Single photoelectron resolution for the calibration of photomultiplier systems

The Pierre Auger Observatory is a hybrid detector to measure ultra-high energy cosmic rays by an array of 1600 surface detectors and 24 fluorescence telescopes. Every night the response of the telescopes to light from a LED source, distributed through fibers and illuminating the camera, is measured. This relative calibration monitors the response of each pixel and the time stability. The absolute calibration is one way to perform an end-to-end calibration including effects of the UV filter transmittance, losses at the corrector ring elements, mirror reflectivity, shadowing effects of the camera, light collection efficiency, photomultiplier tube (PMT) gain, and the response of the analog and digital electronics. The absolute gain of each PMT is obtained from the variance of the recorded signals and taking into account the single photoelectron resolution (SER) and the bandwidth of the analog electronics. Thus, the knowledge of the SER and the quality parameters of the optics allow us to calculate the sensitivity piece by piece and to cross-check the absolute calibration. This work presents the results of measurements of the single photoelectron spectra recorded on site with our standard setup. The results are consistent with the Monte Carlo simulations of the multiplication processes at each dynode.