SiPM developments for the Time-Of-Propagation detector of the Belle II experiment

Belle II is a particle physics experiment working at a high luminosity collider that expects a hard irradiation environment in the next few years. The Time-Of-Propagation modules surround the Belle II tracking detector on the barrel part for particle identification. Each module contains a finely fused silica bar, microchannel plate photomultiplier tube (MCP-PMT), and high-speed readout electronics. These MCP-PMTs will have a lifetime of about few years at the nominal luminosity of the accelerator due to the high photon background degrading the quantum efficiency of the photocathode. An alternative to the MCP-PMTs can be silicon photomultipliers (SiPM). The SiPMs, in comparison to MCP-PMTs, have a lower cost and higher photon detection efficiency, but also a higher dark count rate that strongly depends on the accumulated neutron radiation. The dark count rate can be mitigated by annealing the irradiated devices and/or by lowering the temperature. We tested SiPMs from different producers, different dimensions and different cell pitches to understand their functionality and behavior in several conditions, e.g. irradiation up to 5 · 1011 1 MeV neutron equivalent (neq) cm-2 and after strong annealing for 60 days at 150 °C. Dark count rate studies demonstrate significant recovery of the degradation of SiPMs when annealed. In the photon spectra analysis, we are able to extract photon peaks and estimate breakdown voltages, which are consistent in different conditions. In time resolution studies, the SiPMs achieve a 100 ps level, and the results are compatible in all tested conditions.