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Question Potential G4 bug!? Optical Photon not getting stopped 

Keywords: Scintillation OptcialPhoton Error at Runtime
Forum: Processes Involving Optical Photons
Date: 30 Mar, 2006
From: Joerg Orboeck <>


I am running a similar G4 project as the underground_physics example.
That means a a detector with three metal vacuum cans one in the other.
Inside a LXe volume surrounded by teflon and three 1 inch^2 PMTs on top. 
In order to perform light collection efficiency (LCE) simulations for that
detector, I have defined the photo cathodes of the PMTs as SDs. 
I defined the optical properties like reflectivity etc. for all 
materials and switch on the scintillation effect for the LXe. 
In principle it is working now, i.e. in an interactive session I've seen 
a large number of scint-photons are produced and tracked through the 
detector, bouncing off the teflon
walls etc. As data structure I am using a ROOT TTree. All seems to be 
working fine in an interactive session as well as for a test run with 
a small number of events, started from a point source below the bottom 
of the innermost can. Now after these tests being successful, I started 
a run of 1 million events (reduction factor about 1000, i.e. 1M events 
started => ~1k events create a signal in the detector) I run into a 
problem after >300k events. The process appears to continue running 
(still using >80% CPU), but no more events are processed and no additional 
data is written to file. The ROOT file that was created, indicates that until
the problem occured everthing was doing fine. The tree is filled with
reasonable energies and "No of PMT hit" values and so on. I have tried
running it on our Mac G5 Xserve (OSX 10.4, Geant4.7.1, gcc4) as well my 
Linux64 notebook (AMD64 3200+, Debian Linux64, Geant4.8.0, gcc4.0.3), 
but I am running into the same problem on both machines. 

Looking a bit more closely into that matter, i.e. saving the RndmSeeds
and re-running that one event with max. verbosity indicates that out the
thousands of optical photons created, there is one "going nuts" :) in 
the detector. That means it is bouncing around for eternity (>3 million 
steps, tracklength > 7 KILOMETER!!!, until I stopped the run). 

I have no idea what is going on! Could that be a Geant4 bug??? 

I have created a tar-ball of my project, that can be downloaded here:

Untaring this file will create a folder UFXE_2. In there is a 
macro "particle_source.mac" that runs the simulation the way I did. 
(UFXe particle_source.mac test.root) and the "currentEvent.rndm" 
contains a set of RndmSeeds that should reproduce the problem as 
described above.   

Any advice or help offered is highly appreciated. 


Greets Joerg

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1 Feedback: Re: Potential G4 bug!? Optical Photon not getting stopped   (Peter Gumplinger - 20 Apr, 2006)
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