[neutron-mc] McStas Polarisation details

[Dalgliesh, RM (Robert)]

Hi Klaus,
It's exactly this sort of situation I'm trying to imagine. On CRISP (at ISIS) we make use of a horizontal polarisation direction and then analyse in the vertical so that we can probe off specular scattering from a horizontal sample. For this I would write a spin rotator component to do the same job as the physical device. This is very trivial. The problem I have with allowing the spin vector to be rotated is with what happens as more polarising components are added and if they are rotated with respect to one another or positioned absolutley. In the long run this is likely to get very messy.
I thought that another alternative to do this would be to apply the inverse of ROT _A_CURRENT_COMP, as Peter originally suggested, in the component but as ROT _A_CURRENT_COMP is cummulative it would depend on where you polarised your beam as to what happens.
 
Rob
 

________________________________

From: neutron-mc-bounces at risoe.dk on behalf of Klaus Lieutenant
Sent: Mon 22/08/2005 15:32
To: McStas users list at neutron.risoe.dk
Subject: Re: [neutron-mc] McStas Polarisation details



Dear Robert,

why should the neutron spin be decoupled from the ROTATED command ? This
command takes care that all parameters (velocity, ...) of the neutron
are transformed to the local co-ordinate system of the component. This
is also necessary for the spin.
Imagine you have polarised a beam into a certain direction and you like
to analyse the polarisation direction afterwards with a component that
is rotated relative to the polariser. For this analyser, you need to
know the polarisation direction in its co-ordinate system, i.e. a
rotation of the spin co-ordinates is necessary.

Best regards,

   Klaus




Dalgliesh, RM (Robert) wrote:

>Peter,
>I suppose that's what I was trying to get to but wrote a totally garbled e-mail. If the field/polarisation direction is defined locally within the component then I guess we have got back to applying the ROT_A_CURRENT_COMP transform locally, as you first suggested. As far as I can see the ROT_A_CURRENT_COMP is a cumulative transform so this shouldn't be a problem but the neutron spin does then need to be decoupled from the ROTATED command.
>
>Regards
>
>Rob
> 
>
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