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Question Small energy deposition in vacuum at parallel world boundaries 

Forum: Fast Simulation, Transportation & Others
Date: 03 May, 2017
From: Nicholas Kinnaird <Nicholas Kinnaird>

Hi, I'm seeing a bug in the geant simulation where occasionally (every ~20/30 events) particle steps in vacuum geometry are ended with the physics processes of ionization or bremsstrahlung, depositing a very small (~=0) amount of energy in the vacuum. This problem seems to occur at boundaries of parallel world geometry, and disappears if the parallel world is removed.

I'm not so concerned with the small number energy dep since I imagine geant probably just calculates some very small value from the vacuum material, but I am very surprised that steps are being halted by ionization or bremsstrahlung processes instead of transportation/coupled transportation/parallel world processes/step limiter, etc.

Attached are a couple of pics showing the issue, in two a histogram of post process ids which halted steps within the simulation, including processes ionization (one zoomed out and one zoomed in - 2 is ionization, the rest are transportation, etc.), and the other some output showing a single ionization event even though vacuum is the material the step is contained within.

Has anyone seen something like this before? Is this expected behavior at all?

geant version: v4_10_2_p02a

   Attachment:

      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2017/05/03/13.31-36490-vacenergydep.png
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2017/05/03/14.04-9444-postProcessAll.png
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2017/05/03/14.04-95252-postProcessIon.png

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