[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 ISI=
S) we make use of a horizontal polarisation direction and then analyse in t=
he 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 allow=
ing the spin vector to be rotated is with what happens as more polarising c=
omponents 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 bu=
t as ROT _A_CURRENT_COMP is cummulative it would depend on where you polari=
sed 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 co=
mponent then I guess we have got back to applying the ROT_A_CURRENT_COMP tr=
ansform locally, as you first suggested. As far as I can see the ROT_A_CURR=
ENT_COMP is a cumulative transform so this shouldn't be a problem but the n=
eutron spin does then need to be decoupled from the ROTATED command.
>
>Regards
>
>Rob
> =

>
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