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  2. SIM-505

Warning about stuck step in G4PropagatorInField in ALICE productions, inside TGeoPCone solids

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      Ivana Hrivnacova reported the following (by email):

      ALICE has started physics validation with Geant4.10.00.p01, and in the first tests we are getting frequently these warnings:

      -------- WWWW ------- G4Exception-START -------- WWWW -------
      *** G4Exception : GeomNav1002
           issued by : G4PropagatorInField::ComputeStep()
      Particle is stuck; it will be killed.
       Zero progress for 51 attempted steps.
       Proposed Step is 1.42936e-05 but Step Taken is 1.42936e-05
      in volume TPC_WBAR
      *** This is just a warning message. ***
      -------- WWWW -------- G4Exception-END --------- WWWW -------

      The warnings happen in about 15% of events, and they all report just two volumes: TPC_Drift and TPC_WBAR. (I put below the parameters of these volumes.)
      We ran checking overlaps, and there are no overlaps reported in this part of geometry.

      We were getting these warnings also with previous productions in 2011, and we reported on them in ALICE presentation in the LHC Detector Simulations Workshop in 2011. But at that time we did not spot just few particular volumes.

      Would you have an idea what could go wrong here, or tips how to debug the problem?

      =================================
       
      TPC_WBAR
      *** Shape bar: TGeoPgon ***
         Nedges = 1
      *** Shape bar: TGeoPcon ***
         Nz    = 2
         phi1  =     0.00000
         dphi  =    20.00000
          plane 0: z=   -3.00000 Rmin=  122.86184 Rmax=  127.86184
          plane 1: z=    1.50000 Rmin=  122.86184 Rmax=  127.86184
       
      Positions (non rotated) of TPC_WBAR in TPC_Drift:
      posXYZ   X_Y_Z="  8.5069;   1.5000;  256.6000"
      posXYZ   X_Y_Z="  8.5069;   1.5000; -256.6000"
       
      TPC_Drift
      *** Shape TGeoPcon: TGeoPcon ***
         Nz    = 6
         phi1  =     0.00000
         dphi  =   360.00000
          plane 0: z= -260.00000 Rmin=   74.50000 Rmax=  264.40000
          plane 1: z= -253.60000 Rmin=   74.50000 Rmax=  264.40000
          plane 2: z= -253.60000 Rmin=   76.67740 Rmax=  258.00000
          plane 3: z=  253.60000 Rmin=   76.67740 Rmax=  258.00000
          plane 4: z=  253.60000 Rmin=   74.50000 Rmax=  264.40000
          plane 5: z=  260.00000 Rmin=   74.50000 Rmax=  264.40000

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            japost John Apostolakis
            ivana Ivana Hrivnacova
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