[mcstas-users] Spin random

[Петр Коник]

Dear Peter,

Thanks for quick answer! 

Can you elaborate a bit more on a situation where this component is useful? I understand what you are saying, but still can’t get why would I want this? For example, if I have an ideal polarizer, normally it would just set P=1 for every trajectory in the unpolarized beam. With spin_random I would lose half of all trajectories after such a polarizer.

My general feeling is that trajectories were never meant to be identified as real neutrons, so I’m struggling to understand the rationale behind this component.

I’m running 3.5.24, I will check the fix, thanks!

Best regards,
PK

> On 2025. Apr 24., at 15:37, Peter Kjær Willendrup <pkwi at dtu.dk> wrote:
> 
> 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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