Observation of light emission from Hamamatsu R11410-20 photomultiplier tubes

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.