Dose Discrepancy with and without boundary skipping in regular navigation

Geometry, Fields and Transportation
1

I have set up a project with a voxelized volume, with the goal of determining the total dose to a subset of the voxels. I was initially using nested parameterisation with G4PSEnergyDeposit3D, and defined dose as the sum of energy deposit in the voxels divided by the total mass of those voxels.

I recently discovered the G4PhantomParameterisation class, and found that it is significantly more CPU efficient when the option SetSkipEqualMaterials(true) is set. However, I also found that the dose values were consistently a factor of ~2 greater than those from the simulations using nested parameterisation.

I then tried using the phantom parameterisation without boundary skipping, i.e. SetSkipEqualMaterials(false), and the dose results suddenly matched the nested parameterisation, but the CPU speedup disappeared, of course.

Finally, I tried defining a simple voxelized phantom with a single material and I still observed the 2x higher dose when using boundary skipping.

Has anyone noticed something like this?

2

I have used G4PhantomParameterisation many times and got the correct dose deposition, with or without SkipEqualMaterials. Can you send me (privately if you prefer) you code?

3

Hello Pedro,

Thank you very much for your response. I was preparing my code to send it to you by removing all of the auxiliary classes that were irrelevant (or so I thought) to the issue, only to notice that the problem went away.

It turns out that the cause of the issue is that I had a parallel world in my geometry. I am not sure though why this would only impact the results when boundary skipping is turned on.

4

I have also run with parallel geometries without problem. You may send me your code if you want me to see if the problem is en the Geant4 code

5

Hello Pedro,

I have actually figured out a solution to my problem. There were two issues:

  1. When registering physics for the parallel world, it is necessary to include the true argument, e.g.:

physics->RegisterPhysics(new G4ParallelWorldPhysics(parallelWorldName,true));

  1. The phantom parameterization needed to be in the parallel world, not the primary world.