Noise in Photomultiplier Tubes

Direct measurements of the statistical gain fluctuations occurring in the electron multiplication process in a sampling of multiplier phototubes are reported. The results are expressed in terms of the effective reduction of the photocathode quantum efficiency, i.e. the noise factor, as determined by a comparison of the experimentally observed output signal-to-noise ratio with the expected ratio based on independent direct current measurements of photocathode quantum efficiency. The spread in the measured magnitudes of the resultant noise factor, in nominally identical tube types, is sufficient to demonstrate that both photocathode quantum efficiency and electron multiplier noise factor should be specified if optimum detector performance is to be assured.