From: Alfredo Ferrari (alfredo.ferrari@cern.ch)
Date: Mon Oct 20 2003 - 17:42:29 CEST
Hi Ludwik
this is a perfectly normal behavior: as in all neutron codes FLUKA does
not generate "exclusive" neutron events. Rather, it contains inclusive
distribution of secondary particles (neutrons in particular) and samples
from them. Your first example is a case where there is one outgoing
neutron and the energy sampled from the inclusive spectrum of ALL
emitted neutron came back to be very low (the inclusive spectrum
contains also n,2n, n,3n etc neutrons). The average kerma energy for
a neutron in the 1st FLUKA group is .17 MeV and you got it as deposited
energy correctly. The second case is a typical "in-scattering", where the
neutron stays in the same group (very likely on heavy materials because of
the small energy loss due to elastic) => identical average group energies,
and the energy deposition is the computed average kerma for this group
again.
There is nothing wrong, simply you cannot follow exclusive individual
neutron histories as in most other neutronic codes. The neutron
spectra/multiplicities, energy deposition all come out correct for a
sufficiently large number of histories.
Please note that many of the stoppers which prevent to run fully exclusive
neutron histories below 20 MeV are intrinsical to the way the nuclear data
files (ENDFB, JEF, JENDL) are organized. They do not contain the exclusive
infos at all, but only inclusive ones.
Ciao
Alfredo
On Mon, 20 Oct 2003, Ludwik Pienkowski wrote:
> Dear FLUKA authors,
>
> I observe a strange energy leak for low energy neutrons.
> The neutrons of the energy 19.5 MeV irradiate a very large block
> of lead (ntest.inp and other files are attached to this email)
> and the secondaries emitted during the cascade were watched
> in the mgdraw routine using the entry USDRAW. The attention
> was put on (n,n') reactions, i.e. the reactions that only
> one neutron is emitted. It was noted that for many cases the
> emitted neutron kinetic energy is much smaller than the energy
> of the primary neutron. The results are shown in the ntest001.log
> file. For example the strange case is:
> event En En" tv tvcms tvheavy time
> (MeV) (MeV) (MeV) tvrecl tvbind (ns)
> 2 19.50 0.95 0.17 0.00 0.00 0.00 0.00 0.27400
> The value of the En-En' is equal to 18.55 MeV and this energy
> is not shown in the other available variables. Where is this
> missing energy?
> The other strange result is as following:
> event En En" tv tvcms tvheavy time
> (MeV) (MeV) (MeV) tvrecl tvbind (ns)
> 5 18.52 18.52 0.17 0.00 0.00 0.00 0.00 0.36955
> The energies of both neutrons are virtually equal each other,
> however, the variable tv (excitation energy) is 0.17 MeV. What
> is the source of this energy?
>
> Best regards,
> Ludwik Pienkowski
>
> P.S.The attached files:
> 1.ntest.inp
> 2.ntest001.log
> 3.mgdraw-ntest.f
> The program was linked and run with the commands:
> lfluka -o ntest -m fluka mgdraw-ntest.f
> rfluka -e ntest -N 0 -M 1 ntest
>
> ================================
> Heavy Ion Laboratory
> Warsaw University
> ul. Pasteura 5A
> 02-093 Warszawa
> Poland
> pienkows@slcj.uw.edu.pl
> ================================
>
>
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