[mcstas-users] Spin random

Thu Apr 24 16:16:18 CEST 2025

Hi again Peter,

Think of the component as mostly intended for “unit-testing” - with an “arbitrary” definition of the polarisation, e.g. with a partly polarised set of neutron rays arrive from another simulation package.
( I’ve taken the liberty of adding Erik in cc: - he might have other insights or thoughts to add - pol in McStas was mostly his work in fact. ;-) )

And I absolutely agree that most polarised simulations would start by introduction of e.g. Pol_guide_vmirror, Pol_mirror or some other polariser. In that sense there is no other reason to include such a the Set_pol component other than showing that  “this description in fact also works”.

To illustrate that both models of describing an unpolarised beam is indeed working, please have a look at the test instrument Test_Pol_mirror.instr. (I think Pol_mirror received the same edit a longer time ago and should be in functional order in your 3.5.24.)

By running a scan:
mcrun -c Test_Pol_Mirror.instr polarize=0,1 -N2

you can check that the net polarisation after the mirror is the same if “rays start unpolarised” (polarize=0) and “rays start polarised, but with average polarisation 0” (polarize=1), see attached screenshots comparing reflected and transmitted polarisation between the two cases.

Best and hope this makes better sense?

Peter

[Transmitted_ beams.png][Reflected_beams.png]

On 24 Apr 2025, at 15.46, Петр Коник <104pet104 at gmail.com> wrote:

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

<image001.gif>

Technical University of Denmark

<image002.gif>

Department of Physics
Fysikvej
Building 307
DK-2800 Kongens Lyngby

Main office at
ESS DMSC
Building 305
DK-2800 Kongens Lyngby

Direct +45 2125 4612
Mobil +45 2125 4612
Fax +45 4593 2399
pkwi at fysik.dtu.dk

Peter Kjær Willendrup
Forskningsingeniør, Specialkonsulent

DTU Physics

[image001.gif]

Technical University of Denmark

[image002.gif]

Department of Physics
Fysikvej
Building 307
DK-2800 Kongens Lyngby

Main office at
ESS DMSC
Building 305
DK-2800 Kongens Lyngby

Direct +45 2125 4612
Mobil +45 2125 4612
Fax +45 4593 2399
pkwi at fysik.dtu.dk

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