Effects of dark counts on Digital Silicon Photomultipliers performance

Digital Silicon Photomultipliers (dSiPM) are novel light detector that integrates single-photon avalanche photodiodes and CMOS logic into a single silicon chip and have been used for developing new, high performance detectors for Positron Emission Tomography (PET). As a solid-state devices they suffer from thermal excitation what leads to the appearance of noise events called dark counts. However, it is unclear what effect the dark counts have on the count rate performance of dSiPM. Therefore, it is necessary to investigate the event loss caused by these dark counts and to come up with optimal configuration of these devices. Here, the effects of dark counts on the performance of are evaluated. Due to the trigger architecture of dSiPM, dark counts cause start of acquisition sequence of the device. Processing of these dark counts leads to dead time of dSiPM what cause the loss of true gamma events. We studied how trigger level, validation level and validation length influence the loss of events due to dark counts. We found that validation time should be kept long (40 ns) to minimize the loss of events. Use of high trigger level and validation level also reduce the event loss caused by dark counts. However, with the high validation level, detection of events with low number of optical photons is reduced as it more difficult for these events to pass the validation threshold. The RTL refresh option was also tested to reduce the effect of dark counts. We found that this option resulted in the achieving maximum sensitivity, i.e. the highest fraction of correctly recorded true events, of dSiPM regardless of used validation and trigger levels. In cases when the scintillation light is spread over several dies, we found that the use of RTL refresh option combined with a low validation level in order to guarantee the individual validation of all required dies ensures higher sensitivity than the use of Neighbor Logic (NL). Finally we verified the dead time of dSiPM and found that is longer than specified and equal to 50 ns.