Effect of Photocathode-to-First Dynode Voltage on Photomultiplier Noise Pulses

The experiments of Pagano, in which the noise count at a high discriminator setting was found to reach a minimum at a V/sub C//V/sub D/ ratio of approximate unity, were repeated. Two low-noise photomultipliers (EMI 9514A, a venetian blind type, and EMI 9524S, of box and grid structure) were studied under conditions where all single electron noise was counted. The results do not show a minimum in the noise vs V/sub C//V/sub D/ graph, but instead a steadily rising curve tending toward a plateau. For the 9514A tube, the curve can be explained by a combination of the Schottky effect and an increase of the number of electrons produced at the first dynode as V/sub C/ is increased. (D.L.C.)