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

[Peter Willendrup]

Hello Markos,


Thanks for pointing this out.

First of all, you are right: Negative events should never occur! :)

My feeling is indeed that this relates to V_sample in the sense that this component uses the runtime function randvec_target_rect - which returns a solid-angle multiplier... (Some vector-geometry calculations internal to randvec_target_rect likely has a missing fabs() somewhere...)

I haven't looked yet, but my guess is that the immediate workaround is to use focus_r instead of focus_rw and focus_yh.

I will have a closer look and fix this - likely next week... In turn, this likely means another update release, 1.12c...

Cheers,

Peter


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Peter Willendrup - Development engineer

RISØ DTU
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On 21/10/2010, at 13.49, Markos Skoulatos wrote:

> 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
> <FLEX2112_horiz_foc.instr><negative counts k is 1.256.bmp>_______________________________________________
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