A leading-edge timing discriminator is appropriate when the optimum time resolution is not essential. "Walk" is normally the dominant limitation on time resolution with this method. The rise time of the analog pulse at the discriminator input can be used as a rough estimate of the contribution "walk" will make to the time resolution when a wide range of pulse amplitudes must be processed.

The intrinsic timing jitter of a scintillation detector is inversely proportional to the square root of the pulse amplitude. Consequently, the discriminator threshold can be set to cut off the lowest pulse amplitudes, which have the worst intrinsic jitter. If a very narrow range of pulse amplitudes is being analyzed, the "walk" contribution will be very small, and the discriminator threshold can be set at the level that yields the best time resolution. As shown in Fig. 2(a), the minimum time resolution will typically occur when the threshold is set somewhere between 10% and 40% of the anode pulse-height.

Figure 2. A Comparison of Leading-Edge Timing with Constant-Fraction Timing for a Narrow
Pulse-Height Range. The source was ²²Na, with the selected equivalent electron energy in the scintillator = 340 keV. The time resolution (FWHM) is Δt.