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

[Markos Skoulatos]

Hi Peter,

thank you very much, indeed your suggested workaround works. I checked
various wavelengths, especially extreme cases and everything seems
fine.
Klaus Habicht pointed out another solution which strangely enough
still enables one to use focus_xw and focus_yh but the target should
be defined as "target_x = ..., target_y = ..., target_z = ..." instead
of "target_index = +..." (so it seems to be quite a combination that I
managed to set up and get these negative events!)

cheers, Markos

On Thu, Oct 21, 2010 at 3:09 PM, Peter Willendrup <pkwi at risoe.dtu.dk> wrote:
> 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
>
>
> --
> -------------------------------------------------------------------
> Peter Willendrup - Development engineer
>
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> Materials Research Division
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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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>
>