  Radiation damage in lead tungstate crystals reduces their transparency. The
calibration that relates the amount of light detected in such crystals to
incident energy of photons or electrons is of paramount importance to
maintaining the energy resolution the detection system. We report on tests of
lead tungstate crystals, read out by photomultiplier tubes, exposed to
irradiation by monoenergetic electron or pion beams. The beam electrons
themselves were used to measure the scintillation light output, and a blue
light emitting diode (LED) was used to track variations of crystals
transparency. We report on the correlation of the LED measurement with
radiation damage by the beams and also show that it can accurately monitor the
crystals recovery from such damage.
