Radiation damage effect on time resolution of 6 series-connected SiPMs for MEG II positron timing counter

We studied the impact of radiation damage on scintillation counters read out by six SiPMs connected in series focusing on their time resolution. Six SiPMs from AdvanSiD were irradiated step by step with a 37 MBq 90Sr source. At the integrated dose expected at the end of the MEG II experiment, the time resolution at 30 °C was measured to be worse by ∼29%. However, the deterioration is found to be reduced to ∼6% at 10 °C since thermal dark noise is suppressed. We also tested the effect of connecting in series differently damaged SiPMs. We observed a position dependence of the time center in a counter, which worsened the time resolution.