Photocathode-Uniformity Tests of the Hamamatsu R5912 Photomultiplier Tubes Used in the Milagro Experiment

The Milagro experiment observes the extensive air showers produced by very high energy gamma-rays impacting the Earth's atmosphere. Milagro uses 898 Hamamatsu R5912 Photomultiplier Tubes. To complete our Monte Carlo simulations, we tested the photocathode uniformity of our PMTs. The main finding was that the PMT gain and detection efficiency are a function of the distance from the center of the photocathode. Both quantities become considerably smaller as the illumination position nears the edge of the photocathode.