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
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--
Leonel Morejon
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Helmholtz-Zentrum Berlin für Materialien und Energie GmbH
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Postadresse:
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http://www.helmholtz-berlin.de
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Received on Mon Dec 12 2016 - 21:10:39 CET