Monitoring DC anode current of a grounded-cathode photomultiplier tube

The Pierre Auger Observatories (PAO) for the highest energy cosmic rays wil l make use of both the Cherenkov and Air Fluorescence techniques. Surface D etectors (SD) and Fluorescence Detectors (FD) will have to operate in a des ert-type environment during at least 15 years. In order to avoid dust depos ition, due to electrostatics, and other practical inconveniences derived fr om biasing the cathode with a negative potential, the 15 000 PMTs of the FD will operate in the grounded cathode configuration. Despite the fact that the anodes Frill remain at high voltage with respect to ground, the DC anod e current, which varies with background light, will have to be recorded. We have developed a current monitoring system based on a novel optocoupled fe edback circuit that allows sensitive, linear, and temperature-independent m easurements of the DC anode current. A distinctive feature of this circuit is that it uses optical coupling between passive components at high voltage and active components near the ground potential. This represents a substan tial improvement over classical solutions which require the supply of power to an active circuit at high voltage. We report on the first tests perform ed with both active and passive biasing networks which demonstrate the vali dity of this new method. (C) 1999 Elsevier Science B.V. All rights reserved .