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Question Tracking of low energy particles near a boundary 

Forum: Event and Track Management
Date: 16 Dec, 2013
From: Nicola Mori <Nicola Mori>

Hi, I'm looking at very low energy particles moving towards the boundary of a volume, i.e. a delta ray electron produced in volume A and moving towards volume B. As far as I understand tracking in Geant4, the step length of such a particle may be limited by the presence of the boundary between A and B: in this case, the particle is moved to the boundary and G4Navigator::fExitedMother is set to true. This will have the effect of "blocking" the current volume A when searching for the volume entered by the particle (B in this case) with G4VoxelNavigation::LevelLocate; this seems to be fine since the particle is leaving volume A so it makes no sense to check if it is entering volume A.

However, in this scheme a problem could arise when the direction of a low energy particle is heavily modified by e.g. multiple scattering. Such a particle may be moved to the border, triggering all the above described procedures; if now the modified momentum direction makes the particle re-enter in volume A, then the voxel navigator will identify the wrong post-step volume since A is blocked.

I'm investigating a strange behaviour in my simulations which seems to stem from the above described situation, but I don't know if such a situation can effectively happen given the internals of the tracking algorithm. I'm inspecting the code flow with gdb to check it, but I'd appreciate if some tracking expert could shed some light on this topic.


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1 None: Re: Tracking of low energy particles near a boundary   (Tom Roberts - 16 Dec, 2013)
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