The calibration of the absolute sensitivity of photomultiplier tubes in the high resolution Fly's eye detector

Abstract The High Resolution Fly's Eye Experiment (HiRes) measures cosmic rays from 10 17 eV to beyond 10 20 eV by the atmospheric fluorescence light produced in their cascade showers. The light is detected with photomultiplier 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 PMTs are essential to this experiment. We have developed at the University of Utah a facility to calibrate the absolute gain and quantum efficiency of the tubes to the accuracy of better than three percent. This facility has proved to be capable of testing a large number of photomultiplier tubes efficiently and economically. In addition, it can be used to map the gain and the quantum efficiency of a PMT as a function of where the incident photons hit the tube's window. Although this facility is developed for HiRes, it is applicable to other experiments that utilise PMTs. We shall describe in this report the operation principles, the apparatus and some typical results.