Several controls are available on timing discriminators to optimize performance. Common to all discriminators is the Threshold adjustment. All ORTEC discriminators have front-panel adjustable potentiometers for threshold control, except those that incorporate a computer-controlled DAC. The Threshold should be adjusted above the noise to reduce false triggering. In some timing applications, it may be desirable to raise the Threshold and eliminate the lower pulse amplitudes which typically produce worse timing resolution.

Constant-fraction discriminators, used either in the constant- fraction or the ARC timing mode, have additional adjustments. Of principal importance is the delay selection. Various models have external cable delays, internal cable delays, or internal lumped-constant delay lines. Internal delays are the most convenient, but external cable delays allow better optimization of the timing performance in exacting experiments.

When using detectors having a constant rise-time signal, the delay is nominally equal to the time taken by the detector signal to rise from the intended triggering fraction (e.g., 20%) to 100% of its maximum amplitude. As the selection of the delay is critical, experimentation is appropriate to determine the optimum value. For example, a 36-cm delay cable was found to be optimum when using a Hammamatsu R1332 or Burle 8850 PMT with the Model 583B CFD and a 12.9 cm³ BC418 scintillator.

Selection of the delay for germanium detectors is more difficult and can best be determined experimentally. In general, the larger the germanium detector, the longer the delay. A 10% HPGe detector may require a 20-ns delay, while a 70% relative efficiency detector may require a 35-ns delay. However, there is still a large spread in optimum delay, even among detectors of similar size. Often a delay unit consisting of various lengths of high-quality coaxial cable is used to set the delay for a germanium detector. When using an external delay unit, its insertion delay as well as the delay of the interconnecting cables must be counted as part of the delay.

The optimum delay when timing with silicon charged-particle detectors is dependent on the preamplifier. Generally, the charge collection time for this type of detector is much faster than the rise time of the preamplifier. Because the preamplifier output delivers the signal to the constant-fraction discriminator, the proper delay is based on the rise time of the preamplifier output signal. If additional amplification follows the preamplifier, the delay must be appropriate for the signal fed to the constant-fraction discriminator.

The final critical adjustment on a constant-fraction discriminator is the Walk Adjustment. Referring to Fig. 4, the Walk Adjustment corresponds to setting the Zero-Crossing Reference. Most units have the Walk Adjustment available on the front panel, while in a few units the walk must be adjusted using a printed wiring board potentiometer.

Most constant-fraction discriminators have a special Monitor output, which can be used to optimize the walk adjustment. The constant-fraction discriminator is connected as shown in Fig. 11 when its input is taken from an actual detector and its output is used to trigger a fast oscilloscope. The Monitor output signal is delayed a few nanoseconds and connected to a 50-Ω input on the oscilloscope. The Monitor signal will generally be one of two types. ORTEC Models 583B and 935 have a well-shaped monitor signal like that shown in Fig. 12a. Other discriminators provide a truncated monitor signal from the zero-crossing detector output like that shown in Fig. 12b. The well-formed Monitor signal is an output from the transformer pulse shaping circuit used in Models 583B and 935. In Fig. 12a, the walk adjustment is optimized when all pulse amplitudes cross through the baseline at the same time. In Fig. 12b, the walk adjustment is optimized when the noise on the baseline between pulses is centered between the high and low logic levels of the zero-crossing detector output. A further fine tuning of the walk adjustment can be achieved by minimizing the peak width observed in the time spectrum from a time-to-amplitude converter.

Figure 11.  System Interconnection to View Walk Adjustment.

(a)

(b)

Figure 12.  Monitor Signals when Triggered by the Constant-Fraction Discriminator Output Signal for (a) Passive Pulse Shaping and (b) for Active Pulse Shaping.