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None the local time goes back -- the true step length is negative 

Forum: Processes Involving Optical Photons
Date: 03 Aug, 2012
From: khoi nguyen <khoi nguyen>

dear all,

i'm trying to simulate the scintillation process for electron in liquid argon with geant4.9.5 .

in the physics list for electron, i turn on :

1. G4eIonisation (with livermore model)

2. G4Scintillation

for the optical photons produced in scintillation process, i turn on G4OpWLS (wavelength shifting process).

i want to shoot a test electron with E < 100 keV, let it go through the ionisation process, deposit some energy Edep, which will then be used by G4Scintillation to generate some scintillation photons (with energy ~ 10 eV). those photons propagate in space and undergo wavelength-shifting process (energy shifted down to ~ 3 eV).

i tried the following combinations for e- and opticalphoton (0 = off, 1 = on)

1. G4eIonisation 1, G4Scintillation 1, G4OpWLS 0 : i shoot a test e- with any energy (from 1 keV up to 100 GeV) --> the program works fine !

2. G4eIonisation 1, G4Scintillation 0, G4OpWLS 1 : (since no scintillation --> no photon --> therefore) i shoot some test photons with E ~ 10 eV, and E ~ 3 eV --> fine !

3. G4eIonisation 1, G4Scintillation 1, G4OpWLS 1 : i shoot a test electron with energy E and let it create some scintillation photons.

    i. if E < 30 keV --> fine !

    ii. E >= 40 keV --> GEANT4 complains :  
         a. G4ParticleChange::CheckIt : "the local time goes back !!"

         b. G4VParticleChange::CheckIt : "the true step length is negative !!"

         c. G4Exception: ... step length and/or energy deposit was illegal

you can see this in the log file in the attachment.

it seems to me that the problem doesn't always happen, but whenever it does, it's always due to the G4OpWLS process.

i went and check the source code of the method G4ParticleChange::CheckIt

http://www.lcsim.org/software/geant4/doxygen/html/classG4ParticleChange.html#a8cbb79b15a330626f4a0fa02c234ca73

i can't find any G4cout command which spits out the line "the local time goes back !!" .

i do find that line in here though :

http://geant4.cern.ch/support/source/geant4/source/track/src/G4ParticleChange.cc

--> my hypothesis: the documentation in the web is more up-to-date than my version --> i went here:

http://geant4.cern.ch/support/download.shtml

downloaded the patch-01 at the end of the page, followed the instruction to recompile everthing from scatch --> re-ran --> the problem is still there. still the annoying line "the local time goes back", which i can't find in G4ParticleChange::CheckIt .

would be very grateful if you guys could help me nail the problem down.

thanks a lot .

cheers khoi

   Attachment:
      http://hypernews.slac.stanford.edu/HyperNews/geant4/get/AUX/2012/08/03/06.09-59676-test.txt

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1 Feedback: Re: the local time goes back -- the true step length is negative   (Gumplinger Peter - 03 Aug, 2012)
(_ None: Re: the local time goes back -- the true step length is negative   (khoi nguyen - 03 Aug, 2012)
(_ None: Re: the local time goes back -- the true step length is negative   (khoi nguyen - 07 Aug, 2012)
(_ Feedback: Re: the local time goes back -- the true step length is negative   (Gumplinger Peter - 07 Aug, 2012)
(_ None: Re: the local time goes back -- the true step length is negative   (khoi nguyen - 14 Aug, 2012)
(_ None: Re: the local time goes back -- the true step length is negative   (khoi nguyen - 14 Aug, 2012)
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