The Performance of High-Gain First-Dynode Photomultipliers

The performance of photomultipliers having first dynodes of cesium-activated GaP with gains of from 30 to 50 is discussed. The resolution capabilities are high enough so that the pulse height distribution obtained from low intensity scintillations breaks up into separate peaks for one electron, two electrons, etc. This high resolution makes it possible to separate unwanted single electron dark pulses from the scintillation signal pulses very effectively. The full width at half maximum (FWHM) of a single electron pulse height distribution is typically 40% to 50%. Methods of measuring the resolution of these tubes are discussed together with some of the results obtained. These high gain dynodes appear to have good life characteristics. Their life and fatigue properties are as good or better than those of CuBe dynodes.