Re: Energy deposited in a vacuum with USRBIN

From: Alberto Fasso' (fasso@SLAC.Stanford.EDU)
Date: Sat Feb 16 2008 - 04:39:44 CET

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    Hi Jeff,

    when an electron is stopped because of a transport energy cutoff, its
    energy is entirely deposited at the stopping point. The reason for that,
    as you have guessed, is to keep a correct energy balance.
    Even if that could be considered unphysical, depositing energy in
    vacuum would be even more unphysical: therefore electron cutoff is
    not allowed to work in vacuum.

    In some problems where one wants to study the photons but not the electrons
    which have generated them, setting a very high electron cutoff in a region is a
    convenient way to get rid of the electron component. (In reality,
    that could be done for instance by a magnet).
    But for the reason explained above, the "killing region" cannot be a
    vacuum region. In order to interact as little as possible with the
    other particles, it is a good practice to make such a region very thin
    and filled with a very low density material.


    On Thu, 14 Feb 2008, Jeff Wilkinson wrote:

    > I think I know what is going on with a simulation I'm running, but I wanted
    > to check with the experts, just in case.
    > I'm simulating an industrial x-ray source at 400 kV. The electron beam is
    > incident on a tungsten target enclosed in vacuum. To speed up the calculation
    > I added a very thin sphere of low density material around the target to
    > absorb electrons (Thanks Alberto!). The transport cutoff for electrons is
    > very high in this thin region and low for photons. That stops the secondary
    > electrons ejected from the target quickly before they have chance to slow
    > down the calculation by interacting with the rest of the regions. It works
    > great and comparing results with and without the shell shows no difference
    > other than a 20% decrease in calculation time.
    > When I looked at the results of USRBIN for ENERGY particles for the volume
    > that included the target, sphere and filter structures I was surprised to see
    > that Fluka was claiming that the electrons were depositing energy in the
    > vacuum region leading up to the absorber shell. Turning the absorber back
    > into vacuum (it was hydrogen) gave the expected result and no energy was
    > shown as being deposited in the vacuum. The photon flux is the same in both
    > cases.
    > I'm guessing that Fluka was trying to maintain energy balance when it stopped
    > transporting the electrons in the absorber by assigning their energy along
    > the track from the target to the shell. Is that correct?
    > Thanks for the help,
    > Jeff Wilkinson

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