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Warning Bad transportation for suspended tracks 

Keywords: G4Transportation navigation suspended tracks
Forum: Geometry
Date: 12 Dec, 2005
From: Paulo Mora de Freitas <Paulo Mora de Freitas>

I've been tracking a rare and strange bug in our simulation program for the ILC, Mokka, and it seems to be related with a problem in the G4Transportation process when dealing with suspended tracks.

Each time the tracking changes the track being tracked the G4Transportation process has to reset several private attributes. It's done in the beginning of the G4Transportation:: AlongStepGetPhysicalInteractionLength method. In the current Geant4 relase (7.1-patch01) this reset is done if and only if the track step number is equal to 1.

The problem arises when a suspended track comes back to be tracked. In this case the G4transportation doesn't reset these several attributes. So the suspended track restarts with a bad G4Touchable which lets to error in the navigation.

Here you have a example: In this case a back scatter starts at 
(308,-713,2.5e+03):
...
...
   25      308     -715  2.5e+03     0.397  0.00721     0.17      6.28
     TPCEnd eIoni
   26      308     -715  2.5e+03     0.388  0.00885     0.23      6.51
 TPCChamber Transportation
   27      306     -715 2.47e+03      0.38  0.00668     27.9      34.4
 TPCChamber eIoni

At this point, as it enters in the tracker region, Mokka suspends the track. When it came back to be tracked again we get the following steps:

    28      306     -715 2.47e+03     0.379 0.000621     1.85      36.3
   SiBarrel Transportation
   29      306     -715 2.47e+03     0.358   0.0146   0.0466      36.3
   SiBarrel eIoni

The particle was in the TPCChamber volume before being suspended. As soon as it's restarted, the Transportation decides that it's on the boundary of the SiBarrel volume. The problem is that the SiBarrel volume normally is in the Ecal barrel, far away from the TPCChamber. The Ecal barrel starts at R= ~1700 mm! Both volumes are not neighbors, so it should not be a problem of ktolerance on the boundary.

This problem is reported as problem report #820. You can get a patched G4Transportation copy (for 7.1-patch01 release only) by myself in the URL

http://polywww.in2p3.fr/~mora/Geant4/ProblemReport820/

 while waiting for a official one.

 Cheers, Paulo.

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