In low background and low threshold particle astrophysics experiments usingobservation of Cherenkov or scintillation light it is common to use pairs orarrays of photomultipliers operated in coincidence. In such circumstances, forinstance in dark matter and neutrino experiments, unexpected PMT noise eventshave been observed, probably arising from generation of light from one PMTbeing detected by one or more other PMTs. We describe here experimentalinvestigation of such coincident noise events and development of new techniquesto remove them using novel pulse shape discrimination procedures. When appliedto data from a low background NaI detector with facing PMTs the new proceduresare found to improve noise rejection by a factor of 20 over conventionaltechniques, with significantly reduced loss of signal events.
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