[mcstas-users] negative counts and peaks in McStas 1.12b

[Markos Skoulatos]

Dear all,

we are modeling a cold neutron triple axis spectrometer in McStas (for
the FLEX upgrade in Berlin) and in the last few days I noticed some
unusual behaviour in the complete instrument (consisting of
source--->guides--->velocity selector--->more guides--->doubly
focusing PG monochromator--->V sample--->horizontally curved PG
analyser--->monitors).
The most striking of all is the fact I get negative counts on my
energy monitors, after the sample, for long wavelengths (below ki=1.36
A^-1) . I initially thought there is something wrong with the V sample
(which still might be the case) until I noticed that the problem
happens when the absolute value of the 2*theta angle of the
monochromator (defined as A2 on this attached instrument) gets greater
than roughly 90 degrees. So perhaps the monochromator has something to
do with this. I tried extensively changing the A4 angle (2-theta angle
of the sample) as well as instrument configurations (ie scattering
senses at sample and analyser) in order to find some values where the
program can run. I failed to make it run for cold wavelengths.
My results for debugging purposes can be summarised as follows:
1. For ki>1.37 A^-1 things seem to run smoothly
2. At ki=1.36 A^-1 peaks start going below zero
3. In the magic range 1.35>ki>1.33 the energy monitors after the
sample and at the detector are negative, however the monitor after the
analyser is positive!!
4. For ki<1.32 all monitors after the V sample are inverted (negative).

I attach a picture where you can see what I mean negative counts
(simulation with ~1e7 trajectories).
Has anybody some experience on simulations at colder wavelengths with
monochromator take off angles above 90 degrees or has anyone noticed
these negative counts? Perhaps McStas doesn't like this angle to be
above 90 because then neutrons have a backward direction
(back-scattering)?
Even if I have some mistake in my file I can't see why my program runs
at specific wavelengths (for example ki=1.55 is fine). In any case I
think negative counts should not be there.

cheers,
Markos Skoulatos
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