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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Received on Mon Dec 12 2016 - 18:57:49 CET