Multi-Pixel Photon Counters for TOF PET detector and its challenges

Multi-Pixel Photon Counter (MPPC) is a Geigermode APD developed by Hamamatsu Corp. We proposed that it could be a suitable photo-sensor for next-generation time-of-flight PET detectors due to mainly its high photon detection efficiency. Therefore, we concentrated on the measurement of coincidence timing performance of various MPPCs in conjunction with LYSO crystal scintillators. With 3mm × 3mm devices of 50µm sub-pixels coupled to 3mm × 3mm × 10mm crystals, we have demonstrated a strong dependence of timing performance on over-voltage and temperature, and analyzed how changes in photon detection efficiency and dark counts would explain the measurements. The best coincidence timing resolution we have achieved between two identical LYSO/MPPC detectors was 240ps in FWHM. This was worse than the timing resolution of 220ps obtained with Hamamatsu H6533 fast PMT, and contradicted the expected improvement from higher photon detection efficiency. The contradiction could be explained by slow rise-time of MPPC pulse shape, transit time spread, dark counts and electronics noise from large capacitance of MPPC. In particular, the slow rise-time of MPPC pulse suggested that the need for a very low trigger threshold on the timing circuit. Since it in turn makes the detector system more sensitive to noise, this poses additional challenges for ganging multiple devices together into a commercially viable time-of-flight PET block detector. We will discuss it in detail including other challenge imposed by MPPC characteristics.