  Owing to their high photon detection efficiency, compactness, and low
operating voltage, silicon photomultipliers (SiPMs) have found widespread
application in many fields, including medical imaging, particle physics, and
high-energy astrophysics. However, the so-called optical crosstalk (OCT)
phenomenon of SiPMs is a major drawback to their adoption. Secondary infrared
photons are emitted inside the silicon substrate spontaneously after the
avalanche process caused by the primary incident photons, and they can be
detected by the surrounding photodiodes. As a result large output pulses that
are equivalent to multiple photoelectrons are observed with a certain
probability (OCT rate), even for single-photon events, making the charge
resolution worse and increasing the rate of accidental triggers by
single-photon events in applications such as atmospheric Cherenkov telescopes.
In our previous study, we found that the OCT rates of single-channel SiPMs was
dependent on the thickness of their protection resin window, which may be
explained by photon propagation inside the resin. In the present study, we
measured the OCT rate of a multichannel SiPM and those of neighboring channels
caused by photon propagation. Both OCT rates were found to be dependent on the
protection-window thickness. We report our OCT measurements of a multichannel
SiPM and comparisons with a ray-tracing simulation.
