Re: Thermal Neutron Capture Gammas

From: Alberto Fasso' (fasso@SLAC.Stanford.EDU)
Date: Fri Mar 23 2007 - 00:13:56 CET

  • Next message: Tang, Alfred: "RE: estimating neutron rate using USRTRACK"

    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)
    etc.
    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
    *Custom Weighting.
    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
    track"

    FLDSCP is normally used to simulate a drift. Is that what you intend to do?
    Why?

    Alberto

    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).
    >
    >
    > Lindley
    >
    >
    > ----- Original Message -----
    > From: Alberto Fasso' <fasso@slac.stanford.edu>
    > Date: Friday, March 23, 2007 6:26 am
    > Subject: Re: Thermal Neutron Capture Gammas
    > To: Fluka Discussion <fluka-discuss@fluka.org>, 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
    fasso@slac.stanford.edu
    

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