Test and characterization of SiPMs for the upgrade of MEGII high resolution Timing Counter

The MEG II experiment based at PSI (Zuerich, Switzerland) has been committed and is taking data since 2021 to improve sensitivity on the decay μ++ → e++γ. The pixelated Timing Counter (pTC), consisting of two arrays of 512 5 mm thick scintillator pixel each, read out by 6 3×3 mm22, 50 μm cell, Silicon Photomultipliers (SiPMs) from AdvanSiD, for a total of 6144 SiPM, achieves an overall resolution in the positron impact time of ∼43 ps when exploiting multiple measurements. To additionally improve this resolution, 4×4 mm22, 40 μm cell SiPMs have been selected to substitute a fraction of the old ones (about 1000 of them overall). By means of an automated test system, a first group of them has been characterized (measuring their breakdown voltage and their I-V curves), to match as much as possible SiPMs with the same gains in each pixel, in order to maximize the pixel time resolution. Such automated test system will be presented, together with some preliminary results on single pixel time resolution and the expectations of the average time resolution of the pTC in the coming years.