[neutron-mc] Single crystal peak in McStas1.6-ill

[Gutmann, MJ (Matthias)]

Hi,

	I am trying to use the single crystal together with a TOF detector
and observe a strange peak shape for some parameter values in the
simulation. The peak shape changes if an additional TOF monitor is inserted
at the sample position. Is this to be expected?

The instrument is as follows:

1) Flat lambda source.
2) An m=0 guide to mimic a rectangular collimator of 1cm^2 cross section at.
3) A TOF monitor before the entrance of that collimator.
4) A single crystal that is rotated by an angle theta with respect to the
   incident beam, fulfilling Bragg's law for a given wavelength (dspacing
   is calculated).
5) A TOF detector at the sample position (presently commented out).
6) A TOF monitor at a distance L2 to mimic a single pixel of a flat area
   detector. If the pixel is not in the center of the area detector (center
   positioned at an angle gamcen with respect to the incident beam) it is
   slightly tilted. The distance sample-pixel is recalculated as 
   L2/cos(Gam - gamcen). Gam is the angle of the pixel with respect to the
   incindent beam.

Input parameters are 

1) Gam: Angle of pixel.
2) Gamcen: Angle of center of imaginary area detector.
3) L1: Distance source - sample.
4) L2: Distance sample - center of imaginary area detector.

The "double peak" occurs e.g. for Gam = 60.0, Gamcen = 37.5, L1 = 8.3, 
L2 = 0.225. 10^8 neutrons were used. The shape changes if the TOF monitor
at the sample position is uncommented.

I've tried several things, e.g. taking the collimater out, changing the size
of the focussing window, changing the size of the crystal, use different
sources, untilted pixel. One thing I noticed is that the double peak occurs
if the focussing window is smaller than the cross-section of the sample. It
corresponds to the situation where the sample is not fully illuminated by
the beam.

The files used in the simulation are attached.

Many thanks for your help,

Matthias Gutmann


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