Hits can not switch with the energy deposition

Hi everyone. I am doing the optical simulation of a Scintillator and a detector connect with the scintillator. And the particlegun generate 1 MeV gamma ray into the scintillator. The scintillator is wrapped by air with 100% reflectivity. I think that the hits of sensitive detector should be near the total energy deposition x scintillation yield(I set it to 40000/MeV). However, when 1MeV is total deposited, the hits are 2000~3000 less than the expectation(about 40000).
The behavior confuses me a lot. Do you have any hint?

absorption, statistical uncertainty, scattering in the air before entering the scintillator? just guessing :slight_smile: