From: Alberto Fasso' (fasso@SLAC.Stanford.EDU)
Date: Fri Mar 23 2007 - 00:13:56 CET
Everything is clear now. Counting gammas or neutrons or events has no
meaning. You should always count the corresponding weights.
I have looked at your input and I have found some errors.
The card DEFAULTS should be placed at the beginning of input. See the manual:
2) IMPORTANT! Option DEFAULTS must be issued at the very beginning of
input. It can be preceded only by a GLOBAL card and by commands such as COMMENT or TITLE. This is one of the rare cases, like GLOBAL,
MAT-PROP and PLOTGEOM, where sequential order of input cards is of
importance in FLUKA (see 7}).
In the COMPOUND definitions, if both WHAT(1) and WHAT(2) have a positive sign,
WHAT(1) is the number of atoms in the molecule. Same for WHAT(3) and WHAT(5)
So, since yout LIQUIDS compound is made of 9 atoms of C and 12 of H, it is
not correct to write
COMPOUND +0.3381203 6.0+0.6618797 3.0 0.0 0.LIQUIDS
but you should have:
COMPOUND +9.0 6.0 +12.0 3.0 0.0 0.LIQUIDS
But if you prefer instead to use mass fractions, you should put WHAT(1) and
WHAT(3) with a negative sign:
COMPOUND -0.9 6.0 -0.1 3.0 0.0 0.LIQUIDS
In fact, 9 atoms of C with mass 12 plus 12 atoms of H of mass 1 make a total
of 120. And 9*12/120 = 0.9, 12*1/120 = 0.1. I don't understand where you got
those numbers 0.3381203 and 0.6618797.
Finally, I am surprised by seeing
USERWEIG 0.0 0.0 4.0
According to the manual, WHAT(3) = 4.0 means that you multiply your fluence of
yield scores by a function FLUSCW (and that is quite normal) but also that
"The routine FLDSCP is also called, applying a shift to the current binned
FLDSCP is normally used to simulate a drift. Is that what you intend to do?
On Fri, 23 Mar 2007, Lindley Anne Winslow wrote:
> Thank you Alberto.
> I am sorry I forgot the material. I have attached my setup this time.
> It is Liquid Scintillator C9H12. I couldn't find a reference for (n,alpha) on carbon12 quickly but my understanding is (n,gamma) on hydrogen should be the dominant process. Gammas should be generated for both these materials and some are.
> The number quoted below is just the counted number of events with gammas.
> The sum of the weights of these particles is ~2100 which is the same as the weights of the incoming neutrons.
> I also see very little deuterium or carbon13 produced. When I run thermal neutrons in this environment I would expect no fewer capture gammas than neutrons entering the volume (and not escaping).
> ----- Original Message -----
> From: Alberto Fasso' <firstname.lastname@example.org>
> Date: Friday, March 23, 2007 6:26 am
> Subject: Re: Thermal Neutron Capture Gammas
> To: Fluka Discussion <email@example.com>, Lindley Winslow <LAWinslow@lbl.gov>
> > Dear Lindley,
> > your question is too vague to give it an answer.
> > Which material(s) are you using? Have you made sure that
> > the cross sections of all of them include gamma production?
> > (See Chap. 10 of the Manual).
> > Are you aware that not all neutron captures produce gammas?
> > For instance you can have a (n,alpha) reaction without gamma.
> > Are you aware that in a (n,gamma) reaction you may have
> > more than one gamma?
> > Have you just "counted" the neutrons and gammas, or have
> > you calculated the sum of their weights?
> > Alberto
> > On Thu, 15 Mar 2007, Lindley Winslow wrote:
> > > Hello all,
> > >
> > > To try to understand FLUKA's treatment of low energy neutrons, I
> > > simulated 10,000 thermal neutrons (group 72)
> > >
> > > I found 5575 scatter and leave the detector.
> > > This leaves 4425 that stay in the detector.
> > > I only see 951 capture gammas from the neutrons that stay in the
> > > detector.
> > >
> > > I would expect that all neutrons that stay in the detector should
> > > capture. Is there another way to lose neutrons from the last group?
> > >
> > > Thank you,
> > > Lindley
-- Alberto FassÚ SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025 Phone: (1 650) 926 4762 Fax: (1 650) 926 3569 firstname.lastname@example.org
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