Pulse shaping and standardizing of photomultiplier signals for optimum timing information using tunnel diodes

Abstract

Accurate timing signals have been derived from fast photomultiplier pulses by differentiating the photomultiplier pulse in order to produce a zero-crossing signal whose time of zero crossing is fixed over a large dynamic range of light level. Detection of the zero-crossing point is accomplished with a balanced-bridge tunnel-diode discriminator, which produces a standardized output of 100-mV amplitude, and a half-width of 4 nsec. A balanced bridge is used to prevent the zero-crossing signal from appearing at the output. For a twenty-to-one range of photomultiplier-input light level, the time shift of the output pulse may be as low as 0.5 nsec with a change in the output amplitude of less than a factor of two. When this discriminator is used with a 14-stage photomultiplier, the sensitivity is great enough to allow direct operation from single photo-electrons. The above numbers are an indication of what one can obtain with the tunnel diode and transistor combination chosen at the time of the original development almost two years ago. Since that time, particularly in the tunnel-diode field, numerous improved devices have become available. We are currently evaluating these in zero-crossing circuits. The zero-crossing photomultiplier pulse has been generated in two ways. The first method uses an overdamped LC-tuned circuit built into the base of a 6810-A photomultiplier. Here the time of zero crossing is controlled by the frequency of the LC-tuned circuit. The decay time of the scintillation plastic used and the rise time of the photomultiplier determine what this zero-crossing time should be. A clipping stub has also been used to produce the zero crossing remotely from the photomultiplier. By this method, one can control the zero-crossing time by both the length and impedance of the clipping stub.