VITESS Module Collimator

The module collimator simulates a soller collimator by allowing for a certain divergence
range (in the x-y-plane) which is described by a triangular distribution
characterising the probability that a neutron of a certain divergence passes through
the collimator.  The maximal possible divergence d_phi then corresponds to the width
w and length l of a soller collimator channel, i.e. tan d_phi = w/l . The FWHM of
the probability distribution (the peak is centred at Divergence 0) is also d_phi.

This method of simulating collimation is a good approximation. Alternatively one can
also use the module guide to characterise the soller collimation device more
realistically: For this one should use the bender option of the guide module to
generate channels of a certain width (and also appropriate spacer widths) while 'no
curvature' and 'total absorption' (i.e. no reflectivity file must be defined) has to
be adjusted.  The simulation of slits can easily performed with the module window.
The module collimator does neither change the neutron position and direction,
nor the frame of reference. 


The following information has to be given as an input:

Collimator divergence [deg]
  allowed x-y divergence (FWHM of the triangular shape);

peak transmission
  maximal probability for passing through the soller collimator (corresponds to an
  average loss due to blocking neutrons by the finite size of soller collimator
  spacers)


case of angular collimation:
This option is needed, if one wants to consider radial collimation (in front of the
detector) after neutrons are scattered at a sample. For a realistic intensity
comparison one has to divide the resulting intensity by a factor 1/n which considers
that not all angle ranges can be covered by a realistic radial collimator system at
one time (because certain angular regions refer to different parts of the
sample). To cover all angles the position of the collimation system has to be
changed n times.

The following input parameters are only considered by the collimator module if
angular collimation is activated:


minimum of angle range [deg]
  certain angles define directions in the x-y-plane which are considered as
  0-divergence lines. The lowest angle must be given as an input.

number of collimation centres
  as described above, each collimation centre is defined by an angle which corresponds
  to divergence 0 (angle 0 corresponds to the positive x-axis direction). The first
  centre is defined by the 'minimum of angle range', the following centres (always
  higher angles) are calculated by considering a gap of (2*(allowed divergence d_phi)+
  angle spacing) between two centres.


angle spacing [deg]             
  additional angular distance between the collimation centres due to the size of
  collimator spacers



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Last modified: Tue May 8 17:08:06 MET DST 2001