Re: Flux

From: Alberto Fasso' (fasso@SLAC.Stanford.EDU)
Date: Mon Feb 25 2008 - 03:18:03 CET

  • Next message: Buthaina Abdalla Suleiman Adam: "[Fwd: Re: Flux]"

    Dear Buthaina,

    you have an incident beam of protons with a momentum of 0.2 GeV,
    which means about 21 MeV kinetic energy. Protons of 21 MeV energy
    have a very short range: about 2.8 mm in beryllium.
    And your energy cutoff for protons is the default, equal to 10 MeV.
    So, as soon as each proton has lost half of its energy, it is ranged
    out without any nuclear interaction. At 10 MeV, the residual range
    is about 0.8 mm, so each of your protons has only a short distance
    available (2.8 - 0.8 = 2 mm) to have a nuclear interaction.
    The proton nuclear cross section between 10 and 21 MeV is anyway
    very small. And I must add that this energy range is
    at the limit of the FLUKA preequilibrium model, so any result must
    be taken with some caution.
    (If your intention was to simulate 200 MeV protons, then you should
    have input -0.2 and not 0.2).
    If you look at the end of your .out output, you will see that:
    - on average, 99% of the proton energy is dissipated as dE/dx
    - there are only very few inelastic interactions (stars): about 1600
       for 1.E5 protons
    - only a very few neutrons are produced: about 1100 for 1.E5 protons

    I would like to add another comment about your input: from a test I
    did, it seems to work, but it would be better to avoid building
    contiguous regions by means of bodies having a common surface (in
    your case, regions regBE4 and regST5 have a common face at z=1.96
    and regST5 and regFe6 at z=201.96).
    You can obtain that by making some of the bodies a little longer
    and cutting them out. Instead of:
    RPP body3 -1.0 +1.0 -1.0 +1.0 0.0 +1.96
    RCC body5 0.0 0.0 +1.96 0.0 0.0 200.0
    RPP body6 -2.375.0 +2.375 -2.375 +2.375 +201.96 +401.96

    * Be target 2nd half
    regBE4 5 +body3 -body4
    * beam tube
    regST5 5 +body5
    * Iron Collimator
    regFe6 5 +body6

    you could have
    ............ make body3 0.04 cm longer, and body5 1 cm longer
    RPP body3 -1.0 +1.0 -1.0 +1.0 0.0 +2.00
    RCC body5 0.0 0.0 +1.96 0.0 0.0 201.0
    RPP body6 -2.375.0 +2.375 -2.375 +2.375 +201.96 +401.96

    * Be target 2nd half -- subtract body5 to make it end at z=1.96
    regBE4 5 +body3 -body4 -body5
    * beam tube -- subtract body6 to make it end at z=201.96
    regST5 5 +body5 -body6
    * Iron Collimator
    regFe6 5 +body6

    This latter definition is more likely to work without problems.

    Best regards,


    On Sun, 24 Feb 2008, Buthaina Abdalla Suleiman Adam wrote:

    > Hi
    > I want to get the flux of neutron at 400cm away from the target outside
    > Steel tube 200cm (regST5) and iron tube200cm (regFe6). My problem is the
    > flux value is zero, even when I made the iron tube shorter than 200cm
    > (10cm, 100cm). Also I made regFe6 just as vacuum to see if I can get any
    > flux but still the same. I don't is this problem due to the number of
    > regions (there is a flux outside regST5)? Please any help, I attached the
    > input file
    > Thank You in Advance,
    > Buthaina Adam

    Alberto Fasso`
    SLAC-RP, MS 48, 2575 Sand Hill Road, Menlo Park CA 94025
    Phone: (1 650) 926 4762   Fax: (1 650) 926 3569

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