  We have shown that high voltage biased Hamamatsu R11410-20 photomultipliers
with a dark count rate above 10 kHz emit single photons. The effect has been
observed in a few units at room temperature and temperatures reduced down to
-60 degrees Celsius. The effect should be taken into account in experiments
aimed on search for rare events with small energy depositions in massive liquid
xenon detectors.
