Cosmic Ray Induced Dark Current in Photomultipliers

Abstract

Čerenkov light flashes produced by cosmic rays traversing the window of an end‐on photomultiplier create a fundamental noise limitation that cannot be reduced by cooling the tube. Although the cosmic ray pulse rate is only 1 or 2 cm⁻²/min, the individual pulses may be hundreds of times larger than single electron pulses. Thus in dc or charge integration photometry, cosmic ray noise can exceed thermionic dark current noise at temperatures as high as +10°C. These assertions are based on the observed behavior of EMI 6256 and 9558 tubes. The implications for photometry of faint astronomical sources are discussed.