Geant4 Version: 11.2.2
Operating System: Ubuntu 24.04.3 LTS
Hello Geant4 community,
I am simulating antiproton transmission through a Moiré grating structure using Geant4 (version 11.2.2). My geometry consists of two silicon gratings with vacuum openings (40 μm wide, 100 μm pitch, 250 μm thick), placed at z = -45 cm, z = 0 cm apart, and a solid silicon counter downstream. The antiproton beam is generated as a parallel beam (10 keV, 5 cm radius) aimed along the +Z axis, starting at z = -60 cm.
Despite setting the geometry and user limits to force small steps (4 μm) in both the wall and opening regions, I observe that the transmission rate of antiprotons through the grating openings is much lower than expected from simple geometric calculations (i.e., the fraction of open area). Most antiprotons are absorbed in the first silicon walls, and very few reach the counter.
Questions:
- Are there known issues or subtleties in Geant4 tracking or geometry that could cause artificially low transmission through narrow vacuum gaps?
Is there a recommended way to ensure accurate tracking of low-energy antiprotons through micron-scale vacuum openings? - Could physics processes (e.g., multiple scattering, step limitation, boundary effects) be responsible for this, and how can I best diagnose or mitigate them?
- Are there best practices for modeling such fine structures to avoid numerical artifacts?Additional info:
I am using G4UserLimits to set the maximum step size to 4 μm in both wall and opening logical volumes.
The geometry is built with G4Box and G4PVReplica for the grating slices.
Sensitive detectors are assigned to both wall and opening volumes.
The physics list is based on FTFP_BERT with low-energy extensions.
Any advice or suggestions would be greatly appreciated!
Thank you,
Piyush