Performance of Low Afterpulsing Probability Multi-Pixel Photon Counters for Time-of-Flight Positron Emission Tomography

Hamamatsu´s Multi-Pixel Photon Counters (MPPCs) satisfy the requirements of simultaneous Time-of-Flight (ToF) PET-MR operation, matching the gain and speed of vacuum photomultipliers while being immune to magnetic fields. They have been used to construct MR-compatible PET detectors that have demonstrated excellent timing performance on a prototype whole-body PET/MR scanner. Charge amplification in MPPCs relies on a Geiger mode avalanche developed in an individual silicon diode microcell. The primary noise sources in MPPCs include dark counts, optical crosstalk and afterpulsing. Afterpulses occur over a time range sufficiently long to increase the tail of the average MPPC output in response to incident optical photons. Hamamatsu Photonics continues to make improvements to their MPPCs aimed at lowering afterpulsing (and thus dark current) by using silicon with reduced impurities and a new junction profile. We characterized the operation of these Low Afterpulsing Probability (LAP) MPPCs and compared the timing performance of a ToF-PET detector block constructed using these devices with a block constructed using standard MPPCs. We found that the LAP MPPCs demonstrated significantly lower afterpulsing than their standard MPPC counterparts, which resulted in lower dark current at a fixed overvoltage. This feature ensured a more stable operating range for a LAP-based ToF-PET detector block. Despite this useful feature, the maximum photon detection efficiency of the LAP MPPC was similar to a standard MPPC, which limited any additional improvement in the CRT of a LAP-based block. Future improvements in the MPPC design that improve the PDE while concurrently retaining the low afterpulsing feature could benefit the timing performance of the ToF-PET detector block.