Re: AW: [fluka-discuss]: Dips in sampled neutron spectrum

From: Francesco Cerutti <Francesco.Cerutti_at_cern.ch>
Date: Wed, 14 Dec 2016 12:17:05 +0100

Dear Klaus,

thanks. Now the picture is clear: you are sampling an energy value just at
the boundary between two groups, which is not a good idea (since by
definition a boundary value challenges the identification of the range it
belongs to), and you are doing so with single precision numbers (as stored
in your arrays, where the D specifier is missing). Apart from using double
precision energy values, a robust solution - assuring your spectrum
reproduction - is rather to take an energy value in the middle of the
respective group

ENERGY=(EBIN(K)+EBIN(K-1))/TWOTWO

(To avoid the undesired case K=1, set SYCUM(1)=-ONEONE, i.e. to a negative
value instead of zero, since FLRNDM(DUMMY) may return 0, never 1).

Also for E>20 MeV, if the energy array is meant to store the histogram bin
limits, it makes anyway more sense to sample a value in between.

Kind regards

Francesco

**************************************************
Francesco Cerutti
CERN-EN/STI
CH-1211 Geneva 23
Switzerland
tel. ++41 22 7678962
fax ++41 22 7668854

On Tue, 13 Dec 2016, Ott, Klaus wrote:

>
> Dear Francesco,
>
>  
>
> I send you the source file.
>
> Side note:  It  should be d2Phi/dOmega/dE  
>
>  
>
> Best regards
>
>  
>
> Klaus
>
>  
>
>  
>
>  
>
> ----Ursprüngliche Nachricht-----
> Von: Francesco Cerutti [mailto:Francesco.Cerutti_at_cern.ch]
> Gesendet: Dienstag, 13. Dezember 2016 15:22
> An: Ott, Klaus <klaus.ott_at_helmholtz-berlin.de>
> Cc: fluka-discuss_at_fluka.org
> Betreff: Re: AW: [fluka-discuss]: Dips in sampled neutron spectrum
>
>  
>
>  
>
> Dear Klaus,
>
>  
>
> my first guess would be indeed related to the group structure. The actual group
> assignment resulting from the energy sampling in your source.f may be responsible
> for the dips. This would mean that the sampled energy value falls in the wrong
> group range. -
>
> Considering that your original spectrum already follows the group structure, this
> hypothesis looks surprising, but a final word cannot be said without looking into
> your source.f.
>
>  
>
> On a side note, I understand that with dPhi you actually mean dE.
>
>  
>
> Kind regards
>
>  
>
> Francesco
>
>  
>
> **************************************************
>
> Francesco Cerutti
>
> CERN-EN/STI
>
> CH-1211 Geneva 23
>
> Switzerland
>
> tel. ++41 22 7678962
>
> fax  ++41 22 7668854
>
>  
>
> On Mon, 12 Dec 2016, Ott, Klaus wrote:
>
>  
>
> >
>
> > Dear Leonel,
>
> >
>
> >  
>
> >
>
> > I used of course the low energy groups for sampling (the original
>
> > spectrum has been calculated by Fluka).  The Fluka output is
>
> > d2/dPhi/dOmega and if you plot this, you see that this curve decreases
>
> > by orders of magnitude. If these values are multiplied
>
> >
>
> > by the energy interval of the respective bin, the effect of the small
>
> > low energy bins
>
> >
>
> > is (roughly) compensated. For neutron sampling I used  the group
>
> > fluence
>
> >  (d2/dPhi/dOmega*2pi*deltaEi) which I integrated (summarized) and
>
> > normalized
>
> >
>
> > and the energy values of the Fluka groups below 20 MeV (255) and
>
> > further 12 high energy bin values  up to 0.5 GeV.
>
> >
>
> >  
>
> >
>
> > Best regards
>
> >
>
> >  
>
> >
>
> > Klaus
>
> >
>
> >  
>
> >
>
> > Von: Leonel Morejon [mailto:leonel.morejon_at_eli-beams.eu]
>
> > Gesendet: Montag, 12. Dezember 2016 17:06
>
> > An: Ott, Klaus <klaus.ott_at_helmholtz-berlin.de>;
>
> > fluka-discuss_at_fluka.org
>
> > Betreff: Re: [fluka-discuss]: Dips in sampled neutron spectrum
>
> >
>
> >  
>
> >
>
> > Dear Klaus Ott,
>
> >
>
> >     It is probably is related to the way you scored and/or sampled.
>
> >
>
> >     Did you take into account that in Fluka usually by default the
>
> > group approach is used for neutrons below 20 MeV?
>
> >
>
> >     Is difficult to guess without your sampling routine, but if you
>
> > sampled linearly from all energies then the lower bins are extremely unlikely.
>
> >
>
> >     Indeed if you sample linearly with energy, even with 10E10 sampled
>
> > particles, the particle density per energy in the low range (20MeV to
>
> > 1e-5eV)  is less than  1E3 part/eV, and this might be ok for bins
>
> > spawning 100-10meV but not for those thinner. Is not enough statistics
>
> > especially because the groups are not linearly spaced, and some spawn 1e-2meV.
>
> >
>
> >     To have them populated equally, you need to sample according to
>
> > the characteristics of the spectrum, Maxwell+1/v.
>
> >
>
> >     But this are just conjectures, is hard to answer w/o seeing your
>
> > routine.
>
> >
>
> > Regards,
>
> >
>
> > Leonel
>
> >
>
> > On 12.12.2016 15:23, Ott, Klaus wrote:
>
> > 
>
> >       Dear Fluka team,
>
> > 
>
> >        
>
> > 
>
> >       I calculated a neutron spectrum outside the shielding of an
>
> >       electron accelerator (1.7 GeV)  and used that spectrum for
>
> >       sampling in a simple geometry:
>
> > 
>
> >       an isotropic irradiating neutron source in the center of the
>
> >       coordinate system, surrounded by vacuum and a sphere just used
>
> >       for USRBDX
>
> > 
>
> >       counting. It is just a test to see, if the sampled spectrum is
>
> >       the same as the original one.
>
> > 
>
> >        
>
> > 
>
> >       What I found is a similar spectrum, but it has several zero bins
>
> >       in the low energy region < 20 MeV.  (This behaviour is not a
>
> >       statistic effect, it occurs also
>
> > 
>
> >       when 7E9 neutrons are sampled and it is not an effect of a wrong
>
> >       integrated and normalized spectrum function used in source.f,
>
> >       using  source.f  outside Fluka, an almost identical spectrum is
>
> >       returned).
>
> > 
>
> >       It seems to be an artefact of the neutron transportation similar
>
> >       to the one described in chapter 10 of the manual, which is in
>
> >       that case a result of the interaction of neutrons with matter.
>
> > 
>
> >        
>
> > 
>
> >       Could you recommend optimized transport/threshold  parameters to
>
> >       reduce/avoid this problem?
>
> > 
>
> >        
>
> > 
>
> >       Best regards
>
> > 
>
> >        
>
> > 
>
> >       Klaus Ott
>
> > 
>
> >        
>
> > 
>
> >       HZB/BESSYII Berlin Germany
>
> > 
>
> >        
>
> > 
>
> >        
>
> > 
>
> >        
>
> > 
>
> >        
>
> > 
>
> >        
>
> > 
>
> >        
>
> > 
>
> >        
>
> >
>
> >
>
> > ______________________________________________________________________
>
> > ______
>
> >
>
> >
>
> > 
>
> >       Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
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> >
>
> > --
>
> >
>
> > Leonel Morejon
>
> >
>
> > --------------------------------------
>
> >
>
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>
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>
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> > ______
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> >
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> > Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
>
> >
>
> > Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher
>
> > Forschungszentren e.V.
>
> >
>
> > Aufsichtsrat: Vorsitzender Dr. Karl Eugen Huthmacher, stv. Vorsitzende Dr.
>
> > Jutta Koch-Unterseher
>
> > Geschäftsführung: Prof. Dr. Anke Rita Kaysser-Pyzalla, Thomas
>
> > Frederking
>
> >
>
> > Sitz Berlin, AG Charlottenburg, 89 HRB 5583
>
> >
>
> > Postadresse:
>
> > Hahn-Meitner-Platz 1
>
> > D-14109 Berlin
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> >
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> > http://www.helmholtz-berlin.de
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>
>  
>
>
> _________________________________________________________________________________
>
> Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
>
> Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher Forschungszentren e.V.
>
> Aufsichtsrat: Vorsitzender Dr. Karl Eugen Huthmacher, stv. Vorsitzende Dr. Jutta
> Koch-Unterseher
> Geschäftsführung: Prof. Dr. Anke Rita Kaysser-Pyzalla, Thomas Frederking
>
> Sitz Berlin, AG Charlottenburg, 89 HRB 5583
>
> Postadresse:
> Hahn-Meitner-Platz 1
> D-14109 Berlin
>
> http://www.helmholtz-berlin.de
>
>

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Received on Wed Dec 14 2016 - 14:39:45 CET

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