Development of a scintillation detector using a MPPC as an alternative to an APD

We conducted a study to examine the performance of the multi-pixel photon counter(MPPC). The MPPC is a novel type of semiconductor photodetector consisting of multiple avalanche photodiode (APD) pixels operated in Geiger mode. Whereas the MPPC offers a great advantage in signal multiplication comparable to that achieved by the photomultiplier tube (PMT), the detection of weak scintillation light signals is difficult due to the severe contamination of dark counts. In this study, we first compared the energy resolutions and energy thresholds of a 3 × 3 mm² MPPC with those of a 3 × 3 mm² APD as scintillation detectors. The MPPC and APD were optically coupled with 5 × 5 × 5 mm³ scintillation crystals of BGO, Tl:CsI, Pr:LuAG, and YAG. It turned out that the APD had better energy resolutions for 662 keV gamma-rays, while the MPPC had lower energy thresholds as measured using a test pulse. Despite the low energy thresholds, it is difficult for the MPPC to detect low energy gamma-rays due to the contamination of dark counts. Secondly, we applied a coincidence technique to discriminate weak gamma-ray signals from dark counts by using scintillation detectors that consisted of a 2 × 2 MPPC-array optically coupled with 10 × 10 × 10 mm³ crystals of GSO, BGO, and Pr:LuAG. With this technique, we demonstrated that dark counts achieved a rejection efficiency of more than 99.8%. As a result, 22.2 keV gamma-rays were successfully detected with a GSO scintillator as measured at +20°C.