| Summary: | We investigated two detector modules, each consisting of a 8 × 8 matrix of GaGG:Ce crystals with a crystal size of 3 × 3 × 20 mm<inline-formula><math display="inline"><semantics><msup><mrow></mrow><mn>3</mn></msup></semantics></math></inline-formula> and a 3.2 mm pitch. The light is collected by a 8 × 8 silicon photomultiplier array, with one silicon photomultiplier matching one crystal. The signals are read out and processed using the TOFPET2 ASIC. Performed laboratory tests of the detectors were performed using a <inline-formula><math display="inline"><semantics><msup><mrow></mrow><mn>22</mn></msup></semantics></math></inline-formula>Na source, where energy and coincidence time resolution with different optical coupling were examined between the crystals and silicon photomultipliers, as well as under various operating voltages. The mean energy resolution of <inline-formula><math display="inline"><semantics><mrow><mn>9.8</mn><mo>±</mo><mn>0.6</mn><mo>%</mo></mrow></semantics></math></inline-formula> at 511 keV was observed and it was shown that the coincidence time resolution of <inline-formula><math display="inline"><semantics><mrow><mn>384</mn><mo>±</mo><mn>33</mn></mrow></semantics></math></inline-formula> ps could be achieved. The results reassure that the GaGG scintillator is a very promising candidate for the development of imaging systems, in particular, ones utilizing Compton scattering where the energy resolution plays a critical role and a moderate timing performance is acceptable.
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