Signal linearity, gain stability, and gating in photomultipliers: application to differential absorption lidars

We present the results of a study that identifies a photomultiplier tube (PMT), divider networks, and gating circuitry for use in the current detection mode, in which the specific objectives were to hold variations in both signal gain over a 25-µs gate period and signal linearity up to 20 mA to less than ±0.1%. The study, aimed at optimizing the performance in a nadir-looking airborne UV differential absorption lidar, is sufficiently general to apply to other critical gated or pulsed PMT applications in which performance at the 0.1% level is required. Signal-induced gain increases peculiar to pulsed or gated signals from PMT's with BeCu dynodes that can have values between 1 and 10% over 25 µs were reduced to less than 0.1% by the use of a 2-in. (5.08-cm)diameter PMT (EMI 9214) with CsSb dynodes. Compliance with the linearity requirement was achieved for gated signals up to 8 mA at a current gain of ~10(7) with the EMI 9214 PMT controlled by a resistive divider network with an inverted taper, in which the linearity data showed no tendency toward overlinearity caused by either space charge effects or induced divider-network voltage changes.