Cosmic ray induced photomultiplier noise

Cosmic rays produce a component of photomultiplier noise which often dominates the high amplitude region of the noise pulse height spectrum and which is not reduced by cooling the tube. Theoretical and experimental investigations were conducted to show that the source of the noise is Cerenkov radiation produced by individual high energy cosmic rays in their passage through the glass envelope of the tube, principally in the glass faceplate on which the photocathode is deposited. The investigation entailed a comparison of theoretical and experimental cosmic ray induced pulse height spectra with photomultiplier orientations of 0 and 90 degrees and theoretical and experimental plots of mean pulse height as a function of orientation. The Cerenkov mechanism was indeed shown to be the main source of high amplitude photomultiplier noise.