The selection of suitable high-efficiency scintillation detectors plays an essential role in improving the quality of PET images. Many studies have used two layers of crystal detectors with different scintillation materials. This research uses GATE simulation to simulate an animal PET system with a two-layer detector. By varying the type of the crystal detectors, essential parameters such as sensitivity, scatter fraction, noise equivalent counting rate, spatial resolution, energy resolution, and time resolution for different scintillation materials were calculated and compared to determine the most efficient solutions. This simulation used scintillating materials; LYSO, GSO, LSO, BGO, YAP, LuAP, and LPS in 12 different states. Results showed that by increasing the distance of the source from the center in the radial direction, the two-layer crystals with different scintillation materials have a better radial spatial resolution than the same material of crystals. As a general result, the PET system???s performance, designed from two-layer crystals with different scintillation materials, lower layer LuAP and upper layer BGO or same-material of BGO, is more desirable than the rest.
