[neutron-mc] McStas Polarisation details

[Dalgliesh, RM (Robert)]

Hi Kristian, Peter,
You make a good analogy. On further thought there is a bit of a problem her=
e. I need to know the absolute relationship between the two polarisation di=
rections in order to define the reflected intensity. If I rotate a mirror w=
here the polarisation direction has been fixed in the x axis direction and =
the mirror is horizontal (x-z plane) then a 90 degree rotation process mean=
s that the plane of the mirror is now in the y-z plane. Then without an ope=
ration on the defined polarisation direction of the mirror it will still th=
ink it is in the x direction. Rotation of the polarisation vector does then=
 make sense if I take a dot product as the two directions will now be at 90=
. This doesn't make much sense intuitively however.
My personal preference would still be to define a polarisation direction wi=
thin a component (similar to that of the neutron) which would then be rotat=
ed such that the polarisation direction of the neutron can be left unchange=
d. I admit that this will need an extra part to the rotation process that w=
ould only be invoked if polarisation is turned on. However, I think this wo=
uld be likely to lead to fewer mistakes later on such as when further polar=
ising components are added. I can foresee particular problems when componen=
ts are position absolutely in space but a rotation of one component has bee=
n necessary, e.g. with a channelled guide.
 =

Rob =


________________________________

From: neutron-mc-bounces at risoe.dk on behalf of Kristian Nielsen
Sent: Mon 22/08/2005 11:00
To: McStas users list at neutron.risoe.dk
Subject: Re: [neutron-mc] McStas Polarisation details



"Dalgliesh, RM (Robert)" <R.M.Dalgliesh at rl.ac.uk> writes:

> It looks as if the neutron polarisation direction is rotated with the
> objects.

Now, it is admittedly a long time since I've been active with McStas :-)
But it looks to me as if you fundamentally misunderstand the use of
ROTATED in McStas.

When you say ROTATED in the instrument file, it means that you
physically change the orientation of the component. When you turn your
coffe cup upside down, certainly all other properties of the cup changes
orientation with it, _except_ for gravity which doesn't change and so
your coffe spills out!

If on the other hand you want to control the polarisation direction of a
polarised mirror relative to the mirror geometry itself, that needs to
be given as a parameter to the component, and the component code must
handle the appropriate calculation itself. That would correspond to
programming a coffe cup where the orientation of the handle could be
changed.

Hope this helps, and sorry if after five years of no McStas work I am
talking complete nonsense.

 - Kristian.

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