The scintillator failed detect photons

Hi all,
I defined a scintillation detectors to detect the gamma ray from the decay of colbat-60. the decay of cobalt-60 could generate two photons with energy 1.17MeV and 1.33 MeV but sometimes the scintillator can only detect one photon. May I ask what’s the probelm. My codes are available in the github: GitHub - renqipan/geant4_tutorial_v11 at pet, and the ''pet" branch.

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Maybe you observe the 0.12% of the decays that yield only one photon?