Anomalous variations in dark pulse rate in GaP first dynode PMTs at constant temperature

The temporal dependence of dark pulse rate in RCA 8850 tubes has been examined at different high voltages at constant temperatures. The experimental results reveal explicitly that at higher applied voltages considerable amount of luminescence occurs possibly as a result of charge buildup on the dynodes, thus causing an anomalous increase in dark pulse rate. For tubes of this type, and under specific operating conditions, the phenomenon might introduce a large unsuspected error in photon counting and in low-level scintillation counting experiments.