[mcstas-users] Finite transmission probability for DiskChopper component

[Marc Seifert]

Dear Peter,

thank you very much for your help. I was able to implement your 
solution, however, it seems necessary to add a SCATTER in the EXTEND 
section of the absorber. This may be caused by my rather complex chopper 
geometry using several windows and additional WHEN conditions. I used 
the Sample_nxs component as it offers pure transmission without 
scattering. The code looks now like this:

COMPONENT window0 = DiskChopper(parameters=values...)
WHEN (condition)
AT (0, 0, z)
GROUP DiskChopper1
EXTEND
%{
if(SCATTERED) do_something();
%}

COMPONENT window1 = DiskChopper(parameters=values...)
WHEN (condition)
AT (0, 0, z)
GROUP DiskChopper1
EXTEND
%{
if(SCATTERED) do_something();
%}

COMPONENT absorber = Sample_nxs(
     TransOnly=1, IncohScat=0, MultiScat=0,
     xwidth=0.5, yheight=0.5, zthick=250e-6,
     nxsFileName="boron10.nxs")
WHEN (condition)
AT (0, 0, z)
GROUP DiskChopper1
EXTEND
%{
SCATTER;
%}

COMPONENT arm = Arm()
WHEN (!condition)    // inverse condition here
AT (0, 0, z)
GROUP DiskChopper1
EXTEND
%{
SCATTER;
%}


Cheers,

Marc


On 11/19/19 1:40 PM, Peter Kjær Willendrup wrote:
> Dear Marc,
>
>> On 19 Nov 2019, at 13.23, Marc Seifert <Marc.Seifert at frm2.tum.de 
>> <mailto:Marc.Seifert at frm2.tum.de>> wrote:
>>
>> Hello everyone,
>>
>> we are simulating a chopper system over a large wavelength spectrum 
>> (0.5 to 10 AA). We would like to consider finite transmission of the 
>> choppers and we expect this to be 1E-7 for 3 AA and up to 0.007 for 1 
>> AA. We would like to simulate the overall impact on the performance 
>> of the instrument by implementing a transmission probability to the 
>> DiskChopper component, but we are not sure which would be the best 
>> approach.
>>
>> Is it possible to check in the EXTEND section of a DiskChopper if the 
>> neutron was absorbed and then restore it with a certain probability? 
>> Or would it be better to change the neutron weight instead of 
>> absorbing it in the component? Additionally with a cutoff at a 
>> certain wavelength in order to not propagate every neutron with very 
>> tiny weight through the whole instrument.
>>
>> Any help is very much appreciated.
>
> I thought of solutions for this in the past, but never actually 
> implemented anything in practice. What I would do is probably 
> something like
>
>
> COMPONENT Chopper=DiskChopper(parameters=values…)
> AT (somewhere)
> GROUP DiscChopper1
>
> COMPONENT Absorber=Incoherent(parameters=values…)
> AT (somewhere)
> GROUP DiscChopper1
>
> - This means that if the chopper opening is not hit, the beam instead 
> hits the following component. Depending on taste, that component could 
> be Incoherent as shown or maybe even better PowderN which allows to 
> tune the branches of statistics going into incoherent scattering, 
> coherent scattering and “tunnelling”, i.e. keeping the ray direction 
> but taking the material cross-sections into account, i.e. “absorbing” 
> the relevant weight.
>
> Hope this helps,
>
> 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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