|Message: Re: Track stuck or not moving with too small voxel sizes||Not Logged In (login)|
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I've reproduced the problem myself.
It does not have to do with the number or size of the voxels.
I think it's a fairly basic problem that we just didn't realize because we are generally focused on external beams hitting our detectors rather than point sources within our detectors.
Here's what I do to reproduce the problem. Place an isotropic point source at the center of the world. Place a nested parameterization box at the center of the world.
If the box is undivided, no problem.
If the box is divided into an even number of voxels, I see many track stuck warnings.
If the box is divided into an odd number of voxels, I do not see track stuck warnings.
The behavior is consistent whether I divided the box by a small even number, such as 2 x 2 x 2 (gives problems) or 3 x 3 x 3 (no problems), or by a large even number, such as 200 x 200 x 200 (gives problems) or 201 x 201 x 201 (no problems).
If the box is divided into an even number of voxels, but I offset my box from the center of the world, even by a tiny bit, the warnings go away.
In my case, I am using 200 MeV protons with a modular physics list consisting only of g4em-standard_opt0. But I do not believe the particle type or physics list is the issue.