[mcstas-users] Spin random

[Peter Kjær Willendrup]

Hi Peter,


On 24 Apr 2025, at 13.55, Петр Коник via mcstas-users <mcstas-users at mcstas.org> wrote:

Can you explain me the intended use of Spin_random component? What are the benefits of assigning polarization to trajectories before the polarizer?

The component was mainly added to allow defining an “unpolarised beam” at the “full beam level” (i.e. with a set polarisation pr. ray but a net 0 polarisation) to allow comparing with all rays having sx=sy=sz=0 (i.e. with each ray unpolarised).

Moreover, when used in combination with Pol_Vmirror component it results in polarization over 1 if two components are rotated 90 degrees relative to each other.

We have recently made a fix to that component (i.e. ~ 1 month ago) that I would think rectifies this problem - see https://github.com/mccode-dev/McCode/commit/55e3989eb05150a7671523749f30dd462e19b266.
The logic is that a given polariser / analyser defines a definition of “up/down”, meaning post to “measurement” the other components should be put to zero.

How recent a version are you running? Chances are that 3.5.25 (which is only on conda-forge at the moment) would fix this.

Otherwise stay tuned for 3.5.26 that I can promote to become a “full” release including .deb packages, Windows installer etc.


Best,
Peter W



Peter Kjær Willendrup
Forskningsingeniør, Specialkonsulent

DTU Physics

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Technical University of Denmark


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Department of Physics
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