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

From: Francesco Cerutti <Francesco.Cerutti_at_cern.ch>
Date: Wed, 14 Dec 2016 17:53:21 +0100

Dear Klaus,

actually what happens is that those neutrons get from the beginning a
different energy, being moved to neighbouring energy groups and
so distorting the spectrum. And the reason is as I previously wrote below.

Let's have three energy groups, from A to B, from B to C, and from C to D,
respectively. If you assign as energy value B, C, and D (which is itself a
bad idea, since a boundary value makes the group identification
ambiguous), and you input for them single precision numbers, it may happen
that your B value is interpreted as falling between A and B, your C value
as falling between C and D, and your D value as falling between C and D
too. So a given spectrum, expected for instance to get N counts in each of
the three groups, turns out to be polluted with N, zero, and 2N counts,
respectively.

Cheers

Francesco

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

On Wed, 14 Dec 2016, Ott, Klaus wrote:

> Dear Francesco,
>
> now it works fine, the sampled spectrum looks like the original one. Thanks a lot!
>
> One last question: Why are neutrons of some energies not transported, if the source routine returns
> energy values from table 10.4.1.1 (energy group structure of the 260- neutron data set)?
> (these are the energy values in my EBIN array)
>
> Best regards
>
> Klaus
>
>
> -----Ursprüngliche Nachricht-----
> Von: Francesco Cerutti [mailto:Francesco.Cerutti_at_cern.ch]
> Gesendet: Mittwoch, 14. Dezember 2016 12:17
> 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,
>
> 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
>>
>>>
>>
>>> Mitglied der Hermann von Helmholtz-Gemeinschaft Deutscher
>>
>>> Forschungszentren e.V.
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>>
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>>
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>>
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>>
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>>>
>>
>>>
>>
>>> --
>>
>>>
>>
>>> Leonel Morejon
>>
>>>
>>
>>> --------------------------------------
>>
>>>
>>
>>> ELI Beamlines
>>
>>>
>>
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>>
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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.
>>
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>>
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>>
>>> Jutta Koch-Unterseher
>>
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>>
>>> Frederking
>>
>>>
>>
>>> Sitz Berlin, AG Charlottenburg, 89 HRB 5583
>>
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>>
>>> Postadresse:
>>
>>> Hahn-Meitner-Platz 1
>>
>>> D-14109 Berlin
>>
>>>
>>
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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
>>
>> http://www.helmholtz-berlin.de
>>
>>
>
> ________________________________
>
> 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 - 19:25:06 CET

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