  Silicon photomultipliers are photon sensors based on an array of pixels, each
consisting of a single photon avalanche photodiode (SPAD). Reading of the array
is not done on a single-pixel basis, but all pixels are read simultaneously.
Because of thermal agitation and other phenomena, each pixel can fire even in
the dark with a small but not zero probability of adding to the output signal a
fixed current. Each pixel has an average random event rate, and the dark noise
rate is due to the sum of any pixel contribution. Statistically, two or more
pixels may fire simultaneously, giving a current signal equal to the sum of the
pixels involved. This eventuality has a lower probability of a more significant
number of fired pixels. This paper aims to find simple formulas that relate
dark noise and the rate of having multiple multiplicity events.
