[mcstas-users] Component Monochromator-reflect

Thu Jun 3 16:02:29 CEST 2021

Dear Erik and Peter,

Thanks for your answers.

Le 02/06/2021 à 13:18, Erik B Knudsen a écrit :
> Dear Thierry,
>
> You could definitely use Single_crystal or, if it is an assembly of 
> crystals you want, a Union of single crystals for mosaic crystals. Is 
> there a specific geometry you are targeting?

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.

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.
- 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.
- 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 ?

Thierry

>
> Please note that Single_crystal has effectively 4 ways of modelling 
> mosaic, depending on your situation.
>
> cheers
> Erik
>
> On 01/06/2021 19:53, Peter Kjær Willendrup wrote:
>> Dear Thierry,
>>
>>
>>> On 1 Jun 2021, at 18.57, Thierry Bigault <bigault at ill.fr 
>>> <mailto:bigault at ill.fr>> wrote:
>>>
>>> I am trying to make calculations with mosaic crystals. The following 
>>> paper states that a new component, called Monochormator-reflect, was 
>>> introduced in order to overcome some limitations of 
>>> Monochromator_curved:
>>>
>>> L. Alianelli, N. Wilson, K.H. Andersen, M. Sánchez del Rı́o, R. 
>>> Felici, A method for detailed simulations of neutron diffraction 
>>> from imperfect crystals, Nucl. Instr. Meth A 529 (2004) 231–233.
>>>
>>> Does anyone know what this became ? Was it included into another 
>>> more sophisticated component that can be used to simulate 
>>> monochromators, like Single_crystal ?
>>
>> I do have a copy of the said component component somewhere, but could 
>> not locate it at first glance. I am also sure both Erik and Emmanuel 
>> has a copy. I am however also sure that it will not work “out of the 
>> box”. :)
>>
>> ( The reason that the component never made it into the official 
>> McStas release was in a way simple:
>>   It relied on an inputfile that was prepared using an IDL 
>> procedure-code, which we thought was a very nasty dependency…)
>>
>> What specific advanced feature(s) are you looking for? We do have a 
>> number of other good alternatives, e.g. Single_crystal or NCrystal.
>>
>>
>> Best,
>> Peter
>>
>>
>> Peter Kjær Willendrup
>> Forskningsingeniør, Specialkonsulent
>> Næstformand for DTU Fysik LSU
>>
>> DTU Physics
>>
>>
>>
>>
>>
>> Technical University of Denmark
>>
>>
>>
>>
>>
>>
>> Department of Physics
>> Fysikvej
>> Building 307
>> DK-2800 Kongens Lyngby
>> Direct +45 2125 4612
>> Mobil +45 2125 4612
>> Fax +45 4593 2399
>> pkwi at fysik.dtu.dk <mailto:pkwi at fysik.dtu.dk>
>>
>>
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>

-- 
Logo ILL <https://www.ill.eu/> 	
Dr. Thierry BIGAULT
Research Engineer
Multlayer Neutron Optics
*Institut Max von Laue - Paul Langevin (ILL)*
71, avenue des Martyrs - CS 20156
38042 Grenoble cedex 9 - France
+33 (0)4 76 20 76 95
bigault at ill.eu

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