[neutron-mc] McStas Polarisation details

[Peter Willendrup]

Hi again,

On Mon, 2005-08-22 at 13:12 +0200, Dalgliesh, RM (Robert) wrote:
> You make a good analogy. On further thought there is a bit of a problem here. 
> I need to know the absolute relationship between the two polarisation directions 
> in order to define the reflected intensity. If I rotate a mirror where 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 means that the
> plane of the mirror is now in the y-z plane. Then without an operation on the
> defined polarisation direction of the mirror it will still think 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.

Possibly I am missing a point here, but:

It seems to me that since components interacting with the neutron spin
are physical device e.g. with little mounted magnets, it does not make
much sense to define a 'polarisation direction' relative to the global
coordinate system (meaning the e.g. the global 'x' direction). Better
would be to define this direction within the local coordinate system,
i.e., rotating the component will rotate the component polarisation
direction since rotating the little physical magnets or coils.

It seems to me that the description above mixes the global coordinate
system of the instrument with that of the component (in which we are
almost always handeling the neutron - in samples for instance, we are to
first order dependent on local geometry, not on where the source
component was situated). 

Regards,

Peter

> My personal preference would still be to define a polarisation direction 
> within a component (similar to that of the neutron) which would then be 
> rotated such that the polarisation direction of the neutron can be left 
> unchanged. I admit that this will need an extra part to the rotation 
> process that would only be invoked if polarisation is turned on. However,
> I think this would be likely to lead to fewer mistakes later on such as
> when further polarising components are added. I can foresee particular
> problems when components are position absolutely in space but a rotation of
> one component has been necessary, e.g. with a channelled guide.



-- 
-------------------------------------
Peter Kjaer Willendrup, cand. scient

Phone: (+45) 46 77 58 62
email: peter.willendrup at risoe.dk
-------------------------------------