Simulation of signal losses in highly pixelated scintillator arrays read out by discrete photodetectors

The performance of scintillation detectors used in Positron Emission Tomography imaging strongly depends on the scintillation light transport from the crystal to the photodetector. In highly pixelated scintillator arrays with individual pixels approaching millimetric cross section, the loss of signal is compounded with crosstalk effects, squandering valuable signal to adjacent pixels, and with light absorption in lateral faces adhesive materials and imperfect reflectors. The purpose of this simulation study is to uncover processes responsible for light losses in scintillator arrays. Four sources of losses through crosstalk between pixels were identified, namely 1) escaping photoelectrons to other pixels after photoelectric interactions, 2) X-ray fluorescence and Auger emission, 3) reflector transparency to scintillation light, and 4) light leakage to other crystals due to adhesive material between reflectors and scintillators in which optical photons can propagate to other crystals. An important source of signal loss and energy resolution degradation was found to be related to the transmittance of the adhesive material used to bond reflectors to scintillators. Moreover, the angular distribution of scintillation photons impinging on the detection face was assessed in order to weigh the proportion of trapped photons through total internal reflection due to the refractive index difference between scintillators and optical coupling medium.