thierry,


On 3 Jun 2021, at 16.02, Thierry Bigault <bigault at ill.fr<mailto:bigault at ill.fr>> wrote:

My aim is to optimize the design of a monochromator for an angle-dispersive reflectometer with a fixed take-off angle, and whose scattering plane is horizontal.
I would like to estimate the performance of crystals (e.g. Si 111) which are both mosaic and bent horizontally, along a cylinder. The bending is expected to result in an additional effective mosaic contribution, which depends on thickness. This should combine with intrinsic mosaic and extinction effects.
Ideally I would also like to look at the harmonics contributions.

As I understood Monochromator_curved cannot do all of these, because it is infinitely thin, it does not calculate structure factors and the reflectivity is given empirically rather than being deduced form the crystal structure.
I started using Monochromator_curved, because I also want the monochromator to focus vertically. For this I use several crystals, which are positioned with different angles but not bent in vertical. This is easily implemented with this component. Maybe the treatment of the vertical direction could be decoupled from the horizontal, if things are getting too complicated.

Concerning Single_crystal, I thought it was not suited, since the first sentence in the component manual says it  "models a thick, flat single crystal..."
But on the web help (http://www.mcstas.org/download/components/samples/Single_crystal.html) it looks quite different:
"The crystal lattice can be bent locally, keeping the external geometry unchanged.
Curvature is spherical along vertical and horizontal axes."
And I see now that one can enter RX and RY parameters, so I guess it should work.

Good point, the documentation for Single_crystal could indeed need an overhaul. I believe that the RX/RY approach could work in your case.

Concerning the other points:
- "4 ways of modelling mosaic, depending on your situation": which one would you recommend in my case ? I will assume isotropic mosaicities (except the contribution of bending). Beyond this I did not try yet to understand the subtleness of the different methods.

This is probably also the relevant parametrisation in most cases. For a deeper understanding of the other models I recommend having a look at the illustrations in the component manual together with the component doc text.

- I do not how Union works, and didn't find it in the manual. Is it something new in McStas 3.0 ? I am learning with 2.6.1 on Windows, and I am far from being a software expert.

The Union subsystem (for now) has a stand-alone manual, see https://github.com/McStasMcXtrace/McCode/blob/master/doc/Union/union_manual_v0.91.pdf .

It implements the possibility of making an assembly of McStas “sample” volumes and handles the scattering-processes with that assembly, for instance allowing any order of multiple scattering back and forth between the elements of the assembly.

There are a number of example instruments included in your 2.6.1 for inspiration.

- If Single_crystal does the job, I was wondering why Lucia Alianelli developped a new component for mosaic crystals. Do you know if the modelling approach is different, or is it better suited to particular cases ? As she published some systematic comparisons with measurements on real crystals, do you know if similar things were done to validate the Single_Crystal component ?

First of all, at the time of this development some doubts had been raised about the validity of certain aspects of the Single_crystal model which later were found to be heavily exaggerated.

Further, Lucia’s work was a central part of her PhD work https://www.esrf.fr/computing/scientific/people/srio/publications/alianelli_thesis.pdf and also became part of the NOP https://www.esrf.fr/computing/scientific/people/srio/publications/alianelli_PhysicaB_NOP.pdf extension of Manuel Sánchez del Río’s XOP package.

Also, Single_crystal claims to be a “perfect imperfect” single crystal, in the sense that the imperfections are described by a gaussian model of mosaic and lattice variations, whereas Lucia’s work takes a more microscopic / discrete approach to the problem.

Wrt. experimental validation of Single_crystal several experiments were done along the way, but the most extensive one probably in connection with Linda Udby’s PhD thesis:
https://www.nbi.ku.dk/english/theses/phd-theses/linda-udby/Linda_phd_final.pdf




Best and hope this helps,

Peter


Peter Kjær Willendrup
Forskningsingeniør, Specialkonsulent
Næstformand for DTU Fysik LSU

DTU Physics


[cid:b6339554-7f28-441d-9f31-5944f811397c at win.dtu.dk]


Technical University of Denmark


[cid:e108e480-fcab-46e2-9531-b38165079572 at win.dtu.dk]



Department of Physics
Fysikvej
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pkwi at fysik.dtu.dk<mailto:pkwi at fysik.dtu.dk>

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