[mcstas-users] Finite transmission probability for DiskChopper component

Peter Kjær Willendrup pkwi at fysik.dtu.dk
Tue Nov 19 13:40:29 CET 2019

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 Kjær Willendrup
Forskningsingeniør, Specialkonsulent
Næstformand for DTU Fysik LSU

DTU Physics

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Technical University of Denmark

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Department of Physics

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pkwi at fysik.dtu.dk<mailto:pkwi at fysik.dtu.dk>

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