Re: Question about STEPSIZE

From: Alberto Fasso' (fasso@SLAC.Stanford.EDU)
Date: Tue May 30 2006 - 20:04:02 CEST

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    Dear Dr. Fan,

    first one remark that is valid for you as well as for any other user who
    wants an explanation about some results: please send always your input
    file and your user routines (if any), and send them as text attachments.
    In general, the person who asks "sees" only a small part of the problem,
    which most often is located somewhere else. In your case, for instance,
    nobody can know what you have coded into your mgdraw file: what is printed as
    "track length of each step" could be correct, or could have been
    misunderstood. And your input could contain commands that have an effect
    on that track length.

    Having said that, I would like to warn you that the user is not supposed
    to change the default stepsize, UNLESS THERE ARE MAGNETIC FIELDS.
    The idea that you could use the STEPSIZE command to reduce calculation time
    is not a good idea. In principle, you could gain some (small) amount of time
    by setting a maximum energy loss per step by means of commands EMFFIX
    and FLUKAFIX. But the first is only for electrons, and both commands anyway
    assure you only that each step is SMALLER than a certain length, not that
    all steps have a given length. And this maximum length cannot be larger
    than that corresponding to a 20% energy loss.
    You need to remember that FLUKA has a very sophisticated algorithm to manage
    the step length, which is already optimized for both accuracy and time, and
    using STEPSIZE you can only do worse. The reason STEPSIZE is available
    is only to make sure that a small region is not missed when tracking IN
    A MAGNETIC FIELD, and that a particle of very low energy does not spiral
    unnecessarily IN A MAGNETIC FIELD.

    Best regards,

       Alberto

    On Tue, 30 May 2006, Fan, Ph.D., Jiajin wrote:

    > Hi all,
    >
    > I'm working on proton and carbon dose calculation. I tried to use STEPSIZE
    > card to change charged particles' stepsize to reduce calculation time. And I
    > use mgdraw.f to output particle track info to test it. But I found what ever
    > minimum step size I gave to input file, particle track is always same. Can
    > anybody tell me what's wrong in my input? Thank you.
    >
    > Stepsize input card (no other input cards which will affect particle step
    > size):
    >
    > * minimum 1.0 cm, maximum 10.0 cm in region 3
    > STEPSIZE 1. 10.0 3.0 0.0 0.0 0.0
    >
    > Track info:
    >
    > * First column: track length of each step; Second column: energy
    > deposit in this step
    > * Last column: (carbon) total kinetic energy
    > 0.019366 2.216800 1.000000 0.000000 0.000000 0.000000 0 0
    > -30 3597.783447
    > 0.012749 1.459835 1.000000 0.000000 0.000000 0.000000 0 1
    > -30 3596.324463
    > 0.019322 2.213185 1.000000 0.000000 0.999730 -0.023238 0 0
    > -30 3593.977539
    > 0.008135 0.932023 1.000000 0.000345 0.999730 -0.023238 0 0
    > -30 3593.045654
    > 0.010283 1.178353 1.000000 0.000000 0.999730 -0.023238 0 0
    > -30 3591.866943
    > 0.003214 0.368304 1.000000 0.000000 0.999730 -0.023238 0 0
    > -30 3591.498779
    > 0.002584 0.296127 1.000000 0.000000 0.999730 -0.023238 0 1
    > -30 3591.203125
    >
    > The minimum step size is much smaller than 1.0 cm set in input file, and
    > output info is always same whatever minimum stepsize value set in input
    > file. I don't know why. Thanks again.
    >
    > *******************************************
    > Jiajin Fan, Ph.D.
    > Research Associate, Radiation Oncology
    > Fox Chase Cancer Center
    > 432 Rhawn Street, Philadelphia, PA 19111
    > Tel: (215) 214-1711
    > Fax:(215) 728-1148
    > Email: jiajin.fan@fccc.edu
    > *******************************************
    >
    >

    -- 
    Alberto Fassò
    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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