THE CALIBRATION OF THE ABSOLUTE SENSITIVITY OF PHOTOMULTIPLIER TUBES IN THE HIGH-RESOLUTION FLYS EYE DETECTOR

The High Resolution Fly's Eye Experiment (HiRes) measures cosmic rays from 10(17) eV to beyond 10(20) eV by the atmospheric fluorescence lig ht produced in their cascade showers. The light is detected with photo multiplier tubes (PMTs). The full HiRes project proposes to use about 30000 PMTs in two sites. More than four thousand tubes are already in operation. Accurate calibrations of the absolute sensitivity of the PM Ts are essential to this experiment. We have developed at the Universi ty of Utah a facility to calibrate the absolute gain and quantum effic iency of the tubes to the accuracy of better than three percent. This facility has proved to be capable of testing a large number of photomu ltiplier tubes efficiently and economically. In addition, it can be us ed to map the gain and the quantum efficiency of a PMT as a function o f where the incident photons hit the tube's window. Although this faci lity is developed for HiRes, it is applicable to other experiments tha t utilise PMTs. We shall describe in this report the operation princip les, the apparatus and some typical results.