From: Jeff Wilkinson (email@example.com)
Date: Fri Feb 15 2008 - 02:59:49 CET
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
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,
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