I am simulating a simple gamma camera geometry with two materials and am detecting much fewer counts than expected.
Analysis of the boundary processes showed that the last photon status, just before the photon was killed, was set to “StepTooSmall” about 1/2 of the time, instead of OpAbsorption or Detection.
So what is this StepTooSmall status, and can I recover the photons killed with that status?
Below is a boundary interaction breakdown by status:
note that Detection are the photons entering my detector, so we get the SameMaterial status as expected)
The killed statuses are FresnelRefraction and StepTooSmall, neither of which make sense to me.
This simulation has every surface set to 100% reflectivity, so there should be no photons absorbed at all, as per my understanding.
The “Reflected” processes are boundary interactions that do not kill the photon (not necessarily reflection - can be reflection, refraction, or transmission)
The total # of photons are (1993+542+1893) + Volume_absorbed photons = 4428 + Volume_absorbed photons.
Note that my simulation has a high opt. photon attenuation length, so the amount of Volume_absorbed is about 10 photons.
Interestingly, some boundary interactions with the StepTooSmall status are not killed, and I cannot find any calls from the StepTooSmall portion of the Geant4 source code to fStopAndKill!
(If this post is in an incorrect category, please let me know.)
I would really appreciate any insight you might have. Thank you!