Single photoelectron time spread measurement of fast photomultipliers

Abstract

Time spread measurements, using single photoelectrons of fast photomultipliers having dynodes with cesium-activated gallium-phosphide secondary emitting surfaces have been investigated. Measurements of the time spread, made by means of a specially developed measuring system with improved time resolution capabilities, on several 8850s and C31024s are presented and discussed. Critical evaluation of the measuring system is given, particularly concerning the influences of the light pulse width and the constant fraction discriminator timing errors on the system time-resolution capabilities. The measuring system incorporates a subnanosecond light-pulse generator capable of operating at a repetition rate of 3 MHz with output light pulses having a full width at half-maximum of less than 200 ps. It also incorporates a constant fraction discriminator which has a time walk less than ±35 ps over a range of input-pulse amplitudes from 35 mV to 8 V. The optimum operating conditions of the photomultipliers have been determined for minimum time spread and relative collection efficiency. The error of the time spread measurements, because of the finite width of the light pulse, is discussed and determined for different pulse widths.