NORMALLY ON PHOTOMULTIPLIER GATING CIRCUIT WITH REDUCED POST-GATE ARTIFACTS FOR USE IN TRANSIENT LUMINESCENCE MEASUREMENTS

A normally on gating circuit with reduced gate-associated artifacts and improved extinction has been developed for focusing electrode-equipped photomultiplier tubes (PMTs) such as the Thorn EMI, Inc. model 9816. Unlike previous gating methods where the focusing electrode is pulsed negative with respect to the photocathode (PC) to gate the tube off, this method holds the focusing electrode at a constant potential of +80 V with respect to the PC and pulses the first dynode from +80 to -246 V. This technique effectively eliminates post-gate signals induced by PC irradiation during the gating period, though at the cost of lower extinction efficiencies. By also pulsing the fifth dynode capacitatively with -326 V, attenuation is restored to greater than 10(7):1. The circuit assembly is compact enough to fit within a standard PMT housing and operates from the PMT dynode current. Turn-off and turn-on times of 55 and 40 ns, respectively, are obtained. The gate is thus suitable for exciting light pulse rejection in low level, submicrosecond time-resolved fluorescence, and phosphorescence measurements.