Re: photon beam divergence

From: Alberto Fasso' (fasso@SLAC.Stanford.EDU)
Date: Wed Aug 31 2005 - 04:21:23 CEST

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    Dear Chiara,

    your mail is not completely clear to me.

    You speak about the beam divergence, but you say that you use the
    source routine to sample the x-y coordinates.
    So, do you want a gaussian distribution for the beam profile (coordinates)
    or for the beam angle (divergence), or for both?

    You say also that you have tried to define the beam divergence using WHAT(3) in
    BEAM card or DIVBM in source but you didn't get good results.
    What did you get exactly? In which way they weren't good?

    While waiting for a more detailed explanation from you, I want to call your
    attention on a couple of points.

    1) At least until some time ago, the beam profile and divergence
    were working correctly only for a beam directed along the positive
    z-direction. I think that this restriction has been removed now (Alfredo please
    confirm!) but I see that you are not using the most recent version, so
    perhaps that could be the reason for your problems.

    2) I see the following lines in your source:
          UBEAM1 = -0.3
          UBEAM2 = 0.3
          VBEAM1 = -0.3
          VBEAM2 = 0.3
          X1 = 0.5
          Y1 = 0.5
    When writing FLUKA user routines, write ALWAYS the numerical constants
    in double precision (with a "d"):
          UBEAM1 = -0.3d0
          UBEAM2 = 0.3d0
          VBEAM1 = -0.3d0
          VBEAM2 = 0.3d0
          X1 = 0.5d0
          Y1 = 0.5d0
    especially if cosines are concerned.
    I don't think if that could be the cause of your problems, but is
    certainly safe

    3) There is a FLUKA routine which does the sampling from a gaussian
    distribution: it is called FLNRRN and it is called as follows:
    where GAUSS is a random number normally distributed.
    That routine has also an entry which allows to sample two such numbers
    with a single call, so you can save some time:
          CALL FLNRR2 (GAUSS1, GAUSS2)


    On Tue, 30 Aug 2005, Chiara D'Ambrosio wrote:

    > Hi everybody!
    > I'm trying to get information about the influence of the beam divergence
    > on the interaction between a photon beam and a sort of
    > "collimated"-detector. Distance between source and detector is 10 mm.
    > I'm using a modified source routine to sample the x-y coordinates of the
    > beam from a gaussian distribution with sigma = 100 um. (z is fixed and
    > photons have negative z direction)
    > I've tried to define the beam divergence using WHAT(3) in BEAM card or
    > DIVBM in source but I didn't get good results.
    > I've used a sampling of the director cosines but I think this is not the
    > right thing to do.
    > Any suggestion will be appreciated!!
    > I append the input and source file.
    > Thanks in advance
    > Chiara D'Ambrosio
    > LNF-INFN Research Division
    > Via Enrico Fermi, 40

    Alberto FassÚ
    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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