Speaker
Description
Silicon photomultipliers (SiPMs) are widely used in photon counting experiments today because of their high photon detection efficiency, compactness, small dead area, and low bias voltage. However, SiPMs tend to have higher dark count rate than conventional photomultiplier tubes, and their temperature dependence is known to be non-negligible. Therefore, it is essential to characterize and calibrate the dark count rate to accurately extract the charge of faint signals. In previous studies, the SiPM dark count rate was usually measured only on short time scales of minutes to hours, and its long-term instability was not discussed. In the present study, we monitored dark current of 128 SiPM channels, which is nearly proportional to the dark count rate, for about a year to find any unexpected failure modes before the construction of the next-generation ground-based gamma-ray observatory, the Cherenkov Telescope Array Observatory, which will use on the order of $ 10^5 $ SiPM channels to detect atmospheric Cherenkov photons. Through this study, we found that the dark current baselines of 50 of the 128 channels frequently changed by 5-20% on time scales of hours to days. We investigated the cause of this multimodal behavior in detail by observing the photon emission from the SiPM surfaces and correlating the brightness change with the dark current. We conclude that multiple avalanche photodiode (APD) cells of a single SiPM carry this baseline change, because certain APD cells were bright in optical images (hot spots) when the dark current shifted to higher baselines. While the dark current shift of up to ~20% is not a problem for gamma-ray observations under the night sky, understanding the cause of the hot spots will be able to reduce the dark current of future SiPM products. In addition, photon counting experiments under very dark conditions might improve the charge resolution by taking into account the dark current instability we found.
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