[neutron-mc] McStas Polarisation details

[Peter Willendrup]

Hi Robert,

On Fri, 2005-08-19 at 16:30 +0100, Dalgliesh, RM (Robert) wrote:
> Sorry for the deluge of e-mails of Peter,

No need to excuse, its' what I'm here for and what the list is there
for. 

> It looks as if the neutron polarisation direction is rotated with the objects. 
> There is a mccoordschange_polarisation function defined in mcstas-r.c which I 
> presume is applied whenever ROTATED is applied. Unfortunately for the situation 
> of a polarising mirror, where the polarisation direction is confined to the plane 
> of the mirror, this isn't particularly useful as I want the neutron polarisation 
> direction to remain fixed and mirror polarisation direction to change. If I want 
> the neutron polarisation direction change I will then force the change by knowing 
> the relationship between the incident polarisation direction and the component itself. 
> I think I'm going to have to try and decouple this functionality somehow but this is 
> now into the nuts and bolts of the mcstas instrument build process. Is there a more 
> detailed manual anywhere with handy hints about doing such things or is just a case 
> of ploughing my way throught the code?

Well. The rotation of the spins ofcourse (and by symmetry the neutron
coordinates) only happens to allow working in a local component oriented
coordinate system (where interactions between component and neutron are
handled in an easier way). With my current overview of the package,
regarding spin variables and polarisation, only the infrastructure is
there. No existing component really changes the spin state, spins
initially set parrallel to gravity it seems. 

Interacting with the neutron spin would (by symmetry as in the case of
neutron coordinates) happen by simply setting the sx,sy,sz variables
within the component (x,y,z / vx,vy,vz in the case of neutron
coordinates). As far as I know, there is next to no documentation
anywhere in manuals etc., only the basic infrastructure was added. So
basically, any input added from you and others will serve as the basis
for what we decide to implement. I think that the possibility to use
interpolated magnetic field input files (mentioned by you elsewhere) is
a reasonably flexible solution, currently exising in the Vitess package
(some of Geza's components work this way).

> My guess would be that the best way of doing this is to ensure that the neutron polarisation 
> direction remains unchanged throughout the instrument unless changed by a component in some 
> way. I would be interested to here other people's thoughts on this especially with regards 
> to He3 cells. 

I agree, it is very relevant to hear opinions on what to focus at before
we really get going. So people, make your remarks! :-)


Regards,

Peter

-- 
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Peter Kjaer Willendrup, cand. scient

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