[neutron-mc] Curved monochromator

[Klaus Lieutenant]

Hi Joerg,

this happened to me and probably to several other McStas users.
The problem is: with the monitors 'DivergenceMonitorguide' and 
'GuideExitLambda' prior to the monochromator you propagate the neutrons 
to a plane perpendicular to the incoming beam positioned at the centre 
of the monochromator. As this is rotated by 23 deg, half of the neutrons 
have now already passed the monochromator and are lost.
The rest will be reflected on one half of the monochromator; but of 
course the focus will be shifted, as practically only one side of the 
monochromator is illuminated.

One solution is to position the monitors in front of the monochromator. 
The latest McStas version also offers the possibility to allow a 'back 
propagation' of the neutrons. You need to add
EXTEND
%{
  ALLOW_BACKPROP;
%}
to the component directly before the monochromator. It sets the flag 
'back propagation is allowed' for the next propagation. After this 
propagation, the flag is automatically reset.

Regards,

   Klaus