Improvement of energy thresholds for scintillation detectors using a monolithic 2 × 2 multi-pixel photon counter array with a coincidence technique

The performance of a large-area, monolithic Hamamatsu multi-pixel photon counter (MPPC) was tested consisting of a 2 × 2 array of 3 × 3 mm² pixels. MPPC is a novel type of semiconductor photodetector comprising multiple avalanche photodiode (APD) pixels operated in Geiger mode. Despite its great advantage of signal multiplication comparable to that achieved with the photomultiplier tube (PMT), the detection of weak scintillation light signals is quite difficult due to the severe contamination of dark counts, which typically amounts to ≳1 Mcps/3 × 3 mm² at room temperature. In this study, a coincidence technique was applied for scintillation detectors to improve the detection efficiency for low energy gamma-rays. The detector consisted of a 10 × 10 × 10 mm³ crystals of GSO, BGO, and Pr:LuAG optically coupled with the 2×2 MPPC-array. With this technique, we demonstrated that the contamination of dark counts was reduced with a rejection efficiency of more than 99.8%. As a result, 22.2keV gamma-rays were successfully detected with a GSO scintillator as measured at +20 °C.