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