Control Circuit for Arbitrary Gating of Side-on Photomultipliers

Abstract

In many experiments that use pulsed laser excitation, it is necessary to gate a photomultiplier tube either on or off to prevent saturation of the detector while recording much weaker events that follow only a few microseconds later. Several methods of gating photomultipliers have been reviewed in the literature, and commercial devices are also available. These circuits, in general, operate by adjusting the potentials of either the photocathode or the dynodes to inhibit collection of photoelectrons or electron multiplication, respectively. When the potential of the photocathode is varied to achieve the off state, and the tube is then illuminated, a space charge may be generated which can subsequently affect the anode current after the tube is returned to the on state. This undesired optical memory effect is eliminated in gating circuits that utilize intermediate dynodes as gating elements rather than the photocathode. Many gating circuits also require capacitive coupling to isolate the drive amplifier from the high dc potentials of the dynode chain. This coupling approach, however, induces an electrical memory effect and sets pulse width and duty cycle restrictions on the gating waveform. Consequently, such drive circuits cannot perform both normally off and normally on gating functions. Also most gating circuits typically require a second power supply whose adjustment critically influences the gain of the photomultiplier. Phototransistors have been used in place of capacitors to effectively overcome these difficulties, by optically coupling the gating signal to the dynode chain. Unfortunately, in this case the > 10 μs transition times between the on and off states are longer than desired in many applications because of the inherently slow response of the opto-electric coupling elements. These difficulties can be eliminated in dc-coupled gating circuits that control the appropriate dynode potentials.