An Amplitude-Independent Nanosecond Timing Discriminator for Fast Photomultipliers

A circuit has been developed which reduces amplitude-dependent timing errors to less than 0.2 nsec over a dynamic amplitude range of more than 100 to 1. The anode signal is clipped, and a pedestal added, allowing adjustment of the discriminator to the true "zero-crossing" point. Tunnel diodes are used as threshold devices and pulse shapers. A version of this circuit adapted to RCA-2067 photomultipliers is described which features a pulse-recognition threshold variable from < 1 to 10 mA and a pulse-pair resolution of < 15 nsec. In tests performed with a mercury-light pulser and in coincidence experiments with plastic scintillation counters, the time delay through this discriminator was found to be essentially independent of the signal amplitude over the linear range of the photomultipliers.