Dark Rate of a Photomultiplier at Cryogenic Temperatures
  When cooled below room temperature, the pulse response of a photomultiplier in the absence of light (dark rate) initially decreases, but then turns around near 250 K and continues to rise all the way down to 4 K. When the photomultiplier is cold, its dark response is burst-like. We have measured the characteristics of the dark response of a photomultiplier. 
H.O. Meyer