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Warning Stepping Problem with Shared Boundary 

Keywords: step stepping track tracking infinite loop particle stuck
Forum: Event and Track Management
Date: 21 Jan, 2004
From: Keven L. Ates <atescomp1@comcast.net>

In my calorimeter simulation, I generally have two material boundaries that "touch" each other--like a liquid Xenon tower against its Aluminium cryostat. Occasionally, a particle apparently becomes "stuck" between the boundaries. I'm guessing a step places the particle at the interface. At this point, the stepping process enters an infinite loop.

I'm not sure whether:

1: The program cannot decide which material its in.
   or
2: Each following steps remain in the plane of the interface and the
   particle is bouncing around between the two materials.

Each step is identical except for the volume it reports it is in, which swaps between the two volumes at the interface.

Currently I'm handling the problem by doing:

    if (pStep->GetTrack()->GetCurrentStepNumber() > 9999)
        pStep->GetTrack()->SetTrackStatus(fStopAndKill);

in my G4SteppingAction::UserSteppingAction(const G4Step * pStep). However, I believe this will result in the loss of the particle's track and energy deposit, resulting in a loss of precision in my analysis.

While this is disturbing, it is fairly uncommon to occur. Still, it seems that there should be a solution, like:

1: randomly selecting the material and sticking with it
2: determining which material would be the most likely interactor
3: analyzing both until it leaves the interface or interacts with
   one of the materials.

Currently, I'm using G4 v5.2 and am planning to upgrade to v6. As I've seen this also appear in other simulations, has this problem been addressed yet?

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1 More: Re: Stepping Problem with Shared Boundary   (Bob Weller - 19 Mar, 2004)
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